GB2590948A

GB2590948A - Rectifier Assembly

Info

Publication number: GB2590948A
Application number: GB2000243.2A
Authority: GB
Inventors: Bell Adrian; Riley Mark; Spargo Anthony
Original assignee: Cummins Generator Technologies Ltd
Current assignee: Cummins Generator Technologies Ltd
Priority date: 2020-01-08
Filing date: 2020-01-08
Publication date: 2021-07-14
Also published as: GB202000243D0

Abstract

A rectifier assembly 28 for an electrical generator comprising a main machine and an exciter, the rectifier assembly comprising a mounting plate 30 and a three-phase full-wave rectifier circuit 32, 34, for converting an AC output of the exciter to a rectified DC output for supply to the main machine where the rectifier circuit is mounted on the mounting plate, and the mounting plate is arranged to be attached to a rotor of the exciter. The rectifier assembly may be provided as a single rectifier module that can be attached to one diametrical side only of the exciter rotor. The diodes of the circuit may all be mounted on the same mounting plate and may comprise a first diode module for rectifying one half cycle of a three-phase AC output of the exciter, and a second diode module for rectifying the other half cycle of a three-phase AC output of the exciter with each diode module comprising three diodes. The mounting plate may be flat in construction and made from a thermally conductive material with a removable housing. Surge protection and a snubber may also be provided. The assembly may also comprise a plug and socket connector. The exciter rotor may comprise a rotor core where the mounting plate is arranged to be attached to the rotor core.

Description

RECTIFIER ASSEMBLY

The present invention relates to a rectifier assembly for an electrical generator, and in particular a rectifier assembly for converting an AC output of an exciter to DC for supply to rotor windings.

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 generator shaft. Rotating diodes are provided in order to convert an AC output of the exciter to DC for supply to the rotor windings.

The rotating diodes used in an electrical generator are typically connected in a full wave bridge circuit, with two diodes being used for each phase. For example, in a three-phase machine, six diodes are provided, connected to form a three-phase full-wave bridge rectifier circuit.

In some known configurations, the rotating diodes are mounted on the generator shaft, either directly or via a heat sink. This can allow the diodes to be mounted to a rotating component of the generator at a point between the exciter and the main machine. However, such an arrangement is relatively complex to manufacture, and may increase the length of the generator.

In other known configurations, the rotating diodes are provided as part of a rectifier assembly which can be mounted to the exciter rotor. In one known configuration, two half-moon base plates are mounted in diametrically opposed positions on the exciter rotor to form an annular rectifier assembly extending around the shaft. Each half-moon base plate comprises three stud diodes connected to provide three-phase half-wave rectification. Each base plate rectifies a respective half cycle of the AC exciter output, to provide a three-phase full-wave rectification. This arrangement can avoid the need to connect the diodes to the shaft.

US 5,737,210, the subject matter of which is incorporated herein by reference, discloses a rectifier assembly which comprises two crescent shaped rectifier modules so that the rectifier assembly can be mounted directly to the housing of the exciter rotor. The rectifier module assembly occupies the space between and around the generator shaft and the exciter rotor windings.

A problem in the known rectifier assemblies is that they tend to be relatively expensive to produce due to the number and size of the components involved. Furthermore, each rectifier assembly tends to be designed for a specific product, increasing inventory costs. In addition, the existing designs may be time consuming to set up and difficult to service. In some cases, servicing may require removal of the rotor of the main machine, leading to long repair times.

It would therefore be desirable to provide a rectifier assembly which is easy to manufacture and install, which can be produced at relatively low cost and/or which is easy to service.

According to one aspect of the present invention there is provided a rectifier assembly for an electrical generator comprising a main machine and an exciter, the rectifier assembly comprising: a mounting plate; and a three-phase full-wave rectifier circuit for converting an AC output of the exciter to a rectified DC output for supply to the main machine, wherein the three-phase full-wave rectifier circuit is mounted on the mounting plate, and the mounting plate is arranged to be attached to a rotor of the exciter.

The present invention may provide the advantage that, by mounting a three-phase full-wave rectifier circuit on a mounting plate which is arranged to be mounted on a rotor of the exciter, it may be possible to provide a compact, robust rectifier assembly that can be produced at a lower cost than previously known rectifier assemblies. Furthermore, the time taken to set up the rectifier assembly on the exciter and/or to service the rectifier assembly may be reduced.

Preferably the rectifier assembly is provided as a single rectifier module. Thus, all of the components of the rectifier assembly may be provided as a single self-contained unit. This can facilitate the setting up and/or servicing of the rectifier assembly.

Preferably the rectifier assembly is arranged to be attached to one diametrical side only of the exciter rotor. For example, the rectifier assembly may be arranged to be attached to an end face of the rotor core, on one side only of the shaft. This may facilitate assembly and servicing.

The rectifier circuit may be a bridge rectifier circuit. The rectifier circuit is preferably arranged to convert a three-phase AC output from exciter rotor windings to a full-wave rectified DC output for supply to rotor windings in the main machine.

