Download Electrical Measurement Safety

Introduction to Motor
Troubleshooting
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
1
Motors are the number one
consumer of electrical power
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
2
Power consumption based on size
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
3
Standard motor operating conditions
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
4
Special purpose motor operating conditions
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
5
Motor failure
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
6
Troubleshooting fuses
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
7
Taking motor voltage measurements
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
8
Measuring motor voltage unbalance
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
9
Measuring motor control circuit transformer
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
10
Measuring motor current
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
11
Measuring temperature
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
12
Measuring motor insulation
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
13
Megohmmeter reading interpretation
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
14
Motor power measurements
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
15
Testing motor capacitors
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
16
Electrical measurements on
adjustable speed drives
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
17
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
©2004 Fluke Corporation
Measurement
category
Working voltage
(dc or ac-rms to gnd)
Peak impulse
transient
(20 repetitions)
Test source
(Ohm = V/A)
CAT I
600V
2500V
30 ohm source
CAT I
1000V
4000V
30 ohm source
CAT II
600V
4000V
12 ohm source
CAT II
1000V
6000V
12 ohm source
CAT III
600V
6000V
2 ohm source
CAT III
1000V
8000V
2 ohm source
CAT IV
600V
8000V
2 ohm source
Introduction to Motor Troubleshooting
18
Adjustable speed drive – theory
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
19
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
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
20
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
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
21
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
quality analyzer
253 (avg) volts @ 20MHz B/W
243 volts @ 3KHz B/W
226 volts @ 60 Hz
Using an analog meter
223 volts @ 100 Hz
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
22
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
1 470
+462
1381
©2004 Fluke Corporation
2
1381
3
= 460
3
11
460
Introduction to Motor Troubleshooting
X 100 = 2.39 %
23
PWM drives – measurement 3:
Overvoltage reflections at the motor terminals
Normal PWM
waveform
Leading edge of
normal PWM
pulse
PWM waveform
with reflected
voltages
Leading edge of
PWM pulse with
reflected voltage
(ringing)
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
24
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 MG11993 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
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
25
PWM drives – measurement 3:
Overvoltage reflections at the motor terminals
Possible
remedies for
overvoltage
reflections
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
26
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 to
the 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
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
27
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
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
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
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
29
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
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
30
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
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
31
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.
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
32
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.
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
33
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?)
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
34
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?)
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
35
PWM drives – measurement 9:
ASD “trip” problems – overvoltage
Overvoltage transient capture with a power quality analyzer.
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
36
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
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
37
PWM drives – measurement 10:
ASD “trip” problems – undervoltage
Check for flat topping of the input voltage
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
38
PWM drives – bonus measurement:
ASDs and IEEE-519 measurements
Check for VTHD < 5 %
at the point of
common coupling
(PCC), not the ASD
©2004 Fluke Corporation
Check power factor.
Utilities may start
charging for distortion
power factor.
Introduction to Motor Troubleshooting
Check for ITHD < ? % at
PCC, not the ASD
(% depends on short circuit
current ratio of PCC)
39
PQ troubleshooting:
Transformer solutions – Three phase filter traps
Series resonant tuned
LC shunt filter with series
line reactor.
©2004 Fluke Corporation
Series resonant tuned
LC shunt filter.
Introduction to Motor Troubleshooting
40
PQ troubleshooting solutions:
Active harmonic compensation devices
• Samples load current for harmonic content
• Generates harmonic frequencies
demanded by load
• Source supplies 60 Hz only
• Current distortions typically less than 5 %
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
41
©2004 Fluke Corporation
Introduction to Motor Troubleshooting
42