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Rotor resistance calculation for a slipring motor


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I need a starting torque of 250% FLT for a slipring motor 60HP,415V,1500 rpm with Rv 573V & Ir 49A for a wire drawing machine. A resistance of 3.5 ohms can help in generating the required torque? I think it will not. Reason is K value is Rv/rt3*Ir = 5ohms(approx) ie a resistance of 2K will be 10 ohms. Is it correct? Kindly help me.;e;
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Hello asgkrishnan


Welcome to the forum.

As you add resistance to the rotor circuit, you shift the maximum torque to a higher slip (lower speed). You do not change the maximum torque.

If you need to have 250% torque over the whole speed range from zero to 100% speed during start, then you will need to have a number of resistance stages with the max torque speeds effectively steped up the speed range.

Download our Electrical Calculations software from http://www.LMPhotonics.com/busbar32.zip and you can calculate the resistors for multistage rotor resistance starters. The more stages you use, the higher the average start torque due to less droop before the next stage take over.


Best regards,

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Thank you very much sir for your quick reply. As suggested by you, Iam not able to down load the file busbar32.zip. Elect calculation software . I think some problem is there. Thank you. But you did not clarify my doubt that what value of K is to be taken for whole resistance calculation i.e., 2K or 1K or 0.7K . for rotor resistance speed control. In cranes, for long travel, cross travel etc total resistance value is taken 2K. and for thyristor drive it is 0.7K. Kindly clarify.
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hello asgkrishnan


For you application, it depends on the torque speed curve that you want. If you want maximum speed at zero speed, you use a much higher value of resistance than if you want the maximum torque at say half speed.

The slip at which maximum torque is developed is the slip where the rotor reactance is equal to the rotor resistance.

You can calulate the rotor reactance at 100% slip by taking the rated open circuit rotor voltage divided by the rated short circuit rotor current. As you reduce the rotor slip, the reactance reduces.


Ther is no magic K figure that I am aware of, you need to consider the application and use the rotor resistance to create the torque curve(s) that you require.


I am sorry that you couold not download the software, I have found this to be a problem when using MS Explorer 6. If you use a browzer like Mozilla Firefox, you may not have the same problem.


Have a good day,

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