Introduction to Motor Troubleshooting

1. Introduction to Motor Troubleshooting

2. Motors are the number one consumer of electrical power

3. Power consumption based on size

4. Standard motor operating conditions

5. Special purpose motor operating conditions

6. Motor failure

7. Troubleshooting fuses

8. Taking motor voltage measurements

9. Measuring motor voltage unbalance

10. Measuring motor control circuit transformer

11. Measuring motor current

12. Measuring temperature

13. Measuring motor insulation

14. Megohmmeter reading interpretation

15. Motor power measurements

16. Testing motor capacitors

17. Electrical measurements on adjustable speed drives

18. Making safe measurements • Voltage ratings • Category ratings • Safe practices • De-energize circuits • Use protective gear • Do not exceed instrument voltage and category ratings • Use 3-point test method • Test known live circuit • Test target circuit • Test known live circuit again • Avoid holding the meter

19. Adjustable speed drive – theory

20. Pulse-width modulated inverter • Theory of operation • DC converter section supplies constant DC level • Rms motor voltage is varied by the width of the PWM pulse • Motor drive signal frequency is controlled by the modulation frequency

21. Motors – measurement 1: Low voltage Low voltages • Check for voltage drops across connectors, or • Check for heated connections Analog meters • Reads the average voltage of the modulation frequency of the PWM drive • Meter may not have IEC-61010 safety rating Digital multimeter • Current DMMS read higher than analog meter on PWM drives because they responding to the entire frequency spectrum of the drive signal. • These DMM are not giving inaccurate readings. • Exception: New DMMs that include low-pass filters

22. Motors – measurement 1: Low voltage Using the ASD display 230 volts (calculated) Using a true-rms meter 247 volts 20 KHz B/W Using an averaging meter 230 volts @ 5KHz B/W Using a voltage tester 227 volts @ 400 Hz B/W Using an oscilloscope 255 (avg) volts @ 20MHz B/W Using a power 253 (avg) volts @ 20MHz B/Wquality analyzer 243 volts @ 3KHz B/W 226 volts @ 60 Hz Using an analog meter 223 volts @ 100 Hz

23. 11 X 100 = 2.39 % 460 1381 = 460 3 Motors - measurement 2:Voltage & current unbalance • Check for voltage unbalance (< 2 %) first, then • Check for current unbalance (< 10 %) Max. Deviation (V or I) % (V or I) Imbalance = X 100 Average (V or I) For example: 449 470 +462 1381 1 2 3

24. PWM drives – measurement 3:Overvoltage reflections at the motor terminals Normal PWM waveform Leading edge of normal PWM pulse Leading edge of PWM pulse with reflected voltage (ringing) PWM waveform with reflected voltages

25. PWM drives – measurement 3:Overvoltage reflections at the motor terminals Overvoltage reflections at the motor terminals. • Damages the motor windings • Shorten cable if possible • If motor is worth repairing, consider rewinding with better insulated wire such as TZ Q (by Phelps Dodge) • If new motor is required, use one that meets NEMA MG1-1993 Part 31 specifications (can tolerate sustained voltage peaks of 1600 V and rise times >100 ns) • Use filtering if none of the above is feasible • Try to mitigate overvoltages to <900 V for standard motors

26. PWM drives – measurement 3:Overvoltage reflections at the motor terminals Possible remedies for overvoltage reflections

27. PWM drives – measurement 4:Motor shaft voltages Bearing currents: occur when shaft voltages exceed insulating capability of the grease • Higher breakdown voltages of 8 -15 V occur due tothe fast edge of the PWM pulse • First signs of this problem = noise and overheating caused by pitting and loosened metal fragments • Use an oscilloscope to view shaft voltages measured between the motor shaft and the grounded frame using stranded wire or a carbon brush

28. PWM drives – measurement 4:Motor shaft voltages Bearing currents: occur when shaft voltages exceed insulating capability of the grease • Make the measurement after the motor has heated up • Simplest solution is to lower the carrier frequency to less than 10 kHz, or down to 4 kHz if possible • Shaft grounding devices, bearing insulation, faraday shield in the motor, conductive grease or filtering between the ASD and the motor