The rectifier circuit preferably comprises a plurality of diodes (for example, six diodes) which are preferably all mounted on the same mounting plate. Mounting the diodes on the same mounting plate may help to provide a compact arrangement which is easy to assembly and service.

In one embodiment the rectifier circuit comprises a first diode module for rectifying one half cycle of a three-phase AC output of the exciter, and a second diode module for rectifying the other half cycle of a three-phase AC output of the exciter. In this case each diode module may comprise three diodes. Both diode modules are preferably mounted on the same mounting plate.

However it will be appreciated that other arrangements are also possible. For example, the rectifier circuit may comprise a single three-phase full-wave diode module, or a plurality of individual diodes, or diode modules containing a different number of diodes.

In known rectifier assembly designs, the diodes are typically provided as individual stud diodes. However, such diodes tend to be costly and inefficient from a space perspective.

In a preferred embodiment of the invention, the rectifier circuit is implemented using silicon diode chip technology. Thus, the rectifier circuit may comprise at least one semiconductor diode chip, such as a planar passivated silicon diode chip. For example, where the rectifier circuit comprises two (or more) diode modules, each diode module may comprise a semiconductor diode chip. This may help to provide a compact, robust product that involves a lower production cost than previously known rectifier assemblies.

Preferably the mounting plate is constructed from a thermally conductive material.

For example, the mounting plate may be constructed from a metal such as aluminium or steel, although other materials such as a thermally conductive plastic could be used instead. This can help to dissipate heat produced by the rectifier circuit during operation of the generator. For example, heat may be at least partially dissipated through the mounting plate to the rotor of the exciter.

Preferably the mounting plate is substantially flat, which may facilitate construction and assembly and minimise space requirements.

The rectifier assembly may further comprise a housing, and the housing may be arranged to enclose the three-phase full-wave rectifier circuit. For example, the housing may be arranged to be attached to the mounting plate. This may help to protect the rectifier circuit, and may allow the rectifier assembly to be provided as a single self-enclosed module which is easy to assemble and service.

The housing may be removably attachable to the mounting plate, for example using a bolt or screw. This may facilitate assembly and servicing. Alternatively or in addition the housing may comprise a removable lid.

During operation of an electrical generator, surges in voltages may occur due to external events. Such surges could potentially damage the diodes in a rectifier assembly. It is therefore known to use a surge protection device, such as a resistor, varistor or Zener diode, to protect against voltage surges. Such devices are typically designed to fail before the failure of the diodes, in order to protect the diodes.

In a preferred embodiment of the invention, the rectifier assembly further comprises a surge protection device, such as a resistor, varistor or Zener diode. The surge protection device is preferably packaged together with the other components of the rectifier assembly. For example, the surge protection device may be at least partially enclosed by a housing. This may help to provide a single self-contained module.

Alternatively, the surge protection device may be located on the mounting plate away from the rectifier circuit. This may be desirable where failure of the surge protection device could potentially cause damage to other components located nearby.

The electrical generator may comprise exciter rotor leads which provide an output from exciter rotor windings and/or main machine rotor leads which provide an input to rotor windings in the main machine. Thus, the rectifier assembly may further comprise means for (electrically) connecting the rectifier circuit to exciter rotor leads and/or main machine rotor leads. The connecting means are preferably integrated with the rectifier assembly. For example, where the rectifier assembly is provided as a rectifier module, the connecting means may be integrated with the rectifier module.

In one embodiment, the connecting means is in the form of an electrical terminal such as a threaded stud for receiving a ring terminal, or a threaded hole to which a ring terminal can be screwed, or any other appropriate electrical terminal.

In another embodiment, the connecting means is part of a plug and socket connector, with another part of the connector being provided on exciter rotor leads and/or main machine rotor leads. For example, the rectifier assembly may comprise a socket to which a plug on the exciter rotor leads can be connected, or vice versa. Similarly, the rectifier assembly may comprise a socket to which a plug on the main machine rotor leads can be connected, or vice versa. This can allow the rectifier assembly to be connected using "plug and play" connections, thereby facilitating assembly and servicing.

The connecting means may be provided through a rectifier housing. For example, a plug or socket may pass through a hole in the rectifier housing. This may facilitate connection of exciter rotor leads and/or main machine rotor leads and may help to provide a single compact module that is easy to connect.

In some generator applications, switching transients can lead to failure or reduction in the life span of diodes in the rectifier circuit. It is therefore known to use a snubber to reduce the rate of rise of the voltage in the event of a voltage surge. Typically, the snubber is connected to the output of the rectifier assembly, for example in parallel with a surge protection device. In existing machines, this is typically done by physically connecting the snubber across the output terminals of the rectifier assembly using wires and plugs. This can allow the snubber to be added when it is required and removed when not. However, this may result in increased complexity and assembly time.

In a preferred embodiment of the invention, the rectifier assembly further comprises a snubber. The snubber is preferably arranged to reduce a rate of change of voltage across the rectifier circuit, in comparison to the case where the snubber is not provided. Preferably the snubber is integrated with the rectifier assembly. For example, where the rectifier assembly is provided as a rectifier module, the snubber may be integrated with the rectifier module. This can help to provide a single compact module.

