A new induction motor open-loop speed control capable of low frequency operation

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 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 non-linear torque-speed estimate, is also introduced. This method reduces the steady state speed error to almost zero. If is also shown that a linear torque-speed approximation is a special case of the non-linear 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 start up time, using the same PWM-VSI without additional hardware, the proposed drive also exhibits self-commissioning capability.