A motor capacitor (start capacitor or run capacitor) is an electrical capacitor that alters the current to one or more windings of a single-phase AC induction motor to create a rotating magnetic field. There are two common types of motor capacitors, run capacitors and start capacitors. The units of capacitance are labeled in microfarads (uF). Older capacitors may be labeled with obsolete terms ‘mfd’ or ‘MFD’ which also means microfarad.
Some single-phase AC electric motors require a ‘run capacitor’ to energize the second-phase winding (auxiliary coil) to create a rotating magnetic field while the motor is running.
Start Capacitors briefly increase motor starting torque and allow a motor to be cycled on and off rapidly. A start capacitor stays in the circuit long enough to rapidly bring the motor up to a predetermined speed, which is usually about 75% of the full speed, and is then taken out of the circuit, often by a centrifugal switch, that releases at that speed. After this happens, the motor works more efficiently with a run capacitor.