Electrical machines

There is disclosed a rotor for an electrical machine, the rotor having a current-carrying winding comprising a plurality of circumferentially distributed winding portions which lie in at least one plane perpendicular to the rotor axis, and extend from a radially inner region to a radially outer region. A commutator is provided by surfaces of the winding portions at the inner region, and the winding is formed from a plurality of conductive sections, each having ends which lie at said outer region, interconnections between the winding sections being made only by way of those ends. Thus, soldered connections made between the winding sections will be remote from the commutator, which is the main source of heat for overheating, and moreover will be at positions where air cooling due to rotor movement is greatest. Air gaps between the winding portions at the outer region aid this cooling affect even further.

BACKGROUND OF THE INVENTION 
One aspect of this invention relates to electrical machines which convert 
mechanical energy into electrical energy, or vice versa, by an interaction 
between a magnetic field and an electric current. Examples of such 
machines are electric motors, dynamos and alternators. A further aspect of 
the invention relates to rotors for such machines. 
In a dynamo having a field coil to produce the dynamo's magnetic field, it 
is well known to vary the electrical output of the dynamo by varying the 
field current. This way of varying the electrical output of an electrical 
machine is not possible, however, in the case of a machine whose magnetic 
field is provided by one or more permanent magnets. 
BRIEF DESCRIPTION OF THE INVENTION 
In accordance with one aspect of the present invention, there is provided 
an electrical machine in which a current carrying member is acted upon by 
and movable relative to magnets disposed to opposite sides of the current 
carrying member, a pair of magnetic field diverting members being disposed 
to the opposite sides of the current carrying member between the magnets 
and the current carrying member, and the diverting members being coupled 
together for movement in opposite directions relative to the magnets to 
vary the proportion of the magnetic field produced by each magnet which 
flows through the current member. 
Thus, the magnetic field can be produced by permanent magnets and yet the 
degree of interaction between the magnetic field and the electric current 
can be varied, by diverting the magnetic field. 
Preferably, the field diverting members can be moved to vary the degree of 
diversion of the magnetic field progressively. Thus, in the case of a 
motor, the speed and/or torque can be varied progressively, and, in the 
case of a generator, the electrical output can be varied progressively. 
Preferably, the machine has on opposite sides of the current carrying 
member respective first and second series of alternately oppositely 
polarised magnets each spaced from said current carrying member, each 
diverting member having a series of elements movable in the spaces between 
the magnets of the corresponding series and the current carrying member. 
Preferably, each series of magnets and each series of diverting elements 
are each arranged in a circle, the diverting member being rotatable about 
the centers of the respective circles, and the series of magnets and 
diverting elements are preferably centred on an axis of the current 
carrying member. Preferably, the current carrying member is a rotor 
rotatable about the axis, the series of magnets and the diverting member 
or members forming a stator assembly. 
In accordance with another aspect of the invention there is provided an 
electrical machine in which a disc-shaped current carrying rotor is acted 
upon by and rotatable relative to a series permanent magnets arranged in a 
circle centred on the rotor axis to provide a stator producing a magnetic 
field with which the current in said rotor interacts, said series 
comprising at least first and second circular subseries which are 
relatively rotatable about the axis to vary the interacting field. 
Preferably the two series comprise corresponding pluralities of regularly, 
circumferentially spaced magnets, each spaced from the current carrying 
member, means being provided for displacing one sub-series relative to the 
other so as to alter the state of mutual registration between said first 
and second sub series. The magnets in each sub-series are preferably 
alternately oppositely polarized, the maximum field being achieved when 
magnets of the first and second sub-series of like polarization are 
mutually registered, the minimum field being achieved when magnets of the 
first and second sub-series of opposite polarization are mutually 
registered. 
The machine may have a further series of permanent magnets, again arranged 
in a circle centred on the rotor axis and again divided into first and 
second sub-series, the current carrying member being sandwiched between 
the two series of permanent magnets. The two displaceable sub-series may 
be coupled together for movement in the same, or more preferably, opposite 
directions. 
For the or each series of magnets there may be provided a corresponding 
series of magnetically permeable fixed pole pieces arranged inacircle, 
themagnets of one sub-series being attached to first portions of the pole 
pieces, and the magnets of the displaceable sub-series being registrable 
with second portions of the pole pieces. 
