Input Phase Imbalance Test - Danfoss VLT HVAC Drive FC 100 Service Manual

High Power Service Manual for VLT
Adjustable Frequency Drives

6.3.7 Input Phase Imbalance Test

Theoretically, the current drawn on all three input phases should be equal. Some imbalance may
be seen, however, due to variations in the phase to phase input voltage and, to some degree,
single phase loads within the adjustable frequency drive itself.
A current measurement of each phase will reveal the balanced condition of the line. To obtain an
accurate reading, it will be necessary for the adjustable frequency drive to run at its rated load or
at a load of not less than 40%.
1. Perform the input voltage test prior to checking the current, in accordance with proce-
dure. Voltage imbalances will automatically result in a corresponding current imbalance.
2. Apply power to the adjustable frequency drive and place it in run.
3. Using a clamp-on amp meter (analog preferred), read the current on each of three input
lines at L1(R), L2(S), and L3(T).
Typically, the current should not vary from phase to phase by more than 5%. Should a
greater current variation exist, it would indicate a possible problem with the line power
supply to the adjustable frequency drive or a problem within the adjustable frequency
drive itself.
One way to determine if the line power supply is at fault is to swap two of the incoming
phases. This assumes that two phases read one current while the third deviates by more
than 5%. If all three phases are different from one another, swap the phase with the
highest current with the phase with the lowest current.
4. Remove power to adjustable frequency drive.
5. Swap the phase that appears to be incorrect with one of other two phases.
6. Reapply power to the adjustable frequency drive and place it in run.
7. Repeat the current measurements.
If the phase imbalance moves with swapping the leads, then the line power supply is suspect.
Otherwise, it may indicate a problem with the gating of the SCR. This may be due to a defective
SCR or in the gate signals from the power card to the module, including the possibility of the wire
harness from the power card to the SCR gates. Further tests on the proper gating of the SCRs
require an oscilloscope equipped with current probes. Proceed to testing the input waveform and
input SCR in accordance with their procedures.
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FC Series
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6 Test Procedures
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