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

Abstract

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.

Implementation of a Fuzzy PI Controller for Speed Control of Induction Motors Using FPGA

“…This is the reason why variable frequency drives are needed to vary the rotor speed of an induction motor [10]- [12]. The most popular algorithm for the control of a three phase induction motor is the V/F control approach based on the space vector pulse width modulation (SVPWM) technique to drive a voltage source inverter using fuzzy PI control [13].…”

On-line dead time compensation technique for open-loop PWM-VSI drives

“…In the past, Koga et al showed that the dead-time effect results in an equivalent increased stator resistance affecting the stability of constant volts per hertz drives [14]. For this type of drive, it is known that the torque is proportional to the square of the fundamental voltage [12]. Assuming that the output voltage has an error with respect to the commanded value then (8) which can be approximated to (9) where is a proportionality constant given by the torque when there is no voltage error.…”

“…Here, it is important to point out that the amount of volt seconds lost (gained) per fundamental cycle does not depend on the magnitude of the commanded voltage. Hence, its impact will be much more severe for lowoutput fundamental voltages, as is the case in constant volts per hertz drives operating at low speeds [12].…”

“…Notice that the time functions cos and sin are required to generate the commanded voltages in a constant implementation [12], therefore, its use in (4) and (5) does not represent an extra cost.…”

“…Hence, it is evident that to achieve good speed accuracy in a drive it is essential to have an even better accuracy in the synthesized output voltage. For example, for the machine used in the experiment, to obtain a 0.3% speed accuracy it is necessary to synthesize the PWM output voltage to better than 0.4 V, which represents a 0.2% accuracy [12]. Accurate experimental verification of the influence of only the dead-time error on the torque-speed curve of the machine is quite difficult since it is impossible to separate accurately the individual effects that account for torque variations such as voltage and slip.…”