Capacitor 101 – Johnstone Supply Support

1. Capacitors do not increase voltage

As a motor spins the magnetic fields create a Back EMF. This Back EMF comes from the motor, not the capacitor, but does become measurable at the capacitor.

2. No current goes through a capacitor** - Just IN and OUT

Not all analogies are perfect, but... The capacitor is like an expansion tank in a boiler system. It absorbs the voltage irregularities that come into it, then smoothly lets voltage back out of it. The µF specification is the "gallons/expansion" that the tank can hold and the 370/440V specification is the amount of "pressure" it is designed to handle.

3. The greater the capacitance spec (µF), the more current goes on the start winding

The greater the µF of the capacitor, the more stored energy there is and therefore the higher the start winding amperage.
If a capacitor has failed and measures 0µF, it would be the same effect as having an open start winding.
So... Never oversize a capacitor. It can quickly cause damage to a compressor by increasing the amount of current on the start winding, and therefore causing early failure.
An undersized capacitor results in less stored energy available and therefore a lower start winding amperage. There is collateral damage though. If the start winding is not doing the correct amount of work (less), then the run winding has to compensate and do more work. This will heat up the run winding causing damage to the run winding and thermal overload situations.

4. The voltage specification is what it can handle, not what it will produce

Since a capacitor does not produce energy, but just stores it, a higher Voltage specification is allowable. You can replace a 370V with a 440V cap, but do not replace a 440V cap with a 370V.

5. Testing a capacitor under a load will give an accurate dynamic µF reading

Measure the amps of the motor start winding coming off of the capacitor. Multiply it times 2653 (60Hz constant). Now divide that number by the voltage you measure across the capacitor.