# 120 VAC Motors with start or run capacitors



## E&I (Jan 24, 2012)

Could someone explain how each capacitor works in its specific application.
How it affects waveforms any help anyone can provide. Thanks


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## JRaef (Mar 23, 2009)

E&I said:


> Could someone explain how each capacitor works in its specific application.
> How it affects waveforms any help anyone can provide. Thanks


Why are you concerned about wave forms? 

In a single phase induction motor, there is no relative "rotation" of the magnetic fields to make it spin. Capacitors are used as one possible methough of creating a pseudo phase shift in the motor circuit to start it rotating. Once it is rotating it may or may not need the capacitor any longer, it depends on which design of motor you are referring to. If it is one type that only uses the cap to start the motor, it is switched out of the circuit at about 80% speed, so it is only affecting "waveforms" for a fraction of a second.

There are several well done simplistic training websites for single phase induction motors that will explain all of the different ways to make them start and run, I suggest using your favorite search engine to find them and read up.


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## gottspeed (Mar 8, 2010)

Makes sense. My furnace blower crapped out and would only rotate at like 6 RPM, I noticed I could spin it the other way and it would continue going the wrong direction. Had continuity across the capacitor. The supplier urged me to get a new blower but I got er going with a 2 dollar cap. Ha ha. Glad I'm an electrician.


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## erics37 (May 7, 2009)

gottspeed said:


> Makes sense. My furnace blower crapped out and would only rotate at like 6 RPM, I noticed I could spin it the other way and it would continue going the wrong direction. Had continuity across the capacitor. The supplier urged me to get a new blower but I got er going with a 2 dollar cap. Ha ha. Glad I'm an electrician.


Start and run caps are among the first things I check when a single-phase motor stops working. If the cap is bad but the motor megs okay then it's a lot cheaper and easier to replace the cap than the whole motor.


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## Aussielec (Apr 1, 2012)

Put quite simply the cap is there to improve motor torque. On a standared cap start motor the cap is only used for the initial start where higher torque is required. Cap start and cap run motors function pretty much the same except now you have increased torque not just when the motor starts but while it is running aswell. 

As far as the wave forms go it creates a phase shift in the associated winding causing the magnetic field to move across the rotor bars at a different angle thus improving the rotating magnetic field of the motor.


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