GB2547622A - Excitation system - Google Patents

Abstract

An excitation system is disclosed for providing excitation to a main rotating electrical machine such as a synchronous generator. The excitation system 18 comprises an exciter 20 and a permanent auxiliary generator 40 nested inside the exciter. The exciter rotor 24 is located radially outwards of an exciter stator 22 and the aux. generator pm rotor 44 is located insider the aux, generator stator 45. By nesting the auxiliary generator within the exciter and locating the exciter rotor radially outwards of the exciter stator, a compact design using a reduced number of components may be achieved. As shown in fig 5 a common stator core 50 may be shared by the exciter and the auxiliary generator. Rotating diodes 29, including a heat sink, may mount via a mounting member 52 to the rotor cup 30 and fan blades, circumferentially space around the mounting member, may provide a cooling air flow to the exciter. The exciting system may be mounted externally to the main machine and may be removable therefrom.

Description

EXCITATION SYSTEM

The present invention relates to an excitation system for providing excitation to a rotating electrical machine, such as a synchronous generator.

Synchronous 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 is 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, and thereby to control the current supplied to the rotor windings.

The power for the exciter is usually derived from the output of the main machine (known as self-excitation). However, certain applications may require the generator to have short circuit maintenance and/or enhanced overload capability. In order to achieve this, it is known to use a separate permanent magnet generator (PMG) mounted on the generator shaft to provide the power for the exciter. The use of a PMG means that power can be supplied to the exciter independently of the output of the main machine.

The main machine, exciter and PMG are generally provided in series along a rotary shaft. The PMG and the exciter take up axial space on the shaft, and it is generally desirable to minimise the space that is occupied so that the size of the overall apparatus is minimised. This is particularly the case where the PMG is being retro-fitted to the generator, in which case the space constraints may already be defined. GB 2498874 discloses apparatus for providing excitation to a rotating electrical machine in which a PMG is nested inside an exciter. This arrangement can allow the overall size of the apparatus to be reduced, in comparison to the case where the PMG and exciter are provided in series.

While the arrangement of GB 2498874 is effective in reducing the size of the excitation system, it has been found that further improvements may be desirable.

In particular, it may be desirable to reduce the complexity of the system, to allow better utilization of raw materials, to achieve better cooling, and/or to improve transient and/or low voltage performance.

According to a first aspect of the present invention there is provided an excitation system for providing excitation to a main machine, the excitation system comprising an exciter and an auxiliary generator nested inside the exciter, the exciter comprising an exciter rotor which is located radially outwards of an exciter stator.

The present invention may provide the advantage that, by nesting the auxiliary generator within the exciter and locating the exciter rotor radially outwards of the exciter stator, a compact design using a reduced number of components may be achieved. Furthermore, since the exciter rotor is on the outside, it may have a higher moment of inertia than would otherwise be the case. This may help to provide more stable excitation to the main machine, thereby improving the transient stability when the main machine is supplying a variable load. In addition, a higher rotor moment of inertia can help the excitation system to continue providing excitation even in low voltage or short circuit conditions, thus enhancing low voltage ride through performance.

The main machine is preferably a rotating electrical machine, such as a synchronous generator, with field windings which are to be excited by the excitation system.

The exciter rotor may comprise a rotor core and rotor windings. The rotor may act as the armature and produce an output for exciting the main machine. The exciter stator may comprise a stator core and stator windings. The stator windings may act as the field windings for the exciter, and the auxiliary machine may be arranged to produce the excitation for the stator windings.

The auxiliary generator may comprise an auxiliary generator rotor and an auxiliary generator stator. The auxiliary generator rotor may comprise permanent magnets. Thus the auxiliary generator may be a permanent magnet generator. However other types of machine, such a switched reluctance machine, could be used. Alternatively or in addition the auxiliary machine may comprise a toothed metal ring such as that described in GB 2496674, the contents of which are incorporated herein by reference. In this case the teeth in the toothed metal ring may function as flux-switching teeth.

In the case of a permanent magnet generator, the permanent magnets may be ferrite magnets. Using ferrite magnets, rather than rare earth magnets, may help to address issues relating to the cost, price volatility, supply security and environmental impact of rare earth magnets. However any other appropriate type of permanent magnet, including rare earth magnets, could be used instead if desired.

The auxiliary generator stator may be located radially outwards of the auxiliary generator rotor. This may allow at least some of the stationary parts of the system to be co-located, which may simplify the design and/or allow the sharing of some components.

