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PSC Motor Question- Run Cap Related. #75902 03/16/07 03:19 PM
Joined: Aug 2004
Posts: 55
W
wendel Offline OP
Member
Here's the excellent ECN schematic of a PSC motor: https://www.electrical-contractor.net/ubb/Forum15/HTML/000084.html

My question regards the failure of the run capacitor.

What are the usual failure modes for a PSC run capacitor... short to ground, loss of capacitance due to dielectric failure or...?

Would any capacitor failure mode cause the aux/run windings of the motor to fail due to excessive current or heat?

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Re: PSC Motor Question- Run Cap Related. #75903 03/16/07 09:30 PM
Joined: Jul 2004
Posts: 9,575
G
gfretwell Offline
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If C1 is open the run winding will burn out because the motor will not try to start. It just sits there burning (no rotating field)
If the capacitor shorts both windings are in a race to burn up because they are creating a field the armature can never null. The motor will lump over slowly but never be able to find that sweet spot in the field where it can survive.


Greg Fretwell
Re: PSC Motor Question- Run Cap Related. #75904 03/20/07 01:25 AM
Joined: Oct 2000
Posts: 2,721
Scott35 Offline
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Adding to "gfretwell"'s reply:

If the Capacitor fails in an "Open Circuit" state (like physically removing the component), the following should occur:

  1. If the Motor is Running at the time of failure, the Rotor will continue to spin at close to the set speed - governed by the selected speed tap connected at the "Input Autotransformer", then speed should slowly taper down, input Current should increase, and overall winding heat will increase,
  2. If the Motor is _NOT_ Running - and an attempt to start the Motor is made with the Capacitor failed open, the Rotor will be subjected to the common "Single Phase Problem" of a "Magnetic Field Which Varies In Intensity, But Is Stationary In Time" (Non-Rotational) - so the results will be a Non-Spinning Rotor, and there _SHOULD_ be a minimal input Current draw on the Main Winding, which corresponds to the total Impedance of the Main / Run Winding (Winding Z + whatever is steady from not inducing VA to the Rotor).


If the Capacitor fails in a "Closed Circuit" state (like physically shunting around the component), the following should occur:

  1. Motor should remain running, but Rotor may spin erratically, as the Auxiliary Winding will draw Current solely according to the Resistance of the Auxiliary Winding, and not as the value of both the Capacitive Reactance of the Capacitor (XC) and the Aux. Winding's Resistance (R).
    In this case, the Current drawn will be higher than under normal circumstances,
  2. Motor will start and achieve rotation, but will experience odd behavior, as listed above,
  3. Speed reduction may be achieved also, with similar reactions as described in list #1 above.


Nevertheless, with a Shorted Capacitor, the Motor could easily lose smoke in the Auxiliary Winding after some period of time, and possibly barbecue the Main Winding from eccentric behavior.

Open Capacitor would effect the Main (Run) Winding, by requiring it to Induce the Rotor with Volt-Amps solely (not in conjunction with the Aux. Winding), which could also result in loss of smoke after a period of time.

Scott35


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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