The motor is used to start the capacitor during the motor start phase, and once the rotor reaches the predetermined speed, the motor starting capacitor is disconnected from the circuit. This speed is typically about 75% of the maximum speed of the motor type. These capacitors typically have capacitance values in excess of 70μF. They come in a variety of voltage ratings depending on their application.
Some single-phase AC motor designs use a motor to run a capacitor that remains connected to the auxiliary coil even after the start capacitor is disconnected by the centrifugal switch. These designs operate by generating a rotating magnetic field. The motor operating capacitor is designed for continuous operation and maintains power when the motor is energized, which is why it is avoided to use electrolytic capacitors, but to use low loss polymer capacitors. The capacitance value of the capacitor is typically lower than the capacitance of the startup capacitor, typically in the range of 1.5μF to 100μF. Choosing the wrong capacitor value for the motor results in a non-uniform magnetic field, which can be observed as uneven motor speed, especially under load. This can cause additional noise, reduced performance and increased energy consumption, as well as additional heating, which can cause the motor to overheat.
The motor starting and running capacitors are used for single phase AC induction motors. As long as the single-phase power supply is more practical than the three-phase power supply, such as in household appliances, such an electric motor is used. However, they are not as effective as three-phase AC induction motors. In fact, single-phase AC motors are two to four times less efficient than three-phase AC motors, which is why they are only used for smaller power motors. Typical applications that utilize starting and running motor capacitors include power tools, washing machines, tumble dryers, dishwashers, vacuum cleaners, air conditioners and compressors.
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