GB1559004A

GB1559004A - Brushless dc motors

Info

Publication number: GB1559004A
Application number: GB2513676A
Authority: GB
Original assignee: Danfoss AS
Current assignee: Danfoss AS
Priority date: 1975-06-18
Filing date: 1976-06-17
Publication date: 1980-01-09

Description

(54) BRUSHLESS D. C. MOTORS (71) We, DANFOSS A/S, a Danish Company, of DK6430 Nordborg, Denmark, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to brushless D.C.

motors.

A known type of brushless self-starting D.C. motor comprises a permanent magnet rotor and a starter winding, and a magnetic field-dependent component subjected to the rotor field to determine the angular position of the rotor. The operating circuit of this type of motor comprises an electronic control element connected between a D.C. source and the stator winding, the control element controlling the flux generated by the stator winding in dependence on the angular position of the rotor, the magnetic field-dependent parameter changes of the magnetic field-dependent component being convertible to a control signal for the control element.

An operating circuit of this kind for a brushless D.C. motor is known, wherein a Hall generator is used as magnetic field-dependent component so that a signal is developed from the rotor field even when the rotor is stationary and starting is made independent of the rotary speed. By way of a control circuit comprising a plurality of transistors, the Hall voltage controls the control element that is in series with the stator coil. However, by reason of its numerous terminals, a Hall generator involves additional wiring expense for the motor and the operating circuit as well as between the operating circuit and the motor, even if there is only one stator coil. The Hall generator requires a constant current supply, the Hall current, which reduces the overall efficiency.Further, a Hall generator is dependent on temperature, which can result in displacement of the switching point of the control element, a transistor operated as a switch and thus to a reouction m the motor efficiency. The efficiency of the operating circuit, the D.C. motor and the total required expense play a decisive role in many cases, particularly when using the D.C. motor for hermetically encapsulated refrigerators which, in relation to their useful life, are frequently in operation and are made in large numbers, particularly in the case of accumulator-driven refrigerators for leisure use, e.g. in caravans, boats, holiday cottages etc. and also for batteryoperated household appliances such as tape machines.

Brushless D.C. motors with more than two stator coils are also known. In these the magnetic alternating field of the oscillating coil of a continuously oscillating LC-oscillator alternately induces a control voltage in control coils which are disposed in the control circuit of a stator coil and are distributed over the periphery of the stator, so that a metallic segment rotating with the rotor consecutively couples the magnetic field of the oscillating coil to all the control coils or shields it from all the control coils except one. This involves a still higher expense with regard to wiring of the control coils and their accommodation at the stator as well as the construction of the coupling or screening means.

The present invention provides a brushless self-starting D.C. (direct current) motor connected to a mechanical load having, in use, alternate working and idle strokes, the motor comprising a permanent magnet rotor including at least two poles of opposite senses, a sensing coil arranged to sense the rotor position by means of the change with rotor angle of rotor field with the coil, and an operating circuit connected to the sensing coil and stator winding to control the supply of stator current in accordance with rotor position, wherein the sensing coil includes a pre-magnetised core, the relationship between the level of pre-magnetisation of the core and the rotor field linked with the core is such that on rotation of the rotor the core saturates in one direction in response to the filed from one of the said at least two poles but does not saturate in the opposite direction in response to the opposite pole, the motor is such that on de-energisation the rotor assumes a position with its pole axis at an acute angle to the pole axis of the stator, and the motor and mechnical load are arranged so that in starting from its de-energisation rotor position the motor starts on an idle stroke of the mechanical load.

The motor can be connected to a refrigeration compressor, the suction strokes of which are the idle strokes.

The peripheral air gap between rotor and stator can be non-uniform.

A permanent magnet can be provided near the periphery of the rotor.

The rotor can have an asymmetrical pole distribution.

The core can, at least in part, be of ferrite material.

The core can include a permanent magnet.

The motor can have just two stator coils.

One end of each stator coil and one end of the sensing coil can be commonly connected.

The two stator coils can be wound in close juxtaposition and can be substantially identical.

