DESIGN OF SOFT STARTER FOR THREE PHASE INDUCTION MOTOR - PROJECT REPORT
CHAPTER -1
INTRODUCTION
Induction motor is a highly efficient electrical machine working closed to its rated torque and speed. Due to robustness, reliability, low price and maintenance free, induction motors (IMs) are used in most of the industrial applications. The influence of these motors (in terms of energy consumption) in energy intensive industries is significant in total input cost[1]. Starting a de-magnetised induction motor from standstill is a demanding and complex process. At the instant of switching all the energy necessary to magnetise the motor, to provide the acceleration force, and to supply the kinetic energy of the rotor and load, must be present together with the energy to overcome the mechanical and electrical losses[2]. Whenever a induction motor is started, the electrical system experiences a current surge and mechanical system experiences a torque surge[3]. When line voltage is applied to the motor during starting, the line current can be of 5-7 times larger than that in the normal operation and the starting torque can be 2 times larger than that in the normal operation. The big starting current will cause power voltage drop and influence the normal operation of other equipment connected in the same power line, and even cause the electrical network to lose stability[4].Since starting current is determined by the impedance of the motor while starting, reduction of the stator voltage will reduce the starting current requirement. If the starting voltage is reduced to 50% of its nominal value, the starting current will also be reduced by the same percentage, in accordance with Ohm’s Law I=E/Z ,where I is the starting current, E is the voltage applied to the motor and Z is the locked rotor impedance of the motor. Since Z is essentially a fixed value at the instant of starting, any change in voltage will directly affect the starting current [3].
Methods of starting ac induction motors can be broken down into four basic categories:
- Full-voltage (across-the-line) starting
- Electromechanical reduced-voltage starting
- Solid-state reduced-voltage starting, and
- VFD starting
Electromechanical reduced-voltage starting has been in existence nearly as long as the induction motor itself. This starting method encompasses autotransformer starting, wye–delta (star–delta) starting, and resistor/reactor starting. Each of these methods requires the use of some type of mechanical switch or contact. Electromechanical starting is the most common method of reduced voltage starting used in industry today. Solid state starters, on the other hand, have only been in existence since the early 1970s. This method of starting uses programmable logic controllers in combination with sophisticated power electronic circuits to provide reduced voltage and/or torque. Advances made in the electronics industry with new high-power diodes and SCRs have led to the development of both electronic soft starters as well as inverter controlled VFDs. In each of these cases, smooth, electronically controlled starts can be achieved with a high degree of process control.
In my
dissertation work, I have designed a solid state thyristor based soft-starter.
Because thyristor based soft starters are economical, simple, and reliable. By
using thyristor based starting, the initial inrush current of motor can be
reduced significantly. Additionally they offer smooth acceleration, ease in
implementation of current control, and energy savings with a partial load can
be available. The proposed work
is first simulated using matlab
simulink 7.13 environment and later the hardware is developed. For the
developed work it has been seen that the hardware and simulation results
matches to a greater extent.
CHAPTER-2
LITERATURE SURVEY
2.1 Review of Literature
I have referred more than twenty research papers on soft starting of three phase induction motor. The literature survey is presented by considering few papers.
Ø
John Larabee, Brian Pellegrino, Benjamin Flick
presented a paper titled “INDUCTION
MOTOR STARTING METHODS AND ISSUES”, members of IEEE which is a copyright
material of IEEE .Paper No. PCIC-2005-24 Siemens Energy & Automation,
In this paper, the most common starting methods and their recommended applications are specified. Many methods can be used to start large AC induction motors such as full voltage, reduced voltage either by autotransformer or Wye - Delta, a soft starter, or usage of an adjustable speed drive can all have potential advantages and trade offs. Choosing the proper starting method for a motor will include an analysis of the power system as well as the starting load to ensure that the motor is designed to deliver the needed performance while minimizing its cost.
In order for the load to
be accelerated, the motor must generate greater torque than the load
requirement. In general there are three points of interest on the motor's speed-torque
curve. The first is locked-rotor torque
(LRT) which is the minimum torque which the motor
will develop at rest for all angular positions of the rotor. The
second is pull-up
torque (PUT) which is defined as the minimum
torque developed by the motor during the period of acceleration from
rest to the speed at which breakdown torque occurs. The last is the breakdown torque (BDT) which is defined as the maximum torque which the motor will develop. If any of these points are below the
required load curve, then the motor
will not start. The time it takes for the motor
to accelerate the load is dependent on the inertia of the
load and the margin between the torque of
the motor and the load curve, sometimes called accelerating torque. In general, the longer
the time it takes for the motor to accelerate the load, the more heat
that will be generated in the rotor bars, shorting
ring and the stator winding. This heat leads
to additional stresses in these parts and can
have an impact on motor life.
The full voltage starting
method, also known as across the line starting is one of the easiest
method to employ, it has the lowest
equipment costs, and is most reliable. This method utilizes a control to close a
contactor and apply full line voltage to the motor
terminals. This method will allow the motor to generate its highest starting torque and provide the shortest acceleration times. This method also puts the
highest strain on the power system due to the high starting currents that can
be typically six to seven times the normal full load current of the motor. If
the motor is on a weak power system, the sudden high power draw can cause a
temporary voltage drop, not only at the motor terminals, but the entire power
bus feeding the starting motor. This voltage drop will cause a drop in the
starting torque of the motor, and a drop in the torque of any other motor
running on the power bus. The torque developed by an induction motor varies
roughly as the square of the applied voltage.
Besides electrical variation of the power bus, a potential
physical disadvantage of an across
the line starting is the sudden loading seen by
the driven equipment. This shock loading due to transient
torques which can exceed 600% of the locked rotor torque can increase the wear on the equipment, or
even cause a sudden failure if the load
can not handle the torques generated by the motor during
staring.
The reduced voltage methods are intended to reduce the impact of motor starting current on the power system by controlling the voltage that the motor sees at the terminals. It is very important to know the characteristics of the load to be started when considering any form of reduced voltage starting. This method provide a solid means of easing the required energy draw, but at the expense of motor generated torque during starting. This method may also lead to having to increase the size of the motor in order to generate the torque required for the load.
Adjustable
Frequency Drives type of device gives the
greatest overall control and flexibility in starting induction motors giving the most torque for an amount of current.
It is also the most costly. The drive varies
not only the voltage level, but also the frequency, to allow the motor to operate on a constant volt per hertz
level. This allows the motor to generate full
load torque throughout a large speed range, up to 10:1. During starting, 150% of rated current is typical.
This allows a significant reduction in the power required to start a load and reduces the heat generated in the motor, all of which add up to greater efficiency. Usage of the AFD also can allow a smaller motor to be applied due to the significant increase of torque available lower in the speed range. The motor should still be sized larger than the required horsepower of the load to be driven. The AFD allows a great degree of control in the acceleration of the load that is not as readily available with the other types of reduced voltage starting methods. The greatest drawback of the AFD is in the cost relative to the other methods. Drives are the most costly to employ and may also require specific motor designs to be used. Based on the output signal of the drive, filtered or unfiltered, the motor could require additional construction features.
The
characteristics such as initial current, initial torque and cost of the system
for different starting methods are compared. The
selection of the best method to
use will be based on the overall power system constraints, cost for
the equipment, and the driven equipment.
Ø Chia- chou yeh and Nabeel A.O. Demerdash presented a paper on “FAULT TOLERANT SOFT STARTER CONTROL OF INDUCTION MOTOR WITH REDUCED TRANSIENT TORQUE PULSATIONS” IEEE Transactions on energy conversion, vol. 24, no. 4, December 2009.
Fault-tolerant operation of induction motors fed by soft starters when experiencing thyristor/silicon-controlled rectifier open-circuit or short-circuit switch fault is presented in this paper. This fault-tolerant approach is applicable to any soft starters that control small to large integral horsepower induction motors. This paper consists of five other sections. The principles of fault-tolerant operations are presented in Section II, while the small-signal modeling of the motor–soft starter controller system is developed and discussed in Section III. Simulation and experiment results obtained under both healthy and faulty conditions, including results from using the present fault-tolerant control approach, are given in Sections IV and V, respectively. Finally, conclusions are presented in the final section of this paper.
A circuit configuration of the conventional industrial-type three-phase soft starter topology is explained in this paper. It comprises of a set of back-to-back connected thyristors/SCRs (A1, A2, B1, B2, C1, C2) in series with each phase of the motor’s stator phase windings, respectively. Snubbers consisting of series-connected resistors and capacitors are connected in parallel in the circuit to reduce any switching transients. The voltage feedback loop is used to control the starting acceleration profiles of the motor currents and torque. It controls the duration of the starting period, which varies for different types of applications. A small-signal model representing the induction motor–soft starter controller is developed and described. More specifically, the derivations of the open-loop and closedloop (feedback) system transfer functions of the small-signal model consisting of the controller and the plant (induction motor+ soft starter) are detailed here.
The softstarter controller consists of a set of PI regulators for the voltage and current loops with unity feedback. It is because of tuning of the PI regulators that the small-signal model is being developed for transient response and stability purposes. In order to incorporate the electrical behavior of the soft starter into the voltage and current control loop systems, a mathematical expression describing the nonlinear nature of the soft starter is derived and detailed here. The main thrust of the soft starter is to provide voltage excitation to the motor, where the rms value of this impressed voltage is dependent on the firing angle profile. As a result, a mathematical expression that describes the relationship between the impressed rms voltage and the firing angle is developed in this paper.
Simulation results on motor starting performance under both healthy and faulty conditions were obtained for a 1.492-kW, 460-V, four-pole, three-phase induction motor. Three simulation cases were conducted for comparisons, namely: 1) healthy case with conventional control; 2) faulty case with conventional control; and 3) faulty case with proposed fault-tolerant control. The simulation work was carried out in a MATLAB–Simulink environment. The motor was simulated in the dq frame of reference representation. Meanwhile, the load is of the fan type: TL = kLω2m, where ωm denotes the motor speed in mechanical radians (mech.rad) per second and kL is the load coefficient in N·m/(mech.rad/s)2 .
The three-phase healthy soft starter using conventional open-loop voltage control was first simulated. From the results of the three-phase motor currents we can observe that the motor currents have smooth starting profiles as the firing angle is decreasing, with low starting currents. To demonstrate the fault impact on motor performance due to a short-circuit SCR switch fault while using the open-loop voltage control method in a two-phase SCR mode (since the SCR switching of the shorted phase produces negligible effect), the results of the motor phase currents are depicted . Note that the high motor current unbalances during the starting transients. This is not the case for the present fault tolerant two-phase control. The three-phase motor currents have reasonably smooth starting profiles, which indicate significant starting transient improvement due to the open-loop two-phase control. To further demonstrate the fineness of the present approach, the transient current space vector plots of the motor currents in the form of _is = iq + jid = 2/3(ia + ibej2π/3 + icej4π/3 are used, for the healthy case using three-phase open-loop control, the faulty case using two-phase open-loop control, and the faulty case using proposed fault-tolerant closed-loop control, respectively. The current space vector plot due to three-phase open-loop control exhibits a circular locus, while the two-phase open-loop control in the event of a fault results in an elliptic locus with higher starting currents. An improved starting transient of the current space vector plot with a circular locus can be observed for the proposed closed-loop two-phase control approach. The developed motor torque profiles for all three cases are depicted. Again, one can see a smooth starting profile with minimum torque pulsations. However, this is not the case under open-loop two-phase control with fault. An improved starting torque profile for the proposed approach is illustrated. It is shown that the starting transient torque pulsations are significantly reduced. The simulation result for the case of open-circuit SCR switch fault is not presented, since the motor does not produce a starting torque during the occurrence of such fault. However, if the faulty switch is isolated by the corresponding bypass contactor, the results of which will be equivalent to the case of a short-circuit switch fault with only two controlled SCR phases in operation.
