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The effect of rotor skew, cusp and cogging on motor startup

Publication date: 
May 2015
Chuck Yung, EASA Senior Technical Support Specialist
Type of media: 

Anyone who has spent much time reading the EASA AC Motor Redesign book, or motor design textbooks, will recall that there are certain stator-rotor slot combinations that can detract from the performance of an electric motor.

Most of these are readily identified by counting the number of stator slots and rotor bars, then comparing the difference to the table presented in this article. The number of poles is a factor, which is why we should always consult the table when changing motor speed.

Topics discussed in this article include:

  • Stator slot-rotor bar combinations
  • Motor starting
  • What is rotor skew
  • Benefits of rotor skew
  • Cusps
  • Cogging
  • Operating on reduced voltage
Rotor bar shapes