WO2018002609A1

WO2018002609A1 - Auxiliary windings in a rotating electrical machine

Info

Publication number: WO2018002609A1
Application number: PCT/GB2017/051883
Authority: WO
Inventors: Peethamparam Anpalahan; Adrian Bell; Atul Jain
Original assignee: Cummins Generator Technologies Limited
Priority date: 2016-06-29
Filing date: 2017-06-28
Publication date: 2018-01-04
Also published as: GB2566192A; GB201611272D0; GB201819787D0; GB2566192B

Abstract

AUXILIARY WINDINGS IN A ROTATING ELECTRICAL MACHINE A stator for a rotating electrical machine is disclosed, the stator comprising main windings (18) for supplying power to a load and auxiliary windings (20) for supplying power to an exciter. The auxiliary windings comprise a first group of windings (24) arranged to couple to a fundamental component of a magnetic flux, and a second group of windings (26) arranged to couple to a third harmonic of the magnetic flux. By providing a first group of windings arranged to couple to a fundamental component of a magnetic flux, and a second group of windings arranged to couple to a third harmonic of the magnetic flux, the auxiliary windings may be at least partially distributed about the stator, which may lead to better utilisation of the slot space. (Fig. 7)

Description

AUXILIARY WINDINGS IN A ROTATING ELECTRICAL MACHINE

The present invention relates to auxiliary windings for supplying power to an exciter in a rotating electrical machine such as an electrical generator.

Electrical generators operate by rotating a magnetic field produced by a rotor relative to windings in a stator in order to generate an AC output in the stator windings. The rotor's magnetic field may be produced by passing a DC current through windings in the rotor. This DC current may be generated by an exciter mounted on the shaft of the generator. An automatic voltage regulator (AVR) may be provided to control the exciter, thereby to control the current supplied to the rotor windings. This arrangement requires a power source to be provided for the exciter. In order to supply power for the exciter, it is known to provide auxiliary windings in the stator. The auxiliary windings are independent of the main windings, and are able to supply power to the exciter independently of the generator output.

However auxiliary windings may not provide the short circuit maintenance or enhanced overload capability required by some applications, such as those where the generator is used to start an induction motor. In such situations it is known to use a separate permanent magnet generator (PMG) to power the exciter. However this adds to the cost and overall size of the machine.

WO 2016/007482 discloses an auxiliary winding for a generator, in which the auxiliary winding is configured to utilize the fundamental component of the flux in the air gap of the alternator along with selected spatial harmonic components to provide power to an automatic voltage regulator during all operating conditions. However the disclosed arrangement requires a large amount of slot space to be made available for the auxiliary winding. This reduces the amount of space for the main windings, thus reducing the overall performance of the machine.

Furthermore, it may be difficult to achieve the correct balance between the fundamental component of the flux and the selected spatial harmonic

components. In addition, the overhang windings may be difficult to cool. According to a first aspect of the present invention there is provided a stator for a rotating electrical machine, the stator comprising main windings for supplying power to a load and auxiliary windings for supplying power to an exciter, wherein the auxiliary windings comprise a first group of windings arranged to couple to a fundamental component of a magnetic flux, and a second group of windings arranged to couple to a third harmonic of the magnetic flux.

The present invention may provide the advantage that, by providing a first group of windings arranged to couple to a fundamental component of a magnetic flux, and a second group of windings arranged to couple to a third harmonic of the magnetic flux, the auxiliary windings may be able to provide enhanced overload and/or short circuit maintenance capability, while being at least partially distributed about the stator. This may lead to better utilisation of the slot space, allowing more space to be made available for the main windings. This in turn may help to improve the overall performance of the machine. Furthermore, the auxiliary windings may have end windings which are more distributed, thereby facilitating cooling. In addition, by providing two groups of windings, it may be easier to adjust the relative contribution made by each harmonic. The magnetic field is preferably produced by a rotor, and preferably crosses an airgap between the rotor and the stator. The rotor may have a number of poles with a pole pitch. The fundamental component and/or third harmonic of the magnetic flux may be determined by the pole pitch. The stator may comprise a plurality of stator slots. In this case at least some of the main windings and the auxiliary windings may be located in the same stator slots. For example, the auxiliary windings may be located beneath the main windings within the slots, or above the main windings, or a combination thereof. Preferably the first and second groups of windings are interleaved. Thus some of the windings in one group may have a space between them, and the space may be occupied by some of the windings from the other group. Preferably the windings from one group are located in slots which are not occupied by the windings from the other group. This may help to distribute the auxiliary windings around the stator slots, thereby utilising the slot space more efficiently and allowing more space to be made available for the main windings.

The first and second groups of windings may be connected in series. This can allow any voltage contribution from either or both groups of windings to be supplied to the exciter.

Preferably the first group of windings comprises a plurality of coils. Each coil is preferably wound through two separate stator slots. Thus a coil may comprise two coils sides running through respective stator slots, and end windings extending between the stator slots.

Preferably the coils in the first group of windings are connected in series, to allow a voltage contribution from any of the coils to be supplied to the exciter.

Preferably, in the first group of windings, adjacent coil sides in adjacent coils comprise turns of windings which run in the same direction. This can allow both of those coil sides to contribute to the output. The adjacent coil sides in adjacent coils may form a set (e.g. a pair) of coil sides, in which set the turns of windings all run in the same direction. Preferably a plurality of such sets of coil sides is provided, and the directions of the turns of windings alternate between each subsequent set. A distance between a midpoint of one set of coil sides and a midpoint of another set of coil sides is preferably (approximately) equivalent to a pole pitch. This can allow the coils to couple to the fundamental component of the magnetic flux.