Experimental tests had been carried out using the same 1.492-kW induction motor that was used in the earlier simulation work. A test setup of the soft starter system is illustrated, while the induction motor test rig is shown.
The test setup consists of a three-phase back to back connected SCRs, voltage and current sensors, signal conditioning circuit boards, a gate driver, and a Texas Instruments Incorporated DSP board [22] (Model: TMS320F2812) for controlling the triggering sequences of the SCR switches. The test cases were similar to those carried out for the earlier simulation work.
Ø Jack Bowerfind And Sylvester J. Campbell, presented a paper on “APPLICATION OF SOLID-STATE AC MOTOR STARTERS IN THE PULP AND PAPER INDUSTRY” IEEE Transactions On Industry Applications, Vol. Ia-22, No. 1, January/February 1986
Solid-State power devices, such as thyristors, have for many years been accepted universally in the pulp and paper industry as the prime means of supplying controlled power to dc motors. These same power devices, with extensions in their associated control and application technology, are now being supplied to eliminate both electromagnetic line starting and conventional methods of reduced voltage starting of ac squirrel cage induction motors. Additional control and protective functions are being realized with these solid-state starters that are not possible or feasible with previous motor starters. The purpose of this paper is to review these new solid-state starters as to what they are, how they operate, protection features, options, enclosures, rating methods, service conditions, maintenance requirements, and application considerations. Solid-state motor starters, up to 575 V, generally consist of six power semiconductors connected between the line and the motor, plus gating or triggering circuitry and various combinations of control for sequencing, protecting, and status indication.
Solid-state reduced voltage motor starters can be divided into two basic parts, namely, the power section and the logic control.
The power section is comprised of two major components: the thyristor and the heat sink. Thyristors are solid-state switches that allow current to flow in only one direction. When connected back to back, they can be controlled to turn on as required and thus control the voltage that is applied to the ac motor.
The control logic for a basic solid-state reduced voltage motor starter comprises the following basic components:
1) Trigger circuit
2) Amplifier circuit
3) Current limit circuit
4) Current trip circuit.
The purpose of the trigger circuit is to generate a turn-on command for the thyristor. The trigger pulse is designed to inject a high level of current pulse energy into the thyristor gate terminal for a short time and then to continue supplying a lower amount of gate current until the thyristor is fully turned on. The amplifier's function is to supply an analog signal to the triggers, which will control the firing angle phase delay. One of the most advantageous features of a solid-state reduced voltage starter is the current limit circuit .Current limiting with thyristors allows infinitely adjustable values over the entire design range. By definition, a motor starter must provide a means of motor running overload protection. Solid state starters lend themselves very nicely to this task because the electronic intelligence is already there. In a well-designed product, it is no longer necessary to select and install a heater element to match the motor rating. The electronic current allows the starter to be calibrated to the motor full load amperes by properly setting a few switches or turning a dial. The thermal response can also be tailored to approximate the motor heating characteristics. In this paper he has also highlighted on maintenance requirements of soft starter and its application.
Solid-state starters have the capability to reduce no-load or light-load motor losses by sensing the motor load and reducing the voltage applied to the motor during these conditions. The entire effect is a reduction of losses within the motor itself when the motor is at other than full load or at other than rated voltage. Thus, soft starter helps us to save energy.
Ø G. Zenginobuzt, I. C a d i d , M. Ermist and C. Barlak presented a paper on “SOFT STARTING OF LARGE INDUCTION MOTORS AT CONSTANT CURRENT WITH MINIMISED STARTING TORQUE PULSATIONS” TUBITAK-METU Information Technologies and Electronics Research Institute, TR 0653 1 Ankara – Turkey
Direct-on-line starting of large ac motors may present difficulties for the motor itself and the loads supplied from the common coupling point because of the voltage dips in the supply during starting, especially if the supply to which the motor belongs is weak. AC motor starters employing power semiconductors are being increasingly used to replace electromagnetic line starters and conventional reduced voltage starters because of their controlled soft-start capability with limited starting current. Among these, thyristorized soft starters which apply reduced voltage to the motor are cheap, simple, reliable, and occupy less volume, and hence, their use is a viable solution to the starting problem of medium voltage large, ac motors in applications where the starting torque requirement of the load is not high. When the motor in service is continuously supplied from the thyristorized soft starter, it also serves to the minimisation of a wide variety of electric transients that the motor is exposed due to disturbances in electric supply system. These disturbances can be as small as momentary voltage fluctuations to as large as actual voltage intemptions. If the voltage depression is severe, the main circuit breaker or the soft starter will disconnect the motor from the electrical supply. The stoppage of an essential service motor in a continuous process may result in a costly shut-down. Therefore, before the motor reaches zero back-up supply (usually motor-generator set), or back to the electrical supply after its recovery. In conventional systems, this will lead to severe reclosing transients in torque and current. A satisfactory reclosing performance requires controlled switching of motor terminals to the electrical supply.
In this paper, some control strategies are proposed to eliminate electromagnetic torque pulsations both at starting and reclosing, and to keep the line current nearly constant at a preset value over the entire starting period. Torque pulsation elimination strategy defines the triggering instants of the soft starter thyristors on the first supply voltage cycle just after energising the motor.
The proposed current control strategy is composed of successive cosinusoidal and constant function segments of triggerring angle of thyristors. All control, protection and monitoring functions are implemented on an 8-bit microcontroller. Transient performance analyses of the resulting soft starter are carried out by means of a hybrid ABC/dq model which takes into account three-phase, two-phase and disconnected modes of operation in terms of actual stator variables. Simulations have been carried out on both a medium-voltage, large IM driving a centrifugal pump, and a low-voltage universal machine set. Theoretical results are verified experimentally on a custom-design test-bed composed of a universal machine set and a shaft torque measuring system.
The schematic diagram and operation of soft starter is mentioned in this paper. The mathematical model used in digital simulation of the system are given in this section. Numerical solution method and the necessary transitions between various operation modes of the starter will also be described. This paper also explains how to eliminate the torque pulsations and control current. The soft starter described in this paper can detect the occurrence of a voltage sag or supply interruption, thus blocking the firing pulses of thyristors. The reclosing delay time can be programmed on a pc based controller, and its numerical value depends on experience by simple voltage records in the field. In a thyristor soft starter, since the switching instants and magnitude of applied voltage are controllable, reclosing torque transients can be nearly eliminated.
The performance of the developed soft starter is tested in the laboratory on a 5 kVA, 0.4 kV, 8.1A universal machine set equipped with a custom-design shaft torque and speed measuring system in the dynamic state. Torque and speed measuring system between IM and the driven machine is shown. The torque transducer is coupled to the shaft of IM by using double Rigiflex couplings, to the driven machine by a flexible coupling, and installed to the test bed with base-plate mounting . The ratings of two interchangeable strain gauge type measuring elements of the torque transducer with slipringless transmission by rotary coils are 100 Nm and 1000 Nm.
In order to avoid magnification and reduction of shaft torque pulsations in the dynamic state, the stiffness coefficients and moment of inertia of the torque measuring element, couplings and shafts used in the torque measuring system are chosen carefully by taking into account parameters of the shafts and rotors of IM, and the driven machine. By this way, the ratio of forcing frequency to resonance frequency has been adjusted to Fr/Fr=3.3. Since Fr is brought nearly to 165 Hz, shaft torque pulsations in the range 0-60 Hz can be successfully measured with unity magnification factor. The author concludes by saying that, using the proposed strategies, a good acceleration profile can be tailored by smooth, pulsation-free torques over the entire starting period.
2.2 Problem statement
The electric motor designer cannot ignore the effects of inrush current when designing the motor, particularly in the larger motors. When an ac induction motor is started across the line, the electrical current demanded by the motor instantaneously reaches a value of five to six times its normal full-load running current; this is true whether the motor is fully loaded or unloaded. This instantaneous increase in starting current is called inrush current. Inrush current occurs because, at the instant of starting, the impedance of the ac induction motor winding is very low. As the rotor of the motor starts to accelerate, the impedance increases, and therefore the current starts to diminish. The foregoing is true only if the voltage applied to the motor is held at a fixed value. Therefore, it becomes obvious that if the voltage applied to the motor are reduced at the start, the inrush current would be reduced. Or better yet, if we could gradually apply the voltage to the motor from zero on up, the inrush current would be very small [5].
Reducing the inrush current during starting will greatly increase motor life. Most motors are limited to a certain number of starts per unit of time. One of the major concerns that limit the number of starts is the motor's end-turn movement. Inrush causes a tremendous magnetic force on the end-turns, causing them to physically move. This movement causes abrasion between end-turns, gradually eroding away the insulation. This movement can also cause the wires to fracture at the point of where they leave or enter the stator slots.
Historically, the majority of large motor burnouts occurs at the end-turns, and this generally can be traced to the movement caused by inrush currents[5]. Over the years, many methods have been developed and used to reduce the inrush current, such as the autotransformer, wye-delta motor starting and in more recent years, the reduced-voltage solid-state starter. In my work, I am designing a three phase solid state reduced voltage starter for three phase induction motor. With this method, torque pulsations are reduced and the sudden increase in the motor current is made to increase gradually. This linear increase in the current does not create any hot spots, thus protecting the motor windings and system damage. With reduced torque pulsations and starting current, the motor insulation, conductors life extends and there by the motor life is extended and at the same time the starting losses are reduced.
CHAPTER -3
INDUCTION MOTOR AND SOFT STARTER
3.1 Introduction to Induction Motor
An induction
motor or asynchronous motor is
a type of alternating current motor where power is supplied to the rotor by
means of electromagnetic induction. An electric motor converts electrical power
to mechanical power in its rotor (rotating part). There are several ways to
supply power to the rotor. In a DC motor, this power is supplied to the
armature directly from a DC source while, in an induction motor, this power is
induced in the rotating device. An induction motor is sometimes called a rotating transformer because the
stator (stationary part) is essentially the primary side of the transformer and
the rotor (rotating part) is the secondary side. Unlike the normal transformer
which changes the current by using time varying flux, induction motors use
rotating magnetic fields to transform the voltage. The current in the primary
side creates an electromagnetic field which interacts with the electromagnetic
field of the secondary side to produce a resultant torque, thereby transforming
the electrical energy into mechanical energy.
