GB2247362A - Exciting arrangement for homopolar machine - Google Patents

Abstract

Permanent magnet rings (20) are used to produce an excitation field through an air gap (21), a rotor (2), an air gap (10). a stator core (12), cylindrical member (11), and end plates (3). A D.G. auxiliary winding (22) is mounted on the exterior of the cylindrical member (11) to generate an auxiliary excitation flux which is additive or subtractive. The permanent magnet (20) is magnetically coupled to the core of the rotor (2) via an air gap (21) extending perpendicularly to the machine axis. Alternatively the magnet may be mounted on the end of the rotor. The left hand side of the fig. illustrates the prior art. <IMAGE>

Description

INPROVEMENTS IN ELECTRICAL MACHINES This invention relates to a rotating electrical machine, and in particular to a homopolar machine which is specifically adapted to act as a generator, although motor operation is possible.

A homopolar rotating electrical machine is described in British Patent Application Serial No. 2225174A. In that specification, there is described a machine in which a lobed rotor is used with fixed excitation windings to produce a unidirectional or homopolar fluctuating magnetic field directed radially out towards a surrounding cylindrical stator which supports the armature windings. The flux is linked with the stator windings which are housed in axially extending slots defined in the inner cylindrical surface of the stator.

Substitution of excitation windings by permanent magnets for excitation is known (see for example "Permanent-magnet a.c.

generators" by Binns and Kurdali - Proceedings of the IEE Volume 126, No. 7, July 1979, pages 690 - 696). However, while the use of permanent magnets can provide improved efficiency, the flux density cannot be altered.

The present invention provides an electrical machine having a rotor mounted for rotation within a stator, and having excitation means comprising at least one permanent magnet and at least one auxiliary excitation winding arranged to produce in combination with the permanent magnet or magnets a magnetic excitation field. In this way, it is possible to produce a magnetic flux which can be varied while, at the same time, taking advantage of the improved efficiency afforded by the use of a permanent magnet or magnets. In a preferred embodiment of the machine, means are provided for varying the electrical current passed through the auxiliary winding during operation of the machine to maintain a substantially constant output voltage.Such varying means preferably includes means for sensing the output voltage so that the voltage may be maintained substantially constant when the output current or the power factor vary during operation of the machine as a generator.

The invention also includes a homopolar electrical machine comprising a lobed rotor mounted on a shaft within a cylindrical casing so as to be rotatable about a common axis of the shaft and the casing, a substantially cylindrical stator core divided into a plurality of sectors each of which is an integral body defining an even number of axially extending slots in its inner surface, armature windings arranged in the slots, and excitation means for producing a fluctuating magnetic field between the periphery of the rotor and the stator during rotation of the rotor, wherein the excitation means comprise at least one permanent magnet and at least one auxiliary excitation winding arranged in combination to produce the said magnetic field.

Preferably, the or each permanent magnet is located between an end wall of the casing and the rotor, with the auxiliary winding coaxial with the rotor shaft and encircling the casing. The magnet is preferably adjacent the rotor and it is magnetically coupled to it via an air gap which extends generally perpendicularly to the axis of rotation of the rotor. In the preferred embodiment of the invention, the casing constitutes a structure of magnetic material forming part of a magnetic circuit including the rotor and the stator, the casing connecting the stator to the permanent magnet on an opposite face thereof from the rotor. The auxiliary excitation winding generates an auxiliary excitation flux in the magnetic circuit.

The invention will now be described by way of example with reference to the accompanying drawing in which: Figure 1 is a diagrammatic axial section through a rotating electrical machine, the left-hand side of the Figure showing the prior construction, and the right-hand side showing features of a machine in accordance with the present invention.

Referring to Figure 1, the general construction of the machine is similar to that shown in British Patent Application No. GB 2225174A and, indeed, the left hand side of Figure 1 corresponds exactly to the machine shown in that Application. No other views of the machine are included in the present Application, since they are shown in Application No. 2225174A, the disclosure of which is incorporated herein by reference. Novel features of the preferred embodiment of the present invention are shown in the right-hand side of Figure 1.

Briefly, the parts of the machine which are common to that of the above mentioned prior art machine include a shaft 1 mounting a lobed rotor 2 which extends through a machine casing defined by end plates 3 and a cylindrical member 11.

Mounted inside the casing is the stator core 12 which has slots in its internal surface for receiving armature windings 14.

The shaft 1 is divided into two parts which between them support the rotor 2. One end of each part of the drive shaft is spigoted into one end face of the rotor. The rotor 2 is built up from laminations of sheet steel and has two diametrically opposed lobes which carry rotor windings (not shown). Any number of pairs of lobes may be formed on the rotor 2 depending on the required frequency and speed of rotation of the machine.

Fan blades 16 are mounted between each pair of lobes for cooling. The rotor winding has wires (not shown) connected at their ends to a common wire to form a conventional insulated squirrel cage winding for induction rotor operation. This provides for damping when the machine is used as an alternating current generator.

Each part of the shaft 1 is fitted into a respective end plate 3 and is supported in bearings 5 and 6 housed in housings 7 fixed to the exterior of the end plates 3. A drive pulley 8 is attached to the shaft 1.

In the prior art construction, an excitation field is provided by excitation coils 4 encircling the shaft 1 and an inwardly directed bush of the end plate 3. These coils 4 are supplied with a direct current and produce a magnetic field having the polarity indicated on the respective shaft part in Figure 1. The lines of flux produced by the excitation winding 4, as in a conventional homopolar machine, extend from the left hand end plate 3 to the cylindrical member 11, and then into the stator core 12, across an air gap 10 between the rotor and the stator, into the rotor core 2, and then through the left hand part of the shaft 1, and across the air gap 9 back to the end plate 3.

