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Title:
PERMANENT MAGNET RELUCTANCE MACHINES
Document Type and Number:
WIPO Patent Application WO/2009/056879
Kind Code:
A1
Abstract:
An electrical machine comprising a rotor (11) with magnetic saliency but without windings, a stator (101) having stator teeth, the stator carrying armature windings (106, 111) wound with a pitch corresponding to a plurality of stator teeth and arranged around the stator to make one or more electrical phases, the stator also including a field magnet means, interspersed between every two stator teeth, wherein the field magnet means comprises at least one field winding (191, 192) and at least one permanent magnet (104, 105), and a field current control means such that variation of the direct current in the field windings changes the number of magnetic poles in the machine. The armature coils of the machine can be configured into one or more armature windings and circuits are described to excite the armature coils in the most efficient manner depending on the load or application requirements of the machine.

Inventors:
POLLOCK, Charles (Technelec Ltd, Edison HouseStation Approach, Oakham Leicestershire LE15 6QW, GB)
POLLOCK, Helen (Edison House, Station Approach, Oakham Leicestershire LE15 6QW, GB)
Application Number:
GB2008/051009
Publication Date:
May 07, 2009
Filing Date:
October 29, 2008
Export Citation:
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Assignee:
TECHNELEC LTD (Edison House, Station Approach, Oakham Leicestershire LE15 6QW, GB)
POLLOCK, Charles (Technelec Ltd, Edison HouseStation Approach, Oakham Leicestershire LE15 6QW, GB)
POLLOCK, Helen (Edison House, Station Approach, Oakham Leicestershire LE15 6QW, GB)
International Classes:
H02K21/04; H02K19/10
Foreign References:
US6242834B12001-06-05
Attorney, Agent or Firm:
HARDING, Richard (Marks & Clerk, 4220 Nash CourtOxford Business Park South, Oxford Oxfordshire OX4 2RU, GB)
Download PDF:
Claims:
Permanent Magnet Reluctance Machines

CLAIMS

1. An electrical machine comprising a rotor without windings, a stator having stator teeth and slots between each stator tooth, the stator carrying armature windings wound with a pitch corresponding to a plurality of stator teeth and arranged in a first set of slots around the stator to make one or more electrical phases, the stator also including a field means to establish field flux to link the armature windings, the field means comprising at least one field winding and at least one permanent magnet arranged in a second set of slots around the stator between the slots of the first set, each slot in the second set comprising either a field winding or a permanent magnet, and a control means for supply of current to or from the at least one field winding such that the magnitude of the flux linking the armature windings is controllable.

2. A machine according to claim 1 where the number of permanent magnets in the field means is an even number.

3. A machine according to claim 1 or 2 where the permanent magnets of the field means are magnetised in a direction parallel to the air-gap.

4. A machine according to claim 1, 2 or 3 where the permanent magnets of the field means extend radially through substantially all of the stator.

5. A machine according to claim 1,2,3 or 4 where any two permanent magnets in the field means are separated by two stator teeth.

6. A machine according to claim 1,2,3 or 4 where any two permanent magnets in the field means are separated by six stator teeth.

7. A machine according to claim 1,2,3 or 4 where any two permanent magnets in the field means are separated by ten stator teeth.

8. A machine according to any of the preceding claims such that the flux associated with the permanent magnet flux acts on a first set of stator teeth and links at least one first armature coil and passes through the rotor of the machine.

9. A machine according to any of the preceding claims such that when current flows in the field winding an additional flux is produced which acts on a second set of stator teeth and links at least one second armature coil and passes through the rotor of the machine.

10. A machine according to claims 8 and 9 where at least one first armature coil is connected in series with at least one second armature coil to make at least one armature winding which carries electrical current in synchronism with rotation of the rotor.

11. A machine according to claims 8 and 9 in which the at least one first armature coil is connected to a first electronic control means and the at least one second armature coil is connected to a second electronic control means such that the current in the at least one first armature coil can be controlled independently of the current in the at least one second armature coil when the machine is operating as a motor or a generator.

12. A machine according to claims 8 and 9 where at least one first armature coil is connected to a first electrical control circuit and at least one second armature coil is connected to a second electrical control circuit, the first electrical circuit carrying current in synchronism with rotation of the machine under all conditions and the second electrical circuit carrying current in synchronism with rotation of the machine only when at least one field winding is excited.

13. A machine according to any of the preceding claims which is used as a generator in which the emf induced in the at least one first armature coil provides an initial emf which is used to excite at least one field winding to increase the magnetic field excitation system in the generator as load is increased.

14. A machine according to claims 8 and 9 in which the at least one first armature coil is connected to a first electronic control means and the at least one second armature coil is connected to a second electronic control means such that the current in the at least one first armature coil can be controlled independently of the current in the at least one second armature coil when the machine is operating as a motor or a generator, wherein the first electronic control means is connected to a first power system and the second electronic control means is connected to a second power system.

15. A machine according to claim 14 wherein the first power system is a battery and the second power system is a rectified ac supply.

16. A machine according to any of the previous claims where the armature coils are arranged into two or more electrical phases windings.

17. A machine according to claim 16 with two electrical armature phase windings arranged in a stator with sixteen stator teeth and containing two pairs of permanent magnets and two field coils, with a rotor with ten rotor teeth.

18. A machine according to claim 16 with three electrical armature phase windings arranged in a stator with twenty four stator teeth and containing three pairs of permanent magnets and three field coils and a rotor with ten rotor teeth.

19. A machine according to claim 16 with three electrical armature phase windings arranged in a stator with twenty four stator teeth and containing three pairs of permanent magnets and three field coils and a rotor with fourteen rotor teeth.

20. An electrical machine substantially as hereinbefore described with reference to Figure 4-30 of the accompanying drawings.

Description:

Permanent Magnet Reluctance Machines

Electrical machines of many types operate with the interaction of stator and rotor magnetic fields. Electronically commutated brushless motors have become common in recent years due to the absence of brushes which greatly improves their reliability. In addition to the induction motor, which is used extensively in many industrial applications with the benefit of electronic frequency control, there are also two further, major categories of electronically commutated motors:

(i) Permanent magnet synchronous machines (sometimes referred to as brushless ac or brushless dc) have a stator containing armature windings and a rotor carrying permanent magnets. As a motor, the magnetic field produced by the stator currents interacts with the magnetic field of the rotor magnets to produce torque. The rotor magnets may be mounted near to the surface of the rotor and magnetised in a radial direction. In such a motor the air-gap flux density is limited by the magnet flux density which is usually less than the saturation flux density of steel. Alternatively the magnets are buried within the rotor structure with paths of high and low reluctance to guide and focus the magnet flux and create magnetic poles at the surface of the rotor with a flux density higher than that of the magnet itself.

