(2) 1401036342 Predictive Maintenance of Mechanical Failures Using Electrical Measurements for Instantaneous Torque. a Modern Approach.

Predictive Maintenance of Mechanical Failures, using Electrical Measurements for Instantaneous Torque.A Modern Approach Ernesto Wiedenbrug, Ph.D. SM IEEE Baker Instrument Company Abstract: Historically, vibration technology was the only means available to assess many mechanical failures in the field. However, there are conditions in which it is physically impossible to reach proximity to the system in question, being either motor, or the driven load. Using examples from recent field experiences, the new technology’s approach, and the breakthrough value of the instantaneous torque signal will be evaluated diagnosing several cases in the field: looseness in a 1.5 hp coupling, - a deteriorated 1250Hp submerged pump, - diagnosing the pumping of sand in deep well pumps, failure modes of - mechanical unbalance and bearing failures for an in-duct mounted 5hp fan in nuclear environments, - rotor-bar count independent assessment of eccentricity for cryogenically cooled pumps for line operated applications. Variable Frequency Drive (VFD) applications pose particular challenges to maintenance due to their dynamic behavior, which is exemplified with one application. Here, the instantaneous torque signal is used to diagnose all three of the following dynamic maintenance challenges: - regeneration, - dynamic overloading during acceleration, and - oscillations (hunting) during steady speed operation. Comparisons of the signal qualities for current and torque signals for the identical data acquisition runs will clarify the advantages of the instantaneous torque signal in the time domain, in the frequency domain, and in the demodulation domain. Overall, the concept of torque signature analysis will be explained in such a way that the reader will be capable of identifying looseness, oscillations, cavitation and more, just by looking at torque signatures. This will allow the attendees to forecast applications, in which transient torque monitoring could ensure proper root cause analysis for predictive maintenance; allowing them to identify the most suitable technology for the conditions that they are facing. Key words: Predictive Maintenance, Torque, Instantaneous Torque Signal, VFD Diagnostic, Vibration. I. INTRODUCTION on the instantaneous torque signature instead of the current signal as a predictive maintenance tool lies on the ease of assessment – both for trained and untrained personnel. This paper will visit a large variety of field case studies that illuminate some of the uses of the torque signal in the time domain, in the frequency domain, and in the demodulation domain. The reported case studies cover a large variety of motor ratings (1.5hp to 1250hp) and a large variety of industrial profiles (manufacturing, pulp and paper, coal fired power plants, nuclear environments, deep-well pumping). The chosen case studies aim at fault types that relate mainly to the mechanical field for lineoperated and for VFD driven loads (Looseness, Cavitation, Pumping of Sand, Mechanical Unbalance, Bearing Failures, Eccentricity, VFD-Regeneration, VFDDynamic Overloading, VFD-Oscillations). This wide palette of case studies will allow the reader to develop a good understanding of the range of applications to this aggressively expanding technology. Frequently, professionals newly exposed to this breakthrough technology question the advantages of the instantaneous torque signal when compared to the current signal (the standard method of analysis used in the 90’s). Comparison of signal quality of the calculated instantaneous torque for the identical data capture will show the advantages of the instantaneous torque signal. Safety has always been at the top most priority for field professionals. The new regulations on safety and exposure to arc-flash [2] slow down the speed of capture of data for the electrical professionals, unless the data can be taken with a closed MCC-door. Slow data capture processes are the main reason why portions of predictive maintenance programs are stopped of being used, and turn into dead capital lacking any benefit. Good technical solutions to this real-life impediment have to offer setups that allow closed-MCC-door testing. II. LOOSENESS Electrical On-Line predictive maintenance is a field of rapid technical development. The same theoretical background which made the first torque controlled VFDs possible can be utilized to monitor the shaft torque by measuring only the stator currents and voltages [1]. The first diagnostic use of the instantaneous torque signal as a part of an off-the-shelf solution for field maintenance reached the market in 1999. Since then it has been shown at an increased rate that the quality of the obtained information is much higher than the quality of the current signals used until that time. The main benefit of relying In many cases, the motor and the driven load are relatively inaccessible. Automated machinery is frequently relatively inaccessible in manufacturing environments. Due to safety considerations, it is very uncommon that maintenance professionals investigate the functioning of automated machines