29. PWM drives – measurement 4:Motor shaft voltages Bearing currents: occur when shaft voltages exceed insulating capability of the grease • Make the measurement after the motor has heated up • Simplest solution is to lower the carrier frequency to less than 10 kHz, or down to 4 kHz if possible • Shaft grounding devices, bearing insulation, faraday shield in the motor, conductive grease or filtering between the ASD and the motor

30. PWM drives – measurement 5:Leakage currents (common mode noise) • Leakage currents flow in capacitive coupling between the stator windings and frame ground. The faster rise times and switching frequencies of the PWM pulse can increase leakage • Interferes with 4-20 ma control signals and PLC communications • Increased leakage currents pose potential safety problems and may cause ground fault protection relays to trip

31. PWM drives – measurement 5:Leakage currents (common mode noise) • To measure, place current clamp around all three motor conductors at the inverter output • Use an oscilloscope to examine the CMN waveform • Possible solutions: special EMI suppression cables, isolation transformers on the line input, or a common mode choke

32. PWM drives – measurement 6:Testing the IGBT output waveshape 1. Connect the scope common lead to the dc+ bus and measure each of the three phases at the inverter’s motor output terminals. Check for clean-edged square waves with no visible pulse noise. Verify that all three phases have the same appearance. 2. Check the negative conducting IGBTs by connecting the common lead to the dc- bus and performing step 1 on each phases at the inverter’s motor output terminals.

33. PWM drives – measurement 7:Testing the IGBT outputs for leakage • Measure voltage from earth ground to the inverter’s motor output terminals with the drive powered on and speed set to zero (motor stopped). • If leaky, the voltage will be elevated 3 or 4 times normal. • Perform this measurement on a known good drive to determine what is normal for that drive.

34. PWM drives – measurement 8:ASD “trip” problems – overloading Cause of overloading = too much motor current • Verify motor load is not causing the problem • Check for excessive current unbalance (possible shorted phase windings) • Verify ASD trip points are set correctly • Is dc bus voltage being regulated properly? • Leaky capacitors (too much ripple, too little inrush current) • Link inductor OK? (waveform different or same on both sides?)

35. PWM drives – measurement 9:ASD “trip” problems – overvoltage • Check for high line voltage and/or long term variations • Check for line transients • Lightening protection in place? • Proper wiring and grounding? • Isolation from transient producing loads? • Verify ASD trip points are set correctly • Is load regenerative (cranes, elevators)? If so, is dynamic braking installed and working properly? • Is dc bus voltage being regulated properly? • Leaky capacitors (too much ripple, too little inrush current) • Link inductor OK? (waveform different or same on both sides?)

36. PWM drives – measurement 9:ASD “trip” problems – overvoltage Overvoltage transient capture with a power quality analyzer.

37. PWM drives – measurement 10:ASD “trip” problems – undervoltage • Check for low line voltage and/or long term variations • Verify ASD trip points are set correctly • Is dc bus voltage being regulated properly (dc link capacitors and/or reactor) • Check for voltage sags

38. PWM drives – measurement 10:ASD “trip” problems – undervoltage Check for flat topping of the input voltage

39. PWM drives – bonus measurement:ASDs and IEEE-519 measurements Check for VTHD < 5 % at the point of common coupling (PCC), not the ASD Check power factor. Utilities may start charging for distortion power factor. Check for ITHD < ? % at PCC, not the ASD (% depends on short circuit current ratio of PCC)

40. PQ troubleshooting: Transformer solutions – Three phase filter traps Series resonant tuned LC shunt filter with series line reactor. Series resonant tuned LC shunt filter.

41. PQ troubleshooting solutions: Active harmonic compensation devices • Samples load current for harmonic content • Generates harmonic frequenciesdemanded by load • Source supplies 60 Hz only • Current distortions typically less than 5 %