In a preferred embodiment of the invention, the exciter and the auxiliary generator share a common stator core. By using a common stator core for the exciter and the auxiliary generator, the number of components in the system can be reduced. This in turn can reduce the amount of raw material needed, and reduce the cost and complexity of manufacture.

This feature may be provided independently, and thus, according to another aspect of the present invention there is provided an excitation system for providing excitation to a main machine, the excitation system comprising an exciter and an auxiliary generator nested inside the exciter, wherein the exciter and the auxiliary generator share a common stator core.

Preferably exciter stator windings and auxiliary machine stator windings are provided on the common stator core. For example, the exciter stator windings may be provided on a radially outwards side of the stator core, and the auxiliary machine stator windings may be provided on a radially inwards side of the stator core. By sharing a common stator core in this way, the cost and complexity of the system may be reduced.

The common stator core may comprise stator slots on its radially outwards side for exciter stator windings and/or stator slots on its radially inwards side for auxiliary machine stator windings.

The common stator core may be laminated. For example, the stator core may be formed from laminated sheets of steel. In this case each lamination in the stator core may be produced from a single piece of raw material. This arrangement may allow single blow stamping of the stator laminations for both the exciter and the auxiliary machine. This may help to reduce the manufacturing time, and/or to reduce the amount of raw material consumed.

In any of the above arrangements, the excitation system may comprise means for connecting the stator to a non-rotating part of the main machine. For example, in the case of a common stator core, the common stator core may comprise bolt holes for bolting the common stator core to a non-rotating part of the main machine, such as a non-drive end bracket. In the case of separate exciter and auxiliary machine stator cores, the auxiliary machine stator core may be bolted to the exciter stator core, and the exciter stator core may comprise bolt holes for bolting the stator to a non-rotating component of the main machine. The use of bolts can allow the excitation system to be readily removed from the main machine. However it will be appreciated that other means for connecting the stator or stators to the main machine could be used instead.

The excitation system may comprise means for connecting the exciter rotor to a rotating component of the main machine, such as the main machine shaft. This can allow rotational energy from the main machine to be transferred to the exciter rotor.

In a preferred embodiment the excitation system further comprises a rotor cup. The rotor cup may be arranged to connect the exciter rotor to a rotating component of the main machine.

Preferably the rotor cup at least partially encloses the excitation system. This may allow the excitation system to be provided as a self-contained unit and/or may provide a certain level of ingress protection. For example, the rotor cup may comprise a cylindrical outer wall and a cylindrical inner wall, and the excitation system may be at least partially contained between the inner wall and the outer wall. An end wall may be provided to connect the inner wall and the outer wall. The end wall may be disc-shaped, or some other shape.

Preferably the rotor cup is arranged to transfer rotational energy within the excitation system. Thus the rotor cup may allow rotational energy to be transferred to rotating parts of the excitation system.

The excitation system may comprise means for attaching the rotor cup to a shaft of the main machine. For example, the rotor cup may be arranged to be bolted to the shaft of the main machine. This may be achieved, for example, by providing the rotor cup with a joint which can be bolted to the shaft. The joint may be provided, for example, at the end (axially) of an inner wall of the rotor cup. This can allow rotational energy to be transferred from the shaft of the main machine to rotating parts of the excitation system.

Preferably the rotor cup supports the exciter rotor. For example, the exciter rotor may comprise a rotor core which is attached to the inside of an outer wall of the rotor cup. This can allow the rotor cup to transfer rotational energy to the exciter rotor.

Preferably the rotor cup supports the auxiliary machine rotor. For example, the auxiliary machine rotor may comprise a rotor core which is attached to the inside of an inner wall of the rotor cup. This can allow the rotor cup to transfer rotational energy to the auxiliary machine rotor.

An exciter for a rotating electrical machine typically includes rotating diodes to convert an AC output of the exciter to DC for supply to the field windings of the main machine. In a preferred embodiment, a mounting member is provided for mounting rotating diodes, and the mounting member is attached to the rotor cup. For example, the mounting member may be attached to an end wall of the rotor cup. This can provide a convenient way of transferring rotational energy to the rotating diodes. However the rotating diodes could alternatively be mounted elsewhere such as on the rotor cup.

