GB2059175A - Electrical generator - Google Patents

Abstract

The present invention relates to an electrical generator which comprises a magnetic flux-return member 13 and a disc-type winding 1 serving as a stator, and permanent magnets 6 serving as the rotor 2 and arranged to generate a magnetic field parallel to the axis of the winding. The magnets 6 and the flux- return member 13 are located on opposite sides of the disc-type winding 1 and the axial position of the flux-return member 13 is adjustable 12 to vary the magnetic flux gap between the magnetic flux-return member 13 and the magnets 6, and thus the generator out-put voltage. The adjustment may be effected by a screw or a cam mechanism, or automatically in response to the torque exerted on the stator by the rotor to provide an output regulated responsive to changes in rotor speed. As shown, the winding 1 and the flux- return member 13 are both mounted on an adjustment screw 9 and move together. <IMAGE>

Description

SPECIFICATION Electrical generator This invention relates to electrical generators, particularly alternators, having disc-type windings.

A known form of d.c. motor has a disc-type armature which rotates between permanent magnets filled to a stationary frame, the magnetic field running parallel to the rotational axis of the armature. Motors of this kind have high power/weight and power/volume ratios and low inertia.

An alternator could also be constructed using a disc-type winding and permanent magnets. However, a conventional method of controlling output voltage of an alternator involves the control of current flow in a field winding thereof and this method could not, of course, be utilised in such above-mentioned alternator.

An object of the present invention is to provide an electrical generator such as an alternator, having a disc-type winding and permanent magnets yet with which output voltage thereof can be controlled in a simple and effective manner.

According to the invention therefore there is provided an electrical generator comprising a magnetic flux return member; a disc-type winding serving as one of a stator and a rotor of the generator; at least one permanent magnet serving as the other of the stator and the rotor and arranged to generate a magnetic field parallel to the axis of said winding, said winding and the or each magnet being rotatable one relative to the other about said axis and the or each magnet and the flux return member being located on opposite sides of the disc-type winding; and means for adjusting the magnetic flux gap between the magnetic flux return member and the or each said magnet.

With this arrangement, advantages of cost and efficiency arising out of the use of the disc-type winding/permanent magnet combination can be obtained and, at the same time, voltage control can be effected in a simple and effective manner by adjustment of the magnetic flux gap.

In a preferred form of the present invention the generator is an alternator with the winding and the flux return member comprising the stator while the magnet or magnets are incorporated in the rotor.

Conveniently, the rotor is mounted directly on the drive output shaft of a prime mover (such as an internal combustion engine) so that bearings are not required in the generator itself.

With the disc-type winding a two orfour pole machine may be inefficient, whereby a 50/60 Hz generator operated from a standard 3000/3600 r.p.m.

or 1500/1800 r.p.m. internal combustion engine may be impractical. However, the generator of the invention may be particularly suitable for use in applications, particularly although not exclusively in an alternator-based welding set, in which an accurate 50/60 Hz output is not essential. N In such a welding set, an alternator according to the invention may be driven by an i.c. engine at any convenient speed since the generator frequency is, in general terms, unimportant although it should be fixed. The output of the alternator may be rectified with an inductor or inductors interpose between the alternator and the rectifier, such inductor or inductors acting to limit the short circuit current and to give desired current/voltage characteristics for welding.The requisite size and cost of the inductor or inductors would normally decrease with increase in frequency, and the number of poles used would preferably be selected to give optimum size and cost parameters for the equipment.

The rectifier may be omitted to provide an a.c.

welder.

It has been suggested in the welding art that a frequency of 400 Hz for an a.c. welder provides better results than 50 Hz and to this end a sixteen pole alternator according to the present invention would be suitable.

In the case where a generator according to the present invention is used in a welding set as described above or in any other application in which pre-adjustement of open-circuit voltage is required, the said adjustment means may comprise a simple mechanism, which may be manually operable or power operable and which acts, when operated, to move the magnet or magnets and the flux return member axially relative to each other to adjust the magnetic flux gap therebetween and hence to adjust the open circuit output voltage to suit requirements.

