A new induction motor V/f control method capable of high-performance regulation at low speeds

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.