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The proposal for multi-phase motor drive system was at first presented in
1969 (Ward and Härer, 1969). An upsurge in the research on multi-phase motor drive
is seen in recent years due to the advent of cheap and reliable semiconductor
devices such as IGBTs and MOSFETs. This paper takes up the issue of control of a
six-phase voltage source inverter. Two configurations of a six-phase inverter are
investigated leading to symmetrical six-phase supply and quasi six-phase supply. Simple
square wave control is considered, instead of Pulse Width Modulation (PWM) mode.
The existing 180° conduction mode of operation is elaborated and a novel
150° conduction mode is proposed in this paper. Phase-to-neutral voltages are
investigated and their harmonic analysis is done. A comparison of two configurations is
provided based on the quality of output voltages. A prototype six-phase inverter is built in
the laboratory and simple analog circuit based gate control is developed. The
experimental setup and results are elaborated in detail.
The advantages of voltage source inverter fed induction motor drive are widely
recognized. However, variable speed AC drives are still limited to low and medium power
applications. High power applications require expensive power switching devices. The fast
power switching devices are not readily available in the high power range. Moreover,
reliability is an important aspect in the high power applications. Multi-level inverters have
been developed for high power applications, but their complexity limits their use. Another
solution lies in the usage of multi-phase motors. Such motors offer several advantages over
the three-phase counterparts. These include reduction in the amplitude and increase in
the frequency of torque pulsation, reduction in the rotor current harmonics, reduction in
the DC link current harmonics, reduction in the current per phase without increasing
the voltage per phase, and increasing the torque per ampere for the same volume
machine. Since the current per phase reduces in multi-phase motors, small rating
semiconductor switches can be used. A detailed review of the
research on multi-phase motors is presented in Jones and Levi (2002); Singh (2002); Bojoi et al. (2006); Levi et al. (2007); and
Levi (2008). Five-phase and six-phase motors are most popular in the area of
multi-phase motors and a number of papers have been published in recent years on these two types
of motors. Vector control principles of five-phase motors have been investigated in
Toliyat et al. (2000); Xu et al. (2002); Shi and Toliyat (2002); and Iqbal et al. (2003). Space vector PWM techniques for a five-phase Voltage Source Inverter (VSI) feeding a
five-phase motor are discussed in Ryu et al. (2004), and Iqbal and Levi (2006). There
are two types of six-phase motors, namely, symmetrical and quasi six-phase. Six-phase
motors are obtained by splitting the phase belt of a conventional three-phase motor into two
equal halves with a phase separation of 60° (symmetrical) or 30° (quasi). Most of the
available literature deals with quasi six-phase configuration (Gopakumar et al., 1993a and 1993b; Xu and Ye, 1995; Zhao and Lipo, 1995; Oriti et al., 1997; Bakshai et al., 1998;
and Hadiouche et al., 2003).
This is due to the fact that all the air-gap harmonics of
order are eliminated in this configuration. Consequently, all the rotor
copper losses produced by these harmonics as well as all torque pulsation of the
order are eliminated. A comparison of inverter fed symmetrical and quasi six-phase induction
motor configuration is presented in Singh et
al. (2003). Six-step operation (180°
conduction mode) and PWM mode of inverter is considered in
Singh et al. (2003). A simple implementation of vector control scheme for a quasi six-phase induction motor is
presented in Singh et al. (2005) where PWM operation of two three-phase inverter supplying
the motor is considered. Symmetrical six-phase configuration of VSI is considered in
Correa et al. (2003a and 2003b). This paper comprehensively presents the operation of
a six-phase inverter for both symmetrical and quasi six-phase output voltage
configurations. The inverter operation is elaborated for 180° and 150° conduction angles. With the
advent of high performance Digital Signal Processors (DSPs), the control became more
flexible but their use is still limited in industries due to their cost, delicacy and stringent
power quality requirements. Some of the applications require cheap and robust controllers
and then one has no option other than the conventional analog circuit based systems.
This paper thus proposes a very simple, cheap, reliable and robust controller for a
six-phase VSI. Experimental setup is illustrated and detailed experimental results are provided.
It is found that the proposed 150° conduction mode offers better harmonic
performance compared to the existing 180° conduction mode. |
