# How a capacitor works

I have an AC capacitor from the ceiling fan.

When I connect its end to a socket (AC line 110V), it gets charged (as touching the terminals produces a spark).

But at this point I am unable to understand this mechanism because I have read that capacitors don't charge storage for AC and act almost like a short circuit at t(0+). [neglecting the phase shift and assuming frequency above 50 Hz].

Could someone please explain where I am going wrong?

-
If you don't know how it works, don't plug it into the mains!! –  exscape Jul 11 '12 at 19:22
I saw the technician doing that.... But I dont think it was wise enough to ask him the underlying equation for the phenomenon ;) –  perilbrain Jul 11 '12 at 19:25

Capacitors do store charge. In fact, that's basically what a capacitor does, with the added characteristic that its voltage will be proportional to the amount of charge it has stored.

A capacitors doesn't store AC, but it does store whatever charge is on it given the voltage at the time it was disconnected. Since the AC voltage can vary from zero to fairly high peaks, it is somewhat random what the capacitor will be charged to. The peaks of the AC line is the square root of 2 times the RMS voltage. For example, 115 VAC has peaks of ±163 V. A capacitor could get charged to anywhere in that range.

-
So it means there is a probability distribution for the capacitor instantaneous voltage? ie. (163 to -163V) as per the example?? –  perilbrain Jul 11 '12 at 19:41
@Peril: Yes. And, due to the AC line being a sine wave, the probability is not even accross the range. It is weighted more towards the peaks because the sine spends proportionately more time there. If it were a triangle wave, then the probability would be even accross the range. –  Olin Lathrop Jul 11 '12 at 19:43
Thanks a lot.....Understood (Rate of change in sinusoidal voltage)/(Rate of discharge) <<1 –  perilbrain Jul 11 '12 at 19:49

There are two common uses of plastic non-polarized caps in AC motors

1. Start Capacitors

• briefly increase motor starting torque
• allow a motor to be cycled on and off rapidly
• Furnace motors switch off the cap. after it reaches 2/3 of full speed or so.
2. Run Capacitors

• are used in variable speed fans, which use single phase electric motors and need a capacitor to energize a second-phase winding.
• If this cap is missing or improperly sized; the motor will get hotter, lose power, & become noisy.
-

It's not quite correct to say that capacitors, as used in electrical circuits, store electric charge. In normal usage, a capacitor has no net electric charge even when "charged".

It is correct to say that a capacitor stores electric energy by keeping electric charge separated. When you connect a capacitor to a source of electric current, electric charge flows from one plate of the capacitor to the other plate via the external circuit. We say the capacitor is "charged" but we don't mean that it is electrically charged but, like a rechargeable battery, energy "charged".

When connected to an AC source, the capacitor alternately charges and discharges, i.e., energy is alternately stored in and then released by the capacitor. Work is alternately done on and then by the capacitor.

-