Sohail Ansari

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Capacitor Start Induction Motor

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Depending upon whether capacitor remains in the circuit permanently or is disconnected from the circuit using centrifugal switch, these motors are classified as,
           1. Capacitor start Induction run motor
           2. Capacitor start capacitor run motors
 1. Capacitor start Induction run motor:
 In case of capacitor start capacitor run motor, there is  centrifugal switch which is put to open after starting the motor that means capacitor remain temporarily in the circuit.
The connection of capacitor start motor is shown in the Fig.. The current Ilags the voltage by angle Φm while due to capacitor the current Ist leads the voltage by angle Φst. Hence there exists a large phase difference between the two currents which is almost 90, which is an ideal case. The phasor diagram is shown in the Fig.1(b).
                                         Fig 1.  Capacitor start Induction Run 
       The starting torque is proportional to ‘α ‘and hence such motors produce very high starting torque .
       When speed approaches to 75 to 80% of the synchronous speed, the starting winding gets disconnected due to operation of the centrifugal switch. The capacitor remains in the circuit only at start hence it is called capacitor start Induction run motors.
2. Capacitor start Capacitor run motor:
In case of capacitor start capacitor run motor, there is no centrifugal switch and capacitor remain permanently in the circuit. This improves the power factor.
       The schematic representation of such motor is shown in the Fig.
                              Fig. 2 Capacitor start capacitor run motor
 The phasor diagram remains same as shown in capacitor start induction run motor. The performance not only at start but in running condition also depends on the capacitor C hence its value is to be designed so as to compromise between best starting and best running condition. Hence the starting torque available in such type of motor is about 50 to 100% of full load torque.
       The direction of rotation, in both the types can be changed by interchanging the connection of main winding or auxiliary winding. The capacitor permanently in the circuit improves the power factor. These motors are more costly than split phase type motors.
       The capacitor value can be selected as per the requirement of starting torque, the starting torque can be as high as 350 to 400 % of full load torque. The torque-speed characteristics is as shown in the Fig.3.
                Fig.3  Torque speed characteristic of capacitor split phase motor
 Applications :
       These motors have high starting torque and hence are used for hard starting loads. These are used for compressors, conveyors, grinders, fans, blowers, refrigerators, air conditions etc. These are most commonly used motors. The capacitor start capacitor run motors are used in celling fans, blowers and air-circulations. These motors are available up to 6 kW.


  1. Ajay Kumar says:

    thnx bro….

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