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Samurai Appliance Repair Man's Blog

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What does the run capacitor do in split-phase compressor circuits?

Samurai Appliance Repair Man


The run capacitor is wired in series with the start winding. When the compressor first starts, the run capacitor is shunted by the closed PTC (or whatever start relay or device is being used) so current just flows through the main and start windings which are in parallel with each other. The start winding is there to create that split-phase magnetic field that's out of phase with the main winding field that rotates around the stator. This rotating magnetic field is what starts the rotor turning from a dead stop.


After the compressor has started and is up and running, the PTC opens which removes the shunt from the run capacitor. Now the run capacitor and start winding are in series with each other and both are in parallel with the main winding.


At this point, the run capacitor does a couple things:

1) limits current flow through the start winding

2) smooths out the back EMF pulses from the main winding.

The net effect of the run capacitor is to reduce the current draw of the compressor while running (which also helps it run a bit cooler) and to help the compressor run smoother and more efficiently.

If the run capacitor were shorted (ie., replaced by a wire), the compressor would still start but the compressor would draw A LOT more current, run very rough, and quickly kickout on thermal overload.

I have never seen a run capacitor fail short.

Learn more about refrigerators and how to troubleshoot them in the Refrigerators training course at the Samurai Tech Academy.

Source: Sub Zero 632-S S/N prior to 1810000


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Well......I thought I was done with the Run capacitor questions, but the new compressor (Embraco FF10HBK) arrived and it makes no mention of one.  Further research on the various charts of Embraco specifically omit a Run cap for this unit.  I realize it's a RSIR motor, but after reading Samurai's analysis on the benefits of a Run cap, I'm kind of scratching my head as to whether this would be a good addition, or would it fly in the face of the mfgr's specs and cause more problems?

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Samurai Appliance Repair Man


Further research on the various charts of Embraco specifically omit a Run cap for this unit.  




Do you have a link to the spec sheet you're using for this compressor?


RSIR (resistive start inductive run) just means that the compressor is capable of running without a run cap, not that a run cap is prohibited. The run cap is probably optional for this compressor. 


Where did you get this compressor model as the replacement?


Also, remember: we are technicians, not engineers. We do not write design specifications; we determine whether or not components are operating within given design specifications. If the original design did not include a run cap, don't add one. We do not do field design modifications without specific instructions from the manufacturer. Although you may have a perfectly valid technical reason to do so, it also opens you up to liability if something goes horribly wrong because you altered a UL-approved design-- insurance will not cover you in this situation nor should it. 

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Well color me silly and call me a rabbit... I thought this was how start caps were wired in (with the relay taking the cap out of the circuit after starting).

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Samurai Appliance Repair Man


Well color me silly and call me a rabbit.



Silly wabbit, caps are for kids. :0

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