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MCD 500 Service Manual
2 Troubleshooting
When a protection condition is detected, the MCD 500 will write this to the event log and may also trip or issue a warning. The soft starter's response to
some protections may depend on the Protection Action settings (parameter group 16).
If the MCD 500 trips you will need to reset the soft starter before restarting. If the MCD 500 has issued a warning, the soft starter will reset itself once
the cause of the warning has been resolved.
Some protections cause a fatal trip. This response is pre-defined and cannot be overridden. These protection mechanisms are designed to protect the
soft starter, or can be caused by a fault within the soft starter.
2.2 Functional Tests
Use the tests in this section to identify the cause of problems with the soft starter.
2.2.1 Power Circuit Test
This procedure tests the soft starter's power circuit, including the SCR, Interface PCB and Main Control PCB.
1.
Disconnect the soft starter from mains voltage (L1, L2, L3), control voltage (A1, A2, A3) and from the motor (T1, T2, T3).
2.
Use a 500 VDC insulation tester to measure the resistance across each phase in both directions (L1-T1, L2-T2, L3-T3 and vice versa). Low voltage
ohm meters or multimeters are not adequate. The resistance should be between 250 kΩ and 500 kΩ and equal for all measurements.
•
If the resistance is below 250 kΩ for any measurement, the SCR on that phase may be faulty. For internally bypassed units (MCD5-0021B
- MCD5-0215B) the bypass contactor on that phase may be closed. Replace the faulty SCR or bypass contactor.
•
If the resistance is above 500 kΩ for any measurement, the Main Control PCB or Interface PCB may be faulty or there may be a faulty
connection between these two PCBs. To isolate the fault, perform the PCB integrity test.
3.
Investigate the likely cause of SCR damage to prevent a repeat SCR failure. Analysis of the MOVs on the Backplane PCB and connected across
each controlled phase can provide a good indication of the mode of SCR failure.
•
If the MOVs and/or surrounding circuitry on the Backplane PCB show signs of physical damage, the most likely cause is overvoltage.
•
If the MOVs and/or surrounding circuitry on the Backplane PCB do not show signs of physical damage, the most likely cause is over-
current.
The modern SCRs used in MCD 500 soft starters are extremely reliable and it is extremely unlikely that they will fail due to faulty manufacture. SCR
damage is almost always caused by external influences. Often these influences can be identified but in other cases the identification may prove difficult
or impossible because the damaging event was temporary in nature. See
2.2.2 Start Performance Test
This procedure tests that the MCD 500 soft starts correctly. This test is performed using an AC voltmeter. During Start mode, the Run LED (green) on
the LCP should flash.
1.
Connect the MCD 500 to mains voltage, control voltage and to a motor.
2.
Measure the voltage across each phase (L1-T1, L2-T2, L3-T3). This should be close to the nominal mains voltage (phase voltage for in-line
connection and line voltage for inside delta connection).
•
If the voltage is zero, the SCR on that phase may have failed.
•
If the voltage is not equivalent to the nominal mains voltage, the bypass contactor may be damaged and should be replaced (models MCD5-0021B
- MCD5-0215B only).
NB!
There is no need to replace the Main Control PCB or the Backplane PCB just because an SCR has been damaged.
Consider replacing these parts only after first replace the damaged SCR(s) and checking for correct operation.
MG.17.L1.02 - VLT
®
Avoiding Damage
for information on typical causes of SCR damage.
is a registered Danfoss trademark
2 Troubleshooting
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