Induction
motors are widely used, especially polyphase induction motors, which are
frequently used in industrial drives.
Induction motors are now the preferred choice for industrial motors due to
their rugged construction, absence of brushes (which are required in most DC
motors) and the ability to control the speed of the motor.
Induction motor can be
classified based on the type of the rotor used and they are
- Squirrel
cage induction motor and
- Slip-ring
or wound-rotor induction motor.
3.2 Construction of Induction Motor
Induction
Motor has a Stator and a Rotor. The construction of Stator for any induction
motor is almost the same. But the rotor construction differs with respect to
the type which is specified above.
3.2.1
Stator
The stator is the outer
most component in the motor which can be seen. It may be constructed for single
phase, three phase or even poly phase motors. But basically only the windings
on the stator vary, not the basic layout of the stator. It is almost same for
any given synchronous motor or a generator. It is made up of number of
stampings, which are slotted to receive the windings. In the construction of a
three phase stator, the three phase windings are placed on the slots of
laminated core and these windings are electrically spaced 120 degrees apart.
These windings are connected as either star or delta depending upon the
requirement. The leads are taken out usually three in number, brought out to
the terminal box mounted on the motor frame. The insulations between the
windings are generally varnish or oxide coated. The stator consists of wound
'poles' that carry the supply current to induce a magnetic field that
penetrates the rotor. In a very simple motor, there would be a single
projecting piece of the stator (a salient pole) for each pole, with
windings around it; in fact, to optimize the distribution of the magnetic
field, the windings are distributed in many slots located around the stator,
but the magnetic field still has the same number of north-south alternations.
The number of 'poles' can vary between motor types but the poles are always in
pairs (i.e. 2, 4, 6, etc.).
3.2.2
The Rotor
There are three types of rotor:
a)
Squirrel-cage rotor
b)
Slip ring rotor
c)
Solid core rotor
a) Squirrel-cage
rotor
The
most common rotor is a squirrel-cage rotor. It is made up of bars of either
solid copper (most common) or aluminum that span the length of the rotor, and
those solid copper or aluminium strips can be shorted or connected by a ring or
some times not, i.e. the rotor can be closed or semiclosed type. The rotor bars
in squirrel-cage induction motors are not straight, but have some skew to
reduce noise and harmonics. This kind of rotor consists of a cylindrical
laminated core with parallel slots for carrying the rotor conductors, which are
not wires, as we think, but thick, heavy bars of copper or aluminium or its
alloys.
The
conductor bars are inserted from one end of the rotor and as one bar in each
slot. There are end rings which are welded or electrically braced or even
bolted at both ends of the rotor, thus maintaining electrical continuity. These
end rings are short-circuited, after which they give a beautiful look similar
to a squirrel thus the name. One important point to be noted is that the end rings
and the rotor conducting bars are permanently short-circuited, thus it is not
possible to add any external resistance in series with the rotor circuit for
starting purpose. The rotor conducting bars are usually not parallel to the
shaft, but are purposely given slight skew. In small motors, the rotor is
fabricated in a different way. The entire rotor core is placed in a mould and
the rotor bars & end-rings are cast into one piece. The metal commonly used
is aluminium alloy. Some very small rotors which operate on the basis of eddy
current, have their rotor as solid steel without any conductors.
b) Slip ring rotor
A
slip ring rotor replaces the bars of the squirrel-cage rotor with windings that
are connected to slip rings. When these slip rings are shorted, the rotor
behaves similarly to a squirrel-cage rotor; they can also be connected to
resistors to produce a high-resistance rotor circuit, which can be beneficial
in starting.
c) Solid core rotor
A
rotor can be made from solid mild steel. The induced current causes the
rotation.
3.3 Theory of Induction Motor
1)
When a voltage is
applied to the stator of an induction motor, a magnetic field begins to rotate in
the stator. For example, this rotating magnetic field may be 1800 rpm. When the
rotor is at a stand still (0 rpm) the rotating magnetic field of the stator
cuts the conductor of the rotor at a maximum rate, inducing maximum voltage and
current in rotors conductor.
2)
As the rotor gains the
speed, for example 600 rpm, the conductor of the rotor are being cut by the
stators magnetic field at a slower rate since the rotors rpm is catching up to
the magnetic fields rpm. (1800rpm-600 rpm=1200 rpm) this cause less voltage
current to be induced in the rotors conductor.
3)
When the reaches full
speed (at no load) the rotor conductors are being cut at the slower rate. Note:
the rotor of the induction motor can never reaches synchronous speed. If the
rotor speed equals 1800 rpm then the rotor conductor would be cut at a rate of
0 rpm. This would result in no induced voltage and current, without current
there would be no magnetic field and there for no torque.
4)
When the mechanical
load is applied to the motor, the rotor slows down from full speed, for example
1400 rpm. The rotor conductors are now being cut at a rate of 400 rpm. This
would cause the induced voltage and current in the rotor to increase as well as
the magnetic field in the rotor conductors.
3.4 Equivalent Circuit of Induction Motor
To
analyze the operating and performance characteristics of an induction motor, an
equivalent circuit can be drawn. We will consider a 3–phase, Y connected
machine, and the equivalent circuit for the stator is as shown in Figure 3.1
Figure 3.1: Equivalent circuit for stator.
Where:
V1= Stator Terminal
Voltage
I1= Stator Current
R1 = Stator Effective
Resistance
X1 = Stator Leakage
Reactance
Z1 = Stator Impedance (R1 + jX1)
I1 = Exciting Current
(this is comprised of the core loss component = Ig, and a
magnetizing current = Ib)
E1 = Counter EMF
(generated by the air gap flux) The counter EMF (E2) is equal to the
stator terminal voltage less the voltage drop caused by the stator leakage
impedance.
E2 = V1 -
I1 (Z1)
E2 = V1 -
I1 (R1 + j X1)
In an analysis of an
induction motor, the equivalent circuit can be simplified further by omitting the
shunt reaction value, gx. The core losses associated with this value
can be subtracted from the motor power and torque when the friction, windage
and stray losses are deducted. The simplified circuit for the stator then
becomes as shown in Figure 3.2
Figure 3.2: Simplified circuit for the stator.
To
complete the circuit, the component for the rotor equivalent must be added. The
rotor equivalent circuit is as shown in Figure 3.3
Figure 3.3: Rotor equivalent circuit.
Simplifying the rotor component will yield the following
per phase approximate equivalent circuit as shown in Figure 3.4. This circuit
will be used in the motor analysis in the sections following.
Figure 3.4: Simplified circuit.
Where,
I2 = Rotor
Current
R2 = Rotor
Resistance
X2 = Rotor
Reactance
Z2 = Rotor
Impedance (R2 / S + jX2)
XX = ZX =
Air Gap Impedance
What the stator sees in the air gap is the equivalent of
putting impedance (equal to R2/S + jX2) across E2.
In the analysis, all components in the motor equivalent circuit are referred to
the stator. Solving for the Rotor Impedance in parallel with the Air Gap
Impedance will yield the following simplified equivalent circuit as shown in
Figure 3.5
Figure 3.5: Simplified equivalent circuit.
3.5
Starting
of Induction Motor in the First Place
Basically, motors convert electrical energy drawn from the power supply into a mechanical form, usually as a shaft rotating at a speed fixed by the frequency of the supply. The power available from the shaft is equal to the torque (moment) multiplied by the shaft speed (rpm).
From an initial value at standstill, the torque alters, up or down, as the machine accelerates, reaching a peak at about two-thirds full speed, finally to become zero at synchronous speed. This characteristic means that induction motors always run at slightly less than synchronous speed in order to develop power - the ’slip speed’ and, hence the term asynchronous. Figure 3.6 shows induction motor torque/speed curve, illustrates this most important characteristic.
Figure 3.6:
Torque/Speed curve for the induction motor
As for each type of motor, so each load coupled to an induction motor has its own speed/torque curve as shown in Figure 3.7
Figure 3.7: Torque/Speed Curve
- Coupled load
The acceleration of a motor-load system is caused by the difference between the developed torque (motor) and the absorbed torque (load) and is shown by the shaded area in Figure 3.8
Figure 3.8: Torque/Speed Curve
- Accelerating Torque
Obviously, the larger the difference, the faster the acceleration and the quicker full speed is reached - and, coincidentally, the greater stresses experienced by the supply and drive systems during the acceleration process. An "ideal" start would accelerate the load with just sufficient force to reach full speed smoothly in a reasonable time, and with minimum stress to the supply and drive mechanisms. The Torque/Speed Curve for High starting torque/High efficiency motor Starting is as shown in Figure 3.9.
The motor speed/torque characteristic is controlled by the rotor resistance, a motor with high rotor resistance can generate its peak torque (pull-out torque) at standstill giving the high break-away torque characteristic, which reduces steadily as the speed increases and becoming zero at synchronous speed. At the other end of the scale, a motor with a very low rotor resistance will produce a low starting torque but will generate its peak torque closer to the synchronous speed. Consequently this type of motor runs at full power with higher operating efficiency and low slip speed. Increasingly, modern induction motors to combine the twin requirements of high starting torque and efficient full-speed operation within a single motor by techniques such as double-cage or deep bar design, and this motor characteristic is ideal for use with soft starter control [2].
Figure 3.9: Torque/Speed
Curve - High starting torque/High efficiency motor Starting
The torque of a three phase motor is proportional to the internal air gap flux which consists of two components:
1) Stator winding voltage and
2) Rotor voltage that is created through the generator action of the field voltage cutting the rotor bars of the stator and thus inducing current into the bars.
Thus, with a 50% drop in motor terminal voltage, the rotor voltage will also be reduced by 50%. That condition will hold true when the motor inrush is allowed to flow freely on start-up. Therefore, the torque produced is proportional to both the field and rotor voltages. For rated voltage at per unit volts, the torque will be (field volts x rotor volts), or, 1.0 x 1.0 = 1.0 which is 100% of rated torque. With a 50% reduction in terminal voltage, torque will be 0.50 x 0.50 = .25, or, 25%. For that reason the torque of the motor is proportional to the square of the voltage with normal inrush current allowed. Since the rotor voltage is proportional to the IR drop within the rotor (IR = current in amps x rotor resistance in ohms), the rotor voltage achieved will be proportional to the rotor current. A reduced voltage starter will allow a “soft” start by virtue of the torque being reduced. The soft start will lower shock loading throughout the mechanical system of the load.
The following relationships exist between motor voltage, inrush current and developed torque:
1) Starting torque will vary approximately with the square of the applied voltage.
2) Inrush current will vary approximately with the applied voltage.
3) Rotor inrush current, which is dependent upon rotor design, will be approximately 50% of the total motor inrush current.
4) Running torque will be approximately proportional to the rotor current.