during their operation. Commonly, people do not visit the inside of the machine’s enclosure unless a known fault has to be fixed. On the other hand, many faults are only noticed once they reach such grave proportions.160V pump motor was detected and diagnosed in a Progress Energy power generation plant in . which cannot be explained by the mechanical understanding of the driven load. A very short downtime of less than one hour sufficed to readjust the load. Clearly. On-line technology allows measuring the electrical input to the motors.5hp motor. after fastening the tube. than if it had been found at an earlier stage. The accurate load torque estimate of steady state operation has been the key tool with which a defective submerged 1.250hp 4. as that they stop functioning and production. This happens dozens of times per second (so frequently indeed. describes the steady state operation of this application satisfactorily. Figure 1 shows the instantaneous torque graph (torque vs. A mechanical fault reaching this state will take much more downtime to fix. like this one. time) of a tube application in a manufacturing environment. The achievable accuracy of such load estimates when operated at line frequencies has been published previously in papers [3-4] in laboratorycontrolled studies. The concept is to use the motor as a sensor for the driven load. Figure 2: Torque signature of previously loose. The torque bubbles are gone. and are almost impossible to diagnose there. only elevate the noise-floor in the frequency domain. which is a safe distance of the automated equipment. that it cannot be counted in this graph without zooming in). The optimal type of fault that is found in the time domain is a fault type with relatively high transient energy levels. This signature shows that on the driven load. The fact that the amplitude of the torque bubbles is varying from bubble to bubble. and diagnosing the driven process. A basic understanding of the mechanical setup of this load. made identification of a deteriorated submerged pump possible. then it cannot be diagnosed well neither in the frequency domain. the cost of the motor itself is not sufficient to warrant a predictive maintenance schedule. Looking at the size of the highest peak of the torque bubble to the lowest valley. A 1. onto which ‘torque bubbles’ are added. This signature shows a steady state average torque. as well as the duration of a torque bubble change too are the hallmark of signatures that cannot be found easily in the frequency domain. In an industrial application the outlined techniques [3-4] of monitoring instantaneous torque at the motor control cabinets. If the fault lacked consistent inner frequencies. These ‘torque bubbles’ represent a very fast changing torque. there are extremely large pulsations of load. allows diagnosing looseness and rattling. 1). Figure 2 shows the torque signature of the same load. There are no reoccurring patterns in the time domain. CAVITATION Figure 1: Torque ripple of load showing looseness. 1.5hp 4-pole motor drives the load. we see that the very dynamic torque changes in a range of full rated torque. rattling load (see Fig. This fault type may or may not have consistent inner frequencies. however. Random-like patterns. and the frequency of occurrence (distance from bubble to bubble). and the torque signature The instantaneous torque techniques outlined in [3] have found industrial application. III. nor in the demodulated domain. through instantaneous torque signature analysis. is much larger. The red line shows the upper limit of the rated operation (rated torque). hence there are no clear distinguishable peaks resulting in the frequency domain. manufacturing environment. The cost of downtime. The advantage of this early diagnosis is that the fault was found during operation of the machine and at the MCC. however. Using the instantaneous torque signal it was possible. Decreasing the speed of the pump can frequently lower the amount of sand being pumped. 4. it can be usual for sand to fall into the intake of the pump.600Nm vs. to diagnose cavitation of this submerged pump. IV. 5) to the pump rusted over time and broke. 7ft inner diameter) showed operation at a torque level of 27% below its two twin systems (23. The verification of the sand signature was done using a lab setup of a VFD driven submerged pump. The defective pump. which caused the endbell to fall 20ft down into the water pit. The pump was operated . The endbell’s function is to assure laminar water flow. sample of sand (right). Depending on the soil. Figure 3 shows the healthy and faulty torque signatures of the pumps side by side. PUMPING OF SAND Figure 3: Healthy (left) and Faulty (right) pump torque signatures. Deep well water pumps are in use in most community water supply systems across the United States.40s) and increasing sand pumping (80s – 120s). Data was acquired for 120 seconds. The instantaneous torque signal is obtained through calculations from the low voltage side of the PTs and CTs of this 4160V motor and allows a clear diagnosis of cavitation by predictive maintenance professionals of a remote pump by connecting to low voltage signals. Figure 7: Instantaneous torque signatures (red) and