In the above configuration, at least two adjacent coil sides within a set may be separated by at least one stator slot (that is, at least one slot may be left between the two adjacent coil sides). This can allow one or more coils from the second group of windings to be located in one or more slots between the two adjacent coil sides. This can help to distribute the auxiliary windings around the stator slots, thereby utilising the slot space more efficiently and allowing more space to be made available for the main windings. Preferably the coils in the first group of windings have a pitch which enables them to couple to the fundamental component of the magnetic flux.

In one embodiment, the coils in the first group of windings have a pitch of approximately one pole. Thus each coil may have two coil sides which are separated by a distance of approximately one pole. This can allow the coils to couple to the fundamental component of the magnetic flux.

In the above embodiment, the coils in the first group of windings may be arranged in sets, with the coils in a set staggered from each other, preferably such that there is at least one slot between two coils in a set. This can allow one or more coils from the second group of windings to be located in one or more slots between the two coils in a set. In another embodiment the coils in the first group of windings have a pitch of approximately two thirds of a pole (i.e. 2/3 short pitched). In this embodiment, each alternate coil may be wound in the opposite direction (e.g. clockwise or anticlockwise). This can allow adjacent coil sides in adjacent coils to comprise turns of windings which run in the same direction.

Adjacent coil sides in adjacent coils may form a set (e.g a pair) of coil sides with turns of windings which run in the same direction. A plurality of such sets of coil sides may be provided, with the direction of the turns of windings alternating between each subsequent set. A distance between a midpoint of one set of coil sides and a midpoint of another (e.g. adjacent) set of coil sides may be equivalent to a pole pitch (or a multiple thereof). Thus this configuration can allow coupling to the fundamental component of the magnetic flux using 2/3 short pitched coils.

Using 2/3 pitched coils rather than full pitched coils may provide the advantage of less physical interference between the overhangs, and lower overhang length, leading to lower costs and lower weight, as well as being easier to wind.

Preferably the second group of windings comprises a plurality of coils. Each coil may comprise two coils sides running through respective stator slots, and end windings extending between the stator slots. Preferably the coils in the second group of windings are arranged in sets, with each set of coils comprising at least two concentric coils. This can allow the second group of windings to be spread out over a greater number of stator slots than would otherwise be the case. Thus the windings may be more distributed, which can help to utilise the slot space more efficiently. Furthermore a concentric arrangement may facilitate winding, since the end windings do not need to cross over. Preferably the coils in a set are wound in the same direction (e.g. clockwise or anticlockwise).

Preferably the sets of coils in the second group of windings have an average pitch which enables them to couple to the third harmonic of the magnetic flux.

Preferably adjacent coil sides in a set of coils form a set (e.g. a pair) of coil sides in which the turns of windings run in the same direction. A plurality of such sets of coil sides may be provided, with the directions of the turns of windings alternating between each subsequent set. Preferably a distance between a midpoint of one set of coil sides and a midpoint of another set of coil sides is equivalent to one third of a pole pitch. This can allow the coils to couple to the third harmonic of the magnetic flux.

Preferably an inner coil in a set of coils comprises coil sides which are separated by at least one stator slot. This can allow a coil side in a coil in the first group of windings to be located in the stator slot. This can allow the windings to be more distributed.

Preferably the number of coils in a set of coils in the second group of windings is equal to the number of stator slots between two adjacent coil sides in the first group of windings. Thus a set of coil sides in the second group of windings may be located in stator slots between two adjacent coil sides in the first group of windings. This can help to maximise the number of slots used for the auxiliary windings, thereby distributing the windings more evenly about the stator and utilising the slot space more efficiently. In some embodiments the second group of windings may have a greater number of coils than the first group of windings (for example, to facilitate coupling of the second group of windings to the third harmonic). As a consequence, the voltage induced in the first group of windings may be less, or else an increased number of turns may be required in the first group of windings, which may impact on the slot space.

In one embodiment, some of the coils in the second group of windings are arranged to couple to the fundamental component of the magnetic flux, as well as the third harmonic. This may allow the number of turns in the first group of windings to be reduced, helping to achieve a better balance between the number of windings in each group, and thus leading to more efficient usage of slot space.

For example, some of the coils in the second group of windings may have a greater number of turns than the other coils in the second group of windings. Preferably the coils with a greater number of turns are arranged to couple to the fundamental component of the magnetic flux. For example, the coils with a greater number of turns may be spaced at an interval of one pole or a multiple thereof. This can allow the second group of coils also to contribute to the voltage produced from the fundamental component.

Preferably the rotating electrical machine has a plurality of poles with a pole pitch, and the fundamental component of the magnetic flux is determined by the pole pitch. The number of poles may be, for example, 2, 4, 6, 8, or some other number.

Any appropriate number of stator slots may be provided for the windings. For example, a stator with 36, 72, 96, or some other number of stator slots may be used. If a larger number of stator slots is available, a greater number of coils may be provided in each group, and the coils may be further distributed around the stator slots, thereby improving the usage of the slot space.

