Project:

High Performance Switched Reluctance Drives

From October 1998 to February 2002
Project Leader(s): Prof. B.C. Mecrow
Sponsors: EPSRC (GR/M07120), in collaboration with Switched Reluctance Drives Ltd (a world leader in the technology).

The aims of this research project are to investigate drives for doubly salient reluctance machines employing fully pitched windings with the following objectives, given in order of priority:-

  • Research of unipolar and bipolar drive concepts, with both hardware development and simulation.
  • Development of fully pitched winding switched reluctance machine design, using both finite element simulation and by construction of a demonstrator.
  • Application of the concept of fully pitched windings to hybrid stepping motors and drives, with prototyping arising from the modification of existing commercial drives and short pitched winding machines.

 

Switched reluctance and stepping motors have developed around the concept of short pitching each phase winding, generally around a single stator tooth. By employing such a winding torque is derived due to the rate of change of self inductance of the excited phases. Utilisation of the electric circuit is poor, due to the fact that each phase winding is restricted to periods when the self inductance is rising, so that any one winding can only contribute to positive torque for a maximum of one half of the time. Alternatively, if the machine is wound with a fully pitched winding then it can derive virtually all its torque from a changing mutual inductance between phases. The machine therefore becomes a dual of the conventionally wound machine, which operates entirely upon rate of change of self inductance. Because of the fundamental change in operating mode, each phase of the machine can contribute to positive torque production for considerably greater than one half of the cycle of rotation, leading to a more efficient utilisation of the winding.

Prototype Machines

Two fully pitched winding SRM drives have been built; the first used a rewound 12-8 machine within a D132 frame, and the second is a purpose built three phase machine, built within a D100L frame. Extensive modelling, using the finite element method in conjunction with a drive simulator, enabled an optimised design to be produced. The figure below shows the lamination profile of the second prototype. It is a 12-8 machine, with an outside diameter of 150 mm and a stack length of 150mm.

The following conclusions were drawn for this design:-

  • 4 magnetic poles reduces end-winding length without reducing the torque per unit phase current. It provides an optimal number of magnetic poles for this winding arrangement and frame size.
  • The 4 pole fully pitched winding machine will produce greater torque than a 4 pole conventional SRM of the same stack size, providing the stack length is greater than 30% of the outside diameter.
  • Three phase excitation maximises mean torque production, but as in a conventional SRM there is typically a large torque ripple.

 

The design has tapered teeth to reduce magnetic saturation and small slot closures to facilitate winding retention. Studies showed that these closures slightly reduced torque ripple, with minimal effect upon mean torque produced. Such features are not usually used on conventional SRMs, where a solid pre-formed winding is placed over the teeth. Both prototypes have been subjected to extensive testing, including measurement of static torque and flux-linkage characteristics and thermal testing using both embedded thermocouples and rate of change of resistance monitoring.

To date the following conclusions can be made from this research:-

  • Fully pitched winding SRMs offer superior performance to conventional SRMs, providing the stack length is more than 30% of the outside diameter. In a standard D100L frame size a 35% increase in rated torque was measured in comparison to conventional SRMs, producing a machine with a performance midway between an SRM and a rare earth permanent magnet machine.
  • Fully pitched winding SRMs produce torque to the changing mutual coupling between phases. With idealised excitation sequences maximum torque production occurs with bi-directional excitation of all phases. However, bi-directional excitation of the machine requires an excessive converter volt-ampere rating, and therefore, despite high torque production at low speed, is not a favoured excitation sequence.
  • Unidirectional excitation from asymmetric half bridge converters is simple to implement and works well under both current and voltage controlled operation. In terms of peak values of current and voltage the drive has an identical converter volt-ampere requirement to a conventional SRM drive, but because the device duty cycles are higher, total converter losses are almost doubled.
  • New converter topologies have been introduced which reduce the losses in the converter to that comparable with a conventional SRM drive. These operate by reducing the number of simultaneously conducting power devices from four to two. It is demonstrated that a modified 3-phase bridge converter is suitable for this task.
  • Direct torque control of fully pitched winding SRMs and conventional SRMs has been achieved using a flux-linkage controller. Torque control has been possible over a wide speed range because the controller can implicitly take into account the limited rate of change of flux, imposed by the converter d.c. link voltage.
  • A simulation method has been developed and implemented for a range of converter topologies and excitation sequences. This simulation has used a set of novel transformations to enable de-coupled magnetic characterisation of the machine, thus greatly simplifying a rather complex process of magnetic interaction between phases. The simulation has been shown to be an accurate method of analysing drive performance.
  • Application of fully pitched windings in hybrid stepping motors has been investigated. The winding has been demonstrated on an experimental machine and shown to be much more efficient than the conventional arrangements used, with approximately 40% less winding loss. Test results have demonstrated that it can successfully emulate all MMF patterns produced by a conventional hybrid stepping motor.

 

References

  1. Mecrow, B.C.: "Fully Pitched Winding Switched Reluctance and Stepping Motor Arrangements", Proc. IEE Part B, Jan 1993.
  2. Mecrow, B.C.: "New winding configurations for doubly salient reluctance machines", IEEE Industry Applications Society Transactions, to be published 1996.
  3. Barrass, P.G., Mecrow, B.C. and Clothier, A.C., "Bipolar Operation of Fully Pitched Winding Switched Reluctance Drives", IEE Seventh International Conference on Electrical Machines and Drives, Durham, September 1995, pp252-256.
  4. Barrass, P.G., Mecrow, B.C., and Clothier, A.C., "The Unipolar Operation of Fully Pitched Switched Reluctance Motor Drives", ICEM, Paris, Sept. 1994.
  5. Barrass, P.G., High Performance Switched Reluctane Motor Drives, Ph.D. thesis, Oct. 1995.
  6. Davis, R.M., European Patent Application No. EP 0 692 862 A2, 1995.
  7. Kron, G., A Short Course in Tensor Analysis for Electrical Engineers, John Wiley & Sons, Inc. 1942
  8. Torque Control of the Doubly Salient Reluctance Motor, U. Steiert and H. Späth, European Transactions on Electrical Power Engineering, Vol. 3, Number 4, 1993
  9. A Balanced Commutator for Switched Reluctance Motors to Reduce Torque Ripple, R.S. Wallace and D.G. Taylor, IEEE Transactions on Power Electronics, Vol. 7, Number 4, 1992
  10. Optimum Commutation Current Profile on Torque Linearization of Switched Reluctance Motors, D.S. Schramm, B.W. Williams, T.C. Green, ICEM Conference Proceedings 1992, Vol. 2, Page 484, 1992
  11. Neural Networks used for Torque Ripple Minimisation from a Switched Reluctance Motor, D.S. Reay, T.C. Green, B.W. Williams, EPE Conference Proceedings 1993, Vol 6, Page 1, 1993
  12. Barrass, P.G., and Mecrow, B.C., "Torque Control of Switched Reluctance Drives", International Conference on Electrical Machines, Vigo, September1996.
  13. Mecrow, B.C., and Clothier, A.C., "Increased Torque Production in Hybrid Stepping Motors", International Conference on Electrical Machines, Vigo, September1996.

Staff

Professor Barrie Mecrow
Professor of Electrical Power