GB2183102A

GB2183102A - Alternating current rotary convertors

Info

Publication number: GB2183102A
Application number: GB08528282A
Authority: GB
Inventors: Harold Aspden; Francis James Mumford
Original assignee: Individual
Current assignee: Individual
Priority date: 1985-11-16
Filing date: 1985-11-16
Publication date: 1987-05-28
Also published as: GB2183102B; GB8528282D0

Abstract

An AC rotary convertor has a stator 16 and a rotor comprising a shaft 10, a cylindrical laminated ferromagnetic core 13 and a rotor winding 14 inductively-coupled with stator winding 17. Both windings have solenoidal form so that energy is transferred by transformer action between the rotor and stator. The rotor is capable of continuous rotation and the use of slip rings and brush connections can be avoided by providing connecting leads (not shown) through the bearing section of the shaft. One application is in a system in which the rotary convertor provides the excitation for a coupled alternator, the excitation current being fed through the bearing sections to the rotating field windings of the alternator. Variable speed drives using the invention are also described. <IMAGE>

Description

SPECIFICATION Alternating current rotary convertors This invention relates to electrodynamic machines in which alternating current is converted, in the sense that it is transformed, not only by the usual transformer principle with primary and secondary windings in a rigidly coupled unit, but by use of this same principle in a system in which these two windings are in relative rotary motion.

The conversion can involve current and voltage transformation but, essentially, it involves transfer of energy between a non-rotating and a rotating frame of reference.

The object of the invention is to overcome problems in conventional electrodynamic machines (a) when conditions are such that the use of brush connections or slip-rings would present hazards or imply reliability and servicing problems, e.g. in aircraft or spacecraft and (b) where special circumstances, such as high currents, make the use of brushes or slip-rings impractical. There are, however, other objectives. Speed regulation of the propellor drive in ship propulsion poses special problems, which may be overcome by the use of this invention. Also, there are features which allow the rotary transformer action to be exploited so as to provide regulated voltage outputs or higher current or voltages than are normally available from alternators of standard design.

Brushless AC generators are known in which there is a stationary armature and a rotatable main field winding, the latter being excited by a transformer and rectifier system attached to the rotary shaft structure. U.K. Patent Specification 827,576 provides such a system, designed for use in an aircraft.

Such systems serve to supply energy for use in the rotor as an auxiliary excitation means, the primary role of the rotor being to transfer power between a mechanical drive and the electrical system coupled to the stator windings.

The subject invention, however, is concerned with a system in which the rotary action has iittle effect on the power transfer between stator and rotor. The transfer occurs with the rotor at rest, but because the power on the rotor side can be supplied to or from another rotating machine conected to the convertor provided by this invention, we can dispense with the need for slip-rings or brushes on that other machine.

To achieve this result in an optimum way, it has been found that the rotor winding ought to be solenoidal and this leads to a number of inventive features in preferred design embodiments.

According to the invention, an alternating rotary convertor having a stator and rotor capable of continuous rotation and in which electric energy is transferred by transformer action between inductively-coupled primary and secondary windings, is characterized by one winding being part of the stator assembly and the other winding being part of the rotor assembly, the rotor winding being of solenoidal form and having a central axis common with that of the rotor.

The stator winding may also be solenoidal or it could be sectionalized, for example by series connected windings on each of a plurality of limbs of the stator. The inductive-coupling may comprise a laminated ferromagnetic core, wrapped around the rotor shaft to form a radially laminated cylinder.

The insulation between adjacent laminations could be a bonding agent providing a rigid rotor unit, able to withstand the centrifugal forces set up by rotation at operating speeds.

These features of the invention can be applied directly by coupling the rotor to the rotor of another electrical machine the rotor windings of which are connected to the rotor windings of the convertor, the combined rotor assembly then sharing the same bearings. This exploits the solenoidal construction, which has advantages in permitting substantial and possibly main energy transfer th roug h the convertor system. It does, however, have certain disadvantages in highly powered machines, because the combined rotor length may become too long and intermediate bearings are then needed.

Accordingly, the convertor can assume the form of a self-standing machine and be coupled to another machine, either in the usual way via flange couplings, perhaps providing some resilience, or even in a mode in which the two machines share a common intermediate bearing.

A further feature of the invention therefore provides that a connection lead for a winding on the convertor passes through the cylindrical space bounded by the bearing surface. Thus, by bringing leads through holes drilled in the bearing section of the rotor shaft, the windings can be connected through to external suppliesorto rotorwindings on a connected machine.