(ii) Reluctance machines such as switched reluctance machines or flux switching machines having laminated steel rotors with a salient pole structure (at least one radially aligned regions of high magnetic reluctance alternating with at least one radially aligned region of low magnetic reluctance. The rotor of a reluctance machine carries no magnets or windings and can therefore be very robust. When operating as a motor the stator assembly carrying coils is energised to create a magnetic field which causes the rotor to rotate into a position where the axis of low magnetic reluctance is aligned with the magnetic field of the stator.

Machines of the permanent magnet synchronous type have the following advantages over reluctance machines:

(i) The presence of the permanent magnet material on the rotor provides a source of magnetic flux without copper losses, thus providing a more efficient machine;

(ii) The rotor poles can be shaped to provide a sinusoidal or trapezoidal flux so that appropriate control of the current in the stator delivers very smooth torque at all rotor angles whereas the reluctance machine torque varies strongly with position producing significant torque ripple;

(iii) At each rotor position, the torque produced by the machine will vary linearly with the amount of current in the armature winding whereas the torque in a reluctance machine varies non- linearly with the current in the stator windings;

(iv) If the machine is used as a generator the rotation of the permanently magnetised rotor immediately induces a voltage in the armature windings without requiring any additional energy source such as a battery, to set up an initial magnetic field.

However, the simple rotor structure of the reluctance machines has some advantages over the permanent magnet machine :

(v) The reluctance rotor is very robust and can spin to high rotational speeds without any risk of magnets becoming detached or needing the complication of a carbon fibre sleeve to retain the magnets;

(vi) As the speed of a permanent magnet rotor increases, the internally generated EMF within the armature windings increases. This means that it becomes more difficult to drive current into the windings and the torque available from the machine decreases with higher speeds. Reluctance machines do not suffer from this, and can therefore be controlled to deliver power over a much wider speed range.

(vii) If a permanent magnet machine is used as a generator, the armature voltage is proportional to speed. If the machine is to be used as a battery charger then at low speeds the machine may not generate enough voltage to charge the battery and at higher speeds the armature voltage may be too high and excessive current would flow into the battery causing damage to either the battery or the generator. A reluctance machine can operate as a generator over a much wider speed range since the field current is supplied through the stator windings and can be varied inversely with the speed to produce a constant voltage generator.

Recently many machines have been proposed which try to combine the advantages of both permanent magnet and reluctance machines while overcoming some of the disadvantages of each machine.

The paper "A Novel Permanent Magnet Motor with Doubly Salient Structure" by Y. Liao, F. Liany and T. Lipo in IEEE Transactions in Industry Applications, Vol.31, No.5, September/October 1995 describes a doubly salient permanent magnet motor in which two permanent magnet sections are inserted in the salient pole stator structure to provide an additional flux to augment the flux produced by the stator windings. The advantage offered by that machine is that the stator windings no longer need to carry field excitation current in addition to torque producing current. This reduces the losses in the windings. However, the machine proposed by Liao et al had a pre- determined amount of field flux set by the size and shape of the permanent magnet section and its operating modes are therefore limited as a result. A paper "Design Considerations and Test Results for a Doubly Salient PM Motor with Flux Control" by F. Leonardi, T.Matsuo, Y. Li, T.A. Lipo and P. McCleer in IEEE IAS Annual Meeting in 1996 describes a machine in which a field winding and permanent magnet is inserted into the stator structure of a three phase 6/4 switched reluctance motor. The machine provides the benefit of a field winding in which the field current is able to adjust the total field flux from the magnet. However, the stator is made from several pieces which are difficult to assemble for a low cost motor. Furthermore, the MMF of the field winding and the MMF of the magnet are effectively in series, both acting to push flux through a single magnetic circuit. The flux density in the magnetic circuit is therefore always limited by the saturation flux density of the permanent magnet section.

A paper " Flux-Reversal Machine: A New Brushless Doubly Salient Permanent Magnet Machine" by R. Deodhar, S. Anderson, I. Boldea and T.Miller published at IEEE Industry Applications Annual Meeting 1996 describes a machine with permanent magnets mounted on the surface of the stator teeth. This machine allows for control of the permanent magnet flux by the current in the stator windings. In this machine the MMF produced by the magnet and the coil are in series and consequently there is a high risk of demagnetisation of the magnet. Furthermore, the air-gap flux

density is limited to the flux density of the permanent magnet material, which limits the torque production of the machine.

Figure 1 shows a flux switching machine with a stator 1 and rotor 2 according to the prior art. The stator 1 is made from laminated steel and in this example has 12 stator teeth 3. There are 12 slots in the machine to carry the machine windings. Alternate slots 4 in the stator carry armature field windings and alternate slots 5 carry field windings. The rotor 2 has six teeth 6 which, as the rotor rotates within the stator varies the mutual coupling between the armature winding and the field winding. The operation of the flux switching machine has been described in published papers. In a paper "Low cost high power density, flux switching machines and drives for power tools", in IEEE IAS Annual Meeting 2003 by H. Pollock, C. Pollock, R. Walter and B. Gorti, the operation of the machine with field winding in both series and shunt configurations relative to the armature switching circuit is described. In a paper "Flux switching machines for automotive applications" by C. Pollock, H. Pollock, R. Barron, J. Coles, D. Moule, A. Court, R. Sutton, published in IEEE Transactions in Industry Applications Vol. 42 No. 5, September 2006, pp 1177 - 1184, the operation of the machine as a motor with bifilar armature windings is described.

Figure 2 shows a flux plot of the prior art machine in Figure 1 when only the field windings are energised such that the field slots 5 carry current. The field winding comprises 3 or 6 coils connected in series or parallel or may be one long coil wound as a consequent pole winding. The conductors in the field slot are arranged such that the current in three of the field slots 11 are opposite in direction to the currents in the three other field slots 12. As a result of this connection pattern a six pole magnetic field is produced which passes through the stator teeth to the rotor. The six pole pattern also links the armature winding, the conductors of which are located in slots 4 of the stator. As the rotor rotates the field flux linking the armature winding varies from a positive to a negative value and back to positive as the rotor turns through each rotor tooth pitch. This variation in armature flux linkage leads to an armature emf which means that the machine can be used as a motor or as a generator.

The flux switching machine shown in Figure 1 and Figure 2 and as described in the prior art has the advantage that it is simple to manufacture. It also has a rotor structure which carries no windings or magnets and is very robust. The flux switching machine, with field winding, allows control of the field flux over the operating conditions of the machine. However, the efficiency of the machine is reduced by the losses in the field winding.

