Sensorless position detection for vector-controlled induction motor drives using an asymmetric outer-section cage

This paper describes a new method of obtaining a rotor position signal from a cage induction machine operating without a mechanical sensor. The method is based on introducing a circumferential variation in the resistance of a high-resistance outer-section cage of an induction-machine rotor. Simulation results using a linear "double-cage" machine equivalent show that the method is feasible and provides incremental rotor position tracking with good dynamics. The paper describes an implementation of the method for a 30 kW double-cage machine having variable-gauge copper wire in the outer slots. It is shown that, while the rotor position-dependent signals are robust to changes in load, interference harmonics arising from slot saturation and rotor slot harmonics cause problems for rotor position tracking. These problems are discussed in relation to the present method and that of designed asymmetries in general.