Torsional Stress Analysis of Squirrel Cage Windings of Asynchronous Motors

In the last decades the use of asynchronous machine in industrial and traction applications has been widespread also aided by the development of electrical drives and control techniques. It has been noted that distorted feeding voltages causes, in some cases, the breaking of squirrel cages. In order to investigate these phenomena, in the paper, authors have studied the mathematical model of the mechanical behaviour of the rotor cage of an asynchronous motor. A calculus procedure for obtaining an analytical solution of this mathematical model is suggested. The analytical solution expresses the natural torsional frequencies of the rotor cage as a function of the bar overhang and stiffness. The theoretical procedure presented can be used as a design guideline in order to prevent the breaking of the end-bar junctions when the motor is supplied by PWM inverters. The theoretical procedure is completed by a numerical analysis.