The operating circuit can comprise an oscillation circuit in which the inductance of the sensing coil is connected as part of a resonant circuit to produce oscillations modulated by the rotor field linkage, current control means connected to the stator winding, and means connected to control the current control means in accordance with the output of the oscillation circuit.

The oscillation circuit can be such that there is, in use, an ON-OFF modulation of oscillation.

The means connected to control the current control means can comprise a demodulator or drive a rectangular waveform from the envelope of the oscillations.

The demodulator can comprise rectifier means and a smoothing capacitor.

The current control means can comprise power transistor switches.

The means connected to control the current control means can be operative to apply the rectangular waveform to a first power transistor switch and the rectangular waveform inverted to a second power transistor switch.

A 1800 phase-splitter can be connected between the output of the oscillation circuit and the current control means.

Each power transistor switch can be preceded by a respective transistor pre-amplifier.

A respective current limiting resistor can be connected between each transistor pre-amplifier and its associated power transistor switch.

A respective diode can be connected in anti-parallel with each power transistor switch.

Means can be provided to control the current control means additionally in response to a physical parameter external to the circuit.

The means to provide the additional control in response to a physical parameter can comprise a blocking circuit responsive to the physical parameter and connected between the output of the oscillation circuit and the current control means.

The physical parameter can be temperature.

Brushless D.C. motors constructed in accordance with the invention will now be described with reference to the accompanying drawings, of which: Fig. 1 illustrates a brushless D.C. motor and the operating circuit of the motor; Fig. 2 shows the BH characteristic of a sensing coil; Figs. 3 and 4 show alternative forms of core for the sensing coil; Fig. 5 shows a motor with bifilar-wound stator coils; Figs. 6 and 7 illustrate stator and rotor arrangements for achieving a stable holding point for the rotor when the stator coil is switched off; Fig. 8 is a circuit diagram of the operating circuit, and Fig. 9 shows a control signal as a function of the rotary angle of the rotor before and after demodulation.

Referring to Figure 1, a brushless D.C.

motor has a stator 1 provided with two coils 2 and 3. Each coil 2,3 is in series with a D.C.

source 6 and a respective control element 4, 5 in the form of a power transistor operated as a switch. A respective diode 7, 8 is connected in anti-parallel with each control element 4, 5. The angular or rotary position of a rotor 9 in the form of a permanent magnet is determined by means of a sensing coil 10 which is disposed in the vicinity of the rotor 9 and has a premagnetised saturable core. The sensing coil 10 has one end connected to the same pole of the D.C.

source 6 as that to which the coils 2 and 3 are directly connected and its other end connected to a control unit 11 so that merely four connecting leads 12, 13, 14 and 15 suffice for the motor.

Special provision is made, as hereinafter described, for the rotor 9 to assume the illustrated rest position when the coils 2, 3 are de-energised; in this rest position, the pole axis of the rotor makes an acute angle with the pole axis of the stator 1. The field of the rotor 9 in this position of the rotor 9 results in an induced field in the core of the sensing coil 10 such that the control unit supplies the control element 4 with a switchon control signal. The control element 4 is thereupon switched on and a current flows in the lead 12 in the direction indicated by the arrow. The resulting South pole S of the coil 2 repels the South ole S of the rotor 9 so that the rotor 9 beings to rotate in the direction indicated by the curved arrow.After half a revolution of the rotor 9, the induced field in the core of the sensing coil 10 has changed such that the control unit 11 no longer applies the switchsn signal at the control input of the control element 4 and a switch-on signal is instead applied to the control input of the control element 5. As a result, the current through the control element 4 ceases, the coil 2 is de-energised and a current flows through the coil 3. The lower pole face of the stator therefore becomes so magnetised that its South pole faces the South pole S of the rotor and imparts to the rotor 9 a further impulse in the same rotary sense.Each time a control element 4 or 5 is switched off, the series-connected coil 2 or 3 can become discharged by transformer action into the D.C. source 6 by way of the other coil 3 or 2, respectively, and the appropriate diode 8 or 7, respectively. In this way one obtains a higher efficiency.

During each rotation of the rotor 9, these procedures are repeated.

