EMF-based rotor flux detection in induction motors using virtual short circuits

This paper presents the practical implementation of EMF-based flux-detecting algorithms for induction machines using current measurement only. The basic idea is to measure the reaction of the stator current on the change of the rotor flux during the "short circuit at machine terminals" inverter state. A generalization is possible by a combination of two different active states of the inverter. By a linear transformation of the electrical equations for both inverter states, a mathematical zero state is constructed. The use of this virtual short circuit is necessary at higher speed when the inverter output voltage is fully utilized. Both variants were implemented in real time on an induction motor drive with an insulated gate bipolar transistor (IGBT) inverter and floating-point digital signal processor (DSP). In order not to disturb the current controller, the algorithms use naturally occurring inverter switching states only. Measurement results during load steps demonstrate the excellent flux information of the method.