A Multilevel Inverter with Hexagonal and 12-sided Polygonal Space Vector Structure for Induction Motor Drive Abstract- This paper proposes a multilevel inverter which produces hexagonal voltage space vector structure in lower modulation region and a 12-sided polygonal space vector structure in the over-modulation region. Normal conventional multilevel inverter produces 6n±1 (n=odd) harmonics in the phase voltage during over-modulation and in the extreme square wave mode operation. However, this inverter produces a 12- sided polygonal space vector location leading to the elimination of 6n±1 (n=odd) harmonics in over-modulation region extending to a final 12-step mode operation. The inverter consists of three conventional cascaded two level inverters with asymmetric dc bus voltages. The switching frequency of individual inverters is kept low throughout the modulation index. In the low speed region, hexagonal space phasor based PWM scheme and in the higher modulation region, 12-sided polygonal voltage space vector structure is used. Experimental results presented in this paper shows that the proposed converter is suitable for high power applications because of low harmonic distortion and low switching losses. I. INTRODUCTION Multilevel inverters have gained popularity in recent years in high power drives because of low switching losses and low harmonic distortion in the output voltage [1]. Various inverter topologies, like NPC, cascaded H-bridge and flying capacitor based multilevel inverter topologies, have been proposed in the literature [2]-[7]. In order to control the output voltage of such inverter topologies, sine-triangle PWM (SPWM) and conventional space vector PWM (SVPWM) are quite popular [8]. SVPWM technique offers some advantage over SPWM in terms of better utilization of dc bus voltage, reduced switching frequency and lesser harmonic current ripple. In SVPWM technique, the desired reference voltage vector is realized by switching adjacent voltage vectors produced by the inverter. The adjacent voltage vectors lie on the vertices of a hexagon. So in the extreme modulation range there is a possibility of producing (6n±1) harmonics in the phase current waveform. If the switching frequency is very high, then such a problem is practically non-existent as the current harmonics reside around the switching frequency and its sidebands. But, if the switching frequency is low, which is mostly the case in high power drives, the (6n±1) harmonics can reappear in the current waveform and can produce torque pulsation in the drive . The problem is particularly severe in over-modulation region where the (6n±1) harmonics constitute a major portion of the total current, and the voltage space vector locations produced by a conventional inverter lie along the edges of a hexagon. So the presence of low order harmonics such as 5 th and 7 th needs special compensated current control schemes, for high performance drive applications [9]. In this respect, polygonal voltage space vector structures with sides more than six, is very desirable for multilevel inverter fed drives. An extension of the hexagonal voltage space vector structure is to produce a 12-sided polygonal voltage space vector structure [10]-[12]. The inverter structure and the space vector diagram proposed in [10] are shown in Fig. 1. In this arrangement, the space vector boundary is a 12-sided polygon, and any point inside this polygon is realized by time averaging the adjacent voltage vectors at the vertices of the 12-sided polygon and the zero voltage vectors at the center of the polygon. This scheme results in lesser harmonic distortion compared to hexagonal space vector structure and the inverter can be operated at lesser switching frequency. Moreover, in the over-modulation region 6n±1 (n=odd) harmonics are totally absent leading to a more sinusoidal current waveform. However, one drawback of this scheme is that for any modulation index, the space vectors that are switched lie either on the vertices of the 12-sided polygon or at the center (zero vector locations). This produces high dv/dt on the phase voltage and causes EMI problems with greater switching stresses on the devices. However, in this scheme[10] it is also observed that, out of many inverter space vector locations possible in a four level asymmetrical dc voltage inverter, only a few are selected that lie on the vertices of a 12-sided polygon, and many vector locations are not switched. In the present work, the redundant switching states are used so that the inverter operates as a four level inverter (Fig.2), in the linear modulation range with (6n±1) harmonics highly suppressed. In the over-modulation region, the 12-sided polygonal space vector locations and the upper hexagonal voltage space vector structures are used for PWM control, extending to a final 12-step mode operation, in the extreme limit. II. INVERTER STRUCTURE The overall configuration of the inverter consists of cascaded combination of three two-level inverters, fed from Anandarup Das Student Member, IEEE Email: danand@cedt.iisc.ernet.in K.Sivakumar Student Member, IEEE Email: ksiva@cedt.iisc.ernet.in Gopal Mondal Student Member, IEEE Email: mgopal@cedt.iisc.ernet.in K Gopakumar Senior Member, IEEE Email: kgopa@cedt.iisc.ernet.in Centre for Electronics Design and Technology, Indian Institute of Science, Bangalore-560012, India.   k ,(((