WO2007015065A1

WO2007015065A1 - Synchronous motors

Info

Publication number: WO2007015065A1
Application number: PCT/GB2006/002804
Authority: WO
Inventors: David Ronald Gibson
Original assignee: Stored Energy Technology Limited
Priority date: 2005-07-30
Filing date: 2006-07-26
Publication date: 2007-02-08
Also published as: GB2428916B; EP1911147A1; GB2428916A; JP2009504126A; GB0515718D0

Abstract

A salient pole synchronous motor (24) has three phases of stator coil (26) and a common neutral conductor (28). Drive voltages are applied at (30). A power supply (32) excites the coils (26) by AC drive voltages. The inductance of each phase is detected at (34) and a comparison at (36) reveals information about the relative positions of the stator and rotor of the motor (24).

Description

Synchronous Motors

The present invention relates to synchronous motors. In particular, the present invention relates to rotor position sensing in salient pole synchronous motors.

During normal operation, a synchronous motor rotates in synchronism with the frequency of the supply. Varying the drive voltage does not vary the speed. Torque output can be controlled by controlling the in phase supply current.

At medium and high speed, the back EMF produced in the motor coils is significant, compared with the voltage drop in the windings and consequently, power factor control can be used to maintain synchronism. Power factor control is a technique which detects quadrature current and uses negative feedback to minimise it, resulting in the motor running in phase, which is the most efficient operating condition. However, at start-up and at low speed, any back EMF is insufficient for power factor control to be used. The motor can be forced into synchronism by a high drive current, but will then not operate efficiently.

It has previously been proposed to use a shaft encoder, based on optical or Hall effect sensing techniques, to detect the rotor position and thus allow synchronism to be established, even at slow speeds. The use of an encoder introduces additional reliability issues, particularly if the motor is in use in an environment which is hostile to the technology on which the shaft encoder is based.

The present invention provides a salient pole synchronous motor, comprising: a first portion which provides, in use, a substantially constant magnetic field, a second portion having a plurality of coils which, when excited, generate magnetic flux to interact with the first portion field to create relative rotation between the first and second portions, means operable to excite the coils by drive voltages supplied to the coils in a plurality of phases, means operable to detect the inductance of each phase, and means operable to compare the inductance values to obtain information about the relative positions of the first and second portions.

Preferably, the first portion is a rotor and the second portion is a stator.

The comparison means preferably compares the inductance values of each pair of phases to obtain position information from the results of the comparison. The comparison means may obtain coarse position information as a range of possible relative positions, the range being determined by identifying which of each pair of phases has the greater inductance value, there being a predetermined range associated with each combination of phases identified by the comparisons.

The comparison means is preferably operable to obtain a position within the associated range by extrapolation from the relative values of the inductances of at least one pair of phases. Alternatively, there may be means operable to compensate the detected inductance values in accordance with changes in the drive voltages, whereby to obtain a position within the associated range by measurement of the inductance value.

Detection means are preferably associated with at least one coil of each phase, and comprise means operable to measure current in the coil winding. The measurement means preferably comprise a toroid of magnetic material around a conductor carrying coil winding current, and a tap coil around the toroid, whereby the voltage created in the tap coil is dependent on the coil winding current.

The coil excitation means may provide pulse width modulated excitation voltages, there being filter means provided at the tap coil output to substantially remove the PWM modulation from the output, and wherein the measurement means measures the amplitude of the filtered output.

Preferably, when the motor is started from rest, the coil excitation means is operable to provide, in sequence, a first and a second quadrature excitation pulse to at least one phase, the first and second quadrature excitation pulses having opposite polarity and the detection means being operable to detect the inductance values so caused, there being discrimination means which uses the relative sizes of the induction values to discriminate between relative positions which are 180° apart.

Preferably the quadrature pulses are provided to all phases. The inductance values of all phases are preferably summed for each excitation pulse, and the summed values are compared to discriminate as aforesaid.

