What is a Motor Run Capacitor?

A Motor Run Capacitor keeps a single-phase motor running when the start capacitor drops out of the circuit, helping to maintain the torque on the auxiliary windings. They are found in many 120 V or 240 V single-phase electric motors, such as air conditioning compressors, refrigerators and pumps that need a high starting torque.

Capacitors Store Energy, Release It at a Fast Rate:
In a 60-cycle alternating current system, capacitors store and release electricity. They do this at a rate of sixty times per second.
The sizing of capacitors is critical to their performance. Just like a battery, a capacitor is sized to meet its operating load. A small size will not provide adequate voltage, and a large size will overheat the motor windings, shortening their life span.

When replacing a capacitor, it's important to match the microfarad rating (mfd or uF), voltage and dimensions of the original. For example, a replacement start capacitor must have the same microfarad mfd rating and voltage as the original capacitor to be considered correct for the application.

Once a motor has started, the starting capacitor is connected to the motor's circuit for only a few seconds to give it a "starting kick" or "torque boost." When this happens, the start capacitor is disconnected from the circuit by a centrifugal switch. Once the motor reaches a set speed (typically 75% of its full power) the switch is open, allowing the motor to continue to run on normal current.