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

High Power Service Manual for VLT
Adjustable Frequency Drives
A1 > 1.1 A
A2 > 0.40 A
T1> 300 μs
Given all the other tests above were successful, a missing gate signal indicates the power card is
defective. Replace the power card in accordance with the disassembly procedures in Section 7 or
8.
A distorted signal may be due to a defective gate on that particular SCR that is loading down the
supply. Replace the SCR module which corresponds to the incorrect gate signal reading.

6.3.10 Output Phase Imbalance Test

Checking the balance of the adjustable frequency drive output voltage and current measures the
electrical functioning between the adjustable frequency 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 adjustable frequency 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
adjustable frequency drive and the motor, or, if applicable, a defective motor overload.
If the output current is unbalanced as well as the voltage, the adjustable frequency drive is not
gating the output properly. This could be the result of a defective power card, gate drive, con-
nections between the gate drive card and IGBTs, or the output circuitry of the drive improperly
connected.
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 voltmeter, measure AC output voltage at adjustable frequency drive motor ter-
minals 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 at which the adjustable frequency drive is running. The volts/hertz ratio is relatively
linear (except in VT mode) so at 60 Hz the voltage should be approximately equal to the AC 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.
2. Next, monitor three output phases at adjustable frequency drive motor terminals 96
(U), 97 (V), and 98 (W) with the clamp on the ammeter. An analog device is preferred.
To achieve an accurate reading, run the adjustable frequency drive above 40 Hz as this
is normally the frequency limitation of such meters.
®
FC Series
NOTE!
Use an analog voltmeter for monitoring output voltage. Digital voltmeters are sen-
sitive to waveform and switching frequencies and commonly return erroneous
readings.
MG.90.L1.22 - VLT ® is a registered Danfoss trademark
6 Test Procedures
6
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