GB2062977A

GB2062977A - Alternator

Info

Publication number: GB2062977A
Application number: GB8034835A
Authority: GB
Original assignee: Northern Engineering Industries PLC
Current assignee: Rolls Royce Power Engineering PLC
Priority date: 1979-11-10
Filing date: 1980-10-29
Publication date: 1981-05-28
Also published as: GB2062977B

Abstract

An alternator, e.g. for direct driving by wind or water, has either a permanent magnet field 14 or is self- exciting, the alternator having at least 12 poles and having a core in slots in which a winding is located. It may have two contra-rotating rotors 12, 26 and a compensating field coil 20. For single- phase working the winding is a sinuous multi-turn coil laid into the slots of differing pitch and cross-overs are not necessary. For multi-phase working a similar winding system may be used or a conventional winding. Where permanent magnets are used the flux in the air gap is preferably kept constant by using tapered claw members or skewed slots. In Figure 5, a rotating field comprises tangentially magnetised permanent magnets 60 interspersed by wedge-shaped poles 62. <IMAGE>

Description

SPECIFICATION Alternator The invention relates to alternators.

An alternator according to the invention has two relatively rotatable parts at least one of which is a rotor, the alternator having at least twelve poles and a field system which enables the alternator to be self-starting.

Examples of alternators (all of which have two rotors for direct coupling to respective sets of oppositely handed blades of a windmill or watermill) will now be described with reference to the accompanying drawings in which: Figure 1 is a longitudinal section through the alternator; Figures 2 and 3 are similar sections showing details of two forms of inner rotor; Figures 4, 5 and 6 are scrap views partly in section showing further details; Figure 7 is a development of the outer rotor showing the winding; Figure 8 is a development of the outer rotor showing the relationship of outer rotor poles to the teeth of the inner rotor; Figures 9 and 10 are circuit diagrams showing two basic circuits for the alternator; and Another embodiment of alternator is shown in Figures 11, 12 and 13, which are views similar to Figures 1,4 and 5, respectively.

One alternator developed to date has 42 poles, and the excitation is provided by permanent magnets 14 (see Figures 1,2 and 3). The general construction of the inner rotor 12 is similar to a type known as a "Lundell" type rotor, which the field of the permanent magnets 14 is essentially axial. The flux trapped in collector rings at each end is transmitted to the air gap 16 by means of claws 18 equally disposed around the periphery. A similar type of construction is used in some forms of eddy current clutch and brake units wherein the magnetic field is generated by means of the coil in the form of a torus laid within the embrace of the claws.

We have found that it is possible to improve the performance of a permanent magnet rotor of this type by including a toroidal compensating coil 20 wound and connected to assist the permanent magnets 14. The compensating coil 20 is fed from the d.c. terminals of a bridge rectifier 22 inserted in one of the alternator output lines (see Figures 2 and 10). The compensation is therefore proportional to the load current and reduces the regulation of the machine by counteracting the de-magnetising effect of the load current in the winding 24 of the outer rotor 26.

A serious disadvantage of the multiple claw rotor design is that excessive flux leakage occurs owing to the large area of the interpole gaps. The main advantage of this form of construction is that it is possible to magnetise the permanent magnets 14 in situ after assembly.

The winding process for a machine with a very large number of poles and small physical size is tedious and labour intensive if the conventional basket pattern is used because of the very large number of coils and interconnections which are required.

In this alternator a large coil with the appropriate number of turns is first wound and then the coil is formed into a sinuous shape and positioned in the slots. In this example a compromise between the size of the coil and its handleability dictated that the machine should be wound with six coils grouped in pairs, each pair embracing 14 poles. It is advantageous to connect all 6 coils in series to eliminate circulating currents between coil groups if the machine is not perfectly "centred". This form of winding has a particular advantage when winding machines intended for single-phase output. By long and short pitching of the coils of this type of winding a complete winding without cross-overs can be formed. The winding thus has a very short overhang so that the stray reactance of the machine and the amount of copper required are reduced.When the coils are full-pitched, cross-overs are then necessary.