The rotating diodes may conveniently be included in one or more diode assemblies. In this case the mounting member may comprise one or more mounting points for mounting the diode assemblies. For example, the mounting member may be generally annular, and the mounting points may be located on a radially inwards side of the mounting member. Preferably a diode assembly is mounted to the or each mounting point. However the diode assemblies could alternatively be attached directly to the rotor cup.

In operation, heat may be generated by currents passing through windings in the exciter and/or auxiliary machine. Furthermore, heat may be generated by the rotating diodes as they rectify the AC output of the exciter. It may therefore be desirable to provide cooling within the excitation system.

In a preferred embodiment, the mounting member includes one or more heatsinks for cooling the rotating diodes. For example, a heatsink may be provided radially outwards of a mounting point for mounting a diode assembly. The heatsinks may include fins for cooling. This arrangement may help to improve cooling and/or to reduce the number of components by allowing the mounting member also to function as a heatsink.

In a preferred embodiment, the mounting member includes one or more fan blades. For example, the fan blades may be located circumferentially between two or more mounting points and/or diode assemblies. The fan blades may be arranged to provide air flow through the exciter. For example, the fan blades may provide air flow over stator windings and/or rotor windings, and/or through the exciter airgap, thereby assisting with cooling. This arrangement can help to reduce the number of components by allow the mounting member also to function as fan.

Preferably one or more holes are provided in the rotor cup. The holes may allow air flow into the exciter. Thus, where a fan is provided, the holes may allow air to be drawn from outside of the rotor cup to inside the rotor cup, to assist with cooling.

Alternatively or in addition a hole in the rotor cup may allow access to a diode assembly. This may facilitate servicing of the diodes. For example, the diode assembly may comprise surface mounted diodes, which may be mounted on a radially inwards surface of the diode assembly. In this case a hole in the rotor cup may be located radially inwards of the diode assembly, to allow an operator to access the diodes for servicing.

The excitation system may be provided as a self-contained unit. This may facilitate fitting of the excitation system to a main machine. For example, the excitation system may be retro-fitted to an existing synchronous machine which was previously self-excited in order to enhance the performance of the machine.

The excitation system may be arranged to be fitted externally to the main machine. For example, the excitation system may be fitted outside of a frame or housing of a main machine and/or on the other side (axially) of a bracket such as a non-drive end bracket. This can facilitate retro-fitting and servicing of the excitation system.

The excitation system is preferably removable from the rotating electrical machine. For example, at least part of the excitation system may be removable by undoing bolts which connect the rotor cup to a rotating part of the main machine (such as the shaft) and/or bolts which connect the stator to a nonrotating part of the main machine (such as a non-drive end bracket). This may allow the excitation system to be removed for servicing without the need to remove the main rotor.

According to another aspect of the invention there is provided a rotating electrical machine comprising a main machine and an excitation system in any of the forms described above.

The main machine may comprise a shaft, and the excitation system may comprise a rotor cup attached to the shaft. The stator core may be attached to a non-rotating component of the main machine, such as a non-drive end bracket.

According to another aspect of the invention there is provided a method of manufacturing an excitation system for providing excitation to a main machine, the method comprising assembling the excitation system with an auxiliary generator nested inside an exciter, and with an exciter rotor located radially outwards of an exciter stator.

According to another aspect of the present invention there is provided a method of manufacturing an excitation system for providing excitation to a main machine, the method comprising assembling the excitation system with an auxiliary generator nested inside an exciter, wherein the exciter and the auxiliary generator share a common stator core.

The method may further comprise fitting exciter stator windings and auxiliary machine stator windings on the common stator core.

The method may also comprise stamping a lamination for the common stator core from a single piece of raw material.

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 the present specification terms such as “radially”, “axially” and “circumferentially” are generally defined with reference to the axis of rotation of the 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 with an exciter and a PMG;

Figure 2 shows parts of an excitation system in accordance with a first embodiment of the invention;

Figure 3 shows parts of an excitation system in accordance with a second embodiment of the invention;

Figure 4 shows a lamination forming part of a combined stator core;

Figure 5 shows a mounting member for mounting diode assemblies; and Figure 6 shows parts of a synchronous generator including an excitation system.

Overview

Figure 1 shows an overview of a synchronous generator with an exciter and a PMG for supplying power to the exciter. Referring to Figure 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 relative to the main stator causes an AC output to be generated in the main stator windings.

The main rotor is magnetised by passing a DC current through the rotor windings. 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, and rotation of the exciter rotor 7 relative to the exciter stator 8 generates an AC output in the exciter rotor windings. The AC output of the rotor 7 is converted to DC by the rotating diodes 9, and the DC output of the rotating diodes is fed to the main rotor 3.