According to Fig. 2, the core of the sensing coil 10 is premagnetised up to the point A on the BH characteristic curve and the core material is so selected that the BH curve is practically right-angled. A comparatively small additional flux is therefore sufficient to saturate the core to a stage at which the inductance of the coil is practically zero. This sudden change in inductance in the one or other direction is utilised in the control unit 11 for deriving the control signals for the control elements 4 and 5. The premagnetisation ensures that the core is brought to (positive) saturation only once during each rotor revolution up to the point C and becomes unsaturated only once up to the point D.A high inductance of the sensing coil 10 results in switching-on of one of the control elements 4, 5 with simultaneous switching-off of the other, and a low inductance of the sensing coil 10 results in switching-on of the other control element with simultaneous switching-off of the one control element.

The core of the sensing coil 10 can, as shown in Figs. 3 and 4, have one part 16 of ferrite and a permanent magnet 17 for premagnetisation. The sensing coil 10 is only diagrammatically illustrated in Fig. 1.

In practice, it is arranged relative to the rotor 9 so that it is magnetised up to the point C or D in that position of the rotor where the pole axes of the rotor and stator come together. The axis of the sensing coil 10 can be directed towards the rotor, for example, radially or towards its end.

The stator coils 2, 3 can be bifilar as shown in Fig. 5, that is wound in close juxtaposition and identical. In this way one obtains a closer magnetic coupling between the coils 2, 3 and thus a better return flow of the magnetic energy stored in the coils after they are switched off, this, in turn, leading to an increase in the motor efficiency.

To achieve a stable holding point when the coils 2, 3 are de-energised, that is, when the motor is switched off, the stator as shown in Fig. 6 can include a permanent magnet 18 which is arranged so that (when the motor is switched off) the poles axis Ps of the stator 1 makes an acute angle to the pole axis P1 of the rotor, which ctan also have several permanent magnets. In this way it is ensured that the coil which is first energised when the motor is switched on immediately exerts a torque on the rotor 9 and the motor starts by itself.

The same can be achieved as shown in Fig. 7 by means of a corresponding asymmetrical distribution of the north and south poles of the rotor 9. Another possibility for a corresponding asymmetrical distribution of the field in the air gap of the motor consists of allowing the air gap to converge or diverge in the peripheral direction. These features can also be combined.

A circuit diagram for the operating circuit is represented in Fig. 8. An oscillator 19 contains frequency-defining elements in the form of an LC (inductance-capacitance) resonant circuit of which the inductance and Q factor is defined by the sensing coil 10. In response to these parameters of the sensing coil 10, particularly its inductance but also its Q factor which, in turn, depend on the rotary angle of the rotor, the feedback characteristics of the transistor-oscillator are so selected that the oscillations of the oscillator occur or stop as shown at the top of Fig. 9. This means that during half a turn of the rotor 9 the oscillator oscillates with a frequency of about 100 kHz and during the following half turn of the rotor the oscillations stop.It is also possible to design the oscillator 19 so that the amplitude and frequency of the oscillations gradually or suddenly increase and decrease during one revolution of the rotor, without the oscillations stopping altogether.

In a demodulator 20 which is disposed donwstream of the oscillator 19 and comprises rectifiers 21, 22 and a smoothing capacitor 23, the output signal of the oscillator 19 is converted to a rectangular signal as shown at the bottom of Fig. 9. The pulse repetition frequency of the rectangular signal always corresponds to the rotarv speed of the rotor, the oscillator frequency at 100 Hz being considerably higher than the pulse repetition frequency of the rectangular signal or the rotor speed.

Following the demodulator 20 there is a phase-splitting stage 24 with two series connected transistors 25, 26 of which the output signals on the collector side are likewise rectangular but displaced 1800 from one another and are each fed through a current limiting resistance 27, 28 to the inputs of a pre-amplifier stage 29.

The pre-amplifier stage 29 contains for each power transistor 4 or 5 a pre-amplifying transistor 30 or 31 in series with a current limiting resistance 32 or 33.

By reason of the phase-displaced retangular signals of the phase-splitting stage 24, the power transistors 4 and 5 are switched on and off in anti-phase by way of the preamplifying transistors 30 and 31, so that the power transistors alternatively apply the two coils 2, 3 to the D.C. source 6.

