GB2107937A - D.C. Homopolar machines with superconducting field coils - Google Patents

Abstract

Rotor 13 carries conductors 14 connected between sets of slip rings 19 and 20 which co-operate with brush sets 21 and 22. Fixed superconducting field coils 15 and 16 are each split into axially-spaced sections 15A, B and C and 16A, B and C to increase the uniformity of the field at the surface of the slip rings and minimize the field component perpendicular to that surface. The number, length and spacing of the field coil sections is chosen to give the required field. Rings of ferromagnetic material may be disposed to further modify the field. At the outer ends of the slip ring sets the slip ring surfaces may be inclined to follow the field lines more closely. <IMAGE>

Description

SPECIFICATION D.C. Homopolar machines with superconducting field coils The present invention relates to D.C.

homopolar machines with superconducting field coils and a drum rotor.

Machines of this kind, of which an example is described in our U.K. Patent Specification No.

1,181,821, have a drum rotor which carries rotor conductors arranged on a cylindrical surface and extending between slip-rings at opposite axial ends of the drum. Direct current flows between these rotor conductors and an external circuit by way of fixed brushes engaging the slip-rings. The field in which the rotor conductors rotate is generated by superconducting coils, normally two coils coaxial with the drum and arranged at opposite ends of the drum. These coils are arranged to generate a radial field which is cut by the rotor conductors. The machine may thus operate as a motor when direct current is supplied to the rotor conductors or as a generator when the rotor is driven by a prime mover. The superconducting coils may be inside or outside the cylindrical surface defined by the rotor conductors.

To increase the voltage output of such machines several rotor conductors can be connected in series by the use of segmented slip rings with each conductor connected between a segment of one slip ring and a segment of the other, the segmented slip ring then cooperating with brushes spaced around the circumference of the rotor as described in the aforesaid specification No. 1,181,821. Alternatively separate pairs of slip rings are provided for the several conductors and at each end of the drum the slip rings are axially spaced along a cylindrical surface.

In either case the high magnetic field generated by the superconducting field coils (typicaliy of 1 Tesla or more in the region of the brushes) may result in a substantial field component perpendicular to the surface of contact between each slip ring and the associated brush gear.

When the component is in excess of 0.1 Tesla a significant voltage gradient is generated across the surface of contact which causes uneven current distribution and consequently uneven brush wear and degraded brush performance. The presence of transverse voltage gradients is a severe practical limitation on the number of stages which can be provided in a homopolar machine of this kind and hence on the output of such a machine.

In accordance with the present invention there is provided a D.C. homopolar machine having a drum rotor and first and second superconducting .field coils arranged coaxially with the drum at opposite ends thereof characterized in that each field coil is composed of two or more axially separated sections such that the curvature of the magnetic field in the region of the rotor slip rings is reduced.

With the conventional single superconducting coil the magnetic field lines, as seen in a section containing the axis of the coil, approximate to circles centred on the conductors of the coil and hence the radial component of the field is substantial except in the mid-plane of the coil. By using axially-separated sections the field in the region of the slip rings is made less curved so that it approximates to a field parallel to the axis and thus has only a small radial component perpendicular to the contact surface between the slip ring or slip-rings and the brush gear.

To increase the uniformity of the field in the region of the slip rings or to extend this uniformity over a greater axial length rings of ferromagnetic material can be placed coaxially with the field coils to influence the field distribution. Towards the axial limits of the region where the field is approximately parallel to the axis the slip ring surfaces can be inclined to follow the curvature of the field. This latter measure has been used in superconducting machines with disc rotors to minimize field components perpendicular to the slip-ring surface.This would not however be practicable in machines with a drum rotor especially when a plurality of slip rings are to be arranged side-by-side over a substantial axial distance because each slip ring would then require a different inclination of its contact surface and the costs of manufacture and mounting of the slip rings would become excessive.

The invention will now be described in more detaii with the aid of examples illustrated in the accompanying drawings, in which: Fig. 1 is a side elevation, partially in section, of a D.C. homopolar machine in accordance with the invention, Figs. 2 and 3 are diagrams of modifications of the field coil and slip-ring arrangements using rings of ferromagnetic material to modify the magnetic field, and Fig. 4 is a diagram of a further modification of the slip-rings.