short-time average (blue) for no sand pumping (0s . causing it to erode over time. The bolts which attach the endbell (Fig. Figure 4: Defect 1250hp pump. Depending on the size of the corns of sand. The slow turning pump in question (273rpm. pulled for repairs. An example of pump erosion on the impeller caused by sand and a sample of the sand is shown in Figure 6. Figure 5: Endbell (input funnel) of the pump. Being able to observe whether sand is being pumped also allows forecasting pump degradation. Figure 6: Sand-eroded impeller (left).400Nm) yet significantly higher levels of calculated torque ripple. for the first time.North Carolina. is shown in Fig. it will grind inside of the pump. 30. and its loss resulted in increased cavitation with decreased water flow. One of the most critical environments for predictive maintenance is the nuclear industry. The amplitude of this peak was found to be a very reliable measure of the severity of unbalance. do not reach the outside of the duct. the amount of pumped sand is above any acceptable level for the field. saturation and skew. granted that the amplitudes of the current spectra calculated before do not always show for every motor design. Predicting failures of motors or bearings is of very high importance in most environments. Additionally it shows the sidebands at ±2 x rpm (in green) to the BPFO. and Figure 10 According to [5]. Lacking steadily wired up sensors. and the short time averaged curve in blue. however. More modern work [6. which were connected to the outside of the duct. The first case was comparing the baseline. while it shows a clear strong ripple for the sand corns that are introduced. two different failure modes were planted onto a ductmounted fan. They came to the conclusion that the frequencies observed by [5] show variations due to skew. The failure pattern in the torque signature shows a high peak at 107Hz. Frequency signature of 5hp motor with BPFO 107Hz. Between seconds 100 and 115. MECHANICAL UNBALANCE shows the typical bearing signature pattern. Figure 9: Prepared bearing with scratch on outer race. by analyzing the calculated instantaneous torque signal. by using the stator current’s frequency components created by rotor bar pass frequencies. The measurements of currents and voltages were done at the MCC. Increasing amount of sand that was let into the intake during the remaining 80 seconds. regardless of the amount of eccentricity of the motor. at stage 3. The torque spectrum shows a clear 1x peak for this 4 pole motor. V. Figure 9 shows the bearing with planted 3rd stage failure. The result of this investigation showed that it is possible to find deteriorating bearing failures for this application. Over the years it was noted that in-duct mounted fans were failing due to bearings at a usual rate. well balanced operation of the fan with a mechanically unbalanced operation. At this time this technology can be used for base lining and identifying worsening mechanical unbalance conditions. The mounting bars of the motor to the duct are dampening the vibration signatures to the point that failure modes. BEARING FAILURES The second failure mode that was investigated in this study was of an outer bearing failure. were not able to observe the fault. which can easily be diagnosed on the motor’s case. frequency signature at 1 x rpm. The further increase of sand between seconds 115 and 120 reaches such high level that the pump is almost starting to bind. VI. it is possible to detect stator line eccentricity. In an effort of finding alternate methods of failure prediction. and it’s third harmonic 314Hz (in red). Torque vs. and the unbalanced torque signature to the right. ECCENTRICITY Figure 8: In-duct mounted fan (left) with fastening. which will not show the signatures. There are rotorbar-stator slot combinations. Figure 7 shows the instantaneous torque curve in red. however. balancing and unbalancing bolts. Babour and Thomson [7] investigate the effects of rotor slot design (open or closed bridge). 7]. VII. which explained the historic difficulty in predicting these motor’s failures. Figure 8 shows the fan to the left. This bearing had a BPFO of 107Hz.without introducing sand into the intake for the first 40 seconds. Figure 10: Torque vs. rotor bar . focuses on the possible use of these frequencies. but their failure was not predicted successfully using top of the line portable vibration monitoring equipment. It is very noticeable that the short time average is flat while only water is pumped. the taking of vibration data is limited to the outside of the air-duct in which the motor is mounted. The vibration sensors. Typically. This dynamic eccentricity disappears the instant that the motor is lifted out of those extreme low temperatures. While there is no log in proximity to the saw. (1) Figure 11: AM-demodulated torque signature of a motor with eccentricity. trying to stop the load from running that fast. application is a 60hp conveyor belt. it was necessary to develop an alternate method that does not disagree with IEEE research results. which feeds logs into a saw. it keeps constant low and high torque values. Cutting the log takes one second. the resolution displayed in Fig. The 120Hz signature stands very high. . These low temperatures have the effect of introducing a bend into motor shafts. 