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Slip Ring Motor Resistance Calculation


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we Have a slip ring induction motor with the following details

11kv/174 amps stator

rpm 994/6pole

rotor voltage 1425

rotor current 1125 amps

rotor resistance between phases 0.00647 ohm

The application is for centrifugal fan .the fan rated torque is 21582 NM .


Earlier we had speed variation from 60 to 100%.speed is being varied with grid resistance (grr).but the speed linearity was not equal in the final stages.whenever we reduced one step fell from full speed the speed dropped down to 940 rpm and affected process perfomance.



we have procurred one new grr which allows speed variation of 20% in each step.We have not put the grr in to service as it needs shutdown of the plant.


The present grr resistance is in delta with two branches parallel with the following resistance values

r1 0.069 ohms parallel with r11 0.069

r2 0.138 ohms parallel with r12 0.138

r3 0.277 ohms parllel with r13 0.277

r4 0.554 0hms parllel with r14 0.554

r5 1.109 ohms parallel with r15 1.109


the speed is varied in 32 steps from 60 to 100%


my queries are


what will be the current in each stage

how to calculate these resistance values for each steps







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Hello ram


This is quite a complex problem because the speed and power of the motor is related to the speed torque curve of the motor in addition to the slip curve of the motor with the different resistors in circuit.


You should plot the speed torque curve of the load, and superimpose the speed torque curves of the motor with the different values of rotor resistance. The speed with each resistance is the intercept between the two curves.


The current is dependent on the slip losses at the intercept points.


Best regards,

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