Referring first to Fig. 1, this shows a machine with an internal stator 10 and a rotor 11 in the form of a cylindrical drum which encloses the stator 1 0. The rotor 11 is attached to a drive shaft 12 which is mounted in bearings at one end of the machine. The other end of the roller 11 is journalled at 13 on the stator structure. The rotor 11 carries a plurality of bar conductors 14 which.

extend parallel to the axis of the rotor and are disposed uniformly around the periphery of the rotor. The rotor conductors 14 rotate in the magnetic field produced by two super-conducting coils 1 5 and 16 mounted on the stator, each of which is divided into three axially-separated sections which are identified as 1 5A, 1 SB, 1 SC and 1 6A, 1 6B, 1 6C. The coils 1 5 and 1 6 are mounted near the ends of the stator in order to produce a magnetic field which extends generally radially in the region of the rotor conductors 14.

The superconducting coils 1 5 and 16 are cooled by liquid helium contained in an inner annular jacket 1 7 and the space 1 8 between the jacket 1 7 and the wali of the stator 10 is evacuated.

Arrangements for the support of the windings 1 5 and 1 6 and the circulation of the liquid helium form no part of the present invention and have been omitted from the drawings for simplicity and clarity.

Sets of slip-rings 1 9 and 20 encircle the rotor 11 at opposite ends of the conductors 14. The slip-ring sets 19 and 20 cooperate with brush sets 21 and 22, respectively, to transfer the current between the rotor conductors 14 and an external circuit. Each conductor 14 is connected between a pair of slip-rings, one from each set. Thus for example a bar conductor 1 4N is shown as connected by lead 23 to slip-ring 1 9N and by conductor 24 to slip-ring 20N, these being the Nth slip-rings in their respective sets.

The splitting of the coils 1 5 and 16 into a number of spaced sections enables the magnetic field in the region of the slip-ring sets 19 and 20, respectively, to be made more nearly uniform and parallel to the axis by choice of the number and lengths of the individual coil sections and the spacing between them. Thus in Fig. 1 three sections are shown for each coil, the outer sections 15A, 15C and 1 6A, 1 6C being approximately twice as long as the inner sections 15B and 16B. Preferably the sections 15C and 1 6A are somewhat ionger than the sections 15A and 16C, respectively.

In Fig. 2 is shown a superconducting coil consisting of two sections 25A and 25B with an axial gap 26 between them. Slip-rings 27 lie in a cylindrical surface surrounding the annular coil sections 25A and 25B. A typical magnetic flux line is shown at 28. A ring 29 of ferromagnetic material has been placed in the region of the gap 26 at a greater radius than the coils 25A and 25B to modify the magnetic field. In the absence of the ring 29 the flux line 28 would follow the path shown by a broken line but the ring 29 smooths out this curve to follow the full line path which is more nearly parallel to the surface of the slip-rings 27.

Fig. 3 again shows the two superconducting coil sections 25A and 25B and the flux line 28. In this case the use of a ferromagnetic ring 30 beyond the end 27E of the slip ring set of Fig. 2 extends the region in which the flux line 28 has only a small component perpendicular to the slipring surfaces so that the slip-ring set can be extended to terminate at 27F. Fig. 4 shows that even without ferromagnetic rings 29 and 30 an improvement in the magnetic field is possible in the region of the slip rings 27 by splitting the field coil into sections 25A and 25B. Moreover the slip ring set can be extended by the addition of end clip rings 31 and 32 whose surfaces are inclined to the axis of rotation of the machine in order to follow the curvature of the flux lines 28.

Claims (8)

1. A D.C. homopolar machine having a drum rotor, rotor conductors mounted on the rotor, slip rings for conveying current to and from the rotor conductors, and first and second superconducting field coils arranged coaxially with the drum, wherein each field coil is composed of at least two axially separated sections whereby the curvature of the magnetic field of the field coils in the region of the slip rings is reduced.

2. A machine as claimed in claim 1 wherein the slip rings are mounted on the rotor in two sets, each rotor conductor being connected between a slip ring of one set and a slip ring of the other set, and the rings of each set being axially spaced along the surface of the rotor.

3. A machine as claimed in claim 2 in which each set includes slip rings at its ends with surfaces which are aligned with the magnetic field direction.

4. A machine as claimed in claim 2 comprising a ring of ferromagnetic material axially aligned with the outer end of each of the slip-ring sets to extend the uniformity of the magnetic field over the said outer end.

5. A machine as claimed in any of claims 1 to 4 wherein each field coil is in three sections of which the middle section is axially shorter than the end sections.

6. A machine as claimed in any of claims 1 to 5 comprising at least one ring of ferromagnetic material surrounding the gap between two axiallyseparated sections of field coil.

7. A machine as claimed in claim 1 or 2 comprising rings of ferromagnetic material coaxial with the field coils arranged to increase the uniformity of the magnetic field in the region of the slip rings.

8. A machine as claimed in claim 1 in which each field coil comprises multiple sections so sized and distributed axially as to reduce the magnetic field curvature in the region of the slip rings.