Motor Start Capacitors

A motor start capacitor provides a strong initial boost of energy to get a single-phase motor turning. It increases starting torque so motors reach running speed quickly and reliably, then disconnects from the circuit once the motor is up to speed. Typical applications include pumps, compressors, fans, and air conditioners.

A start capacitor is only in the circuit briefly during startup and is rated for short, intermittent duty — so it carries a high capacitance value but should never run continuously. A run capacitor stays energized the whole time the motor runs and is built for continuous duty at a lower capacitance. The two are not interchangeable.

Frequent start capacitor failure usually points to an underlying issue rather than a defective part: too many starts per hour (short-cycling), a failed or welded start relay or potential relay that leaves the capacitor in the circuit too long, the wrong MFD or voltage rating, or excessive ambient heat. Correcting the root cause and matching the original MFD and voltage ratings will maximize service life.

Match the microfarad (MFD) range and the voltage rating printed on the original capacitor. Start capacitors are often listed as a range (for example, 88–108 MFD); staying within that band is important for correct starting torque. Use a capacitor whose voltage rating equals or exceeds the original. Confirm the case size fits your mounting.