My questions, as stated in the title, is how does skewed conductor on the rotor of squirrel cage decreases crawling affect?

I've gone through articles in about cogging and crawling affect on induction motor, but I still don't fully understand how skewed conductor help decreases the affect of the low torque created by the 7th harmonic. The only explanation that I've found is...

"Skewing primarily helps in removing crawling phenomenon by reducing slot harmonics, as if rotor bars are skewed by such an angle where the rotor bar terminations fall under two same poles (order of harmonic X no. of poles) created by harmonic fluxes in stator and as a result production of same polarity emf takes place in the termination and they effectively gets cancelled.."

So my understanding is since slot harmonics depends on the number of rotor bars, by skewing them there is now less of the rotor bars and henced improve performance. So by doing this mechanical design, electrically we see an improvement in motor efficiency..am I on the right track?


1 Answer 1


The crawling effect failure of the motor to accelerate to full speed when starting. Rather than accelerating to the intersection of the load to the intersection of the load torque vs. speed demand curve and the motor torque vs. speed capability curve at a point near synchronous speed, the motor accelerates to a slower operating point. That operating point is at a low point in the motor curve where the curves cross at a lower speed. That low point is partly the result of harmonic torque subtracting from the fundamental torque of the motor. Rather than accelerating to point k on the curve shown below, the motor only accelerates to point g.

enter image description here

Skewing the rotor slots prevents a large area of a rotor slot from being aligned with a large area of a stator slot. When that occurs, there is less stator iron area that is only separated from the rotor iron by the narrow stater-rotor air gap. That means that some angles of the rotor position with respect to the stator have a higher reluctance. That reluctance variation causes the harmonic torque components that distort the motor's normal torque vs. speed capability curve.

The slot harmonics are influenced by the number of rotor slots compared to the number of stator slots, the width of the stator and rotor slots, the amount of skew and, to a lesser extent, the shape of the slots near the stator and rotor surfaces.

The impact of the harmonic torque components is influenced by the shape of the fundamental torque vs. speed capability curve, particularly any dip in the curve between the locked-rotor and pull-out points. That is influenced by the depth and shape of the rotor bars.

See also:

Why is there a pull-up torque during the starting of an induction asynchronous motor

Skew angle in squirrel cage induction motor


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