Figure 3 shows a further flux switching motor from the prior art in which the field coils are replaced by permanent magnets. This motor was described in a paper "A permanent magnet flux switching motor for low energy axial fans", by Y. Cheng, C. Pollock and H. Pollock published in IEEE IAS Annual meeting, 2005, Vol. 3, pp 2168-2175. The stator 20 carries four permanent magnet sections where the field windings would have been located and four slots 22 carry the armature winding. The stator has eight teeth 23. The permanent magnets are magnetised with their magnetic axis parallel to the air gap. Alternate magnets have opposing magnetic poles so that magnets 21 and 24 will have like poles facing each other. This creates a 4 pole magnetic field. The armature coils are also arranged so that the armature current flows in opposite directions in alternate armature slots. The resultant field created from the combination of armature mmf and field magnets passes from four of the stator teeth to the four rotor teeth. By reversing the direction of the current in the armature winding the flux will switch into the other four stator teeth. The machine of Figure 3 has no field winding and no field losses and therefore provides a machine of high efficiency. However, the field flux produced by the magnets is relatively constant and cannot be adjusted with load or speed. This means that the

operating speed range of such a motor is limited because the emf will increase with speed and at high speed the armature current will be reduced and resulting torque output will be very small.

It is the object of this invention to provide a flux switching machine of high efficiency with some field flux produced by permanent magnets but with the unique feature that the field winding can be used to increase and decrease the amount of field flux linking the armature windings so that a flux switching machine can have the benefit of high efficiency and controllable field flux.

It is a further object of this invention to provide a doubly salient reluctance machine which overcomes the limitations of the prior art and incorporates both a field winding and permanent magnets in the stator of the machine so that the field winding MMF and the permanent magnet MMF can act in parallel magnetic circuits, avoiding the limitation of the prior art which uses the field winding mmf and the permanent magnet MMF in a single magnetic circuit. As a result the invention teaches that total flux in the machine is the combination of two magnetic circuits which interact to allow the armature flux linkage to be the combination of the field flux produced by the magnet and the flux controlled by the field current to give a machine of high efficiency while retaining as much as possible of the very simple and robust stator structure of a reluctance machine without magnets.

According to the present invention there is provided an electrical machine comprising a rotor without windings, a stator having stator teeth and slots between each stator tooth, the stator carrying armature windings wound with a pitch corresponding to a plurality of stator teeth and arranged in a first set of slots around the stator to make one or more electrical phases, the stator also including a field means to establish field flux to link the armature windings, the field means comprising at least one field winding and at least one permanent magnet arranged in a second set of slots around the stator between the slots of the first set, each slot in the second set comprising either a field winding or a permanent magnet, and a control means for supply of current to or from the at least one field winding such that the magnitude of the flux linking the armature windings is controllable.

An electrical machine according to the invention will commonly have an even number of permanent magnets in the field means and the permanent magnets are magnetised in a direction parallel to the air-gap.

In a machine according to the invention the permanent magnets of the field means extend radially through substantially all of the stator.

A machine according to the invention may have two of the permanent magnets in the field means separated by two, six or ten stator teeth.

The magnets in the stator structure will usually be magnetised in a direction parallel to the air-gap of the machine and in machines with more than one magnet, the magnets will be arranged around the stator so that alternate magnets have opposite magnetic polarity, such that the permanent magnet flux acts primarily on a first set of stator teeth and links at least one first armature coil and passes through the rotor of the machine. The field winding in a machine according to this invention will be used to produce an additional magnetic field which acts primarily on a second set of stator teeth, linking at least one second armature coil, such that as the value of the direct current in the field winding changes, the magnetic field of the field winding works in combination with the magnetic field of the magnet to change the number of magnetic poles in the machine.

In a first aspect of the invention the at least one first armature coil is connected in series with the at least one second armature coil to create an armature winding, such that the armature winding emf is

the combination of the emf induced in each armature coil, this armature winding then connected to an electronic control means to operate the machine as a motor or a generator, carrying current, varying in magnitude and direction in synchronism with rotation of the rotor.

In a second aspect of the invention the at least one first armature coil is connected to a first electronic control means and the at least one second armature coil is connected to a second electronic control means such that the current in the at least one first armature coil can be controlled independently of the current in the at least one second armature coil when the machine is operating as a motor or a generator.

In a further aspect of the present invention there is provided a double salient reluctance generator with a simple and robust stator structure in which permanent magnet sections are inserted in the stator to provide an initial emf to self excite some of the armature windings of the generator thus avoiding the need for a separate dc power supply or battery.

In a further aspect of the invention, there is provided a machine in which the at least one first armature coil is connected to a first electronic control means and the at least one second armature coil is connected to a second electronic control means such that the current in the at least one first armature coil can be controlled independently of the current in the at least one second armature coil when the machine is operating as a motor or a generator, wherein the first electronic control means is connected to a first power system and the second electronic control means is connected to a second power system. The first power system may be a battery and the second power system may be a rectified ac supply. A machine constructed according to this aspect of the invention could operate using the first electronic control means when only a battery supply is available and would therefore have high efficiency. When an ac power system was also available the machine could operate using the second electronic control means.

In a further aspect of the invention the armature windings of the machine are arranged into two electrical phases with two pairs of permanent magnets and two field coils.

In a yet further aspect of the invention the armature windings of the machine are arranged into three electrical phases with three pairs of permanent magnets and three field coils.

For a better understanding of the present invention reference will now be made to the accompanying drawings in which:

Figure 1 shows a prior Art Flux Switching Machine with 12 stator teeth and 6 rotor teeth

Figure 2 shows a flux plot in the prior art flux switching motor when the field winding is excited with a total MMF of 2400 At.

Figure 3 shows a further prior art flux switching motor with 8 stator teeth and 4 rotor teeth and four permanent magnet sections in the stator.

Figure 4 (a,b,c) shows a machine according to the invention with 12 stator teeth and 6 rotor teeth and two permanent magnet sections embedded within the stator and the location of field and armature coils.

Figure 5 shows a flux plot of the machine in Figure 4 with no current in any windings at a rotor position where the rotor is aligned with a first set of stator teeth

Figure 6 shows a flux plot of the machine in Figure 4 with no current in any windings at a rotor position where the rotor is aligned with a second set of stator teeth

Figure 7 shows a flux plot in the machine of Figure 4 with no current in any windings, at a rotor position, midway between aligned positions with stator teeth.

Figure 8 shows a flux plot of the machine in Figure 4 with 2400 At in the field winding at a rotor position where the rotor is aligned with a first set of stator teeth.

Figure 9 shows a flux plot of the machine in Figure 4 with 2400 At in the field winding at a rotor position where the rotor is aligned with a second set of stator teeth.

Figure 10 shows the total Armature flux linkage at different rotor angles and at different values of Field MMF in the prior art flux switching motor shown in Figure 1 and 2.

