Performance Improvement of Induction Motor Current Controllers in Field-Weakening Region for Electric Vehicles

A novel flux weakening strategy and an expert controller utilizing fuzzy inference mechanism are proposed to improve robustness of current control in the flux weakening region. Because of variable parameters, high ratios of fundamental-to-sampling frequencies and interval delay caused by pulsewidth modulation (PWM) and digital discrete, the effects of the current control technologies are degraded in the existing indirect field-oriented control system. It is especially obvious when motors work in the weakening region. Highly oscillatory or unstable response may occur if current regulators do not obtain proper current commands. Thus, an expert controller changing current command before current regulators is proposed to avoid unreasonable tuning. Based on the simple and formal inference rules, the expert controller could be trigged in parallel. Even in the case of invalid performance target, the suboptimal parameter could be substituted. As a part of the current control, a novel flux weakening strategy is proposed for engineering applications. Simulation and experiment results are provided to verify the stability of current control.

I. Introduction

Cage Induction Motors (IM) are widely accepted as the most promising candidate for the electric propulsion of hybrid electric vehicles (HEVs) and electric vehicles (EVs), due to their reliability, ruggedness, low maintenance, low cost, and ability to operate in the extended high speed [1]. Fig. 1 shows the typical block diagram of IM drives based on indirect field oriented control (IFOC). Extended speed range operation beyond the base speed is accomplished by the flux weakening strategy [2]. At present, most of the research on the current control in the flux weakening region is below 2 to 3 times of the base speed. Moreover, most of the existing research focuses on flux weakening strategy optimization or current regulators design. Fig. 1.

Scheme of IFOC including flux weakening strategy.