In the arrangement of Figure 1, power for the exciter 6 is drawn from a separate permanent magnet generator (PMG) 11 mounted on the same shaft as the main machine and the exciter. The PMG comprises rotor 15 and stator 13. The rotor has a number of permanent magnets, which rotate to generate electrical power in windings in the stator 13. The output of the stator 13 is fed to the exciter 6 via an automatic voltage regulator (AVR) 1. The AVR senses the output of the main stator 4, and controls the amount of power fed from the PMG to the exciter. 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.

In an alternative arrangement, the main stator may provide the power for the exciter under normal operating conditions, while the PMG may contribute some or all of the power for the exciter under overload conditions. Such an arrangement is disclosed in International Patent Publication Number WO 2008/096117, the contents of which are incorporated herein by reference.

Although for simplicity only single lines are shown in Figure 1, the generator is typically a three-phase generator producing a three-phase output. However in general a generator having any number of phases could be used instead.

Embodiments of the present invention relate to an excitation system comprising a combined exciter and PMG design in which the PMG is nested inside the exciter.

First embodiment

Figure 2 is a cutaway drawing showing parts of an excitation system in accordance with a first embodiment. The excitation system 18 comprises an exciter 20 for providing excitation to a main machine (not shown in Figure 2), as well as a permanent magnet generator 40 for providing excitation to the exciter.

In the arrangement of Figure 2, the entire permanent magnet generator 40 is located radially inwards of the exciter 20. Nesting the permanent magnet generator inside the exciter in this way allows the overall length of the exciter/PMG combination to be reduced, thereby producing a compact overall design.

Referring to Figure 2, the exciter 20 comprises an exciter stator 22 and an exciter rotor 24, with an airgap 23 between the two. In contrast to a conventional exciter, the exciter rotor 24 is located on the radially outwards side and the exciter stator 22 is located on the radially inwards side.

The exciter rotor 24 comprises a rotor core 27 and rotor windings 28. The rotor core 27 is cylindrical, and faces the airgap 23 on its radially inwards side. The rotor windings 28 are wound on the radially inwards side of the rotor core 27.

The exciter stator 22 comprises a stator core 25 and stator windings 26. The stator core 25 is cylindrical, and faces the airgap 23 on its radially outwards side. The stator windings 26 are wound on the radially outwards side of the stator core 25.

The permanent magnet generator 40 comprises a PMG stator 42 and a PMG rotor 44 with an airgap 43 therebetween. The PMG stator 42 comprises a PMG stator core 45 and PMG stator windings 46. The stator core 45 is cylindrical, and faces the airgap 43 on its radially inwards side. The stator windings 46 are wound on the radially inwards side of the stator core 45. The PMG rotor 44 comprises a PMG rotor core 47 and permanent magnets 48. The rotor core 47 is cylindrical, and faces the airgap 43 on its radially outwards side. The permanent magnets 48 produce a magnetic flux which crosses the airgap 43.

In the arrangement of Figure 2, the whole excitation system is contained within a rotor cup 30. The rotor cup 30 comprises a cylindrical outer wall 32, a discshaped end wall 34, and a cylindrical inner wall 36. The exciter rotor core 27 is attached to the inside of the outer wall 32. The PMG rotor core 47 is attached to the inside of the inner wall 36 of the rotor cup 30. The rotor cup 30 is connected to the shaft of the main machine by a bolted joint 38.

The exciter 20 of Figure 2 also includes three diodes assemblies 29. The diode assemblies 29 contain rotating diodes for converting the AC output of the rotor windings 28 to DC for supply to each phase of the rotor of the main machine.

The diode assemblies 29 are mounted on a mounting member 52 at three different locations spaced 120° apart. The mounting member 52 is attached to the inside of the end wall 34 of the rotor cup. The mounting member 52 includes heatsinks 94 which are located radially outwards of the diode assemblies 29.

Fan blades are spaced circumferentially around the mounting member, between the diodes assemblies 29. The fan blades cause air to flow over the stator windings 26 and the rotor windings 28, and into the airgap 23, as the rotor rotates, thereby assisting with cooling.