Figure 11 shows the Flux linking each of the six armature coils in the machine shown in Figure 4 over one rotor pitch when there is no current in the field windings.

Figure 12 shows the Flux linking each of the six armature coils in the machine shown in Figure 4 over one rotor pitch when there is an MMF of 800 At in the field windings in addition to the MMF created by the two permanent magnets.

Figure 13 shows the variation in flux linking a first armature winding in a machine according to the invention over one rotor pitch at various values of field winding MMF.

Figure 14 shows the variation in flux linking a second armature winding in a machine according to the invention over one rotor pitch at various values of field winding MMF.

Figure 15 shows the variation in flux linking a first and second armature winding, and the total armature flux linkage in a machine according to the invention at the maximum flux linkage position over a range of field winding MMF.

Figure 16 shows the variation in total armature flux linkage with field winding MMF at the maximum flux linkage position for a design according to the invention compared to a prior art machine of the same dimensions.

Figure 17 shows three possible coil connections for the armature coils in a machine according to the invention.

Figure 18 shows a possible electronic switching circuit for a machine according to the invention. Figure 19 shows a further electronic switching circuit for a machine according to the invention. Figure 20 shows a further electronic switching circuit for a machine according to the invention.

Figures 21, 22 and 23 show electronic switching circuits for the armature and field windings in a machine according to the invention.

Figure 24 shows an electronic switching circuit for the connection of the machine according to the invention from separate power systems.

Figure 25, 26 and 27 shows a three phase machine according to the invention.

Figure 28 shows a flux plot of the machine in Figure 25 with no current in the field winding.

Figure 29 shows a flux plot of the machine in Figure 25 with current in the field winding.

Figure 30 shows a plot of the flux linking the three armature phase windings as the rotor turns through 36°.

Figure 4a shows an example of a machine according to the invention. The stator 101 compromises two laminated stator sections 102, 103 which may be linked by a thin section of steel to maintain rigidity. The two laminated sections are separated by two permanent magnet blocks 104, 105. The permanent magnet blocks are magnetised with their magnetic axis parallel to the air gap i.e. in a vertical direction in Figure 4a. The pole faces of 104 and 105 touching stator section 102 will have the same magnetic polarity e.g. both north and the pole faces of the magnets 104, in contact with stator section 103 will also have the same magnetic polarity, though opposite to the poles on the other stator section e.g. both south poles. Each laminated stator section carries 5 slots. Slots 106, 107, 108, 109, 110 and 111 contain armature windings and slots 121, 122, 123, and 124 carry field windings. A field winding would usually be made up of two field coils, connected in series or in parallel. One field coil 191 would span slots 121 and 122, the second field coil 192 would span slots 123 and 124 as shown in Figure 4b.

The armature winding in slots 106, 107, 108, 109, 110, and 111 will usually comprise six coils each spanning adjacent armature slots, as illustrated further in Figure 4c. A first coil 151 would span slots 106 and slot 111, each coil side occupying half of the available slot area in slots 106 and 111 such that the next armature coil 152 spans slots 107 to 106 continuing around the machine with coils 153, 154 and 155 with a sixth armature coil 156 spanning slots 111 to 110. Methods of connection of the six armature coils will be discussed later.

The rotor of the machine according to the invention, illustrated by the example in Figure 4 has six salient teeth and is usually made of laminated steel. There are no magnets or windings on the rotor.

Without any current in the field windings or armature windings the teeth of stator section 102 will all form north poles and the teeth of stator section 103 will be south poles, created under the influence of mmf of the magnets. Thus the magnetic field under magnet only field excitation is a two pole magnetic field. Figure 5 and Figure 6 show flux plots of the two pole magnetic field set up by the magnets at two rotor positions where the six rotor teeth are aligned with alternate sets of stator teeth.

Figure 7 shows the flux plot in the example of the machine according to the invention when the rotor is at a position mid- way between two sets of stator teeth. There is no current in the field winding or the armature winding. The flux pattern is still a two pole pattern with all the teeth in section 102 acting as north poles and all the teeth in section 103 acting as south poles.

Figure 8 and Figure 9 shows flux plots of the machine according to the invention when current is now flowing in the field coils 191 and 192, spanning slots 121 & 122 and 123 & 124. The current direction in slot 121 is into the paper creating a magnetic field in the steel behind the field slot 121 which opposes the north pole of magnet 104. The current direction in slot 122 is then opposite of that in slot 121 since the two slots are filled with conductors of a single coil. The magnetic field pattern shown in Figures 8 and 9 are completely different from the magnetic field pattern in Figures 5, 6 and 7. The addition of the field current has changed the two pole magnetic field into a six pole magnetic field. The six pole magnetic field in Figures 8 and 9 is similar to the six pole pattern in a machine of the prior art.

The machine according to this invention therefore has a field magnet means comprising permanent magnets and field coils which interact in a way so that the addition of field current changes a two pole field system into a six pole field system.

Figure 10 shows the armature flux linkage in a machine according to the prior art and illustrated in Figures 1 and 2. The graph shows the armature flux linkage as the rotor turns through one electrical cycle which with six rotor teeth represents a rotor angle of 60 mechanical degrees. Four graphs are shown in Figure 10, lines 141 and 142, each calculated with increasing values of field current. Line 141 shows the armature flux linkage with an MMF of 1200At in the field winding. This corresponds to six field coils spanning the six field slots 5 in Figure 1, each field coil carrying 200At of MMF. It can be observed that there is a large increase in armature flux linkage as the field MMF increases from 1200At to 2400At. However, beyond this MMF the stator steel then saturates and there is limited further increase in armature flux linkage. The variation in armature flux linkage with rotor rotation induces an emf in the armature winding allowing the machine to be used as a motor or generator by connection of an appropriate switching circuit to the armature windings.

Figure 4c showed the six armature coils 151, 152, 153, 154, 155 and 156 in the machine according to the invention, each armature coil spanning two stator teeth. As the rotor rotates there is a variation in the flux linking each of the armature coils. The unusual feature of the machine according to the invention is that the variation in armature flux linkage in each of the armature coils is not all identical. Unlike the prior art machine the flux linking each armature coils is different as the magnetic field pattern changes from 2 pole to 6 pole due to increasing field current.