In one preferred embodiment, the snubber is provided as part of a connecting means. For example, the snubber may be physically attached to and/or electrically connected across a socket or plug which is used to connect the rectifier circuit to main rotor windings. This can allow the snubber to be provided as an integrated part of the rectifier assembly which can easily be installed or replaced by replacement of the connector.

Preferably the exciter rotor comprises a rotor core, and the mounting plate is arranged to be attached to the rotor core. The mounting plate may be attached directly to the rotor core, for example using bolts. Alternatively the mounting plate may be attached to the rotor core using stand-offs in order to provide a gap between the two. The gap may provide a passage for airflow and/or clearance for rivets or other features of the exciter rotor. Attaching the mounting plate to the rotor core may facilitate assembly and servicing, may help to avoid or reduce any increase in shaft length, and may allow the rotor core to act as a heat sink for the rectifier circuit.

Preferably the rectifier assembly is arranged to be attached to an end face (axially) of the rotor core, on one side only of the shaft (diametrically), which may facilitate assembly and servicing.

In one embodiment of the invention the mounting plate comprises one or more bolt holes for attaching the rectifier assembly to the exciter rotor (for example, the rotor core). In this case the rectifier assembly may be attached to the rotor using bolts which pass through the bolt holes and into corresponding holes in the rotor.

In another embodiment of the invention, a clamping member is used to attach the rectifier assembly to the exciter rotor. Thus, the rectifier assembly may further comprise a clamping member. In this case, the clamping member may be arranged to be attached to the exciter rotor (for example the rotor core), and the mounting plate may be arranged to be clamped by the clamping member.

The clamping member may comprise, for example, a base arranged to be attached to the exciter rotor, a lip arranged to engage with the mounting plate, and/or a retaining clip arranged to engage with the mounting plate. The base may be arranged to be attached to the exciter rotor for example using one or more bolts. The retaining clip may be removably connected to the base, for example by being screwed onto a rod on the base. This can allow the rectifier module to be assembled to the exciter rotor using a single nut.

According to another aspect of the present invention there is provided an exciter for a rotating electrical machine, the exciter comprising an exciter stator, an exciter rotor, and a rectifier assembly in any of the forms described above.

Preferably the exciter rotor comprises a rotor core, and the mounting plate is attached to the rotor core. For example, the mounting plate may be bolted to the rotor core, or attached using a clamping member.

Preferably the rectifier circuit is connected to exciter rotor windings, for example, using a connector which connects exciter rotor leads to the rectifier circuit.

According to another aspect of the invention there is provided an electrical generator comprising a main machine and an exciter, the exciter comprising a rectifier assembly in any of the forms described above. Preferably the exciter rotor comprises rotor windings which supply a three-phase AC output to the rectifier assembly. Preferably the main machine comprises a main rotor with rotor windings which receive a rectified output from the rectifier assembly.

Corresponding methods may also be provided. Thus, according to another aspect of the invention there is provided a method of assembling a rectifier assembly for an electrical generator comprising a main machine and an exciter, the method comprising providing a rectifier assembly comprising: a mounting plate; and a three-phase full-wave rectifier circuit for converting an AC output of the exciter to a rectified DC output for supply to the main machine, wherein the three-phase full-wave rectifier circuit is mounted on the mounting plate; the method further comprising attaching the mounting plate to a rotor of the exciter.

The mounting plate may be mounted to the rotor, for example, using bolts or a clamping member. The method may further comprise connecting the rectifier circuit to exciter rotor leads and/or main machine rotor leads.

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 disclosure, terms such as "radially", "axially' and "circumferentially" are generally defined with reference to the axis of rotation of the electrical generator unless the context implies otherwise.

Preferred embodiments of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which: Figure 1 shows schematically parts of a known synchronous generator; Figure 2 shows parts of a known rectifier assembly; Figure 3 shows parts of an exciter with a rectifier assembly; Figures 4A and 4B shows parts of a rectifier assembly in an embodiment of the invention; Figure 5 shows a connection diagram for a rectifier assembly; Figures 6A and 6B shows parts of a rectifier assembly in another embodiment of the invention; Figure 7 shows parts of a rectifier assembly in another embodiment of the invention; Figures 8 to 10 show parts of a rectifier assembly in an embodiment of the invention; Figures 11 to 15 show rectifier assemblies in other embodiments of the invention; and Figures 16 and 17 illustrate an alternative technique for mounting a rectifier assembly on an exciter rotor.

Figure 1 shows schematically parts of a known synchronous generator. The generator includes a main machine 2 and an exciter 6. The main machine 2 comprises 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 3 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 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 the exciter 6. The exciter 6 comprises exciter rotor 7, exciter stator 8, and rectifier assembly 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 rectifier assembly 9. The DC output of the rectifier assembly 9 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 automatic voltage regulator (AVR) 10. 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. As an alternative, a separate permanent magnet generator (PMG) may be located on the shaft 5 and may provide the power for excitation of the exciter field.

Although for simplicity only single lines are shown in Figure 1, the generator is typically a three-phase generator producing a three-phase output.