13 we see the same torque signal. The deceleration takes 1 second (from 0. This behavior matches the expectations. by remembering that Force = mass · acceleration.7s to 1. since the thermal bend clicks back into straight position. At 0. the conveyor’s 6 pole motor runs at maximal rated speed (1200rpm). This is calculated as a function of voltage level. 11 shows the frequency spectrum of the AM demodulated torque signature for a motor with dynamic eccentricity. and from seconds 3. 12. The additional advantage is that dynamic eccentricity shows at the same frequencies where it shows for vibration technology: at 1x and 2x electrical. which causes the PLDs to slow the conveyor’s speed down to cutting speed of 250rpm. respectively. which are not very well tempered. Lacking a steady state. which is the operation of the first 0. A. Three sections of interest were identified in this figure. In Fig.7 to the end. VFD – REGENERATION Variable Frequency Drives (VFDs) show the largest challenge for diagnosis. This figure also shows the dynamic rated torque boundary. but represented with a higher resolution. The first section. Figure 12: Torque and Speed vs. frequency and nameplate information of the motor. Since the rotorbar pass frequencies are not reliable for eccentricity detection. Three common diagnoses will be discussed with the example of Fig. This means that during this period (the deceleration). This Pulp and Paper Industry Figure 13: Dynamic torque and rated torque vs.7s). while the display of Fig. During the constant deceleration and the constant acceleration. making an accurate assessment of eccentricity based on the rotorbar pass frequencies unreliable. and at 2.7 seconds.7s the PLDs accelerate back to 60Hz within one second. shows that the torque is very negative. Our research has shown that the AM demodulated torque signal is a reliable way of identifying dynamic eccentricity. This situation is challenging because accelerometers do not function at cryogenically low temperatures.design and saturation. Fig. taking energy from the driven load and passing it to the electrical grid. the motor is operating with a negative torque. It keeps a steady average value during the steady state operations. Looking at the red portion of Figure 12 we see the torque signal. and the fault does not show when the motor is elevated to higher temperatures. time for VFD driven conveyor belt.7 seconds. the analysis has to be done in the time domain. 12 is used for understanding the trends of the system. due to their very dynamic changing of operating condition. 13 is chosen to scrutinize the particulars of the system. VIII. time. Negative torque means here that the motor is operating as a generator. Cryogenic pump motors are operated at liquid CO2 temperatures. the log is in proximity to the saw. This line represents the equivalent of full rated operation’s rated temperature stress to the motor. The blue line shows the constant higher speed at 1200rpm during these times. In this case.3s during the 4s of 60Hz speed.3s is prohibitive.3s acceleration. Clearly. which the system passed. If this 0. Time domain The same process. 14 it is basically impossible to distinguish the quality of the signatures of the good and the poor operation. The torque signature is calculated from the current signatures shown. The fundamental responsibility of identifying oscillation and equipment deterioration will always be the field maintenance groups’. a. XI. Overloading is the fundamental reason for overheating motors. The data displayed is the identical data measurement.This assessment is of interest in some cases. The PID settings have the function of counteracting the ‘talking’ between the load and the VFD. to run motors of 4 poles and up to up to 110% of rated frequency. XI. an example displaying the data of both. . who may do it themselves. Apart of having been on the market for more than twice as long. 15. All that needs to be done is to extend the 1s acceleration from 20Hz to 60Hz to a 1. The rated torque is 220Nm. Cost-effective maintenance fundamentals game is to avoid unnecessary wear before it causes the need of repair or a possible unplanned outage. Since the VFD manufacturers cannot know in advance what mechanical system properties are going to be faced by the motor. the mechanical system is constantly accelerated and decelerated. then the cruising speed of the conveyor can be adjusted higher. Ensuring that the PIDs are set to avoid oscillations is the responsibility of field maintenance. Changing the settings for the acceleration (Hz/seconds) to a slower acceleration will keep the motor from overheating. This type of oscillation is typical in VFDs when the mechanical system ‘talks’ with the electrical regulation of the VFD. IX. both torque signatures show tremendous differences. COMPARING CURRENT AND TORQUE SIGNALS Current signature analysis is an older technology than torque signature analysis. 13 shows that the instantaneous torque surpassed the rated torque for some period of time. 13 shows that the torque is not constant while the motor is running at a set speed. having as little of torque ripple as feasible. in which the cryogenic pump manufacturer checks for low temperature induced eccentricity. 