Preferably, after the motor has started, the discrimination means monitors relative position information obtained by the comparison means, in order to detect each 180° of rotation and thereby to maintain discrimination between positions which are 180° apart.

The first portion preferably includes permanent magnets for creating the substantially constant magnetic field. The second portion coils are preferably connected in three phases.

The present invention also provides a control arrangement for a salient pole synchronous motor of the type which comprises: a first portion which provides, in use, a substantially constant magnetic field; and a second portion having a plurality of coils which, when excited, generate magnetic flux to interact with the first portion field to create relative rotation between the first and second portions; the control arrangement comprising: means operable to excite the coils by drive voltages supplied to the coils in a plurality of phases, means operable to detect the inductance of each phase, and means operable to compare the inductance values to obtain information about the relative positions of the first and second portions.

The comparison means preferably compares the inductance values of each pair of phases to obtain position information from the results of the comparison. The comparison means may obtain coarse position information as a range of possible relative positions, the range being determined by identifying which of each pair of phases has the greater inductance value, there being a predetermined range associated with each combination of identified phases.

The comparison means is preferably operable to obtain a position within the associated range by extrapolation from the relative values of the inductances of at least one pair of phases. Alternatively, there may be means operable to compensate the detected inductance values in accordance with changes in the drive voltages, whereby to obtain a position within the associated range.

Preferably the quadrature pulses are provided to all phases. The inductance values of all phases are preferably summed for each excitation pulse, and the summed values are compared to discriminate as aforesaid. Preferably, after the motor has started, the discrimination means monitors relative position information obtained by the comparison means, in order to detect each 180° of rotation and thereby to maintain discrimination between positions which are 180° apart.

The present invention also provides a method of controlling a salient pole synchronous motor which comprises:

a first portion which provides, in use, a substantially constant magnetic field, a second portion having a plurality of coils which, when excited, generate magnetic flux to interact with the first portion field to create relative rotation between the first and second portions, the method including the steps of: exciting the coils by drive voltages supplied to the coils in a plurality of phases, detecting the inductance of each phase, and comparing the inductance values to obtain information about the relative positions of the first and second portions.

The comparison preferably compares the inductance values of each pair of phases to obtain position information from the results of the comparison. The comparison may obtain coarse position information as a range of possible relative positions, the range being determined by identifying which of each pair of phases has the greater inductance value, there being a predetermined range associated with each combination of identified phases.

The comparison preferably obtains a position within the associated range by extrapolation from the relative values of the inductances of at least one pair of phases. Alternatively, the detected inductance values may be compensated in accordance with changes in the drive voltages, whereby to obtain a position within the associated range.

Detection is preferably associated with at least one coil of each phase, and may measure current in the coil winding.

The coil excitation may provide pulse width modulated excitation voltages, there being filter means provided to substantially remove the PWM modulation from the detected output, and wherein the measurement measures the amplitude of the filtered output. Preferably, when the motor is started from rest, the coil excitation provides, in sequence, a first and a second quadrature excitation pulse to at least one phase, the first and second quadrature excitation pulses having opposite polarity and the detection detects the inductance values so caused, and uses the relative sizes of the induction values to discriminate between relative positions which are 180° apart.

Preferably, after the motor has started, relative position information obtained by the comparison means is monitored, in order to detect each 180° of rotation and thereby to maintain discrimination between positions which are 180° apart.

Examples of the present invention will now be described in more detail, by way of example only, and with reference to the accompanying drawings, in which:

Fig. 1 is a schematic, simplified diagram of a part of the rotor and stator of a salient pole synchronous motor of the type with which the present invention may be implemented;

Fig. 2 is a diagram of the apparatus according to the present invention;

Fig. 3 is a detector circuit for use with the apparatus of Fig. 2;

Fig. 4 is a diagram of signals obtained by detector circuits of Fig. 3; and

Figs. 5 and 6 are flow diagrams of the start-up and running procedures implemented in accordance with the present invention. Background

Fig. 1 illustrates part of a salient pole synchronous motor of the type with which the present invention may be implemented. The motor 10 has a first portion 12 in the form of a rotor of permanent magnets 14 which provide a substantially constant magnetic field in the environment of a second portion 16, which is in the form of a stator. The stator has a ring of coils 18 mounted on pole pieces 20.