Figures 2A and 2B show parts of a known rectifier assembly for converting the AC output of an exciter to DC for supply to the rotor of a main machine. Figure 2A is a view of the rectifier assembly from the main machine side. Referring to Figure 2A, the rectifier assembly comprises two semi-circular annular backplates 12A, 12B each with three diodes 16. The diodes 16 are stud diodes which pass through holes drilled at spaced locations in the backplates 12A, 12B. The backplates 12A, 12B are made from metal, and act as heatsinks for the diodes. Terminals 20 are provided which are used to make electrical connections to the exciter rotor windings. Wire links 22 are provided to connect the exciter rotor windings to the diodes in the required configuration. Also shown in Figure 2A are two varistors 24 which are used to protect the rectifier from voltage spikes and surge voltages that may occur during operation of the machine.

Figure 2B is a view of the rectifier assembly from the rectifier side. Referring to Figure 23, in this arrangement the stud diodes are encapsulated in mouldings 14A, 14B to which the backplates 12A, 12B are attached. The mouldings 14A, 14B are attached to the exciter core via bolts 18. The bolts 18 pass through the rotor core, and screw into threaded inserts captivated within the mouldings. Bolts 26 are used to connect the main rotor windings.

Figure 3 shows parts of an exciter with a rectifier assembly of the type shown in Figures 2A and 2B in place. The two backplates 12A, 12B are in diametrically opposed positions on the exciter rotor to form a substantially annular rectifier assembly extending around the shaft 5. The terminals 20 are attached to the exciter rotor windings. The exciter rotor windings are connected to three diode anodes in one backplate (the positive plate) and three diode cathodes in the other backplate (the negative plate) using the wire links 22 to give full wave rectification from AC to DC. The bolts 26 are used as output terminals for connecting the rectified output to the main rotor windings.

In operation, the rectifier assembly rotates together with the exciter rotor, and rectifies the AC output of the exciter rotor to provide a DC output for supply to the rotor of the main machine.

Rectifier assemblies of the type shown in Figures 2 and 3 provide a convenient way of attaching rectifier diodes to a rotating component of the machine without significantly increasing the shaft length. However, the rectifier assemblies are relatively expensive to produce due to the number and size of the components involved. Furthermore, each rectifier assembly tends to be designed and sized for a specific product, which increases inventory costs. In addition, the rectifier assemblies may be difficult to service, in some cases requiring removal of the rotor of the main machine to access the rectifier assembly, which can lead to long repair times.

Embodiments of the present invention provide module-based rectifier assemblies using silicon diode chip technology. This can allow a compact, robust product to be produced at a lower production cost than prior designs. The module-based rectifier assemblies also enable a faster servicing and setup time on the machine.

Figures 4A and 43 shows parts of a rectifier assembly in an embodiment of the invention. Referring to Figure 4A, the rectifier assembly 28 comprises a mounting plate 30 on which are mounted two diode modules 32, 34. The mounting plate 30 is formed from a flat, rectangular piece of heat conducting material such as metal. The diode modules 32, 34 are secured to the mounting plate 30 using bolts 35. Bolt holes 36 are provided in the mounting plate 30 in order to secure the rectifier assembly to the exciter rotor. In addition, a hole 38 is provided in order to secure a rectifier housing to the mounting plate.

In the arrangement of Figure 4A, each of the diode modules 32, 34 comprises three diodes which in this example are formed using planar passivated silicon diode chip technology. The diode modules include terminals 40 which provide connections to the diodes. In this arrangement, wires 42 from the exciter rotor windings and wires 43 to the main rotor windings are attached directly to the diode modules using ring-terminals and bolts 41.

Also shown in Figure 4A is a varistor 44 which is used to protect the diodes from voltage spikes and surge voltages. The varistor 44 is secured to the mounting plate 30 using a bolt 45. Wires 48 are used to connect the diodes to the varistor 44.

Figure 43 shows the rectifier assembly of Figure 4A with the rectifier housing in place. Referring to Figure 4B the rectifier housing 46 fits around the diode modules in order to provide protection. The rectifier housing 46 is secured to the mounting plate 30 using a bolt 47. The rectifier housing is formed from an electrically insulating material such as a moulded heat resistant plastic. The wires 42, 43, 48 pass through holes in the housing 46.

In use, the rectifier assembly 28 is secured to the exciter rotor using bolts which pass through the bolt holes 36. The wires 42 connect the exciter rotor windings to the diodes and the wires 43 connect the diodes to the main rotor windings.

The exciter rotor windings are connected to three diode anodes in one diode module (the positive module) and three diode cathodes in the other diode module (the negative module). The other ends of the diodes are connected to the main rotor windings. Thus, each diode module 32, 34 provides half-wave rectification of the three-phase exciter output, to give full-wave three-phase rectification. The backplate 30 acts as a heatsink to dissipate heat produced in the diode modules 32, 34.

The rectifier assembly 28 shown in Figures 4A and 43 can therefore be provided as a single module which can replace the two "half-moon" components shown in Figures 2 and 3. This can allow a compact, robust product to be provided at a lower production cost.

