GB2370424A

GB2370424A - Varying air gap alternator

Info

Publication number: GB2370424A
Application number: GB0031628A
Authority: GB
Inventors: Michael Inman; Ian Ashley Hunt
Original assignee: AEA Technology PLC; Accentus Medical PLC
Current assignee: Accentus Medical PLC; Ricardo AEA Ltd
Priority date: 2000-12-22
Filing date: 2000-12-22
Publication date: 2002-06-26
Also published as: GB0031628D0

Abstract

Unwanted ripple on the output voltage is reduced in an alternator 11 of salient pole type by setting the centre of curvature of the stator pole surface non-coincident with that of the rotor pole surface to provide for a smooth buildup, collapse and reversal of magnetic field in the stator pole components.. The stator may carry more than one winding to provide a plurality of outputs, voltage regulation associated directly with one output serving to regulate the other outputs by virtue of magnetic coupling.

Description

Electrical Generator

The invention relates to an electrical generator and more particularly to an electrical generator comprising an alternator of the salient pole type for use in motor vehicles.

The conventional alternator used on motor vehicles provides an output at a nominal 14 volts. In the automotive industry the principal battery technology is currently the lead-acid battery system with a nominal 12 volt output and open circuit voltage of 13.2 volts. The charging voltage required for a lead-acid battery is temperature-dependent but typically varies between 13.613.8 volts, that is a nominal 14 volts due to the open circuit voltage of the battery. The industry adopted terminology refers to battery voltages in multiples of the nominal 12 volts and to alternator outputs and charging voltages as multiples of the nominal 14 volts.

It will also be appreciated that as battery technology develops higher voltage standards such as 42 volts or 56 volts for charging 36 volt and 48 volt batteries respectively will be introduced. Current battery systems can be referred to as 12V/14 V systems. The demands on the alternator have steadily increased as the number of components requiring electrical power has increased and some of these, in particular, require high power such as electrically heated windshields and seats. For a development such as a non-thermal plasma reactor for treating exhaust gases to remove noxious components, there is a still further demand for electrical power and, in this particular case, the voltage is required to be of the order of kilovolts to tens of kilovolts.

As the demand upon the alternator increases, so the importance of its efficiency also increases, particularly

in view of the concern to reduce fuel consumption. It is noteworthy that improved efficiency has a double effect in that an alternator of low efficiency generates more heat and requires appropriate forced gaseous or liquid cooling by for example air, oil or water, this in itself causing a power drain on the engine.

There is also an emerging need to provide an electrical generator capable of supplying electrical outputs at more than one voltage and, in particular, at a nominal 14 volts for operating conventional motor vehicle equipment designed for operation at this voltage and at a higher voltage such as 42V for equipment requiring higher power. Use of a higher voltage offers a saving both in reducing power loss through the alternator windings and connecting cabling and also in that smaller diameter cabling can be used. This is because for a given alternator power output a higher voltage results in a lower current and power loss is given as the product of the square of the current and resistance of the windings.

Lower electrical losses are associated with a reduced requirement for forced cooling.

The low efficiency of existing generators for use on vehicles is illustrated in the leaflet'ISAD: A New Approach to Energy Engineering', ISAD 9.98 (Pe), published by ISAD Electronic Systems, Koln, Germany and also in Figures 13 and 14 in the article'A Vehicle Electric Power Generation System with Improved Output Power and Efficiency'by Liang et al published in the 1998 IEEE Industry Applications Conference, volume l, pep 50-56,1988. In principle, improved efficiency might be expected by adapting the salient pole type of alternator for use as an alternator in motor vehicles. However, we have found that such an alternator, particularly when used for multi-phase alternating current generation

superimposes a serious ripple on the output voltage which introduces problems for both rectification and voltage step up.

It is an object of the present invention to provide an electrical generator in which these problems are addressed.

The invention provides an electrical generator comprising a stator having a plurality of radially projecting pole components of magnetisable material with windings thereon, a rotor having a plurality of pole components radially extending such as, on rotation of the rotor, to pass sequentially by and in facing relationship to the pole components of the stator, the pole components of the rotor also being of magnetisable material with windings thereon, means for rotating the rotor relative to the stator, the rotor and stator windings being appropriately connected to provide an electrical voltage output when there is relative rotation between rotor and stator, wherein the respective centres of curvature of the facing surfaces of the pole components of the rotor and the stator are spaced apart.

