Zero speed sensorless control signals of induction motors with closed rotor slots

Realising control of standard induction machines at low and zero speed without mechanical sensors implies the use of parasitic nonfundamental wave effects. Different schemes have been proposed in the past which exploit the inherent saliencies of saturation and/or rotor slotting to detect the main flux and the rotor position. They all use a high frequency or transient signal in addition to the fundamental wave excitation which is impressed by the inverter. The machines reaction on this high frequency excitation is measured and the flux and/or rotor position signal is extracted by special algorithms. However, this transient electrical behaviour and the resulting control signals of an induction machine are strongly influenced by the design of the machine and especially the shape of the lamination. The two most prominent saliencies present in standard induction machines are caused by saturation and slotting, enabling thus either sensorless flux or rotor position detection. Especially the rotor position detection suffers from a strong dependence on the slot geometry of the lamination. Yet the details for this influence are not quite clear and have not been addressed in literature. In this paper measurements were performed on machines giving a comparison of closed and opened rotor slots. With the help of measurement windings placed along the air gap the spatial distribution of the transient leakage flux caused by the high frequency excitation is determined to make an interpretation of the transient behaviour more accurate. The measurements made in this paper are based on a transient excitation of the machine with voltage pulses. The results are however also applicable to high frequency sinusoidal excitations.