In operation, the rotor cup 30 is caused to rotate by rotation of the main shaft. Thus the rotor cup 30 transfers rotational energy from the shaft to the exciter rotor 24. This causes the exciter rotor 24 to rotate around the outside of the exciter stator 22. Rotation of the exciter rotor 24 relative to the stator 22 generates an AC output in the rotor windings 28. This AC output is converted to DC by the diode assemblies 29. The DC outputs of the diode assemblies 29 are fed to the rotor of the main machine. Thus, in the exciter 20, the rotor functions as the armature while the stator windings function as the field windings.

Power for the exciter stator 22 is provided by the PMG 40. The PMG rotor 44 is attached to the rotor cup 30, and thus rotates together with the exciter rotor 24. Rotation of the permanent magnets 48 relative to the PMG stator windings 46 generates an AC output in the stator windings 46. This AC output is fed to the exciter stator windings 26 via an AVR (not shown). The AVR converts the AC output of the PMG to DC, and controls the amount of power fed to the exciter, thereby allowing control of the field of the main machine.

As well as serving to transfer rotational energy, the rotor cup 30 also functions to enclose the exciter 20 and PMG 40, thereby providing a certain level of ingress protection. However a hole 35 is provided in the rotor cup 30 to allow air flow into the exciter. The hole 35 also allows access to the diode assemblies 29. The diode assemblies have surface mounted diodes designed for ease of service.

The exciter stator core 25, the exciter rotor core 27, the PMG stator core 45 and the PMG rotor core 47 may suitably be constructed from steel laminations. The PMG stator core 45 may be attached to the exciter stator core 25, for example using bolts. The exciter stator windings 26, the rotor windings 28 and the PMG stator windings may be formed from enamelled copper wires. The rotor cup 30 may be formed from a metal such as steel, and may for example be cast and/or machined as appropriate. Of course, the skilled person will appreciate that in each case any other suitable material or construction technique may be used instead.

In a preferred embodiment, the permanent magnets 48 are ferrite magnets, rather than rare earth magnets. By moving to larger and cheaper ferrite magnets, the cost of the magnets can be reduced. Using ferrite magnets may also help to address issues relating to the price volatility, supply security and environmental impact of rare earth magnets. Of course, any other appropriate types of permanent magnet, including rare earth magnets, could be used instead if desired.

The arrangement described above provides the advantage that, by nesting the PMG within the exciter, the length of the excitation system can be reduced. This in turn can provide a reduction in the overall length of the generator. For example, in some implementations, a 5% reduction in overall machine length has been achieved, in comparison to a non-nested exciter/PMG design. Reducing the length of the excitation system can also allow retro-fitting of the excitation system to a generator which was originally self-excited (i.e. without a PMG), within the same space.

In contrast to conventional excitation systems, in the arrangement described above the exciter rotor is located radially outwards of the exciter stator. This allows the stator of the exciter and the stator of the PMG to be co-located in the middle (radially) of the excitation system. Co-locating the non-rotating parts in the middle of the excitation system can facilitate assembly and help to provide a compact arrangement and/or a reduced number of parts.

Furthermore, since the exciter rotor is located outwards of the stator, the rotor has a higher moment of inertia than would otherwise be the case. This may provide better transient stability when the generator is supplying variable loads. A higher rotor moment of inertia can also help the excitation system to continue providing excitation even in low voltage or short circuit conditions. Thus the arrangement described above may provide enhanced low voltage ride through performance.

Second embodiment

In a second embodiment of the excitation system, a further enhancement of the design is achieved by using a common stator core for both the exciter stator windings and the PMG stator windings.

Figure 3 is a cutaway drawing showing parts of an excitation system in accordance with the second embodiment. Parts of the excitation system which are in common with the first embodiment are given the same reference numerals.

Referring to Figure 3, the excitation system comprises an exciter designated generally by reference numeral 20, and a PMG designated generally by reference numeral 40. The exciter 20 includes a rotor comprising rotor core 27 and rotor windings 28, in a similar way to the first embodiment. The exciter 20 also comprises a stator comprising stator core 50 and stator windings 26. The PMG 40 includes a rotor comprising rotor core 47 and permanent magnets 48. The PMG 40 also includes stator windings 46. However, rather than being wound on a separate stator core, in the arrangement of Figure 3 the PMG stator windings 46 are wound on the same stator core 50 as the exciter windings 26.

In the arrangement of Figure 3, the stator core 50 is cylindrical. The stator core 50 faces the exciter rotor on its radially outwards side, and the PMG rotor on its radially inwards side. The exciter stator windings 26 are provided on the radially outwards side of the stator core 50, and the PMG stator windings are provided on the radially inwards side.

