skewing and saturation

[dazhoid]

Hi,
I have a 250 kW induction motor which has a high saturation level since rotor was changed from a non-skewed to a skewed one (one slot pitch)... Is there a correlation between saturation and skewing ??

Thanks for your answers,

 

[electricpete]

Out of curiosity, what is the indication of saturation that you see?

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[electricpete]

The induction motor handbook:

The Induction Machine Handbook section 4.13
The term outside the square brackets in (4.62) is zero if there is no skewing. Consequently the term inside the square brackets is the compensated or the magnetizing mmf which tends to be small ....and thus the skewing s small and the angle..

Therefore, we define F2skew by (4.63) [not transcribed]

the uncompensated rotor-produced mmf, which is likely to cause large airgap flux densities varying along the stator stack length at standstill (and large slip) when the rotor currents (and F2m) are large. Thus, heavy saturation levels are expected along main flux paths in the stator and rotor core at standstill as a form of leakage flux, which strongly influences the leakage inductances as shown later in this book.

There is a lot more that I can't easily transcribe (equations and words). The gist that I get is that the stator load current flows to match/cancel the mmf of the rotor load current. But since the stator is not skewed, it cannot possibly cancel the axially-varying component of the rotor mmf. This extra uncanceled component of mmf on top of the normal airgap exciting mmf apparently leads to saturation.

That's what I get out of the book. But it's a little above my head.


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[electricpete]

Just to clarify, the heading of the section is rotor skewing so the conclusion applies to skewed rotors.

The expression given for rotor mmf that includes a "compensated" portion which doesn't vary with y (axial distance) and an "uncompensated" portion which does vary with y. The uncompensated axially-varying portion of rotor mmf is the one that can't be cancelled by stator mmf.

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[electricpete]

The Electric Motor Handbook (Beaty and Kirtly) and Handbook of Small Electric Motors (Yeadon) both give expressions for increase in rotor leakage reactance associated with skew. I think this is another way of describing the same thing discussed above. An increase in rotor leakage reactance means there is flux associated with the rotor that is not linked with the stator (and hence cannot be cancelled/compensated by stator load current). That extra flux (which I believe is the axially-varying component of the rotor flux) increases flux density in various parts of the iron (I suspect more in the rotor than the stator)

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[dazhoid]

Thanks. I assumed there was higher saturation because a vibration saturation harmonic was much higher...