Basic Capacitor Question

[Jeff Matthews]

Friday, my outside A/C condensor unit fan stopped spinning. I remember this happened once before, and my friend (who knew a little about A/C) and I replaced the motor capacitor. Remembering what we did before, like a good monkey, I replaced the capacitor on this unit w/a spare I had, and of course, all went well.

What I want to know is what is the capacitor's function. I'd like to know WHY I replaced it (other than it's what we did before). That way, I might be able to diagnose a bad capacitor in the event of other electronics failures.

[jtkinney]

The capacitor is there to provide enough torque to start the fan spinning. AC fan motors don't need to have as much torque while running as they do at start-up. To boost the starting torque, an extra winding is added to the motor and the phase of that winding is offset using the capacitor (current leads voltage through a capacitor) to help spin up the motor. Once the motor is running there is a switch that opens to keep the start circuit from overheating.

Hopefully this oversimplification helps.

[Jeff Matthews]

Yes, I think it helps. If I understand correctly, the energy to start-up is greater than the energy to run, so instead of using enough energy to start-up as a constant, energy is saved by introducing a capacitor for a short spurt of extra energy. ???

If my understanding is correct, it seems that maybe capacitors are used for other functions, too. I tried to read up, and one thing I saw was that they are used to protect from DC current.

Thanks for your reply. Another question is since this is the case re: start-up energy, do all electric fans make use of capcacitors?

[BEC]

http://dnr.louisiana.gov/sec/execdiv/techasmt/ecep/hvac/d/d.htm

[Jeff Matthews]

Thanks, Bob. I read the link. What I do not get is why isn't the electrical wire run directly to the motor, instead of through the capacitor? It would seem A/C carries enough volts to run the fan, so what is the purpose of the capacitor?

[BEC]

Jeff,

When running inductive loads (like a motor) we get into a situation needing power factor correction. You can do a google search on "power factor correction" and get the explaination and formulas and such. Been too long since I studied that stuff for me to be able to talk much about it.

Bob

[fini]

Been too long since I studied that stuff for me to be able to talk much about it.

Bob

Plus, Bob would have to kill you if he went into greater detail.

[Deang]

There's no such thing as a "basic capacitor question".

[jtkinney]

It's a phase shift thing, not a boost of energy thing (which capacitors can also do). In a multi pole motor the capacitor shifts the phase of one of the poles relative to the next one. As Bob said, it gets into the power factor of the motor.

With a DC motor you have to give it full voltage to get full torque. With AC you can shift the phase of the poles and ramp up to full torque.

[Jeff Matthews]

Thanks for all your input. I can see that I'll have to study much more if I really want to understand this. I was just curious because if I have a motor go out or something like that in the future - of some component whatever it may be - I was wondering if a general statement would help me diagnose a capacitor failure as opposed to say, a burnt motor.