A Tool to Detect Faults in Induction Motor

 

1. INTRODUCTION

 

The operators of electrical drive systems are under continual pressure to reduce maintenance costs and prevent unscheduled outages that can result in lost production and revenue. The application of condition based maintenance strategies rely on specialized monitors to reliably provide a measure of the health of the drive system. Thus, unexpected failures and consequent downtime may be avoided and/or the time between planned shutdowns for planned maintenance may be increased. Maintenance and operational costs are thus reduced. During the past twenty years, there has been a substantial amount of fundamental research into the creation of condition monitoring and diagnostic techniques for induction motor drives.

 

New methods have been developed, which are now being used by the operators while, at the same time, research is continuing with development of new and alternative on-line diagnostic techniques. This paper describes the development of an instrument that will reliably diagnose problems such as broken rotor bars or abnormal levels of airgap eccentricity in induction motor drives during normal in-service operation. The main focus of the paper is on the key features embedded into the intelligent hand-held CSA instrument and the interactive communication possibilities via networking.

2.  DETECTION OF BROKEN BARS

 

It is well known that a 3-phase symmetrical stator winding fed from a symmetrical supply with frequency f1, will produce a resultant forward rotating magnetic field at synchronous speed and if exact symmetry exists there will be no resultant backward rotating field. Any asymmetry of the supply or stator winding impedances will cause a resultant backward rotating field from the stator winding. When applying the same rotating magnetic field fundamentals to the rotor winding, the first difference compared to the stator winding is that the frequency of the induced electro-magnetic force and current in the rotor winding is at slip frequency, i.e. s·f1, and not at the supply frequency:

 

The rotor currents in a cage winding produce an effective 3-phase magnetic field with the same number of poles as the stator field but rotating at slip frequency f2 = s·f1 with respect to the rotating rotor. With a symmetrical cage winding, only a forward rotating field exists. If rotor asymmetry occurs then there will also be a resultant backward rotating field at slip frequency with respect to the forward rotating rotor. As a result, the backward rotating field with respect to the rotor induces an e.m.f. and current in the stator winding at:

This is referred to as the lower twice slip frequency sideband due to broken rotor bars. There is therefore a cyclic variation of current that causes a torque pulsation at twice slip frequency (2sf1) and a corresponding speed oscillation, which is also a function of the drive inertia. This speed oscillation can

reduce the magnitude (amps) of the f1(1-2s) sideband but an upper sideband current component at f1(1+2s) is induced in the stator winding due to rotor oscillation. The upper sideband is enhanced by the third time harmonic flux. Broken rotor bars therefore result in current components being induced in the stator winding at frequencies given by:

 

These are the classical twice slip frequency sidebands due to broken rotor bars. Due to the variables that affect the frequency of these sidebands and their magnitude in amps (normally in dB in a CSA system) any reliable diagnostic strategy has to consider the following:

 

2.1. Detection Of Airgap Eccentricity:

Airgap eccentricity may be detected by identifying the characteristic current signature pattern being indicative of abnormal levels of airgap eccentricity and to then trend that signature. The specific frequencies of the current components indicative of airgap eccentricity may be calculated by:

 

With nd = 0, equation (3) gives the classical rotor slot passing frequency components - a series of components spaced at twice the supply frequency, 2·f1, apart. With nd = ±1, equation (3) gives additional components that were initially thought (1985/1986) to be only a function of dynamic airgap eccentricity. Extensive experimental tests subsequently proved that as static eccentricity increased the components that were theoretically supposed to be only a function of dynamic eccentricity also increased in magnitude. Finite element studies and further laboratory tests and industrial case histories have proved that these components are in fact a function of the combination of static and dynamic airgap eccentricity, namely the overall airgap eccentricity. The signature pattern of specific rotor slot passing frequencies and the two components from equation (3) with nd = ±1 can be used to identify abnormal levels of airgap eccentricity.