In one embodiment the main windings are three phase windings, although a different number of phases may be used instead. In the case of a single phase machine, the magnetic flux in the stator is pulsating rather than rotating. Therefore, in a single phase machine, it is desirable to position the auxiliary windings to maximise the voltage induced in the windings. According to another aspect of the present invention there is provided a stator for a single phase rotating electrical machine, the stator comprising main windings for supplying single phase power to a load and auxiliary windings for supplying power to an exciter, wherein the auxiliary windings comprise a plurality of coils arranged to couple to a fundamental component of a magnetic flux and to a third harmonic of the magnetic flux.

This aspect of the invention may provide the advantage that the auxiliary windings can provide enhanced overload and/or short circuit maintenance capability in a single phase machine.

Preferably the coils are arranged in sets, with each set comprising a plurality of concentric coils. This can help to distribute the coils about the stator, thereby leading to more efficient usage of slot space. Furthermore, a concentric arrangement of coils may facilitate winding, since the end windings do not need to cross over.

Preferably the coils in a set comprise substantially the same number of turns. This can help to distribute the turns about the stator, thereby allowing more efficient usage of the slot space.

Preferably the coils in a set are wound in the same direction.

Preferably a distance between a midpoint of each set of coils is equivalent to a pole pitch or a multiple thereof. This can allow the coils to couple to the fundamental component of the magnetic flux.

Preferably each set of coils comprises a set of coils sides with turns running in one direction, and a set of coil sides with turns running in the opposite direction. Preferably a distance between a midpoint of one set of coil sides and a midpoint of the other set of coil sides is equivalent to one third of a pole pitch. This can allow the coils to couple to the third harmonic of the magnetic flux.

Preferably the auxiliary windings are located in at least some of the same slots as the main windings. This can allow the auxiliary windings to be better coupled to the magnetic flux in a single phase machine. This is turn may maximise the voltage in the auxiliary windings and/or allow the auxiliary windings to use fewer turns, resulting in lower copper loss and better performance. Any compromise on the slot space may be mitigated through the use of multiple distributed coils.

According to another aspect of the present invention there is provided a rotating electrical machine comprising:

a rotor, the rotor comprising a plurality of poles; and

a stator in any of the forms described above.

Preferably the magnetic flux is produced by the rotor. The machine may further comprise an exciter for supplying electrical power to the rotor. An output of the auxiliary windings may be fed to the exciter via an automatic voltage regulator (AVR).

Corresponding method aspects may also be provided. Thus, according to another aspect of the invention there is provided a method of winding a stator for a rotating electrical machine, the method comprising winding the stator with main windings for supplying power to a load and winding the stator with auxiliary windings for supplying power to an exciter, wherein the auxiliary windings comprise a first group of windings arranged to couple to a fundamental component of a magnetic flux, and a second group of windings arranged to couple to a third harmonic of the magnetic flux. According to another aspect of the present invention there is provided a method of winding a stator for a single phase rotating electrical machine, the method comprising winding the stator with main windings for supplying single phase power to a load, and winding the stator with auxiliary windings for supplying power to an exciter, wherein the auxiliary windings comprise a plurality of coils arranged to couple to a fundamental component of a magnetic flux and to a third harmonic of the magnetic flux.

Features of one aspect of the invention may be provided with any other aspect. Apparatus features may be provided with method aspects and vice versa.

In this specification, terms such as "radially", "circumferentially" and "axially", as well as angles, are generally defined with reference to the axis of rotation of the electrical machine.

Preferred features of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows an overview of a synchronous generator;

Figure 2 shows the overload characteristics of a typical generator of the type shown in Figure 1 ;

Figures 3A and 3B are axial cross sections through parts of a rotating electrical machine illustrating the magnetic flux distribution;

Figure 4 shows parts of a three phase synchronous generator with auxiliary winding control;

Figures 5 and 6 are cross sections through a main stator in embodiments of the invention;

Figures 7A to 7C illustrate a winding schematic for auxiliary windings in one embodiment of the invention;

Figures 8A to 8C illustrate a winding schematic for auxiliary windings in another embodiment of the invention;

Figure 9 shows schematically the arrangement of the auxiliary windings in the embodiment of Figures 8A to 8C;

Figure 10 shows parts of a single phase synchronous generator with auxiliary winding control;

Figures 1 1 and 12 illustrates a winding schematic for auxiliary windings a single phase synchronous generator; and

Figure 13 shows schematically the arrangement of the auxiliary windings in the embodiment of Figures 1 1 and 12. Overview

Figure 1 shows an overview of a synchronous generator 1 . The generator includes a main machine 2 comprising a main rotor 3 and a main stator 4. The main rotor 3 is located on a shaft 5 which is driven by a prime mover such as a diesel engine (not shown). The main rotor develops a magnetic field, so that rotation of the main rotor 3 relative to the main stator 4 causes an AC output to be generated in main stator windings located in the main stator.

The main rotor 3 is magnetised by passing a DC current through rotor windings located in the main rotor. This DC current is generated by an exciter 6, which comprises exciter rotor 7, exciter stator 8, and rotating diodes 9. The exciter rotor 7 is mounted on the shaft 5. Rotation of the exciter rotor 7 relative to the exciter stator 8 generates an AC output in exciter rotor windings located in the exciter rotor. This AC output is converted to DC by the rotating diodes 9, and the DC output of the rotating diodes is fed to the rotor windings in the main rotor 3.

