Design and modeling of a Variable Impedance Induction Motor

Basic principles of a new type of induction motor, called Variable Impedance (Z) Induction Motor (VZIM), are introduced. The main goal of implementing this new structure of induction motors is to counteract the existing compromise in the design stage of induction motors between high starting torque at standstill and high breakdown torque and high efficiency at rated speed by combining these features, using a special motor structure. A sectionalized cage rotor with different bar shapes for each sub-cage section is used. In a 3-section rotor configuration, two sub-cages with high and medium resistance are placed inside the stator bore at standstill, to increase the starting torque and to reduce the starting current. During start-up of the motor, a centrifugal mechanism moves the rotor into the axial direction and puts the third section of the rotor cage with low resistance, together with the middle sub-cage, inside the stator bore to reduce the rotor losses at rated speed, which leads to increased efficiency at nominal operation. A 22 kW, 4 pole prototype motor has been built and tested. In this paper the design procedure, the simulation and measurement results of the test motor are presented.