When the coils 18 are excited by an alternating drive voltage, they generate a magnetic flux which interacts with the field created by the rotor 12 to create relative rotation between the rotor and stator 12, 16, as indicated by the arrow 22, the axis of rotation being perpendicular to the plane of the drawing. The requirements for driving a salient pole permanent magnet synchronous motor are well known in themselves.

Overview

Fig. 2 shows a salient pole synchronous motor 24, in accordance with the present invention. This has three phases of stator coils illustrated symbolically in Fig. 2 as three coils 26 connected between a common neutral conductor 28 and respective drive voltages applied at 30. Features of the rotor are omitted, for clarity, but is arranged with the stator coils 26 and other stator components (not shown) in the manner illustrated in Fig. 1. Means are provided in the form of a power supply 32 to excite the coils 26 by alternating drive voltages supplied to the coils 26 in the respective three phases. Detector means indicated generally at 34 are provided to detect the inductance of each phase, during operation. These will be described in more detail below. Comparison means 36 compare the inductance values to obtain information about the relative positions of the stator and rotor of the motor 24.

The comparison means 36 forms part of a controller 38 which also includes a control function 40 for the power supply 32, a compensation circuit 42 to provide compensation information to the comparison means 36, from the power control 40, a 180° discriminator 44 and a supervisor 46. The supervisor 46 is provided with input and output connections 48, for example to receive instructions from an operator and to provide position information to an operator.

Drive voltages to the coils 26 are provided as a pulse width modulated (PWM) pulse train. Thus, the pulse lengths of a high frequency carrier from the power supply 32 are controlled by the arrangement 40 to be equivalent, in the coils 26, of an applied sinusoidal excitation.

Detecting Inductance

The example being described requires the inductance of each phase of the motor 24 to be detected. The inductance changes cyclically as the rotor turns, as can be understood from Fig. 1. Magnetic coupling between the permanent magnets 14 and the coils 18 will change as the coils turn from a position in which they are aligned with one of the magnets 14, through intermediate positions, until being aligned with the next magnet 14 around the stator 12. In one experimental motor we have constructed, the inductance of each phase is in the order of 200 μH. Measurement of inductances of this order requires the use of relatively high frequency signals. We have realised that the carrier frequency of the PWM pulses is typically at a sufficiently high frequency (such as 10 kHz). The manner in which the inductance of each phase is detected is illustrated simply in Fig. 2 and in more detail in Fig. 3. Fig. 3 shows the arrangement for detecting the inductance of a single phase. A similar arrangement is provided for each phase. In Fig. 3, conductor 50 is providing the connection between the power supply 32 and one of the coils 26. An iron powder toroid 52 is placed around the conductor 50 and is chosen to have sufficient reluctance to prevent saturation at the operating current in the conductor 50. A tap coil 54 is provided on the toroid 52 and the output is tuned by a parallel capacitor 56 to the PWM carrier frequency. Consequently, a waveform at the PWM carrier frequency is extracted to appear at 58, as illustrated schematically in Fig. 3. This waveform is an amplitude modulated sinusoid at the PWM carrier frequency, the amplitude modulation arising from the changes in inductance, noted above. The amplitude modulated waveform is applied to a rectifier 60 and then a filter 62 to leave a DC level which rises and falls with the inductance of the corresponding phase, again as schematically illustrated in Fig. 3. This information about the inductance is applied to an analogue-to-digital converter 66 to provide a digital value representing the instantaneous inductance of the corresponding phase. This inductance measurement is provided to the comparison means 36. Fig. 3 also indicates two further inputs to the comparison means 36, from the similar circuits for the other two phases. Alternatively, a single converter could be used, connected to the three phases in turn, to extract sufficient information for the comparisons to be made.