It is important for there to be an adequate circumferential extent of the facing surfaces of each rotor pole component relative to the circumferential extent of the facing surfaces of the stator pole components. For reducing ripple on rectified output voltage we have found it desirable that the circumferential extent of the facing surfaces of each rotor pole component corresponds substantially to the circumferential extent of two facing surfaces of adjacent stator pole components together with the gap therebetween and the gaps on each side thereof.

We have found that voltage ripple is further reduced by arranging that the radius of curvature Ro of the outwardly facing surfaces of said facing surfaces is less than Ri-G, where Ri is the radius of curvature of the inwardly facing surfaces of the said facing surfaces and G is the minimum width of the gap between the said facing surfaces.

Conveniently, the rotor is inside the stator so that the said outwardly facing surfaces are on the rotor and the said inwardly facing surfaces are on the stator.

A valuable feature of this form of construction is that the rotor can be made of laminated construction to reduce loss through eddy currents, and similarly, the stator can also be made of laminated construction to further reduce loss through eddy currents.

Also, the windings can be arranged to generate multi-phase alternating current outputs such as a three phase output.

Preferably for this, the rotor has four or more circumferentially equispaced pole components arranged in multiples of two pole components with windings such that when current is passed through the windings the magnetic polarity of one pole is opposite to that of the next circumferentially adjacent rotor pole, and the stator has twelve (or more) circumferentially equispaced pole components.

We have appreciated the importance of control of the flow of magnetic flux from the rotor pole components into the stator pole components and smooth transfer as rotor and stator rotate relatively. For this purpose, the

stator and rotor pole components taper sharply from their respective facing surfaces towards a stem portion, the circumferential extent of which is narrower than that of the respective facing surfaces and onto which stem portions the windings are wound.

To provide electrical outputs at more than one voltage, the stator carries a plurality of separate windings each of which has a different number of turns around the pole components to provide a corresponding plurality of different outputs at different voltages.

A voltage regulator connected to one of the outputs, serves to regulate all the voltage outputs by virtue of the close magnetic coupling between the respective windings.

A specific construction of electrical generator embodying the invention will now be described by way of example and with reference to the drawings filed herewith, in which: Figure 1 is a longitudinal sectional view of the generator which is a three phase dual voltage alternator, Figure 2 is rear end view of the generator, Figure 3 is a diagrammatic plan view of a rotor lamination and a stator lamination, showing generally their position relative to one another, but, for ease of illustration, showing a larger gap between rotor and stator than is adopted in practice, Figure 4 is a part sectional view showing magnetic field lines in a rotor and stator lamination (with their separation drawn to scale in this Figure) at a particular

relative rotational disposition, and Figure 5 is a graphical representation of rectified electromotive force (EMF) against electrical angle for generators having a number of different rotor configurations.

Referring to Figure 1, the alternator 11 of this example comprises a laminated rotor 12 fixed between washers 13 and 14 on a shaft 15 mounted in a respective front bearing 16 and rear bearing 17. The rotor 12 supports coil windings 18. Closely surrounding the rotor 12 is a laminated stator 19 which supports a dual set of three phase coil windings 21.

A finned cover box 22 at the rear end of the alternator 11 provides an enclosure and heat dissipation for the components within which include various connection stubs and a rectifier unit. Within an inner enclosure 23 inside the cover box 22 is mounted a brush box 24 with brushes which operate in conjunction with insulated slip rings 25 mounted on the rear end of the shaft 15.

The rotor 12 comprises a stacked assembly of rotor laminations 26 a plan view of which is shown in Figure 3.

Each lamination 26 is electrically insulated from adjacent laminations by a thin electrically insulating layer of, for example, varnish. It will be seen that the stack of laminations 26 forms four pole components one of which is indicated at 27 in Figure 3. Each pole component has a stem portion 28 around which the windings 18 are wound, the sense of the windings being such that the magnetic polarity reverses from one pole component to the next adjacent pole component.

