Impact of the point of operation on sensorless control of induction motors based on the INFORM method

All control schemes of induction machines that work without mechanical sensor even in the region of zero fundamental stator frequency have to rely on parasitic effects in the machine. Ode of these schemes is the INFORM method (`Indirect flux detection by online reactance measurement´.It detects local differences in the transient stator impedance of the machine by evaluating the current change due to voltage pulses applied to the terminals of the machine. The INFORM method is usually applied to detect the saliencies caused by main flux saturation and therefore does not need a special rotor design. To estimate the main flux position in the whole magnetic and electric range of operation it is however necessary to determine the change of the saliencies in the machine with respect to the point of operation. In this paper, these dependencies are shown in the signals obtained from evaluating the transient current change as well as in flux-change measurements made with measurement coils at different spots in the machine. It is shown that some of the dependencies can be used as additional input for the sensorless control and that others can be eliminated with a simple structure