There is also a blocking circuit 34 between the phase-splitting stage 24 and the pre-amplifier stage 29. This blocking circuit contains a voltage divider consisting of a fixed resistance 35, an ajustable resistance 36 set to provide a desired value and a thermistor 37. The voltage occurring at the tapping point 38 of the voltage divider controls, by way of preliminary stage containing transistors 39, 40 and optionally by way of a further stage inserted in lead 41 shown in broken lines, two parallel output transistors 42, 43 operated as switches which, in turn have their collector-emitter paths connected in respective control circuit of one of the pre-amplifier transistors 30 and 31.The output signals of the phase-splitting stage 24 are fully effective or inoperative at the input circuits of the pre-amplifying transistors 30, 31 depending on whether the transistors 42, 43 are blocked or conductive in responsive to the temperature of the thermistor 37, so that the motor will be stopped or started in accordance with the temperature.

In the preferred use of the motor in a refrigerator, the output signals of the phase-splitting stage 24 are not blocked at a high temperature. On the other hand, at a low temperature the transistors 42 and 43 are conductive so that the pre-amplifying transistors 30 and 31 are blocked because their bases are then practically directly connected to the positive pole of the D.C.

voltage source 6. As a result, the motor stops. By including an inverting stage in the line 41, the converse operation can be achieved, for example when using the motor for a pump in a heating installation or the like.

A brief summary of what has been described will now be given.

The described brushless self-starting D.C.

motors include a sensing coil having a core which is saturable in one direction by the rotor field. This sensing coil very sensitively responds to the magnitude of the rotor field by a change in its inductance and Q factor irrespective of the rotary speed of the rotor.

The change is very simply and advantageously utilised for deriving the control signal by means of an LC oscillator with an LC resonance circuit of which the inductance is formed by the sensing coil.

The sensing coil determines the frequency and occurrence of the oscillations of the oscillator and the control signals are derived from the application or interruption of the oscillation. This can avoid additional losses caused by constant oscillation of the oscillator. The oscillator frequency is considerably higher than the rotor speed. The sensing coil is comparatively insensitive to temperature variations.

Special coupling or screening means and control coils are unnecessary even in the case of several stator coils, of which the control circuits are successively controlled by impulses derived from the parameter change of the sensing coil by an appropriately constructed operating circuit.

In order to connect the sensing coil to the operating circuit, only one terminal for the sensing coil need be led out of the motor.

The other terminal can be internally connected to the motor terminal provided for the D.C. source.

Preferably, the core of the sensing coil consists, as described, at least partially of ferrite. Ferrite can be saturated by means of a comparatively small flux so that one obtains a sudden change in the parameter of the sensing coil with a change in its flux.

Consequently this change can be brought about even by a weak field of the rotor magnet. As the core of the sensing coil is premagnetised, a small additional flux will saturate the core. The core is saturated by an additional flux of one polarity only and is not saturated by flux of the other polarity so that there is a clear indication of the rotary angle.

For the purpose of premagnetisation, the core of the sensing coil may, as described, comprise a permanent magnet. This saves energy for maintaining a premagnetising current. Preferably, provision is made, as described, for the stator to have, apart from a first coil, only one more coil, a second coil which is in series with a second control element at the D.C. source, and for the second control element to be likewise controllable by control signals derived from the parameter changes of the sensing coil.

This number of stator coils represents a particularly favourable compromise with regard to efficiency and expense. As far as the motor is concerned, it is sufficient to have a single additonal coil ensuring a more uniform torque and thus a and a single additional motor terminal for controlling the second coil if, as described, a junction of the stator coils connected to the D.C. source is connected to a terminal of the sensing coil. As far as the operating circuit is concerned, in the simplest case it is sufficient to have an inverting stage and the second control element in addition in order to control the second stator coil in countersequence to the first. The motor delivers a high starting torque at a low starting current. The control elements can be designed for a correspondingly low starting current and the rotor magnet can be thinner without resulting in demagnetisation.This likewise contributes to an increase in the efficiency. The same applies to a preferably laminated construction of the stator and/or rotor, which contributes to a reduction in eddy current losses. The D.C. motor is particularly suitable for driving the piston compressor of a refrigerator because its pulsating torque accurately corresponds to the torque requirement of the compressor.

