Field Weakening Strategy for a Vector-Controlled Induction Motor Drive Near the Six-Step Mode of Operation

This paper proposes a field weakening strategy for a vector-controlled induction motor drive that enables the six-step mode of operation. This field weakening scheme can extract maximum possible torque without exceeding the machine voltage and current ratings and can utilize the available dc-link voltage completely. The main motive of this paper is to maintain current control even if the inverter output voltage is saturated, i.e., it enters the six-step mode of operation. Below base speed, the drive will operate as a normal rotor-flux-oriented vector-controlled drive. In field weakening region, when the inverter terminal voltage gets saturated and enters the six-step mode of operation, the q-axis current controller modifies the flux current reference instead of directly generating the q-axis voltage reference. The reason for this is that at saturated condition, there is only one degree of freedom, i.e., either d-axis voltage or q-axis voltage can be controlled independently. The inverter gate control is generated from a synchronous PWM strategy which is active in both linear and overmodulation zone. Hence, low switching frequency operation of the inverter is possible, which makes it a very attractive scheme for medium voltage drive like traction. The scheme is verified experimentally and the results are presented.