A novel concept of series connected multi-phase, multi-motor drive systems
There are many applications, such as paper mills, locomotive traction and machine tools, which require high performance control of more than one electric motor. These multi-motor drives are generally available in two configurations. The first one consists of a number of three-phase voltage source inverters (VSI) 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 VSIs. The second method comprises one inverter, which feeds multiple parallel-connected three-phase motors. However, this configuration does not allow independent control of each motor and is suitable only for traction. This thesis explores a novel concept for multi-motor drive systems, based on utilization of multi-phase machines and VSIs, and series connection of all the machines in the group. Application of a single multi-phase VSI in conjunction with multi-phase machines generates additional degrees of freedom. The research presented here utilises these additional degrees of freedom to control a number of machines independently within a novel multi-phase multi-machine drive. 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. It is shown that it is possible to connect the machines in such a way that what one machine sees as the flux/torque producing components the other machines see as non-flux/torque producing components, and vice versa. Therefore it is possible to connect in series a number of multi-phase machines and independently control each machine while supplying them from a single multi-phase inverter. Different configurations of the multi-motor drive are possible depending on certain properties of the supply phase number. In general, higher the supply phase number is, higher the number of connectable machines is. However, some phase numbers are more favourable than others, as discussed in detail in the thesis. Simulation studies are provided for five, six, seven, nine, ten and fifteen phase configurations in order to verify the concept. It is shown that the concept is independent of the type of AC machines used and the only requirement is that they all have sinusoidal distributed magnetomotive force. Current control in both the stationary and rotating reference frames is considered and it is concluded that current control in the rotating reference frame requires compensation of the additional voltage drops caused by the series connection. Two possible methods of compensating for these voltage drops are suggested and verified by simulation. Finally, a laboratory rig is described, which utilises two three-phase inverters connected in such a way as to form a single six-phase inverter. A six-phase two-motor drive comprising a symmetrical six-phase induction machine and a three-phase induction machine or a three-phaseP MSM is investigatede xperimentally. An analysis of the performance of the two-motor drive is presented and it is shown that decoupled control of each machine is achieved.