Why capacitor is required for Single phase motor

Single phase Motor Capacitor
Single phase Motor Capacitor

Why capacitor is required for Single phase motor:

Single phase motors are not a self-starting motor, single phase power supply cannot create rotating magnetic field because of its nature (only one phase). So that to rotate the single phase motor we have to give rotary moment or manual rotation to get continuous rotation. But at that same time we can run the motor but adding extra starting winding and the winding will be connected in series with the capacitor. Technically it is called split phase capacitor method. We are going to use the capacitor property (in capacitor voltage lags the current by 90 deg ). Here the supply voltage will be phase shifted by 90deg. hence by adding capacitor we get the two phase simultaneous from our single phase supply. Hence the motor starts rotating.

Circuit diagram for single phase motor:

Single phase Motor Capacitor
Single phase Motor Capacitor
Single phase Motor Capacitor Waveform
Single phase Motor Capacitor Waveform

Here you can see the two winding are shown in the circuit diagram, one is starting winding and another one is running winding. In that, the starting winding is connected in series with the capacitor. You can see the wave form diagram, how capacitor creates phase shift of the input voltage.

How to calculate capacitor value for single phase motor:

You can use any type of capacitor except DC capacitor. Two important criteria should be considered while selection capacitor for single phase motor.

No:1 is Rating: Value of capacitance.

In this, we are going to see some home appliances capacitor run motors capacitance value. The capacitor value is directly proportional to the motor rating. i.e

  • in our home ceiling fan single phase motor rating is 45 watts and 2.5 micro farad capacitor uses to start the motor.
  • Our Home exhaust fan use 4 micro farads and the rating is 200 Watts.
  • At the same time 0.75HP single phase motor use 10 micro farad capacitor. Like that….
  • 3 HP single phase motor uses 42 micro farad capacitor.

The capacitor value is depending upon the reactive power supplied to the auxiliary winding. The auxiliary winding receives reactive current and it does not support to torque development in the motor.

No2: is Voltage rating:

You should choose the voltage rating of the capacitor at 440 Volts. It should not be 220 volts. If it is 220 volts then your motor does not work or give desired output.

Also see: Condition for Transformer Parallel Operation


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