The control elements are preferably power transistors operated as switches. In contrast with, for example, thyristors which, in principle, could likewise be used, the control energy for transistors is less because the quenching means are dispensed with. A transistor operated as a switch can transmit higher outputs in relation to its energy loss.

The power transistors can therefore have correspondingly small dimensions.

It is also favourable if, as described, a diode is connected anti-parallel to each power transistor. In this way, it is possible to increase the efficiency by returning to the D.C. source through one of the stator coils and the associated diode that energy stored in the other stator coil after blocking of the associated power transistor.

Further, it is advantageous if, as described, the wires of the stator coils are wound in close juxtaposition and are identical. This gives a higher coupling factor between the coils and thus a better return flow to the D.C. source through one of the coils of the energy stored in the other coil that is switched off and this again contributes to an increase in the efficiency.

Provision is made as described, for the static magnetic field distribution between the rotor and stator to be so selected in the peripheral direction that the pole axis of the stationary rotor includes an acute angle with the pole axis of the stator, and for the stationary position of the rotor in relation to that of an operating element which is driven by the motor and executes alternate operating and idling strokes to be so selected that the motor starts at no load, e.g.

giving a suction stroke of the compressor of a refrigerator. This will result in a particularly low starting current.

To achieve the magnetic field distribution ensuring self-starting, the air gap between the stator and rotor can differ in the peripheral direction and/or there may be provided a permanent magnet near the rotor periphery and/or an asymmetrical permanent magnet pole distribution in the rotor.

The change in oscillator oscillation can be very reliably determined so as to derive therefrom a control signal with alternating amplitude for effecting the switching on and off of the control elements synchronously with rotation of the rotor. On a fall in the amplitude of the oscillation of the oscillator, its energy consumption is reduced correspondingly. The efficiency of the operating circuit is increased to the same extent. As the sensing coil is comparatively insensitive to temperature variations substantially no fluctuations in the rotary field of the stator that might influence the efficiency of the motor can occur as a result of temperature variations.

It is favourable if, as described, the oscillating condition of the oscillator can be met in response to the induction occasioned in the sensing coil and the control signal is derivable from the intermittent oscillator oscillation. Starting and stopping of the oscillator oscillation can be determined even more reliably so as to derive the control signal therefrom. During the intervals between oscillations of the oscillator, its energy consumption is still further reduced, which again results in an increase in the efficiency.

As the core of the sensing coil is saturable in one direction by the magnetic field of the rotor, there are very intensive changes in the sensing coil inductance and thus a correspondingly marked starting and stopping point for the oscillator oscillation.

Downstream of the oscillator, there may be, as described, a demodulator which converts the intermittent oscillator oscillation to a rectangular signal. A rectangular signal effects more rapid switching over of the switching elements at a defined instant of time.

The demodulator may, as described, comprise a rectifier arrangement and a smoothing capacitor. This results in a particularly simple construction of the demodulator.

Preferably, the rectangular signal can be fed to both power transistors but to one of them with a 1800 phase displacement. In this way one ensures that the two control elements operate in precisely opposite cycles and the stator coils are therefore also switched on and off in opposite cycles but nevertheless synchronously with the stator rotation. The power transistors are very rapidly switched over so that commutation losses are avoided.

An inverting stage effecting the 1800 phase displacement may, as described, be disposed between the demodulator and the power transistors. An inverting stage represents a particularly simple way of producing a 1800 phase shift.

It is favourable if, as described, the inverting stage comprises two series connected transistors from the outputs of which the 1800 phase-displaced rectangular signals are derivable. In this way one obtains an amplification in the output of the rectangular control signal for both power transistors.

Each power transistor may, as described, be preceded by a pre-amplifying transistor.

This results in reliable uncoupling between the control circuit and the power section.

For this it is favourable if a current-limiting resistance is disposed between each power transistor and - pre-amplifying transistor.

This limits the starting current of the motor so that one can use weaker and therefore cheaper power transistors.

