Rotor position determination in a switched reluctance machine
A switched reluctance drive is controlled without using a physical rotor position detector. The control method causes current to flow continuously in a single winding when the rotor is turning and detects position by finding either the peak of the current waveform or some other feature which can be uniquely related to the position of the rotor.
1. A method of determining the position of a moving rotor of an electrical machine having a stator with at least one phase winding, the method comprising:
applying a voltage to the phase winding, the voltage waveform having a period which is independent of the period of movement of the rotor, which voltage establishes a current in the phase winding, the current waveform being cyclic with movement of the rotor;
detecting a predetermined feature in a cycle of the current waveform; and
deriving rotor position from the occurrence of the feature.
2. A method as claimed in claim 1 in which the current is continuous in the cycle.
3. A method as claimed in claim 2 in which the continuous current is established for a plurality of cycles.
4. A method as claimed in claim 1 in which the electrical machine is a switched reluctance machine.
5. A method as claimed in claim 1 in which the current is established by voltage pulses applied across the phase winding.
6. A method as claimed in claim 5 in which the voltage pulses are of one polarity and the voltage across the phase winding between the pulses is non-zero and of the opposite polarity.
7. A method as claimed in claim 1 in which the current is substantially in steady state.
8. A method as claimed in claim 1 in which the feature in the cycle of the current waveform substantially coincides with a change of gradient of the current waveform.
9. A method as claimed in claim 1 in which the feature in the cycle of the current waveform coincides with the start of rising inductance in an inductance profile of the machine.
10. A method as claimed in claim 1 in which the feature in the cycle of the current waveform coincides with the position of minimum inductance of the phase of the machine.
11. A method as claimed in claim 1 in which the feature in the cycle of the current waveform coincides with the position of maximum inductance of the phase of the machine.
12. A method as claimed in claim 1 in which the rotor is coasting when the method is initiated.
13. A position determination system for a moving rotor of an electrical machine having a stator with at least one phase winding, the system comprising:
means for applying a voltage to the phase winding, the voltage waveform having a period which is independent of the period of movement of the rotor, which voltage establishes a current in the phase winding, the current waveform being cyclic with movement of the rotor;
means for detecting a predetermined feature in a cycle of the current waveform; and
means for deriving rotor position from the occurrence of the feature.
14. A system as claimed in claim 13 in which the means for applying is operable to establish a continuous current in the cycle.
15. A system as claimed in claim 14 in which the means for applying is operable to establish the current continuously for a plurality of cycles.
16. A system as claimed in claim 13 in which the electrical machine is a switched reluctance machine.
17. A system as claimed in claim 13 in which the means for applying a voltage is operable to generate voltage pulses of one polarity and a voltage across the phase winding between pulses which is non-zero and of the opposite polarity.
18. A system as claimed in claim 13 in which the means for detecting a predetermined feature is operable to detect a change of gradient of the current waveform.
19. A system as claimed in claim 13 in which the means for detecting is operable to detect the point in the current waveform coincident with the position of the start of rising inductance in an inductance profile of the machine.
20. A system as claimed in claim 13 in which the means for detecting a predetermined feature is operable to detect a point coincident with the position of maximum inductance in the phase of the machine.
21. A system as claimed in claim 13 in which the means for detecting a predetermined feature is operable to detect a point coincident with the position of minimum inductance in the phase of the machine.
22. A computer program product which when loaded on a processor is arranged to execute the method of claim 1 .