Discontinuous Space Vector PWM Strategies for a Seven-Phase Voltage Source Inverter Mohd. Arif Khan, SK Moin Ahmed, Atif Iqbal * Member IEEE, Haitham Abu Rub * Senior Member IEEE, SK Moinoddin Department of Electrical Engineering, Aligarh Muslim University, Aligarh, 202002, India * Electrical and Computer Engineering Programme, Texas A &M University at Qatar, Doha, Qatar Corresponding Author: atif.iqbal@qatar.tamu.edu Abstract-This paper presents discontinuous space vector PWM (DPWM) strategies for a seven-phase voltage source inverter (VSI). Space vector model of a seven-phase VSI shows that there exist 128 space vectors with different lengths and maps into fourteen sided polygons. A number of possibilities could arise to implement modulation of inverter legs due to large number of available space voltage vectors. Two strategies are adopted here; one utilising only large set of space vectors and another using large and two middle sets of space vectors to implement discontinuous space vector PWM. The former offer best utilisation of available dc link and the later offers nearly sinusoidal output. A significant reduction in switching losses is observed. A generalised formula is presented to determine the exact amount of reduction in the switching losses by using DPWM. Simulation results are provided to validate the concept followed by their experimental implementation. The experimental set-up is illustrated and the experimental results matches the simulation results. I INTRODUCTION Speed controlled electric drives predominately utilise three- phase ac machines. However, since the variable speed ac drives require a power electronic converter for their supply, the number of machine phases is technically not limited. This has led to an increase in the interest in multi-phase (more than three-phases) ac drive applications, especially in conjunction with traction, EV/HEVs and electric ship propulsion. Supply for a multi-phase variable speed drive is in the majority of cases provided by a Voltage Source Inverter. There are two methods of controlling the output voltage and frequency of inverters namely; square wave mode and pulse width modulation mode. A number of PWM techniques are available to control a three-phase VSI [1]. However, Space Vector Pulse Width Modulation (SVPWM) has become the most popular one because of the easiness of digital implementation and better DC bus utilisation, when compared to the ramp- comparison sinusoidal PWM method. SVPWM for three-phase voltage source inverter has been extensively discussed in the literature [1]. The same does not apply to multi-phase VSIs, since there are few SVPWM techniques available. SVPWM for a five-phase inverter is taken up in [2-9] and SVPWM for six- phase inverters are elaborated in [10-13]. Seven-phase inverter for a seven-phase brushless dc motor is illustrated in [14] and space vector PWM to generate sinusoidal output is elaborated in [15]. More than seven-phase for instance nine-phase [16] and twelve phase [17] inverters are also available in the literature. In principle, there is a lot of flexibility available in choosing the proper space vector combination for an effective control of multi-phase VSIs because of a large number of space vectors. This paper analyses Discontinuous SVPWM technique to provide variable voltage and frequency output from a seven- phase VSI. This modulation technique is known to offer remarkable advantages compared to the continuous SVPWM in terms of significantly reduced switching losses. Modelling of a seven-phase VSI is reviewed in terms of space vector representation. The model obtained is decomposed into three two dimensional orthogonal sub-spaces. The switching combinations yield 128 space vectors spanning over fourteen sectors. Two different schemes are investigated in this paper. The outer large length space vectors are used to implement the Discontinuous SVPWM method at first followed by using six active space vectors. The six active vector application yield sinusoidal output voltages and the other method produce low order harmonics in the output voltages. A comparison is done for the two schemes developed in the paper in terms of THD. Simulation results are provided to support the analytical and theoretical findings. Experimental validation of the concept is also provided in the paper. II MODELLING OF A SEVEN-PHASE VSI Power circuit topology of a seven-phase VSI is shown in Fig. 1. Each switch in the circuit consists of two power semiconductor devices, connected in anti-parallel. One of these is a fully controllable semiconductor, such as a bipolar transistor or IGBT, while the second one is a diode. The input of the inverter is a dc voltage, which is regarded further on as being constant. The inverter outputs are denoted in Fig. 1 with lower case symbols (a,b,c,d,e,f,g) while the points of connection of the outputs to inverter legs have symbols in Fig. 1. Seven-phase voltage source inverter power circuit capital letters (A,B,C,D,E,F,G). A complete space vector model of a seven-phase VSI is reported in [19]. A brief review is presented here. The total number of space vectors available in a seven-phase VSI is 2 7 =128. Out of these 128 ｩ IEEE 2009 402 Preprint of IECON 2009 Proceedings