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how to calculate the external resiatnce for rotor of slip ring motor


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i want to installed a new slip ring motor to o.h crane main hoist of 3 ton .

the motor is


5 kw , 935 rpm , 6 pole , 50 hz , 400v stator in delta , 12.5 A

rotor is 90v , 35 A


how can i design the external resisatnce box and how calculate the step of resisatnce for each step. waht should be the wattage for each resistor element





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Hello tahira


There are a number of reasons why people use slip ring motors and the selection of the resistors is very dependant on the characteristics that you are after.


The important point to consider, is that the torque produced by the motor is related to the power disipated in the rotor resistors at that speed.


At any speed, maximum torque is produced when the resistive and reactive components of the rotor circuit are equal.


You can determine the effective inductance of the rotor from the open circuit rotor voltage and the short circuit rotor current. In this case, 90v 35A.

(Under short circuit conditions, the rotor current is essentially limited by the inductive reactance of the circuit)


As the rotor accelerates, the frequency of the current flowing in the rotor reduces, and therefore the rotor reactance reduces also. Therefore for maximum torque at that speed, the resistance must also be reduced.


If you require maximum torque across the full speed range, then size the resistors as above at a number of evenly spaced speeds between zero and full speed.


Sometimes, the resistors are used to provide a means of speed control by varying the torque output from the motor. In this case you need to approximate the torque output at the different speeds and select the resistors to provide that torque.


The power ating of the resistors is a simple calculation of I x R and the time that that power is dissipated for.


Best regards,

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A thought,

Is this topic worthy of a page on the web site??

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Asad, Please email or U2U me if you are still chasing this information. I have some software that may help you.


best regards

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  • 4 months later...
It depends on what type of control system you are employing.you need to be aware that it is possible for the motor to run over syncronous in the lower direction under heavy loaded conditions and that reducing external resistance in the lower direction (loaded) will cause the motor to run slower (above syncronous). I.E the amount of external resistance which gives slow speed hoist will give fast speed lower under loaded conditions. Some hoist control systems employ counter torque where-by in the lower direction the motor is connected to hoist and rotor the rotor contactors are controlled by either monitoring rotor voltage or frequency.
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