Compressor Electrical Diagnostics - Manitowoc Q450 Service Technician's Handbook

Compressor Electrical Diagnostics

The compressor does not start or will trip repeatedly
on overload.
Check Resistance (Ohm) Values
NOTE: Compressor windings can have very low ohm
values. Use a properly calibrated meter.
Perform the resistance test after the compressor
cools. The compressor dome should be cool enough
to touch (below 120°F/49°C) to assure that the
overload is closed and the resistance readings will be
accurate.
SINGLE PHASE COMPRESSORS
1. Disconnect power from the condensing unit and
remove the wires from the compressor terminals.
2. The resistance values must be within published
guidelines for the compressor. The resistance
values between C and S and between C and R,
when added together, should equal the resistance
value between S and R.
3. If the overload is open, there will be a resistance
reading between S and R, and open readings
between C and S and between C and R. Allow the
compressor to cool, then check the readings again.
THREE PHASE COMPRESSORS
1. Disconnect power from the condensing unit and
remove the wires from the compressor terminals.
2. The resistance values must be within published
guidelines for the compressor. The resistance
values between L1 and L2, between L2 and L3,
and between L3 and L1 should all be equal.
3. If the overload is open, there will be open readings
between L1 and L2, between L2 and L3, and
between L3 and L1. Allow the compressor to cool,
then check the readings again.
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