The same comment applies for multiphase windings with full-pitched coils.

Figure 7 shows two such coils 30,32. The ends A of the coil 30 shown in the development are of course in reality joined, the coils being continuous.

The same is true of the ends B of the coil 32. The start of the coil 30 is shown at 34 and the finish 36 of the coil 30 is connected to the start 38 of the coil 32. The finish of the coil 32 is shown at 40.

A pole pitch is represented at 42.

In each coil the coil is laid first in two slots separated by a single slot as at the slots 50, 52, for example (a short pitch) and then in two slots 52, 54, separated by three slots (a long pitch).

The two coils 30,32 form one of three pairs of coils. Each pair embraces 14 poles, the total being 3 x 14 i.e. 42 poles. All six coils are connected in series as mentioned above.

In a permanent magnet excited alternator running at very low speeds from standstill, there is a tendency for the inner rotor poles to "lock" to the teeth of the outer rotor. It is necessary to design the inner rotor pole shape in relationship to the teeth of the outer rotor so that the air gap flux remains constant at all times. In this example the claws of the inner rotor are tapered so that they embrace a constant tooth area in all relative angular positions of the rotors (see Figure 8). A similar effect could be obtained in an alternative construction by a skew arrangement of the slots in the outer rotor, so that a full slot pitch of skew was "seen" by the inner rotor poles. If these measures are not adopted the machine rotates at an uneven speed, inducing severe torsional oscillations at the root of the propellor blades with consequent risk of fatigue failure.

In another alternative design for permanent magnet rotors a major proportion of the flux leakage can be avoided. The magnets 60 are aligned with their magnetic axes tangetial to the rotor in groups opposing each other, with mild steel wedge-shaped pole pieces 62 interposed between the magnets to form a complete circle. The broad edges of the pole pieces 62 are presented in the air-gap of the machine to form the pole surfaces alternately north and south as required for electro-magnetic generation. This is shown in Figures 4, and 6. The assembly is clamped between part of an aluminium hub 70 and an aluminium ring 72 by mild steel bolts 74. It is necessary that power be available from the machine over a wide range of wind speeds.It is advantageous to control the machine output so that the machine operates at a substantially constant current set at a value close to the maximum current rating of the machine. This allows greater power than the rated power of the machine to be extracted when winds are very strong but the frequency and voltage both rise with the wind speed. Provided the load 100 (Figure 9) is suitably arranged and control properly organised, variation of voltage and frequency do not cause any problems. Suitable loads are of a resistive nature, such as, immersion water heaters or resistance air heaters.

The power available from the wind is proportional to the cube of the wind speed and the notional power of the permanent magnet alternator is proportional to the wind speed. The possbility arises of making the machine self-exciting and disposing of the permanent magnets (Figure 3). The advantage of this design of machine would be that, with "series" excitation, the output power would be approximately proportional to the square of the speed up to the point where the load control system became operative. With constant current load control, the output would be proportional to the wind speed.

Alternatively, where the user wished to operate the machine at a constant voltage, i.e. to supply a large lighting load using incandescent filament lamps, the machine could be arranged to be selfexcited with the "shunt" type of excitation winding having an "on board" amplifier and control circuit to maintain the output voltage constant. In other words the amplifier and control circuit are mounted on one of the rotors of the alternator.

Considering the utilisation of the materials in the construction of the machine, it would be advantageous to wind the machine for three-phase operation, leading to some 58% better output for a given quantity of copper and iron. In orderto ruse a two-wire transmission system, the three-phase supply would be rectified "on board" and transmitted as d.c. to the load point. Such an arrangement could equally well apply to either permanent magnet or self-excited machines, or to a compound design, as required.

Figure 3 shows a detail of the inner rotor where the permanent magnets 14 are for the most part replaced by soft iron pole-pieces 80 and a "series" excitation winding 82 is provided as mentioned above. In such a modification some initial excitation would be provided by a small permanent magnet.

In Figures 1,2 and 3 there are also shown the following parts: The fixed shaft is shown at 200 carrying roller bearings 202,204, supporting the outer rotor 26, and roller bearings 206,208 supporting the inner rotor 12.