3.6 Soft Starter
A
soft starter is a solid state motor starter that is used to start or stop a
motor by notching the voltage waveform, thereby, reducing the voltage to each
phase of a motor and gradually increasing the voltage until the motor gets up
to full voltage/speed all at a fixed frequency. A soft starter reduces the
voltage by “notching” the applied sinusoidal waveform. A notch is a
non-technical term for the zero voltage area in the middle waveform. As the
notch decreases in size, the Vrms increases along with Irms.
An initial voltage, determined by the user, is ramped up to full voltage by
varying the firing angle depending on the preset profile of the soft starter.
Soft starters can be controlled via open-loop or closed-loop control.
Electrically it can be any system that reduces the torque by virtue of reduced
voltage, or a change in the motor connection.
Changing the motor connection means altering the way the windings are configured so that a reduced torque is put out from the motor, even though the voltage is normal. Case in point would be a Y-Delta (Star-Delta) starting method, or a Part Winding start. Both of those methods require a motor that has been designed to be capable of starting that way, and as such they are not universally available.
Changing the motor terminal voltage reduces the torque because the motor output torque (at a fixed frequency) varies by the square of the applied voltage. So if 50% voltage is applied to a motor, it will produce 25% of it's available torque at that point. If it is a Design B motor, the Locked Rotor Torque at Start-up is typically 160% of Full Load Torque, so starting at 50% Voltage will reduce that to 40% of FLT, limiting the torque shock to the load.
3.7 Necessity of Starter
In
general, there are two reasons to use a soft starter: the power distribution
network may not be able to handle the inrush current of the motor and/or the
load cannot handle the high starting torque. As a rule of thumb, a motor
utilizes around 600-800% of its full load
current (
Applications such as conveyors may not be able to handle a sudden jolt of torque from an across-the-line start. Utilizing soft starters reduces the wear and tear on belts, conveyors, gears, chains, and gearboxes by reducing the torque from the motor. The torque decreases as a square of the reduced voltage, and follows this approximation:
Since soft starters are generally controlled and monitored by a microprocessor, a soft starter can add many features and protections fairly easily. It can offer a choice of the starting time, limited speed control, and energy savings. Power monitoring, such as three-phase current, three-phase voltage, power, power usage, power factor, and motor thermal capacity usage, can be implemented with current transformers, a voltage meter, and an internal clock.
For starting 3
Phase Induction Motor if rated voltage is given to the stator of motor
directly, starting current will flow through the motor winding which is 5 to 6 times the rated current. This starting or initial high
current is objectionable, because it will produce large line voltages drop,
which in turn will effect the operation of other electrical equipment connected
to the same line. The starting current is controlled by applying a reduced
voltage to the stator winding during the starting time, and then full normal
voltage is applied, when the motor has run up to its rated speed.
Figure 3.10: Motor Current vs. Speed
Figure 3.10 shows the impact of using a soft start. For this motor, the initial current when it is started is 600 percent, or six times the motor’s full load current rating. The soft starter can be set to reduce this current, for example in this case to 300 percent. This limits the inrush current on the utility line. As a result of the reduction in current, the motor’s ability to generate torque is also reduced as shown in Figure 3.11. The upper curve shows the same motor started across the line. The initial torque is about 180 percent with a peak torque of over 300 percent. With the soft start limiting the current, the torque speed curve is reduced, reducing mechanical stress. Thus if we reduce the current from 600 percent to 300 percent, the torque varies as the square of this reduction. The torque is thus reduced to 25 percent of the across the line starting torque.
Figure 3.11: Motor Torque vs. Speed
3.8 Soft Starting Methods
Soft starting methods can be
divided into the following functional categories based on their physical
characteristics and function [8].
1)
Full
Voltage Techniques
2)
Reduced
Voltage Techniques
3)
Soft starter Technique
4)
Variable Frequency
Drive
3.8.1 Full Voltage Techniques
Full voltage starting can be used whenever the driven load can withstand the shock of instantaneously applying full voltage to the motor and where line disturbances can be tolerated. Full voltage starting uses a main contactor to apply the motor stator windings directly across the main system voltage. This type of starting method provides the lowest cost, a basic and simple design of controller, resulting in low maintenance and the highest starting torque. These methods include the direct-on-line method and conventional circuit introductions that connect the full terminal voltage to the motor stator.
a) Direct-On-Line (DOL)
DOL is the traditional and
simplest method of motor starting and most other methods are baselined against
it. It is also often called across the line start. This method is the direct
connection of the terminal voltage to the motor stator with no additional
components, and also for this reason it is most economical in terms of
installation cost and ease of use. It is also one of most reliable and robust
methods.
Of all the starting methods
it produces the highest inrush current, usually six to eight times the rated
current, and the highest starting torque; and due to the high starting torque
it has the shortest acceleration time (apart from the shunt capacitor start)
.The DOL method is most commonly used for small motors relative to the size of
the generation and system, due to the fact that the startup of a small motor
will only have a low impact on the system, and in particular the voltage drop
and other drawbacks include the mechanical stress put on the motor’s load and the low startup
efficiency due to the high reactive power consumed at startup. This approach is
typically not suitable for large motors.
b) Shunt Capacitors
Connecting a capacitor in
parallel to the motor can help compensate the reactive demand from the motor
during startup by supplying a leading current and thus improve the power factor
while still achieving high starting torque because of the full voltage. This
provides some relief to the supply source. The shunt capacitance may be left
connected if they are properly rated so as to provide power factor correction;
or removed as the motor approaches rated speed. In reference the authors state
that the shunt capacitance that is left connected should never be sized larger
than the motor’s magnetizing current. Capacitor starting may be used in
conjunction with other starting methods.
3.8.2
Reduced Voltage Techniques
Reduced voltage starting may be required if full
voltage starting creates objectionable line disturbances on the distribution
system or where reduction of mechanical stress to gear boxes or belt drive
systems is required. It must be noted that when the voltage is reduced from
nominal, a decrease in inrush current will occur at a rate of 12% for every 10%
decrease in voltage. The starting torque will also decrease at a rate of 20%
for every 10% decrease in voltage [8]. This phenomenon also occurs in the
opposite manner when the voltage is increased. During
these reduced voltage startup methods, the thermal capability due to the
increased time to reach rated speed must be taken into consideration. Transient
current surges are produced from open circuit transitions, but with closed
circuit transitions these switching transients are avoided. The reduced voltage methods can be implemented through
conventional circuitry, such as resistors and
transformers.
a) Star-Delta:
The
star-delta (wye-delta) starting method controls whether the lead connections
from the motor are configured in a star or delta electrical connection. The
initial connection should be in the star pattern that results in a reduction of
the line voltage by a factor of 1/3 (57.7%) to the motor and the current is
reduced to 1/3 of the current at full voltage, but the starting torque is also
reduced 1/3 to 1/5 of the DOL starting torque . The motor must be delta
connected at rated voltage. The transition from star to delta transition usually
occurs once nominal speed is reached, but is sometimes performed as low as 50%
of nominal speed. The star-delta method is usually only applied to low to
medium voltage motors. The operation of the star-delta method is simple and
rugged, and is relatively cheap compared to other reduced voltage methods with
only additional contactors added to the cost .However, the system cannot be
modified once installed without considerable rework. Closed-circuit transitions
can be performed to avoid open-circuit current surges.
b) Autotransformer
An
autotransformer uses tap changes to reduce the low voltage as needed on the low
side connected to the motor terminal. Therefore, the current can be reduced
during startup; however the torque is also reduced as the square of the voltage
and needs to be taken into consideration to ensure enough torque is supplied
during acceleration. The 10% of the full load torque margin should be supplied
at all points on the speed-torque characteristic curve. As the motor speeds up,
the line is switched to the full voltage. Common taps range from 50-80% of the
rated voltage provides a table describing the currents at the typical 50%, 65%,
and 80% of full voltage as 25%, 42%, and 64% of the current realized at full
voltage during startup, respectively. A practical advantage of the
autotransformer is the ability to provide different tap changes so that a wide
range of applications, which vary in their starting torque and inrush current
needs, can be performed. The star-delta starter is electrically equivalent to
an autotransformer tapped at 57.7%. However, autotransformers have a higher
cost than other conventional electromechanical starting methods.
c) Primary Resistor or Reactor
A switchable primary series
resistor or reactor bank can be added at the motor terminals to limit the
current or limit change in the current, respectively. The resistor bank will
cause a drop in voltage across it reducing the current. The heat dissipated
from the resistor also needs to be taken into consideration. Series resistor
starting is usually only performed for small motors. When using a series
reactor bank, it will oppose the inrush current initially and reduce the
terminal voltage proportionally. The most advantageous characteristic of the
series reactor starting is that the voltage increases over time as a function
of the rate of change of the current without additional control. The added
reactance will also further increase the starting reactive power and thus lower
the starting efficiency. Switching transients will also occur if it is
connected in an open circuit.
3.8.3.
Soft Starter Techniques
A
soft starter is any solid-state electronic circuit based device that
manipulates the supply voltage prior to connecting to the motor terminals. Many
different topologies for soft-starters exist, and many of them are presented in
the literature. The standard soft-starter consists of thyristors that
manipulate the source signal via control of the thyristor firing angles. The
soft-starter can be based on controlling different starting characteristics,
including current, torque, and voltage, and can be easily adjusted based on the
different loadings. Two basic types of soft starters are the voltage ramping
and current limiting types.
The
voltage ramping soft-starter is able to gradually increase the voltage from a
preset level, as low as 0V, to the rated voltage; the result being a very
smooth startup. The current profile generally follows the voltage profile for
voltage ramping soft-starters. One of the drawbacks of the ramping soft-starter
is increased harmonics, which produce extra heating. The current limiting
soft-starter senses the current at the motor so that the fire angle can be
controlled in a manner that the voltage is regulated to maintain the desired
current. Typical values of the current limit are 175% to 500% of the rated
current of the motor. The soft starter adds significant flexibility in
operation (e.g. acceleration time and winding heat) and interoperability, due
to the fact that it is more sensitive to the mechanical load characteristics.
This
also results in lower maintenance cost and increased lifetime of the mechanical
load, and can result in improved energy efficiency. However, the tradeoff is
the increased operational complexity, and soft starters are generally expensive
devices. Limitations include harmonics produced by the electronics and the
installation distance from the motor. They are common in higher power
applications due to their operation flexibility and improved lifetime of equipment.
3.8.4
Variable Frequency Drive
The VFD works on the principle that the AC line voltage is converted to a DC voltage. This DC voltage is then inverted back to a pulsed DC whose RMS value simulates an AC voltage. The output frequency of this AC voltage normally varies for 0 up to the AC input line frequency [15].The variable frequency drive (VFD) enables low starting currents because the motor can produce exactly the required torque at rated current from zero to full speed. The VFD soft-start provides smooth, step-less acceleration of motor and load while controlling inrush current and starting torque. One VFD can be used to start multiple machines [20].
3.9 Comparison of Different Starting Methods of Induction Motor
Table
3.1 shows the comparison of different starting methods of induction motor.
