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

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?