The convertorfinds a primary use when coupled to an electrical alternator or synchronous motor.

The features of the invention therefore extend to installations of this kind in which, for example, the convertor constitutes the excitation supplied for a DC main field in the alternator or motor rotor. In this case a rectifier is incorporated in the rotor assembly of either machine to assure rectification of the current produced by the convertor rotor winding before this is fed to the main excitation winding.

Other examples, include a configuration in which the main field excitation is AC operated, so as to dispense with the rectifier. The AC in the stator of the alternator or motor will then have a frequency related to the difference between the frequency of the AC in the convertor and the corresponding speed of the prime mover connected with the rotor system.

Other variations on this theme are possible, as we shall see from the following description. However, it is noted that an important potential application of the invention is to be found in machines which are large and yet seek to exploit the advantages of superconductivity. There are limits on the amount of current that can be fed through slip-rings in order to provide the powerful magnetic fields directly available from the use of superconductors and without the use of ferromagnetic cores. Provided the machine design can be inverted, as it were, so that the stator constitutes the DC superconductive system, the stip-ring problem is overcome.

However, then there is a need to extract the AC power from the rotor in the alternator mode of operation. This need is met by the convertor provided by this invention. Such a configuration also overcomes the problem of having a rotor kept superconductive, which is the usual approach to machine design of this general form.

Further advantages emerge from the fact that the convertor used to overcome the brush or slip-ring problem is itself a transformer. If used as the main energy transfer unit, this means that it offers versatility in determining voltage and current output or input ratings. In effect, depending upon the application, the convertor serves to eliminate the use of slip-rings and constitutes one transformer stage in the connected transmission lines.

The invention and its various features will now be described by reference to the accompanying drawings in which: Fig. 1 shows an end view of a cylindrical rotor with a stator in eight radial sections and solenoidal rotor and stator windings.

Fig. 2 shows the configuration in Fig. 1,with the stator winding divided into eight separate sections, one being wound around each limb of the stator.

Fig. 3 shows a sectional side view of the convertor assembly illustrated in Fig. 1.

Fig. 4 shows an enlarged sectional view of the bearing section at one end of the convertor illustrated in Fig. 3, indicating how the leads supplying the rotor winding are connected through the body of the rotor shaft at the bearing section.

Fig. 5 shows a schematic installation ih which a convertor provides the DC excitation field supply to an alternator rotor.

Fig. 6 shows a schematic installation in which a convertor and a synchronous motor combine to provide the drive for a ship's propellor, regulated by a variable frequency supply and a fixed frequency main power supply.

Fig. 7 shows an alternator coupled to a prime mover or load and to three convertors, each of which operates to transfer the main electrical power of a different phase in a three-phase system.

Fig. 8 shows a variant design of the convertor system of Fig. 3, in which a light weight rotor is used and the connections to the stator winding are made through a bearing interface.

Referring now to Figs. 1,3 and 4, a rotor shaft 10 is mounted between two bearings 11,12. Wrapped around the body of the rotor shaft in spiral fashion there is a laminar ferromagnetic core section 13.

This forms an integral and rigid cylindrical unit. The continuous ferromagnetic sheet used to form the core is coated by an insulating layer strong enough to withstand the spiral assembly and further coated by a heat-setting insulating substance. Once wound, the core is heat treated to form a rigid assembly able to withstand the centrifugal forces of rotor rotation.

Closely wound around this ferromagnetic core section 13, there is a rotor winding 14 insulated from the core by insulation (see Fig. 4).

The magnetic flux path of the core section 13 is closed by eight stator units 16 which have laminar construction and are also ferromagnetic. A solenoidal stator winding 17 is arranged so as to provide a close inductive coupling with the rotor winding 14, the central axes of both windings being coaxial with the rotor shaft.

A connecting lead 18 passes through a hole in the rotor shaft coextensive with the bearing surface and then enters a tapered slot, extending longitudinally along the shaft, to connect with a connector 19 joining the lead 18 to a flat conductive disc 20. This disc is insulated from the end faces of the rotor core section but bonded thereto. The disc provides a conducting path between the lead 18 and a ring section at its rim, which provides a start point for the helically wound rotorwinding 14. This winding progresses along the body of the rotor and at the other end (not shown) it connects with a similar disc and via that with the other lead 21. Lead 21 passes also through a hole in the rotor shaft at the bearing section and emerges along a tapered slot in the shaft surface. Itthen progresses along a slot longitudinally to the other end of the rotor.These tapered slots are coextensive with the pole sections of the stator units, to minimize the need for bending of the leads 18 and 21, whilst permitting connection to the disc 20.

