Energy efficient PWM induction machine drives for electric vehicles.

The viability of any electric vehicle is critically dependent on it having an acceptable range
between charges, a feature which is ultimately dictated by the capacity of the battery energy
store. Considerable improvements in vehicle range are possible, however, by ensuring the most
effective use of this limited energy resource through the minimisation of the losses in the electric
drive-train, i.e. the combined machine and power electronic controller. A particular consideration
is that, for the majority of the time, the electric drive-train will be operating at part load.
The thesis investigates the operation of induction motor based electric traction drive-trains, with
a view to minimising the system loss over typical driving cycles. The study is based around a
26kW induction motor and IGBT inverter drive, which is typical of the technology used to power
a small urban vehicle.
A potential advantage of an induction motor based drive-train is the ability to vary the level of
excitation field in the motor, and therefore the balance of iron and copper loss. The control of
the supply voltage magnitude necessitates the use of some form of modulation on the output of
the power converter. The method of modulation employed will influence the harmonic content
of the supply to the motor, the level of parasitic harmonic loss in the machine and the switching
losses of the power semiconductors.
A theoretical study supported by experimental work on a DSP controlled drive is presented and
used to determine the most appropriate modulation strategy at a given operating point to achieve
an optimal balance between the motor copper, iron and harmonic loss and inverter switching and
conduction loss. It is shown that compared to the established method of constant flux and fixed
inverter switching frequency control, a significant reduction in the traction system loss can be
achieved. Some different modulation schemes involve varying amounts of computational
overhead in a DSP, the implementation of candidate modulation and control schemes has also
been investigated to ensure the defined scheme is practically realisable.