A. Munozgarcia et al., A NEW INDUCTION-MOTOR V F CONTROL METHOD CAPABLE OF HIGH-PERFORMANCE REGULATION AT LOW-SPEEDS/, IEEE transactions on industry applications, 34(4), 1998, pp. 813-821
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.