Capacitor Start Induction Run Motor - Construction & Working

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 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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