15 b shows a signature that does not match a good pump’s profile by any means. Such an oscillation represents an unnecessary wear on the system. NEMA MG1 [9] allows. is also used for overall functioning of the pump. while figures 15 a-b show the torque signatures of the same two pumps. time signatures of two pumps. In Fig. Figures 14 a-b shows the current and voltage vs. while the operating torque keeps an average of 300Nm and a peak surpassing the 400Nm. at the same timestamp. The motor can operate as a generator without any trouble. Our basic understanding of pump operation gives us the knowledge that steady state operation is expected. it also boasts the enticing simplicity of not requiring a synchronous measurement of multiphase currents and voltages. The frequently asked question is why to trouble oneself with the added complexity of the torque calculation. the conveyor is designed for push-pull.0Hz to 60. Even though the motor sustains no damage under this operation mode. Commonly. and all the parts that hold it in place. The electro-mechanical system can start swinging. Equation (1) shows that the torque will drop by the percentage by which the acceleration time is extended. This small change of cruising speed will not affect the health of the rest of the mechanical system noticeably. The second segment of Fig. including the conveyor belt itself. X.1Hz will make up for the lower acceleration rate’s lost productivity. VFD – DYNAMIC OVERLOADING The reason why this system was monitored is because the motor was found to be running too hot. however. deal with the VFD manufacturer’s field service group. This mode is frequently called ‘hunting’ in the field. Setting the cruising speed from 60.3s per log. there is no problem in slowing down the acceleration for such an application. current and torque signatures will be shown. so that this part of the diagnosis was only a check. This swinging will stress the whole mechanical system. A closer look shows that the time when the motor is overloading is during the acceleration. and we see that the frequency of log cutting is below one log every 7s. which is a common ‘rated’ setting. The solution is very simple. because not all loads are designed for a push-pull operation. or deal with a third party for this task. they cannot pre-set the VFD to the optimal state. The answer will be shown for the three most commonly used domains of analysis: • Time domain • Frequency domain • Demodulated domain For each of these domains. In Fig. The needed adjustment here would need to make up for 0. or oscillating. nor does it need additional computation. It results in a reduced production of 0. VFD – OSCILLATION (HUNTING) The section marked by C in Fig. The torque is showing a low amplitude oscillation. At this point it is set at 60Hz. but not all loads can withstand this operation (even in its dynamic form shown here). however. Every VFD produced during the last 10 years has PID settings that can be adjusted. which is marked B. If the signals start being close to the noise floor (as they are with the current’s spectrum). Figure 16 a-b shows the current vs. b. The signal to noise ratio is the measure of how ‘good’ the signal is. Figure 14: Voltage and current vs. Both these signatures were obtained from the currents and voltages read at the same instant. The result of the comparison of torque and current signatures versus time is that the former hands a simple. This measure represents how high the seeked signals stand above the noise floor. than the current’s signal to RMS ratio. The first measure is the signal to noise ratio. frequency figure on the top. time signatures. The staggering difference between healthy operation and faulted operation makes it not necessary to even compare one signature with the other. This data can be used to define two measures of the signal quality. time signatures a) (top) unfaulted. b) (bottom) faulted pump signature.8 times larger than the current’s. This . is a good case to show the RMS Signal Noise Torque (Nm) 0. and the dark blue lines are drawn at the noise floor. Signal to RMS ratio: The torque Signal to RMS ratio is 125 times larger. and the torque vs.0005 Signal to noise ratio: The torque’s signal to noise ratio is 4.0012 Current (A) 5 0.strengths of the torque signal compared to the current signal vs. Frequency domain The bearing fault case. intuitive and overwhelming assessment. This means that the signal of interest is 125 times larger. than the current signal stands. Table I: Signal to noise ratio and Signal to RMS ratio.025 0. then no reliable assessment can be made. The yellow lines mark the maximal signal of this spectrum. The light blue lines mark the size of the relevant signals (the useful signature). Figure 15: Torque vs. This means that the torque signal stands roughly 5 times taller above the noise. Figure 16: Faulted bearing spectrum signatures. a) (top) unfaulted.5 0. XI. The relevant signatures are marked with the vertical arrows. a) (top) Current b) (bottom) Torque. frequency signature of the bottom. b) (bottom) faulted pump signature. when compared to the other signals of the