The stator 19 also comprises a stacked assembly of stator laminations 29, a plan view of which is also shown in Figure 3. The alternator 11 of this example is for providing outputs in three phase alternating current supply, so the stator laminations 29 are configured to form 12 pole components of which one is referenced 31 in Figure 3. Each pole component 31 has a stem portion 32 on to which the coil windings are wound. The windings encircle the pole components in successive groups of three, the sense of the windings reversing on each successive group of three pole components, and the groups being successively displaced by one pole component position for each of the three phases.

Figure 3 shows the relationship between Ro, Ri and G where Ro is the radius of curvature of the outwardly facing surfaces, Ri is the radius of curvature of the inwardly facing surfaces and G is the minimum width of the gap between the facing surfaces.

It will be seen from Figure 4 (described more fully below) that the pole components of the rotor 12 and stator 19 have facing surfaces 33 and 34 respectively, those, 33, on the rotor being outwardly facing and those, 34, on the stator being inwardly facing.

Figure 4 shows the rotor 12 in a symmetrical position relative to the stator 19 so that the magnetic field from one rotor pole component (say North) passes at maximum concentration into the stator pole component adjacent which the rotor pole component is temporarily centered, divides and returns around the periphery of the stator 19 and back into the rotor pole components (South) which are perpendicular to the North pole component. As the rotor moves on round, the magnetic field in the

stator pole component in which it was just at maximum collapses (and eventually reverses) whilst that in the next adjacent pole component builds to a maximum, and so on.

The shape of the pole components both on the rotor 12 and the stator 19 are critical in determining how the magnetic field in the stator pole components builds, collapses and reverses as the rotor 12 rotates. A graduated rather than sudden transition is promoted by arranging for the outwardly facing surfaces 33 on the rotor pole components to have a circumferential extent corresponding to that of the inwardly facing surfaces of two adjacent stator pole components taken together with the gap between them and the gap on each side. This is discussed further below in connection with Figure 5. The smoothness of the transition is further enhanced by the configuration as seen in cross-section in which the pole components taper sharply from their facing surfaces to the region of their stems 28 and 32. As may be seen in Figure 4, this promotes transfer of a fringing magnetic field into stator pole components adjacent to those at which rotor pole components are centred. However, the ideal is to approach a sinusoidal form to the buildup, collapse and reversal of magnetic field in the stator pole components. We have found a significant improvement in this respect is achieved by arranging for the radius of curvature of the outwardly facing surfaces 33 on the pole components of the rotor 12 to be less than that which would make the surface concentric with the inwardly facing surface 34 of the stator pole components. Put another way, the respective centres of curvature are spaced apart and the radius of curvature Ro of the outwardly facing surfaces of the pole components of the rotor 12 is less than Ri-G where Ri is the radius of

curvature of the inwardly facing surfaces of the pole components of the stator 19 and G is the size of the minimum gap between rotor pole component and stator pole component. The effect, as can be seen from Figures 3 and 4, is that the gap between the respective rotor and stator pole components is at a minimum at the centre of the rotor pole component and increases towards each periphery of the rotor pole component. with this configuration we have found undesirable ripple effects on the output voltages are reduced to an acceptable level.

This is further illustrated in Figure 5 which shows the effect upon ripple in the rectified output EMF of the rotor configuration features discussed above.

Thus, curve A shows the rectified output EMF from an alternator in which the gap between rotor and stator is uniform (i. e. the outwardly facing surfaces of the rotor pole components are concentric with the inwardly facing surfaces of the stator pole components) and the circumferential extent of the rotor pole components is less than that shown in Figures 3 and 4, being 55.6 degrees rather than the chosen 64.6 degrees of the configuration shown in Figures 3 and 4.

Curve B shows the effect of a rotor with pole components having an increased circumferential extent to 66.6 degrees, that is closer to the circumferential extent of the examples shown in Figures 3 and 4, but with a uniform gap between concentric facing surfaces on the rotor and stator pole components. As can be seen from Figure 5, there remains a significant ripple, although this is much reduced as compared with that of curve A.

Curve C shows the effect of introducing a nonuniformity into the air gap between rotor and stator, with Ro < Ri-G as defined with reference to Figure 3. The improvement in reduced ripple is evident. Curves D and E illustrate the effect for the same rotor configuration as that of curve C, but in alternators operating at different voltage outputs.

