Troubleshooting PSC Fan Motors

Common problems are usually electrical or mechanical failures

A popular style of fan motor used on older refrigeration equipment is the permanent split capacitor (PSC) motor. It is an electric motor made up of essentially two magnets: one is stationary (the stator), and the other rotates (the rotor).

The stator is made of two insulated copper conductors (windings), which are wrapped around an iron core. One winding is referred to as the main (run) winding, and the other is the secondary (start) winding. A capacitor (run) is also placed in series with the secondary (start) winding to help align the phase angle created by the voltage and current flowing through these two windings. As AC current flows through the windings of the stator, it becomes magnetized. An induced magnetic field is then created within the rotor, causing it to also become magnetized. Since AC current is flowing through the stator, its poles are continuously changing from north to south, usually 120 times per second. The poles of the rotor will remain the same and will constantly be repelled and attracted by the changing poles of the stator, causing the rotor to rotate freely within the stator.

Troubleshooting and replacing these motors is a common task for refrigeration technicians, and problems generally fall into one of two categories: electrical failure or mechanical failure. A common electrical failure is either an open winding or an open run capacitor. A common mechanical problem is tight or worn bearing(s). Occasionally, problems with these motors may not be easily recognized; for example, the motor may seem to operate normally for a while and then shut down. It is easy for a technician to overlook this problem, since the motors seem to be operating normally at first.

Generally, troubleshooting these motors is a simple task. If the motor is not operational when it should be, first verify the proper voltage is applied to the motor. If the proper voltage is applied and the motor is still not operational, then remove the applied voltage. Be sure to verify the voltage is no longer applied to the motor before proceeding. Then spin the fan blade attached to the motor's shaft. The fan blade should spin freely; if not, the bearing within the motor may be tight and the motor needs to be replaced. Also, check if the motor’s bearings are too loose. This is done by holding the shaft and observing the movement between the shaft and the bearing. A horizontal movement of approximately 1/8-inch is normally OK; however, there should be no vertical movement. If any vertical movement is noticed, then the motor needs to be replaced.

If the bearings of the motor seem OK, then disconnect all electrical wiring leading to the motor remove the run capacitor from the circuit, and check its capacitance. It should be within ±10% of the rating stated on the body of the capacitor. If the capacitor is open or shorted, that is the likely cause. Replace the capacitor and test the operation of the motor again. If the capacitor test is good, then with a standard ohmmeter, measure the resistance of the motor's windings. There should be a measurable resistance across both the run and start windings, with the start winding having a slightly higher resistance. However, if an infinite or zero resistance is measured on any of these measurements, then the motor is electrically defective and needs to be replaced.

Finding the cause of the motor’s failure is generally not difficult; however, replacing them can occasionally be quite frustrating. Depending on the age of the motor and its location, removing and replacing the motor can be quite problematic.

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