effect of skew on load angle

[dazhoid]

Hi,

After my question on the influence of skewing on saturation, another question has came to me : what is the influence of skewing on the load angle of an induction machine ?

We usually think in 2D, where the load angle is the phase angle between stator and rotor mmf, but when the rotor is skewed this load angle should wary in the axial direction (assuming that rotor bar currents do not change in axial direction)...
and if the load angle varies, that means that the torque developed in the machine changes in the axial direction, which is quite hard to imagine (that would mean a torsion force on rotor structure)...

Or do rotor currents change in axial direction in order to keep a constant load angle ???

Thanks for your remarks.

 

[electricpete]

Take a snapshot in time and examine the profile of the rotor mmf along the axial dimension. If the peak of the rotor mmf is at 12:00 position at one end of the motor, then it may be at 12:15 position in the middle and 12:30 position at the other end (rotates by one bar pitch).

The stator mmf has no such variation (perahps tapered toward both ends due to leakage, but not a skew toward one end or the other).

IF (*) we wanted to determine the torque as a function of torque angle, the rotor mmf angle to use would correspond to that at the middle of the rotor. Toward one end the angle is slightly bigger and toward the other end slightly smaller, but the average in the middle.

* I am not so sure that talking about torque angle is productive in the context of induction motors. If we assume the polarity conventions under which a sync motor has a torque angle in the range 0 < TorqueAngle<90, then we would see that an induction motor has a torque angle in the range 90<TorqueAngle<180. Question: How can it operate in a range which is "unstable" (increasing load causes increasing torque angle causes decreasing torque)? Answer: The torque is not controlled by the torque angle, it's controlled by the slip which has a stable characteristic.

I don't work with drives. There may be some other defintion of torque angle out there in the drives world that is different than what I am familiar with. The definition I use is the angle between stator mmf and rotor mmf.

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