Modelling and simulation of induction motors for variable speed drives, with special reference to deep bar and saturation effects.

[摘要] Variable speed motors are achieved by varying the voltage of a DC machine or byvarying the frequency of an AC machine, the former method being the simpler of thetwo. DC motors have the major disadvantage of brushes and commutators which requireregular downtime for maintenance, a fact already recognised by Tesla [1] in 1888. Thusthe AC motor, in particular the induction motor, is of a more rugged design and doesnot suffer from the commutator problem of its DC counterpart. Recent advances in thetechnology of the power electronics used to supply a variable frequency to the motorhas allowed the induction motor to be a viable alternative to the DC motor in variablespeed applications.Problems have been encountered in industry when an inverter is injudiciously selectedto be combined with a motor. Such problems were highlighted by difficulties beingexperienced with some 400 kW inverter drives. The inverters had been bought from onesupplier and the motors from another. When this system was coupled together, therewas excessive heating in the motors and the overall plant was only able to operate wellbelow its capacity, incurring a substantial weekly loss of income. The motor and inverterwere evidently incompatible, and since the inverter could not be modified, the motorwas redesigned to make it less susceptible to the harmonics present in the inverterwaveform,These problems have led to the development of a variable speed drive simulation packageat the University for use by the local industry which can accurately model the completesystem of inverter, motor and its associated load. It is envisaged that this package couldbe used to predict the performance of a drive system and highlight problems that mayoccur. To be able to do this, an accurate model of the motor is required.This investigation gives the development of an induction motor model which is suitable .for variable speed drive system simulations. The model accounts for the deep bar effectby using lumped parameter circuits and includes saturation of the leakage paths usingonly information which is typically available from motor design data. A complete analysisis given of the different lumped parameter models and their suitability for use in thisapplication. The thesis also shows the utilisation of the deep bar model to simulatereswitching transients and double cage motors. The author hopes that the models usedin the simulation package wallow industry to predict problems prior to their occurrence,alter the designs and thereby avoid costly remanufacture of the system.

[发布机构] University of the Witwatersrand