In operation, the system acts as an electrical transformer in the normal way. This action is not impeded by the rotation of the rotor. The air gap between the stator and core is larger than would be the case in a rigid transformer assembly, causing the convertor to have more inductance and require a magnetizing current component higher than normal for a static transformer. However, this gap is minimized and windage effects are minimized by the use of the planar disc connection. This is particularly advantageous where high rotor winding currents and relatively low voltage conditions apply.

However, for a high voltage, current design the connection could be made by cutting a slot radially in the end face of the core and bringing the lead through this slot to make the winding connection at the rim.

In application, the leads emerging from the free end of the flange section of the rotor shaft are deemed to be connected to electrical apparatus rotating with the shaft. Such apparatus could take the form of an alternator having its own end bearings and in this case the leads would need to pass through the alternator bearing section in the same way as is shown in Fig. 4.

Fig. 2 shows a modified version of the apparatus just described. The stator winding is not solenoidal.

There are eight stator limbs 22 and each has its own winding 23. In this configuration it is easierto inspect the rotor by removing one of the eight stator assemblies. Also, there are cooling advantages and some electrical design advantages. For example, the windings 23 can be connected in different configurations, e.g. in series to produce a high voltage or each may supply its own circuit. The arrangement shown in Fig. 3 does, however, have the advantage of requiring less conductor material for the same rating and so dissipates less heat.

Referring to Fig. 5a convertor of the form just described, denoted 50, has alternating current fed into its stator and feeds alternating current from its rotor, through the bearing assembly, but, either in the convertor orthe connected machine 51, this is rectified en route by rectifier 52. This rectifier rotates with the rotor system. The direct current developed excited the main field of the rotor winding in machine 51,which is an alternator. The alternator rotor is shown connected to a driven load 53 or a prime mover such as a turbine. The main AC power connections are made to the stator windings of 51.

Thus, in operation as a generator, the system uses the convertor 50 to provide DC excitation with no slip-rings or brushes in the DC supply circuit.

This has particular advantages if the risk of arcing at high currents is a consideration. Also, it allows high currents to be induced in the convertor rotor winding, which can be rectified to drive a superconductive rotorwinding in the alternator 51.

Fig. 6 shows the system used without the rectifier to drive a load 54, depicted as the propeller of a ship.

This involves lower speeds that apply in normal alternator systems. Thus, supposing a turboalternator on the ship already produces power at, say, 50 Hz, it is possible to feed this directly to the alternator stator 55. The convertor 56 is supplied with a relatively small amount of power by the variable frequency source 57. This power is transferred at this variable and controlled frequency to the convertor rotor, whence it passes through the bearing system to the alternator rotor. By exciting the convertor at a frequency progressively reducing from 50 Hz, the synchronous action of the motor drives the load at a progressively increasing speed.

The action takes place without the heavy currents needed to generate high torque passing through brushes or slip-rings or needing rectification.

Fig. 7 depicts a polyphase alternator system which does not use a rectifier in the rotor assembly.

Typically, a turbo-alternator 58 drives an alternator 59. The alternator in this case is of superconductive design and has a superconducting stator winding which produces a main field excitation from a DC source 60. The main power is developed at high current and high voltage in the alternator rotor winding. This is fed through the bearings to an inline configuration of three coupled convertors of the form aiready described. There are connecting leads applicable to three-phase power passing through the first bearing to convertor 61. Convertor 61 produces from its stator winding the main power output of one phase. Similarly, the connecting leads forthe remaining two phases pass through the bearing system connecting to convertor 62, where the second phase output is developed from the stator.The third phase connections then pass through to the third convertor 63 to develop the third phase output.

Fig. 8 shows a configuration in which there is a convertor with a ferromagnetic core and stator system, but in this case the ferromagnetic unit is all static. The rotor consists of a solenoidal rotor winding 80 supported on an insulating tubular sleeve 81. This light weight rotor is supported by two bearings 82 and 83, as shown. The connecting leads for the rotor winding are not shown, but they pass through the rotor shaft 84 in the part through the bearing 82. The statorwinding could be a solenoidal coil fixed to the stator and enveloping the rotor winding. However, this would have additional inductance, owing to the double air gap between the windings and the ferromagnetic core 85.

