Service manual

control wiring and repeat the test. If this test is successful,
then continue. Remember to check the customer
connections. If still unsuccessful, change the control card.
2. Measure the 10 V DC control voltage at terminal
50 with respect to terminal 55. The meter must
read between 9.2 and 11.2 V DC.
An incorrect reading here could indicate that a fault in the
customer connections loads down the supply. Disconnect
control wiring and repeat the test. If this test is successful,
then continue. Remember to check the customer
connections. If still unsuccessful, change the control card.
Replace the LCP with a known good one. If the problem
persists, replace the control card in accordance with the
disassembly procedures.
6.4.4
Input Imbalance of Supply Voltage
Test
Theoretically, the current drawn on all three input phases
must be equal. Some imbalance may be seen, however,
due to variations in the phase to phase input voltage, and
single phase loads within the frequency converter.
A current measurement of each phase reveals the
balanced condition of the line. To obtain an accurate
reading, the frequency converter must run at its rated
load, or at a load of not less than 40%.
1. Perform the input voltage test before checking
the current, in accordance with procedure.
Voltage imbalances automatically result in a
corresponding current imbalance.
2. Apply power to the frequency converter and
place it in run.
3. Using a clamp-on ammeter (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 indicates a possible
problem with the mains supply to the frequency
converter, or a problem within the frequency
converter. One way to determine if the mains
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 frequency converter.
5. Swap the phase that appears to be incorrect with
one of other two phases.
6. Reapply power to the frequency converter and
place it in run.
7. Repeat the current measurements.
If the imbalance of supply voltage moves with swapping
the leads, the mains supply is suspect. Otherwise, it may
indicate a problem with the gating of the rectifiers.
6.4.5 Input Waveform Test
Testing the current waveform on the input of the
frequency converter can assist in troubleshooting mains
phase loss conditions or suspected problems with the SCR/
diode modules. Phase loss caused by the mains supply can
be easily detected. In addition, the SCR/diode modules
control the rectifier section. If one of the SCR/diode
modules become defective or the gate signal to the SCR
lost, the frequency converter responds the same as loss of
one of the phases.
The following measurements require an oscilloscope with
voltage and current probes.
Under normal operating conditions, the waveform of a
single phase of input AC voltage to the frequency
converter appears as in Illustration 6.15.
0.0 ms
5 ms/Div.
130BX142.10
Input A
Illustration 6.15 Normal AC Input Voltage Waveform
The waveform shown in Illustration 6.16 represents the
input current waveform for the same phase as shown in
Illustration 6.15 while the frequency converter is running at
40% load. The two positive and two negative jumps are
typical of any 6-diode bridge. It is the same for frequency
converters with SCR/diode modules.
0.0 ms 5 ms/Div.
130BX143.10
Input B
Illustration 6.16 AC Input Current Waveform with Diode Bridge
Test Procedures
VLT
®
HVAC Basic Drive Service Manual
54 MG90X202 - VLT
®
is a registered Danfoss trademark
6
6