Figure 11 shows the graphs of the flux linking each of the six armature coils in Figure 4c when there is no current in the field winding. The magnetic field pattern is a two pole pattern under these conditions. Line 161 is the magnetic flux linkage in coil 151 and lines 162, 163, 164, 165, 166 are the armature flux linkages in coils 152, 153, 154, 155, and 156 respectively, all calculated when there is no current in the field winding. The flux linking coils 151, line 161, and the flux linking coil 154, line 164, has a significant variation with rotor rotation whereas the flux linking coils 152, 153, 155 and 156 (Lines 162,163,165 and 166 respectively) shows minimal variation with rotor angle. The flux linking 152, Line 162, and the flux linking 153, Line 163, is a constant negative value at all rotor positions. This is because both stator teeth spanned by each coil 152 and 153 are north poles at all rotor positions. The flux linking coils 155, Line 165 and the flux linking 156, Line 166, are both positive, and also show little variation with rotor position as the stator teeth spanned by coils 155 and 156 are south poles at all rotor positions when there is no current in the field winding. It can be concluded from Figure 11 that when there is no current flowing in the field winding an emf is induced in armature coils 151 and 154 but negligible emf will be induced in coils 152, 153, 155 and 156.

Figure 12 shows the flux linking each of the armature coils when the field winding MMF is increased to 800At. This is an equivalent of 400At in each of the two field coils 191 and 192 spanning 121 to 122 and 123 to 124 respectively. This is therefore the same slot current (400 A/slot) which occurs in a prior art flux switching machine with six coils of 200At (400A/slot) and a total MMF of 1200At.

Lines 171, 172, 173, 174, 175 and 176 correspond to the armature flux linkage in armature coils 151, 152, 153, 154, 155 and 156 respectively. Lines 171 and 174 have larger variations in flux linkages compared to lines 161 and 164 in Figure 11. The increase in field current has therefore increased the magnetic flux linking the two armature coils spanning the two permanent magnet sections. Furthermore the addition of field current excitation has caused a change from two pole magnetic field towards a six pole magnetic pattern and has reduced the dc flux in coils 152, 153, 155 and 156 but

significantly increased the alternating component of the flux linkage in these armature coils. When field current flows in the field winding an armature emf will be induced in coils 152, 153, 155 and 156.

The armature emf in coils 152, 153, 155 and 156 is calculated by the rate of change of the armature flux linkage graphs 172, 173, 175 and 176. Each of these coils will therefore have an equal emf and can be connected in series or parallel providing care is taken to orientate the armature connections so that the positive emf is towards the same end of the winding.

Figure 13 shows the total armature flux linkage variation in coils 151 , 154 at four different field current MMF values illustrated by lines 200, 201, 202, 203. Figure 14 shows the summation of the armature flux linkages in coils 152, 153, 155 and 156, illustrated by lines 210, 211, 212 and 213 at field winding MMFs of OAt, 800At, 1600At and 2400At respectively. It can be seen that correct addition of the armature flux linkages cancels the dc flux components of the 2 pole magnetic field. Since armature emf is the rate of change of flux linkage, the emf associated with the armature coils 152, 153, 155 and 156 is zero with no field current and then increases as field current increases.

Figure 15 shows the variation in peak value of the combined armature flux linkage in coils 151 and

154 (line 221) compared to the peak value of the combined armature flux linkage in coils 152, 153,

155 and 156 (line 222). Line 221 starts above zero indicating that there is a variation in armature flux linkage with no field winding MMF. Line 222 is zero at a field MMF of zero. Line 221 increases slowly with field current indicating that the peak to peak flux linkage in coils 151 and 154 increases slightly with field current. Line 222 increases steeply with field current to a value which is just under twice the value of line 221 at the equivalent field MMF. Since line 222 is the combined flux linkage of four armature coils, rather than two, as in line 221, this is an expected result. Line 223 in Figure 15 shows the total flux linkage if all six armature coils are combined.

Figure 16 shows a comparison between a machine according to the invention and an identical size of flux switching machine according to the prior art. Both machines have 12 stator teeth and six rotor teeth. Line 230 corresponds to the prior art machine and line 231 corresponds to the machine according to the invention. Both machines have outside diameters of 65mm and the total armature flux linkage is quoted per metre of stack length. It is clear from Figure 16 that the armature flux linkage in a machine according to the invention is significantly higher than in the prior art machine for the same field winding MMF. Some key comparisons are shown in Table 1.

Table 1

A machine according to the present invention offers the feature of a simple and robust machine with a field winding system combining a simple permanent magnet field and a variable excitation field winding resulting in a field controlled flux switching machine which is simple to construct and produces significantly more armature flux linkage than a prior art machine for a given field winding MMF.

Figure 17 shows some possible methods for the connections between the armature coils 151-156. Figure 17a shows the arrangement where all the armature coils 151-156 are connected in series. It is important to ensure that the induced voltages of all the series connected coils are in phase so that the voltage across the series combination increases with each additional coil. This is represented by the dot notation on each coil. In Figure 17a the complete armature winding has two end connections 300 and 301. When all the armature coils are series connected the current through them will be equal. The electromechanical energy conversion in coils 151 and 154 may however be different from the electromechanical energy conversion in 152, 153, 155 and 156. This is because the emf induced in 151, 154 follows a different relationship with field current than 152, 153, 155 and 156 as was illustrated by Figure 15. This does not prevent the machine configured in this way from operating very successfully as a motor or generator. The emf across the complete armature winding will be higher for a given field current than a similar machine constructed according to the prior art.

Figure 17b shows a further connection method for the armature coils in a machine according to the invention. Coils 151 and 154 are connected together in series to make a first armature winding 401 with connections 310 and 311. Armature coils 152, 153, 155 and 156 are connected together in series to make a second armature winding 402 with end connections 312 and 313. The induced emf across 310 to 311 will follow a characteristic like line 221 in Figure 15. As field current increases the induced emf between 310 and 311 will increase slowly. The induced emf across 312 to 313 will start at zero with zero field current but will increase faster with field current, as illustrated by line 222 in Figure 15. The armature emfs can be further adjusted by different numbers of turns in coils 151 and 154 compared to coils 152, 153, 155 and 156.

Figure 17c shows another possible connection method for the armature coils. Assuming coil 151 and coil 154 have the same number of turns then, at any given speed and field current, their induced emfs will be equal and the two coils can be connected in parallel to create a first armature winding 401 with end connections 310 and 311. Similarly if coils 152, 153, 155 and 156 each have the same number of turns then these four coils will have the same emf at any given speed and field current and they can be connected in parallel to create the second armature winding 402 with end connections 312 and 313.

Further connection patterns not shown in Figure 17 are possible. These include the coils of the first armature winding connected in series while the coils of the second armature winding are connected in parallel. Alternatively the second armature winding could comprise two of the coils in series and then connected in parallel with the other two coils in series. In another arrangement coils 151 and 154 could be connected in parallel and then connected in series with a series or parallel combination of coils 152, 153, 155 and 156 to create a single armature winding. In general many connection arrangements of the armature coils are possible, providing always that any coils connected in parallel have the same induced emf in magnitude and phase and when coils are connected in series, the emfs are cumulative in magnitude by ensuring that the emfs are in phase in each series connected coil group. It is also possible that each armature coil may be made up of two closely coupled coils (sometimes referred to as bifilar strands). This bifilar arrangement is known for flux switching motors as it provides an electronic control circuit of low cost.

