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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.
•
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.
Current Unbalance
Current Unbalance checks for the difference between
current levels of the phases for 3-phase systems. To
calculate the unbalance value:
% unbalance = (maximum deviation from
average / average of three phases) x 100 %
Tips:
•
Current unbalance should be <6 % and depends on
load current and circuit capacity. Excessive current
unbalance can point to or cause drive rectifier problems
that result in the motor overheating. Current unbalance
can be caused by voltage unbalance. For example, 1 %
of voltage unbalance can result in 3 % to 4 % of current
unbalance.
•
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.
Harmonics (MDA-550 only)
The MDA-550 provides harmonics analysis. Harmonics are
periodic distortions of the voltage and current sine wave.
Harmonics occur when multiples of the fundamental
waveform are superimposed over the fundamental. You can
consider the signal as a combination of various sine waves
with different frequencies. The contribution of each of these
components to the full signal is shown as a bar. For
example, a 5th harmonic is 300 Hz (5 x 60) for 60 Hz
systems or 250 Hz (5 x 50) for 50 Hz systems. The effect of
these harmonics is distortion of the voltage or current. The
sum of all distortions from the 2
harmonic divided by the fundamental component is
expressed as the Total Harmonic Distortion (THD).
Motor Drive Analyzer
Motor Drive Input
nd
th
harmonic to the 50
15
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