PSC Motor
The permanent split capacitor motor used in many blowers and condenser fans, and how it behaves as duct resistance rises.
Safety
What it is
A permanent split capacitor motor is a single phase induction motor with a run capacitor permanently in series with the start winding. It is simple, inexpensive and common on condenser fans and older indoor blowers.
Speed is selected by tapping different windings, which is why a PSC blower has several coloured leads labelled high, medium and low.
Why it matters
A PSC motor does not hold airflow when the duct system fights it. As static pressure rises, CFM falls and the motor actually draws less current. That behaviour is the opposite of a constant airflow ECM and changes how you interpret readings.
| Static pressure | PSC airflow | PSC amp draw |
|---|---|---|
| Design, around 0.5 inches water column | At the table value | At nameplate |
| High, from a dirty filter or coil | Falls noticeably | Falls below nameplate |
| Very high | Falls sharply | Low |
How it works
The run capacitor shifts the phase of the current in the start winding, creating the rotating magnetic field the rotor follows. The capacitor stays in circuit at all times, which is where the name comes from. Starting torque is modest, so PSC motors are used where the load starts easily.
How to diagnose it
Test the run capacitor with a capacitance meter after discharging
A capacitor below its tolerance is the single most common cause of a PSC motor failing to start or running hot
Clamp motor amps and compare to the nameplate
Well below nameplate usually means high static, above nameplate suggests a bearing, winding or capacitor problem
Spin the shaft by hand with power off and locked out
Roughness or drag indicates worn bearings
Check winding resistance between the leads
Open or shorted windings confirm motor failure
Measure total external static pressure
Confirms whether the motor is being asked to work against a restricted duct system
Discharge the capacitor first
Tools involved
- Capacitance meter
- Clamp ammeter
- Manometer
- Multimeter
Symptoms
Practice & Mastery
- PSC Motor Troubleshooting Scenarios2
Practice is part of HVAC Pro — $5/month or $50/year.
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