Figure 5 shows a possible connection diagram for the rectifier assembly of Figures 4A and 43. Referring to Figure 5, the exciter comprises a stator field winding 49 which energises three-phase exciter rotor windings 50. The exciter rotor windings 50 are connected in a star configuration with one end of each winding connected to a common connection point. The other ends of the exciter rotor windings are connected to the three diode anodes in the diode module 32 and the three diode cathodes in the diode module 34. The other ends of the diodes are connected to output terminals which provide the rectified DC output to the main rotor windings. The varistor 44 is connected across the output terminals. The varistor 44 is used to protect the diodes from voltage spikes and surge voltages.

Rather than using a single varistor, if desired a separate varistor could be used for each of the diode modules 32, 34. Furthermore, other surge protections devices such as one or more resistors or Zener diodes could be used as well or instead. If desired, other configurations of the exciter rotor windings and/or the diodes, such as a delta configuration, could be used instead.

The arrangement shown in Figures 4A and 4B can be provided at relatively low cost, since the only connection between the wires (exciter, main rotor and varistor) are the ring-terminals and bolts as shown. However, this arrangement may be difficult to assemble and to remove or service.

Figures 6A and 6B shows parts of a rectifier assembly in another embodiment of the invention. Referring to Figure 6A, the rectifier assembly comprises a mounting plate 30 on which are mounted two diode modules 32, 34. The mounting plate 30 and diode modules 32, 34 may be substantially in the form described above with reference to Figures 4A and 4B. However, in the arrangement of Figure 6A, stud connectors 51 are connected to the terminals 40. The stud connectors 51 extend outwards from the diode modules 32, 34, away from the plane of the mounting plate 30. The stud connectors 51 are made from an electrically conductive material and provide terminals for connecting the various windings to the diodes. Two types of stud connector may be used, one for the input terminals (from the exciter rotor) and one for the output terminals (to the main machine rotor and varistor). In the arrangement shown, the varistor 44 is secured to the mounting plate 30 using an L-shaped bracket.

Figure 6B shows the rectifier assembly of Figure 6A with the rectifier housing in place. Referring to Figure 6B, the rectifier housing 56 fits around the diode modules in order to provide protection. The stud connectors 51 pass through holes in the housing 56 and provide external terminals for connecting the diodes in the diode modules. The stud connectors 51 are threaded to allow ring terminals to be fastened to them. This can allow the stud connectors to receive leads which connect the diodes to exciter rotor windings and main rotor windings. This may facilitate assembly and serviceability of the rectifier assembly.

Figure 7 shows parts of a rectifier assembly in another embodiment of the invention. Referring to Figure 7, the rectifier assembly 52 comprises mounting plate 54 on which are mounted two diode modules 32, 34. The diode modules 32, 34 may be substantially in the form described above with reference to Figures 4A and 4B. In this example, the rectifier assembly does not include a surge protection device.

In the arrangement shown, the mounting plate 54 includes two arms 55 which are bent at an angle to the rest of the mounting plate. Each arm has a bolt hole which is used to connect the rectifier assembly to the exciter rotor using a bolt 57.

When the rectifier assembly 52 is connected to the exciter rotor 7, the mounting plate 54 is at an angle to the rear surface of the rotor. This can provide additional space for mounting the rectifier assembly, and in particular may allow the rectifier assembly to avoid coming into contact with the shaft 5. In this example, standoffs 59 are used to provide the appropriate clearance between the mounting plate 54 and the exciter rotor. The stand-offs 59 can also be used to captivate the bolt 57 so that the rectifier assembly can be removed by removing the threaded nuts only. This may allow removal and replacement of the rectifier to be achieved from the rectifier side only of the exciter rotor.

Figures 8 to 10 show parts of a rectifier assembly in an embodiment of the invention. Referring to Figure 8, the rectifier assembly 58 comprises a mounting plate 60 on which are mounted two diode modules 32, 34. As in previous embodiments, the mounting plate 60 is formed from a flat piece of thermally conductive material such as a metal. The diode modules 32, 34 may be substantially in the form described above in the previous embodiments. The mounting plate 60 includes bolt holes 62 which are used to bolt the diode modules 32, 34 to the mounting plate 60. Bolt holes 64 are also provided for bolting the mounting plate to the exciter rotor 7. In additional, bole holes 66 are provided in the mounting plate for mounting a rectifier housing.

Also shown in Figure 8 is part of the exciter rotor core 68. The exciter rotor core 68 includes slots 69 which are used to accommodate exciter rotor windings (not shown in Figure 8). In the arrangement shown, the mounting plate 60 is bolted directly to the exciter rotor core 68. Bolts (not shown) pass through the bolt holes 64 and screw into threaded holes in the rotor core. Since the mounting plate 60 is in contact with the rotor core 68, the rotor core can also act as a heat sink for the diode modules 32, 34.

In this embodiment the mounting plate 60 is arc-shaped rather than rectangular.

This allows the mounting plate 60 to extend part way around the shaft of the machine, circumferentially. This may help to optimise the contact area between the mounting plate and the exciter rotor core 68, improving stability and heat transfer.