The stator core 50 shown in Figure 3 is formed from laminations which are punched from sheets of electrical steel. Figure 4 is diagram of a lamination 80 forming part of the stator core 50. Referring to Figure 4, the lamination 80 comprises a plurality of teeth 82 on its outside edge, the teeth 82 defining exterior stator slots 84. The lamination also comprises a plurality of teeth 86 on its inside edge, the teeth 86 define interior stator slots 88. In the assembled machine, the exciter stator windings 26 are located in the exterior stator slots 84, and the PMG stator windings 46 are located in the interior stator slots 88. Bolt holes 89 are provided in the laminations for bolting the stator core to the main machine.

The arrangement described above can allow the laminations for the stator core to be stamped from a single sheet of electrical steel. Single blow stamping from a single sheet of electrical steel gives a complete PMG and exciter lamination in a single process, which reduces the time and complexity of manufacture. Furthermore, sharing a common stator core can provide better utilisation of electrical steel. For example, in one implementation it has been found that the volume of electrical steel for the combined stator can be reduced by 33% compared to the case of separate stators. It has also been found that a reduction in scrap volume can be achieved. In addition, a single impregnation process can be used for the combined stator, further reducing manufacturing costs.

Referring back to Figure 3, the excitation system is contained within a rotor cup 30, in a similar way to the first embodiment. The exciter rotor core 27 is attached to the inside of the outer wall 32 of the rotor cup 30. The PMG rotor core 47 is attached to the inside of the inner wall 36 of the rotor cup 30. The rotor cup 30 is connected to the shaft of the main machine by bolted joint 38. A mounting member 52 is also provided, which is attached to the end wall 34 of the rotor cup using bolts 53. The mounting member 52 is used to hold the diode assemblies 29.

Figure 5 shows the mounting member 52 and diode assemblies 29 in more detail. Referring to Figure 5, the mounting member 52 is a single component that attaches to the rotor cup 30 using bolt holes 90. The mounting member 52 has three mounting points 92 on its radially inwards side. The mounting points 92 are spaced 120“apart circumferentially around the mounting member. A diode assembly 29 is mounted on each of the mounting points 92.

The mounting member 52 also includes three heatsinks 94 for cooling the diode assemblies. The heatsinks 94 are provided on the radially outwards side of the mounting points 92. The heatsinks include a plurality of fins for cooling.

The mounting member of Figure 5 also includes plurality of fan blades 96. The fan blades 96 are interspaced between the mounting points/heatsinks 92, 94. In operation, the mounting member 52 and diode assemblies 29 rotate together with the rotor cup 30. The fan blades 96 provide a driving pressure for airflow movement inside the closed cup end of the exciter rotor. In addition, air flow over fins in the heatsinks 94 provides cooling for the diode assemblies 29.

The mounting member 52 is formed from a single piece of material. For example, the mounting member may be cast and/or machined from metal. This allows a single component to be used which performs three separate functions, namely, a mount for the diode assemblies, a heatsink for the rotating diodes, and a fan to force airflow through the exciter. This can allow improved cooling with a reduced number of components in a compact design.

The diode assemblies 29 may suitably be formed from heat resistant plastic. Of course, any other appropriate materials and construction techniques could be used for the mounting member and diode assemblies. The mounting member of Figure 5 may be used in any of the embodiments described above.

Generator with excitation system

Figure 6 is a cutaway drawing showing parts of a generator including an excitation system as described above. Referring to Figure 6, the generator comprises a main machine 16 and excitation system 18. The main machine 16 comprises a stator 54 with stator windings 56, and a rotor 58 with rotor windings 60. The rotor 58 is mounted on a shaft 62 which is driven by a prime mover such as a diesel engine (not shown). A bearing 64 is provided at the non-drive end of the machine to support the shaft 62. The bearing is supported by a non-drive end bracket 66. The main machine is enclosed within a frame 68.

In the arrangement of Figure 6 the excitation system 18 is attached to the nondrive end of the main machine 16. The rotating components of the excitation system are attached to the main machine by bolting the rotor cup 30 to the shaft 62 using bolts 70. This can allow the bearing 64 to be used for the rotating components of the excitation system 18 as well as for the shaft 62 of the main machine 16. The non-rotating components of the excitation system are attached to the main machine by bolting the stator core 50 to the non-drive end bracket 66 using the bolt holes 89 in the stator laminations. A channel may be provided through the shaft 62 to allow the rotating diode assembly 29 to be connected to the main rotor windings 60. Connections are also provided to an AVR (not shown) which is typically located on the outside of the main machine housing.