In the arrangement of Figure 1 , power for the exciter 6 is drawn from the main stator 4, via an AVR 10. This is referred to as a self excitation. The AVR 10 controls the level of excitation supplied to the exciter stator 8. By controlling the relatively low power which is fed to the exciter stator, control of the high power in the main rotor is achieved through the rectified output of the exciter rotor.

Synchronous generators normally have a maximum rated load, which is the maximum load that the generator can supply under steady state conditions. In addition, certain applications may require the generator to have short circuit maintenance and/or enhanced overload capability. Applications demanding this include, amongst others, marine applications, mobile light towers and military prime power generating sets, and applications which require induction motor starting capability.

Figure 2 shows the overload characteristics of a typical generator of the type shown in Figure 1 . Of note is the fact that the generator has no short circuit capability, i.e. at zero per unit volts, the generator produces zero per unit current. Overload capability is poor, with the generator achieving only ~1 .8 per unit current at 90% rated voltage. Various techniques exist to improve the overload capability of generators. These include providing a separate permanent magnet generator (PMG) on the shaft in order to provide the power for the exciter, and providing auxiliary windings in the stator. In the latter case, the auxiliary windings are independent of the main windings, and thus are able to supply power to the exciter even in overload conditions where the voltage in the main windings is reduced.

An advantage of auxiliary windings is that the length of the machine does not need to be increased, and the overall cost is lower compared to the cost of a PMG. However a disadvantage of auxiliary windings is that the output power available for the exciter is considerably reduced in short circuit or near short circuit conditions, due to a collapse in the fundamental component of the magnetic flux in the stator. Furthermore, known auxiliary winding arrangements compromise the space available for the main stator windings, thereby reducing the maximum power of the machine.

Figure 3A is an axial cross section through parts of a rotating electrical machine illustrating the magnetic flux distribution during normal, rated operation. Referring to Figure 3A, the machine comprises a rotor 22 located inside stator 12. The rotor in this example is a salient pole rotor with four poles, spaced apart by 90°. The rotor 22 produces a magnetic flux, which passes predominately between adjacent rotor poles via the stator. This establishes a magnetic flux in the stator 12, as indicated by flux lines 23. The magnetic flux in the stator has a

fundamental component determined by the pole spacing.

Figure 3B illustrates the magnetic flux through the same machine when the machine is in a short circuit operating condition. Referring to Figure 3B, it can be seen that, in this condition, the fundamental component of the magnetic flux collapses due to the interaction between the rotor and the stator. Instead, a number of shorter magnetic flux paths are established, with the third harmonic being the dominant component.

Embodiments of the present invention are designed to provide improved short circuit and overload capability by coupling the auxiliary windings to the fundamental component and the third harmonic of the magnetic flux, while maximising the space available for the main windings in the stator.

Three phase embodiments

Figure 4 shows parts of a three phase synchronous generator with auxiliary winding control. Referring to Figure 4, the generator comprises a main machine 2, an exciter 6 with rotating diodes 9, and an AVR 10. The main machine 2 comprises main stator windings 18 which supply a three phase load 19. In addition, auxiliary windings 20 are provided in the main stator. The auxiliary windings 20 receive part of the magnetic field produced by the main rotor, and supply single phase power for the exciter 6 via the AVR 10. An output of the main stator windings 18 is sensed, and the sensed value is fed to the AVR for use in controlling the amount of excitation supplied to the exciter. Figure 5 is a cross section through a main stator in an embodiment of the invention. The stator of Figure 5 is part of a three phase synchronous generator such as that shown in Figure 4. Referring to Figure 5, the stator 12 comprises stator core 14 with a plurality of stator slots 16 spaced about its inner

circumference. In this example 36 stator slots are provided, with a stator pitch of 10°, although of course a different number of slots and a different slot pitch could be used if desired. A plurality of main stator windings 18 are located in the stator slots. The stator windings 18 are double layer distributed windings, comprising a number of coils distributed around the stator slots in a known manner. Each slot contains a first layer 18_! of windings from one coil and a second layer 18₂ of windings from another coil.

In the arrangement of Figure 5, auxiliary windings 20 are provided within the same stator slots as the main stator windings 18. The auxiliary windings 20 are located on the inside of the slots 16 relative to the slot openings, and underneath (radially outwards of) the main stator windings 18. The auxiliary windings are distributed about the stator, and are located in at least the majority of the stator slots. The auxiliary windings are in the form of a plurality of coils having coil sides which run through the stator slots, and overhang windings which run outside of the stator slots. Figure 6 shows an alternative arrangement in which the auxiliary windings 20 are located above (radially inwards of) the main stator windings 18, adjacent to the slot openings. Figures 7 A to 7C illustrate a winding schematic for auxiliary windings in one embodiment of the invention. The winding schematic is designed for use with a three phase synchronous generator such as that shown in Figure 4.

In the embodiment of Figures 7A to 7C, the auxiliary windings are divided into two groups 24, 26. The first group of windings 24 is designed to couple principally to the fundamental component of the magnetic flux, while the second group of windings 26 is designed to couple principally to the third harmonic. This can allow the auxiliary windings to continue providing excitation even when the fundamental component of the magnet flux has collapsed due to a short circuit or overload condition.

