Rotor position detection of an electrical machine
An electrical machine is controlled without using a physical rotor position detector. When more than one phase is used simultaneously to produce sufficient torque for the application, the increased leakage flux and the reduced permeability of parts of the magnetic circuit distort the information gleaned from diagnostic pulses. By storing sets of characterizing data appropriate to the number of phases in use, the control system is able to determine the rotor position more accurately.
1. A method of determining rotor position in a polyphase electrical machine in which the phases are separately excitable, the method comprising:
applying a diagnostic pulse to an idle phase;
integrating applied voltage in the idle phase to derive a value associated with idle phase flux linkage;
measuring idle phase current to produce a measured current value;
compensating the measured current value for the current due to leakage flux in the idle phase; and
deriving a rotor position from the compensated current and the phase flux linkage.
2. A method as claimed in claim 1 in which compensating for current due to leakage flux includes:
measuring current in the idle phase substantially coincident with applying the diagnostic pulse to derive a value indicative of current due to leakage flux; and
subtracting the value indicative of current due to leakage flux from the measured current value to derive the compensated current.
3. A method as claimed in claim 1 in which compensating for current due to leakage flux includes:
storing pre-characterized values of current for values of rotor position, the values of current representing current due to phase flux linkage and current due to leakage flux;
wherein the rotor position is derived by accessing the stored value of rotor position for the measured current value.
4. A method as claimed in claim 3 in which the values of current are derived empirically.
5. A method as claimed in claim 3 in which the values of current are derived by finite element analysis of the machine.
6. A method as claimed in claim 1 in which the diagnostic pulse is of substantially constant flux.
7. A method as claimed in claim 1 including applying the diagnostic pulse to the idle phase when at least two other phases are excited together.
8. A system for determining rotor position in a polyphase electrical machine in which the phases are separately excitable, the system comprising:
means for applying a diagnostic pulse to an idle phase;
means for integrating applied voltage in the idle phase to derive a value associated with idle phase flux linkage;
means for measuring idle phase current to produce a measured current value;
means for compensating the measured current value for the current due to leakage flux in the idle phase; and
means for deriving a rotor position from the compensated current and the phase flux linkage.
9. A system as claimed in claim 8 in which the means for compensating for current due to leakage flux includes:
means for measuring current in the idle phase substantially coincident with applying the diagnostic pulse to derive a value indicative of current due to leakage flux; and
means for subtracting the value indicative of current due to leakage flux from the measured current value to derive the compensated current.
10. A system as claimed in claim 8 in which the means for compensating for compensating for current due to leakage flux includes:
means for storing pre-characterized values of current for values of rotor position, the values of current representing current due to phase flux linkage and current due to leakage flux; and
means for deriving the rotor position by accessing the value of rotor position for the measured current value.
11. A system as claimed in claim 8 in which the electrical machine is a switched reluctance machine working as a motor or a generator.
12. A system as claimed in claim 8 in which the means for applying is operable to apply a diagnostic pulse of predetermined magnitude of flux linkage.
13. A system as claimed in claim 8 in which the means for applying is arranged to apply the diagnostic pulse to the idle phase when at least two other phases are excited together.
14. A computer readable medium loaded with a program operable to perform the method of claim 1 .
15. A method as claimed in claim 1 in which the electrical machine is a switched reluctance machine working as a motor or a generator.