Comparison Between Phases

The inductance of each phase will vary substantially sinusoidally as the motor rotates. Fig. 4 illustrates the inductance of two phases of a three phase motor as they change across a complete sinusoid. The two phases are 120° apart in phase. They cross at two positions 68A, 68B. Consequently, a simple comparison of which inductance is instantaneously the greater will reveal simple position information, indicating whether or not the current position lies between the positions representing the crossing positions 68, or outside that range. ^■

This comparison is effected by the comparison means 36. Specifically, the comparison means 36 compares the inductance values of each pair of phases. Identifying which of each pair of phases has the greater inductance value allows coarse position information to be determined as a range of possible positions, there being a different range associated with each possible logical combination of results. (In fact, this reveals position information which repeats every 180° of rotor position, as will be explained more fully below).

Thus, with three phases, 120° apart in phase and called phase A, phase B and phase C, if phase A is greater than phase B, the rotor is between 165° and 75°. If phase B is greater than phase C, the result is between 45° and 135°. If phase C is greater than phase A, the result is between 105° and 15°. Thus, from these combinations (and the inverse combinations), the rotor position becomes known to within 30°. Thus, an instantaneous measurement of the inductance of each phase, and comparison of each pair of phases, identifies the rotor position to a range of 30°, within 180°. The full logic tables of the decision taken by the comparison means 36 is as follows:

TABLE 1 - COMPARISON OF PHASE PAIRS

TABLE 2 - COMBINATIONS OF COMPARISON RESULTS

As has been noted, the technique thus far described will result in the same result from any two rotor positions set 180° apart.

Returning to Fig. 4, it becomes apparent that additional information can be extracted by looking at the absolute values of a pair of phases, between crossing points. Having decided on the 30° range, as above, and assuming constant amplitude, the relative values of the two inductances shown in Fig. 4 (such as a ratio of the two values) corresponds uniquely with a particular rotor position within the 30° range. A ratio is vulnerable to changes in amplitude which will arise in practice. Consequently, this use of absolute values can be seen as an approximate but reasonable extrapolation of position between the two positions which are represented by the crossing positions 68 and are thus known absolutely.

Alternatively, the comparison means 36 is provided with information by the compensation circuit 42 and relating to the drive currents being applied to the phases. This allows compensation of a measured inductance value to recover a more precise position from the 30° range set, by measuring the instantaneous inductance value of a single phase.

180° Discrimination

It has been noted that the technique so far described cannot distinguish between each pair of equivalent positions which are 180° apart. This additional information is provided, as follows.

The 180° discriminator 44 maintains a logic flag which indicates which of the two possible 180° arcs the rotor is currently in. The output of the comparison means 36 is provided to the 180° discriminator 44 to allow the discriminator 44 to detect the completion of each half turn (six 30° ranges) and change the value of the flag.

Monitoring the output of the comparison means 36 allows relatively straightforward maintenance of the correct flag value, once the initial value has been set correctly. This is achieved by means of the sequence of steps illustrated in Fig. 5. An instruction to start the motor is received at step 70 by the supervisor 46. Before generating drive voltages in the normal way, it is necessary to establish the current position of the rotor. This is achieved in two stages, firstly to measure without discrimination between pairs of positions separated by 180°, and then to discriminate between the two positions of the measured pair. The first step (step 71) is achieved in a manner which will be described below (with reference to steps 84 to 90). For the second step (step 72) the control arrangement 40 is instructed to cause a first quadrature current pulse to be injected into each phase. The comparison means 36 is also instructed that quadrature current pulses are being injected. The comparison means 36 receives at 74 three induction values LA⁺, LB⁺ and Lc⁺ created by the first quadrature current pulses. We have realised that non-linearity in the magnetic circuits of any practical machine, and a tendency to saturate, will lead to measurable differences between the inductances of the three phases when fed with quadrature current pulses, but these pulses do not contribute to the torque output of the motor. Second quadrature current pulses of opposite polarity are then instructed at step 76 and further inductance values L_A ^', L_B ^" and Lc^' are received at 78. The inductance measurements for the two polarities of quadrature pulses will differ, depending on whether the resultant magnetic field tends to add to the field provided by the rotor, or be opposed to it. The comparison means 36 then compares, at step 80, the sum of the two sets of three values. This results in two possible outcomes, allowing the flag held by the discriminator to be initiated at step 82A or 82B to the appropriate value by detecting the initial position of the rotor.

