Experimental study using trail rotors to study the inter-bar current of three-phase squirrel-cage induction motors

At present, many types of induction motors (IMs) are used as the power sources of industrial machines. Generally, middle and small size IMs have skewed slots in the rotor bar, which is made of aluminum die cast, to reduce abnormal starting torques and electromagnetic vibration and noise. The rotor iron and rotor bars are shorted, and an electrical potential difference is generated between the rotor bars by skewing, so that the current, which is called inter-bar current, flows in the direction of rotation. However, many unclear issues exist with regard to the inter-bar current. Therefore, we have experimentally studied the effect of the inter-bar current on the characteristics of an IM. In a previous paper, a trial rotor was investigated by using the finite element method (FEM). In this paper, the effects of the inter-bar current on the characteristics of an IM are demonstrated through experiments with many test motors, which include some trial rotors that follow the previous results. In particular, it is shown that the stray load torque in the reverse rotational test is remarkably reduced when the rotor is heated to about 500◦C (abbr. post-heating), and that its influence extends the improvement of the starting torque and running performance. Furthermore, the resistance between the rotor bars are measured using ultra-low resistance meter. © 2019 The Institute of Electrical Engineers of Japan.