How To Check Capacities of a Capacitor Under Load


Unfortunately, some capacitor checkers we use in the field are ineffective at testing for a bleeding capacitor because there is no load on the capacitor at the time that it’s being tested.

The good news is that there is a fairly reliable way to test a capacitor under load
(Figure 3).

While the compressor is running, measure the ac voltage across the run capacitor. You will be reading the voltage that the compressor is generating. The term for this is “back electromotive force.” Measure the amperage being drawn through the start wire between the capacitor and the compressor start terminal.

Be sure to keep your amp meter away from the components in the control box — that could distort your reading. Use the voltage and amperage readings you’ve obtained in the following formula:

Formula: Amps X 2,650 ÷ Voltage = Actual microfarads.

If the solution to your test gives you a microfarad rating that is 5% below the capacitor’s labeled rating, be suspicious. If the results of your test show the capacitor to be 10% or more below the labeled rating on the capacitor, replace it.
Of course, if the run capacitor is swollen or is leaking oil there is no need to test it. Just replace it.

To check the start-assist components just place your amp meter on the wire on either side of the capacitor or solid-state start-assist device and watch for an amperage spike for about ¼ sec as the compressor starts. If there is no amperage spike when the compressor starts, more investigation is necessary.