In one embodiment, the magnet or magnets are rotatable and the flux return member (and winding) is fixed, and a screw mechanism or a cam mechanism is connected to the centre of the flux return member for effecting axial adjustment of the position of same. Such mechanism may also provide the fixed mounting for the winding.

In the case where a generator according to the present invention is used in a context in which automatic regulation of the output voltage under load is required, this may be achieved by using a torque-sensitive device which responds to an increase in torque on the winding and flux return member as current is drawn from the winding in a fixed magnetic field, to effect a compensatory reduction in the spacing of the flux return member and magnet or magnets.In accordance with one embodiment, the winding and flux return member are mounted on an adjustment screw or cam mechanism such that angular adjustment of the position thereof in the direction of action of generated torque causes both the winding and flux return member to move axially towards the rotatable magnet or magnets, and a spring acts on the flux return member or adjustment mechanism to urge the former away from the magnet or magnets to a maximum gap position. With this arrangement, as the current in the winding increases the generated torque opposes the action of the spring and closes the gap thereby to increase the output voltage. To give the best speed of response without instability a damping device may be incorporated and this may be of a hydraulic, magnetic, pneumatic or other nature.

The torque-sensitive device may constitute the said adjustment device or may be provided additionaly to a said adjustment device which is of the kind as mentioned above, which can be used to pre-adjustthe open-circuit voltage.

With a generator according to the present invention, magnets may be positioned on opposite sides of the disc-type winding or alternatively and preferably one or more magnets may be provided on one side only. In the former case the magent or magnets positioned on one side of the disc-type winding serve as the flux return member. In the latter case the member is in the form of a flux return plate provided on the opposite side of the winding. In the case where the magnet or magnets rotate and the plate is axially adjustable, the plate may be fixed relative to or rotatable with the magnet or magnets as desired. Where the plate is fixed it may be mounted relative to the winding so as to be positionally adjustable together with same. The plate may be formed from iron, mild steel, sintered iron or laminations.

The generator of the invention may be a single or multi-phase alternator.

The invention will now be described further by way of example only and with reference to the accompanying drawing which is a diagrammatic sectional view of one form of an alternator according to the invention.

The alternator comprises a stator 1 and a rotor 2 mounted within a bell housing 3 secured to the casing of an internal combustion engine 4 around the drive output shaft 5 thereof. The rotor 2 comprises a plurality of permanent magnets 6 fixed to a structure 7 which is mounted on the shaft 5 so as to be supported thereby. The stator 1 is a disc-type winding having one face 8 thereof extending parallel to the magnets 6 and perpendicular to thee axis of the drive shaft 5. The stator is mounted at its centre on the end of a screw 9 which is coaxial with the shaft 5 and is passed through a threaded bore 10 in a wall of the housing 3. The screw 9 is secured, externally of the housing 3, to one end of an adjustment shaft 11 which has, at its opposite end, a radially extending manual adjustment handle 12.

From the foregoing it will be noted that the magnet assembly (i.e. the rotor 2 made up of the magnets 6 and structure 7) is supported solely to the drive shaft 5, and the stator 1 (i.e. the winding) is supported solely by the screw 9. No bearings therefore utilised within the alternator.

On the opposite side of the winding 1 to the magnet assembly 2 there is a flux return plate 13 in the form of a disc which is secured to the screw 9.

An air gap which forms the magnetic flux gap is maintained between the face of the reflux return plate 13 and the face of the magnets 6 and this air gap can be adjusted by rotating the screw 9, with the handle 12, so as to rotate the plate 13 (and the winding 1) to move same towards or away from the magnet assembly 2, the full range of axial move ment of the plate 13 preferably corresponds to rotation of the screw 9 through less than 360". The position of the winding 1 between the flux return plate 13 and the magnets 6 is not important provided, of course, that the winding 1 does not contact the magnets 6. The winding 1 can therefore be fixed to the plate 13 and move with it or fixed to the housing 3 and spaced from the plate 13.

The winding 1 has output terminals 14 accessible externally of the housing 3, and in the case where the alternator is to be used for welding purposes, these terminals 14 may be connected via an inductor or inductors and a rectifier, where included, to welding electrodes (not shown).

The open-circuit output voltage can be adjusted, or preset, by adjusting the magnetic flux gap between the magnet assembly 2 and the flux return plate 13 in steps or continuously with the handle 12.