When the flux passes into the rotor 2 it is concentrated by the lobes so as to produce a fluctuating, radial, magnetic field between the periphery of the rotor and the cylindrical member 11. The design of the components is such that as much soft steel, or other magnetically soft material, as possible is used for the components forming this flux path.

In the preferred embodiment of the present invention shown in the right-hand part of Figure 1, the excitation coils 4 are replaced by a permanent magnet ring 20 attached to the respective end plate 3. The magnet ring is coaxial with and encircles the shaft 1, and extends from the end plate 3 to within a small distance of the axially directed end face of the rotor 2, leaving a small air gap 21 therebetween. It will be noted that in this particular embodiment the end plate has no inwardly directed bush surrounding the shaft 1.

As before, the magnetic field direction produced by the permanent magnet ring 20 is shown by the letters "N" and "S" on the shaft 1. The shaft 1 is of non-magnetic material in this case.

In order to allow the strength of the excitation field to be varied, an auxiliary winding 22 is provided to contribute to or to detract from the field produced by the permanent magnet ring 20. In the present embodiment, this auxiliary winding 22 is an annular winding mounted on the outside of the cylindrical member 11 and coaxial with the axis of the shaft 1 and the machine. It will be appreciated that the excitation field produced by the permanent magnet ring 20 has a magnetic circuit comprising the respective end plate 3, part of the cylindrical member 11, the stator core 12, the air gap 10 between the stator and the rotor, the rotor core, and the air gap 21. In other words, there is direct magnet coupling between the inner end face of the magnet ring 20 and the rotor core 2 via the air duct 21.It should be noted that it is conceivable that the permanent magnet 20 can be instead mounted rotatably on the rotor 2, with the air gap lying instead between the outer end of the magnet ring 20 and the end plate 3. Other auxiliary winding configurations are envisaged, the main requirement being that it should be located so as to produce an auxiliary flux in the magnetic circuit just described. As shown, the auxiliary winding 22 in this embodiment generates a magnetic flux component whose plurality is in the same direction in the magnetic circuit as that of the permanent magnet ring 20, although this is easily reversed by altering the direction of direct current passed through it.

Although only one half of the machine in accordance with the present invention is shown in Figure 1, it will be understood that, in practice, the other half is identical in most respects. Thus, the complete machine has two magnet rings 20 on opposite sides of the rotor 2 and two auxiliary windings 22 each producing an auxiliary flux in a respective magnetic circuit.

By varying the direct current and its polarity in the auxiliary windings 22, the total magnetic flux in the magnetic circuits described above may be varied. Control circuitry, not shown, is provided for determining the current in the auxiliary windings 22, and includes means for sensing the output voltage, or an output voltage change, when the machine is operating as a generator. In such a manner, it is possible to maintain the output voltage constant with varying output current and power factor.

The use of permanent magnets yields a considerably stronger excitation flux than in the prior art machine in the available space.

A second embodiment of the invention consists of the modification of the machine shown in Figures 3 and 4 of the above-mentioned Application Serial No. 2225174A.

Naturally, other embodiments of the invention are possible within the scope of the claims.

Claims (10)

1. An electrical machine having a rotor mounted for rotation within a stator and having excitation means comprising at least one permanent magnet and at least one auxiliary excitation winding arranged to produce in combination a magnetic excitation field.

2. A machine according to claim 1, for use as a generator, the machine including means for varying the electrical current passed through the or each auxiliary winding during operation of the machine to maintain a substantially constant output voltage.

3. A machine according to claim 2, wherein the varying means is operable to maintain the said output voltage substantially constant when the output current or the power factor vary during operation of the machine.

4. A homopolar electrical machine comprising a lobed rotor mounted on a shaft within a cylindrical casing so as to be rotatable about a common axis of the shaft and the casing, a substantially cylindrical stator core divided into a plurality of sectors each of which is an integral body defining an even number of axially extending slots in its inner surface, armature windings arranged in the slots and excitation means for producing a fluctuating magnetic field between the periphery of the rotor and the stator during rotation of the rotor, wherein the excitation means comprise at least one permanent magnet and at least one auxiliary excitation winding arranged in combination to produce the said magnetic field.

5. A machine according to claim 4, wherein the or each permanent magnet is located between a respective end wall of the casing and the rotor.

6. A machine according to claim 4 or claim 5, wherein the or each auxiliary winding is coaxial with the rotor shaft and encircles the casing, and wherein the shaft is formed of a non-magnetic material.

7. A machine according to any preceding claim, wherein the excitation means comprises at least one permanent magnet mounted adjacent the rotor and magnetically coupled to the rotor via an air gap which extends generally perpendicularly to the axis of rotation of the rotor.

8. A machine according to claim 7, wherein the said at least one permanent magnet is located in a magnetic circuit formed by the rotor, the stator, and a structure of magnetic material connecting the stator to the permanent magnet or an opposite face thereof from the rotor, and wherein the respective auxiliary excitation winding is arranged so as to generate an auxiliary excitation flux in the magnetic circuit.

9. A machine according to claim 8, wherein the said auxiliary winding is an annulus located externally of the said structure of magnetic material.

10. An electrical machine constructed and arranged substantially as herein described and shown in the drawing.