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The rotor in the squirrel cage induction motor are shorted end to end by aluminium bars; however the are placed parallel at a certain skew angle.

Wikipedia states that "The conductors are often skewed slightly along the length of the rotor to reduce noise and smooth out torque fluctuations that might result at some speeds due to interactions with the pole pieces of the stator ."

However I am unable to understand the above statement.

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    \$\begingroup\$ Why is helical gearing quieter than straight gearing? Same here. \$\endgroup\$
    – user16324
    Commented Aug 31, 2015 at 11:24
  • \$\begingroup\$ Can you be more specific ?I understood with respect to gears but I am not able to apply the analogy here \$\endgroup\$ Commented Aug 31, 2015 at 11:29
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    \$\begingroup\$ Analogy: Poles=Teeth. \$\endgroup\$
    – Dzarda
    Commented Aug 31, 2015 at 11:57

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The problem is that the rotor and stator must have spaces called slots, in the iron for the conductors. The conductors and air in the slots have a lower reluctance than the reluctance of the iron. There are more slots than there are poles in the stator. The windings are distributed among the slots to form distributed poles. Their is a different numbers of slots used for the stator and rotor. Those two features prevent the rotor from becoming locked or "cogged" in the minimum reluctance position that would otherwise exist.

Even with proper selection of the number of stator and rotor slots, there can be reluctance variations at different rotor angles that cause torque fluctuations and inflections in the torque vs. speed curve. Skewing the rotor slots mitigates that effect.

Here is a picture of the rotor and stator of a 4-pole, permanent-split capacitor, single-phase induction motor.

Rotor and stator of a 4-pole, permanent-split capacitor, single-phase induction motor

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  • \$\begingroup\$ What is the best skew angle? What is consideration when we would like to design a rotor-stator? Would it be relative to the stator? I also have the same question about this. \$\endgroup\$
    – Sitorus
    Commented Nov 9, 2020 at 2:58
  • \$\begingroup\$ @Sitorus; Lipo, "Introduction to AC Machine Design" mentions skewing the rotor slots by one rotor slot pitch. Skewing the rotor slots by one stator slot pitch is also mentioned. Skewing the stator slots in addition to the rotor slots is also a possibility. There are recommended combinations for the number of stator and rotor slots and certain combinations to be avoided. Recommendations vary by the number of poles. \$\endgroup\$
    – user80875
    Commented Nov 9, 2020 at 14:18
  • \$\begingroup\$ "skewing the rotor slots by one rotor slot pitch", make sense. Thank you for your explanation. \$\endgroup\$
    – Sitorus
    Commented Nov 14, 2020 at 14:40
  • \$\begingroup\$ If you don't mind, I have question regarding universal or DC motors, which the rotors has their own winding (rather then using permanent magnet), but not skewed as in the induction motor. Is any explanation about it? Are they different? \$\endgroup\$
    – Sitorus
    Commented Nov 14, 2020 at 14:43
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Due to the interference between the stator and the magnets(in case of BLDC) the torque ripple will happen.It will tends to the noise and vibration at lower speeds. To avoid this problem they made angle in the stator slots(called skew angle) by this the inerface between pure stator portion without coil doesn't meet the magnet. So there is no ripple in torque and smoother motion in low speedsenter image description here

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