Analytical modeling of variable impedance induction machines

A new concept of a variable impedance (Z) induction motor (VZIM) is introduced in order to combine high starting torque and high efficiency at the nominal operating point. In this type of induction motors a sectionalized cage rotor with different bar shapes for each sub-cage section is used. At standstill, sub-cages with high resistances are placed inside the stator bore to increase the starting torque and reduce the starting current. By increasing the rotational speed, a centrifugal mechanism moves the rotor in the axial direction and brings the other sections of the rotor cage with low resistances inside the stator bore to reduce the rotor losses at rated speed. This leads to increased efficiency at nominal operation. An analytical model is presented to predict the performances of the proposed variable impedance induction machines. A 5.5 kW, 4 pole prototype motor has been built and tested. Measurement results prove the accuracy of the presented analytical model.