In Figures 7A to 7C, linear representations of the slots 16 in the stator 12 are shown. Figure 7A illustrates the first group of auxiliary windings 24. This group of windings consists of four coils 24† to 24₄, each comprising a number of turns of wire. Each coil has a span of nine slots (equivalent to 90 ° or one pole pitch). The coils are arranged in two sets, with each set spaced 18 slots apart from the other (equivalent to 180 ° or two pole pitches). The two coils within each set are staggered from each other by 3 slots (30°). In this example, the first coil 24† is wound between slot number 3 and slot number 12, the second coil 24₂ is wound between slot number 6 and slot number 15, the third coil 24₃ is wound between slot number 21 and slot number 30, and the fourth coil 24₄ wound between slot number 24 and slot number 33. However it will be appreciated that this slot numbering is arbitrary and the coils may be located in any slots having the appropriate spacing between them.

In the arrangement of Figure 7A, the coils 24_! to 24₄ are connected in series. The directions of the turns of wire in each coil (relative to the input/output terminals) are indicated by the arrows. It can be seen that adjacent coil sides in adjacent coils (for example, those in slots 3 and 6) are in the same direction. These adjacent coil sides form a set of coil sides in which the turns are in the same direction. One such set of coil sides is indicated by the dotted line 25 in Figure 7A. Four such sets of coil sides are provided, with the direction of the turns alternating between each subsequent set. The distance between the midpoint of one set of coil sides and the midpoint of the subsequent set of coil sides is equivalent to one pole. As a consequence, the coils 24_! to 24₄ couple to the fundamental component of the magnetic field (see Figure 3A).

Figure 7B illustrates the second group of auxiliary windings 26. This group of windings consists of 12 coils 26_! to 26₁₂, each comprising a number of turns of wire. The coils 26i to 26i₂ are arranged in six sets, with each set spaced 6 slots apart from the other (equivalent to 60 ° or 1 /3 short pitched). Each set comprises an inner coil with a span of two slots and an outer coil with a span of four slots.

This concentric arrangement facilitates the winding process as the ends of the windings do not cross over. The coils 26-_\ to 26₁₂ are connected in series. The directions of the turns of wire in each coil (relative to the input/output terminals) are indicated by the arrows.

In the arrangement of Figure 7B, adjacent coil sides in a set of coils (for example, those in slots 1 and 2) are in the same direction, while the other coil sides in the set (for example, those in slots 4 and 5) are in the opposite direction. Thus 12 sets of coil sides are provided, with the direction of the turns alternating between each subsequent set. One such set of coil sides is indicated by the dotted line 27 in Figure 7B. The distance between the midpoint of one set and the midpoint of the subsequent set is three slots, which is equivalent to 30 ° or one third of the pole pitch. As a consequence, the coils to 26₁₂ couple to the third harmonic component of the magnetic field (see Figure 3B).

In addition, two of the sets of coils (in this example the second set of coils 26₃, 26₄ and the fifth set of coils 26₉, 26₁₀) have a larger number of turns than the other coils in the second group. This is indicated by the double arrows in those coils. The two sets of coils with a larger number of turns are spaced apart by 180 °. This allows those sets of coils also to couple to the fundamental component of the magnetic flux, for reasons that will be explained below. Figure 7C illustrates the winding configuration with both groups of auxiliary windings in place. The two groups of windings are connected in series, as indicated by the connection between terminals 4 and 2. The terminals 1 and 3 provide the output to the AVR.

Referring to Figure 7C, it can be seen that the two groups of windings 24, 26 are interleaved, with the windings from one group located in slots which are not occupied by the windings from the other group. This is made possible by the staggered configuration of the four coils in the first group of windings 24.

Furthermore, the coils within each group of windings are distributed over a number of slots. As a consequence, it is possible for the windings to be distributed around almost all of the stator slots. This results in more efficient usage of the slot space by the auxiliary windings, so that a greater number of turns can be provided in the main stator windings. This in turn can allow the overall performance of the machine to be improved.

In the arrangement shown in Figures 7A to 7C, a lower number of coils are present in the first group of windings 24 than in the second group of windings 26. It is therefore necessary for the coils in the first group of windings to have a greater number of turns, in order to produce the required output for the exciter. This is indicated by the double arrows in the first group of windings. Therefore, with reference to Figures 5 and 6, the amount of slot space which needs to be made available for the auxiliary windings 20 is determined by the number of turns in the first group of windings 24.

As mentioned above, in the second group of windings 26, two of the sets of coils have a larger number of turns than the other coils in the group. Those two sets of coils are spaced 180° apart, and thus couple to the fundamental component of the magnetic flux. This allows those coils to supplement the output of the first group of windings 24. This can allow the number of turns in the first group of windings to be reduced. This in turn allows the amount of slot space which needs to be made available for the auxiliary windings to be reduced. As a

consequence, a greater number of turns can be provided in the main stator windings, thereby improving the performance of the machine. Ideally, the coils in the second group of windings with an increased number of turns have the same or a similar number of turns as the coils in the first group of windings. This can allow an optimum distribution of the windings about the slots. The actual number of turns is chosen to achieve the desired output from the fundamental component of the magnetic flux. The number of turns in the other coils in the second group of windings can then be chosen to achieve the desired output from the third harmonic.

Figures 8A to 8C illustrate a winding schematic for auxiliary windings in another embodiment of the invention. In this embodiment, the auxiliary windings are again divided into two groups 24, 26. The first group of windings 24 is designed to couple principally to the fundamental component of the magnetic flux, while the second group of windings 26 is designed to couple principally to the third harmonic.

