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. 

Skewing bars 
Skewing the rotor or stator laminations can re­duce the flux path variations to reduce the magnitude of the harmonics. This smoothes out the speed vs. torque curve (see Figure 1). Skew is the angular twist of a rotor or stator slot away from the axial direction, illustrated in Figure 2. 

There are other ways of skewing bars, such as herringbone (or V-) shaped bars, as well as straight bars that are off­set in the center of the stack. But most often, the bars will be angled as shown in the figure. 

The purpose of the skew is to reduce har­monics in the air gap flux that are introduced by a certain stator-rotor slot combination. The rotor is more commonly skewed than the stator, because coil insertion becomes more difficult, and slot opening cross-section decreases, with skewed slots. In an ideal situation, the motor manufacturer would have an unlimited number of stator and rotor lamination options; however, the cost would be prohibitive. They are limited to laminations that can be used for as many different designs as possible. The same ro­tor laminations may be used for both a 4- and a 6-pole motor, but the stator-rotor slot combination may not be favorable for both speeds. 

Degrees of skew 
There are, of course, varying degrees of skew, but typically the rotor is skewed whichever dis­tance is greater: one stator slot or one rotor slot. Too little skew may not effectively reduce the har­monics, while too much will only further increase losses. If there are more stator slots than rotor bars, the bars will normally be skewed one rotor slot. If there are more rotor bars than stator slots, the offset will be one stator slot. 

Rather than using a skewed rotor design, one other way that manufacturers have overcome torque dips due to the stator-rotor slot combina­tion is to design a stator winding with higher flux densities, which results in a higher overall torque. The increased torque may result in overall starting and acceleration torque levels that meet or exceed the requirements of the application, even with the potential torque loss from the harmonics. Of course, the tradeoff to this is a higher locked rotor current, but manufacturers overseas follow IEC standards that do not have the limits for starting currents that NEMA does. 

While a skew will smooth the torque curve and reduce noise, the advantages are not without a cost. The skew will increase the stray load losses, decreasing the overall efficiency, and torque. Since the rotor bars are not parallel to the shaft, there is not a uniform circumferential force in the air gap. This introduces a small axial com­ponent to the flux that reduces the overall torque, since it is no longer parallel to the shaft. Figure 3 illustrates the effect. 

Will often run hotter 
Motors with skewed rotors will very often run hotter due to the increased losses, but the torque or noise benefits are an acceptable tradeoff for the manufacturer. However, it should be noted that if a rotor that was not originally skewed is re-stacked with a skew, the motor may run hotter than expected. 

With straight rotor bars, there is virtually no induced voltage between rotor bars and lamina­tions. But with a skew, there is some induced voltage, and current can flow through the lami­nations, increasing the heat. Some skewed bars have an insulating coating that helps to minimize stray currents. 

In addition, the bar length is slightly more for a skewed bar than a straight bar, so the resistance is slightly higher in skewed rotors. This, in turn, will increase I2R losses, increasing the heat and result­ing in slightly less torque.