Operation After Start-Up

Once the motor has been started, and the discriminator flag has been set, position information is obtained in accordance with the sequence set out in Fig. 6. At step 84, three inductance values L_A, LB and Lc are received from each of the three phases, as described above. These values are reported at a frequency which is high relative to the rotation frequency of the rotor, so that many position measurements are taken during each rotation. At steps 86A, B, C the three comparisons between each pair of phases are made. Coarse position is then determined at 88 according to the outcome of the comparisons, as noted above. The sequence then may revert to step 84, or execute additional, optional steps, as follows.

At step 90, extrapolation between coarse positions is implemented, for example by one of the techniques described above. The 180° degree flag maintained by the discriminator 44 is checked at 92. A determination of a unique position is made at 94, based on the coarse position determined at 88, the extrapolation performed at 90 and the check performed at 92.

At step 96, a determination is made as to whether a complete half turn has been completed and if so, the 180° flag is changed at 98.

In a further optional sequence, the current speed may be determined at

100. If current speed is greater than a threshold w, the supervisor 46 may implement at step 102 a change to power factor control of the motor 24.

After any selected optional steps 90 to 102 have been completed, control returns from step 104 to step 84 for the complete sequence to repeat.

Possible Alternatives

The various functions implemented by the block diagram of Fig. 2 may be implemented by many different technologies, including hardware, software and mixtures thereof. Other arrangements could be provided for measuring inductances. Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

1. A salient pole synchronous motor, comprising: a first portion which provides, in use, a substantially constant magnetic field, a second portion having a plurality of coils which, when excited, generate magnetic flux to interact with the first portion field to create relative rotation between the first and second portions, means operable to excite the coils by drive voltages supplied to the coils in a plurality of phases, means operable to detect the inductance of each phase, and means operable to compare the inductance values to obtain information about the relative positions of the first and second portions.

2. A salient pole synchronous motor as claimed in claim 1 wherein the first portion is a rotor and the second portion is a stator.

3. A salient pole synchronous motor as claimed in any preceding claim wherein the comparison means compares the inductance values of each pair of phases to obtain position information from the results of the comparison.

4. A salient pole synchronous motor as claimed in claim 3 wherein the comparison means obtains coarse position information as a range of possible relative positions.

5. A salient pole synchronous motor as claimed in claim 4 wherein the range is determined by identifying which of each pair of phases has the greater inductance value, there being a predetermined range associated with each combination of phases identified by the comparisons.

6. A salient pole synchronous motor as claimed in claim 5 wherein the comparison means is operable to obtain a position within the associated range by extrapolation from the relative values of the inductances of at least one pair of phases.

7. A salient pole synchronous motor as claimed in any preceding claim wherein the comparison means is operable to compensate the detected inductance values in accordance with changes in the drive voltages, whereby to obtain a position within the associated range by measurement of the inductance value.

8. A salient pole synchronous motor as claimed in any preceding claim wherein the detection means are associated with at least one coil of each phase.

9. A salient pole synchronous motor as claimed in claim 8 wherein the detection means comprises means operable to measure current in the coil winding.

10. A salient pole synchronous motor as claimed in claim 9 wherein the measurement means comprises a toroid of magnetic material around a conductor carrying coil winding current, and a tap coil around the toroid, whereby the voltage created in the tap coil is dependent on the coil winding current.

11. A salient pole synchronous motor as claimed in any preceding claim wherein the coil excitation means provides pulse width modulated excitation voltages.

