GB2027284A - Improvements in electrical generators - Google Patents

Abstract

A generator, e.g. an alternator driven by a vehicle engine, has a rotor (10) capable of retaining a small degree of remanent magnetisation. The stator has a winding or windings (11, 11<1>) in which an emf is induced by the remanent magnetisation to provide a current e.g. to charge a battery 16 through a rectifier arrangement (not shown). The battery 16 is arranged to apply current to other windings (14, 14<1>, 15, 15<1>) at an appropriate rotor position to increase the rotor magnetisation, the application of this magnetisation-increasing current being controlled by an element responsive to the position of the rotor. As shown, the rotor (10) carries a small magnet (12) to operate a reed-relay (13) to allow current to flow as desired in the magnetisation- increasing windings (14, 14', 15, 15'). The reed-relay limits current flow to short intervals so that demagnetisation of the rotor is avoided. <IMAGE>

Description

SPECIFICATION Improvements in electrical generators The present invention relates to self-excited electrical generators, and particularly, but not exclusively, to alternators for use in aircraft and automobiles.

Some of the disadvantages of known alternators are that special arrangements are needed to excite the rotor, usually in the form of a d.c. supply current from which passes by way of brushes to windings for magnetising the rotor. Brushes and slip-rings wear unduly at altitude, and some alternators have avoided the problem by using rectifiers mounted on the rotor. This constitutes a hazard, since it is difficult to check or replace such rectifiers during flight and failure of the electrical supply in some modern aircraft leads to disaster.

In UKPS 1171541 there is described and claimed an electrical generator, including a rotor, and two or more circuits, the second circuit being so positioned that a current flowing therein, as a direct or indirect result of an e.m.f. induced in the first circuit on turning the rotor, enhances the magnetisation of the rotor which induced that e.m.f. and the said circuits including current-control means which allow the occurrence of an e.m.f. induced in the first circuit to cause current to flow in the second circuit but prevents substantial currents flowing in the first circuit, in a direction which would tend to demagnetise the rotor, as a result of e.m.f. induced in the second circuit. Such a generator is herein referred to as "of the type described".

According to the present invention there is provided a generator of the type described including a current-control means having an element responsive to rotor position to interrupt the circuit path including the second, magnetisation enhancing, conductor except when the rotor is positioned for such magnetisation enhancement.

The element responsive to rotor position may be a sensor responsive to a marker on the rotor shaft. The marker may be a magnet or body of magnetisable material or a radiation reflector e.g. for light. The element may be reed-relay operated directly by said magnet or a magneto-resistor or opto-electronic device as appropriate.

The interruption may be a separation of switch contacts, as for reed-relay, or the introduction of a very high impedance element as for the magneto resistor or opto-electronic device. The generator may include means to trim the operation of the element responsive to rotor position for optimum operation.

The generator may have associated there with an accumulator charged and replenished by current caused to flow in the first circuit by the e.m.f. induced therein on turning the rotor, to thereby supply the magnetisation enhancing current for the second circuit.

Embodiments of the invention are described by way of example, with reference to the accompanying drawings in which Figure 1 is a simplified schematic diagram of the basic form of a first embodiment of the invention, and Figure 2 shows trimming arrangements for reed-relays.

In Fig. 1, a salient pole rotor 10 of an alternator (not shown) is not a permanent magnet but has small remanent magnetisation, assumed to be of the polarity shown. On rotation of the rotor by a suitable drive, such as a coupling from an aircraft engine, an electromotive force (e.m.f.) is induced in the conductors 11 and 11'. These conductors can be formed into a single turn coil in a first circuit to deliver the output of the alternator, through a rectifier (not shown), to charge an accumulator.A small permanent magnet 1 2 is attached to the rotor shaft and for a few degrees of rotation around the illustrated position it actuates a reed-relay switch 1 3. This allows a burst of current from a cell 1 6 of the accumulator to flow through control resistor 1 7 and the conductors 14, 14', 15, 15' together forming a second circuit. The current in the second circuit increases the magnetisation of the rotor in the sense shown. since the induced e.m.f. depends on the speed of the shaft as well as the degree of rotor magnetisation, the process can charge and replenish the accumulator provided the apeed is high enough even with remanent magnetisation. It has been found that replenishment occurs at practical speeds.The reed-relay prevents subsequent rotor de-magnetisation by the isolation of the circuit containing conductor 11 from that containing conductor 1 4. The rectifier prevents the accumulator energising the first circuit. Suitable accumulators for aircraft use are readily available.

Instead of a single replenishment during each revolution, several complete circuits of control resistor, cell and reed-relay can be arranged around the stator.

Instead of a reed-relay to perform the switching, a magnetoresistor or opto-electronic switch, which have no moving parts, may be substituted.

An alternator functioning in this way may, of course, have a plurality of coil pairs similar to 11, 11' and 14, 14' and a plurality of salient poles. A separate output winding may also be used.