 

2.2. Detection Of Shorted Turns In Lv Stator Winding:

The objective is to reliably identify current components in the stator winding that are only a function of shorted turns and are not due to any other problem or mechanical drive characteristic. There has been a range of papers published on the analysis of airgap and axial flux signals to detect shorted turns and the detailed mathematics can be found in the references. Previous studies have proved that the following equation gives the components in the airgap flux waveform that are a function of shorted turns:

 

It is not the purpose of this article to reproduce existing theory since the focus is on experimental verification from an industrial perspective. The diagnosis of shorted turns via CSA is based on detecting the frequency components given by equation (2) in that these rotating flux waves can induce corresponding current components in the stator winding. A very recent paper by one of the authors of this paper gives full details of the application of CSA to diagnose shorted turns in low voltage stator windings. Motors with different winding designs, pole numbers and power ratings were tested until failure under different load conditions. This is the first time results have been presented from motors prior to actual failure from shorted turns. The reader is referred to  for full details.

 

2.3. Detection of mechanical influences:

Changes in airgap eccentricity results in changes in the airgap flux waveform. With dynamic eccentricity the rotor position can vary and any oscillation in the radial airgap length results in variations in the airgap flux. Consequently this can induce stator current components given by:

This means that problems such as shaft/coupling misalignment, bearing wear, roller element bearing defects and mechanical problems that result in dynamic rotor disturbances can be potentially detected due to changes in the current spectrum.

 

2.4. The influence of gearboxes:

As seen from above, mechanical oscillations will give rise to additional current components in the frequency spectrum. As will be shown in the following, gearboxes may also give rise to current components of frequencies close to or similar to those of broken bar components. Hence, to perform a reliable diagnosis of a rotor winding for motors connected to a gearbox, the influence of gearbox components in the spectrum need be considered.

Specifically, slow revolving shafts will give rise to current components around the main supply frequency components as prescribed by equation (5) where the rotational speed frequency of the shaft, rotating with Nr rpm, may be calculated as fr = Nr/60. For instance, consider the case where a 60 Hz motor rated 300 HP, 575 V, 885 rpm is connected to a gearbox. The output speed from the gearbox is 19.39 rpm. The shaft speed of 19.39 rpm gives rise to two frequency components symmetrically distributed 0.32 Hz around the main supply frequency, i.e. the specific rotational speed frequencies are 59.67 Hz and 60.32 Hz. Harmonics of these are distributed symmetrically around f1 at 0.64 Hz, 0.97 Hz, 1.29 Hz, 1.62 Hz and so forth. From equation (2) it can be seen that the frequency range in which current components due to broken rotor bars may be present is defined by:

where sfl is the full load slip. For instance, for the machine in this example the full load slip is 0.0166 yielding a maximum lower and maximum upper frequency for a broken bar current component of 58 Hz and 62 Hz respectively. Hence, up to the 6th harmonic of the rotational frequency of the shaft are present

in the full load frequency band. Furthermore, to accurately measure and trend the number of broken rotor bars, as will be shown later in this publication, it is imperative that the rotational speed frequencies of gearbox shafts be detected, identified and omitted from the broken bar analysis.

3.  DEVELOPMENT REQUIREMENTS

 

To help assist maintenance personnel with the maintenance of critical induction motor drives a diagnostic system must include the following strategic attributes:

Ø  The monitor must be reliable and must not give false indications

Ø  The monitor must be easy and safe to operate

Ø  The monitor must be cost effective and provide real benefit

Ø  The monitor must eliminate the need for an on-site expert

Ø  The monitor must be able to work in autonomous (stand alone mode) and controlled (in conjunction with a user friendly software program) modes

Ø  The monitor must perform “on the spot” diagnosis of the motor’s condition

Ø  The monitor must be able to acquire, interpret and store several sets of data prior to uploading data to a database program.

