Replacing a Slip Ring Motor with a VFD fed squirrel cage Motor

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 What is Slip Ring Motor:

It is also a Induction Motor in which

1. The Slip of the Motor can be changed by inserting the external resistance to achieve the maximum              Torque at that point.

2. By this method the maximum torque or the Pull Out Torque can be achieved at starting point only.

3. The pullout torque can be achieved during starting with the minimum starting current . The value is        much lower compared to a cage rotor motor. 


This type of motor we use for those applications where the starting torque requirement is too high or the number of starts / stop is very high ( Intermittent Duty ). Few examples are Crushers, Cranes, Ball Mill, Rolling mill.

Hence, to replace the same with a cage motor, one must do it by a VFD and VFD suitable Motor.


For replacement, below points to be taken care:

1. For the same output the frame size of slip ring motors are higher than the cage motor. Hence, the mechanical dimension like shaft dimension, mounting details must be obtained beforehand to check the possibility of replacement.

2. Complete nameplate data of the Slip Ring motor including RV and RA value.


3. The Speed ramp up time as the process may demand a specific acceleration.


4. The fuse rating of protection unit.


5. The torque value and the current drawn data must be available during resistance change.


6. The absolute Pull out torque value of the cage motor should be 1.15 times higher than the absolute value of pull out torque of slip ring motor.


7. The details of the external resistance of the existing motor.



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