12. A salient pole synchronous motor as claimed in claim 11 wherein filter means are provided at the tap coil output to substantially remove the PWM modulation from the output, and wherein the measurement means measures the amplitude of the filtered output.

13. A salient pole synchronous motor as claimed in any preceding claim wherein when the motor is started from rest, the coil excitation means is operable to provide, in sequence, a first and a second quadrature excitation pulse to at least one phase.

14. A salient pole synchronous motor as claimed in claim 13 wherein the first and second quadrature excitation pulses have opposite polarity and the detection means are operable to detect the inductance values so caused.

15. A salient pole synchronous motor as claimed in claim 14 wherein there is provided discrimination means which use the relative sizes of the induction values to discriminate between relative positions which are 180° apart.

16. A salient pole synchronous motor as claimed in any of claims 13 to 15 wherein the quadrature pulses are provided to all phases.

17. A salient pole synchronous motor as claimed in claim 16 wherein the inductance values of all phases are summed for each excitation pulse, and the summed values are compared to discriminate between relative positions which are 180° apart.

18. A salient pole synchronous motor as claimed in claim 17 wherein after the motor has started, the discrimination means monitors relative position information obtained by the comparison means, in order to detect each 180° of rotation and thereby to maintain discrimination between positions which are 180° apart.

19. A salient pole synchronous motor as claimed in any preceding claim wherein the first portion includes permanent magnets for creating the substantially constant magnetic field.

20. A salient pole synchronous motor as claimed in any preceding claim wherein the second portion coils are connected in three phases.

21. A control arrangement for a salient pole synchronous motor of the type which comprises: a first portion which provides, in use, a substantially constant magnetic field; and a second portion having a plurality of coils which, when excited, generate magnetic flux to interact with the first portion field to create relative rotation between the first and second portions; the control arrangement comprising: means operable to excite the coils by drive voltages supplied to the coils in a plurality of phases, means operable to detect the inductance of each phase, and means operable to compare the inductance values to obtain information about the relative positions of the first and second portions.

22. A control arrangement for a salient pole synchronous motor as claimed in claim 21 wherein the comparison means compares the inductance values of each pair of phases to obtain position information from the results of the comparison.

23. A control arrangement for a salient pole synchronous motor as claimed in any of claims 21 to 22 wherein the comparison means obtains coarse position information as a range of possible relative positions.

24. A control arrangement for a salient pole synchronous motor as claimed in claim 22 wherein the range is determined by identifying which of each pair of phases has the greater inductance value, there being a predetermined range associated with each combination of identified phases.

25. A control arrangement for a salient pole synchronous motor as claimed in any of claims 21 to 24 wherein the comparison means is operable to obtain a position within the associated range by extrapolation from the relative values of the inductances of at least one pair of phases.

26. A control arrangement for a salient pole synchronous motor as claimed in any of claims 21 to 25 wherein the comparison means is operable to compensate the detected inductance values in accordance with changes in the drive voltages, whereby to obtain a position within the associated range.

27. A control arrangement for a salient pole synchronous motor as claimed in any of claims 21 to 26 wherein the detection means are associated with at least one coil of each phase.

28. A control arrangement for a salient pole synchronous motor as claimed in claim 27 wherein the detection means comprises means operable to measure current in the coil winding.

29. A control arrangement for a salient pole synchronous motor as claimed in claim 28 wherein the measurement means comprises a toroid of magnetic material around a conductor carrying coil winding current, and a tap coil around the toroid, whereby the voltage created in the tap coil is dependent on the coil winding current.

30. A control arrangement for a salient pole synchronous motor as claimed in any of claims 21 to 29 wherein the coil excitation means provides pulse width modulated excitation voltages.

31. A control arrangement for a salient pole synchronous motor as claimed in claim 30 wherein there is provided filter means at the tap coil output to substantially remove the PWM modulation from the output, and wherein the measurement means measures the amplitude of the filtered output.

