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Current Unbalance
Current Unbalance checks for a difference between current
levels of the phases in 3-phase systems.
The value is calculated by dividing the maximum rms
current deviation of one of the phases and the average rms
current of all phases. Current unbalance should be <6 %
and depends on load current and circuit capacity.
Make sure the phase currents are equal. If one of the
phases shows a failure, it can cause the motor to run hot,
not start after stopping, and loose efficiency. A phase failure
can be due to motor drive output malfunction or bad
connection between motor drive and motor and can cause
motor overheating.
changes the readings that show in the top of the
screen to the peak-peak values of each phase as well as
the highest Crest Factor (ratio between peak and rms value)
of one of the phases.
Motor Input
The functions for motor input are identical to motor drive
output, except that the phase-dc bus measurements are left
out for voltage modulation, as it is not practical to use the dc
bus as reference on the motor input.
Use the motor input functions to do the same
measurements and check the influence of the cable
between the motor drive and the motor and then document
the measurements separately in a report. The voltage
modulation measurements are useful to show voltage
peaks that are too high when the cabling is not properly
matched.
Motor Shaft (MDA-550 only)
The Motor Shaft function detects bearing flash-over that can
damage motor bearings. The measurement requires a
connection to the rotating shaft of the motor. For this
measurement, brushes are included as accessories.
Alternatively, you can use a stranded wire probe. Fluke
recommends the VP410 10:1 Voltage Probe. See Figure 5.
Figure 5. Motor Shaft Test Setup
W Caution
For safety, stop the motor.
To set up:
1. Remove the black protective cap and black insulation
sleeve from the probe tip.
2. Place the brush on top of the voltage probe.
3. Turn the screw to tighten the brush on the probe.
Motor Drive Analyzer
Motor Input
Voltage probe
Magnetic
probe holder
21
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