Connections to the winding 24 of the outer stator 26 are made through fixed slip-rings 210,212,214 engaged by brushes 215,218,220, respectively, carried by the outer rotor 26. A further slip-ring 215 and brush 217 would be provided for three-phase operation.

The ring 214 is an earth connection. Insulation is provided at 219. The winding 20 or82 (Figures 2 and 3 only) is connected to one side of the output from the winding 24through sliprings 222,224 carried by the inner rotor 12 and engaged by brushes 226,228, respectively. The rectifier bridge shown in Figure 10 is omitted from Figure 2 for simplicity.

The outer rotor 26 carries wind propellor blades (not shown) attached to brackets such as the bracket 250 secured to the rotor.

The inner rotor 12 has similar brackets 252 to which its wind propeller blades are secured.

The blades are oppositely handed so that the rotors rotate in opposite senses on the shaft 200.

The propellor and alternator assembly is mounted on a mast (not shown) and can rotate about a vertical or near vertical axis so as to face the wind whatever its direction. The sliprings 215,210,212 (Figure 2) are connected to sliprings (not shown) forming part of the propellor and alternator assembly and power is picked up by fixed brushes (not shown) engaging the sliprings and carried by the mast at its upper end.

The outer stator 26 has a laminated core 300 of electric steel stampings in which the slots are formed for the winding 24.

The construction shown in Figure 7 is that used when there is one slot per pole per phase i.e. 3 slots per pole pitch.

In other constructions, where there is a greater number of slots per pole pitch available, the coils would occupy a spatially similar relationship. For example if there are 6 slots per pole pitch, a total of four coils would be used for a single-phase output.

The first coil would miss two slots and then eight slots. The second coil would miss four slots and then six slots. The third coil would miss six slots and then four slots; and the fourth coil would miss eight slots and two slots.

In general, single phase machines are wound as for two phase working, as indicated in Figures 9 and 10 conforming with known normal practice to better utilise the materials of the machine.

Figures 11, 12 and 13 shows the inner rotor 300 as : comprising a main component 302 carrying rings 304,306 between which the poles 308 are retained.

Each pole 308 is made up of stamped laminations which are assembled and held in a press while a rod 310 extending through the stack is welded at each end at 312. In addition, a bolt 314 is passed through the stack and nuts are tightened at each end.

The poles 308 are progressively assembled around the rotor 300 in relation to the ring 306, which has a groove 316 to receive the nuts at ends of the bolts 314, and ceramic magnets 318 are interposed between the poles 308. The other ring 304 is positioned as shown and nuts 320 are tighened on studs 322 to hold the assembly together.

Alternatively the mounting rings 304 and 306 may be of a non magnetic material such as stainless steel manufacture in sheet form machined accurately to locate on the ribs of the inner rotor hub 302 and precision drilled to provide accurate location for studs passing lengthwise through the lamination pack at 310 and 314. The whole assembly then secured by tightening nuts at either end of the two studs. The drive connection between hub and rotor would as before be provided by studs 322.

As shown in Figure 13 the magnets 318 are arranged so that adjacent magnets have similar adjacent polarities. For example two south polarities adjacent one another with a pole 308 between those polarities.

The outer ends of the poles 308 have successively alternate north and south polarities, as shown.

Figure 11 shows the inner rotor 300 in position within an outer rotor 330 which comprises core plates 332 and a winding 334. In this case the core plates 332 provided skewed slots between poles (not shown) analogous to the skewed slots referred to above.

The winding here is a basically conventional winding typically for three-phase operation. Alternatively, for single-phase or two-phase operation, the winding may be as described in relation to Figure 7, for example.

Typically, there are 42 poles in the inner rotor shown in Figures 11 to 13 for an output of 7.50 kilowatts at 50 hertz at a relative rotational speed of the rotors at 142.80 revolutions per minute.

Alternatively, at faster running there may be 16 poles on the inner rotor, for example.

Outputs may range in general between 3 kW and 25kW.

