Output Phase Imbalance Test - Danfoss VLT Series Service Manual

5.2.8 Output Phase Imbalance Test

Checking the balance of the drive output voltage and current
measures the electrical functioning between the drive and the
motor. In testing the phase-to-phase output, both voltage and
current are monitored. It is recommended that static tests on
the inverter section of the drive be conducted prior to this
procedure.
If the voltage is balanced but the current is not, this indicates
the motor is drawing an uneven load. This could be the result
of a defective motor, a poor connection in the wiring between
the drive and the motor, or, if applicable, a defective motor
overload.
If the output current is unbalanced as well as the voltage, the
drive is not gating the output properly. This could be the result
of a defective power card, gate drive, connections between
the gate drive card and IGBTs, or the output circuitry of the
drive improperly connected.
Use an analog volt meter for monitoring output
voltage. Digital volt meters are sensitive to
waveform and switching frequencies and
commonly return erroneous readings.
The initial test can be made with the motor connected and
running its load. If suspect readings are recorded then the
motor leads may have to be disconnected to further isolate
the problem.
1. Using a volt meter, measure AC output voltage
at drive motor terminals 96 (U), 97 (V), and 98
(W). Measure phase to phase checking U to V,
then U to W, and then V to W.
All three readings should be within 8 VAC of each other. The
actual value of the voltage depends on the speed the drive is
running at. The volts/hertz ratio is relatively linear (except in VT
mode) so at 60Hz the voltage should be approximately equal
to the line voltage applied. At 30 Hz it is about half of that and
so on for any other speed selected. The exact voltage reading
is less important than balance between phases.
5-20
VLT is a registered Danfoss trademark
NOTE
2. Next monitor three output phases at drive motor
terminals 96 (U), 97 (V), and 98 (W) with clamp
on ammeter. Analog device is preferred. To
achieve accurate reading, run drive above 40Hz
(as this is normally the frequency limitation of
such meters.)
The output current should be balanced from phase to phase
and no phase should be more than 2 to 3% different from
another. If the above tests are successful, the drive is operating
normally.
3. If a greater imbalance exists than described
above, disconnect motor leads and repeat
voltage balance test.
Since the current will follow the voltage, it is necessary to
isolate between a load problem and a drive problem. Should a
voltage imbalance in the output be detected with the motor
disconnected, it is necessary to test the gate drive circuits for
proper firing. Proceed to the gate drive signals test (5.2.9).
If the voltage was balanced but the current imbalanced when
the motor was connected, then the load is suspect. There
could be a faulty connection between the drive and motor or
a defect in the motor itself. Look for bad connections at any
junctions of the output wires including connections made to
contactors and over loads. Also, check for burned or open
contacts in such devices.