What Is a Run Capacitor?

A run capacitor is an electrical capacitor that energizes the auxiliary coil of an alternating current electronic motor, creating a rotating magnetic field while the motor runs. Run capacitors are sometimes used in single phase AC motors, and they typically have a voltage range of 370V to 440V.

Unlike start capacitors, run capacitors are designed to keep the auxiliary coil powered during the motor startup phase as well as during normal operation. As a result of this, the capacitance of a run capacitor is typically set lower than that of the start capacitor. Replacing a run capacitor should be done with care. Choosing the wrong capacitance value may lead to an unsteady magnetic field, which in turn may lead to increased energy consumption, drop in performance and additional noise. Installing the wrong run capacitor may also cause the motor to overheat.

Run capacitors are sometimes used in washing machines, air conditioners, dishwashers and tumble dryers. When testing a capacitor, most technicians rely on digital or analog multimeters. The unit's capacitance can be measured by placing the probes across both its terminals. In addition to a loss in performance, faulty capacitors may reveal themselves with physical signs such as leaky oil or a bulgy side.