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Abstract-A novel open-loop speed control method for induction motors that provides high output torque and nearly zero steady-state speed error at any frequency is presented. The control scheme is based on the popular constant volts per hertz (V/f) method using low-cost open-loop current sensors. Only stator current measurements are needed to compensate for both stator resistance drop and slip frequency. The scheme proposed fully compensates for the current-resistance (IR) voltage drop by vectorially modifying the stator voltage and keeping the magnitude of the stator flux constant, regardless of changes in frequency or load. A novel slip frequency compensation, based on a nonlinear torque-speed estimate, is also introduced. This method reduces the steady-state speed error to almost zero. It is also shown that a linear torque-speed approximation is a special case of the nonlinear estimate and that it leads to large speed errors for loads greater than rated. It is shown that, by using the proposed method, the speed can be accurately controlled down to, at least, 1.2 Hz with load torques of more than 150% of rated value. Since the only machine parameter required, the stator resistance, is automatically measured at startup time, using the same pulsewidth modulated voltage-source inverter without additional hardware, the proposed drive also exhibits self-commissioning capability.

The application of vector control to a conventional synchronous reluctance motor (VCSynRM) is presented with emphasis on the effects of saturation and iron losses. It is shown experimentally that these parasitic effects can significantly influence the performance of a VCSynRM. A simplified steady-state d-q model including saturation and iron losses is presented, and experimental results concerning optimal torque/ampere and optimal efficiency operation are shown to be in general agreement with the predictions of the model.