MAINTENANCE
110
Repair of 300Amp Assembly
Magnetek recommends having an entire 300
amp drive chassis available as a spare part
in order to get a faulty unit back in operation
as quickly as possible. It is also possible to
have the internal SCR Power Bridge
assembly as a spare part for replacement in
the field. Field repair of individual SCR parts
is possible but requires the aid of alignment
jigs, proper tools and considerable patience
for proper success. Whole drives or SCR
Power Bridge assemblies may be returned
to the factory for repair.
Snubber Circuit
Snubber capacitors are mounted on a PCB
attached to the underside of SCR heat
sinks. To service these components, the
entire SCR heat sink power bridge assembly
must be removed from the chassis for
access. Follow the steps below for power
bridge removal. The snubber resistors are
the large tubular devices located behind the
AC line fuses, F1-F3. These can be
replaced without removal of the power
bridge assembly.
SCR Testing
Running PCU Diagnostics #998 should be
the first test to determine if an SCR is faulty.
If this test indicates a failure or if the drive
has blown fuses, continue the procedure
below to verify the need for repairs. The
Heat Sink Assembly is composed of three
Phase Assemblies, each containing four
“hockey puck” type SCRs with heat sinks
and clamps. Three phase assemblies are
combined with upper and lower DC buss
bars and front and back PCBs into a unified
power bridge assembly, then placed into the
drive chassis. A hockey puck packaged
SCR must be clamped at high pressure in
order to operate or to be properly tested with
an ohmmeter. Ohmmeter testing of the
SCRs can and should be done before any
disassembly or removal of the SCR Power
Bridge
Ohmmeter SCR Testing
1. Using a multi-meter on the X100 range,
press the positive probe to the positive
(+, right side) DC bus bar and the
negative probe to each of the three AC
bus bars (load side of AC line fuses) to
measure the resistance between them.
The reading in each case should be
greater than 300 K-ohms. Since most
SCRs fail by shorting, a low resistance
could indicate a defective SCR. This
procedure, continued in steps 2) & 3),
checks a pair of SCRs mounted within
the same heat sink half-phase
assembly. If found defective, that half-
phase assembly must be removed for
further repair.
2. Press the positive multi-meter probe to
the negative (–) DC bus bar and the
negative probe to each of the three AC
bus bars. Again, all readings should be
greater than 300 K-ohms.
3. Reverse the multi-meter probes and
repeat the above two steps. Again, all
readings should be greater than 300 K-
ohms.
4. Check SCR gate resistance for each
device.
a. Pull off the WHITE gate lead faston
from the connection to the Armature
Interface PCB. Use a multi-meter to
measure the resistance from the
white SCR gate wire, using the (+)
ohmmeter lead, and the (-)
ohmmeter lead to the red SCR
cathode wire. Compare reading
results per Table 16: SCR Gate
Resistance Testing.
b. Repeat the above for all 12 SCR
devices.
5. If any of the above ohmmeter readings
indicate a damaged SCR device, the
entire Power Bridge Assembly must be
removed for repair access of the
individual SCRs.
Removal and Re-installation of the 300
Amp SCR Power Bridge
IMPORTANT
When removing wires and connectors during
dis-assembly, make sure that they will stay
in position or are properly labeled for
reconnection.
1. Remove the entire Power Bridge in the
following sequence:
a. Remove the ribbon cable connector
J24 and Current Transducer cable
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