Figure 18 shows an electronic control circuit for one aspect of the invention. The circuit of Figure 18 can be used in conjunction with the armature connection scheme of Figure 17a. All the armature coils are connected together to make a single armature winding with terminations 300 and 301. The circuit of Figure 18 is a single phase inverter with four power transistors or IGBTs 321, 322, 323 and 324 which can control the flow of alternating current through the armature winding by appropriate switching of the four IGBTs. End connection 300 of the armature winding is connected to the node

between the two IGBTs 321 and 322 and end connection 301 is connected to the node between IGBTs 323 and 324. The field winding 325 comprising the parallel or series combination of the two field coils 191 and 192 in a machine according to the invention is connected between the positive dc supply and the positive supply rail of the inverter. In this position, the current flowing in the inverter switches is drawn in series through the field winding. In this series configuration the field current varies as the armature current varies giving a characteristic like a series connected brushed motor. A diode 326 or a capacitor 327 is also usually present to provide a path for the field current during switching transitions of the armature switched. The operation of this circuit is identical to circuits to control the prior art flux switching machine. A machine according to this invention which changes the number of magnetic poles as the field current is increased can therefore be configured to have a single field winding and a single armature winding and can be controlled using the same inverter circuits as used on prior art flux switching motors. This means that the inverter circuit can be one of low cost but the unique magnetic geometry of the machine according to the invention leads to a machine of high efficiency, combined with low manufacturing costs.

Figure 19 is an alternative electronic control circuit which can be used to control the first aspect of the machine according to the invention. This circuit is also from the prior art and gives independent control of the armature and field currents with the windings forming parallel (or shunt) paths within the converter. The armature circuit employs four IGBTs (or mosfets or other transistor switch) 321, 322, 323 and 324 connected to the first end 300 and second end 301 of the armature winding. A further IGBT 328 controls the current through the shunt field winding 330. A diode 329 carries the field current when the IGBT 328 is turned off. The whole circuit is connected to a dc power source 331. The dc power source may be a battery or may be the output from a rectifier to convert ac to dc. Some smoothing capacitance may also be present though it does not have to be large.

The shunt field winding 330 in Figure 19 will typically have higher numbers of turns and use thinner wire compared to a field winding 325 designed for series connection as in Figure 18. In either case the field winding will be made up of the series or parallel combination of field coils 191 and 192.

As an alternative the field winding could be excited by a separate excitation circuit.

Figure 19 has an advantage over Figure 18 in that it is suitable for motoring or generating operation of the machine according to the invention. In Figure 19 the inverter switches 321, 322, 333 and 334 can be controlled so that current flows out of terminal 300 when the emf in the armature winding at terminal 300 with respect to 301 is positive. As energy is returned to the dc power supply Figure 19 has the advantage that the field winding is not in series with the generated current. The shunt field winding in Figure 19 can easily be controlled independently of the generated armature current to control the magnetic configuration of the machine.

Figure 20 is a circuit suitable for use with the second aspect of the invention. The machine according to the second aspect of the invention uses the fact that the armature coils can be connected into two armature windings as shown in Figure 17b and 17c. The first armature winding comprises coils 151 and 154 and has an emf with no field current. This first winding has terminals 310 and 311 and is connected to a first inverter circuit with electronic switches 351, 352, 353 and 354. The second armature winding comprising series and or parallel combinations of armature coils 152, 153, 155 and 156 with terminals 312 and 313 is connected to a second inverter circuit comprising electronic switches 355, 356, 357 and 358. A further IGBT 328 controls the current through the shunt field winding 330. A diode 329 carries the field current when the IGBT 328 is turned off. Thus Figure 19 allows the current in the first and second armature windings to be independently controlled.

Furthermore the magnitude of the field winding is controlled by pulse width modulation of switch 328. This allows maximum benefit to be obtained from the second aspect of the invention.

When the machine is used as a motor and when the load on the motor is relatively small there is an advantage to be able to operate the machine with high efficiency. Under light loads the machine according to the invention can be operated in its two pole mode. The field winding does not need to be energised and there will be no emf induced in the second armature winding. Under these conditions maximum efficiency is obtained as there are no field losses and there is also no current in the second armature winding. All the motor torque up to approximately 30% of full load torque could be produced from excitation of the first armature winding only. The losses in the machine, particularly copper losses in the windings are minimised and the machine operates with high efficiency.

As the load on the machine increases the current in the field winding can be introduced and the second inverter circuit of Figure 20 can be used to increase the armature current in the second armature winding. As the load on the machine increases the configuration of the machine changes from predominately two pole to predominately six pole and operation at maximum efficiency is obtained for all loads.

The circuit in Figure 20 is also suitable for operation of the machine according to the second aspect of the invention as a generator with high efficiency, particularly over a wide range of load conditions. The induced emf present in the first armature winding even with no field current is particularly valuable to induce voltage in the generator when no external excitation is present.

Figure 21 shows a further variation to the circuit of Figure 20 in which the field winding, 325, is in series with the current flowing into the inverter circuit controlling the second armature winding of a machine according to a second aspect of the invention. This circuit is more suited to a motor rather than a generator. At light loads the first armature circuit and first armature winding can be used for torque production. The emf in this winding is induced from the permanent magnet flux and does not require field current. As the load on the motor increases the second armature inverter can be used with increasing armature current. The field current also flows in the field winding 325 as this second inverter draws power from the supply, 331, automatically changing the machine from a two pole flux pattern to a six pole flux pattern and increasing the emf and electromechanical power conversion in the second armature winding.

Figure 22 shows a further circuit suitable for implementation of the second aspect of the invention. US Patent 6, 140, ,729 describes a flux switching motor in which the armature windings comprise two closely coupled coils (bifilar strands). Bifilar strands can also be applied to the first or second aspect of the invention to control the first and second armature windings according to the invention. In Figure 22 the closely coupled coils are shown for use with the second aspect of the invention. Both coils 151 and 154 would be wound with bifilar strands to make closely coupled coils 371 and 372. The two closely coupled coils, 371 and 372, making up the first armature winding are connected to two electronic switches, usually mosfets, 361 and 362 to create the first armature inverter. Operation of the first armature inverter can be performed in a manner similar to US 6,140,729. Using this first armature inverter alone, the machine according to the invention will have a dominant two pole magnetic field produced by the permanent magnets and will operate with high efficiency.

