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Chapter 6: Troubleshooting
Electrical Theory & Applications for HVACR
3.
Compressor motor’s overload trips soon aŌer drawing LRA
4. Resets aŌer two minutes
The service technician then turns off power to the refrigeraƟon system to let the motor cool down.
AŌer studying the electrical diagram again, the technician lists some of the possible causes that
correlate to every symptom listed. If a possible cause does not correlate to
every
symptom, it cannot
be a possible cause.
Possible Causes:
1. Open start winding
2. Open run winding
3. Open run capacitor
4. Open start capacitor
5. Compressor mechanically stuck
6. PotenƟal relay contacts between 1 and 2 stuck open
NoƟce that every possible cause listed correlated to all the symptoms. Now all the service technician
has to do is to check the six possible causes to find out which one is causing the symptoms, instead of
blindly checking the enƟre system.
With the power off, the technician takes a wire off of the start winding and measures the windings
resistance. The technician finds that it has 4 ohms, meaning that it is not open. The technician now
takes a wire off the run winding and measures the windings resistance. The technician finds that it is
an open winding because of the infinity reading on the ohmmeter. The compressor has to be
replaced. With either winding open, the compressor has no phase shiŌ for starƟng and will lock its
rotor drawing LRA unƟl the overload trips.
If either capacitor was bad, the motor may not have had enough phase shiŌ to start. In certain cases,
the motor may slowly turn. If the compressor was mechanically stuck, such as something wedged
between the piston and cylinder, the motor would lock its rotor and draw LRA. If the contacts
between terminals 1 and 2 of the potenƟal relay were stuck open for some reason, the start capacitor
would be out of the circuit. This again will probably not cause enough phase shiŌ to start the motor
turning. The motor would again draw LRA. NoƟce that in every case, all the symptoms were met.
What about an open overload or an open potenƟal relay coil between terminals 2 and 5 of the
potenƟal relay? If the overload were opened at the beginning, maybe from too high of a compressor
amp draw, the compressor would not hum or draw LRA. This would not correlate with all the
symptoms listed and could not be a possible cause. If the coil of the potenƟal relay were open, the
contacts between 1 and 2 of the potenƟal relay would stay in their normally closed posiƟon and not
open. This would cause the start capacitor to be in the circuit all the Ɵme, and the motor would turn,
draw higher than normal amperage, eventually open the overload and probably burn out the start
capacitor. These symptoms do not correlate with the original symptoms listed, thus cannot be a
possible cause.
Once the service technician has replaced the compressor and the system is up and running, it is
important to run a system check to see what caused the compressor overheaƟng that opened the