30
Theoretically, the input current drawn on each of the three input phases
should be equal. These currents will vary, however, due to variations
in phase-to-phase input voltage and due to some single phase loads
within the drive.
Given that the input phase voltages are equal, the input currents phase-
to-phase should not vary more than 5%. Current imbalances in excess
of 5% may indicate one of the SCR/diode modules is not conducting
properly. When the VLT is lightly loaded, it may not be possible to
detect a current imbalance. If suspect, the modules should be statically
tested. Refer to the Static Test procedures beginning on page 23.
A current sensor is in line with each phase of the output. These hall
effect devices generate a current that is proportional to the current
being drawn in each respective phase. The VLT relies on this feedback
for proper waveform control and for providing fault protection.
A simple test of the signals can be made with an AC voltmeter. The
voltage level present is relative to the current signal generated. At very
light loads the voltage may be no more than 100 to 300 millivolts. The
importance of this test is to verify that all three sensors are functioning
and the signals are approximately equal when compared to each other.
With an AC voltmeter, connect the negative (-) meter lead to pin 3 of
connector MK15 on the interface board. With the positive (+) meter
lead compare the voltage readings at the right side of resistors R28,
R29, R30.
Severe imbalances in the readings indicates an uneven current draw
by the motor. See "Testing for Output Phase Imbalance" on page 27 if
a large imbalance exists.
TESTING INPUT RECTIFIERS
TESTING FOR CURRENT
FEEDBACK