Closely coupled coils 373 and 374 are made from series or parallel connection of closely coupled coils within armature coils 152, 153, 155 and 156. These are connected in a manner similar to US

6,140,729 to two further mosfets 363 and 364. Operation of mosfets 363 or 364 in phase with mosfet 361 or 362 respectively with respective duty cycles chosen to optimise efficiency automatically draws the field current through the field winding 325. Diode 326 provides a path for the field current in 325 to free-wheel when mosfet 363 or mosfet 364 is turned off. Figure 22 therefore provides a circuit of low cost which provides independent control of the first and second armature windings of the machine according to the invention and therefore allows the currents in the first and second armature windings to be optimised for efficiency.

Figure 23 is a further example of a circuit which can be used to control the first and second armature windings of a machine according to the invention. The first connection end 310 of the first armature winding 401 is connected to the node between inverter switches 411 and 412. The first connection end 312 of the second armature winding 402 is connected to the node between armature switches 413 and 414. The second connection ends 311 and 313 of the first and second armature windings are connected together to the node between inverter switches 415 and 416. Since the emf induced at the first ends of the first and second armature windings are substantially in phase the operation of inverter switches 415 and 416 can be shared between the two armature windings. However, the current in each of the two armature windings can be independently controlled using the inverter switches connected to the first end of each armature winding.

The machine according to both the first and second aspects of the invention can be manufactured easily because the magnets are simple rectangular blocks. Furthermore, the two magnets 104 and 105 have parallel magnetic axes and are both magnetised in the same direction. The machine can therefore be completely assembled with the magnets demagnetised. After assembly of all parts and windings are complete an external magnetic field can be used to simultaneously magnetise both magnets. The method of magnetising the permanent magnets according to the first and second aspects of the invention is a third aspect of the invention.

In all the examples shown the stator teeth associated with the first and second armature windings are equally spaced. In a fourth aspect of the invention the stator teeth linked by the first and second armature windings have an offset angle relative to the rotor pole pitch. The offset angle introduces a phase angle between the induced emf of the first and second armature winding and enables the torque output of the machine to be distributed more evenly over all angles of rotor rotation. This effectively reduces the torque minimum which is present in a symmetric flux switching machine if all the armature winding emfs pass through zero at the same time. An offset of 5 mechanical degrees gives an offset in the electrical phase of first and second armature of 30 degrees which provides a significant reduction in torque ripple. To benefit fully from this fourth aspect of the invention a circuit offering independent control of the first and second armature winding should be used.

A fifth aspect to the invention uses the fact that the first and second armature windings can be connected to completely different power switching circuits or electronic control means, each one connected in turn to different power systems. In some applications it is useful for a motor to be able to operate from either a battery supply or a mains ac supply source. Since the battery is usually a low voltage, such as 9, 12 or 24 V and the mains ac supply is usually a significantly higher voltage such as 120V or 230V, the design of a single motor to operate successfully from both power systems is difficult. Furthermore it is important to maintain electrical isolation between the ac supply and the battery terminals. Furthermore if the machine is operating as a motor from a battery supply it is vital that the machine has high efficiency so that the life of the battery is maximised. A machine according to the invention can be configured to solve this problem.

The first armature winding 401 is connected to a first electronic control means 501 such as shown in Figure 24 and is in turn connected to a battery supply 510. The machine can operate as a motor or generator taking power from the battery or returning energy to the battery. In both modes the machine will operate with high efficiency since there is no field losses. The machine will have reduced number of poles and may not be able to deliver maximum power but can be designed to be sufficient for the required task.

The second armature winding 402 can be connected to a second electronic control means 502 which is connected to a higher voltage ac supply 511 via a rectifier 512. The field winding can be connected between output of the rectifier and the second electronic control means as shown in Figure 24. Alternatively a shunt field winding circuit as illustrated in Figure 19 could be used. When the higher voltage, mains ac supply, is present the machine can operate as a motor drawing energy from the ac supply. If it is required to generate power back into the ac supply the rectifier would have to be replaced by an inverter as is well known in the art. When operating from the ac supply it is beneficial to also excite the field winding to allow the machine to reach its full potential power capability. When the field winding is excited the machine will change from the lower number of poles to its full number of magnetic poles.

In a further operating mode of the two electronic control means, energy can be drawn from the ac supply into the second electronic control means to excite the field winding and/or the second armature winding. The first electronic control means can operate independently such that energy is is delivered to the battery, charging the battery. By correct control of the switches in the electronic control means this battery charging can be obtained when the machine is turning or when the machine is stationary. This aspect of the invention uses the fact that the first and second armature windings are mutually coupled and both are also coupled to the field winding. Energy can therefore be transferred from one winding to another to charge the battery.

Asymmetrical rotor teeth are common in the prior art and can be used very successfully with all aspects of the invention.

The machine according to this invention is particularly effective as a generator. The machine has a simple and robust stator structure making it suitable for easy manufacture. Furthermore the flux of the permanent magnet sections 104, 105, inserted in the stator induces an initial emf to self excite some of the armature windings of the generator. This provides an emf from the rotation of the rotor without requiring an excitation source thus avoiding the need for a separate dc power supply or battery.

The invention can be applied to other flux switching configurations as well as the example given here. For example a flux switching machine with 12 stator teeth and 6 rotor teeth could have four permanent magnets and one field winding spanning the remaining two field slots. The magnetic field pattern would be four pole while there was no field current in the remaining two field slots and would then become six poles when field current was applied during operation of the machine.

The invention can also be applied to a flux switching motor with eight stator teeth and four rotor teeth. Armature windings occupy every alternate slot, each spanning two stator teeth. The field magnet means is interspersed between the armature slots and could comprise two permanent magnets and two field winding slots carrying a single field coil. The two magnets would occupy field positions separated by 90 degrees and would be magnetised parallel to the air-gap but have like poles facing the 90 degree section of stator steel separating the two magnets. The field pattern from the magnets alone

would be a two pole pattern. However, this would become four pole when field current is supplied to the field winding. The invention can therefore be applied to machines with eight teeth as well as twelve teeth on the stator providing that the magnets are inserted into the stator in positions where they can have opposite magnet faces in contact with the section of stator steel which separates the magnets.

In general the method according to the invention can be applied to a flux switching machine with 4*n stator teeth, where n = 2, 3, 4 The rotor of such machines will have 2*n rotor teeth, the spacing between rotor teeth (rotor pole pitch) therefore being 180/n degrees. Between the 4*n stator teeth there are 4*n slots, each alternate slot containing armature conductors so that there are 2*n armature slots. There are therefore two stator teeth between every armature slot. These two teeth are separated by a field magnet means which will either be a slot containing field conductors or a permanent magnet. A machine according to the invention will have 2*p permanent magnets, where p=l, 2, 3... (p<n) and 2*(n-p) remaining slots containing field conductors. The permanent magnets will be positioned in pairs, the angular spacing between the two magnets of a pair being an odd number of rotor pole pitches, i.e. r*180/n, where r =1,3,5... (r < n).

