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Electric motor basics: Why are rotors skewed?

  • December 2003
  • Number of views: 10714
  • Article rating: 4.1

Cyndi Nyberg 
Former EASA Technical Support Specialist 

Stator windings in three-phase motors are de­signed to have the amount of flux that the core needs to produce the desired output. The number of turns and size of wire are limited by the core dimensions. However, in the squirrel cage rotor, there are many more variables in the design. One of the variables is the shape of the rotor slot. Many rotor designs are skewed. So, why are ro­tors skewed? 

As the rotor turns, discontinuities on the sur­face of the rotor and stator disrupt the magnetic flux path of the motor. The flux path variation shows up in the form of harmonics that affect the performance of the motor. The difference be­tween the number of stator slots and rotor slots has a significant impact on the harmonics. The motor may be noisy, or there may be stray torques that lower the torque during starting or acceleration. The stator-rotor slot difference is why a motor winding that is redesigned for a dif­ferent speed may have problems, and why it is important to check the stator-slot-rotor-bar com­bination before proceeding with the redesign. 

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EASA Technical Manual

More information on this topic can be found in EASA's Technical Manual
  • Section 2: AC Machines

Related Reference and Training Materials

Principles of Medium & Large AC Motors (NEMA Standards)
  • Section 8: Squirrel Cage Rotors

Principles of Medium & Large AC Motors (IEC Standards)
  • Section 8: Squirrel Cage Rotors

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