32. A control arrangement for a salient pole synchronous motor as claimed in any of claims 21 to 31 wherein when the motor is started from rest, the coil excitation means is operable to provide, in sequence, a first and a second quadrature excitation pulse to at least one phase.

33. A control arrangement for a salient pole synchronous motor as claimed in claim 32 wherein the first and second quadrature excitation pulses have opposite polarity and the detection means are operable to detect the inductance values so caused.

34. A control arrangement for a salient pole synchronous motor as claimed in claim 33 wherein there is provided discrimination means which use the relative sizes of the induction values to discriminate between relative positions which are 180° apart.

35. A control arrangement for a salient pole synchronous motor as claimed in any of claims 32 to 34 wherein the quadrature pulses are provided to all phases.

36. A control arrangement for a salient pole synchronous motor as claimed in claim 35 wherein the inductance values of all phases are summed for each excitation pulse, and the summed values are compared to discriminate between relative positions which are 180° apart.

37. A control arrangement for a salient pole synchronous motor as claimed in claim 36 wherein after the motor has started, the discrimination means monitors relative position information obtained by the comparison means, in order to detect each 180° of rotation and thereby to maintain discrimination between positions which are 180° apart.

38. A method of controlling a salient pole synchronous motor which comprises:

39. A method of controlling a salient pole synchronous motor as claimed in claim 38 wherein the comparison compares the inductance values of each pair of phases to obtain position information from the results of the comparison.

40. A method of controlling a salient pole synchronous motor as claimed in any of claims 38 to 39 wherein the comparison obtains coarse position information as a range of possible relative positions.

41. A method of controlling a salient pole synchronous motor as claimed in any of claims 38 to 40 wherein the range is determined by identifying which of each pair of phases has the greater inductance value, there being a predetermined range associated with each combination of identified phases.

42. A method of controlling a salient pole synchronous motor as claimed in any of claims 38 to 41 wherein the comparison obtains a position within the associated range by extrapolation from the relative values of the inductances of at least one pair of phases.

43. A method of controlling a salient pole synchronous motor as claimed in any of claims 38 to 41 wherein the detected inductance values are compensated in accordance with changes in the drive voltages, whereby to obtain a position within the associated range.

44. A method of controlling a salient pole synchronous motor as claimed in any of claims 38 to 43 wherein detection is associated with at least one coil of each phase, and measures current in the coil winding.

45. A method of controlling a salient pole synchronous motor as claimed in any of claims 38 to 44 wherein the coil excitation provides pulse width modulated excitation voltages.

46. A method of controlling a salient pole synchronous motor as claimed in claim 45 wherein there is provided filter means to substantially remove the PWM modulation from the detected output, and . wherein the measurement measures the amplitude of the filtered output.

47. A method of controlling a salient pole synchronous motor as claimed in any of claims 38 to 46 wherein when the motor is started from rest, the coil excitation provides, in sequence, a first and a second quadrature excitation pulse to at least one phase.

48. A method of controlling a salient pole synchronous motor as claimed in any of claims 38 to 47 wherein the first and second quadrature excitation pulses have opposite polarity and the detection detects the inductance values so caused.

49. A method of controlling a salient pole synchronous motor as claimed in claim 48 wherein, the detection uses the relative sizes of the induction values to discriminate between relative positions which are 180° apart.

50. A method of controlling a salient pole synchronous motor as claimed in any of claims 47 to 49 wherein the quadrature pulses are provided to all phases.

51. A method of controlling a salient pole synchronous motor, as claimed in claim 50, wherein the inductance values of all phases are summed for each excitation pulse, and the summed values are compared to discriminate between relative positions which are 180° apart.

52. A method of controlling a salient pole synchronous motor as claimed in claim 50 or 51 , wherein after the motor has started, relative position information obtained by the comparison means is monitored, in order to detect each 180° of rotation and thereby to maintain discrimination between positions which are 180° apart.

53. A salient pole synchronous motor, substantially as described above, with reference to the accompanying drawings.

54. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.