Rotor Slots in Squirrel Cage Induction Motor – Top 5 Factors to be Considered by Electrical Design Engineers while Specifying Number of rotor slots

While manufacturing squirrel cage rotor motors, one of the most important aspects which Design Engineers should consider is the number of rotors slots in reference to the number of selected stator slots. Rotor slots are abbreviated using Sr while stator slots are abbreviated using Ss.

Electrical Design Engineers should carefully select appropriate combination of stator and rotor slots. Since improper combination might result in crawling, cogging, noise production, vibrations, or synchronous cusps.


Crawling in electrical machines refers to the phenomenon in which harmonic torques are generated that cause machine to continuously run at harmonic asynchronous speeds such as 1/7th, 1/11th, 1/13th of the sub-synchronous speed. In order to avoid crawling Sr should remain between 70-130% of the value of Ss.


Cogging in electrical machines happen when stator and rotor slots are equal. This consequences that speeds of all the harmonics generated by rotor as well as stator coincide, which in turn has corresponding synchronous torques of rotor and stator that cancel out each other. As a result of this net torque becomes zero and motor will not start due to zero net torque.

To avoid cogging:

  • Sr ≠ Ss
  • Sr ≠ ++ 3
  • Sr ≠ 3p, 6p , 9p, 12p

Synchronous Cusps

The term synchronous cusps refers to the harmonic effects that can happen in machine as a result of appropriate combination of stator and rotor slots. To avoid synchronous cusps:

  • Sr ≠ Ss ± p
  • Sr ≠ Ss + 2p
  • Sr ≠ Ss ± 5p

Noise production

Production of noise because of machinery operations is undesired. Since it is unpleasant for ears as well as it has environmental impacts. To avoid noise:

  • Sr ≠ Ss = ±1
  • Sr ≠ Ss ±2
  • Sr ≠ Ss ± (p ± 1)
  •  Sr ≠ Ss ± (p ± 2)

Vibration production

The production of vibrations in machines impact the motor parameters in longer run, as well as these are dangerous especially when the level of vibrations is very high. To avoid vibrations:

  • Sr ≠ Ss = ±1
  • Sr ≠ Ss ±2
  • Sr ≠ Ss ± (p ± 1)
  •  Sr ≠ Ss ± (p ± 2)


FactorsRotor Number of Slots
CrawlingSr should remain between 70-130% of the value of Ss
CoggingSr ≠ SsSr ≠ ++ 3Sr ≠ 3p, 6p , 9p, 12p
Synchronous CuspsSr ≠ Ss ± pSr ≠ Ss + 2pSr ≠ Ss ± 5p
NoiseSr ≠ Ss = ±1Sr ≠ Ss ±2Sr ≠ Ss ± (p ± 1) Sr ≠ Ss ± (p ± 2)
VibrationSr ≠ Ss = ±1Sr ≠ Ss ±2Sr ≠ Ss ± (p ± 1) Sr ≠ Ss ± (p ± 2)

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