 

An instrument was developed with these features in mind. The keyword for condition based-monitoring is reliability and in the case of the instrument this includes a number of crucial features:

Ø  Unambiguous diagnosis of a fault over a range of motor ratings

Ø  Correct estimation of the slip for any given load conditions for a range of motor designs and power ratings

Ø  Clear discrimination between the unique current signature patterns caused by a fault and any current components induced due to normal characteristics of the drive system.

Ø  Current components caused by the effect of mechanical load (for example coal crushers, conveyors and gearboxes in the drive train) must be reliably diagnosed since they can be misinterpreted as components from broken rotor bars etc.

Ø  Eliminate the need for an expert to interpret the acquired data by applying reliable, advanced diagnostic algorithms to the spectra.

 

Correct estimation of the slip together with discrimination between current components due to mechanical phenomena and current components due to broken rotor bars is, without doubt, the most critical aspect of current signature analysis. In the case of diagnosis of broken rotor bars, without a correct estimation of the operating slip, the sidebands due to broken rotor bars cannot be identified. Furthermore, as stated earlier, current components due to mechanical phenomena may appear at frequencies close to those caused by broken rotor bars. Hence, correct identification of these components and following elimination of these from the analysis process allows for reliable diagnosis of broken rotor bars.

 

For diagnosis of abnormal levels of airgap eccentricity, correct identification of the current signature pattern defined by equation (3) involves correct estimation of the operating slip. Furthermore, it requires knowledge of the number of rotor slots, R, for the individual machines tested. This number is not readily accessible via the nameplate data but, nonetheless, known by the OEM and often listed in the machine datasheets.

 

Finally, the data spectrum has to be of a sufficient resolution allowing for diagnosis of broken rotor bars at very light loads. The resolution of the current spectrum for diagnosis of broken rotor bars is 10 mHz and the resolution of the current spectrum for diagnosis of airgap eccentricity is 0.1 Hz. In the following, data acquired with the new instrument will be presented. Rather than representing data via screen captures from the instruments application software, the data presented here has been exported to Microsoft Excel.

3.1. Basic MCSA Instrument:

Fig.1. Picture of CS Meter prototype

Instrument description:

            The above figure 1 shows the prototype CS meter.  The instrument is battery operated (supports approximately 3 hours of operation).  A plug allows for connection of an external power supply for operation or changing purposes.  The instrument has a LED color display and a key pad providing an environment for interaction between user and CS meters software for acquisition and diagnosis of data.  A series port is located on the right side of the instrument allowing for communication between CS meter.

 

4. CASE STUDIES

 

4.1 CASE STUDY I: No Broken Rotor Bars:

 

            Figure 2 shows part of the frequency resolved current spectrum for a 575 V, 100 HP fan motor, operating in a cement plant. The full load speed is 1780 rpm yielding a frequency interval of 58.66 Hz to 61.33 Hz for detection of broken rotor bars. The motor was operating at 71 Amps, corresponding to approximately

80% full load. As can be seen from Figure 2, the spectrum is completely free of any current components around the main supply frequency, f1, and consequently, the frequency range in which current components due to broken rotor bars are expected, are empty. The motor thus shows no signs of broken rotor bars.

Figure 2: Current spectrum obtained on a healthy motor. The spectrum

no signs of broken rotor bars.

 

4.2 CASE STUDY-II: Signs of Rotor Asymmetry:

            Figure 3 shows part of the frequency resolved current spectrum for yet another 575 V, 100 HP fan motor operating in a cement plant. The full load speed is 885 rpm yielding a frequency interval of 58 Hz to 62

Figure 3.  Current spectrum obtained on a healthy motor showing signs of

initial rotor asymmetry.

 

 

4.3 CASE STUDY III: Broken Rotor Bars:

            Figure 4 shows part of the frequency resolved current spectrum for coal mill rated 440 V, 180 HP operating in a utility plant. The full load speed is 885 rpm yielding a frequency interval of 58 Hz to 62 Hz for detection of broken rotor bars.

