Abstract
A general pulsewidth modulation (PWM) method for control of four-switch three-phase inverters is presented. The proposed vector PWM offers a simple method to select three or four vectors that effectively synthesize the desired output voltage, even in presence of voltage oscillations across the two dc-link capacitors. The method utilizes the so called space vector modulation, and includes its scalar version. Different vector combinations are compared. The effect ofWye and delta motor winding connections over the pulse width modulator is also considered. The common mode voltage generated by the four-switch three-phase converter is evaluated and compared to that provided by the standard six-switch three-phase inverter. Simulation and experimental results are presented to demonstrate the feasibility of the proposed approach.
Index Terms—Common mode voltage, four-switch three-phase inverter, vector and scalar pulsewidth modulation (PWM).
I. INTRODUCTION
ASTANDARD three-phase voltage source inverter utilizes three legs [six-switch three-phase voltage source inverter (SSTPI)], with a pair of complementary power switches per phase. A reduced switch count voltage source inverter [fourswitch three-phase voltage source inverter (FSTPI)] uses only two legs, with four switches, as shown in Fig. 1(a). Several articles report on FSTPI structure [1]–[3], [5]–[9], regarding inverter performance and switch control. The FSTPI structure generates four active vectors in the plane, instead of six, as generated by the SSTPI topology. This paper presents a general method to generate pulsewidth modulated (PWM) signals for control of four-switch, threephase voltage source inverters, even when there are voltage oscillations across the two dc-link capacitors. The method is based on the so called space vector modulation, and includes the scalar version. The proposed method provides a simple way to select either three, or four vectors to synthesize the desired output voltage during the switching period. In the proposed approach, the selection between three or four vectors is parameterized by a single variable. This permits to implement all alternatives, thus allowing for a fair comparison of the different modulation techniques. The influence of different switching patterns on output voltage symmetry, current waveform, switching frequency and common mode voltage is examined. The paper also discusses, how the use of the Wye and delta connections of the motor windings affects the implementation of the pulsewidth modulator. The utilization of an induction motor, with its windings connected in delta is studied here as an alternative to reduce the dc-link voltage used, in respect to the Wye connection. For the Wye connection it is investigated, how the common-mode voltage can be reduced. The reduction of the common mode voltage permits to mitigate effects of common mode currents, which commonly are responsible for damage of motor bearings and bearing lubrication [10]–[13]. Simulation and experimental results illustrate the use of the FSTPI to supply a three-phase induction motor.

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