A Speed Range Extension Scheme for Induction Motor Drive with Open-end Stator Windings with Maximum Torque Per Ampere Control

This paper presents a speed range extension scheme for induction motor drives with open-end stator windings having a DC source fed inverter at one end and a floating capacitor fed inverter at the other end. The Maximum Torque Per Ampere (MTPA) control scheme is used for speed control which ensures higher efficiency as compared to the conventional V/f and Field Oriented Control (FOC) schemes. In the proposed scheme, the capacitor fed inverter provides the reactive component of the higher voltage to be applied to the motor at higher speeds to overcome the increased back EMF. Hence, a higher speed range of operation can be realized with a given DC source voltage without resorting to field weakening. Thus, the capability of the motor to deliver rated torque is not compromised at higher speeds. The proposed scheme enhances the utilization of the DC source fed inverter as it is used only for supplying active power and the reactive power requirement is fully compensated by the capacitor fed inverter. This feature is specifically advantageous in applications like battery powered electric vehicles. The proposed scheme is validated by simulation and then experimentally verified on a laboratory prototype.

I. Introduction

Today, many applications require electric motor drives with wide speed range. Electric vehicles, for example, demand wide operational speed range while maintaining a desired torque output with a limited voltage supply. The induction motor (IM) is considered a good choice for many such applications due to its high reliability, rugged construction and almost maintenance free operation [1]–[3]. For motor operation at higher speeds, the voltage applied to the motor also has to be increased to overcome the increased back EMF. Since the output voltage of the inverter is limited, the conventional method of operating the drive above base speed is by resorting to field weakening to reduce the back EMF generated at higher speeds [4]–[6]. However, the motor will not be able to generate rated torque in the field weakening region. In addition, the field weakening operation also results in reduction in the efficiency of the drive.