The excitation system 18 of Figure 6 is enclosed by a cowl 74. The cowl prevents external bodies from coming into contact with the rotating rotor cup 30. The cowl 74 includes louvre vents 76 which allow air to enter the generator. Axial air inlets 78 are provided in the non-drive end bracket 68 to allow air flow into the main machine 16. A fan (not shown) at the drive end of the main machine 16 draws air though the main machine in order to provide cooling. In addition, fan blades in the mounting member 52 draw air through the hole 35 in order to cool the exciter.

In the arrangement of Figure 6 the excitation system 18 is externally mounted to the main machine 16 using bolts. Thus the excitation system 18 can be removed from the main machine by undoing the bolts, without the need to remove the main rotor. This can allow the excitation system to be easily serviced. Furthermore, the hole 35 can provide access to the diodes in the diode assemblies 29, thereby providing easy serviceability.

In Figure 6 the excitation system is shown with a common stator core 50 as in the second embodiment described above. However, the excitation system of the first embodiment could be attached to the main machine in a similar way.

In contrast to conventional excitation systems, in the embodiments described above the exciter rotor is located radially outwards of the exciter stator. Since the exciter rotor is located outwards of the stator, the rotor has a higher inertia. This may provide better transient stability when the generator is supplying variable loads. Furthermore, a higher rotor moment of inertia can help the excitation system to continue providing excitation even in low voltage or short circuit conditions. Thus the embodiments described above may provide enhanced low voltage ride through performance.

It has also been found that the exciter design described above can provide superior motor starting capability and better transient response, in comparison to a series connected PMG and exciter.

Preferred embodiments of the excitation system use ferrite magnets rather than rare earth magnets. In one embodiment it has been found that a move to larger and cheaper ferrite magnets can result in a cost reduction for the magnets of approximately 70%.

Overall, the present design can provide lower manufacturing costs through a reduced number of components and shorter manufacturing time, as well as better transient performance, better cooling, easy serviceability, and ability to retro-fit the excitation system to an existing self-excited generator without the need for additional space.

In the above description, preferred embodiments of the invention have been described by way of example. However it will be appreciated that the invention is not limited to these embodiments, and variations in detail will be apparent to the skilled person. For example, parts of one embodiment may be provided with any other embodiment. While embodiments have been described with reference to a synchronous machine, the invention is applicable to any rotating electrical machine, including all types of generators and motors, for which it is desired to provide excitation. While in a preferred embodiment a permanent magnet generator is used to provide the excitation for the exciter, other types of auxiliary generator such as a switched reluctance machine could be used instead.

Claims (50)

1. An excitation system for providing excitation to a main machine, the excitation system comprising an exciter and an auxiliary generator nested inside the exciter, the exciter comprising an exciter rotor which is located radially outwards of an exciter stator.

2. A system according to claim 1, wherein the auxiliary generator comprises an auxiliary generator rotor and an auxiliary generator stator.

3. A system according to claim 2, wherein the auxiliary generator rotor comprises permanent magnets.

4. A system according to claim 3, wherein the permanent magnets are ferrite magnets.

5. A system according to any of claims 2 to 4, wherein the auxiliary generator stator is located radially outwards of the auxiliary generator rotor.

6. A system according to any of the preceding claims, wherein the exciter and the auxiliary generator share a common stator core.

7. An excitation system for providing excitation to a main machine, the excitation system comprising an exciter and an auxiliary generator nested inside the exciter, wherein the exciter and the auxiliary generator share a common stator core.

8. A system according to claim 6 or 7, wherein exciter stator windings and auxiliary machine stator windings are both provided on the stator core.

9. A system according to any of claims 6 to 8, wherein exciter stator windings are provided on a radially outwards side of the stator core, and auxiliary machine stator windings are provided on a radially inwards side of the stator core.

10. A system according to any of claims 6 to 9, wherein the common stator core comprises stator slots on its radially outwards side for exciter stator windings and stator slots on its radially inwards side for auxiliary machine stator windings.

11. A system according to any of claims 6 to 10, wherein the stator core is laminated.

12. A system according to claim 11, wherein each lamination in the stator core is produced from a single piece of raw material.

