Electromagnetic Force Calculation and Structural Static Analysis of the Rotor in Squirrel-Cage Induction Motor with Broken Bars during Startup

During startup, a motor will draw anywhere from five to eight times the normal current required to run under full-load condition. An additional wearing effect of repeated starts or over an extended period of time is to cause expansion and contraction of material, and it will tend to become brittle and crack. In this paper, the electromagnetic force and structural static of the rotor in induction motor during startup are analyzed. A new method combining the Maxwell stress and the principle of the Lorenz force was proposed for electromagnetic force, and the effectiveness of the method has been proved by the torque obtained from local Jacobian derivative method (LJDM) and experiments. The local magnetic stress of each node for the healthy and faulty rotor can be got based on the transient magnetic fields. Based on the result of magnetic force, the distribution characteristics of rotor deformation are achieved, and some valuable conclusions can be drawn on the optimized design of the induction motor. Finally, the usual point of bar breakage and the reason for successive occurrence of the cage fracture can be founded from the perspectives of magnetic stress and structure mechanics.