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 provi des high output torque and nearly zero steady-state speed error at any frequency is presented. The control scheme is based on the popular co nstant volts per hertz (V/f) method using low-cost open-loop current s ensors. Only stator current measurements are needed to compensate for both stator resistance drop and slip frequency. The scheme proposed fu lly compensates for the current-resistance (IR) voltage drop hy vector ially modifying the stator voltage and keeping the magnitude of the st ator flux constant, regardless of changes in frequency or load, A nove l slip frequency compensation, based on a nonlinear torque-speed estim ate, is also introduced. This method reduces the steady-state speed er ror to almost zero, It is also shown that a linear torque-speed approx imation 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 d own to, at least, 1.2 Hz with load torques of more than 150% of rated value. Since the only machine parameter required, the stator resistanc e, is automatically measured at startup time, using the same pulsewidt h modulated voltage-source inverter without additional hardware, the p roposed drive also exhibits self-commissioning capability.