Figure 8A illustrates the first group of auxiliary windings 24. This group of windings consists of four coils, each comprising a number of turns of wire. In this embodiment each coil has a span of six slots (equivalent to 60° or 2/3 short pitch). The coils are connected in series, with each alternate coil being connected with the turns running in the opposite direction (clockwise and anticlockwise). The directions of the turns of wire in each coil are indicated by the arrows.

Referring to Figure 8A, it can be seen that adjacent coil sides in adjacent coils (for example, those in slots 3 and 6) are in the same direction. These adjacent coil sides form a set of coil sides in which the turns are in the same direction.

One such set of coil sides is indicated by the dotted line 25 in Figure 8A. Four such sets of coil sides are provided, with the direction of the turns alternating between each subsequent set. The distance between the midpoint of one set and the midpoint of the subsequent set is equivalent to one pole. As a consequence, the coils couple to the fundamental component of the magnetic field, in a similar way to the windings shown in Figure 7A.

Figure 8B illustrates the second group of auxiliary windings 26. This group of windings is the same as those shown in Figure 7B. Thus this group of windings couples principally to the third harmonic of the magnetic field. However the second and fifth coils also couple to the fundamental component.

Figure 8C illustrates the winding configuration with both groups of auxiliary windings in place. The two groups of windings are connected in series, as indicated by the connection between terminals 4 and 2. The terminals 1 and 3 provide the output to the AVR.

Referring to Figure 8C, it can be seen that the two groups of windings 24, 26 are interleaved, with the windings from one group located in slots which are not occupied by the windings from the other group. This is made possible by the 2/3 short pitched configuration of the four coils in the first group of windings 24.

Furthermore, the coils within each group of windings are distributed over a number of slots. As a consequence, it is possible for the windings to be distributed around almost all of the stator slots. This results in more efficient usage of the slot space by the auxiliary windings, and less compromise on the performance of the machine.

An advantage of the arrangement of Figures 8A to 8C, compared to that of Figures 7A to 7C, is that the 2/3 pitch coils shown in Figure 8A are easier to wind than the full-pitch coils shown in Figure 7A. This is because the end windings of the 2/3 pitch coils do not interfere to the same extent with the end windings of the coils in the first group of windings or with each other. Furthermore, since the span of the coils is less, less wire is required. This results in lower cost, reduced weight, and reduced space requirements for the end windings.

Figure 9 is an axial cross-section of the stator 12, showing schematically the arrangement of the auxiliary windings in the embodiment of Figures 8A to 8C. In Figure 9, a cross represents turns going into the plane of the paper, and a dot represents turns coming out of the plane of the paper.

In the embodiments described above, the six groups of 1 /3 short pitched coils capture third harmonic contents during short circuit conditions, hence giving increased short circuit capability, while the full pitch or 2/3 short pitch coils provide fundamental voltage for no-load, rated load, and overload conditions. The embodiments described above provide the advantage that the auxiliary windings are distributed around almost all of the stator slots, resulting in more efficient usage of the slot space and less compromise on the performance of the machine. Furthermore, by providing a first group of windings for the fundamental component, and a second group of windings for the third harmonic, it is possible to adjust the short circuit condition and normal load condition almost

independently. In addition, the overhangs of auxiliary winding are distributed, and hence are better cooled due to the exposure of more surface area to remove heat. The distributed short pitched winding has reduced overhang length compared to other configurations of auxiliary windings, which gives the benefit of cost reduction and reduced weight, as well as being easier to wind.

Overall, the embodiments described above can provide enhanced overload capability, short circuit capability and motor starting capability without requiring a separate PMG. This provides the advantage of lower cost and shorter axial length of the overall machine compared with a machine with a PMG excitation system. Single phase embodiments

In the three phase embodiments described above, the magnetic flux within the stator rotates due to the interaction of the various phases. Thus it is possible to distribute the auxiliary windings within the stator, without reducing their ability to couple to the magnetic flux. By contrast, in a single phase generator, there is a pulsating magnetic flux in the stator. Therefore, in a single phase machine, it is desirable to position the auxiliary windings to maximise the voltage induced in the windings.

Figure 10 shows parts of a single phase synchronous generator with auxiliary winding control. Referring to Figure 10, the generator comprises a main machine 30, an exciter 32 with rotating diodes 33, and an AVR 34. The main machine 30 comprises single phase main stator windings 36 which supply a load 38. In addition, auxiliary windings 40 are provided in the main stator. The auxiliary windings 40 receive part of the magnetic field produced by the main rotor, and supply power for the exciter 32 via the AVR 34. An output of the main stator windings 36 is sensed, and the sensed value is fed to the AVR for using in controlling the amount of excitation supplied to the exciter.

In some cases a generator may be provided in either single phase or three phase versions. In this case it may be desirable for both versions to use the same basic stator configuration, in order to reduce costs. This typically results in empty slots being available in the single phase version.

In known single phase generators with auxiliary windings, the auxiliary windings are typically located in slots which are not used by the main stator windings. However this may reduce the voltage produced in the auxiliary windings, since they are less well coupled to the pulsating magnetic flux. Furthermore, existing auxiliary windings for single phase machines do not couple to the third harmonic, and thus do not provide enhanced overload or short circuit capability.

Figure 1 1 illustrates a winding schematic for auxiliary windings a single phase synchronous generator such as that shown in Figure 10. In Figure 1 1 , the main stator windings 36 are shown in solid lines, while the auxiliary windings 40 are shown in dashed lines. The arrangement of the windings in slots 7 to 12 of Figure 1 1 is shown in more detail in Figure 12.

