Control of a switched reluctance machine
The windings of a switched reluctance machine are supplied from a converter which is capable of providing bipolar energization. The energization pattern supplied to the windings is dependent on the number of stator and rotor poles in the machine and is selected to produce unipolar energization in the rotor poles. This in turn reduces the losses in the rotor which would otherwise result from bipolar energization. The machine may also be operated to provide energization patterns which are adapted to provide optimum performance in different parts of its operating speed range.
1. A method of operating a reluctance machine having a stator defining stator poles, the stator having windings which are arranged as at least three separately energisable phases, and a rotor defining rotor poles, the method comprising: energising the windings such that the flux in each of the rotor poles is unipolar for successive energisations of the windings when the rotor poles are in substantial alignment with the energised stator poles, wherein the flux is unipolar for successive energisations such that a unipolar direction for the flux is maintained through each of the corresponding rotor poles for a complete cycle of rotation of the rotor.
2. A method as claimed in claim 1 wherein the energising includes energising 2n stator poles per phase, where n is a positive integer.
3. A method as claimed in claim 1 , in which the machine has six stator poles, four rotor poles and three separately energisable phases, and the energising includes using an energisation pattern for each phase that includes two pulses of energisation of one polarity and two pulses of energisation of the other polarity.
4. A method as claimed in claim 3 , wherein the energising is carried out so that the phase displacement between the energisation patterns of the phases is twice the step angle of the machine.
5. A method as claimed in claim 1 , in which the machine has eight stator poles, six rotor poles and four separately energisable phases, and the energising includes using an energisation pattern for each phase that includes three pulses of energisation of one polarity and three pulses of energisation of the other polarity.
6. A method as claimed in claim 5 , wherein the energising is carried out so that the phase displacement between the energisation patterns of the phases is three times the step angle of the machine.
7. A method as claimed in claim 1 in which the machine has q phases, where q is three or more, wherein the energising includes selecting energisation patterns of the stator windings such that a selected phase is energised according to one polarity for a number of energisations greater or less than q and then energised for the same number of energisations according to the other polarity.
8. A method as claimed in claim 7 wherein the energising is carried out so that the selected phase is energised according to one polarity for q−1 energisations and is energised according to the other polarity for the subsequent q−1 energisations.
9. A method as claimed in claim 1 wherein the energising of the phases is carried out such that the flux in the stator poles is bipolar.
10. A method of operating a reluctance machine comprising energising the stator poles according to a first pattern of energisation polarities of the phases, changing the pattern of energisations while the machine is operating, and energising the windings according to a second pattern of energisation of the phases, wherein the changing of the pattern includes the pattern being changed between alternating polarities of stator poles to groups of stator poles of the same polarity.
11. A method as claimed in claim 10 wherein the changing of the pattern of energisations is carried out as a function of the speed of the machine.