One way to determine if the line power supply is
at fault is to swap two of the incoming phases. 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.
6.
Reapply power to the adjustable frequency drive
and place it in run.
7.
Repeat the current measurements.
If the imbalance of supply current moves with swapping
the leads, then the line power supply is suspect.
Otherwise, there could be a problem with the gating of
the SCR, perhaps due to a defective SCR/diode module.
This result could also indicate a problem in the gate
signals from the inrush card to the module, including the
possibility of the wire harness from the inrush card to the
SCR gates. Proceed to testing the input waveform and
input SCR in accordance with
and
.
6.4.7 Input Waveform Test
Testing the current waveform on the input of the
adjustable frequency drive can help in troubleshooting line
phase loss conditions or suspected problems with the SCR/
diode modules. Phase loss caused by the line power
supply can be easily detected. In addition, the SCR/diode
modules control the rectifier section. If one of the SCR/
diode modules becomes defective or the gate signal to the
SCR is lost, the adjustable frequency drive responds the
same as if one of the phases were lost.
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 adjustable
frequency drive appears as in
.
Figure 6.5 Normal AC Input Voltage Waveform
The waveform shown in
represents the input
current waveform for the same phase as
while
the adjustable frequency drive is running at 40% load. The
two positive and two negative jumps are typical of any 6-
diode bridge. It is the same for adjustable frequency drives
with SCR/diode modules.
Figure 6.6 AC Input Current Waveform with Diode Bridge
With a phase loss, the current waveform of the remaining
phases would take on the appearance shown in
.
Figure 6.7 Input Current Waveform with Phase Loss
Always verify the condition of the input voltage waveform
before forming a conclusion. The current waveform follows
the voltage waveform. If the voltage waveform is incorrect,
proceed to investigate the reason for the AC supply
problem. If the voltage waveform on all three phases is
correct but the current waveform is not, the input rectifier
circuit in the adjustable frequency drive is suspect. Perform
the rectifier circuit test and input SCR test.
6.4.8 Input SCR Test
The SCRs can be disabled as a result of an input, or lack of
input, at power card connector MK106, the external brake
temperature switch. Unless used as an input, a jumper
must be placed between terminals 104 and 106 of MK106.
The following test is to measure the SCR gate resistance.
Test Procedures
Service Manual
MG94A222
Danfoss A/S © Rev. 2014-02-10 All rights reserved.
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