In a specific example, we have found satisfactory results with a configuration in which the circumferential extent of each rotor pole face is 64.6 degrees and Ro is typically 33.74 mm with its centre 6.26 mm from the shaft centre. This feature yields a non-uniform air gap G, which at its extremes measures 1.43 mm with a central minimum at 0.3 mm.

In the example of Figures 1 to 4, two sets of windings, each for three phase alternating current output, are provided on the stator 19. One set provides a 12 volts output. The other set has more turns and provides a multiple of 12 volts such as 48 volts or 96 volts. For use in a motor vehicle, if a 96 volts output is required it is necessary, for safety reasons, to arrange that this appears as-48 and + 48 either side of earth potential. It should be appreciated that other voltage outputs and combinations are also applicable to this invention as will be apparent to those skilled in the art.

A further valuable feature of this arrangement is that voltage regulation (which can be provided by a standard form of voltage regulator, not shown) applied to one such output will automatically regulate the other such output, because of the close magnetic coupling between them.

The invention is not restricted to the details of the foregoing example. For instance, the windings can be arranged to provide single phase output or more than three phase, if required. Relative rotation is, of course, all that is required between rotor and stator and it is possible to fix the rotor (in which case it becomes the stator) inside and mount the stator on a rotatable shaft (in which case it becomes the rotor).

Claims

1. An electrical generator comprising a stator having a plurality of radially projecting pole components of magnetisable material with windings thereon, a rotor having a plurality of pole components radially extending such as, on rotation of the rotor, to pass sequentially by and in facing relationship to the pole components of the stator, the pole components of the rotor also being of magnetisable material with windings thereon, means for rotating the rotor relative to the stator, the rotor and stator windings being appropriately connected to provide an electrical voltage output when there is relative rotation between rotor and stator, wherein the respective centers of curvature of the facing surfaces of the pole components of the rotor and the stator are spaced apart.

2. An electrical generator as claimed in claim 1, wherein the radius of curvature Ro of the outwardly facing surfaces of said facing surfaces is less than Ri G, where Ri is the radius of curvature of the inwardly facing surfaces of the said facing surfaces and G is the minimum width of the gap between the said facing surfaces.

3. An electrical generator as claimed in claim 2, wherein the rotor is inside the stator so that the said outwardly facing surfaces are on the rotor and the said inwardly facing surfaces are on the stator.

4. An electrical generator as claimed in any of the preceding claims, wherein the rotor is of laminated construction to reduce loss through eddy currents.

5. An electrical generator as claimed in any of the

preceding claims, wherein the stator is of laminated construction to reduce loss through eddy currents.

6. An electrical generator as claimed in any of the preceding claims, wherein the windings are arranged to generate multi-phase alternating current outputs.

7. An electrical generator as claimed in claim 6, wherein the windings are arranged to generate three phase alternating current outputs.

8. An electrical generator as claimed in claim 7, wherein the rotor has four circumferentially equispaced pole components with windings such that when current is passed through the windings the magnetic polarity of one pole is opposite to that of the next circumferentially adjacent rotor pole, and the stator has twelve circumferentially equispaced pole components.

9. An electrical generator as claimed in claim 8, wherein the circumferential extent of the facing surfaces of the rotor pole components corresponds substantially to the circumferential extent of the two facing surfaces of adjacent stator pole components together with the gap therebetween and the gaps on each side thereof.

10. An electrical generator as claimed in any of the preceding claims, wherein the stator and rotor pole components taper sharply from their respective facing surfaces towards a stem portion the circumferential extent of which is narrower than that of the respective facing surfaces and onto which stem portions the windings are wound.

11. An electrical generator as claimed in any of the preceding claims, wherein the stator carries a plurality

of separate windings each of which has a different number of turns around the pole components to provide a corresponding plurality of different outputs at different voltages.

12. An electrical generator as claimed in claim 11, wherein a voltage regulator is connected to one of the outputs, said voltage regulator serving to regulate all the voltage outputs by virtue of the close magnetic coupling between the respective windings.

13. An electrical generator substantially as hereinbefore described with reference to, and illustrated in, the drawings filed herewith. 15435 LgCm