The purpose of capacitor on single phase motor

In single phase motors capacitor is used to convert motor to two phase. Otherwise the single phase motor will not have starting torque. Capacitor is connected in series with the starting winding of the motor. The value of the capacitor depends on the starting torque required. Phase advancement in the starting winding with respect to running winding is the key. There is nothing called as 3 phase capacitor or single phase capacitor. Capacitors are individual components and have only two terminals.

Coming to improve the power factor you have to connect the capacitor in parallel with both windings of the motor. The value of capacitor depends on the reactive power of motor.

The purpose of providing a capacitor on single phase motor is to enable the motor to start in a particular direction when its power supply is put ON. It is also possible to start a single phase motor without any capacitor but in that case the motor would not start rotating automatically when power in put ON, rather it would remain stand still until its shaft is rotated externally in either direction. The motor can in such a case be started in any direction as may be required. This is because of Ferrari Arno Effect.

Now, coming to formula for calculating capacity of the required capacitor, let us understand the basics.

  1. The starting winding and the running winding of a 1-phase motor are identical; i.e., same size of conductor and same number of turns.
  2. It is required that (a) if the motor is equipped with a centrifugal switch for taking the starting capacitor out of circuit after the motor has started, a high MFD capacitor is used for starting and a low value capacitor for running and (b) if the capacitor is not just for starting a medium value capacitor is used such that current flowing into the starting or auxiliary winding is kept limited to a safe value.

With above mentioned methods, ensure to use the correct rated capacitor according to the following formula.
i.e Q actual(actual value)/ Q Rated(Rated value) = (V Actual/V Rated)^2 This means that the ratio of the actual value of the capacitor and the rated value is equal to the square of it voltages.

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