Voltage Unbalance - Fluke ScopeMeter 190 Series III User Manual

• When the Motor Drive is on, the waveform does not have a typical sinewave, for example, it
can look more like a camel hump shape. The current readings and waveform shape can vary
as the load changes.
• Compare the measured frequency against the specified frequency intended for that circuit.
Nominal frequency (typical 50 Hz or 60 Hz) should be within 0.5 Hz of specification.
• Select Harmonics to determine the harmonics related to the waveform shape for both voltage
and current (see Harmonics section).

Voltage Unbalance

Voltage Unbalance checks for the difference between the Phase-Phase voltages for 3-phase
systems.
At the simplest level, all three phases of voltage should always have the same magnitude.
Expressing unbalance as a percentage provides one number to describe the situation. To
calculate the unbalance value:
% unbalance = (maximum deviation from average / average of three phases) x 100 %
While voltage unbalance at motor terminals can adversely affect motor operation, it can also
cause problems at the input side of the drive. As little as 2 % to 3 % voltage unbalance on the
input to a motor drive can cause voltage notching and excessive current to flow in one or more
phases. Voltage unbalance can also cause tripping of the current overload fault protection on the
motor drive.
Tips:
• The source of unbalance voltage could be poor installation practices or loads that need to be
correctly optimized. Another common cause of voltage unbalance is single-phase loads
dropping in or out on the same feed as the 3-phase motor drive. To minimize or eliminate this
problem, increase the kVA rating of the transformer or provide a separate feed for the motor
drive.
• 2 changes the readings that are displayed 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.
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