Why AC Capacitor use for a single phase motor but not in a three phase ?

Some types of single-phase motor do not have a rotating magnetic field

, so the rotor does not start rotating automatically. A "motor start and run" circuit is needed, which includes a capacitor toproduce a rotating magnetic field. That rotating field pulls the rotor and it starts to turn. Once the motor has been started, it will continue to spin as long as enough power is supplied to it to overcome the force of the load it is driving. In homes such motors are commonly used in lawn-mowers, for example.

However not all designs of single-phase motor require a "motor start and run" circuit: a "commutator and brushes" motor, which has its field winding in series with its rotor windings, will start turning on its own without needing a capacitor-start circuit. Such motors are commonly used in small domestic hair-dryers, power drills and vacuum cleaners, for example.

However a different kind of capacitor-based circuit is usually fitted to such motors: a "spark-suppression circuit". This is fitted to help suppress the production of high levels of interference at radio frequencies that is sent out from the sparks produced when a "commutator and brushes" motor system is running.

A three-phase motor has a rotating magnetic field so that the motor's rotor is pulled and starts rotating automatically. Hence no "motor start and run" capacitor is needed to start the rotor turning.

However, depending on the type of three-phase motor, to limit the amount of current drawn at motor startup, a different type of starter circuit may be needed, such as a "star-delta starter". Once the motor has been started, it will continue to spin as long as enough power is supplied to it to overcome the force of the load it is driving.

Three-phase motors are not typically used in household appliances but are very common in factories, workshops and other industrial sites because of their higher power-to-weight ratio and better electrical efficiency compared to single-phase motors.

Source(s):

http://www.capacitors-china.com/why-ac-capacitor-use-for-a-single-phase-motor-but-not-in-a-three-phase.html

Why AC Capacitor use for a single phase motor but not in a three phase ?

By: simonzhan

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