Fig. 4 Broken bars and blow holes in the bar to end ring region

 

            Based on the supply current, the instrument predicted sidebands due to broken rotor bars to be positioned at 49.70 Hz and 50.57 Hz. These frequency positions are close to that of the supply frequency. Figure c show the supply frequency to have a somewhat wide declining current component. This is caused by the motor being subjected to smaller changes in load, i.e. smaller changes in supply current, during the data acquisition process. However, the peak detection algorithms embedded in the instrument was able to detect the declining slopes of the supply frequency within the applied search bands and thus disregard these slopes from the analysis thereby correctly identifying the current components due to broken rotor

Figure 5: Current spectrum obtained on a motor subjected to broken rotor

is further subjected to changes in load.

 

4.4 CASE STUDY IV: The Influence of Gear Boxes:

            Figure 6 shows part of the frequency resolved current spectrum for coal mill rated 575 V, 300 HP operating in a utility plant. The full load speed is 885 rpm yielding a frequency interval of 58 Hz to 62 Hz for detection of broken rotor bars. The motor is driving a coal mill through a three-stage reduction gearbox, i.e. the gearbox thus contains three shafts. The output speed of the gearbox at full load conditions is 19.39 rpm and the individual shaft speeds internal to the gearbox are 52.80 rpm and 141.69 rpm at full load conditions.

 

            The fundamental rotational speed frequencies for these shafts at full load conditions are 0.32 Hz, 0.88 Hz and 2.36 Hz respectively. Since part of the spectrum, which may contain current components due to broken rotor bars, span from 58Hz to 62 Hz, only the last two reduction stages may give rise to a series of current components in the part of the current spectrum where broken rotor bar components may be present. Specifically, two components may be found in the upper and lower search band.

 

            The motor was operating at 250 Amp corresponding to approximately 77% load. Based on the gearbox name-plate data, the instrument was able to correctly identify the current components caused by the shafts in the gearbox. The positions of these components are displayed in Figure d. As can be seen, the two current components in each search band are indeed caused by the gearbox. Specifically, the two components in each search band are a 4th harmonic from the 3rd reduction stage and the 2nd harmonic from the 2nd reduction stage. Since the current components in the search band are caused by the gearbox and not by the presence of broken rotor bars, the motor was diagnosed as not being subjected to broken rotor bars.

 

            This example clearly demonstrates that gearbox components need be correctly identified and omitted from the analysis. If the influence of gearbox components is not considered when identifying the presence of broken rotor bar current components in the current spectrum, otherwise healthy machines may be incorrectly diagnosed as unhealthy. This may lead to significant costs and may thus raise questions regarding the value of any condition based monitoring program such a system being a part of.

Figure 6: Current spectrum obtained on a healthy motor driving a coal mill

through a gearbox. Note that two current components are present in the

upper search bands

 

Figure 7: Same as Figure 4 but with current components caused by the

gearbox identified. The dotted lines identify the gearbox components. The

lines constitute the lower and upper search band for broken rotor bars.

 

5. Stator Winding Monitoring

 

            Many cement plants are moving toward condition based maintenance (CBM), also known as predictive maintenance, for major equipment such as generators and motors. CBM allows plant maintenance personnel to avoid in service failures, to identify those machines that require outages for testing and repairs and perhaps most importantly, to ensure that unnecessary motor and generator shutdowns for testing and repairs are avoided on machines which are in good condition (normally the majority of machines in a plant are in good condition).

 

            The result is that availability is increased, the time between major inspection is increased, and overall maintenance costs are reduced, since very few machines  are in trouble at any one time. Because only a few machines need extensive maintenance or testing at any one time, the maintenance effort can be concentrated on the motors and generators most in need.

