Some single phase AC motor designs use motor run capacitors, which are left connected to the auxiliary coil even after the start capacitor is disconnected by the centrifugal switch. These designs operate by creating a rotating magnetic field. Motor run capacitors are designed for continuous duty, and remain powered whenever the motor is powered, which is why electrolytic capacitors are avoided, and low-loss polymer capacitors are used instead. The capacitance value of run capacitors is usually lower than the capacitance of start capacitors, and is often in the range of 1.5 µF to 100 µF. Choosing a wrong capacitance value for a motor can result in an uneven magnetic field, which can be observed as uneven motor rotation speed, especially under load. This can cause additional noise from the motor, performance drops and increased energy consumption, as well as additional heating, which can cause the motor to overheat.
Motor start and run capacitors are used in single-phase AC induction motors. Such motors are used whenever a single-phase power supply is more practical than a three-phase power supply, such as in domestic appliances. They are not as efficient as three-phase AC induction motors, however. In fact, single-phase AC motors are 2 to 4 times less efficient than three-phase AC motors, which is why they are used only for less powerful motors. Typical applications which utilize start and run motor capacitors include power tools, washing machines, tumble dryers, dishwashers, vacuum cleaners, air conditioners and compressors.