AC Motor Testing by PX-5 with and without a VSD
Background
Electric motors are the most prolific load in the electrical system. The Department of Energy
recently estimated that 25 percent of the electricity sold is consumed by industrial electric
motors. These motors are used to power production processes and heating, cooling and
ventilation systems. Note: if consumer products like refrigerators are included more than 50%
of all electricity; it is consumed by motors.
Testing challenges for electric motors
Testing motors while they are operating has always been a technical challenge because the
amount of load on the motor affects the values for current and kwatts. This is due to the power
factor variations as the actual load on the motor varies. Figure 1 shows a graph of the
relationship to load, current, power and power factor. You will notice a motor at 50% load
consumes the same amount of current as one that is 10% loaded.
Figure 1
So, if the engineer wants to measure the amount of actual load on a motor or the amount of
power being consumed he must measure voltage, current, power and power factor.
The second challenge for testing electric motors is the checking of the leakage current to earth.
The earth leakage current is a good indicator of winding insulation integrity. Yet because the
motor is physically connected to the earth, it is a challenge to measure the current.
The third issue with motor testing is the application of VFD/VSDs to the run the motor. These
devices are installed to provide speed control, power control and energy savings. Because they
control the motor by converting the AC to DC and then DC to a controlled AC. Many different
control designs have been used but the most popular is PWM (Pulse Width Modulated). The
PWM output of the VFD/VSD makes it a challenge to measure the electrical parameters
powering the motor. The sketch in Figure 2 shows an illustration of the VSD/VFD.
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Figure 2
The voltage and current waveforms illustrate the PWM characteristics. The simplified
schematic is on the right.
The measurement issue is based on the chopped voltage waveform. Because all new technology
instruments use a phase lock loop to synchronize sampling to the voltage waveform and then
calculate true power, power factor etc from the samples. A phase lock loop cannot synchronize
to the chopped waveform.
The fourth challenge for testing motors is identifying the root cause of motor failure. A
common problem is the motor failure after the installation of a VSD/VFD. The typical
symptom is the winding insulation fails or the bearing fail with a few months after the
VSD/VFD is installed. The failure can be caused by overheating of the windings due to
harmonic currents, voltage transients that exceed the breakdown voltage of windings, bearing
being pitted by the leakage current due to the high frequencies of the VFD/VSD.
The Dranetz-BMI PX-5 is ideally suited to make these tests. The PQX-5 has a motor testing
mode to make the set up easy and the automatic calculation of the motor electrical metering
results. Figure 3 below shows the enunciator panel for the automatic motor test.
Figure 3
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This panel shows the real time readings of critical parameters for motor testing. Including
Negative sequence and derating factor. It also shows the actual horsepower being used real
time. This is very valuable when doing energy audits.
An advanced user might want to add a few more panels to the display. Shown in Figure 4
Calculated leakage
Actual leakage
Figure 4
The PX-5 has the capability to display any measured or calculated parameter real time while it
is monitoring. This gives the user to see where the various parameters are at without changing
the instrument settings or downloading data into a computer.
Measuring and monitoring VSD and VFD outputs.
Figure 5
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This requires the use of other unique features of the PX-5. Shown below in Figure 6 is the setup
screen for synchronizing the PX-5 to the measured signal.
Figure 6
Select the tracking option and then change the sync channel to Channel D. Connect Channel D
to the input voltage source. Another option is to use the internal selection.
A motor can be damaged by excessive voltage transients that are generated by a VFD or VSD
as shown in the following Figure 7.
Dran-View 6.0.21 HASP Hasp-ID: 87337032
Event Details/Waveforms
Volts
500
250
0
-250
-500
16:20:31.77
06/08/2005
Wednesday
16:20:31.78
16:20:31.79
AV
BV
16:20:31.79
CV
Event #1227 at 06/08/2005 16:20:31.773
AV Wave window
Figure 7
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These voltage transients are particularly damaging if they occur at the very peak of the sine
wave as captured in Figure 7.
The following graph in Figure 8 shows the sync channel and the VSD output voltage
waveform.
Dran-View 6.0.21 HASP Hasp-ID: 171071648
Event Details/Waveforms
Volts
750
500
250
0
-250
-500
-750
16:13:00.46
07/04/2005
Monday
16:13:00.47
CV
16:13:00.48
16:13:00.49
DV
Event #21 at 07/04/2005 16:13:00.377
CV Wave window
A
B
C
D
A-B B-C C-A
Vrms 373.7 364.3 369.4 225.8 530.5 551.3 565.3
VPeak 805.2 691.3 756.3 349.7
Figure 8
Electrical motors are designed to operate with high efficiency within some variation in the
electric power supplied.
 voltage variation should not exceed +/- 10%
 frequency variation should not exceed +/- 5%
 combined arithmetic sum of voltage and frequency variation should not exceed 10%
 voltage imbalance between phases should not exceed 1%
Voltage Imbalance % Derating factor
0
1.00
0.5
1.00
1.5
0.97
2.0
0.95
2.5
0.93
3.0
0.89
3.5
0.85
4.0
0.82
4.5
0.78
5.0
0.76
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Conclusions
The PX-5 has several unique features for testing motors with or without VSD/VFD. These
features include the motor testing mode with enunciator panel. The ability to calculate leakage
current, actual horsepower and negative sequences are important capabilities for testing motors.
The PQx-5 can also calculate the derating factor necessary for operating a motor in a high
harmonic environment. When testing the output of a VSD the PX-5 can synchronize to channel
D, so actual voltage and current measurements can be made on the output of the drive. This
allows the engineer to determine the cause of premature motor failures.
These motor testing features are unique to the PX-5 and some in the PG4400. These features
are unique to the Dranetz-BMI intelligent instrument family.
Authors:
Terry Chandler Dranetz-BMI Asia Manager, Tchandler@Dranetz-BMI.com
P.G. Phang Power Quality Thailand LTD PGPhang@powerquality.org
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