In an alternative arrangement, rather than being mounted directly to the exciter rotor core, the mounting plate 60 could be offset on stand-offs a short distance away from the rotor core.

Figures 9 and 10 show the addition of a rectifier housing to the rectifier assembly of Figure 8. In this embodiment the rectifier housing comprises a housing body and a lid. The lid is removable to allow access to the diode modules. Figure 9 shows the rectifier housing with the lid removed. Referring to Figure 9, the rectifier assembly comprises mounting plate 60 and diode modules 32, 34 which are substantially as described above. In addition, a housing body 70 is mounted on the mounting plate 60. The housing body has side walls 71 which extend around the diode modules 32, 34 circumferentially. The housing body 70 is attached to the mounting plate 60 using bolts which pass through holes 72 in the housing body and into the mounting plate. The housing body 70 includes holes 74 which are used to attach the lid.

In the arrangement of Figure 9 the housing body 70 also partially encloses a varistor 76 which is used to protect the diodes from voltage spikes and surge voltages. In this embodiment, the varistor 76 is perpendicular to the mounting plate 60, and extends in an axial direction out of the mounting plate. This can help to achieve a compact design.

Figure 10 shows the rectifier assembly of Figure 9 with the lid in place. Referring to Figure 10, the lid 78 is attached to the housing body 70 using bolts which pass through holes 79 in the lid and into the holes 74 in the housing body 70. In the arrangement shown, electrically conductive studs 80 are used to provide the connection points for the diodes. The studs 80 are connected to the terminals 40 on the diode modules and pass through holes in the lid 78. The studs 80 are threaded to allow ring terminals to be fastened to them. This can allow the studs to receive leads which connect the diodes to the exciter rotor windings and to the main rotor windings. In this embodiment, the varistor 76 also passes through a slot in the lid 78.

In the embodiment described above, the housing body 70 and lid 78 are made from an electrically non-conductive material such as plastic. The various components may or may not be encapsulated by resin and may be designed in such a way that the lid provides the insulation for creepage and clearance of the terminals on the modules.

In the embodiments described above, stud connectors are used to provide terminals for connecting the exciter rotor windings and the main rotor windings to the diodes in the diode modules. However, rather than using stud connectors, wire links could be used to provide the necessary connections directly to the terminals 40 of the diode modules 32, 34. Alternatively, dedicated "plug and play" connectors could be used instead or as well.

Figure 11 shows parts of a rectifier assembly in another embodiment of the invention. Referring to Figure 11, the rectifier assembly comprises mounting plate 60, diode modules 32, 34 and varistor 76 which are substantially as described above. Also shown is a rectifier housing body 82 which is similar to the housing 70 shown in Figure 9 and is also arranged to receive a lid. However, in Figure 11 the rectifier housing body 82 has side walls 83 which are deeper (in an axial direction) than those in Figure 9.

In the arrangement of Figure 11, two connectors 84, 85 are provided, one at either side of the housing body 82. One of the connectors is used for connecting the diodes to the exciter rotor windings and other is used for connecting the diodes to the main rotor windings. In this embodiment the connectors 84, 85 are sockets which are arranged to receive corresponding plugs at the ends of cables connected to the various windings. Of course, it will be appreciated that the plugs and sockets could be provided the other way around. The connectors 84, 85 pass through holes in the side walls 83 of the housing body. The connectors 84, 85 are connected to the terminals 40 on the diode modules 32, 34 using wire links (not shown). In this embodiment the deeper housing body 82 is also able to accommodate the varistor 76.

The connectors 84, 85 of Figure 11 allow the rectifier assembly to be easily connected and disconnected during assembly and for servicing.

Figure 12 shows parts of a rectifier assembly in another embodiment of the invention. Referring to Figure 12, in this embodiment the rectifier assembly comprises a mounting plate 86 and a housing 88 which fits over two diode modules of the type described above. The mounting plate 86 comprises holes 87 for mounting the rectifier assembly on the exciter rotor. Multiple mounting holes 87 are provided in the mounting plate 86 to facilitate the use of the same module on a variety of generators. The housing 88 is fixed to the mounting plate 86 using bolts which pass through holes 89. Wires 92, 93 pass through holes 90 in the housing 88, and are connected inside the housing to the terminals 40 on the diode modules. The wires 92, 93 terminate at the other end in connectors 94, 95. Connector 94 is used to connect the diodes to the exciter rotor windings while connector 95 is used to connect the diodes to the main rotor windings. In this embodiment a varistor 96 is provided outside of the housing 88 and is held in place by a slot 97 in the housing 88 between the two diode modules. Cut-outs 98 are provided at the bottom of the housing 88 near the mounting plate 86 to help with air flow over the top of the diode modules.

In the arrangement shown, the connectors 94, 95 are used to connect the diode modules to exciter rotor leads and the main rotor leads. However, rather than using connectors, the wires 92, 93 could be crimped to the exciter rotor leads and the main rotor leads, or the exciter rotor leads and the main rotor leads could be screwed to connectors in the rectifier assembly, or any other appropriate connection technique could be used.

