WHY DOES MY MOTOR NEED A CAPACITOR?

The best option if you are replacing a starting capacitor or a start/run capacitor is to match the existing device on your system. 

Capacitor Ratings

Motor Run Capacitors energy ratings are expressed in farads - the amount of electrical power stored in the capacitor, where u f = micro farad (10-6 farads) and is the same as micro farads written in other references.

Capacitors are also rated for the voltage range in which the capacitor can safely operate, such as 220 V or 440 V.

You can substitute capacitors on a cooling system but the substitute capacitor must be able to handle the voltage. For example you can't sub a 110 V-rated capacitor into a 220 V system.

Depending on the application, the micro-farad range of starting capacitors varies according to the motor size. Run capacitors typically range in micro-farads from 1.5 to 50 u f. Start capacitors typically range from 20-30 u f up to 250-300. 

You can also check a Motor Run Capacitors to compare its performance with its micro farad rating by using an ohmmeter. In a working capacitor ohms will build-up and then fall off (when the capacitor discharges).

If you reverse the + and - leads of the DC ohmmeter. leads it will repeat. If you do not see any resistance in the capacitor then it has an internal short and it's shot - you need a new one.

HVAC suppliers sell general-use starting capacitors that are intended for use across a range of electrical motors and motor sizes.

But at least some industry sources make a more careful argument explaining that it's best not to install a significantly over sized starting capacitor on an electric motor.

If the start capacitor is too large for the application, the Motor Run Capacitors can actually mask the developed voltage in the start winding and keep the start capacitor in the circuit continuously.The run-start voltage is suppressed below the trigger voltage of the start device. As a result, the start capacitor remains in the circuit as the motor runs.

A secondary, fail-safe method is necessary to ensure that the start device is ultimately removed from the circuit.

 A start device that fails to remove the start capacitor from the circuit has the potential to cause premature failure of the start winding in the compressor. If the capacitor is never removed from the start winding, premature winding failure could occur. As such, care should be taken when selecting capacitor sizes for an application.

Care should also be taken regarding products that tout a "bigger capacitor is better" approach to compressor starting. SUPCO E-Class devices provide a secondary timing safety device to ensure that the start capacitor is dropped from the circuit in a fail-safe mode.

Motor Run Capacitors