Referring to Figure 1 1 , the main stator windings 36 comprise four sets of coils, with each set comprising three concentric coils. The coils within a set are wound in the same direction, while alternate sets of coils are wound in the opposite direction. There is a spacing of, in this case, three stator slots between the coil sides of the inner coil within a set. In this example the sets of coils are contiguous, that is, the outer coil in each set lies in a stator slot which is adjacent to that of the outer coil in the adjacent set. The directions of the turns of windings are shown by the arrows.

In the main stator windings 36 shown in Figure 1 1 , the coil sides in one set and the coils sides in an adjacent set are wound in the same direction. Thus a set of six coil sides is provided, with the turns of windings all running in the same direction. A set of six coil sides is shown in more detail in Figure 12 (in this case, the coil sides running through slots 7 to 12). Referring back to Figure 1 1 , it can be seen that four such sets of coil sides are provided about the stator. The distance between the midpoints of each set of coil sides is equivalent to one pole. Thus the main windings 36 couple to the fundamental component of the magnetic field.

Still referring to Figure 1 1 , it can be seen that the auxiliary windings 40 comprise four sets of coils. Each set comprises three concentric coils, with the coils in a set having approximately the same number of turns. The coils in each set are wound in the same direction (clockwise or anticlockwise), while each alternate set is wound in the opposite direction. The distance between the midpoints of each adjacent set is 90° (one pole). Thus the auxiliary windings 40 couple to the fundamental component of the magnetic field.

In the arrangement of Figures 1 1 and 12, each set of coils in the auxiliary windings 40 comprises one set of coil sides which run in one direction (for example, those in slots 7 to 9), and another set of coil sides which run in the opposite direction (for example, those in slots 10 to 12). The distance between the midpoints of the two sets of coil sides is three slots, which is equivalent to 30 ° or one third of a pole pitch. As a consequence, the auxiliary windings 40 shown in Figures 1 1 and 12 also couple to the third harmonic.

Figure 13 is an axial cross-section through a stator, showing schematically the arrangement of the auxiliary windings in the embodiment of Figures 1 1 and 12. The embodiment described above can capture third harmonic contents during short circuit conditions in a single phase machine, hence giving increased short circuit and/or enhanced overload capability, while also providing fundamental voltage for no-load, rated load, and overload conditions. In the single phase embodiment described above, the position of the auxiliary windings in relation to the stator windings gives the advantage of more voltage in the auxiliary windings compared other positions due to the nature of the pulsating flux in the stator of the single phase generator. As a consequence, the coils require fewer turns, leading to lower copper loss, reduced weight, better performance and lower cost, albeit at the expense of some slot space. However, the distributed short-pitch coils help to optimise the usage of the slot space, improving the machine performance.

Furthermore, if the auxiliary windings were placed in empty slots, slot packers would be required in order to prevent the auxiliary windings from moving. By providing the main stator windings and the auxiliary windings in the same slots, the need for slot packers can be eliminated, thereby increase the ease of manufacture. In addition, the distributed overhangs help with cooling due to the larger surface area.

In the single phase embodiment described above, the same stator configuration as in the three phase embodiment is used, in order to reduce costs. However a different stator configuration could be used for the single phase embodiment if desired. For example, the stator slots which do not receive windings could be omitted.

It will be appreciated that embodiments of the present invention have been described above by way of example only, and various modifications will be apparent to the skilled person. For example, although embodiments have been described with reference to a four pole machine, the invention is also applicable to machines with a different number of poles. Furthermore, although an embodiment with 36 stator slots has been described, any appropriate number of stator slots may be used. If a larger number of stator slots is available, a greater number of coils may be provided in each group, and the coils may be further distributed around the stator slots, thereby improving the usage of the slot space. Other modifications may be made within the scope of the claims.

Claims

1 . A stator for a rotating electrical machine, the stator comprising main windings for supplying power to a load and auxiliary windings for supplying power to an exciter, wherein the auxiliary windings comprise a first group of windings arranged to couple to a fundamental component of a magnetic flux, and a second group of windings arranged to couple to a third harmonic of the magnetic flux.

2. A stator according to claim 1 , wherein the stator comprises a plurality of stator slots, and at least some of the main windings and the auxiliary windings are located in the same stator slots.

3. A stator according to claim 1 or 2, wherein the first and second groups of windings are interleaved.

4. A stator according to any of the preceding claims, wherein the windings from one group are located in slots which are not occupied by the windings from the other group.

5. A stator according to any of the preceding claims, wherein the first and second groups of windings are connected in series.

6. A stator according to any of the preceding claims, wherein the first group of windings comprises a plurality of coils.

7. A stator according to claim 6, wherein the coils in the first group of windings are connected in series.

8. A stator according to claim 6 or 7, wherein adjacent coil sides in adjacent coils comprise turns of windings which run in the same direction.

9. A stator according to claim 8, wherein adjacent coil sides in adjacent coils form a set of coil sides, in which set the turns of windings all run in the same direction.

10. A stator according to claim 9, wherein a plurality of sets of coil sides is provided, and the directions of the turns of windings alternate between each subsequent set.

1 1 . A stator according to claim 10, wherein a distance between a midpoint of one set of coils sides and a midpoint of another set of coil sides is equivalent to a pole pitch.

12. A stator according to any of claims 9 to 1 1 , wherein at least two adjacent coil sides within a set are separated by at least one stator slot.

