Sensorless acquisition of the rotor position angle of induction motors with arbitrary stator windings

The anisotropy of a cage rotor is utilized to determine the angular position of the rotor in an induction machine. The switching transients generated by a pulsewidth-controlled inverter serve as test signals. The response of the three-inverter terminal currents is exploited to derive a quasi-instantaneous rotor position signal. The position is sensed at the inverter through the three-phase motor cable by measuring the current derivatives. The method does not require additional wire connections. It is applicable to induction motors having stator windings connected either in wye or in delta. The results are supported by measurements from an experimental setup.

I. Introduction

The CAGE rotor of induction machines is magnetically anisotropic. This is owed to the presence of its discrete rotor bars, being embedded in the respective slots of the rotor iron. The rotor anisotropies make the high-frequency impedance of the polyphase stator winding vary as a function of the rotor position angle. A transient excitation of the machine, superimposed on the stator voltages in any given operating condition of the machine, therefore produces a response of the stator currents that depends on the rotor position angle. A response signal can be extracted and processed in a suitable manner so as to derive a rotor position signal. Such a signal is independent of the mechanical speed of the rotor; it can be obtained without compromise at very low speed and at zero speed [1]. It is well suited as a feedback signal for closed loop control of rotor position, or speed. Application of interest includes sensorless control of the angular rotor position, or, alternatively, sensorless speed control to promote stable operation in the low speed range and at zero speed [2]. Such performance attributes are difficult to obtain with more traditional sensorless techniques based on the fundamental model of the machine [3].