GB2454171A

GB2454171A - Reluctance machines or the inductor type with permanent magnets integrated into the stator

Info

Publication number: GB2454171A
Application number: GB0721077A
Authority: GB
Inventors: Charles Pollock; Helen Geraldine Phyllis Pollock
Original assignee: Technelec Ltd
Current assignee: Technelec Ltd
Priority date: 2007-10-29
Filing date: 2007-10-29
Publication date: 2009-05-06
Anticipated expiration: 2027-10-29
Also published as: GB2454171B; GB0721077D0

Abstract

An electrical machine of the inductor type includes a field magnet means associated with the stator, the field magnet means interspersed between alternate stator teeth such that current flowing in the armature windings combines with the field produced by the field magnet means to direct the combined flux in a radial direction across the air-gap between stator and rotor, wherein the field magnet means comprises at least one field winding (e.g. associated with slot 41) and at least one permanent magnet , the stator lamination arranged such that there is a continuous path 63 of stator steel from the north face of each of the said magnets to the south face of the same magnet, such that only a portion of the permanent magnet flux links one or more of the armature windings and passes through the air-gap to the rotor. The magnet may be inserted in a slot in the stator outer circumference or closes the field winding slot 41. As the field winding is increasingly energized the shunted flux is reduced so increasing the flux though the armature windings and rotor. At maximum fied current the flux flow though the shunt is reversed. Various magnet arrangements are disclosed and the stator may be such that its radial depth is greater in te h region of the field magnet means than in the armature slot region e.g. a square cross section.

Description

Reluctance Machines With Permanent Magnets Integrated into the Stator 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 region 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 .,aries 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 a motor 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, V. Li, T.A. Lipo and P. McCleer in IEEE lAS 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. Whilst 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.

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.

It is the object of this invention to provide 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 flux produced by the magnet can be controlled by the field current while retaining 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 with magnetic saliency but without windings, a stator having stator teeth, the stator carrying armature windings 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, the field magnet means interspersed between alternate stator teeth such that current flowing in the armature windings combines with the field produced by the field magnet means to direct the combined flux in a radial direction across the air- gap between stator and rotor, wherein the field magnet means comprises at least one field winding and at least one permanent magnet, the stator lamination arranged such that there is a continuous path of stator steel from the north face of each of the said magnets to the south face of the same magnet, such that only a portion of the permanent magnet flux links one or more of the armature windings and passes through the air-gap to the rotor.

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. In normal operation of a machine according to the invention the total field flux is a combination of the flux from the permanent magnets and the flux produced by the current in the field windings.

In a further aspect of the present invention one permanent magnet is positioned in a slot on the outer periphery of the stator directly behind every field slot.

In a further aspect of the invention each permanent magnet is placed at the opening of one or more field slots, near the air-gap surface of the stator. In this aspect of the invention the machine can be further improved by the permanent magnets occupying a significant proportion of the total depth of the field slot so that the field slot is divided by the magnet into two sub-sections.

In a further aspect of the invention the magnets are positioned at an angle to the radial line from the centre of the machine and may not necessarily be magnetised parallel to the air-gap surface of the machine.

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 small permanent magnet sections are inserted in the stator to provide an initial emf to self excite the armature windings of the generator thus avoiding the need for a separate dc power supply or battery.

For a better understanding of the present invention reference will now be made to the accompanying drawings in which: Figure 1 shows a stator (Prior Art) of a flux switching motor with 8 stator teeth and rotor with 4 rotor teeth in which the field magnet means comprises only a field winding in four of the stator slots; Figure 2 shows a stator (Prior Art) of a flux switching motor with 8 stator teeth and rotor with 4 rotor teeth in which the field magnet means comprises four permanent magnets and no field windings; Figure 3 shows a stator of a flux switching motor according to the invention with 8 stator teeth and rotor with 4 rotor teeth in which the field magnet means comprises a field winding in four stator slots and four permanent magnets each with a path of low magnetic reluctance directly linking opposing magnetic faces of each magnet; Figure 4 shows an enlarged portion of the machine of Figure 3.

Figure 5 shows a flux plot of the machine in Figure 4 when there is no current in either the field windings or the armature winding.

Figures 6, 7 and 8 show flux plots of the machine in Figure 4 at progressively higher values of field current.

Figure 9 shows the variation in flux linking the armature of the machine in Figures 3-8, as the field current is increased from zero and also showing the variation in flux linking an armature of a prior art flux switching machine without any permanent magnets.

Figure 10 shows the stator and rotor of a flux switching machine according to a further aspect of the invention.

Figure 11 shows a flux plot of the machine in Figure 10 when there is no current in either the field windings or the armature winding.

Figures 12 and 13 show flux plots of the machine illustrated in Figure 10 with field MMF of 800 At and l600At respectively.

Figure 14 shows a stator with 12 stator teeth and 6 rotor teeth, one armature phase and a field magnet means according to the invention; Figure 15 shows a flux plot of the machine in Figure 14 when there is no current in either the field windings or the armature winding.

Figures 16 and 17 show flux plots of the machine illustrated in Figure 10 with field MMF of 1000 At and l800At respectively.

Figure 18 shows a graph of the flux linking the armature winding plotted against the variation in field MMF in a machine corresponding to Figure 14.

