A motor capacitor, such as a start capacitor or run capacitor (including a dual 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 (µF or uF). Older capacitors may be labeled with the obsolete terms "mfd" or "MFD", which means microfarad.
Motor capacitors are used with air conditioners, hot tub/jacuzzi spa pumps, powered gates, large fans or forced air heat furnaces for example. A "dual run capacitor" is used in some air conditioner compressor units, to boost both the fan and compressor motors.