Table 3.1 Comparison of Different
Starting Methods of Induction Motor
|
Characteristics |
Starter types |
||
|
Full voltage starter |
Reduced voltage starter |
Soft starter |
|
|
Average Starting time |
2-3seconds (depending on inertia) |
3-7 seconds (depending on inertia) |
Adjustable from 1-60 seconds |
|
Starting Current |
5-7 times the rated current |
2-3 times the rated current |
0.5 times the rated current |
|
Starting torque |
0.6-1.5 |
0.2-0.5 |
1.5 |
|
Types |
a) Direct-On-Line (DOL) b) Shunt Capacitors |
a) Star-Delta b) Autotransformer c) Primary Resistor or Reactor |
a) Single phase full wave controlled
soft starter b) Three phase half wave controlled soft
starter c) Three
phase full wave controlled soft starter |
|
Advantages |
- Simple
starter - good
starting torque - low cost
device |
- more
complicated starter - reduction
of starting current |
- easy installation -soft and progressive starting( no
shakes) - no power supply lost -dynamic torque control -control of starting time -mechanically perfect starting - built in protection -long life device. -one control board for whole series. |
|
Disadvantages |
-very high current peak at starting -no possibility of soft and progressive
starting |
- smaller starting torque - loss of power when star commutates to
delta - the stator windings should be prepared
for double connections -6 wires are needed to connect both
connections |
- cost is more compared other starting
methods. |
|
Applications |
- it is usually used for motors of low
and medium powers |
-low power machines starting with load -medium power machines starting with no
load -low power fan and centrifugal pumps |
-pumps -conveyors band - compressors -fans -agitators/mixers -mills/hammers |
3.10 Types of
Soft Starters
Soft starter is based on the phase angle principle. By means of
thyristors it is possible to switch at different points the sine half wave and
supply only part of the mains voltage to the motor. The thyristor
permits the current to flow in one direction only. This requires a second semiconductor
with opposite polarity which supplies the negative current (back-to-back switched
semiconductors).
Soft
starters are divided into groups according to the following criteria [16]
1.
The number of controlled phases.
Ø
One
phase (single-phase controlled soft starters), two phases (two-phase controlled
soft starters) or three phases (three-phase controlled soft starters).
2.
The type of the second semiconductor with opposite polarity.
Ø
If a diode is selected, this is called a half-wave controlled soft
starter.
Ø
If a thyristor is chosen, this is called a full-wave controlled
soft starter.
Figure 3.12, 3.13, 3.14 shows how
the different types of soft starter influence motor voltage and current in
different ways.
3.10.1
Single-Phase Full-Wave Controlled Soft Starter
In case of the single-phase
controlled soft starter shown in Figure
3.12, a phase
angle (Phase L2) is implemented in a phase by means of two back-to-back
thyristors. The phases L1 and L3 are directly connected to the motor. During
start, approximately the 6 x rated motor current still flows in phase L1 and
L3. It is only possible to reduce the current to the 3 x rated current during
the controlled phase.
If this method is compared with a direct start, the run-up time is
longer, but the total average motor current is not considerably reduced. This
means that approximately the same current flows through the motor as during
direct start. This results in an additional motor warm up. Since only one phase
is controlled the network is put under an asymmetrical load in the start phase.
Single and two-phase controlled soft starters are mostly used in a power range
of up to max. 5.5 kW. They are only suitable for avoiding mechanical impact in
a system. The induction motor’s starting current is not reduced by this method.
Figure 3.12: Single-phase full-wave controlled soft starter
3.10.2 Three-phase half-wave controlled soft starter
For a three-phase half-wave
controlled soft starter shown in Figure
3.13, the
phase cut is implemented in all three phases. A thyristor with an anti-parallel
diode serves as a power semiconductor. This means that the phase control is
only implemented in one half-wave (half wave controlled). This means that the
voltage is only reduced during the half-wave when the thyristor conducts.
During the second half-wave, when the diode conducts, the full mains voltage is
applied to the motor. During the uncontrolled half-wave (diode), the current
peaks are higher than during the controlled one. The upper harmonics linked to
this result in a further motor warm-up. Since the current peaks in the
uncontrolled half-wave (diode) and the upper harmonics linked to them become
critical during high performance, half-wave controlled soft starters can only
be applied purposefully up to approximately 45 kW.
Figure 3.13: Three-phase half-wave
controlled soft starter
Starting
3.10.3 Three-Phase Full-Wave Controlled Soft Starter
For this soft starter type shown in
Figure 3.14, the phase control is implemented in all three phases. Two
back to back thyristors are used as power semiconductors. This means that the
phase voltage is controlled in both half waves (full wave control). As a result
of the upper harmonics occurring during phase control, the motor is
nevertheless put under a higher thermal load than during a direct start.
Three-phase full-wave controlled soft starters are applied for up to
approximately 630 kW.
Figure 3.14: Three-phase full-wave
controlled soft starter
3.11 Thermal Load during Start
Figure
3.15 shows the impact of different soft starter types on additional motor warm
up compared to a direct start [16].
Figure 3.15: Motor
warm-up
Item 1/1 marks the motor warm-up
following a direct start. The X-axis shows the multiplication factor of the
start time and the Y-axis shows the multiplication factor of the motor warm-up.
If, for example, the start time is doubled compared to a direct start, this
means that:
·
For the single-phase controlled soft starter, the motor warm up is
increased to the 1.75 x value;
·
For the two-phase controlled soft starter, it is increased to the
1.3 x value;
·
For the half-wave controlled soft starter, it is increased to the
1.3 x value;
·
For the full-wave controlled soft starter, practically no
additional warm-up can be detected.
·
For longer run-up times and higher power, only a fully wave
controlled soft starter can be used.
3.12
Snubbers
Snubbers are an essential part of power electronics. Snubbers are small networks of parts in the power switching circuits whose function is to control the effects of circuit reactances. Snubbers enhance the performance of the switching circuits and result in higher reliability, higher efficiency, higher switching frequency, smaller size, lower weight, and lower EMI. The basic intent of a snubber is to absorb energy from the reactive elements in the circuit.
The benefits of this may include circuit damping, controlling the rate of change of voltage or current or clamping voltage overshoot. In performing these functions a snubber limits the amount of stress which the switch must endure and this increases the reliability of the switch. When a snubber is properly designed and implemented the switch will have lower average power dissipation, much lower peak power dissipation, lower peak operating voltage and lower peak operating current. Snubbers may be either passive or active networks. Passive snubber network elements are limited to resistors, capacitors, inductors and diodes. Active snubbers include transistors or other active switches, often entail a significant amount of extra circuitry and introduce another level of parasitic which must be dealt with (usually with a passive snubber). The basic function of a snubber is to absorb energy from the reactances in the power circuit [17].
3.12.1 Snubber for
protection against dv/dt
The transient over voltages can switch on the power devices. In some cases the power devices can be damaged due to these transient voltages. These transient voltages are very common when the converter is having inductive loads. The power devices can be protected against transient voltages by RC network. This RC network is connected in parallel with the power device. Whenever there is a large spike or voltage transient across the device, it is absorbed by the RC circuit. The Rc circuit acts a low pass filter for the transient voltage. The resistance is of low value so that the transient is absorbed by the capacitor quickly. dv/dt also generates large voltage transients. These rapid voltage variations can also be suppressed by snubber circuit. The capacitor acts as a short for these dv/dt variations [17].
The value of capacitor is given as
Where : Vm is the peak value of the supply voltage
dv/dt is permissible dv/dt
L is the source inductance
and resistance R is given as,
Where: s the damping factor
3.12.2 Snubber for
protection against di/dt
When the power device is turned on the current increases rapidly. This rapid variation of current does not spread uniformly across the junction area of the device. This creates localized hot-spots in the junction and increases the junction temperature. If junction temperature exceeds the permisible value, then the device will get damaged. This rapid variation of current is called di/dt. The devices can be protected against excessive di/dt by using an inductor in series with the device. The inductance opposes for rapid current variations. Whenever there is rapid current variation, the inductor smooths it and protects the device from damage [17].
The value of inductance is given by
Where: di/dt is the maximum value and L is the series inductance including stray inductance.
CHAPTER-4
DSPIC30F2010 DEVICE FAMILY
Overview of dsPIC30F Device Family
The dsPIC30F device family employs a powerful 16-bit architecture that seamlessly integrates the control features of a microprocessor (MCU) with the computational capabilities of a digital signal processor (DSP). The resulting functionality is ideal for applications that rely on high-speed, repetitive computations as well as control. The DSP engine, dual 40-bit accumulators, hardware support for division operations, barrel shifter, 17 x 17 multiplier, a large array of 16-bit working registers and a wide variety of data addressing modes, together provide the dsPIC30F CPU with extensive mathematical processing capability. Flexible and deterministic interrupt handling, coupled with a powerful array of peripherals, renders the dsPIC30F devices suitable for control applications. Reliable, field programmable Flash program memory and data EEPROM ensure scalability of applications that use dsPIC30F devices. At the time of writing, all dsPIC30F devices use Flash program memory technology. The Flash program memory can be electrically erased or programmed [18].
4.1 Features of dsPIC30F2010 Series
The important features of dsPIC30F2010 series are
- High-Performance Modified RISC CPU
- DSP Engine Features
- Peripheral Features
- Quadrature Encoder Interface Module Features
- Analog Features
- Special Microcontroller Features
- CMOS Technology
4.1.1 The RISC CPU
features
The RISC CPU features of dsPIC30F2010 are
- Modified Harvard architecture
- C compiler optimized instruction set architecture
- 84 base instructions with flexible addressing modes
- 24-bit wide instructions, 16-bit wide data path
- 12 Kbytes on-chip Flash program space
- 512 bytes on-chip data RAM
- 1 Kbyte non-volatile data EEPROM
- 16 x 16-bit working register array
- Up to 30 MIPs operation:
- DC to 40 MHz external clock input
- 4 MHz-10 MHz oscillator input with PLL active (4x, 8x, 16x)
- 27 interrupt sources
- Three external interrupt sources
- 8 user selectable priority levels for each interrupt
- 4 processor exceptions and software traps
4.1.2 DSP Engine Features
DSP Engine Features of dsPIC30F2010 are
- Modulo and Bit-Reversed modes
- Two, 40-bit wide accumulators with optional saturation logic
- 17-bit x 17-bit single cycle hardware fractional/ integer multiplier
- Single cycle Multiply-Accumulate (MAC) operation
- 40-stage Barrel Shifter
- Dual data fetch
4.1.3 Peripheral Features
Peripheral Features of dsPIC30F2010 are
- High current sink/source I/O pins: 25 mA/25 mA
- Three 16-bit timers/counters; optionally pair up 16-bit timers into 32-bit timer modules
- Four 16-bit Capture input functions
- Two 16-bit Compare/PWM output functions
- Dual Compare mode available
- 3-wire SPITM modules (supports 4 Frame modes)
- I²C™ module supports Multi-Master/Slave mode and 7-bit/10-bit addressing
- Addressable UART modules with FIFO buffers.
