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I want to determine the resistance of a rotor bar
in a squirrel-cage induction motor, including the skin effect. I tried using
FEMM but the results are not looking right.
I drawed the rotor bar in FEMM, created a boundary
(Prescribed A with all the parameters zero) so that no flux should cross tothe
outside of the bar, as was done in the example on skin effect on the website.
(Is this the right method for this case, or must I draw the whole induction
motor?)
I took a bar length of 1 metre for simplicity
sake (Problem -- depth = 1000), and used a circuit property to have a current of
50 Ampere flow through the bar. (Bar material = copper)
To calculate the DC resistance, I used theBlock
integral in FEMMview to find the area, and then calculated the resistance to be
359 microOHM
When using the block integral for resistive losses
(problem frequency = 0Hz) I get 0,898 Watt which corresponds to my calculation
using P = I^2*R
However, when I change the frequency to 50Hz, the
resistive losses are 0,449 Watt, which is less than the loss at 0 Hz! (I wanted
to use this loss to determine the effective rotor-bar resistance at 50
Hz)
Am I doing anything wrong? or understanding
anything wrong?
I would appreciate any help!
Ivan Slabbert
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