Reduction of common mode voltage in vector controlled induction motor drives

Abstract

The pulsewidth modulated voltage source inverter (PWM-VSI) is becoming popular in recent years due to the wide range of applications. This PWM-VSI is playing a key role in high performance control techniques like vector control. Even though, the vector control gives quick torque response, it exhibits variable switching frequency operation due to the hysteresis controllers. To overcome this problem, later the space vector PWM (SVPWM) was applied to the vector controlled drives. However, the SVPWM technique shows more common mode voltage variations due to the usage of zero states. The pulsed voltages create several unexpected problems such as conducted EMI, shaft voltages, bearing currents and breakdown of motor insulation. newlineThe common mode voltage (CMV) variations can be decreased either by using the active filters or passive filters. But, these filters increase the additional hardware and hence cost also increases. Hence, in the recent years various PWM techniques were developed for reduced CMV variations. As VSI has discrete output voltages, magnitude of CMV takes the values of ±Vdc/6 due to usage of active voltage vectors or ±Vdc/2 due to zero voltage vectors. At higher switching frequencies and higher DC bus voltage levels, excessive common mode voltages can result in high common mode currents and lead to bearing failure of the drive. PWM methods that eliminate the ±Vdc/2 level have been developed such that CMV is limited to ±Vdc/6. However, the existing PWM techniques involve various calculations like complex coordinate transformations, trigonometric calculations and sector identification. In order to reduce this complexity, in the proposed work a simple scalar approach based PWM techniques are developed. The principle of scalar approach based PWM techniques is volt-time balance per carrier cycle. Based on this principle, the reference voltage is generated in each carrier cycle.