4.1.4 Motor Control PWM Module Features
Motor Control PWM Module Features of dsPIC30F2010 are
- 6 PWM output channels
- Complementary or Independent Output modes
- Edge and Center Aligned modes
- 4 duty cycle generators
- Dedicated time base with 4 modes
- Programmable output polarity
- Dead time control for Complementary mode
- Manual output control
- Trigger for synchronized A/D conversions
4.1.5 Quadrature Encoder
Interface Module Features
Quadrature Encoder Interface Module Features of dsPIC30F2010 are
- Phase A, Phase B and Index Pulse input
- 16-bit up/down position counter
- Count direction status
- Position Measurement (x2 and x4) mode
- Programmable digital noise filters on inputs
- Alternate 16-bit Timer/Counter mode
- Interrupt on position counter rollover/underflow
4.1.6 Analog
Features
Analog Features of dsPIC30F2010 are
- 10-bit Analog-to-Digital Converter (A/D) with
- 500 Ksps (for 10-bit A/D) conversion rate
- Six input channels
- Conversion available during Sleep and Idle
- Programmable Brown-out Detection and Reset generation
4.1.7 Special Microcontroller Features:
Special Microcontroller Features of dsPIC30F are as follows
- Enhanced Flash program memory:
- 10,000 erase/write cycle (min.) for industrial temperature range, 100K (typical)
- Data EEPROM memory:
- 100,000 erase/write cycle (min.) for industrial temperature range, 1M (typical)
- Self-reprogrammable under software control
- Power-on Reset (POR), Power-up Timer (PWRT) and Oscillator Start-up Timer (OST)
- Flexible Watchdog Timer (WDT) with on-chip low power RC oscillator for reliable operation
- Fail-Safe clock monitor operation
- Detects clock failure and switches to on-chip low power RC oscillator
- Programmable code protection
- In-Circuit Serial Programming™ (ICSP™)
- Selectable Power Management modes
- Sleep, Idle and Alternate Clock modes
4.1.8 CMOS Technology
The dsPIC30F2010 CMOS technology features are
- Low power, high speed Flash technology
- Wide operating voltage range (2.5V to 5.5V)
- Industrial and Extended temperature ranges
- Low power consumption
4.2 Block Diagram of dsPIC30F2010
The block diagram of dsPIC30F2010 is as shown in the Figure 4.1. The dsPIC30F2010 device contain extensive Digital Signal Processor (DSP) functionality within a high-performance 16-bit microcontroller MCU) architecture.
Figure 4.1: Block
Diagram of dsPIC30F2010
4.3 Pin Diagram of dsPIC30F2010
The dsPIC30F2010 is a 28-pin Enhanced Flash 16-bit Digital Signal Controller, its pin configuration is as shown in Figure 4.2.Each pin is used to perform different functions .
Figure 4.2: Pin Configuration of dsPIC30F2010
Table 4.1 provides a brief description of dsPIC30F2010 I/O pinouts and the functions that may be multiplexed to a port pin . Multiple functions may exist on one port pin. When multiplexing occurs, the peripheral module’s functional requirements may force an override of the data direction of the port pin.
Table 4.1 Pin description ofdsPIC30F2010
4.4 Comparison of dsPIC30F Motor Control and Power
Conversion Family
Table 4.2 provides the comparison of different digital signal controllers ie dsPIC30F series. In the table (4.2) seven dsPIC’s controllers (dsPIC30F2010, dsPIC30F3010, dsPIC30F4012, dsPIC30F3011, dsPIC30F4011, dsPIC30F5015 and dsPIC30F6010) are compared and they are differentiated interms of there pin numbers, memory, PWM channels, A/D converters etc. all these devices supports quadrature encoder interface. The dsPIC30F2010, dsPIC30F3010, dsPIC30F4012 have 28 number of pins, while the pin numbers of dsPIC30F3011 and dsPIC30F4011 vary from 40-44, dsPIC30F5015 has 64 and dsPIC30F6010 has 80 number of pins respectively.
The memory of these devices vary from 12K to 144K. dsPIC30F5015 and dsPIC30F6010 have 8 motor control PWM channels while others have 6 PWM channels. Depending on the requirement of the system output we will select the desired digital signal controller. In this work we are using dsPIC30F2010 controller.
Table 4.2: Comparison
of Different Digital Signal Controllers
CHAPTER-5
BLOCK DIAGRAM OF SOFT STARTER AND ITS
EXPLANATION
5.1 Block Diagram of Three Phase Soft
Starter
The block diagram of three phase soft starter is shown in Figure 5.1. It consists of following blocks:
- TRIACS
- Zero Crossing Detector
- Control Unit
- Power Supply and
- Relays or Bypass Contractor
Figure 5.1: Block Diagram of Three Phase Soft Starter
5.1.1 TRIACS
Thyristors are solid-state switches that allow current to flow in only one direction. Triac is a combination of two SCR’s connected back to back. When connected back to back, thyristors can be controlled to turn on as required and thus control the voltage that is applied to the ac motor. The thyristor gate control signals used to trigger the thyristors are derived from zero crossings of the three phase power source.
5.1.2
Zero
Crossing Detector
A zero crossing detector literally detects the transition of a signal waveform from positive and negative, ideally providing a narrow pulse that coincides exactly with the zero voltage condition. The zero crossing sensor detects the zero crossing of the AC voltage input and allows the SCRs to conduct starting with a zero crossing of the voltage. This avoids harmonic problems associated with the phase angle fired SCR controller. A variable time base firing circuit is used to provide precise control of power delivered to the load.
5.1.3
Control Unit
The controller used in this work is a Digital Signal Controller dsPIC30F2010. The microcontroller is operated at 25MHz crystal frequency. The controller of the soft starter is used for programming the control switching algorithm of the SCRs. It is used to control the switching sequences of the SCR switches. Voltage and current feedbacks are fed into the analog inputs of the DSP board, and the firing pulses in the form of digital signals were accordingly generated. Pulse transformers are usually utilized as the gate driver for sending the gate pulses to the SCRs, as well as providing electrical galvanic isolations between the DSP board and the SCRs.
5.1.4
Power Supply
In the proposed scheme we are using two types of power supplies
1) Three Phase Power Supply
2) Single Phase Power Supply
Three phase power supply is directly connected at the input of the starter. Digital Signal Controller (dsPIC30F2010) needs only 5V supply. A voltage regulator is used to regulate the voltage and it supplies the 5V dc power necessary for the operation of DSP controller. A single phase step-down transformer of voltage ratio 230/12-0-12 is used to step down the voltage from 230V to 12, this is given to the dsPIC30F2010 controller through 7805 voltage regulator.
5.1.5
Relays
As mentioned earlier, at the start of the motor it experiences a very high current and the motor may not run at the rated speed. An electromagnetic relay is used in the proposed scheme. Initially the relay is “pulled out” (normally opened condition), i.e., the supply is connected to the motor through the soft starter. Once the motor reaches its full speed and rated current, the relay will be “pulled-in” (normally closed condition), to bypass these thyristors. During this time, all the thyristors will be in their “turned-off” state and the motor will be directly energized from the supply through the relays.
5.2 Operation
of Soft Starter
The working principle of soft
starter is that by adjusting the voltage applied to the motor during starting,
the current and torque characteristics can be limited and controlled. For
induction motors, the starting torque is approximately proportional to the square
of the starting current drawn from the line. This starting current is
proportional to the applied voltage (V). So the torque can also be considered
to be approximately proportional to the applied voltage. By adjusting voltage
during starting, the current drawn by the motor and the torque produced by the
motor can be reduced and controlled. By using six SCR’s in a back to back
configuration, the soft starter is able to regulate the voltage applied to the
motor during starting from 0 volts up to line voltage. In this method line
frequency is always applied to the motor only the voltage changes.
Soft starters control voltage instead of current. The voltage control is implemented by adjusting either the delay angle, α , or the hold-off angle, γ , of the conduction cycle of the on coming thyristor with respect to either the zero crossing of the supply voltage (α) or the zero crossing of the line current (γ), respectively, as shown in Figure 5.2. The thyristors are then selectively fired to conduct current in the appropriate phase, and naturally commutate off when the current reaches zero. The larger the delay angle, α , or the hold-off angle, γ , the larger the notch width in the applied motor voltage, which consequently reduces the effective or RMS value of such voltage impressed upon the motor. However, improper control of the α or γ firing angles may result in relatively high starting torque and current oscillations. Therefore, optimum starting profiles of the α or γ firing angles have been extensively investigated to produce smooth starting torque and current profiles.
Figure 5.2:
Zero Crossing of the Supply Voltage (α) or the Zero Crossing of the
line Current (γ)
The torque available varies
proportionately with the square of the ratio of the reduced voltage to the
normal line voltage. When the operator depresses the start button, the soft
starter logic issues an on command to the power module, causing the SCRs to
turn on and gently increase the voltage across the motor terminals, or the
current into the motor based on the adjustments made to the soft start logic.
When the SCRs are fully on, the motor reaches full voltage. The complete block
diagram of a typical soft starter is as
shown in Figure 5.1 shown.
This solid-state starter uses six full current rated SCRs as its power devices. The logic circuit monitors three-phase input voltage, three-phase output voltage, and the three output currents. From these inputs, it can provide starting current limitation, running overcurrent protection, phase loss, and undervoltage protection. This starter interfaces with standard control circuits.
A
bypass contactor may be closed to provide higher operating efficiency
after the SCRs are fully on. Figure 5.3
shows a three -phase leg of the soft starter with the SCRs turning
on and becoming the current path for power to flow from the utility to
the motor.
Figure
5.3: SCRs as Current Path
After the motor has come up to
speed, the bypass contactor closes and it becomes the current path for the
motor as shown in Figure 5.4.
At this time, the SCRs no longer
conduct any current. Bypass operation eliminates the SCR losses once the motor
is up to speed, resulting in significantly lower heat generation. Soft starters
with internal run bypass mode are typically much smaller and lighter than
devices without run bypass.
Figure 5.4: Bypass Contactor as Path
CHAPTER-6
SIMULATION SETUP AND ITS RESULT FOR
THREE PHASE SOFT STARTER
6.1 Introduction to Matlab Simulink
In the modern era, almost all the processes and techniques are first simulated before their actual real time implementation. This reduces a significant portion of effort and cost of real time implementation. The performance of the proposed system/process/technique can be evaluated accurately by using proper simulation models. Thus the models should be flexible and accurate to take into account the real time implementation issues as well. With the rapid development in computer hardware and software, new simulation packages which are much faster and more user friendly are now available. One such software is the simulink software of matlab. The main advantage of the simulink over other programming software is that, instead of compilation the simulation model is built systematically by means of function blocks. A set of machine differential equations can be modelled by interconnection of appropriate blocks, each of which performs a specific mathematical operation. Programming efforts are drastically reduced and the debugging of errors is easy[10].
Simulink is a software package for modeling, simulating, and analyzing dynamical systems. It supports linear and nonlinear systems, modeled in continuous time, sampled time, or a hybrid of the two. Systems can also be multirate, i.e., have different parts that are sampled or updated at different rates.