Figure 19 shows the variation in flux per m linking thearmature winding in a machine according to the invention having 12 stator slots and 6 rotor teeth and magnets positioned at the entrance to the

field slots as illustrated by Figure 14.

Figure 20 shows the stator and rotor of a flux switching machine according to a further aspect of the invention.

Figure 21 shows a flux plot of the machine in Figure 20 when there is no current in either the field windings or the armature winding.

Figure 22 shows a flux plot of the machine illustrated in Figure 20 with field MMF of l800At.

Figure 23 shows a graph of the flux linking the armature winding plotted against the variation in field MMF in a machine corresponding to Figure 20.

Figure 24 shows the stator and rotor of a flux switching machine according to a further aspect of the invention.

Figure 25 shows a flux plot of the machine in Figure 24 when there is no current in either the field windings or the armature winding.

Figures 26 shows a flux plot of the machine illustrated in Figure 24 with field MMF of l800At.

Figure 27 shows a graph of the flux linking the armature winding plotted against the variation in field MMF in a machine corresponding to Figure 24.

Figure 28 shows a graph of the percentage increase in armature flux due to the addition of magnets according to the aspect of the invention illustrated by the machine in Figure 24.

Figure 29 shows the stator of a machine according to the invention which is not circular such that the magnets associated with each field slot can be deeper.

Figure 30 shows a reluctance rotor with 5 teeth and a stator of a machine according to a further aspect of the invention in which the stator with 12 slots carries a field winding, permanent magnets positioned according to the invention and a three phase armature winding.

Figure 1 shows a flux switching machine with 8 stator teeth and 4 rotor teeth as described in US Patent 6,788,020. This motor contains a field winding in slots 1,3,5,7 of the stator 10 and an armature winding in slots 2,4,6,8 of the stator 10. The rotor, 11, is a salient pole rotor made from laminated steel with 4 rotor teeth, 9. This motor operates with direct current in the field winding and alternating current in the armature winding. The direct current in the field winding creates a four pole stator flux pattern which links the armature winding in a positive or negative direction as the rotor turns from alignment with stator teeth 21,23,25,27 to alignment with stator teeth 22,24,26,28. This alternating flux linking the armature generates an internal emf in the armature.

The machine can be used as a motor or generator by controlling the armature current to be in phase (motor) or out of phase (generator) with the armature emf. This machine provides a simple and easy to manufacture structure and gives excellent control flexibility with easy variation of both field current and armature current. The machine of Figure 1 has however two major deficiencies. Firstly, the field flux requires a direct current to flow in the field winding throughout its operation and thus the power losses in the field winding due to the resistance of the copper can be quite high. Secondly when used as a generator the machine suffers from the problem that no power can be generated without some external power source such as a battery to provide an initial field current to start the generation of emf in the armature.

Figure 2 shows a further flux switching machine, also from the prior art, as described in a paper "A permanent magnet flux switching motor for low energy axial fans", Y. Cheng; C. Pollock and H. Pollock; Fourtieth lAS Annual Meeting Conference Record, Volume 3, 2-6 Oct. 2005 Page(s):2168 -2175. This motor is the Four pole version of a two pole machine first described in a paper "Design principles of flux switch alternator," S. E. Rauch and L. J. Johnson, AIEE Trans., vol. PAS-74, pp. 1261- 1268, 1955. The stator 30 of figure 2 employs four permanent magnet sections 31,33,35 and 37 interspersed between four laminated stator sections 32, 34, 36 and 38 each carrying a slot for the armature winding. As in the motor of Figure 1 rotation of the rotor 11 causes a cyclical variation in the flux linking the armature winding and hence induces an emf in the armature winding. The emf is proportional to speed and unlike the machine in Figure 1 the field flux produced by the permanent magnets cannot be altered significantly as there is no field winding. The machine of Figure 2 therefore provides a machine of high efficiency since the magnetic field is produced without copper losses in a field winding but the fact that the field cannot be controlled limits the use of the machine to a narrow speed range.

Figure 3 shows the stator 40 and the rotor 49 of a flux switching machine according to a first aspect of the invention. The machine has a rotor 49 with 4 salient teeth. The rotor would usually be made from laminated steel. The stator has 8 stator teeth with slots 41, 43, 45, 47 carrying the field winding arranged such that the current direction in slots 41 and 45 is opposite to the current direction in slots 43 and 47. This current pattern creates a 4 pole magnetic field at the air gap surface between the stator 40 and rotor 49. The stator has 4 further slots 42, 44, 46, 48 which carry an armature winding. The armature winding is arranged such that at any one time current in slots 42 and 46 will be in the opposite direction to the current in slots 44 and 48. The armature winding may be bifilar comprising two strands as described in US Patent number 6,037,740. The stator 40 also has 4 rectangular slots 50 cut into the outer edge of its periphery. Each slot is positioned behind the slots 41, 43, 45, 47 carrying the field winding. Four permanent magnets 51, 53, 55, 57 are located in each of the four slots 50. The permanent magnets are magnetised so that their magnetic axis is tangential or parallel to the air gap between the rotor and stator. Furthermore, the magnetisation direction of magnets 51 and 55 is opposite to the magnetisation direction of magnets 53 and 57. The insertion of magnets in this way is unusual in electrical machine design because a section of stator steel directly connects the north and south faces of each magnet. This effectively creates a magnetic short circuit by providing a very low reluctance path for the magnetic flux from the magnet.