In a modification of the embodiment described above, the handle 12 and shaft 11 are not used but instead a spring is located around the screw 9 and is secured relative to the screw 9 and to the bell housing 3 so as to act to rotate the screw 9 to a position corresponding to the maximum air gap between the plate 13 and the magnet assembly 2.

The direction of rotation of the screw 9 and the magnet assembly 2 are such that the torque generated on the plate 13 and winding 1 as the magnet assembly 2 rotates and current is drawn from the winding 1 opposes the action of the spring so that the magnetic flux gap is reduced and an increase in the output voltage of the alternator is produced which compensates for the effect of the loading thereof.

In the above-described embodiments the winding 1 may be potted in epoxy resin to form a solid annular slab which may be secured by suitable means such as bolts to the housing 3. With a copper strip winding of typically 8 mm thickness, a 1 mm depth of resin covering and 1 mm air gap on each side of the winding gives a total width of space occupied by the winding of 12 mm. If the 1 mm depth of resin covering is avoided to expose the cheeks ofthewindingsfor improved cooling the total width would reduce to 10 mm. The epoxy resin used should have a similar cooefficient of expansion to copper or be sufficiently elastic to compensate for expansion of the copper strip winding to reduce the possibility of failure or distortion in the winding being caused by stress from expansion and contraction of the winding.

In an alternative form of stator construction the winding is secured to annular outer and inner metal rings or, where induced current losses might prove troublesome, of GRP. This form of construction would fully expose the winding to provide improved cooling.

By providing suitable ventilation holes in the structure 7 of the rotor the latter may serve as a centrifrugal fan drawing air through suitably placed vents in the housing 3 and around the winding. The number and size of the vents in the housing can be chosen to provide a desired airflow through the housing. Alternatively or additionally the gap between the rotor and the winding may be reduced by adding non-magnetic material to the rotor in order to direct the maximum amount of air flow across the surface of the winding. Where the generator is used in welding apparatus it may also serve as a welding fume -extractor with the addition of suitable flexible ducting to conduct the welding fumes from the welding area to a fume collector, the ducting incorporating one or more suitable filters. Possibly over heating of the generator may be guarded against by use of thermal tripping devices. Alternatively, the rotor may incorporate vanes attached to the near face of the rotor to form a separate fan.

It is of course to be understood that the invention is not intended to be restricted to the details of the above embodiment and the modifications thereto which are described by way of example only.

Claims (10)

1. An electrical generator comprising a magnetic flux return member; a disc-type winding serving as one of a stator and a rotor of the generator; at least one permanent magnet serving as the other of the stator and the rotor and arranged to generate a magnetic field parallel to the axis of said winding, said winding and the or each magnet being rotatable one relative to the other about said axis and the or each magnet and the flux return member being located on opposite sides of the disc-type winding; and means for adjusting the magnetic flux gap between the magnetic flux return member and the or each said magnet.

2. An electrical generator as claimed in clam 1 wherein said rotor comprises said at least one permanent magnet and the magnetic flux return member is adjustable axially towards and away from said rotor to vary said magnetic flux gap.

3. An electrical generator as claimed in claim 2 wherein said disc-type winding is secured relative to said magnetic flux return member and displaceable axially therewith.

4. An electrical generator as claimed in claim 3 wherein said adjusting means comprises a device for sensing the torque applied to said disc-type winding by current flow in said winding and operable to adjust the position of the magnetic flux return member in dependence thereon.

5. An electrical generator as claimed in claim 4 further comprising means for damping displacement of said magnetic flux return member by said device.

6. An electrical generator as claimed in any of claims 1 to 5 wherein said adjustment means comprises a screw mechanism coupled to said magnetic flux return member.

7. An electrical generator as claimed in claim 6 wherein said screw mechanism is manually actuable.

8. An electrical generator as claimed in any of claims 1 to 7 wherein said magnetic flux return member comprises at least one permanent magnet.

9. An electrical generator substantially as hereinbefore described with reference to the accompanying drawing.

10. An electrical generator substantially as hereinbefore described with reference to the drawing accompanying the specification of patent application No. 7930800.