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Old 02-13-2015, 09:14 AM   #1
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Default Torque vs resistance

Quick question,

In an induction motor why does torque increase as rotor resistance in increased? I know that torque is inversely proportional to speed so increasing resistance reduces the speed hence it should increase the torque but I want a more logical(yet simple if possible) explanation. Thank you.
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Old 02-13-2015, 12:13 PM   #2
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What do you mean by "rotor resistance"?
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Old 02-13-2015, 01:45 PM   #3
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Higher resistance with constant supply voltage means greater current. Greater current flow means greater electromotive force. Greater force with the same length of torque arm means more torque.

Thats what I came up with anyway. Maybe im wrong.
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Old 02-13-2015, 02:12 PM   #4
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Default Rotor don't use resistance per se

Parts of a motor
Stator = stationary part of a motor that contains the windings.
Rotor = rotating part that has bars for the magnetic field (no windings).
Armature = rotating part of a motor that has windings
Commutator = part that conducts voltage to the armature.
So your question should be why does the stator resistance change the torque, see answer above about more electro motive force.
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Old 02-13-2015, 02:21 PM   #5
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The breakdown/maximum torque of a motor occurs when the rotor resistance and rotor inductive reactance are equal. Beyond that the motor stalls.

The resistance is fixed, but the reactance starts out low and increases as slip increases. So if the rotor has been built with a higher internal resistance, there will be more "room" for it to accommodate a higher reactance(caused by higher slip) before the breakdown torque point is reached. This pushes the peak torque point down to a lower rpm.

A wound rotor motor would allow you to adjust the rotor resistance to shift the breakdown torque point up and down the rpm range.
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Old 02-19-2015, 10:46 AM   #6
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Quote:
Originally Posted by John Valdes View Post
What do you mean by "rotor resistance"?
I mean like in a wound rotor induction motor when you add resistance to the rotor circuit the torque increases.
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Old 02-19-2015, 11:07 AM   #7
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By adding more resistance in the rotor circuit, motor goes to a different characterictic point where motor developes less speed. Therefore more torque is generated. It's all about the ratio of speed and torque.
Hope it helps.
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