Types of Single Phase Induction Motors


Types of Single Phase Induction Motors :


     Single phase induction motors are not self starting because there is no rotating magnetic field produced. It is not practicable to provide initial rotation to the rotor, manually. So in practice some provision is made to provide rotating magnetic field at start, to make the motors self starting.


     Single phase induction motors are classified according to the methods of starting used for them.

1. Split phase motor.

2. Capacitor start motor.

3. Capacitor start capacitor run motor.

4. Shaded pole motor.



Split phase induction motor :


     In this motor, in addition to normal stator winding called as Main Winding, stator  carries one more winding called as auxiliary or starting winding.The two windings are displaced in space by 90° electrical with each other. Mostly the rotor is of Squirrel cage type. The arrangement is shown in figure.

Types of single phase induction motors

     The time displacement between the two currents is obtained by designing the auxiliary winding as highly resistive in nature. The main winding is reactive hence carries current, Im which lags supply voltage by large angle while auxiliary winding carries a current, Ist which almost phase with supply voltage. Hence there exists a phase difference between the two alternating current.

 

     The two currents produce the two fluxes which have a phase difference of ' a '. Due to these two fluxes having considerable phase difference in between them, a rotating magnetic field is produced and the motor develops a starting torque


     Because of the high resistance character of auxiliary winding, this motor is also known as Resistance start motor. The auxiliary winding is not designed for continuous operation to prevent the motor from drawing excessive currents from the line and also to protect the winding from damage due to overheating. 


     Hence the auxiliary winding automatically disconnected by the centrifugal switch, when the motor speed ( 75% to 80% of synchronous speed ). As auxiliary winding is in the circuit only at start, hence it is also called as starting winding. As currents Im  and  Ist are splitted from each other by angle 'a' at start, the motor is called as split phase motor, The starting torque is proportional sin a . This motor has moderate starting torque and generally it is 100% to 200% at the full load Torque.




Capacitor start Induction Motor :


     In this motor also, the main and auxiliary windings are space displaced by 90° electrical. The time displacement between the currents in the main and auxiliary windings is achieved by connecting a capacitor in series with auxiliary winding. The feature of the capacitive circuit is to draw the leading current. The current drawn by the main winding lags voltage by angle φm while current drawn by the capacitive start winding leads voltage by angle φst. Hence there exists a large phase difference between I and  Ist which is almost 90°. Thus the motor develops very high starting torque.

Types of single phase induction motors

       Both the capacitor and starting winding are automatically disconnected by centrifugal  switch, when the motor has reached 75% of synchronous speed. Because the capacitor is in the circuit only during the starting period it can be an electrolytic type. The capacitor value is selected as per the requirement of starting torque, the starting torque can be as high as 350% to 400% of full load torque.


Advantages :


(i)  High starting torque.

(ii) Better starting power factor.


Speed Reversal : 


     The direction of rotation can be changed by interchanging the connections of either the main winding or auxiliary winding terminals.


Applications : 


     These motors are used where high starting torque is required such as in compressors, pumps, air conditioners, conveyors etc.



Capacitor start Capacitor run induction motor :


     Two Value Capacitor Motor : To increase the starting torque, two capacitors are used in the auxiliary winding. Theoretically, optimum starting and running performance can be achieved by having two capacitors, one for starting and the other for running both the capacitors Cand Care in the circuit during starting. After the motor picks the speed, the centrifugal switch disconnects the starting capacitor, Cs. The auxiliary winding and running capacitor, Cremain in the circuit during running condition also. The capacitor Cis larger in value and is of electrolytic type, permits high starting torque.

Types of single phase induction motors

     The capacitor Cis smaller in value and is of oil type. This motor is more expensive than a capacitor start motor however it provides the best performance. The starting torque is upto 300% of the full load torque.


Advantages :


(i)  High starting torque.

(ii) Better efficiency and high power factor.


Applications : 


     These are used in compressors, conveyors, pumps and other high torque loads.



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