CAD is the key to low cost switched reluctance

Tom Shelley reports on the development of a technology with potential to revolutionise the low cost end of the motor market

Tom Shelley reports on the development of a technology with potential to revolutionise the low cost end of the motor market

By using advanced computer modelling, it is hoped to produce a range of switched reluctance motors that are cheaper, more efficient, quieter and deliver higher torque than standard induction motors.

Such motors, originally invented in 1838, have iron cores pulled round by firing opposite stator poles in sequence, and save cost by having no rotor windings, magnets or brushes. They rely on the principle that a piece of iron placed in a magnetic field will always place itself in the minimum reluctance position where the magnetic field meets the lowest resistance.

They are, however, hard to design to ensure that they achieve their full potential in terms of torque and efficiency. Until the advent of low cost power electronic switching and computer modelling, the motors were mainly limited to driving synchronous electric clocks.

The present development is being undertaken by the A.O.Smith Electrical Products Company based in Tipp City, Ohio, USA, using OPERA 2d software supplied by Vector Fields, based in Oxford.

Vector Fields says, "The programme was able to create thirty model variants in one hour and then analyse all in one overnight run, providing flux contours and phase winding flux linkage profiles. A simple integral calculation was all that was needed to compute the developed torque, which could then be presented graphically as a function of current and rotor position, taken over every three degrees. From the calculated winding resistance and applied voltage a simple numerical programme could then be used to predict the final motor performance over a range of working load conditions. The empirical design defined promised superb performance characteristics and it had been `developed' in a remarkably short time.

"Practical results have since shown an incredible degree of correlation between predicted and actual values. The method has been used further to refine the design, improving the performance of the switched reluctance motor and providing efficiencies above that of the induction motor, predominantly with flatter characteristics either side of the designed operating conditions. Further advanced analysis included the measurement of the stator forces, normally very difficult to determine, the reduction of which has resulted in a much quieter motor, for an improved environment as well as an extended reliability.

"Once all refining and optimisation of the initial model was complete OPERA 2d was able to scale and check the designs of a range of motors and also variations tailored to meet other applications and the needs of specific clients."

While the company in Ohio has so far declined to reveal exactly what it is up to, its web site reveals it to be a maker of motors for pumps, garage door openers and air compressors. This puts it firmly at the low cost end of the market, presently dominated by AC induction motors.

Vector Fields
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Pointers
* Switched reluctance motors use no rotor windings, magnets or brushes and are therefore potentially much cheaper to manufacture than other types
* In order that they achieve their full potential in terms of efficiency and torque, it is essential to be able to model them in detail
* Using 2D specialised modelling software, it is however possible to create 30 model variants in an hour and then analyse all of them during an overnight run.

Author
Tom Shelley

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