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Motor start-up capacitors and motor running capacitors

Motor start-up capacitors and motor running capacitors

A three-phase motor runs because three-phase electricity – also called three-phase AC – creates a rotating electrical field in the motor. The three individual phases are 120°, i.e. exactly a third of a revolution, apart from each other. Single-phase power supply does not of itself does not satisfy this precondition, so in order to achieve the necessary phase shift in this case it is necessary to use a capacitor. In respect of motor capacitors, a distinction is made between motor start-up capacitors and motor running capacitors. The former is connected only when starting the motor. A soon as the motor is running, the rotational inertia of the rotor coupled with the alternating field of the AC supply ensures that the motor continues to run. The capacitors generally used for this are bipolar aluminium electrolytic capacitors with liquid electrolytes, which must be disconnected one the motor has started. The motor running capacitors on the other hand remain connected at all times. They impart a smoother torque characteristic to the motor, thus enabling selection of a smaller frame size. The capacitors used for this purpose are comparatively larger metal-paper capacitors or somewhat more compact polypropylene film capacitors.

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