Voltage Unbalance; Current Unbalance; Motor Input - Fluke ScopeMeter 190 Series III User Manual

190 Series III
Users Manual

Voltage Unbalance

The Voltage Unbalance checks for a difference between the Phase-Phase voltages for 3-phase
systems. The unbalance value is calculated by dividing the maximum rms voltage deviation of
one of the phases and the average rms voltage of all phases.
Voltage unbalance at motor terminals can adversely affect motor operation and can also cause
tripping of the current overload fault protection on the motor drive.
2 changes the readings that show at 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.

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.
2 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.
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