Slip ring induction motor working principle


Generally we all are know induction motor is most widely used motor in industrial. induction motor is self starting motor and also speed of the induction motor is purely depend on supply voltage. slip ring induction motor is one of the type of 3 phase induction motor. slip ring induction motor is wound rotor type motor. the main difference in the slip ring induction motor apart from other motor is the rotor of the slip ring induction motor is having slip ring and its connected to external resistance.


The slip ring induction motor working in the induction principle. when ever the supply is applied to stator winding and stator winding produced magnetic flux. due to faradays law of electromagnetic induction the rotor winding get induced and produce the magnetic flux current. These motors are usually started with full line voltage . at starting time the induction motor drawn 6 to 7 times full load current but this high inrush current could be control by external resistance connected in rotor circuit. this external resistance in the rotor side should be adjusted to get minimum starting current in the slip ring induction motor. That external resistance  act like a variable rheostat when the motor starting time it  automatically adjusted to high resistance and once motor obtain normal speed that resistance will be reduced. that external resistance adjustment will decrease the rotor current and also decrease  the stator current. that’s why starting current of motor will reduced to normal and it increase the starting torque of the motor. that external resistance also improve the power factor of the motor. the below diagram will show the construction and arrangement of slip ring induction motor.

slip ring induction motor

Speed Control:

speed control of slip ring induction motor is done by external resistance connected in the rotor side. usually the slip ring motor is started with high starting torque with low speed. adding additional resistance to reduce the speed  and rotor current of the motor. even speed control technique is not good for changing torque loads. the speed control of slip ring induction motor can be controlled only from  50% to 100%. it can not be control gradual speed.

Advantages of slip ring induction motor:

  • Excellent starting torque for high inertia loads.
  • its having low starting current because of the external resistance
  • Speed is resistance variable over 50% to 100% full speed

Disadvantages of slip ring induction motor:

  •  Higher maintenance of brushes and slip rings compared to squirrel cage motor
  • complicated construction
  • copper loss is high
  • efficiency low
  • power factor low
  • costly compared to 3 phase squirrel cage induction motor
  • High torque required application
  • low starting current application
  • used in hoist , cranes and elevator applications



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