Figure 4 shows an enlarged view of one of the slots 50 in the stator 40 and also shows one of the magnets 51 with its magnetic axis parallel to the airgap of the machine.

For illustration purposes it will be assumed that the face 61 of the magnet is a north pole and that the face 62 of the magnet is a south pole.

Figure 5 shows a magnetic flux plot of the section of the stator shown in Figure 4 when there is no current in the field slot 41. The flux lines clearly show the majority of the magnetic flux produced by the magnet is directly linking the north and south pole faces of the magnet through the stator steel.

By appropriate choice of magnet dimensions and material and the dimensions of the shorting or bridging section 63, the flux density in the shorting or bridging section can be controlled so that it reaches saturation flux density. When the bridging section 63 saturates a percentage of the magnet (c7 flux is forced to take the longer path through the stator teeth across the air gap to the rotor. In so doing a percentage of the magnet flux links the armature winding. If the rotor were to rotate an emf would be induced in the armature winding. Figure 5 shows that whilst a significant number of flux lines pass through the shorting section of stator steel there is a percentage of the permanent magnet flux which passes through the armature windings to the rotor.

Figure 6, 7 and 8, shows flux plots of the same section of the machine as the current in the field winding is increased. The direction of the current in the field slot 51 is out of the paper such that the field produced by the field winding acts to reduce the magnet flux flowing through the bridging section 63. In figure 7 the mmf of the field winding almost exactly cancels the mmf of the magnet such that there is no flux in the bridging section so all of the magnet flux is forced through the stator teeth to the rotor.

The flux linking the armature therefore increases as the field current increases. In Figure 8 the current in the field increases further and the flux in the bridging section becomes positive such that the total flux passing through the stator teeth to the rotor is greater than the magnet flux alone.

Figure 9 shows a graph of the flux linking the armature winding of the machine (illustrated by Figures 3-8), as the mmf in the field is increased. There are two lines on the graph. Line 71 is the variation in armature flux in a flux switching machine similar to Figure 1 as known in the prior art. Line 72 is the variation in flux linking the armature winding in a machine according to the invention as illustrated in Figures 3 to 8. The graph 72 clearly shows that there is some flux linking the armature winding while the field current is zero and this flux can then be increased further by the field current flowing in the field slots. In this example, in the range 0 to 400 Ampere Turns, the total flux linking the armature winding is higher in a machine designed according to this invention relative to a machine designed according to the prior art. Above 1400 Ampere Turns the steel of the shorting section alongside each magnet behind the field slot begins to saturate and the total flux in the machine according to the invention is lower than a prior-art machine with no magnets behind the field slots. This is caused by saturation of the bridging section of steel which may be narrower in a machine according to the invention due to the space occupied by the magnet. The point at which the two lines cross over can be controlled by choice of the relative radial depth of the magnet and bridging steel sections. In the design illustrated by Figures 3 to 8, operating the flux switching machine at field mmf values up to 400 Ampere Turns, gives higher armature fluxes in the machine according to the invention relative to a machine according to the prior art. Only if the field mmf is increased beyond 1400 Ampere Turns does the flux switching machine with the permanent magnet behind the field slot show lower armature flux values at any given field current. Therefore, in the example given, at field mmf values up to 400 Ampere Turns, the copper losses in the field winding of the machine with the permanent magnet, constructed according to the invention, are lower than a prior art flux switching machine of identical dimensions, making the machine more efficient, particularly under lightly loaded conditions.

A flux switching machine with a magnet positioned in the stator behind each field slot such that the stator steel forms a bridging section directly connecting the north and south faces of the magnet without passing through the rotor has been illustrated. This unusual use of the stator to divert permanent magnet flux away from the rotor, through the bridging section has been shown to provide a permanent magnet assisted flux switching machine in which the armature flux is increased under light load conditions. Unlike other permanent magnet machines the flux can be further controlled by control of the field winding. Thus this invention provides a machine construction with high efficiency under light loads and by use of the field winding, a controllable flux source under high loads is also provided.

In addition to the increased efficiency there are further advantages of a machine according to the

invention over the prior art.

Prior art machines incorporating magnets into the stator of a doubly salient reluctance machine are described in: "A permanent magnet flux switching motor for low energy axial fans", V. Cheng; C. Pollock and H. Pollock, Conference Record of the IEEE Industry Applications Society Annual Meeting 2005, Volume 3, pp:2168 -2175 and in "Design Considerations and Test Results for a Doubly Salient PM Motor with Flux Control", F. Leonardi, Y. Li, 1. Matsuo, l.A. Lipo and P. McCleer,. Conference Record of the IEEE Industry Applications Society Annual Meeting, October 6-11, 1996, pp. 458-463.

In both these prior art examples the stator needs to be assembled from several parts. Unlike these prior art machines, the stator lamination of a machine according to this invention is a single stamped part with additional slots to carry the permanent magnet blocks. This simple construction makes the stator stiffer and easier to assemble without the need of additional mechanical support. This makes it possible to manufacture the machine according to the invention at low cost.