 

5.1 Stator Winding Failure Process.

            To implement CBM one must be aware of the different failure mechanisms that can occur, 'and the symptoms of each failure mechanism. The majority of stator windings fail as a result of gradual deterioration of the electrical insulation. The operation of stator windings at a high temperature for a long time causes the insulation tape layers, which are "glued" together with epoxy or polyester, to debond. This insulation delamination leads to the creation of air pockets in the insulation, and allows the copper conductors to vibrate against one another, as well as against the ground wall. Eventually, the insulation abrades, leading to turn shores (in motors) and or groundwall failure. Another common mechanism occurs if windings are not held tightly in the slot (often the case in non-global VPI windings such. as in large motors and generators). The coil then vibrates in the slot, leading to abrasion of the coil insulation surface and . eventual failure. A further problem occurs 'when partly conductive pollution, from moisture and or oil in combination with dire, settles on the coil surfaces outside the stator slots. This pollution leads to electrical tracking discharges .in the end winding, which eventually punctures the ground wall.  For motor and generator stator windings rated 4 kV and above, it has been known for decades that these failure. processes give rise to partial discharges (PD). PD 0ir.resmall sparks that occur in

high voltage insulation wherever small air pockets exist. Thus, since overheating, coil vibration and pollution create air pockets, PD is a symptom of most high voltage stator winding failure processes.

 

5.2 History of PD Testing :

            For over 40 years, some motor and generator manufacturers as well as utilities have shown that by periodically measuring the PD activity, they can detect high voltage stator winding insulation problems well before failure would occur.  This early warning permits PD test users .to plan  corrective maintenance (such as cleaning or rewedging), change operation (such as restricting load or the number of starts); dipping/baking or rewinding the stator. Such corrective action extends winding life, or at least prevents an unexpected machine failure either during operation or as a result of a dc or ac over voltage test. Partial discharge tests can be performed either when the machine is not operating (off-line) or during normal operation of the motor or generator (on-line). Most motor and generator users prefer an on-line test since no outage is required, and the test can be performed more frequently. In addition, certain problems such as loose coils in the stator slots can only be detected by a partial discharge test  when the machine is operating.

 

5.3 Bus Coupler Test for Motors

            Motors are generally connected to the motor control center relatively long power cables. Research had revealed that most. of the' electrical interference present in motors came from the power system . The on-line PD monitoring system that has now been applied to over 500 motors involves the permanent installation of 80 PF capacitors on the motor terminals (Fig. E), or the installation of high frequency current transformers on the ground side of surge capacitors that are sometimes mourned at the motor terminals. In both cases, the high frequency current pulses which accompany the PD pulses ate detected by these "bus couplers." The bus couplers are installed in a day during a short outage. The capacitors block the 60 Hz voltage while passing the very high-frequency voltage pulses that accompany partial discharge. Each partial discharge within the stator will create a very short voltage pulse which will travel through the stator winding, and eventually appear at the stator terminals where the bus couplers will

detect the voltage pulse. However, PD-like noise can also come from the power system—especially arcing and corona from the switch yard and switchgear. Such signals can be misinterpreted as PD from the stator windings.

 

            Stator winding PD is separated from noise based on the shape of the detected pulses, that is, PD has a Wt rise time, whereas noise has a slow Rise time. Stator winding PD pulses are detected as 3 to 5 ns rise time pulses by the bus couplers, since they are physically close to the stator, and are detected essentially undistorted. Noise pulses must either propagate from the power system to the motor along the power cable, or they couple onto the metal motor enclosure by a radio-type pick-up through the air. The latter is well known to yield

relatively slow rise times, since the motor enclosure was not designed to be an efficient high frequency receiving antenna. The slow rise time response that occurs from a pulse which travels through a power cable to the motor is caused by the frequency- dependent attenuation properties of the cable f8]. Specifically, as a Wt rise time pulse propagates along a power cable, say from sparking at a

poor electrical connection in the switchgear, the pulse rise time lengthens, and the magnitude of the pulse reduces. The result is pulse distortion, which gets worse as the cable gets longer. Power cables can considerably distort a pulse since the semi-conductive conductor shield is a strong absorber of high frequency energy 8]. Fig. 7 shows how a PD pulse is attenuated and its rise time is lengthened as it travels through a power cable.