Figure 13 shows parts of a rectifier assembly in another embodiment of the invention. In this embodiment the mounting plate 100 is extended on one side in order to allow the varistor 96 to be located away from the diode modules. In this embodiment a bolt 102 is used to attach the varistor to the mounting plate. The varistor 96 is connected to the diode modules using wire links. This arrangement may be preferred if there is a risk of the varistor damaging the diode modules in the case of failure. Furthermore, this arrangement may make it easier to replace the varistor when required.

If desired, the mounting plate shown in Figure 13 could be extended on both sides in order to accommodate two varistors, one for each of the diode modules.

Figure 14 shows a rectifier assembly in another embodiment of the invention.

Referring to Figure 14, the rectifier assembly comprises mounting plate 104 and housing 106 which fits over two diode modules of the type described above. The mounting plate 104 includes holes 108 for mounting the rectifier assembly on the exciter rotor. The housing 106 is fixed to the mounting plate 104 using bolts 110.

In this embodiment the mounting plate 104 is extended on one side in order to allow the varistor 96 to be located away from the diode modules, as in the embodiment of Figure 13. In the arrangement of Figure 14, the housing 106 includes holes 112 at locations above the terminals on the diode modules. Connectors 114 pass through the holes 112 in order to connect the diode modules to exciter rotor leads and main machine rotor leads. In this example the connectors 114 are male tab connectors which are arranged to receive female crimp connectors at the ends of the exciter rotor leads and main machine rotor leads.

Figure 15 shows parts of a rectifier assembly in another embodiment of the invention. Referring to Figure 15, the rectifier assembly comprises a mounting plate 120 on which are mounted diode modules 32, 34. In this embodiment a moulded housing body 122 is located on the mounting plate 120. The moulded housing body 122 includes side walls 123 which form part of a rectifier housing.

The diode modules 32, 34 pass through apertures in the moulded housing body 122 in such a way that the terminals 40 of the diode modules are exposed to the inside of the rectifier housing. A varistor 96 is located in the moulded housing body 122, between the diode modules 32, 34. Connectors 124, 125 pass through holes 126 in the side walls 123 at either side.

In use, the connectors 124, 125 are connected to the diode modules 32, 34 and the varistor 96 in the appropriate configuration using wire links (not shown). The connector 124 is arranged to receive a corresponding connector 128 for connecting the diodes to exciter rotor leads 130, while the connector 125 is arranged to receive a corresponding connector 129 for connecting the diodes to the main rotor leads 131. The arrangement shown in Figure 15 may be potted with a potting compound and/or sealed with a lid.

During operation of a synchronous generator, external events such as load transients, connecting other generators, or grid failure can cause transient voltages across the rectifier diodes, which could damage the diodes. It is therefore known to provide a snubber across the rectifier in order to protect the diodes. Snubbers typically comprise a resistor, or a resistor and a capacitor, connected across the output terminals of the rectifier circuit in order to reduce the rate of rise of the voltage in the event of a voltage surge. For example, US 2011/0089779 Al, the subject matter of which is incorporated herein by reference, discloses a rectifier snubber circuit for use with a synchronous generator.

In existing machines, when it is desired to connect a snubber, this is typically done by physically connecting the snubber across the output terminals of the rectifier assembly using wires and plugs. This can allow the snubber circuit to be added when it is required and removed when not. However, this may result in increased complexity and assembly time.

In any of the above embodiments, a snubber can be added to the rectifier assembly as a separate component. The snubber could be mounted separately to the exciter rotor, or the rotor shaft, etc. The snubber could be connected via crimps to the rotor leads. Alternatively, additional modules (similar to the diode modules) could be used as snubbers. These could for example be mounted to the exciter rotor, diametrically opposite from the rectifier assembly.

Alternatively, the rectifier assembly itself may include a snubber connected across the output terminals of the rectifier assembly. For example, the snubber may be a surface mount power resistor which is soldered to the terminals of the connector 125. This can reduce the number of components required and facilitate assembly. Furthermore, retrofitting or replacement of the snubber can be readily achieved by replacement of the connector.

Figures 16 and 17 illustrate an alternative technique for mounting a rectifier assembly on an exciter rotor. In this embodiment, a clamping member is attached to the exciter rotor, and the rectifier assembly is clamped to the clamping member. Figure 16 is a side view of a clamping member for use in this embodiment. Referring to Figure 16, the clamping member 130 comprises a base 132, a lip 134, a threaded rod 136 and a retaining clip 138. The base 132 includes one or more holes which allow it to be attached to the exciter rotor. The lip 134 is arranged to fit into a corresponding slot in the rectifier module. The retaining clip 138 is located on the threaded rod 136 and includes a lip 139 which fits into another slot in the rectifier module. A threaded nut (not shown) is used to hold the retaining clip 138 in place on the threaded rod 136.

