PROSPECTS FOR THE USE OF MULTIPHASE PHASE-POLE-CONTROLLED AC INVERTER DRIVES IN TRACTION SYSTEMS

The use of the multiphase (i.e. having a number of phases more than four) systems «inverter - induction mo- tor», in which the frequency control principle and phase-pole control method are used together, allows to improve considerably a number of technical-and-economic characteristics of the traction systems of transportation facilities: in this case the drive system can have decreased mass-and-overall dimensions and manufacturing cost as well as improved reliability in comparison with DC and 3-phase AC drives. The results of the investigation made by the authors of this paper allow making a conclu- sion about the following: the use of the multi- phase (i.e. having the number of phases more than four) systems «inverter - induction motor» in the traction drives of transportation facilities allows to improve considerably a number of the technical-and-economic characteristics of these traction systems. The use of these multiphase drives is most effective if the classical frequency control principle and phase-pole control method (PPM) (l-4) are used together in them. Expansion of AC drive control fi eld The phase-pole method of a multiphase in- verter control bases upon the following princi- ple: if the electrical angles between the voltages (or currents) of the nearest phases of an inverter are increased by a factor of some whole num- ber (without any change in the inverter voltage frequency), a set of the induction motor me- chanical characteristics can be obtained (Fig. 1). These characteristics differ in the synchronous speed and maximal torque values. Such effect can be obtained only if the phase number «m» (i.e. the number of phases) equals certain values that are more than four (2, 3). The use of this mode together with the frequency control principle in the multiphase systems produces the effect that could not be obtained in the analogous 3-phase systems: that allows a few times increase of the motor maximal torque over all range of the rotation speed regulation without any change in the saturation extent of the motor magnetic circuit. The control fi elds (i.e. the fi elds completed by motor mechanical characteristics that can be obtained in a drive system without any change in the motor magnetic circuit saturation extent) of the 30 kW power system «inverter - induc- tion motor», in which the classical frequency control principle is used (fi eld 1), and of the 12-phase similar system, in which the frequen- cy control method and PPM are used together (fi eld 2), are shown in Fig. 2.