Fig. 2 shows an arrangement for an alternator having several complete circuits for replenishment and/or magnetisation. An end plate 20 of the alternator is attached by lock-screws 21, 22 extending through respective arcuate slots 23, 24 to permit adjustment of the plate 20 by rotation around the rotor shaft 25. Four reed-relays, A, B, C, D are mounted on plate 20 by individual adjusters 26, 27, 28, 29 which permit the reed-relays to be moved radially with respect to shaft 25. The adjusters may include screw threads engaged with plate 20 in a manner not shown. Magnet 1 2 on shaft 25 can act on each reed-relay in turn, as described above.The length of time for which each relay is actuated by magnet 1 2 (dwell) is adjusted using the radially-movable adjusters while the actual times in a rotor revolution at which the relays are actuated can be adjusted by the rotation of plate 20 on loosening lockscrews 21, 22. In this way the incremental magnetisation of the rotor and replenishment of the accumulator can be optimised by "trimming" the exact time and "dwell" of the operation of the reed-relays during the revolution of the rotor.

The exact details of the connections for charging and replenishing the accumulator cells are not shown as suitable arrangements will be apparent to those skilled in the art from the above description of the required operation. Clearly the form of the coils in the first circuit determines the charging rate and that of the coils in the second circuit the replenishment of magnetisation current required. The inductance of the coils should also be considered in preparing the accumulator network to achieve balanced operation. In one network individual cells of an accumulator were assigned to individual second circuits, the accumulator being charged and replenished by a first circuit including all the cells in series.This arrangement ensured an adequate current in each second circuit as it was completed in turn by the action of the rotating magnet 1 2 on a respective reed-relay.

In another embodiment the first circuit in clouded two output windings in the same slot, instead of one winding of the same total number of turns. This improved the output available for charging the accumulator. Remanent magnetisation retention of the rotor was improved when a thick copper strip was placed around the rotor.

In comparison with a brush and slip-ring alternator brush friction was eliminated, reducing the time for the alternator to reach operating speed while the excitation current was substantially reduced from that required with external excitation. The life time or reedrelays suitable for aircraft use can be in the order of 109 operations. This would permit operation for up to a few thousand hours before failure is to be expected. Capacitors may be included to control arcing at the relay contacts. Replacement of the control gear is simple and mechanical wear would not occur as no bearing surfaces are involved.

The use of reed-relays or other devices as mentioned to actually or effectively interrupt the second circuit eliminates or greatly reduces the demagnetisation of the rotor. The ability to limit the flow of magnetisation current to a small controlled fraction of a rotation by relatively quick switching action instead of relying in the decay of capacitive charge or other relatively slow electronic means provides an improvement in performance and precision of operation compared to the control circuits disclosed in UKPS 1171541. The use of electromechanical devices or a mechanical actuator, in contrast to the general trend to solid state devices, produces an improved operation. The improved arrangement can be applied to the various forms of rotor and stator constructions described in UKPS 1171541.

Reed relays can be arranged to provide the gyrator action described earlier by cross-coupled connection of four relays with a centre tapped 1 8v dc. source and by providing four reed-relays operated by the shaft as described above to selectively connect the coils of the cross-coupled relays to the source.

Claims (9)

1. An electrical generator of the type described including a current-control means having an element responsive to rotor position to interrupt the circuit path including the second, magnetisation enhancing, conductor except when the rotor is positioned for such magnetisation enhancement.

2. A generator according to Claim 1 in which the element is a reed-relay and is responsive to a magnet mounted on the rotor shaft at a particular angular position.

3. A generator according to Claim 1 in which the element is a sensor responsive to a marker on the rotor shaft corresponding to the position for magnetisation enhancement.

4. A generator according to Claim 3 in which the sensor is a magneto-resistor or opto-electronic device with an appropriate marker on the rotor shaft to cause the sensor to have a low impedance to magnetisation current at said magnetisation enhancement position of the rotor.

5. A generator according to any one of Claims 1 to 4 in which the magnetisation conductor is energised from an accumulator, or cell thereof, charged and replenished by current in the first circuit to store electrical energy for supply to the second conductor only when said element does not interrupt the circuit path including the second conductor.

6. A generator according to Claim 5 including an accumulator, individual cells of which are charged through respective rectifiers from individual windings of the generator, the cells being permanently series connected to energise the magnetisation conductor through the interceptable circuit path.

7. A generator according to any one of the preceding claims including means to trim the operation of the element responsive to rotor position for optimum operation.

8. A generator according to any one of the preceding claims for use as an engine-driven self-excited alternator or a self-propelled vehi cle or craft providing brush-free operation with rotor magnetisation enhanced only at intervals timed by the rotation of the rotor.

9. A generator substantially as herein de scribed with reference to Fig. 1 or Fig. 2 of the accompanying drawings.