Department of Engineering

Centre for Advanced Electronically Controlled Drives

PROJECTS ON SWITCHED RELUCTANCE DRIVES

High Efficiency Switched Reluctance Drives

Research in the Department has resulted in the discovery that switched reluctance motors with an odd number of phase windings can be designed to operate with much higher efficiency than motors with an even number of phases.  A five phase switched reluctance drive has now been designed with the full benefit of static and dynamic finite element analysis simulation software.  This motor has been constructed in conjunction with a power mosfet power electronic controller.  Efficiencies well above 80% have been recorded over a wide range of the motor's operating range with a full load drive efficiency of 87%.

Low Cost Power Converter Topologies

A new type of power electronic controller has been developed for the switched reluctance motor which allows the power semiconductor switches to be shared between more than one phase winding.  The reduction in the total number of switches required in the power converter results in a very economical design.  This has meant that switched reluctance motors with four and more phases can be considered economically viable.

Very Low Cost Switched Reluctance Drives

The series universal motor and brushed d.c. motor are currently used in many applications as a low cost variable speed drives.  The brushes and commutator make them unreliable but the speed can be controlled simply using a triac controller or a single switch d.c. chopper.  A joint research project between the Universities of Warwick and Cardiff and a consortium of 21 industrial companies began in October 1993 and has, over a three year period, developed the technology of single and two phase switched reluctance drives for low cost applications, including domestic and automotive markets.  The total project budget was over £1 million, employing four research associates.  The project has made considerable advances in the technology of single and two phase switched reluctance drives with the construction of a large number of experimental drives, some of which have been evaluated via test programmes at company premises.  There have been notable innovations in both power electronic circuit topologies and in motor configurations.  The challenge of a wide spread of applications from the industrial partners has considerably enriched the research.  The focus of the project to reduce the manufacturing costs has also acted as a great stimulus to apply innovative thinking to the selection of the motor and drive topology.

Novel Designs of Single and Two Phase Switched Reluctance Drives With High Torque Starting and Reversing

It has long been acknowledged that low phase number switched reluctance motors have the best potential for low cost applications.  However, motors with only two or one phase windings would traditionally have torque dead zones or would have asymmetry introduced such that they have a preferred direction of rotation.  Designs have been developed and patented which allow single phase and two phase switched reluctance drives to be started in either direction from any position against a high torque load.

Low Cost Control Implementation for the Switched Reluctance Drive

The very non-linear characteristics of the switched reluctance drive has always made it difficult to control.  However the development of very low cost analogue controllers has allowed simple and very reliable circuit to be developed.  One circuit uses only one quad comparator IC..  An analogue ASIC containing all the control functions for a complete single phase or two phase switched reluctance drive is being developed.  It has a projected sales cost of less than £ 1 in volumes.

A Novel Electronically Controlled Propulsion Motor

The objective of this research project funded by the Marine Technology Directorate was to design, build and test a ducted propeller with an integral electronically controlled motor drive.  The propeller is mounted inside the rotor of a specially designed switched reluctance motor.  As the motor and propeller form an integral unit the only penetration of the ships hull will be the protected electrical cables running to the motor.  Sealing of the cable entry point is much simpler than the sealing of the rotating shaft in existing systems.  The electrical connections allow total flexibility in the choice of propeller location.  The electrical power is supplied to the static part of the device only and therefore insulation problems will be solved by encapsulation techniques.  A 5 kW prototype unit has been designed, constructed and tested.  A special motor test rig has been constructed to measure the torque on the propeller and the thrust produced in water.  A unit of this type will have applications in small marine craft but will also show the potential of the design for larger vessels.

Analysis and Reduction of Acoustic Noise in the Switched Reluctance Motor

The enormous potential of the switched reluctance motor has not been fulfilled due to the excessive levels of acoustic noise produced by the motor. Extensive experimental tests have been undertaken on several different switched reluctance motors in order to analyse the source of the vibration.  Time domain analysis has been used to demonstrate the links between the shape of the phase current waveforms and the resulting vibrations of the stator of the machine.  It has been discovered that a step change in the applied voltage causes a stator vibration, because of the resulting step change in the gradient of the radial force.  A new method for active cancellation of the vibration has been invented; it induces the creation of two successive vibrations in the stator which cancel each other, producing a dramatic reduction in the vibration of the motor without any associated loss in performance.  The technique is the subject of a patent application. Further reductions in the acoustic noise produced by the motor has been achieved when driving fan loads in HVAC applications by the recent development of a new power electronic circuit topology.

Robust, Variable Structure Controllers for the Switched Reluctance Drive

The switched reluctance drive is usually controlled by monitoring and regulating the current in the phase winding.  It has been discovered that it is much better to control the flux in the switched reluctance motor rather than the current and a robust, variable structure controller has been developed for this purpose. An approximate sliding mode input power controller and another feed-forward sliding mode speed controller are combined with space voltage vector modulation. The resultant drive has rapid and robust speed response.  A switched reluctance drive incorporating the proposed controller requires only one current sensor and can be implemented in a low cost 8 bit micro-computer and a few discrete integrated circuits.  Furthermore, the controller does not require any off-line characterisation of the motor or load characteristics and could easily be applied to switched reluctance motors with any number of phase windings.  A 4 kW 4 phase switched reluctance drive has been constructed to test the performance of the controller.  The results show that a step response from 200 r/min to 1980 r/min needs only 2~ 3 seconds even when driving a high inertia load and that the speed error can be controlled below 1 % even under unknown and dynamic loads.  It has been concluded that modified sliding mode controllers are extremely effective in dealing with the highly nonlinear characteristics of the switched reluctance drive system.

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