When p=l, a machine according to the invention will have a two pole magnetic field when the magnets are magnetised increasing to 2*n magnetic poles when the field slots all carry field current.

When p>l there is more than one pair of permanent magnets, each pair satisfying the above criteria. Furthermore, if the shortest angular spacing between one permanent magnet of a first pair and one permanent magnet of a second pair is also an odd number of rotor pole pitches then a four pole magnetic field will be created when there is no field current. Applying field current to such a machine will increase the number of poles to 2*n. Table 2 summarises some common possible configurations for machines according to the invention.

Table 2 Possible combinations of Permanent magnet, stator teeth and magnetisation patterns in machines according to the invention

The common feature in all the machines illustrated in Table 2 is that the number of magnetic poles in the machine under field only excitation is lower than the number of magnetic poles when the field coils are excited in addition to the permanent magnet fields. The machines also have twice the

number of stator teeth as rotor teeth and the armature emfs will all be substantially in phase and as such are known as single phase flux switching motors.

Flux switching machines with two or three electrical phase windings have also been described in the prior art. In such machines with two electrical phases the induced emf in each armature is separated by approximately 90 electrical degrees. In such machines with three electrical phases the induced emf in each armature is separated by approximately 120 electrical degrees. Typically a two phase machine would have eight stator teeth and either 3 or 5 rotor teeth or would have 16 stator teeth and 6 or 10 rotor teeth. Typically a three phase machine would have 12 stator teeth and five or seven rotor teeth or 24 stator teeth and 10 or 14 rotor teeth. Since the stators of these multiple phase flux switching machines have field excitation systems in alternate stator slots, they therefore have a similar structure to the machine according to this invention, and therefore the invention as disclosed can be applied directly to the stators of two phase and three phase flux switching motors. The armature coils comprising each phase will have different induced voltage characteristics depending on whether they span a permanent magnet or a field slot.

A three phase flux switching motor is shown in Figure 25 with a stator 600 carrying twenty- four stator teeth and a rotor 610 with ten rotor teeth. According to the invention the stator contains six permanent magnets 601,602, 603, 604, 605 and 606. Three permanent magnets 601, 603 and 605 are magnetised so that their North poles face in an anti-clockwise direction. Three permanent magnets 602, 604 and 606 are magnetised so that their North poles face in a clockwise direction. Figure 26 shows that six further slots between the stator teeth contain field windings ideally arranged as three coils 611, 612 and 613. Figure 27 shows the arrangement of the armature coils 701 to 712. Armature Coils 701, 704, 707 and 710 are associated with a first phase winding. Armature Coils 702, 705, 708 and 711 are associated with a second phase winding. Armature Coils 703, 706, 709 and 712 are associated with a third phase winding. Of the phase coils it can be observed that two coils associated with each phase winding span across a slot occupied by a permanent magnet and two coils associated with each phase winding span across a slot occupied by a field coil. For example 701 and 710 are two coils associated with the first phase winding and they each span across a slot occupied by a permanent magnet while 704 and 707 are two coils associated with the first phase winding which span across a slot occupied by a field coil.

Figure 28 shows the field distribution when there is no field current in the field windings. Whilst the pole areas are quite different, the field pattern is substantially six pole, created by the six magnets.

Figure 29 shows the field distribution when field current is flowing in the field windings where the more conventional 12 pole pattern has been restored. In common with the earlier examples of machines according to this invention the increasing field current has increased the number of magnetic poles in the machine.

Referring again to Figure 25 it can be seen that the permanent magnets can be thought of as being arranged in three pairs around the periphery of the machine, 602 and 603, 604 and 605, 606 and 601. Each pair have equal magnetic poles facing each other. The pairs are separated by two stator teeth. Between each pair are six stator teeth. Machines according to the invention will usually have spacings of two or six or ten stator teeth between the magnets. The machine of Figure 25 illustrates an implementation of the invention where spacings of both six and two are present in the one machine.

If the four coils associated with each phase winding are connected in series, the flux linking the three armature phase windings would have the form shown in Figure 30. Plots 751, 752 and 753 show the flux linking the four armature coils of phase winding 1, phase winding 2 and phase winding 3 respectively when there is no current in the field windings. The three plots are approximately sinusoidal. Plots 761, 762 and 763 show the flux linking the four armature coils of phase winding 1, phase winding 2 and phase winding 3 respectively when there is current in the field windings in a direction required to create the higher pole number field. The three plots are still approximately sinusoidal but have an increased magnitude. The armature emfs in this machine are therefore as typically found in a three phase machine, with the major benefit that the magnitude of the induced emf in the armature winding is controllable with the field current independently of the speed. If the direction of the field current were reversed the armature emfs could be reduced below the level created by the permanent magnets alone.

Operation of the three phase machine according to the invention as a motor or generator can be done with a three phase inverter as is known in the industry. The field winding can be separately excited with its own current controller or can be connected in series with the dc supply to or from the three phase inverter driving the armature. The current flowing in the armature phases can be sinusoidal or trapezoidal or any other alternating current shape as is appropriate for the available voltage and speed range of the machine. Further improvements in efficiency can be achieved by operating the machine with two inverters; one inverter connected to armature coils spanning permanent magnets and a separate inverter driving the armature windings spanning slots containing field coils.

A three phase flux switching machine according to the invention can also be made with a similar stator structure to Figure 25 but could have a rotor with fourteen salient pole teeth.

Application of this invention to a three phase flux switching machine leads to a machine of high efficiency since the permanent magnet field has no losses associated with it. However unlike other permanent magnet machines the field flux can be controlled using the additional field windings to allow operation over a wide speed range at maximum efficiency.

Whilst for simplicity and clarity the drawings attached to this description show the stator as having two or more separate sections separated completely by the permanent magnets it is entirely possible to construct the machines from a single laminations which have a thin section of steel bridging the inner or outer or both surfaces of the magnet. Such a section may be useful in assembly of the stator, giving the machine increased structural strength. However the linking pieces of steel in such a lamination will act as magnetic short circuits around the magnet, reducing the amount of flux which usefully links the coils of the first armature winding. The magnet size can be adjusted to ensure that this does not compromise the performance of the machine.

Despite the reduction in structural strength, there is a potential advantage in constructing the machine with two or more sections. The field coils and the armature coils not spanning the permanent magnets can be easily wound on the open sections of the machine and then assembled with the permanent magnet pairs and the remaining armature coils spanning the permanent magnet sections pulled into place. Such a winding system can be achieved with smaller coil overhangs at the ends of the machine and the machine therefore has coils of lower resistance and higher efficiency.