Furthermore, the magnet pieces 51, 53, 55 and 57 can be simple rectangular blocks. These magnet blocks are also easy to manufacture at low cost compared to the curved sections of permanent magnet machines in the prior art, particularly machines where the magnets are mounted on the curved surface of the rotor.

A further advantage of the machine according to the invention is particularly evident when using the machine as a generator. Conventional doubly salient machines such as the switched reluctance machine, do not generate any emf when they are turned by a mechanical prime mover. As a result an additional power source such as a battery is required to first magnetise the stator before any energy can be generated. An example of such a generation scheme is described in "Starting of Switched Reluctance Generators", M. Turner, US Patent 6,906,490. The need for a battery supply is a major disadvantage of using prior art doubly salient machines in off-line or standby power generation applications.

In the machine according to the invention, when the rotor starts to turn, an emf will immediately be induced in the armature winding due to the percentage of magnet flux which links the armature winding. This emf can be used to self excite the generator and then as the armature voltage builds up, the field winding is supplied from the machine's own energy to provide full power generation.

The use of the field winding in addition to the permanent magnet means that the output voltage can be controlled over a wide speed range. This means that a generator can be produced according to the invention which does not need an external battery supply as required in prior art generators such as described in US patent no. 6,906,490.

Furthermore, a generator according to the invention can use the field winding to generate a constant voltage over a wide speed range. This is very difficult to achieve in permanent magnet generators in which the permanent magnet flux is fixed and the voltage increases linearly with speed. At high speeds in prior art permanent magnet generators, the emf is very high. If a prior art permanent magnet generator is used with a wind turbine to charge batteries the speed range over which effective charging of the batteries can take place is very small. If the speed is too low the emf is not sufficient to overcome the internal emf of the battery. If the speed is too high the battery may be overcharged and damaged. The machine according to this invention has a permanent magnet assisted field winding which provides voltage control over a very wide speed range and is very suitable for use as a self excited wind powered generator.

Figure 10 shows a machine according to a second aspect of the invention. A stator 100 has eight stator teeth, 101, with eight stator slots. Stator slots 111, 113, 115 and 117 carry the field winding and stator slots 112, 114, 116 and 118 carry the armature winding. The field slots 111, 113, 115, 117 also contain four permanent magnet blocks, 102, near to the air-gap surface of the field slots. The rotor 103 has 4 rotor teeth, similar to the rotors in Figure 1 to 8.

The permanent magnet blocks are magnetised so that their magnetic axis is parallel to the air-gap surface in a tangential direction. The pole faces of adjacent magnets are opposing each other. The direction of magnetisation is in opposition to any flux which would have leaked across the open field slot when the field winding is energised. Figure 11 shows an enlarged view of one field slot and shows the path of the magnet flux when there is no current in either the field winding or the armature winding. The majority of the permanent magnet flux passes in a path around the field slot.

In common with the first aspect of the invention, the machine therefore incorporates a section of stator steel which provides a low reluctance path between opposing pole faces of each of the permanent magnet sections. Also in common with the first aspect of the invention, introduction of field current in the windings of the field slot reduces the amount of the magnet flux which passes around the back of the field slot (the shorting magnetic bridge) and increases the amount of flux which passes across the air-gap to the rotor, linking the armature winding. When the field current is increased further the direction of the flux in the shorting magnetic bridge changes direction such that the total flux linking the armature is now greater than the total flux produced by the permanent magnet. Figure 12 and 13 show the flux distribution in this machine as the field current is increased to 800 Ampere Turns and 1600 Ampere Turns respectively. In Figure 12 the field current is just sufficient to oppose the magnet flux which would have passed through the bridging section behind the field slot. There is no net flux in the bridging section and all the permanent magnet flux is directed through the rotor linking the armature winding. In Figure 13 the higher field current means that the armature flux is now the sum of the permanent magnet flux and additional flux passing through the bridging section due to the action of the field winding.

Figure 14 shows a further example of a machine according to the second aspect of the invention.

The stator has 12 teeth with six slots 82 used for field windings and six slots 83 used for armature windings. Set into the stator at the opening of each field slot are six magnet blocks, 85, magnetised so that adjacent magnets have like poles facing each other. The rotor 84 has six rotor teeth. The flux distribution in this machine when there is not field current and no armature current is shown in Figure 15. It is clear that when there is no field current in the field winding slots the tangentially magnetised magnets set up opposing magnetic fields. In the example shown containing the feature according to this invention, there is a bridging section of stator steel directly connecting the north and south faces of each of the stator magnets and most of the magnet flux passes through this bridging section. In the example of Figure 15 about 90% of the magnet flux passes through the bridging section and about 10% passes through the air-gap into the rotor. This percentage split can C' be adjusted by the radial thickness of the bridging section and is also dependent on the radial thickness of the air-gap.

Figure 16 shows a further flux plot of the machine of Figure 14 and 15. The field current has been applied to a total effective value of 1000 Ampere Turns. There is now no magnetic flux in the bridging section as the field mmf is in opposition to the permanent magnet mmf in that section. All the effective field flux is passing through the rotor and linking the armature winding.

