Compressor Electrical Diagnostics - Manitowoc STH042 7/15 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 ensure 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 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.
Check Motor Windings to Ground
Check continuity between all three terminals and the
compressor shell or copper refrigeration line. Scrape
metal surface to get good contact. If continuity is
present, the compressor windings are grounded and the
compressor should be replaced.
To determine if the compressor is seized check the amp
draw while the compressor is trying to start.
104 Part Number STH042 7/15