For modeling, Simulink provides a graphical user interface (GUI) for building models as block diagrams, using click-and-drag mouse operations. With this interface, one can draw the models just as with pencil and paper (or as most textbooks depict them). This simulation package helps to solve differential equations and difference equations in a language or program. Simulink includes a comprehensive block library of sinks, sources, linear and nonlinear components, and connectors. We can also customize and create our own blocks.
Models are hierarchical, so we can build models using both top-down and bottom-up approaches. We can view the system at a high level, then double-click on blocks to go down through the levels to see increasing levels of model detail. This approach provides insight into how a model is organized and how its parts interact. After defining a model, we can simulate it, using a choice of integration methods, either from the Simulink menus or by entering commands in matlab’s command window. The menus are particularly convenient for interactive work, while the command-line approach is very useful for running a batch of simulations. Using scopes and other display blocks, we can see the simulation results while the simulation is running. In addition, we can change parameters and immediately see what happens, for “what if” exploration. The simulation results can be put in the matlab workspace for post processing and visualization. Model analysis tools include linearization and trimming tools, which can be accessed from the matlab command line, plus the many tools in matlab and its application toolboxes. Because matlab and Simulink are integrated, we can simulate, analyze, and revise our models in either environment at any point [9].
In my work, I have simulated solid state reduced voltage soft starter applied to induction motor drive systems and it is done using matlab/simulink 7.13 simulation package.
6.2 Block Diagram of Complete Simulation System
The block diagram of complete simulation system of a three-phase voltage regulator for soft starting of induction machine using thyristors is as shown in Figure6.1. Three-phase sinusoidal voltage is generated using voltage source block from Simulink Library. The soft starter or AC controller subsystem block is as shown in the Figure 6.2 The soft starter consists of three sets of back-to-back connected thyristors, which can be readily obtained from the SimPower System Blockset toolbox in Simulink. To turn on the SCR’s, pulses are applied at appropriate firing angles. Gate pulses generated by pulse subsystem are given to the anti-parallel thyristors. The switching signals have either 0 (turn off) or 1 (turn on). The load is taken as a simple three phase squirrel cage induction motor.
The thyristors are initially triggered at a large firing angle (reduced voltage) when soft started, followed by decrease in the firing angle and subsequently increasing the applied voltage to the motor. The initial starting torque at standstill of an induction motor is proportional to the square of the applied voltage. Thus this surely affects the starting of induction motor on load. The pulse subsystem block is as shown in the Figure 6.3. It consists of clock, look-up table, 2 multiport switches and 10 subsystem blocks.
Figure 6.1:
Block Diagram of Complete Simulation System
The clock is used to set the simulation current time. The lookup table parameters are shown in Figure 6.4. Look-up table perform n-dimensional interpolated table lookup including index searches. The table is a sampled representation of a function in N variables. Breakpoint sets relate the input values to positions in the table. The first dimension corresponds to the top (or left) input port. If the break point (BP) is 0 than 0th switch of multiport switch will be selected. The switch selects the top subsystem i-e first subsystem at the top.
Figure 6.2: Soft Starter or AC Controller
Subsystem Block
Figure 6.3:
Pulse Subsystem Block
Figure 6.4:
The Parameters of Lookup Table
Each subsystems are having two pulse generators as shown in Figure 6.5. These two pulse generators generate the pulses to turn on the two SCRs connected back to back. The delay angle and pulse width of these pulse generator is set in the increasing order so that the SCRs turn on initially at a larger firing angle α . Larger firing angle indicates less voltage applied to the motor and less flow of current .The firing angle is reduced as the break point goes on increasing and the current increases linearly with respect to the voltage applied. Due to reduced voltage applied to the motor, current also decreases. Hence the torque pulsations are reduced and the motor can run at its rated speed. Additionally the output voltage quality is poor with high distortion and poor power factor. The problem of higher THD is more prominent at large firing angle. To reduce the effect of harmonics filter is used. The filter subsystem block is as shown in the Figure 6.6. LC combinational filter is used where the value of L is 3mH and C is 1000µF.The designed filter reduces the harmonics and increases the efficiency of the motor.
Figure 6.5:
Pulse Generators
Figure 6.6:
Filter Block
6.3 Simulation
Results
Simulation is done on the complete scheme of electric drive with asynchronous motor, with following machine parameters listed in Table 6.1
Table 6.1
Induction Machine Data
|
Parameters |
Value |
|
Power Pn(VA) |
3.73KW |
|
Voltage Vn(Vrms) |
460V |
|
Frequency (F) |
50Hz |
|
Pole pairs |
2 |
|
Stator Resistance Rs[Ω] |
1.115 |
|
Rotor Resistance Rr[Ω] |
1.083 |
|
Stator Inductance Ls[H] |
0.005974 |
|
Rotor Inductance Lr[H] |
0.005974 |
|
Mutual Inductance M[H] |
0.2073 |
Figure 6.7(a) and Figure 6.7(b ) shows the AC voltage controller output voltage. We can observe from these figures that initially the SCR’s are not turned on completely, they are partially conducting. The output voltage is not sinusoidal till time T=0.1S, after this time the motor attains its rated speed and the SCR’s conducts completely, hence full voltage is now applied to the motor. During this period of time reduced voltage is applied to the motor hence the current flowing through is also less. As the firing angle goes on decreasing, the voltage applied to the motor increases slowly. Due to this the current also increases linearly. The ac controller output current is shown in Figure 6.8(a) and Figure 6.8(b) reduced inrush current decreases the torque pulsations and the motor runs at its rated speed. The speed and torque waveforms is shown in Figure 6.9. The load current or the motor current are shown in Figure 6.10(a) and 6.10(b)
CHAPTER-7
DESIGN AND DEVELOPMENT OF HARDWARE
The system is designed to reduce the starting current and torque pulsations of a three phase induction motor. The application meets the following performance specifications:
Hardware Implementation
The system incorporates the following hardware boards
- Three Phase Soft Starter
- Snubber circuit
- Relays
- Controller dsPIC30F2010
- Regulated power supply
Software Implementation
- Three Phase Soft starter
7.1
Design of Three
Phase Soft Starter
The soft starter circuit is as shown in Figure 7.1. In this circuit TRIACs (BTB04-600SL) are used as power switches. Their current rating is 4A and voltage rating is 600V. TRIAC gate current sensitivity is of 10mA.
Figure 7 .1 Three Phase Soft Starter using TRIAC
7.1.1 Selection of TRIACs
The selection
of TRIAC is done on the basis of voltage , current and power rating of the
load.
The ratings of the induction motor used are:
Voltage rating : 415V
Current rating : 1.9A
Power rating : 1HP
Here, we have selected TRIAC (BTB04-600SL) to meet the requirement. It has following voltage and current rating.
Voltage rating : 600V
Current rating : 4 A
Heat sinks of proper dimensions are used with each TRIAC to keep its temperature within limit.
7.1.2
Optopisolator
The optoisolator is an integrated circuit (IC, chip) that is specifically designed to connect low voltage DC controls to high voltage AC triacs. The optoisolator used in this system is MOC3041 as shown in Figure 7.2. The MOC3041 devices consist of a AlGaAs infrared emitting diode optically coupled to a monolithic silicon detector performing the function of a zero voltage crossing bilateral triac driver. The optoisolator in the triac output boards is a 6-pin chip, but only 4 pins are used: 2 for the DC in, and 2 for the AC out.
Figure 7.2: 3041 Optoisolator
Internally, the optoisolator has three major sections: an
LED input, a zero-crossing detector, and a triac driver output.
When the LED is on, it shines a light inside the
optoisolator to turn on the zero-crossing circuit. This provides the optical
isolation, since there is no electrical connection from the DC input to the AC
output. The second section in the optoisolator is the the zero-crossing voltage
detector circuit. This starts to work when it detects the LED light. It
switches on and off only when the AC voltage is zero. When you turn on or off
the optoisolator, it will turn on or off the AC on the next half-cycle that
crosses zero volts. The third section in
the optoisolator is the triac driver output. This is essentially a mini-triac
that switches AC on and off, with a little power, enough to trigger a triac
(but not enough to drive a load itself). When the zero-crossing circuit turns
on, this triac driver switches the AC on to the gate of the triac, which in
turn switches the AC on to the load.
7.2 Snubber Circuit
The triac is the only bidirectional device able to control various loads supplied by the domestic and industrial mains. It is often designed with a network made of a resistor R and a capacitor C, the snubber circuit. . At turn off the commutation of the triac is the transient phase during which the load current is passing through zero and the supply voltage is reapplied to the triac terminals. The main function of this circuit is to improve the switching behavior of the triac at turn off. The snubber circuit of the system is as shown in Figure 7.3. In order to protect the power switch, a snubber consisting of R=100Ω and C= 0.1µF .
Figure 7.3:
Snubber Circuit
7.3 Relays
A relay is usually an electromechanical device that is actuated by an electrical current. The current flowing in one circuit causes the opening or closing of another circuit. When the coil of a relay is at rest (not energized), the common terminal and the normally closed terminal have continuity. When the coil is energized, the common terminal and the normally open terminal have continuity. In this system HRS4-S DC12V relay is used. Its coil nominal dc voltage is 12V and the resistance of the coil is 320Ω. The relays operating voltage is 9V and release voltage is 1.20V. As mentioned earlier relay is used to connect the motor directly to three phase supply after the operation of the starter.
7.4 Software
Implementation
The hardware related to the system is described as above. Here, a software setup to implement control algorithm will be discussed. All the description and discussion of the software are based on Microchips dsPIC30F2010 Digital Signal Controller.
7.4.1
Processor
Requirements
The following are the lists of the processor requirements
- Strong calculating power for advanced and math-intensive control algorithms
- Capability to support large word lengths for required resolution and dynamic range
- Small interrupt latency and fast branch operation capabilities
- To facilitate quick response to events, including unexpected events
- High-MIPS CPU to increase the range of the sampling frequency
- Integrate the application-specific peripherals with the processor, such as timers , A/D converters ,PWM generators and communication interfaces. Hence, CPU overhead and total system cost are reduced.
7.4.2
Digital Signal
Controller dsPIC30F2010
In this dissertation, the Microchip’s 28 pin High-Performance Digital Signal Controller is used. The dsPIC30F2010 devices contain extensive Digital Signal Processor (DSP) functionality within a high-performance 16-bit microcontroller (MCU) architecture. The DSC core itself has up to 30 MIPS (33.33 ns cycle time) speed and can perform the useful multiply/accumulate (MAC) instruction in a single cycle. DSC has 10-bit high-speed analog-to-digital converter(A/D) which allows conversion of an analog input signal to a 10-bit digital number, a watchdog timer, a serial communication interface(SCI), a serial peripheral interface (SPI). For my work, pin number 2, 3, 4, 15, 16, 17 (RB0, RB1, RB2, RD0, RE8, RE3) are used. These pins function as bidirectional input-output ports, they are used to control the TRIACS and the relays . Pin number 9 and 10 i-e OSC1 and OSC2 oscillator crystal input pin and oscillator crystal output pin respectively are used to give the necessary oscillator frequency. 5V power supply VDD is given through pin number 20.
