17 March 2018

Capacitor Start Induction Run Motor - Construction, Working and Characteristic

Capacitor Start – Induction run Single Phase Motor

  • The phase difference between main winding and starting winding is achieved by introducing an electrolyte capacitor is in series with starting winding. 
  • The capacitor is of electrolyte type. 
  • When the speed of the motor is increased 70 to 80% of full load speed, the centrifugal switch open and starting winding is cut out.
capacitor start induction run motor

Vector diagram

  • As the capacitor is in series with starting winding, its vector IS leads supply by some angle. 
  • The current vector IM lags the supply voltage by some angle ( < 900 ) due to high inductance of the main winding. 
  • The angle between current vector IS and IM is in the range of 800 to 900 which is nearly 300 in the case of split phase motor.

Starting torque

  • The starting torque is directly proportional sine of angle between starting current and main current.
        T a ( Sin θ )
  • The starting torque in the capacitor start – induction run motor is about 3.5 to 4 times that of full load torque.

torque speed characteristic of the capacitor start induction run motor

Advantage of centrifugal switch over magnetic switch

  • It is not affected by the line voltage
  • More reliable
  • Low cost 

Electrolyte Capacitor

  • It is formed by winding two sheets of etched aluminum foil and separated by two layers of insulating layer in the shape of cylindrical. 
  • This whole unit is impregnated with an electrolyte of ethylene glycol. 
  • An anodic film is produced on each foil by the electro – chemical action. 
  • The voltage rating of the capacitor should not necessary be same as that of motor rating. 
  • The power factor of the such capacitor is in the range of 6 to 8 %.

Speed Regulation

  • The speed of the motor varies 10% to 100% from no load to full load.


  • It is available in the range of 1/6 HP to 3 HP.


  • Compressors
  • Elevators
  • Conveyors
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