This thesis describes a method of minimising the total losses of
an inverter-fed 3-phase squirrel-cage induction motor when the
motor ,is subjected to a pulse-width modulated (PWM) voltage
waveform. The inverter is supplied from a d.c. link and
operates at variable frequency to provide speed control of the
motor. Appropriate triggering of the inverter's six main
thyristors generates pulse-width modulated voltage waveforms
for application to the induction motor.
The operation of an induction motor with nonsinusoidal voltage
applied results in a reduction of the motor's efficiency due to
the harmonics present in the waveform. The aim of the project
is to minimise the total losses by obtaining optimum PWM
voltage waveforms rather than by improving the design of the
motor. This requires a thorough examination of motor losses.
The determination of a RiM voltage waveform which may be
produced by the described inverter is subjected to ccnstraints
which characterise the operation of the drive system. The
motor operates with constant airgap flux density throughout its
speed range to obtain maximum output power at fixed per unit
slip. In addition the switching frequency of the thyristors must not exceed a specified limit to avoid short circuiting
of the inverter.
The calculation of the motor's steady-state performances for
both sine wave and PWM supplies is incorporated in a computer
program. The detail of the experimental and theoretical
performances are given and comparison is made between sinusoidal
and PWM voltage wave systems. Good agreement is obtained between
test and calculated results on an inverter-fed 7.5 kw squirrelcage
It is concluded that the degradation of motor efficiency due to the
applied PWM voltage waves is mainly the result of increased copper
losses, which are produced by harmonic currents. The minimisation
of the losses for continuous, constant-flux, operation of the
induction motor is achieved for the given constraints. It is
found that the total losses can be further minimised if the d.c.
link voltage is variable. This permits improved motor performance
but adds complexity and cost to the d.c. link voltage supply.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.