7.4.3
Soft Starter
Control Circuit
The soft starter control circuit is as shown in Figure 7.4. In this control circuit there are two feedback loops, namely the voltage and current loops, which are operating in a parallel fashion. These two feedback loops serve different purposes. The voltage loop is responsible for the starting acceleration of the motor, while the current feedback loop is accountable for mitigating the unbalanced effects of the starting. As a consequence, these two feedback loops are independent of one another, and hence the control feedback loop of each can be designed in an individual manner. Nevertheless, the motor can still function with only one feedback loop available, but at a much inferior performance such as having starting torque pulsations.
Figure 7.4:
Soft Starter Control Circuit
Therefore,
in designing the controller of the control system can be segregated into two
separate individual control systems accountable for the voltage and current loops,
as illustrated in Figure 7.5 and Figure 7.6, respectively. In doing so, the
process of designing both the voltage and current regulators is simplified with
only a single input and a single output in the control systems of both Figure 7.5
and Figure 7.6. Upon observation of the voltage control loop as depicted in
Figure 7.5, the plant of this voltage control system is now represented by the
soft starter. This is due to the fact that the main thrust of the soft starter
is to provide voltage excitation, which is dependent on the firing/delay angle
profile, to the motor. Due to its nonlinear nature, the plant (or soft starter)
is represented by a nonlinear
expression: Vrms = f
(α ) . Meanwhile, the
controller consists of only the voltage PI
regulator with a unity voltage feedback loop. On the other hand, in the
current control system as shown in
Figure 7.6 , the plant consists of both the soft starter and the motor, and the
controller is now a current PI
regulator with a unity current feedback loop.
Figure 7.5 Voltage
Figure 7.6:
Current
7.4.4 Transfer
Function of Controller
The transfer function of the PI controller is straightforward, and it is defined for the voltage and current loops, Gc_V(s) and Gc_I(s), respectively, as follows:
where, kp_V and kp_I are the proportional gains, and ki_v and ki_I are the integral gains of the voltage and current controllers, respectively.
Owing to the separate control loop for the voltage and current controllers, the corresponding kp and ki can be easily designed to achieve the selected bandwidth at the crossover frequency where the gain of the open-loop transfer function equals unity using linear control theory.
7.5 Regulated
Power Supply
The power supply unit consists of a 230/12-0-12V step down transformer. A single phase step-down transformer of voltage ratio 230/12-0-12 is used to step down the voltage from 230V to 12V, this is given to the dsPIC30F2010 controller through 7805 voltage regulator. 7805 voltage regulator is used to provide +5V dc power for dsPIC30F2010 (Digital Signal Controller). Regulator 7805 is available in 3-pin package and contains internal thermal overload protection, short circuit protection.
CHAPTER-8
EXPERIMENTAL SETUP AND RESULTS
The proposed scheme is first simulated and then implemented in the laboratory. The simulations are carried out using MATLAB 7.13. The control scheme of the soft starter is implemented by DSC (dsPIC30F2010). Software program is written in C language. The device is programmed using MAPLAB Integrated Development Environment (IDE) tool. It is a free, integrated toolset for the development of embedded applications employing Microchip’s PIC and dsPIC microcontrollers. For execution of C code MAPLAB C30 compiler is used. In this project 1HP induction motor is used the parameters of which are listed in Table 8.1.
Table 8.1 Induction Machine Data
|
Parameters |
Value |
|
Power Pn(VA) |
0.75 KW |
|
Voltage Vn(Vrms) |
415V |
|
Frequency (F) |
50Hz |
|
Pole pairs |
2 |
|
RPM |
1405 |
|
Current rating |
1.9 A |
|
Efficiency |
75.5 |
In my work, simulation of the three phase soft starter is carried out successfully. The prototype hardware set is tested in the power electronics laboratory and photograph of the complete project setup is shown in Figure 8.5. It is observed that ,when motor starts , the starting current of the motor increases slowly. This slow increase in the current is observed in the ammeter. The setup is tested for different starts and corresponding current is tabulated. The graph of current versus time is plotted for different starts . The soft starting time is fixed to 30 seconds. 415V/ 50HZ three phase power supply is given to the induction machine while testing the developed soft starter. A gap of 5 minutes is given between every start of the induction machine with soft starter and the readings are as tabulated in Table 8.2 and the graph of current versus time is plotted in the Figure 8.1, Figure 8.2, Figure 8.3 and Figure 8.4
Table 8.2: Voltage and Current
|
Voltage(V) |
Linear increase in starting current (A) |
|
415 |
0-6 |
|
415 |
0-5.8 |
|
415 |
0-5 |
|
415 |
0-4.5 |
CHAPTER -9
ADVANTAGES AND
APPLICATIONS OF SOFT STARTER
9.1 Advantages of Soft
Starter
Some of the
advantages of soft starter are
Ø By means of a soft starter it is possible to limit the motor starting current (limited by the amount of starting torque required)
Ø Increased acceleration time can be beneficial for motor and machine.
Ø The starting current is reduced or can be limited.
Ø The torque is adapted to the corresponding load.
Ø For pumps, surges during start and stop can be avoided.
Ø Jerky movements and shocks, which could hamper a process, are avoided.
Ø The wear and tear of belts, chains, gears and bearings is avoided.
Ø By means of the different controls, simplified automation is possible.
9.1.1 Mechanical Advantages
During direct start, the motor develops a very high starting torque. Starting torques of 150 to 300% of the rated torque are typical. Depending on the start type, the drive mechanics can be put under excessive strain due to the high starting torque (‘Mechanical stress’), or the manufacturing process may be unnecessarily hampered by jerky torque impacts.
Ø By using a soft starter, the torque impact which occurs on the mechanical parts of a machine can be prevented.
Ø The start characteristics can be adapted to the application (e.g., pump control).
Ø Simple wiring to the motor (only 3 conductors).
9.1.2 Electrical Advantages
Induction motor starting causes high power surges in the network (6 - 7-fold rated current). This means that large voltage drops can be caused which disturb other users connected to the network. Therefore, electricity companies determine limiting values for motor starting currents.
Ø By means of a soft starter it is possible to limit the motor starting current (limited by the amount of starting torque required)
Ø This reduces the strain on the network.
Ø Possible reduction of network connection fees.
Ø In many cases, however, the electricity company requires the starting current to be limited. This means compliance with the respective regulations.
9.2 Application of Soft
Starter
Typical application of soft starter are listed below
1) Centrifugal fan
For some applications the motor is started with reduced load torque, i.e. unloaded start. Big centrifugal fans are often started with a closed damper and this will make the start easier (shorter) but since the moment of inertia is still present the starting time might be quite long anyway. The use of soft starter helps to decrease the voltage to a low value at the beginning of the start which is low enough to avoid slip but high enough to start up the fan. The soft starter provides the ability to adjust to fit any starting condition, both unloaded and fully loaded starts.
2) Centrifugal Pump
In centrifugal pump soft starter is used for smooth start and stop of the motor. During the start sequence, the soft starter increases the voltage so that the motor will be strong enough to accelerate the pump to the nominal speed without any torque or current peaks. A normal starting current with a soft starter when starting a fully loaded centrifugal pump is approx. 4 times rated motor current. Also during the stop sequence, the soft starter reduces the voltage via a voltage ramp and the motor becomes weaker and weaker. Because of this the water speed slows down very smoothly without creating any pressure waves.
3) Compressor
Smaller compressors are often of
piston type and the load torque increases linearly with the speed. Screw
compressors are often used when there is a bigger need for air flow and this
type has a load torque increasing with the square of the speed.
Drive belts are often used between motor and compressor but direct connections via some type of toothed couplings are also common. Some compressors are started with reduced load. By using soft starter it is possible to limit the starting torque to a level suitable for all different applications. The result is less stress on couplings, bearings and no slipping belts during start.
4) Conveyor belt
Conveyor belts can have a lot of different looks and directions of use. It is a typical constant torque load with low to high braking torque depending on how heavy it is loaded. By using soft starter the starting torque can be reduced to a minimum value still able to start up the conveyor belt. The setting possibility of the soft starter makes it possible to adjust the torque to exactly the level that is necessary for the start. The result is the least possible stress on gearboxes and couplings and no slipping belts during start. This will reduce the maintenance cost to a minimum.
In my dissertation, I have designed a three phase solid state reduced voltage starter for three phase induction motor. Simulation of the three phase soft starter is carried out successfully. Simulation results show that reduced voltage soft starter reduces the inrush current flowing through the motor. The reduced voltage soft starter hardware set is developed and tested in the power electronics laboratory .The developed hardware has worked to the required expectation successfully. The developed soft starter hardware reduces the starting inrush current of the motor by applying reduced voltage at the start of the motor. The different voltages and their current readings are tabulated and the graph of current with respect to time is plotted for different voltages. The plotted graphs concludes that the inrush current during starting is reduced, with reduced inrush current the torque pulsations are decreases to higher extent. Meanwhile the motor terminal windings and insulation are protected from damage. The starting losses of the motor are reduced by using soft starter hence efficiency of the motor and the overall system increases. Solid state soft starters are helping to solve long standing industry problems such as soft start, controlled acceleration, smooth stop, fast stop, and motor starter maintenance and are extending the life and availability of ac motors and their driven processes. Continuing developments leading to higher voltages, higher currents and lower costs will continue to expand the ranges of applications to higher voltages and to higher horsepowers. Also, new types of power devices, such as gate turn-off thyristors may be appropriate for in special applications. New technologies such as VLSI control, with its ability to coordinate multiple inputs and to respond in optimum fashion to system requirements, will undoubtedly further extend the real usefulness of the solid-state motor starter in many industrial applications. More work needs to be done to investigate further the impact that the harmonics have on both motor heating and usable motor torque.
REFERENCES
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[13] G. Zenginobuzt, I. C a d i d , M. Ermist and C. Barlak, “ Soft Starting Of Large Induction Motors At Constant Current With Minimised Starting Torque Pulsations” TUBITAK-METU Information Technologies and Electronics Research Institute, TR 0653 1 Ankara – Turkey
[14] H.H. Goh, M.S. Looi, and B.C. Kok “Comparison between Direct-On-Line, Star-Delta and Auto-transformer Induction Motor Starting Method in terms of Power Quality” Proceedings of the International MultiConference of Engineers and Computer Scientists 2009 Vol II IMECS 2009, March 18 - 20, 2009, Hong Kong
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frequency inverters.
[17] M.H.Rashid Power Electronics , 3rd edition
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[20] “Utilization of
Soft-Starter VFD in Compressor Applications”, Carsten Ritter, Heinz Kobi, Peter
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[21] “Solid-state soft start motor controller and starter”, application paper AP03902001Epplication Paper AP03902001E Application Paper AP03902001E
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