The control signal may, as described, be interruptable in response to a physical quantity. Without considerable additional expense, this permits switching on and off of the motor when the physical quantity exceeds or falls short of a limiting value, the physical quantity being for example the temperature of the motor itself so as to protect it and the energy source from overloading, or the ambient temperature so as to control same by a heating or refrigerating unit driven by the motor.

Thus, the control circuit may, as described, comprise a blocking circuit in the path of the control signal and actuable in response to the physical quantity. When the blocking circuit permits the control signal to pass, the motor can start; otherwise it remains stationary.

WHAT WE CLAIM IS: 1. A brushless self-starting D.C. (direct current) motor connected to a mechanical load having, in use, alternate working and idle strokes, the motor comprising a permanent magnet rotor including a least two poles of opposite senses, a sensing coil arranged to sense the rotor position by means of the change with rotor angle of rotor field with the coil, and an operating circuit connected to the sensing coil and stator winding to control the supply of stator current in accordance with rotor position, wherein the sensing coil includes a pre-magnetised core, the relationship between the level of pre-magnetisation of the core and the rotor field linked with the core is such that on rotation of the rotor the core saturates in one direction in response to the field from one of the said at least two poles but does not saturate in the opposite direction in response to the opposite pole, the motor is such that on de-energisation the rotor assumes a position with its pole axis at an acute angle to the pole axis of the stator, and the motor and mechanical load are arranged so that in starting from its de-energisation rotor position the motor starts on an idle stroke of the mechanical load.

2. A motor as claimed in claim 1, wherein the motor is connected to a refrigeration compressor, the suction strokes of which are the idle strokes.

3. A motor as claimed in claim 1 or 2, wherein the peripheral air gap between rotor and stator is non-uniform.

4. A motor as claimed in claim 1, 2 or 3, wherein a permanent magnet is provided near the periphery of the rotor.

5. A motor as claimed in any preceding claim, wherein the rotor has an asymmetrical pole distribution.

6. A motor as claimed in any preceding claim, wherein the core is, at least in part, of ferrite material.

7. A motor as claimed in any preceding claim, wherein the core includes a permanent magnet.

8. A motor as claimed in any preceding claim, wherein the motor has just two stator coils.

9. A motor as claimed in claim 8, wherein one end of each stator coil and one end of the sensing coil are commonly connected.

10. A motor as claimed in claim 8 or 9, wherein the two stator coils are wound in close juxtaposition and are substantially identical.

11. A motor as claimed in any preceding claim, wherein the operating circuit comprises an oscillation circuit in which the inductance of the sensing coil is connected as part of a resonant circuit to produce oscillations modulated by the rotor field linkage, current control means connected to the stator winding, and means connected to - control the current control means in accordance with the output of the oscillation circuit.

12. A motor as claimed in claim 11, wherein the oscillation circuit is such that there is, in use, an ON-OFF modulation of oscillation.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Preferably, the rectangular signal can be fed to both power transistors but to one of them with a 1800 phase displacement. In this way one ensures that the two control elements operate in precisely opposite cycles and the stator coils are therefore also switched on and off in opposite cycles but nevertheless synchronously with the stator rotation. The power transistors are very rapidly switched over so that commutation losses are avoided. An inverting stage effecting the 1800 phase displacement may, as described, be disposed between the demodulator and the power transistors. An inverting stage represents a particularly simple way of producing a 1800 phase shift. It is favourable if, as described, the inverting stage comprises two series connected transistors from the outputs of which the 1800 phase-displaced rectangular signals are derivable. In this way one obtains an amplification in the output of the rectangular control signal for both power transistors. Each power transistor may, as described, be preceded by a pre-amplifying transistor. This results in reliable uncoupling between the control circuit and the power section. For this it is favourable if a current-limiting resistance is disposed between each power transistor and - pre-amplifying transistor. This limits the starting current of the motor so that one can use weaker and therefore cheaper power transistors. The control signal may, as described, be interruptable in response to a physical quantity. Without considerable additional expense, this permits switching on and off of the motor when the physical quantity exceeds or falls short of a limiting value, the physical quantity being for example the temperature of the motor itself so as to protect it and the energy source from overloading, or the ambient temperature so as to control same by a heating or refrigerating unit driven by the motor. Thus, the control circuit may, as described, comprise a blocking circuit in the path of the control signal and actuable in response to the physical quantity. When the blocking circuit permits the control signal to pass, the motor can start; otherwise it remains stationary. WHAT WE CLAIM IS:

