EUREKA JUNE COVER FEATURE STORY

Variable poles widen induction speeds

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

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