Figure 17 shows the effect of a further increase in the field current. The flux linking the rotor is very high due to the flux from the magnet combining with the flux produced by the field current passing through the steel sections behind the field slots. The position of the magnets at the entrance to the field slots has had the effect of opposing slot leakage flux in the field slots and producing an

increased field flux through the armature winding.

Figure 18 shows a graph of the flux linking the armature winding plotted against the variation in field mmf in a machine corresponding to this aspect of the invention. The Line 172 for a machine with magnets at the entrance to the field slots is higher than the line 171 for a machine according to the prior art with no magnets at the entrance to the field slots.

Figure 19 shows the variation in flux per m linking the armature winding in a machine according to the invention having 12 stator slots and 6 rotor teeth and magnets positioned at the entrance to the field slots as illustrated by Figure 14. The plots in Figure 19 shows that over an electrical cycle spanning 60 mechanical degrees of rotor rotation i.e. a rotor pole pitch, there is a positive and negative variation in the amount of field flux linking the armature winding. The magnitude of the flux linking the armature winding is related to rotor position and at every position is also determined by the magnitude of the field current. Trace 180 shows the magnitude of the armature flux linkage when there is no current in the field winding. Traces 181, 182 and 183 show the armature flux variation with position at field winding mmf values of 1200 At, 2400 At and 3600 At respectively.

This variation in flux linkage in the armature winding therefore induces an emf in the armature winding. This induced emf is the back emf in a motor according to the invention, when current flows in opposition to the emf Alternatively, this induced emf is referred to as a generated emf when current flows in phase with the emf, in a generator according to the invention. The magnitude of the induced emf is proportional to the rate of change of the flux linkage and it therefore going to increase with the speed of rotation of the machine. The magnitude of the induced emf will also increase with the field current as the flux linking the armature winding is seen to increase with field current as illustrated by the other traces in Figure 19. It is particularly important to note from the trace 180 in Figure 19 that when the field MMF is zero, the armature flux is not zero. This small armature flux is due to the flux produced by the magnets alone. When the rotor of the machine is turning, this initial value of flux produces an emf in the armature winding, even if there is no current flowing in the field winding. This is particularly useful in the case of a generator where the need for an auxiliary battery supply to initiate the energisation of the field winding is avoided.

Figure 20 shows a machine illustrating a further aspect of the invention. The machine has a rotor 149 with 4 salient teeth. The rotor would usually be made from laminated steel. The stator has 8 stator teeth with slots 141, 143, 145, 147 carrying the field winding arranged such that the current direction in slots 141 and 145 is opposite to the current direction in slots 143 and 147. This current pattern creates a 4 pole magnetic field at the air gap surface between the stator 140 and rotor 149.

The stator has 4 further slots 142, 144, 146, 148 which carry an armature winding. The armature winding is arranged such that at any one time current in slots 142 and 146 will be in the opposite direction to the current in slots 144 and 148. The armature winding may be bifilar comprising two strands as described in US Patent number 6,037,740. Four permanent magnets 151, 153, 155, 157 are located in each of the 4 slots 141, 143, 145 and 147. The slots in the stator carrying the permanent magnets are shaped so that the permanent magnets are placed at an angle to the radial line from the centre of the rotor. This allows the magnets to have a greater surface area in contact with the steel of the stator. The effect of this angle on the surface area of the magnet in contact with the lamination can be seen by comparison between Figure 20 and Figure 14. The permanent magnets may be magnetised so that their magnetic axis is tangential or parallel to the air gap between the rotor and stator. Alternatively it may be easier for manufacture if the magnets are magnetised with their magnetic axis perpendicular to their longer surface, or an angle between these two angles. Furthermore, the magnetisation direction of magnets 151 and 155 is opposite to the magnetisation direction of magnets 153 and 157. As with previous aspects of the invention, the insertion of magnets in this way is unusual in electrical machine design because a section of stator steel directly connects the north and south faces of each magnet. This effectively creates a magnetic short circuit by providing a very low reluctance path for the magnetic flux from the magnet.

Figure 21 shows the magnetic flux plot for this aspect of the invention when there is no current in the field windings. The magnetic flux produced by the magnet passes around the back of the field coil, creating a magnetic short circuit in common with other aspects of the invention. A percentage of the permanent magnet flux links the armature winding and passes into the rotor.

Figure 22 shows the effect of having field current in the field slots. The flux direction in the shorting section behind the field slots is now reversed and the total flux from both the magnets and the field winding is forced to pass across the air-gap linking the armature winding. Figure 22 illustrates the benefit of the additional magnets and in particular the benefit of positioning these magnets at an angle to the radial line from the centre of the rotor. The magnet surface area is increased relative to the aspect of the machine illustrated in Figure 14. As a result of focussing of the flux produced at the surface of the magnet a greater benefit can be obtained. Figure 23 shows the plot of the flux linking the armature winding in a motor according to this aspect of the invention with an asymmetrically positioned permanent magnet (trace 192) compared to a motor according to the prior art with no magnet at all (trace 191). The benefit of the magnets positioned at an angle to the radial line has resulted in a greater increase in flux over the whole range of field currents.

