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PSC Motors and How to Properly Test a Capacitor


PSC (Permanent Split Capacitor) motors are nifty pieces of technology that you'll most often encounter in Whirlpool's VWM washers. They operate a bit differently from your typical split-phase motors. Rather than having a dedicated start winding and run winding, with each winding having different specifications, PSC motors have two windings that are physically identical. The only difference between them is in what direction the windings are wound.

Another big difference between PSC motors and typical split-phase motors is that PSC motors don't require any kind of start device. Both windings can be kept energized the entire time, with the capacitor phase-shifting the current going through one of the windings in order to create starting torque.

Clearly, the capacitor is the star of the show with these motors, which makes it crucial that you know how to correctly troubleshoot it. A bad capacitor in a PSC motor can cause issues such as agitation in only one direction.

A typical test is to put your multimeter on the ohms setting, then put your probes on either side of the capacitor and watch the resistance increase from very low to very high. Then you reverse the leads and do the same thing again. There's a problem with this test, though: it's not 100% reliable. A capacitor can pass this test and still be faulty. So basically that means it's not a useful test.

To test a capacitor with complete reliability, you have to measure the capacitance. That means setting your multimeter to its capacitance setting, putting your leads on either side of the capacitor, and letting your meter calculate the actual capacitance of the component. Then you compare that reading with your spec, which is almost always printed right on the capacitor itself.

Don't worry -- we're not talking about a specialty tool here. Any multimeter worth its salt should have a capacitance setting. If yours doesn't, perhaps that's a hint that you should upgrade...

Want to learn more about PSC motors, capacitors, motors in general, and much more? Watch this enlightening webinar recording, one of many available only to premium members of Appliantology.

 

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WOOKIE

Posted

I agree 100% the capacitance function on the meter is the only way to test the capacitor.  Years ago when I was at VO-Tech they taught us to use the ohms function and listen for the beep then switch leads to the other terminal and listen to the beep , telling us the cap was good.  WRONG!!!.  In the field I soon  learned that with a weak capacitor the motor would just hum drawing excessive current and,  often melting a solder joint  to the relay contact on the control board.  A weak capacitor can make the motor slow start  ( in the case of a blower motor) and run louder . It also help with the back EMF created by the motor but, that is still a little confusing on how it deals with it. 

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  • Team Samurai
Samurai Appliance Repair Man

Posted

Pop Quiz: In what appliance applications are PSC motors ideally suited? 

WOOKIE

Posted

washing machine because of their efficiency , quiet applications and it's ability to reverse.  Systems that need hi torque and hi current.  Dryers, condenser fan motors, HVAC system's and I believe some microwaves.   Shaded pole motors would be best used in evaporators for the low end and older refrigerator's.

  • Team Samurai
Samurai Appliance Repair Man

Posted

Close! Although PSC motors are less efficient than other type of AC asynchronous induction motors (ie., split phase AC motors), they are inexpensive, reliable, and-- the key thing-- they can handle mechanical loads with varying torque. You would find a varying torque load in washers and dishwashers as the water level increases or decreases. They're also commonly used in HVAC applications where the static pressure could have wide variations requiring a motor that can adjust its torque to the demand. In the single phase/split phase world, that would be a PSC motor; in the three phase world, that would be a BLDC motor, for example. 

More discussion of this here: 

 

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