MODELLING AND CONTROL
OF SERIES CONNECTED



ABSTRACT Numerous industrial applications, such as textile industry, paper mills, robotics and railway traction, require more than one electric drive. Currently, the multimachine drive system is available in two different configurations. The first one comprises n threephase VSIs feeding n threephase motors individually while the second one is a system with n parallelconnected threephase motors fed from a single threephase VSI. The formerdrive system requires n threephase 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 multimotor drive systems based on multiphase machines and multiphase VSIs. The objective is to accomplish independent control of all multiphase 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 multiphase motors and vector control principles. The fundamentals of the concept emerge from the fact that the multiphase 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 multiphase motors in a multimotor drive system. An appropriate series connection of the stator windings converts the flux/torque producing currents of one machine into the nonflux/torque producing currents for the other machines, allowing independent control of each multiphase motor using a vector control scheme. Two multiphase twomotor seriesconnected drive configurations are considered in detail in this thesis. These are the fivephase drive, consisting of two fivephase machines and a fivephase VSI, and the sixphase drive, comprising a symmetrical sixphase machine, a threephase machine and a sixphase 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 fivephase and sixphase VSIs is further reported and a number of space vector PWM schemes are developed. Numerous simulation studies are performed for both fivephase and sixphase twomotor drives using indirect vector control principles and current control in the stationary reference frame (hysteresis and rampcomparison), thus proving by simulation the possibility of independent control under realistic supply conditions. Finally, a laboratory rig is described, which utilises a fivephase inverter. Fivephase twomotor drive systems, comprising two fivephase induction motors and a fivephase induction motor and a fivephase synchronous reluctance motor, respectively, are investigated experimentally. An analysis of the motor’s performance within the fivephase twomotor drive is presented and it is compared with performance of a single fivephase motor drive. The experimental results fully verify the existence of decoupled dynamic control of two machines in the seriesconnected fivephase drive. Dokumenta
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