13. A system according to any of the preceding claims, further comprising means for connecting the stator to a non-rotating part of the main machine.

14. A system according to claim 13 when dependent on any of claims 6 to 12, wherein the common stator core comprises bolt holes for connecting the common stator core to a non-rotating part of the main machine.

15. A system according to any of the preceding claims, further comprising means for connecting the exciter rotor to a rotating component of the main machine.

16. A system according to any of the preceding claims, further comprising a rotor cup.

17. A system according to claim 16, wherein the rotor cup at least partially encloses the excitation system.

18. A system according to claim 16 or 17, wherein the rotor cup comprises a cylindrical outer wall and a cylindrical inner wall, and the excitation system is at least partially contained between the inner wall and the outer wall.

19. A system according to any of claims 16 to 18, wherein the rotor cup is arranged to transfer rotational energy within the excitation system.

20. A system according to any of claims 16 to 19, further comprising means for attaching the rotor cup to a shaft of a main machine.

21. A system according to any of claims 16 to 20 wherein the rotor cup is arranged to be bolted to a shaft of a main machine.

22. A system according to any of claims 16 to 21, wherein the rotor cup supports the exciter rotor.

23. A system according to claim 22, wherein the exciter rotor comprises a rotor core attached to the inside of an outer wall of the rotor cup.

24. A system according to any of claims 16 to 23, wherein the rotor cup supports the auxiliary machine rotor.

25. A system according to claim 24, wherein the auxiliary machine rotor comprises a rotor core which is attached to the inside of an inner wall of the rotor cup.

26. A system according to any of claims 16 to 25, further comprising a mounting member for mounting rotating diodes, wherein the mounting member is attached to the rotor cup.

27. A system according to claim 26, wherein the mounting member comprises a mounting point for mounting a diode assembly.

28. A system according to claim 27, wherein the mounting member is generally annular, and the mounting point is located on a radially inwards side of the mounting member.

29. A system according to claim 27 or 28, further comprising a diode assembly mounted to the mounting point.

30. A system according to any of claims 26 to 29, wherein the mounting member includes a heatsink for cooling the rotating diodes.

31. A system according to claim 30, wherein the heatsink is provided radially outwards of a mounting point for mounting a diode assembly.

32. A system according to any of claims 26 to 31, wherein the mounting member includes a plurality of fan blades.

33. A system according to claim 32, wherein the fan blades are located circumferentially between two or more mounting points.

34. A system according to claim 32 or 33, wherein the fan blades are arranged to provide air flow through the exciter.

35. A system according to any of claims 16 to 34, wherein a hole is provided in the rotor cup.

36. A system according to claim 35, wherein the hole allows air flow into the exciter.

37. A system according to claim 35 or 36, wherein the hole allows access to a diode assembly.

38. A system according to claim 37, wherein the diode assembly comprises diodes mounted on a radially inwards surface of the diode assembly, and the hole is located radially inwards of the diode assembly.

39. A system according to any of the preceding claims, wherein the excitation system is provided as a self-contained unit.

40. A system according to any of the preceding claims, wherein the excitation system is arranged to be mounted externally to the main machine.

41. A system according to any of the preceding claims, wherein the excitation system is removable from the main machine.

42. A rotating electrical machine comprising a main machine and an excitation system according to any of the preceding claims.

43. A rotating electrical machine according to claim 42, the main machine comprising a shaft, and the excitation system comprising a rotor cup attached to the shaft.

44. A rotating electrical machine according to claim 42 or 43, wherein the stator core is attached to a non-rotating component of the main machine.

45. A method of manufacturing an excitation system for providing excitation to a main rotating electrical machine, the method comprising assembling the excitation system with an auxiliary generator nested inside an exciter, and with an exciter rotor located radially outwards of an exciter stator.

46. A method of manufacturing an excitation system for providing excitation to a main rotating electrical machine, the method comprising assembling the excitation system with an auxiliary generator nested inside an exciter, wherein the exciter and the auxiliary generator share a common stator core.

47. A method according to claim 46, further comprising fitting exciter stator windings and auxiliary machine stator windings on the common stator core.

48. A method according to claim 46 or 47, further comprising stamping a lamination for the common stator core from a single piece of raw material.

49. An excitation system or a rotating electrical machine substantially as described herein with reference to and as illustrated in the accompanying drawings.

50. A method of manufacturing an excitation system or a rotating electrical machine substantially as described herein with reference to the accompanying drawings.