The other details shown in Figure 11 are generally similar to those described in relation to Figure 1 above and need not be repeated.

Although the alternators described above all have two oppositely rotatable rotors, the invention is also applicable to alternators in which only one of the rotors is rotated. Instead of wind propellor blades, the rotor or rotors may be driven by water propellor blades or by some other form of power for example a petrol or diesel engine or steam orwater or air turbine.

Claims

1. An alternator having two relatively rotatable parts at least one of which is a rotor, the alternator having at least twelve poles and a field system which enables the alternator to be self-starting.

2. An alternator according to claim 1, in which one of the parts comprises poles formed of laminations there being a permanent magnet between every two adjacent poles, the adjacent polarities of adjacent magnets being similar and the polarities of outer ends of successive poles being dissimilar.

3. An alternator according to claim 1, in which between every two adjacent permanent magnets there is a tapered pole-piece.

4. An alternator according to any preceding claim, in which the permanent magnets are of ceramic material.

5. An alternator according to any preceding claim, in which in one of said parts the field of the permanent magnets is parallel to the axis of relative rotation of said parts.

6. An alternator according to claim 5, in which flux is trapped at each end of the rotor in collector rings and is transmitted to an air gap by claws equally spaced about the rotor.

7. An alternator according to claim 2 or any claim dependent on claim 2, in which a toroidal compensating coil wound and connected so as to assist the permanent magnets, the coil being energisable from rectifier means fed by an output from the alternator.

8. An alternator according to claim 1 or claim 2, in which a multi-turn coil of sinuous shape is positioned in slots in one of said parts.

9. An alternator according to claim 1 or 2, in which two or more multi-turn coils each of sinuous shape are positioned in slots in one of said parts.

10. An alternator according to claim 9, in which the coils are grouped in pairs, each embracing several pairs.

11. An alternator according to claim 7 or claim 10, in which the coils are connected in series.

12. An alternator according to claim 9, 10 or 11, in which the coils are of differing pitches so as to avoid cross-overs.

13. An alternator according to claim 12, in which successive locations of each coil in slots are separated by different numbers of slots so as to provide a shorter pitch followed by a longer pitch.

14. An alternator according to claim 13, in which there are six coils in three pairs each pair embracing 14 poles making 42 poles in all.

15. An alternator according to any preceding claim, in which one of said parts comprises tapered members which embrace a constant tooth area in all relative angular positions of said parts.

16. An alternator according to any claim of claims 1 to 14, in which one of said parts comprises members arranged in skew fashion so that a full slot pitch of skew is seen by the poles of the other of said parts.

17. An alternator according to any preceding claim, in which both said parts are rotatable and are connected directly to respective sets of oppositely handed blades.

18. An alternator according to claim 17, in which the alternator core carried by one of said parts surrounds the poles carried by the other of said parts.

19. An alternator according to claim 1, which is self-exciting one of said parts prising a core and winding means and the other of said parts comprising soft iron pole-pieces surrounded by an excitation winding arranged within an annular assembly of claw members.

20. An alternator according to claim 2 or any claim dependent on claim 2, in which said parts define an air gap in which the flux is constant.

21. An alternator according to claim 1 substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

22. An alternator according to claim 1 substantially as hereinbefore described with reference to Figures 1 and 3 of the accompanying drawings.

23. An alternator according to claim 1 substantially as hereinbefore described with reference to Figures 4,5 and 6 of the accompanying drawings.

24. An alternator according to claim 21,22 or 23, substantially as hereinbefore described with further reference to Figure 7 of the accompanying drawings.

25. An alternator according to claim 21,22 or 23 substatially as hereinbeforedescribed with further reference to Figure 8 of the accompanying drawings.

26. An alternator according to claim 1 substan tialiy as hereinbefore described with reference to Figures 11, 12 and 13 of the accompanying drawings.

27 An alternator according to any preceding claim substantially as hereinbefore described with reference to Figure 9 of the accompanying drawings.

28. An alternator according to any claim of claims 1 to 26 substantially as hereinbefore described with reference to Figure 10 of the accompanying drawings.