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.

Vector diagram

• As the capacitor is in series with starting winding, its vector I_S leads supply by some angle. 
• The current vector I_M lags the supply voltage by some angle ( < 90⁰ ) due to high inductance of the main winding. 
• The angle between current vector I_S and I_M 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. 

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

Rating

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

Application

• Compressors
• Elevators
• Conveyors 

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