A thesis submitted to the Faculty of Engineering, University of the Witwatersrand,
Johannesburg, in fulfillment of the requirements for the Degree of Doctor of
Philosophy. / Variable speed motors are achieved by varying the voltage of a DC machine or by
varying the frequency of an AC machine, the former method being the simpler of the
two. DC motors have the major disadvantage of brushes and commutators which require
regular downtime for maintenance, a fact already recognised by Tesla [1] in 1888. Thus
the AC motor, in particular the induction motor, is of a more rugged design and does
not suffer from the commutator problem of its DC counterpart. Recent advances in the
technology of the power electronics used to supply a variable frequency to the motor
has allowed the induction motor to be a viable alternative to the DC motor in variable
speed applications.
Problems have been encountered in industry when an inverter is injudiciously selected
to be combined with a motor. Such problems were highlighted by difficulties being
experienced with some 400 kW inverter drives. The inverters had been bought from one
supplier and the motors from another. When this system was coupled together, there
was excessive heating in the motors and the overall plant was only able to operate well
below its capacity, incurring a substantial weekly loss of income. The motor and inverter
were evidently incompatible, and since the inverter could not be modified, the motor
was redesigned to make it less susceptible to the harmonics present in the inverter
waveform,
These problems have led to the development of a variable speed drive simulation package
at the University for use by the local industry which can accurately model the complete
system of inverter, motor and its associated load. It is envisaged that this package could
be used to predict the performance of a drive system and highlight problems that may
occur. 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 effect
by using lumped parameter circuits and includes saturation of the leakage paths using
only information which is typically available from motor design data. A complete analysis
is given of the different lumped parameter models and their suitability for use in this
application. The thesis also shows the utilisation of the deep bar model to simulate
reswitching transients and double cage motors. The author hopes that the models used
in the simulation package wallow industry to predict problems prior to their occurrence,
alter the designs and thereby avoid costly remanufacture of the system. / Andrew Chakane 2018
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/24649 |
| Date | January 1990 |
| Creators | Levy, Levy Warren |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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