MICROCONTROLLER BASED CLOSED LOOP CONTROL OF THREE PHASE INDUCTION MOTOR WITH FILTER AT INVERTER OUTPUT TO REDUCE dV/dt AND HARMONICS

 

ABSTRACT

n An induction motor control system fed by an AC/DC rectifier and a DC/AC inverter group is a nonlinear load, causes harmonic distortions in power lines and their control systems.

 

n These harmonic voltages and currents are generated on both load and supply sides of the converters. The harmonics, if not filtered out, enter the ac system and distort the system voltage waveform. And deteriorate the load machines….

 

n Some of the serious harmful effects are extra losses and heating in machines, overvoltages, torque pulsation in the induction motors, interference in the communication lines.

 

n The project is about designing the filter to be placed in the inverter motor drive system driving a three phase induction motor. The optimum filter design is presented to reduce these harmonics at the point of common coupling.

INTRODUCTION

n  Recently, the role of static power converters (rectifiers, inverters, motor control in industrial and domestic applications has grown rapidly due to the ease in which power can be manipulated with high efficiency.

n  As with the most technological innovations, however, a complete understanding and solution of its negative side effects have lagged its introduction to the marketplace.

n  The broader use of power electronics based loads has led to a growth of power pollution because of the non-linear voltage current characteristics of these loads

n  Thus, load currents and voltages are non-sinusoidal and it is necessary to compensate voltage and current harmonics…

 

n The high commutation speeds of the inverter stage call for additional filtering techniques.

 

n In particular, motor’s manufactures still design motors under line frequency drive specifications, while application of high frequencies is known to generate some reliability issue in the motor itself, especially deterioration of bearings due to high common mode currents, as well as deterioration of winding insulation due to high differential mode voltages and dV/dt.

 

BLOCK DIAGRAM
AND ITS EXPLAINATION

       BLOCK DIAGRAMOF THE              PROPOSED SYSTEM

 

n  The block diagram has the following blocks:

n  Three phase full bridge rectifier.

n  Three phase full bridge MOSFET based inverter.

n  Control circuit.

n  Speed sensing unit.

n  Speed selector switch and Display unit.

n  DC regulated power supply.

n  Output filter

n  Three phase induction motor

 

STRATEGIES FOR HARMONICS/EMI SUPPRESSION

n  Grounding capacitance can be connected from both sides of the dc link to the heat sink close to the switching devices providing a physically short path and thus low impedance for RF ground current flowing from the switching device and motor load.

n  The line capacitance can be connected across the dc link as physically close to the switching devices as possible. This capacitance provides low impedance path for the differential mode RF current flowing from switching devices such as the recovery current of the diodes as well as from the cabel-motor load.

n  Line capacitance can be connected across the ac power input terminals as close to the rectifier as possible. These three capacitances serve as another shunt circuit in combination with the capacitance at dc line for differential mode noise compensation…..

 

n  A common mode inductance can be inserted in each phase of the ac input main circuits, providing a high impedance for the RF currents to the power mains.

n  A common mode inductance is inserted in each phase of the ac output main circuit of the inverter, to reduce the dv/dt

n  (In the converter with 6 pulse number the harmonics generated on ac side is given by h=pq, p (pulse no.) q( any integer), therefore harmonics of order 5,7,11,13,… are to be dealt with)

     A combination of LCR elements can be inserted to ac sides and tuned to the frequency pertaining to the harmonic order of 5 and 7.

 

MODIFIED INVERTER DRIVE

TYPICAL NOISE LEVEL WAVEFORM AND EXPECTED LEVEL

THEORITICAL LINE VOLTAGE & LINE CURRENT WITHOUT/WITH FILTER

LCR FILTER IN SINGLE LEG OF 3-PHASE OF IM

 

 

SUMMARY OF FILTER DESIGN PROCEDURE

n  tentatively select C1 such that C1 = ipk/( dV/dt )max

 

n  check is Ton,min from   dV/dt = Vdc*pi/Ton,min   is compatible with the application constraints; if not, higher dV/dt has to be accepted

 

n    Choose L1 such that

     pi*sqrt(L1*C1)<=Ton,min

 

n  Choose R2 = Zc or slightly higher ( R2 = n*Zc with 1<n <2)

 

n  Set O/C protection such that O/Cth > Iphase,peak + Irecovery + Vdc/((n+1)*Zc)

 

Finally, some power dissipation is expected into R2. which cannot be neglected.

It may be shown that, in case R2 = Zc, the dissipated power is:

 

Pdiss = (Vdc^2)/(4*R2) * Ton,min * Fsw

Line to line voltage (Inverter output voltage)
with/without filter

EFFECTS OF HAMONICS ON INDUCTION MOTOR

Two kinds:

n   Heating effect (Motor loss) 

   - reduces efficiency

n   Harmonic Torque

   - Steady Harmonic torque (developed by harmonic stator mmf with harmonic rotor mmf or currents of the SAME ORDER)

   - Pulsating harmonic torque (developed by harmonic stator mmf with harmonic rotor mmf or currents of the DIFFERENT ORDER)

 

 

 

LITERATURE SURVEY

n   D.S. Huang, X.-P. Zhang, G.-B. Huang (Eds.): ICIC 2005, Part I, LNCS 3644, pp. 243 – 252, 2005

n   Sudan Engineering Society JOURNAL, May 2006, Volume 52 No.46

n   Journal of Applied Sciences 5 (2): 249-252, 2005 ISSN 1812-5654© 2005 Asian Network for Scientific Information

n   M. H. Rashid, "Power Electronics, Circuits Devices and Applications, Prentice-Hall International, Inc., New Jersey.

n   E. Pearsson. ‘Transient Effects in Applications of PWM Inverters to Induction Motors’. IEEE Transactions on Industry Applications, 28(5), 1095-1101,Sep/Oct 1992

n   Austinn Bonnett. ‘Analysis of the Impact of PWM Inverter AC Induction Motors’. IEEE Transactions on Industry Applications, vol 1, page 143-152,USA 1996

n   A. von Jouanne, P.N. Enjeti, and J.W. Gray. ‘Filtering Techniques to Minimize the effect of long motor leads on PWM Inverter fed AC motor drive systems’ IEEE Transactions on Industry Applications 32(4):919-926, july/Aug 1996