Fig.7 Schematic of the installation of capacitive PD sensors on a motor terminal.

 

            The difference in magnitude and rise time  Between noise and PD. when detected at the motor terminals  enabled the development of a reliable, on-line method for non-specialists to separate PD from noise. For motors rated 6 kV and higher, a portable instrument called the TGA-B is used to periodically measure the PD during normal operation of the motor. The instrument separates PD from noise, and records the number, magnitude and phase position of the PD. For motor rated 6 kV or more, 15 years of experience has known that typically 2 to 5 years of warning is achieved, thus periodic testing is suitable.

 

            For 4.1 kV motors, which only operate at 2400 V to ground, experience shows that there may be only a few weeks of warning between the time significant PD is detected, and Circuit Breaker when failure occurs £9}. Thus to ensure that impending failures are not missed, an inexpensive instrument was developed. which continuously measures the PD activity in 4.1 kV motors. Again, by taking into account the different pulse shape, PD can be distinguished from noise on a pulse-by-pulse basis, and thus the risk of false indications of stator winding problems caused by noise is greatly reduced. The continuous monitor developedis called Motor  Trac.

 

Fig. 8 Distortion of noise pulses as they propagate along a long power cable

 

 

6. ConcLUSION

 

            A new portable instrument,  called the CSMeter, has been developed for reliable on line detection of broken rotor bars and abnormal levels of air gap eccentricity . The instrument is the first of its kind to be fully portable and provide on the spot diagnostic about the state of the rotor winding and air gap eccentricity problems.

 

            By entering nameplate data,  the instrument allows for reliable  detection of current components due to broken rotor bars and abnormal levels of air gap eccentricity.  As shown in the paper, reliable detection of broken rotor bars,  can also involve the correct identification of current due to mechanical  components in the drive system. The instrument  takes  into account when performing the analysis.

           

            Based on preliminary field testing, it can be concluded  that the CSMeter provide reliable diagnosis  about the state of the rotor winding by estimating the number of broken rotor bars and detecting abnormal levels of air gap eccentricity.

 

            Partial discharge tests can determine which motor and generator stator windings are experiencing insulation problems. A deteriorated winding has a PD activity which can be 30 times or more higher than a winding in good condition. This great difference in PD activity enables even non-specialized maintenance personnel to identify the few motors or generators in a company which need further investigation and/or maintenance.

 

            here are several advantages to on-line partial discharge tests. Machines in good condition require less attention. The overall effect is lower maintenance costs. After implementing on-line partial discharge tests, as well as other monitoring such as flux probing or current harmonic analysis and improved temperature sensing, companies can often confidently extend the outage between major  machine inspections. This saves on outage costs and reduces the risk of a stator fault due to human error during maintenance. Finally, on older machines, if there has been no increase in partial discharge activity over time, then the life of the stator winding can be confidently extended, saving a considerable capital expenditure.

7. REFERENCES

 

1.    J.R. Cameron, W T Thomson and A B Dow, "Vibration and Current Monitoring for Detecting Air gap Eccentricity in Large Induction Motors", Proc IEE Journal, Part B, Vol 133, No.3, May 1986.

2.    W T Thomson and D. Rankin, "Case Histories of Rotor Winding Fault Diagnosis in Induction Motors", 2^nd Int Conf Proc on Condition Monitoring, University College Swansea, March 1987.

3.    W.T. Thomson and M. Fenger, "Current Signature Analysis to Detect Induction Motor Faults", IEEE Industry Application Magazine, July/August Issue, 2001.

4.    S.R. Cambell, et al. "Practical on line partial discharge tests for turbine generators and motors, "IEEE Trans EC, P 281, June 1994.