ABSTRACT

Numerous industrial applications, such as textile industry, paper mills, robotics and
railway traction, require more than one electric drive. Currently, the multi-machine drive
system is available in two different configurations. The first one comprises n three-phase
VSIs feeding n three-phase motors individually while the second one is a system with n
parallel-connected three-phase motors fed from a single three-phase VSI. The formerdrive system requires n three-phase VSIs while the latter system requires the motors to run under identical speed and torque conditions and hence it lacks means for independent control of motors. Thus in the present configurations it is not possible to independently control more than one machine with the supply coming from only one inverter. This thesis considers means for independent control within multi-motor drive systems based on multi-phase machines and multi-phase VSIs. The objective is to accomplish independent control of all multi-phase machines in the group while using a single VSI. Such an independent control is enabled by using an appropriate series connection of stator windings of multi-phase motors and vector control principles. The fundamentals of the concept emerge from the fact that the multi-phase machines, regardless of the number of phases, require only two currents for flux and torque control. Hence there are extra currents, which can be utilised to control the second and subsequent multi-phase motors in a multi-motor drive system. An appropriate series connection of the stator windings converts the flux/torque producing currents of one machine into the non-flux/torque producing currents for the other machines, allowing independent control of each multi-phase motor using a vector control scheme. Two multi-phase two-motor series-connected drive configurations are considered in detail in this thesis. These are the five-phase drive, consisting of two five-phase machines and a five-phase VSI, and the six-phase drive, comprising a symmetrical six-phase machine, a
three-phase machine and a six-phase VSI. Detailed mathematical modelling of both drive systems is performed. It is proved in this way mathematically that independent vector control is indeed possible due to the specific stator winding connection. Detailed modelling of both five-phase and six-phase VSIs is further reported and a number of space vector PWM schemes are developed. Numerous simulation studies are performed for both five-phase and six-phase two-motor drives using indirect vector control principles and current control in the stationary reference frame (hysteresis and ramp-comparison), thus proving by simulation the possibility of independent control under realistic supply conditions. Finally, a laboratory rig is described, which utilises a five-phase inverter. Five-phase two-motor drive systems, comprising two five-phase induction motors and a five-phase induction motor and a five-phase synchronous reluctance motor, respectively, are investigated experimentally. An analysis of the motor’s performance within the five-phase two-motor drive is presented and it is compared with performance of a single five-phase motor drive. The experimental results fully verify the existence of decoupled dynamic control of two machines in the series-connected five-phase drive.