Figure 24 shows a machine illustrating a further aspect of the invention. The machine has a rotor 149 with 4 salient teeth. The rotor would usually be made from laminated steel. The stator has 8 stator teeth with slots 241, 243, 245, 247 carrying the field winding arranged such that the current direction in slots 241 and 245 is opposite to the current direction in slots 243 and 247. This current pattern creates a 4 pole magnetic field at the air gap surface between the stator 240 and rotor 149.

Each field slot is arranged into two sub-sections for example 241a and 241b. Each of these sub-sections would contain conductors carrying field current in the same direction. The stator has 4 further slots 242, 244, 246, 248 which carry an armature winding. The armature winding is arranged such that at any one time current in slots 242 and 246 will be in the opposite direction to the current in slots 244 and 248. The armature winding may be bifilar. Four permanent magnets 251, 253, 255, 257 are located in each of the 4 slots 241, 243, 245 and 247. The slots in the stator carrying the field winding and the permanent magnets have an unusual shape to obtain maximum benefit from the invention and to produce a machine of high efficiency. The field winding would typically be made up of four coils such that if one side of one coil is in sub-section 241a of slot 241, then the second side of the same coil will be in sub-section 247b of slot 247. The other three field coils would span 241b to 243a, 243b to 245a and 245b to 247a. The shape of the field slots therefore allows the field coils to have a shorter pitch which reduces the resistance of the coils and reduces the losses in the field winding. The straighter section of the field slots between each of the two winding sub-sections provides a location slot for the permanent magnets. The surface area of the magnets is greater than the cross-sectional area of the path through which the flux passes, thus creating a focussing of the magnetic flux. The permanent magnets are usually magnetised so that their magnetic axis is tangential or parallel to the air gap between the rotor and stator. Furthermore, the magnetisation direction of magnets 251 and 255 is opposite to the magnetisation direction of magnets 253 and 257. As with previous aspects of the invention, the insertion of magnets in this way is unusual in electrical machine design because a section of stator steel directly connects the north and south faces of each magnet. This effectively creates a magnetic short circuit by providing a very low reluctance path for the magnetic flux from the magnet.

Figure 25 shows the magnetic flux plot for this aspect of the invention when there is no current in the field windings. The magnetic flux produced by the magnet passes around the back of the field coil, creating a magnetic short circuit in common with other aspects of the invention. However, it can be seen that due to the large surface area of the magnet blocks relative to the thickness of the steel sections, it is possible for the magnet to saturate the steel shorting sections and force some of the magnet flux to pass through the air-gap to the rotor and around a path which links the armature winding. The magnetic flux linking the rotor is therefore larger than in some of the earlier aspects of the invention.

Figure 26 shows the effect of having field current in the field slots in addition to the deep magnet.

The flux direction in the shorting section behind the field slots is now reversed and the total flux from both the magnets and the field winding is forced to pass across the air-gap linking the armature winding. Figure 27 shows the graph of the amount of flux linking the armature winding as the field MMF is increased in a motor according to this aspect of the invention with the deep permanent magnet splitting the field slot (trace 196) compared to a flux switching motor according to the prior art with no magnet at all (trace 195). Firstly it can be seen that the magnetic flux linking the armature when no field current is present is higher than with any other aspect of the invention.

Secondly, the percentage increase in magnetic flux at non-zero values of field current is larger than with other aspects of the invention. Thirdly at high values of field current the additional permanent magnet flux gives the machine a higher saturation level of total field flux. Figure 28 shows the percentage increase in armature flux as a result of designing a machine according to this aspect of the invention. At small values of field current there is a very large increase in magnetic flux due to the additional flux of the permanent magnet and as the flux due to the field current increases the additional benefit of the permanent magnet flux decreases but at higher field MMF values when the steel becomes saturated, the permanent magnet and Field MMF act in parallel to provide an additional 8% of magnetic flux which will have a benefit on the machine efficiency.

Figure 29 shows a further aspect of the invention which shows a square shaped stator lamination 350 with 8 slots. Four of the slots 341, 343, 345, 347 would carry the field winding such that the field current pattern creates a 4 pole magnetic field at the air gap surface. Each field slot is arranged into two sub-sections for example 341a and 341b. Each of these sub-sections would contain conductors carrying field current in the same direction. The stator has 4 further slots 342, 344, 346, 348 which carry an armature winding. The armature winding is arranged such that at any one time current in slots 342 and 346 will be in the opposite direction to the current in slots 344 and 348. The armature winding may be bifilar. Four permanent magnets 351, 353, 355, 357 are located in each of the 4 slots 341, 343, 345 and 347. The slots in the stator carrying the field winding and the permanent magnets have an unusual shape to obtain maximum benefit from the invention and to produce a machine of high efficiency. The field winding would typically be made up of four coils such that if one side of one coil is in sub-section 341a of slot 341, then the second side of the same coil will be in sub-section 347b of slot 347. The other three field coils would span 341b to 343a, 343b to 345a and 345b to 347a. The shape of the field slots therefore allows the field coils to have a shorter pitch which reduces the resistance of the coils and reduces the losses in the field winding.

