The use of skewed slots (also referred to as helical or twisted slots) in motor rotors is a very common and critical design feature. Its primary objective is to address issues related to noise, vibration, and additional losses that arise during motor operation. In contrast, if a rotor were to employ straight slots, the rotor bars would occupy identical magnetic field positions along every segment of their axial length. Consequently, should a particular harmonic magnetic field be sufficiently strong, the electromagnetic forces acting upon the rotor bars along the axial direction would be entirely in phase. This implies that the entire rotor would be subjected to a powerful, high-frequency, periodic radial force wave. The principle behind skewed slots is that, by twisting the rotor bars at an angle along the axial direction, the magnetic field phase experienced by the bars varies at different axial positions. Ultimately, this arrangement causes the electromagnetic forces generated by harmonic magnetic fields on the rotor bars to mutually cancel out along the axial axis, thereby significantly attenuating the radial force waves responsible for inducing motor vibration and noise.
For squirrel-cage Induction Motors, skewed slots serve another vital function: increasing starting torque while simultaneously mitigating the phenomenon known as "crawling." At the instant of motor startup, the rotor frequency is extremely high (equal to the supply frequency), resulting in a pronounced skin effect within the rotor bars. Skewed slots disrupt the synchronous coupling between the stator's harmonic magnetic fields and the rotor bars, thereby preventing the generation of asynchronous parasitic torques. In specific slot-combination configurations where "crawling"—a phenomenon in which themotor stalls at a low speed during startup and fails to accelerate further—might otherwise occur, skewed slots effectively eliminate this "dead point," ensuring that the motor can start up smoothly. Furthermore, skewed slots contribute to a reduction in electromagnetic noise; as torque pulsations are smoothed out, the level of electromagnetic noise is significantly lowered.