1. A brushless self-starting D.C. (direct current) motor connected to a mechanical load having, in use, alternate working and idle strokes, the motor comprising a permanent magnet rotor including a least two poles of opposite senses, a sensing coil arranged to sense the rotor position by means of the change with rotor angle of rotor field with the coil, and an operating circuit connected to the sensing coil and stator winding to control the supply of stator current in accordance with rotor position, wherein the sensing coil includes a pre-magnetised core, the relationship between the level of pre-magnetisation of the core and the rotor field linked with the core is such that on rotation of the rotor the core saturates in one direction in response to the field from one of the said at least two poles but does not saturate in the opposite direction in response to the opposite pole, the motor is such that on de-energisation the rotor assumes a position with its pole axis at an acute angle to the pole axis of the stator, and the motor and mechanical load are arranged so that in starting from its de-energisation rotor position the motor starts on an idle stroke of the mechanical load.

13. A motor as claimed in claim 11 or 12,

wherein the means connected to control the current control means comprises a demodulator to derive a rectangular waveform from the envelope of the oscillation.

14. A motor as claimed in claim 13, wherein the demodulator comprises rectifier means and a smoothing capacitor.

15. A motor as claimed in any of claims 11 to 14 wherein the current control means comprises power transistor switches..

16. A motor as claimed in claim 15 wnen dependent on claim 13, wherein the means connected to control the current control means is operative to apply the rectangular waveform to a first power transistor switch and the rectangular waveform inverted to a second power transistor switch.

17. A motor as claimed in claim 16 wherein a 1800 phase-splitter is connected between the output of the oscillation circuit and the current control means.

18. A motor as claimed in any of claims 15 to 17, wherein each power transistor switch is proceded by a respective transistor pre-amplifier.

19. A motor as claimed in claim 18, wherein a respective current limiting resistor is connected between each transistor pre-amplifier and its associated power transistor switch.

20. A motor as claimed in any of claims 15 to 19, wherein a respective diode is connected in anti-parallel with each power transistor switch.

21. A motor as claimed in any of claims 11 to 20 wherein means are provided to control the current control means additionally in response to a physical parameter extend to the circuit.

22. A motor as claimed in claim 21, wherein the means to provide the additional control in response to a physical parameter comprises a blocking circuit responsive to the physical parameter and connected between the output of the oscillation circuit and the current control means.

23. A motor as claimed in claim 22, - wherein the physical parameter is temperature.

24. A brushless self-starting D.C. motor as claimed in claim 1 and substantially as herein described with reference to, and as illustrated by, Figures 1 and 2 of the accompanying drawings.

25. A motor as claimed in claim 24, having a sensing coil, substantially as herein described with reference to, and as illustrated by, Figures 2 and 3 of the accompanying drawings.

26. A motor as claimed in claim 24, having a sensing coil substantially as herein described with reference to, and as illusrated by, Figures 2 and 4 of the accompanying drawings.

27. A motor as claimed in any of claims 24 to 26, having a stator winding substantially as herein described with reference to, and as illustrating by, Figure 5 of the accompanying drawings.

28. A motor as claimed in any ofclaims 24 to 27, and including a permanent magnet to cause the rotor, on deenergisation of the motor, to assume a position with its pole axis at an acute angle to the pole axis of the stator substantially as herein described with reference to, and as illustrated by, Figure 6 of the accompanying drawings.

29. A motor as claimed in any of claims 24 to 28 having a rotor with an asymmetrical pole distribution substantially as herein described with reference to and as illustrated by, Figure 7 of the accompanying drawings.

30. A motor as claimed in any of claims 24 to 29 including an operating circuit substantially as herein described with reference to, and as illustrated by, Figure 8 of the accompanying drawings.