The straighter section of the field slots between each of the two winding sub-sections provides a location slot for the permanent magnets. By locating the field slots in the corners of the lamination, a greater depth is available for the slot. More specifically the depth of the permanent magnets in the slot entrance can be much greater. The surface area of the magnets is greater than the cross-sectional area of the path through which the flux passes, thus creating a focussing of the magnetic flux. The permanent magnets are usually magnetised so that their magnetic axis is tangential or parallel to the air gap between the rotor and stator. Furthermore, the magnetisation direction of magnets 351 and 355 is opposite to the magnetisation direction of magnets 353 and 357. As with previous aspects of the invention, the insertion of magnets in this way is unusual in electrical machine design because a section of stator steel directly connects the north and south faces of each magnet. This effectively creates a magnetic short circuit by providing a very low reluctance path for the magnetic flux from the magnet. However when the field current is applied, the amount of magnet flux which flows in the short circuit is reduced and eventually reaches zero. At higher values of field current the flux in the shorting section starts to have a polarity which adds to the permanent magnet flux. Laminations with extended radial depth in the region behind the field slot are therefore very suited to employing this invention. They may be square, rectangular, hexagonal (in the case of a 12 slot stator) or they may have any external shape providing the radial depth of the stator in the region of the field slot is greater than the radial depth of the stator in the region of the armature slot.

This invention therefore provides an improvement to an electrical machine with a field system on the stator in which the field system contains both a field winding and a permanent magnet, characterised by the stator steel in the region of the field slots being modified to create an additional slot or an adaption of the normal field slot to support one or more permanent magnets wherein sections of the stator lamination create a path or bridging section between the north and south poles of each magnet so that some of the magnetic flux from the magnet is directed away from the air-gap of the machine. When current flows in the field windings the amount of flux flowing in the bridging section reduces and the flux crossing the air-gap of the machine and linking one or more armature windings is increased. At high field currents the total flux from the field system is higher than would have been available from a prior art machine containing only the field winding or only the magnet.

Figure 30 shows a further aspect of the invention implemented in a 3 phase flux switching machine.

The machine has a rotor 379 with 5 salient teeth. The rotor would usually be made from laminated steel. The stator has 12 stator teeth with slots 341, 343, 345, 347, 349 and 35]. carrying the field winding arranged such that the current direction in slots 341, 345 and 349 is opposite to the current direction in slots 343, 347 and 351. As with the previous aspect of the invention, each field slot is arranged into two sub-sections, each of these sub-sections contain conductors carrying field current in the same direction.

Six permanent magnets 361, 363, 365, 367, 369 and 371 are located in each of the 6 slots 341, 343, 345, 347, 349 and 351. The slots in the stator carrying the field winding and the permanent magnets have an unusual shape to obtain maximum benefit from the invention and to produce a machine of high efficiency. The field winding would typically be made up of six coils arranged between the sub-sections of the field slots in a manner similar to previous aspects of the invention. As with previous aspects of the invention, the insertion of magnets in this way is unusual in electrical machine design because a section of stator steel directly connects the north and south faces of each magnet. This effectively creates a magnetic short circuit by providing a very low reluctance path for the magnetic flux from the magnet. The field current then controls the relative proportion of the permanent magnet flux which passes through the shorting steel section behind each field slot.

The stator has 6 further slots 42, 344, 346, 348, 350 and 352 which carry armature windings. The armature windings are arranged to make three electrical phases. Armature Phase 1 comprises a coil spanning slots 342 and 344, in series or parallel with a coil spanning slots 350 and 348. Armature Phase 2 comprises a coil spanning slots 344 and 346, in series or parallel with a coil spanning slots 352 and 350. Armature Phase 3 comprises a coil spanning slots 346 and 348, in series or parallel with a coil spanning slots 342 and 352. Each of the armature slots therefore contain conductors of two armature phases.

The rotor 379 of the three phase flux switching motor has 5 salient poles or teeth. An alternative rotor for this stator would have seven teeth.

Figure 31 shows the magnetic flux plot for this aspect of the invention when there is no current in the field windings. The magnetic flux produced by the magnet passes around the back of the field coil, creating a magnetic short circuit in common with other aspects of the invention. However, it can be seen that due to the large surface area of the magnet blocks relative to the thickness of the steel sections, it is possible for the magnet to saturate the steel shorting sections and force some of the magnet flux to pass through the air-gap to the rotor and around a path which links the armature windings. Since the number of rotor teeth is no longer a factor of the number of stator teeth the flux distribution through armature coils is different and the relative magnitudes of the distribution in the armature phase windings will change with position.

Figure 32 shows a magnetic flux plot for this aspect of the invention when there is no current in the field windings at a second position of the rotor. The flux linking the three armature phases has all changed relative to Figure 31.

Figures 33 and 34 show flux plots at the same positions as Figures 31 and 32 when the field slots carry field current. The flux from the permanent magnets is now prevented from passing through the back iron behind the field slots and all the permanent magnet flux links the armature windings and combines with flux created by the field mmf to give the total armature flux linkages. At both rotor positions represented by Figures 33 and 34 the flux linking the three armature phases have different relative values.