Figure 17 shows part of a rectifier assembly in place on the clamping member. In this embodiment the rectifier assembly comprises a moulded housing body 140 with side walls 142, similar to the arrangement shown in Figure 15. Referring to Figure 17, the rectifier assembly is held in place on the clamping member 130 using the lip 134 and the retaining clip 138. The lip 134 fits into a corresponding slot at the top of the side wall 142, and the lip 139 of the retaining clip 138 fits into a slot 144 at the bottom of the side wall 142. The retaining clip 138 is held in place using a threaded nut which screws onto the rod 136. Also shown in Figure 17 is a hole 146 for a connector. In this example a single connector is used to connect the diodes to the exciter rotor leads and the main rotor leads, although two connectors could be used instead.

In use, the rectifier assembly is slid up the base 132 of the clamping member 130 so that the lip 134 engages with the slot at the top of the side wall 142. The retaining clip 138 is then slid over the threaded rod 136, and the lip 139 is inserted into the slot 144 at the bottom of the side wall. A nut is then tightened over the retaining clip, to clamp the rectifier assembly firmly in place.

The arrangement described above can allow the installation and removal of the rectifier assembly using a single nut. This can allow easy removal and replacement of the rectifier assembly for servicing. Furthermore, this arrangement can provide stress relief for the mounting plate on which the diode modules sit as there are no bolts going through it. This arrangement may also provide clearance for rivets on the exciter rotor.

It will be appreciated that embodiments of the present invention have been described above by way of example only, and variations in detail will be apparent to the skilled person within the scope of the appended claims.

Claims

CLAIMS1. A rectifier assembly for an electrical generator comprising a main machine and an exciter, the rectifier assembly comprising: a mounting plate; and a three-phase full-wave rectifier circuit for converting an AC output of the exciter to a rectified DC output for supply to the main machine; wherein the three-phase full-wave rectifier circuit is mounted on the mounting plate, and the mounting plate is arranged to be attached to a rotor of the exciter.
2. A rectifier assembly according to claim 1, wherein the rectifier assembly is provided as a single rectifier module.
3. A rectifier assembly according to claim 1 or 2, wherein the rectifier assembly is arranged to be attached to one diametrical side only of the exciter rotor.
4. A rectifier assembly according to any of the preceding claims, wherein the rectifier circuit comprises a plurality of diodes which are all mounted on the same mounting plate.
5. A rectifier assembly according to any of the preceding claims, wherein the rectifier circuit comprises a first diode module for rectifying one half cycle of a three-phase AC output of the exciter, and a second diode module for rectifying the other half cycle of a three-phase AC output of the exciter.
6. A rectifier assembly according to claim 5, wherein each diode module comprises three diodes.
7. A rectifier assembly according to any of the preceding claims, wherein the rectifier circuit comprises at least one a semiconductor diode chip.
8. A rectifier assembly according to any of the preceding claims, wherein the mounting plate is constructed from a thermally conductive material.
9. A rectifier assembly according to any of the preceding claims, wherein the mounting plate is flat.
10. A rectifier assembly according to any of the preceding claims, further comprising a housing, wherein the housing is arranged to enclose the three-phase full-wave rectifier circuit.
11. A rectifier assembly according to claim 10, wherein the housing is removably attachable to the mounting plate.
12. A rectifier assembly according to any of the preceding claims, further comprising a surge protection device.
13. A rectifier assembly according to claim 12, wherein the surge protection device is at least partially enclosed by a housing.
14. A rectifier assembly according to claim 12, wherein the surge protection device is located on the mounting plate away from the rectifier circuit.
15. A rectifier assembly according to any of the preceding claims, further comprising means for connecting the rectifier circuit to exciter rotor leads and/or main machine rotor leads.
16. A rectifier assembly according to claim 15, wherein the connecting means is part of a plug and socket connector.
17. A rectifier assembly according to claim 15 or 16, wherein the connecting means is provided through a rectifier housing.
18. A rectifier assembly according to any of the preceding claims, further comprising a snubber arranged to reduce a rate of change of voltage across the rectifier circuit.
19. A rectifier assembly according to claim 18, wherein the snubber is integrated with the rectifier assembly.
20. A rectifier assembly according to claim 18 or 19, wherein the snubber is provided as part of a connecting means.
21. A rectifier assembly according any of the preceding claims, wherein the exciter rotor comprises a rotor core, and the mounting plate is arranged to be attached to the rotor core.
22. A rectifier assembly according to any of the preceding claims, further comprising a clamping member, wherein the clamping member is arranged to be attached to the exciter rotor, and the mounting plate is arranged to be clamped by the clamping member.
23. An exciter for a rotating electrical machine, the exciter comprising an exciter stator, an exciter rotor, and a rectifier assembly according to any of the preceding claims.
24. An exciter according to claim 23, wherein the exciter rotor comprises a rotor core, and the mounting plate is attached to the rotor core.
25. A method of assembling a rectifier assembly for an electrical generator comprising a main machine and an exciter, the method comprising providing a rectifier assembly comprising: a mounting plate; and a three-phase full-wave rectifier circuit for converting an AC output of the exciter to a rectified DC output for supply to the main machine, wherein the three-phase full-wave rectifier circuit is mounted on the mounting plate; the method further comprising attaching the mounting plate to a rotor of the exciter.