Saleh, Mohammad (2013) Development of control techniques for direct AC‐AC matrix converter fed multiphase multi‐motor drive system. PhD thesis, Victoria University.

Abstract

There are numerous industrial applications, such as paper mills, locomotive traction, oil and gas, mining and machine tools, which require high performance control of more than one electric motor simultaneously. When more than one electric motors are employed in an electric drive, it is called, ‘multi‐motor drive’. These multi‐motor drives are generally available in two configurations. The first one consists of a number of three‐phase voltage source inverters connected in parallel to a common DC link, each inverter feeding a three‐phase AC motor. This configuration allows independent control of all machines by means of their own three‐phase voltage source inverters (VSIs). Nevertheless, this configuration needs n number of voltage source inverters for supplying n number of AC machines. The second configuration comprises one inverter, which feeds multiple parallel‐connected three‐phase motors. However, the later configuration does not allow independent control of each motor and is suitable only for traction application. The power converter supplying the drive system, are conventionally, voltage source inverters. However, alternative solution could be a direct AC‐AC converter that can supply the electric drive system. Exploring this alternative solution is the subject of this thesis. Thus for decoupled dynamic control of AC machines working in a group (multi‐motor drive) is possible by employing multi‐phase (more than three‐phase) motors, where their stator windings are connected in either series or in parallel and the combination is supplied from a single multi‐phase power converter. This thesis explores the control techniques of multi‐phase direct AC‐AC converter for such specific series and parallel‐connected multi‐phase motor drives. The research presented here utilises additional degrees of freedom available in a multi‐phase system to control a number of machines independently. The concept is based on the fact that independent flux and torque control of any AC machine, regardless of the number of stator phases requires control of only two stator current components. This leaves the remaining current components free to control other machines within the group. The multi‐phase multi‐motor drive system fed using multi‐phase direct AC‐AC converter need precise Pulse Width Modulation (PWM) technique to independently control the drive system. The subject of this research is to propose PWM techniques for such configurations. The thesis focuses on four different cases; five‐phase, six‐phase (symmetrical and asymmetrical), and seven‐phase system. Five‐phase and six‐phase drive systems consists of two motors, and seven‐phase drive system controls three motors. The thesis presents various PWM techniques aimed at these drive configuration. Carrier‐based, carrier‐based with harmonic injection and direct duty ratio based PWM techniques are presented in the thesis. The independence of control of various motors are shown by simulation and experimentation. Although, the proposed techniques are equally applicable to series‐connected drives and parallel‐connected drives, the thesis focuses on the former drive configuration. Analytical, simulation and experimental approach is used throughout the thesis.

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