Tom Shelley reports on an innovation that makes
induction motors attractive to use over a range of widely varying
speeds and as generators
Multi wound AC induction motors can be efficiently driven over a
very wide range of speeds at constant power by exciting different
numbers of field poles to change the number of alternating
current cycles required to achieve a single rotation.
Smooth running changes between 2, 4, and 6 pole excitation have
been demonstrated in a laboratory prototype, but computer
modelling shows no problem in changing between the excitation of
2, 4, 6, 8, 10, 12, 14 and 16 poles.
The technology is appropriate to any instance where motor speeds
need to be changed over a wide range or power generated from
variable speed inputs. Potential applications already identified
include: traction, machine tool spindles, automotive
starter-generators, hoists, coil winding systems, and wind and
tidal flow power generation.
Inventor Dr Malcolm McCulloch, a lecturer in the Department of
Engineering Science at the University of Oxford, told Eureka that
he had the idea in bed at 6am one day in October 2003. By
mid-day, he had managed to prove that the idea was feasible,
using software specially developed at Oxford for the design of
non-standard motors, and filed the first patent in December. This
was followed by the construction of the first demonstration motor
and driver hardware and software in February 2004.
Multi-pole machines have been constructed in the past, but
normally only with two sets of pole windings. The advantage of
having a large range of possible sets of pole windings are
similar to those associated with a multi-speed mechanical gearbox
- the ability to provide a smooth and even power output while
minimising the source of the power. In a car, this means
minimising the size of the engine, but in an electric motor and
driver, it means minimising wire thicknesses and the ratings of
power electronic components.
The 50W GE motor presently being used for experimental work has
16 slots and 16 coil windings, with pairs of coils connected in
series to give it a maximum of 8 phases. When demonstrated, it
was being driven by 8 of 9 phases available from three 3-phase
bridge pulse width modulated drivers. The motor was being
repeatedly switched between, 2, 4 and 6 phase operation. Hand
tests showed that in 2-phase operation, it ran fast but was
fairly easy to stop, while it was much harder to stop when it was
being run at slower speed under 4-phase and 6 phase operation.
There was no sound or jerk when changing between different phase
combinations on the fly and the motor showed no signs of becoming
excessively hot.
In theory, the motor would also run under 8-phase, although it
then effectively becomes a single phase machine without any
rotating field component. Dr McCulloch explained that in this
instance, the motor cannot be started under 8-phase stimulation,
but will run under it. However, 8-phase operation becomes
potentially very useful when the motor is used for regenerative
braking, because it allows significant currents to be generated
at low speeds.
The project has recently been given the name 'Proteum', loosely
based on the shape
shifting Proteus of ancient Greece, although it is generally
referred to in technical circles as the "Variable Pole
Machine." The next stage in the development process will be
to take make extensive measurements of performance and electrical
behaviour of the prototype on a test rig. This will provide more
insight and help verify the model.
The stage after that will be to design and construct a complete
purpose built motor rated at 1kW. Because the motor used for the
prototype was originally designed for conventional running at a
single speed, it could only be optimised for 2-phase and 4-phase
operation, while 6-phase running functioned but less efficiently.
In future, Dr McCulloch said, "We would hope to built motors
with special stator designs that could be optimised for all
possible phase configurations." It will also be necessary to
develop dedicated controllers.
One of the particular capabilities Dr McCulloch has at his
disposal is a suite of software called OXFEM, originally written
by research student Garry Barnes as part of his Ph.D. project,
but now re-written by Dr McCulloch using Java. Researchers at
Oxford also have a 3D FEA code available named NABLAX.
These packages were particularly written in order to assist the
design of superconducting machines, especially motors. The
researchers are already involved in developing a superconducting
motor as a participant in the European Commission's Supermachines
Research Training Network, more information at
www.supermachines.org. Interestingly, the Proteum technology is
particularly suited for superconducting motors, which are mainly
being researched as a possible means of saving energy at the very
large end of the motor spectrum. Advances in superconductors and
ultraconductors, however, mean that what are presently regarded
as exotic technologies could soon find their way into more
mainstream applications. Ultraconductors, incidentally, are
polymers, first discovered in Russia, but more recently developed
in the US, which have conductivities vastly higher than those of
metals.
Applied to motors made of conventional materials, the technology
is initially targetted at traction applications because it saves
overall weight, and is designed to draw constant power levels
from a small hybrid vehicle engine and/or batteries. Another
obvious potential application is in starter motor/generator units
for motor vehicles, because it runs efficiently at low speed,
when it would use multi-pole operation for starting, and at
higher speed, when it would reduce the number of effective poles
while acting as a generator. Presently, machines constructed for
such applications are based on permanent magnets, which makes
them expensive. Induction motors are cheaper.
Machine tool drives and spindles also have to run over very wide
speed ranges, as do motors for hoists and coil winding. Used as
generators, they should also prove attractive for wind and low
head water power generation, again because of their inherent
ability to extract useful power over a very wide speed range.
Studies suggest that the technology should become increasingly
attractive at larger sizes.
Isis Innovation is working with Dr McCulloch to commercialise the
development. Industrial partnerships and support is sought to
finance the next stages of development.
University of Oxford Department
of Engineering Science
Supermachines Research
Training Network
Ultraconductors
ISIS -
University of Oxford Innovation
Dr
Malcolm McCulloch email
Pointers
* By changing the number of effective poles, it is possible to
make induction motors function efficiently over a very wide range
of speeds at constant power.
* The motors can also be made to function as generators over a
wide range of speeds
* Development is presently at demonstration prototype stage
For more technical
developments see www.eurekamagazine.co.uk