Input Waveform Test; Input Scr Test - Danfoss VLT FC Series Service Manual

Test Procedures
If the imbalance of supply current moves when swapping
the leads, then the mains supply is suspect. Otherwise,
there could be a problem with the gating of the SCR,
possibly due to a defective SCR/diode module. This result
can also indicate a problem in the gate signals from the
inrush card to the module. Check the wire harness from
the inrush card to the SCR gates. Proceed to testing the
input waveform and input SCR in accordance with
chapter 8.5.7 Input Waveform Test and chapter 8.5.8 Input
SCR Test.

8.5.7 Input Waveform Test

Testing the current waveform on the input of the drive
helps in troubleshooting mains phase loss conditions or
suspected problems with the SCR/diode modules. This test
easily detects phase loss caused by the mains supply.
The SCR/diode modules control the rectifier section. If an
SCR/diode module is defective or the gate signal to the
SCR is lost, the drive responds as if losing a phase.
The following measurements require an oscilloscope with
voltage and current probes.
Under normal operating conditions, the waveform of a
phase of input AC voltage to the drive appears as in
Illustration 8.12.
Input A
0.0 ms 5 ms/Div.
Illustration 8.12 Normal AC Input Voltage Waveform
The waveform shown in Illustration 8.13 shows the input
current waveform for the same phase as in Illustration 8.12
while the drive is running at 40% load. The 2 positive and
2 negative humps are typical of any 6 diode bridge. It is
the same for drives with SCR/diode modules.
The same waveform is seen when measuring a 12-pulse
drive between the rectifier and the transformer.
MG94A502
Service Guide
0.0 ms
Illustration 8.13 AC Input Current Waveform with Diode Bridge
With a phase loss, the current waveform of the remaining
phases resembles Illustration 8.14.
0.0 ms
Illustration 8.14 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,
investigate the reason for the AC supply problem. If the
voltage waveform on all 3 phases is correct, but the
current waveform is not, then check the input rectifier
circuit. Perform the rectifier tests. Refer to
chapter 8.4.1 Rectifier Circuits Test.

8.5.8 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.
SCR test part I
1.
2.
3.
A proper reading is between 5–50 Ω.
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Input B
5 ms/Div.
Input B
5 ms/Div.
Remove the power card mounting plate.
In D-sized units, unplug inrush board connector
MK1802. In E-sized units, unplug inrush board
connector MK109.
The plug has 3 pairs of wires, 1 for each SCR
module. Measure the resistance of each pair. Red
is the SCR gate and black is the SCR cathode.
8 8
109