In accordance with a further aspect of the invention, there is provided a 
rotor for an electrical machine, the rotor having a current-carrying 
winding comprising a plurality of circumferentially distributed winding 
portions which lie in at least one plane perpendicular to the rotor axis 
and extend from a radially inner region to a radially outer region, and a 
commutator provided by surfaces of the winding portions at said inner 
region, the winding being formed from a plurality of conductive sections 
each having ends which lie at said outer region, interconnections between 
said winding sections being made only by way of said ends at said outer 
region. 
Accordingly, the winding can be constructed from a number of identical such 
sections, and the risk of damage at the joints between the sections due to 
overheating is minimized since the joints are at the outer region of the 
winding, which is remote from the commutator where most heat is generated, 
the cooling effect due to rotation of the rotor also being greatest in 
this outer region. This is particularly advantageous where soldered joints 
are used. 
The sections are made from metal (e.g. copper) strip (e.g. stampings), the 
surfaces providing the commutator being edge surfaces of said metal strip. 
Preferably, gaps are provided between the winding portions at said outer 
region. This enhances the cooling affect on the winding joints, and also 
aids the procedure for forming he joints, as will be described later 
herein. 
This aspect of the invention is particularly applicable to a wave-wound 
rotor in which the winding extends primarily in a plurality of parallel 
planes perpendicular to the rotor axis, and conveniently in four parallel 
planes. 
The rotor preferably also has ferromagnetic material disposed between the 
winding portions to promote conduction of the magnetic field axially of 
the rotor. 
Preferably, the ferromagnetic material (which may be of, for example, mild 
steel, silicon steel or soft iron) is provided in the form of sheets the 
planes of which extend axially and radially of the rotor. The sheets may 
be provided by laminated layers. 
In this aspect of the invention the magnetic field is preferably provided 
by magnets or coils on both sides of the rotor. A compared with an 
arrangement with magnets or coils on only one side of the rotor, this 
arrangement produces a greater magnetic field and thus, in the case of a 
DC motor, a greater torque for a given current and less speed for a given 
voltage. Because the motor is caused to run slower, friction losses are 
reduced and thus efficiency is increased. The same effect occurs in the 
case of a DC generator, that is to say for a given speed the voltage is 
greater and for a given torque the current is less. Thus, the ohmic losses 
are reduced and thus efficiency is increased.

DETAILED DESCRIPTION 
Referring to FIGS. 1 and 2, a DC motor 10 comprises a pair of stator 
assemblies 12 and a rotor 14 mounted on a shaft 16 having an axis 16a 
which is rotatably mounted by bearing 18 with respect to the stator 
assemblies 12. 
Each stator assembly comprises a steel stator plate 20, and the two plates 
20 are held together by four posts 22 extending through respective holes 
24 in the plates. Eight permanent magnets 26 are bonded in a circular 
arrangement to each stator plate alternately by their north-seeking and 
south-seeking faces, and thus the faces of the magnets facing the rotor 
are alternately north and south-seeking. Furthermore, the magnets on one 
plate which have north-seeking faces facing the rotor are opposite magnets 
on the other plate having south-seeking faces facing the rotor and vice 
versa. Thus, as described so far, each magnet is in two primary magnet 
circuits, one being through the magnet, the rotor, the opposing magnet on 
the other stator plate, that other stator plate, one of the magnets next 
to that opposing magnet, the rotor, one of the magnets next to magnet in 
question, the stator plate on which the magnet in question is mounted and 
back to the magnet in question. The other magnetic circuit is similar, 
except that it passes through the other magnet next to the opposing magnet 
and through the other magnet next to the magnet in question. 
Electric current is passed to the rotor via brushes 28 and commutators on 
the rotor, and flows along generally radial current conductors in the 
rotor. Thus, the electric current interacts with the magnetic fields to 
cause the rotor to rotate. 
A steel diverting member 30 is mounted on each stator plate 20. Each 
diverting member comprises a narrow annular portion 32 from which eight 
petal portions 34 radiate. The annular portion is slidable around an 
annular plastics boss 36 secured to the stator plate. The outer end of 
each petal portion 34 is secured to an annular plastics member 38 which is 
rotatably held on the stator plate by four brackets 40 so that the petal 
portions 34 are slidable on the faces of the magnets 26. A gear segment 42 
is secured to the annular member 38 and a common drive gear (not shown) 
cooperates with the gear segments 42 of both stator assemblies 12 to 
rotate the diverting members in opposite directions. The maximum rotation 
of each diverting member is one sixteenth of a turn, and the arrangement 
is set up such that at one limit of rotation each petal portion 34 
overlies a respective magnet 26, as shown in FIG. 3A, and at the other 
limit each petal portion 34 partly and equally overlies two adjacent 
magnets 26, as shown in FIG. 3B. 