Accordingly, the stator winding 86 illustrated is shown to be wrapped closely around the ferromagnetic core 85. This means that the connection leads to the winding, even for the stator winding in such a configuration, must be brought through the bearing 83, if we are to avoid the use of brushes or slip-rings.

Claims

1. An alternating current rotary convertor having a stator and a rotor capable of continuous rotation and in which electric energy is transferred by transformer action between inductively-coupled primary and secondary windings, characterized by one winding being part of the stator assembly and the other winding being part of the rotor assembly, the rotor winding being of solenoidal form and having a central axis common with that of the rotor.

2. An alternating current rotary convertor according to claim 1, characterized in that the windings are coupled inductively by a cylindrical ferromagnetic core which is radially laminated.

3. An alternating current rotary convertor according to claim 2, characterized in that the laminarferromagnetic core is a multi-turn spiral of one or more sheets, adjacent turns being separated by insulation wound into the spiral.

4. An alternating current rotary convertor according to claim 2 or 3, characterized in that adjacent laminations of the core are bonded together by an insulating bonding agent.

5. An alternating current rotary convertor according to any of claims 1 to 4, characterized in that electrical connection leads for one or both windings pass through the cylindrical space bounded by the bearing surface of one or both of the bearings supporting the rotor.

6. An alternating current rotary convertor according to claim 5, wherein the stator provides a magnetic flux closure path for a ferromagnetic core linking the windings, characterized in that the core is part of the rotor and an electrical connection is formed from a continuous lead which passes through the bearing section of the rotor shaft and emerges along a slot in the rotor shaft coextensive with the pole section of the stator.

7. An alternating current rotary convertor according to claim 5, wherein the stator provides the complete magnetic circuit apart from an air gap and includes a ferromagnetic core linking the windings, one end of which is unsupported and forms a pole face at the air gap, characterized in that the rotor winding has a shaft at one end providing support adjacent the air gap via a bearing, and there being a bearing at the other end ofthe rotor winding encircling the core and providing further support for the rotor.

8. An electrical alternator or synchronous motor installation of the kind in which a rotary magnetic field produced by an excited rotor interacts with a stationary field winding, characterized in that a rotary convertor according to any preceding claim is mechanically coupled to the rotor and has electrical connections between its own rotor windings and those of the excited rotor, these connections forming part of the rotating assembly and being made across rigid interfaces.

9. An electrical alternator or synchronous motor installation according to claim 8, in which the excited rotor is energized by direct current, characterized in that a rectifier also forms part of the rotating assembly and is connected between the output of the rotor winding of the convertor and the input of the excited rotor winding.

10. An electrical alternator or synchronous motor installation according to claim 9, characterized in that the excited rotor has a winding with superconductive properties and the installation includes cooling means for assuring that the excited rotor is maintained at a temperature low enough to assure its superconductive state in operation.

11 An electrical alternator or synchronous motor installation according to claim 8, in which the excited rotor is energized by alternating current and interacts with a stationary field winding also energized by alternating current, characterized by the stator winding of the convertor and the stationary field winding being both connected to be operative at the same electrical frequency.

12. An electrical alternator or synchronous motor installation according to claim 8, in which the excited rotor is energized by alternating current and interacts with a stationary field winding also energized by alternating current, characterized by the stator winding of the convertor having separate alternating current energization means which supply a signal of variable frequency, whereby the relationship between the speed of the alternator or motor and the frequency of the energization of the stationary field winding can be regulated.

13. An electrical alternator or synchronous motor installation according to claim 8, in which the excited rotor is energized by alternating current and interacts with a stationary field winding energized by direct current, characterized by the main energy transfer in the installation being via the stator winding of the convertor.

14. An electrical alternator or synchronous motor installation according to claim 13, in which the stationary field winding energized by direct current has superconductive properties and the installation includes cooling means for assuring that this winding is maintained at a temperature low enough to assure its superconductive state in operation.

15. An electrical alternator or synchronous motor installation according to claim 13 or 14, which operates as a multi-phase system, characterized in that there are as many rotary convertors as there are phases, with their rotors all mechanically coupled to be driven at the same speed as the excited rotor, there being separate connections between each rotor winding and a corresponding phase winding on the excited rotor.