The influence of the skewed rotor slots on the magnetic noise of three-phase induction motors

The skewed rotor slots were originally used to provide starting torque when the motors had the number of the stator slots equal to the rotor slots. This technique was kept later even though knowing that the starting torque could be provided choosing a proper number of rotor slots. It was proved that other negative influences could be reduced when skewed rotor slots are used. The skewed slots generate an additional leakage flux in the machine, reducing the useful flux. So, a reduction of the electromagnetic coupling between stator and rotor occurs. The skew factor indicates the reduction of the electromagnetic coupling. For the harmonics, the electromagnetic coupling is reduced as the order of the harmonic is increased. In some cases, the effect of the harmonics is canceled. At WEG, the investigation and analysis of the causes of the magnetic noise are usual procedures. The electrical machines are analysed during the design stage. Simulation software is used in order to give to the designers the best configuration to minimize magnetic noise and vibration. The two most useful techniques are the proper choice of stator and motor number of slots and the skewing of the rotor slots. The aim of this paper is to analyse the influence of the skewed rotor slots on the magnetic noise. Tests results for seven three-phase induction motors are shown. Its innovative contribution is to provide practical results for the main frequencies of the three-phase induction motor noise spectrum as well as the overall noise for several rotor slots skewing