Electricity: motive power systems – Switched reluctance motor commutation control
Patent
1996-02-22
1998-06-16
Wysocki, Jonathan
Electricity: motive power systems
Switched reluctance motor commutation control
318439, H02P 700
Patent
active
057676380
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of driving an electric motor and to apparatus for carrying out that method. The method and apparatus have particular but not exclusive application to driving switched reluctance motors.
Switched reluctance motors offer a fully-controllable electric motor drive of high torque and simple construction. However, in some potential fields of application switched reluctance motors have been rejected at least partially due to their being relatively acoustically noisy in operation.
In a typical switched reluctance motor, a rotor is mounted for rotation within a stator. The stator has a plurality of inwardly directed salient poles each of which has a corresponding coil winding. The rotor also has a plurality of salient poles directed outwardly towards the poles of the stator. Combinations of windings are connected together, typically in series, each combination constituting a phase winding. Most commonly, phase windings comprise pairs of windings on diametrically opposite stator poles. A drive circuit, known as a power converter, is used to control voltage and current in the windings.
When a phase winding is energised by applying a voltage across it, this creates a magnetic flux linking the stator pole, and exerts a torque on the rotor which tends to turn the rotor to bring its poles into alignment with the poles of the stator whose windings are energised. By energising successive phase windings in sequence around the stator, the rotor can be caused to rotate continuously, the speed of rotation being determined by the frequency at which the successive phase windings are energised. The power converter controls the current in the phase windings to control the torque. Thus, the power converter provides an increase in the current before the rotor reaches its aligned position, limits the current magnitude (especially at low speeds) and decreases the current rapidly near to, but usually just before, the aligned position. Many different power converters are known, but are usually of two kinds. The first kind can apply positive, negative or zero voltage across the windings; the zero voltage state is known as a zero voltage loop or free-wheeling state. The second kind can apply only positive or negative voltage across the windings.
Hitherto, investigation into acoustic noise in switched reluctance motors has centred around analysis in the frequency domain of the noise produced. However, in arriving at the present invention, the applicants carried out a time-domain analysis. Their investigations found that significant components of the generated noise were produced in transient pulses generated in synchronism with the switching between successive stator poles. Further investigation found that these pulses were produced at least partially by vibrations in the stator arising from opposite stator poles being attracted to one another while their windings are energised, causing the stator to be deflected inwardly along the line of the energised poles, the stator then springing outwardly with a release of energy when the coils are de-energised. In fact, the vibrations are initiated by a change in the radial force on the stator; the radial force is related to the flux, which is a result of the voltage applied to the phase winding, and so will change when the applied voltage changes. As a result, any step change in voltage applied to the winding tends to generate noise and/or vibration.
One known switched reluctance motor and controller shown in WO 90/116111 is concerned with improving the efficiency of the motor. The power converter is of the type having a free-wheeling state, and has two switches which operate to control the voltage applied to the windings. The switches are closed simultaneously at a given angle of rotation to connect a winding to a positive voltage. One switch is opened at a fixed angle of rotation to connect the winding to zero voltage, and the other switch is opened at an invariable angle after closure, to apply negative voltage. The motor operates for a relatively long
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Pollock Charles
Wu Chi Yao
The University of Warwick
Wysocki Jonathan
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