Figure 35 shows a graph of the flux linking the three armature phase windings as rotor position changes over one rotor pitch of 72 degrees. Each armature flux linkage graph is the summation of flux linking the diagonally opposite armature coils of the same phase. The Figure clearly shows the three phase distribution of the three armature windings. Lines 401, 402, 403 are the armature flux per m linking the armature coils of phase 1, phase 2 and Phase 3 respectively when only the magnet is present (Field current and Armature Current are zero). These three sinusoidal flux distributions induce three sinusoidal emfs in the three armature phases. Similar to the single phase implementations of the invention, the armature emf can be produced from the permanent magnet flux without any field winding mmf being present. Lines 411, 412, 413 are the armature flux per m linking the armature coils of phase 1, phase 2 and Phase 3 respectively when a field MMF Of 2400 At is present in addition to the magnet MMF. There is a clear increase in the armature flux linkage by the addition of the field current. Lines 421, 422, 423 are the armature flux per m linking the armature coils of phase 1, phase 2 and Phase 3 respectively when a field MMF of 2400 At is present in a stator lamination which has no permanent magnets at the entry to the field slots. For the same value of field MMF the machine according to the invention has 33% more armature flux linkage than a machine without implementation of the invention.

The invention can also be applied to a 3 phase flux switching motor with 12 stator teeth and 7 rotor teeth. US Patent 6,075,302 describes a three phase flux switching motor in which the width of a field slot is reduced and the width of an armature slot is increased. Whilst this improves the alignment of rotor and stator teeth on the teeth associated with the diametrically opposite armature coils of each phase winding, the machine as described in US Patent 6,075,302 suffers from increased slot leakage across the narrower field slot entry. The loss in field flux due to slot leakage eliminates any increased performance achieved by the change in the slot width. Modification of the three phase flux switching machine stator according to this invention overcomes this problem of slot leakage and surprisingly provides a significant boost to performance due to the addition of the permanent magnet.

Figure 36 shows a machine with stator 500 with twelve stator teeth and rotor 501 with seven rotor teeth. The width across the entrance to an armature slot 502 is wider than the width of the gap across the entrance to a field slot 503. Six permanent magnet blocks 504 are positioned in the narrower gap at the entrance to each field slot. In common with other aspects of the invention these permanent magnet blocks are magnetised parallel to the air-gap and adjacent magnets have opposite magnetic polarity.

Figure 37 shows the flux plot of this implementation when field current of 1200 At is applied in addition to the magnet MMF. Note that the area of overlap of stator teeth 505 and 506 with their respective rotor teeth are maximised by the reduced field slot width.

Figure 38 shows the variation of Armature Flux Linkage in one phase of a 3 phase flux switching motor over one rotor pole pitch with 1200 At in the field winding. Line 510 corresponds to a stator with equally spaced armature and field slots and no magnets according to the prior art. Line 511 corresponds to a machine built according to US Patent 6,075,302 with the field slot width reduced relative to the armature slot width. Surprisingly the total flux linking the armature winding is slightly lower than the machine with equally spaced slots. Line 512 shows the machine according to the invention as illustrated by Figure 36 and 37. The flux linkage in the Armature is higher than any

other implementation from the prior art.

All of the illustrations used in this description have been machines with the rotor located on the inside of the stator. All of the concepts and implementations can be equally applied to machines where the rotor (with inwards directed teeth) rotates around the stator with outward directed teeth.

Claims

Reluctance Machines With Permanent Magnets Integrated into the Stator CLAIMS

1. An electrical machine comprising a rotor with magnetic saliency and without windings, a stator having stator teeth, the stator carrying one or more armature windings 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, the field magnet means interspersed between alternate stator teeth such that current flowing in the armature windings combines with the field produced by the field magnet means to direct the combined flux in a radial direction across the air-gap between stator and rotor, wherein the field magnet means comprises at least one field winding and at least one permanent magnet, arranged such that there is a continuous path of stator steel from the north face of each of the said magnets to the south face of the same magnet, such that only a portion of the permanent magnet flux links one or more of the armature windings and passes through the air-gap to the rotor.

2. An electrical machine according to Claim 1 in which alternate magnets have opposite magnetic polarity;

3. An electrical machine according to Claim 2 in which the magnets are magnetised in a direction parallel to the air-gap.

4. An electrical machine according to Claim 2 in which current in the field winding increases the total magnetic flux from the field magnet means which links an armature winding.

5. An electrical machine according to Claim 4 in which the permanent magnets are positioned at an angle to a radial line from the centre of the machine.

6. An electrical machine according to Claim 4 in which each permanent magnet is positioned at the opening of one or more field slots near the air gap surface of the stator.

7. An electrical machine according to Claim 4 in which each permanent magnet is positioned in a slot in the outer surface of the stator cut into the steel section radially outward from a

stator slot carrying a field winding.

8. An electrical machine according to Claim 6 in which each permanent magnet has a radial depth which is a substantial portion of the field slot depth such that it divides the field slot

into two sub-sections each carrying field coils.

9. An electrical machine according to any of the proceeding claims in which the outer shape of the stator lamination is not circular such that the radial depth of the stator in the region of the field magnet means is greater than the radial depth of the stator in the region of armature winding slots.

10. An electrical machine according to Claim 1 in which the width of a field slot is narrower than the width of an armature slot and at least one permanent magnet is inserted in the opening

of the field slot.

11. An electrical machine according to any of the preceding claims which is used as an electric motor.

12. An electrical machine according to any of the Claims 1-11 which is used as a generator.