MCD 200 Series
18
MG.17.D3.02 – VLT is a registered Danfoss trademark
T
e
sts
Power Circuit Test
This procedure tests the soft starter power circuit
including the SCR, Interface PCB and Main Control
PCB. The soft starter must be disconnected from all
power sources. This test is performed using a 500 V
DC insulation tester. Low voltage ohm meters or
multimeters are not adequate.
Test:
•
Disconnect the incoming supply (L1, L2, L3) and
control voltage (A1, A2, A3) from the starter.
•
Disconnect the motor cables (T1, T2, T3) from the
starter.
•
Measure the resistance across each controlled
phase in both directions (L1-T1, L3-T3, T1-L1,
T3-L3). The resistance should be between 30 k
Ω
and 500
k
Ω
and equal across both phases
.
Action
1: If the resistance measured across any of the
two controlled phases is less than 30 k
Ω:
T
he SCR should be replaced.
ATTENTION
There is no need to replace the MCD 200
Interface or Main Control PCBs just because
an SCR has been damaged. Consider replacement of
these parts only after having first replaced the
damaged SCR(s) and checked for correct operation.
The likely cause of SCR damage should also be
determined to prevent a repeat SCR failure. Analysis of
the
MOVs situated on the MCD 200 Interface PCB
and connected across each controlled phase (L1-
T1 and L3-T3) can provide a good indication of the
mode of SCR failure.
MOV Location : MCD 200 007, 015, 018, 022, 030
MOV Location : MCD 200 047, 045, 055
MOV Location : MCD 200 075, 090, 110
Overvoltage is the cause of SCR failure if the MOVs
and/or surrounding circuitry on the Interface PCB
show signs of physical damage.
Overcurrent is the likely cause of SCR failure if the
MOVs and/or surrounding circuitry on the Interface
PCB show no signs of physical damage.
Having determined the mode of the SCR failure the
cause of the damage should be considered to ensure
the damage does not reoccur.
Reliability of the modern SCRs used in the MCD 200
soft starters is such that failures attributable to faulty
manufacture are almost nil. 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. Refer to
4.1 of this manual for information of Typical Causes Of
SCR Damage.
2: If the resistance measured across any of the
two controlled phases is greater than 500 k
Ω
:
The Main Control PCB, Interface PCB or connection
between them items may be faulty.