13. A stator according to any of claims 6 to 12, wherein the coils in the first group of windings have a pitch which enables them to couple to the fundamental component of the magnetic flux.

14. A stator according to any of claims 6 to 13, wherein the coils in the first group of windings have a pitch of one pole.

15. A stator according to claim 14, wherein the coils in the first group of windings are arranged in sets, with the coils in a set staggered from each other.

16. A stator according to claim 15, wherein the coils in a set are staggered such that there is at least one slot between two coils in a set.

17. A stator according to any of claims 6 to 13, wherein the coils in the first group of windings have a pitch of two thirds of a pole.

18. A stator according to claim 17, wherein each alternate coil is wound in the opposite direction.

19. A stator according to claim 17 or 18, wherein:

adjacent coil sides in adjacent coils form a set of coil sides with turns of windings which run in the same direction;

a plurality of sets of coil sides is provided, with the direction of the turns of windings alternating between each subsequent set; and a distance between a midpoint of one set of coil sides and a midpoint of another set of coil sides is equivalent to a pole pitch.

20. A stator according to any of the preceding claims, wherein the second group of windings comprises a plurality of coils.

21 . A stator according to claim 20, wherein the coils in the second group of windings are arranged in sets, with each set of coils comprising at least two concentric coils.

22. A stator according to claim 21 , wherein the coils in a set are wound in the same direction.

23. A stator according to claim 21 or 22, wherein the sets of coils in the second group of windings have an average pitch which enables them to couple to the third harmonic of the magnetic flux.

24. A stator according to any of claims 21 to 23, wherein adjacent coil sides in a set of coils form a set of coil sides in which the turns of windings run in the same direction.

25. A stator according to claim 24, wherein a plurality of sets of coil sides is provided, and the directions of the turns of windings alternate between each subsequent set of coil sides.

26. A stator according to claim 25, wherein a distance between a midpoint of one set of coil sides and a midpoint of another set of coil sides is equivalent to one third of a pole pitch.

27. A stator according to any of claims 21 to 26, wherein an inner coil in a set of coils comprises coil sides which are separated by at least one stator slot.

28. A stator according to any of claims 21 to 27, wherein the number of coils in a set of coils is equal to the number of stator slots between two adjacent coil sides in the first group of windings.

29. A stator according to any of claims 21 to 28, wherein a set of coil sides in the second group of windings is located in stator slots between two adjacent coil sides in the first group of windings.

30. A stator according to any of claims 20 to 28, wherein some of the coils in the second group of windings are arranged to couple to the fundamental component of the magnetic flux.

31 . A stator according to any of claims 20 to 29, wherein some of the coils in the second group of windings have a greater number of turns than the other coils in the second group of windings.

32. A stator according to claim 31 , wherein the coils with a greater number of turns are arranged to couple to the fundamental component of the magnetic flux.

33. A stator according to claim 31 or 32, wherein the coils with a greater number of turns are spaced at an interval of one pole or a multiple thereof.

34. A stator according to any of the preceding claims, wherein the rotating electrical machine has a plurality of poles with a pole pitch, and the fundamental component of the magnetic flux is determined by the pole pitch.

35. A stator according to any of the preceding claims, wherein the main windings are three phase windings.

36. A stator for a single phase rotating electrical machine, the stator comprising main windings for supplying single phase power to a load and auxiliary windings for supplying power to an exciter, wherein the auxiliary windings comprise a plurality of coils arranged to couple to a fundamental component of a magnetic flux and to a third harmonic of the magnetic flux.

37. A stator according to claim 36, wherein the coils in the auxiliary windings are arranged in sets, with each set comprising a plurality of concentric coils.

38. A stator according to claim 37, wherein the coils in a set comprise substantially the same number of turns.

39. A stator according to claim 38, wherein the coils in a set are wound in the same direction.

40. A stator according to any of claims 36 to 38, wherein a distance between a midpoint of each set of coils is equivalent to a pole pitch or a multiple thereof.

41 . A stator according to any of claims 36 to 40, wherein each set of coils comprises a set of coils sides with turns running in one direction, and a set of coil sides with turns running in the opposite direction.

42. A stator according to claim 41 , wherein a distance between a midpoint of one set of coil sides and a midpoint of the other set of coil sides is equivalent to one third of a pole pitch.

43. A stator according to any of claims 36 to 42, wherein the auxiliary windings are located in at least some of the same slots as the main windings.

44. A rotating electrical machine comprising:

a rotor, the rotor comprising a plurality of poles; and

a stator according to any of the preceding claims.

45. A machine according to claim 44, wherein the magnetic flux is produced by the rotor.

46. A machine according to claim 44 or 45, further comprising an exciter for supplying electrical power to the rotor.

47. A method of winding a stator for a rotating electrical machine, the method comprising winding the stator with main windings for supplying power to a load and winding the stator with auxiliary windings for supplying power to an exciter, wherein the auxiliary windings comprise a first group of windings arranged to couple to a fundamental component of a magnetic flux, and a second group of windings arranged to couple to a third harmonic of the magnetic flux.

48. A method of winding a stator for a single phase rotating electrical machine, the method comprising winding the stator with main windings for supplying single phase power to a load, and winding the stator with auxiliary windings for supplying power to an exciter, wherein the auxiliary windings comprise a plurality of coils arranged to couple to a fundamental component of magnetic flux and to a third harmonic of the magnetic flux.