Effect of the Rotor Resistance in a slipring induction motor

This article explains about effect of the rotor resistance in a slipring induction motor. Effect of the rotor resistance and reactance plays vital role to generate torque in an induction motor. Incase of standard induction motor , it is impossible to access rotor circuit from external. But Slip ring induction motor has a special type rotor, which allows to access rotor circuit from external electrical circuit. As we seen in earlier article of Torque equation of an induction motor, it shows that torque generated in an induction motor is depending upon the ratio of its rotor resistance (R₂) and standstill reactance (X₂). When this ration become unity, the slip achieves to it maximum.

The maximum slip in an induction motor , which is generated during its resistance becomes equal to its stand still reactance

Maximum Slip (Sm) = R₂/X₂ =1

The torque speed characteristics of a slip ring induction motor is shown here, This shown how motor operating point is shifting from Point A to B and C, by varying external rotor resistance connected with rotor. When we increases rotor resistance its maximum torque region also is shifter from Point X to Y and Z. But magnitude of maximum torque remains unaffected, As region of maximum torque (S_m) changes, torque speed characteristics of the motor also shifting towards right side , while rotor resistance (R₂) increased.

Further starting torque of the motor is also can be increased by changing rotor resistance. As per statement of starting torque

   

The above statement is shows that starting torque of an induction motor in directly proportional to the rotor resistance. So rotor resistance increases, starting torque is also increases. If Both rotor resistance as well as rotor reactance are kept to equal , starting torque becomes equal to the maximum torque. Such a way effect of rotor resistance in a slipring induction motor as well as standard induction motor takes in place. Hence slip ring induction motor is used to the application , where higher starting torque required such lift, hoists, tractions etc.