spectrum. which was successfully identified with the torque signal. frequency. whereas the second cannot be used realistically for identifying such a fault.0022 0. dynamic overloading and hunting were diagnosed with the time domain signal. The quality of the predictive maintenance program.20 5. again. The torque signature.26E-05 7. which have been implemented since the year 2000 in the many hundreds by single companies.01 testing. allowing the fast and frequent safe monitoring for their critical motors. CONCLUSIONS Even Star-Trek ™ technology shows no advantage in the field. success can also be guaranteed if safety. Two motors with temperature-induced eccentricity (‘bad motors’) are compared to two motors lacking this fault mechanism (‘good motors’). the rotorbar pass frequency is not a reliable identification of eccentricity faults for motors. c. depends to a very large extent of the frequency with which the monitoring takes place. Mechanical unbalance and bearing faults were recognized with the torque vs. however.47E-05 7. and a purely passive port is connected to the front of the cabinet.00308 0.03057 1. Here. Bad Motor #1 Bad Motor #2 Good Motor #1 Good Motor #2 Factor Demodulated Torque Demodulated Current 1* RPM 2* RPM 1* RPM 2* RPM 3.00398 0. and allows for closed-MCC-door Instantaneous torque has been used to identify a multitude of field relevant fault modes. frequency representation. The connection port is a “dead port” (high impedance) while the instrument is not connected. The experience that we gained in the last three years shows that the most consistent and active monitoring of critical motors happens in the companies that implemented this safest and fastest setup. The result of the demodulated comparison is that the current signals show a much larger sensitivity to eccentricity faults. The signatures are investigated at 1x and 2x electrical frequencies. cavitation and pumping of sand were successfully diagnosed with the time-domain torque signal. dust-covered tools on the shelf. The sensor arrays are mounted inside of the MCC (typically during and outage). Table II shows that the difference in amplitudes of the demodulated current’s signatures is maximally of 31% in case the motor shows a fault. The optimal setup for on-line predictive maintenance programs does take this into consideration. SAFETY AND CONNECTIONS Figure 17: Safe connection with EP box. This setup addresses the latest safety requirements of [2] while maximizing data collection speed and ease of use.94E-05 0. Table II: Demodulated Torque and Demodulated Current signature’s amplitudes at 1x and 2x electrical for two healthy. This has the effect that the signs of deterioration will be overlooked much more likely in the frequency domain.35E-05 0. It turns into a low voltage port of 10V maximal peak-peak voltage only during measurement with the instrument.00245 0. XII. The difference between excellent and poor predictive maintenance programs is frequently the same as an active use of the technology compared to never-again used. shows an increased signature of almost 600%. the ease of connection and speed of data acquisition are maximized for tools that give valuable data. The advantages of the modern and proven . XI. This level does barely suffice in the field for defining useful thresholds. the torque signal proves to be of a much better quality for field use than the older technology. than the demodulated current signals do. Table II compares the signatures obtained using the identical data for the 4 motors tested. On the other hand.46E-05 1. Many of these diagnoses are impossible to do successfully using the historic technology of current signature analysis of the motor. the current signature.result can also be interpreted by saying that the fundamental current “dwarfs” the signal of interest.31 1.03091 2.00324 0. Figure [17] show such setups. Dynamic VFD applications showing cases of regeneration. This is a signal of very reliable quality for setting pass-fail criteria. obviously.03150 4. if the focus remains on the spectra of currents.90 1.03109 3. and two faulted motors. Failure of on-line predictive maintenance programs can be guaranteed if the instrumentation is not used.42E-05 0. Eccentricity was diagnosed with the demodulated frequency representation of the torque signal.96E-05 0. As discussed previously. Demodulation domain The demodulation signal’s quality for the current and the torque signals will be evaluated by looking at the field example of eccentricity quality control. if it is never used. Looseness. XIII.96E-05 1. . Mixed Eccentricity in Three Phase Induction Machines: Analysis. XV.pdf. Overall. ACKNOWLEDGEMENTS [9] Eccentricity in Squirrel-Cage Induction Motors. Proceedings of the IEEE Industrial Applications Society meeting in Chicago. R. Nandi.A. and Diagnosis of Electrical Machines. NEMA Standards Publication No. NFPA 70E. XIV. S. CD-Rom. S. Finite Element Study of Rotor Slot Designs with respect to Current Monitoring for Detecting Static Airgap [4] [5] [6] [7] [8] . Wiedenbrug. IEEE Press. S. E. 112. A. Simulation and Experiments. A Dissertation submitted to Oregon State University September 24th. pp. A. Krause. 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