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Chapter 6: Troubleshooting
Electrical Theory & Applications for HVACR
If the motor’s
start capacitor
capacitance is within plus or minus 20 percent of the low and high raƟng
on the label, it is safe to use. If capacitance is below the label raƟng, the capacitor is breaking down
and must be replaced.
Run capacitors are tested using the same procedures described for start capacitors, with a few
excepƟons. A run capacitor is housed in a metal case, therefore, when ohmmeter tesƟng, check that
there is no conƟnuity from either terminal to the case. Because a run capacitor is designed for
conƟnuous duty, there is no Ɵme limit for energizing the capacitor. A
run capacitor
must be within
±10 percent of the raƟng.
TIPS AND SUGGESTIONS
SomeƟmes a PSC fan motor will have a “dead spot” condiƟon. When the motor stops and the rotor
aligns in just the right place, the motor will not start or run. If the rotor is turned leŌ or right from its
posiƟon, it will start and run, but will stop again and cause the same problem someƟme in the future.
Replace the motor with a new one.
A three‐phase motor with a blown fuse in one of the three legs will try to run on the remaining two
legs. This is referred to as “single‐phasing.”
Always check airflow and the rotaƟon of the fan motor to insure that the blade is turning in the right
direcƟon. SomeƟmes a motor has been installed incorrectly.
If a fan motor shaŌ is hard to turn by hand, usually the problem is dry or seized bearings. On
residenƟal units, replace the fan motor. Larger, more expensive motors can someƟmes be taken to a
motor shop for repairs.