Lines representative of the magnetic fluxes in the stator assemblies and 
rotor are shown in FIGS. 3A and 3B, and from a comparison of the two 
drawings it will be noted that the magnetic flux threading the rotor with 
the petal portions 34 in the position shown in FIG. 3B is less than that 
with the petal portions in the FIG. 3A position, because in the former 
case part of the magnetic flux produced by two adjacent magnets is 
"short-circuited" via the respective petal portions 34. A the petal 
portions 34 are progressively moved between the two positions shown in 
FIGS. 3A and 3B, the magnetic flux threading the rotor 14 is progressively 
varied, and thus the torque and/or speed of the motor is progressively 
varied. 
In the second embodiment of the stator assembly, shown in FIG. 4 to 7 (in 
which elements common to this and the first embodiment are indicated by 
like reference numerals) the magnetic field is produced by eight pairs of 
permanent magnets on each assembly, arranged in two concentric circles 
centred on the axis of the rotor. More particularly, eight magnets 60 are 
bonded to each stator plate alternatively by their north-seeking and 
south-seeking faces. As in the first embodiment the magnets 60 on one 
plate which have north-seeking faces facing the rotor are opposite magnets 
on the other plate having south-seeking faces facing the rotor and 
vice-versa. Each stator assembly includes eight further magnets 61 
circumferentially spaced for radial registration with the magnets 60, 
these further magnets being located in corresponding cut-outs 62 formed in 
the inner edge of a plastics retaining ring 63 which is rotatably held to 
the stator plate by brackets 64, so that the circle of magnets 61 can 
rotate relative to the inner circle of magnets 60. Again, a gear segment 
42 is secured to the ring 63 and a common drive gear (not shown) 
co-operates with the gear segments 42 of both stator assemblies 12 to 
rotate the further magnets 61 in opposite directions. The maximum rotation 
of each ring 63 is in this case one eighth of a turn, and the arrangement 
is set up such that at one limit of rotation the circles of magnets 60 and 
61 are in mutual register as shown in figures 4 and 5 with corresponding 
poles of the registered magnets facing the rotor and at the other limit 
each pair of registered magnets has opposite poles facing the rotor, as 
shown in FIG. 7. 
As can be seen from FIGS. 5 and 7, each of the fixed magnets 60 is attached 
to a radially inner portion of a respective magnetically permeable soft 
iron pole piece 65 (omitted from FIG. 4 for the sake of clarity) lying 
adjacent the rotor 14. The magnets 61, when registered with the magnets 
60, each lie adjacent a radially outer portion of a corresponding pole 
piece. These pole pieces concentrated the lines of magnetic flux into the 
rotor. The outer magnets 61 lie radially outwardly of a magnetically 
permeable core 14' of the rotor, and the pole pieces 65 direct the flux 
from these magnets inwardly, as shown in FIG. 5, to flow through the core 
14'. 
In the position of the ring 63 of figures 4 and 5, the magnetic flux 
threading the rotor is maximum, since the magnetic field due to the 
magnets 60 is reinforced by that of the magnets 61. However, when the ring 
63 is rotated to move the magnets 61 away from the pole pieces 65, the 
flux threading the rotor is weakened as some of the flux due to the 
magnets 61 leaks away to form a magnetic closed circuit with the stator 
plate, and some leaks to the next adjacent pole piece. For example, FIG. 6 
illustrates the situation when the ring 63 has rotated one sixteenth of a 
turn to position the magnets 61 mid-way between the magnets 60. This 
figure shows at 66 the flux leaking to the stator plate 20 and at 67 the 
flux which "short circuits" to and form the next adjacent pole pieces 65. 
As the ring 63 is progressively moved away from the position of figured 4 
and 5 the proportion of magnetic flux which leaks increases and thus the 
torque and/or speed of the motor is progressively varied. 
FIG. 7 illustrates the situation when the ring 63 has moved one eighth of a 
turn to place magnets 61 whose North-seeking poles face the rotor adjacent 
magnets 60 whose South-seeking poles face the rotor, and the vice versa. 
In this condition each pair of magnets 60/61 forms a closed magnetic 
circuit with the corresponding pole piece 65 and the stator plate 20, and 
no flux threads the rotor. There is accordingly, no torque applied to the 
rotor. 
The magnets 61 may be quite loosely located in the cut-outs 62. When 
registered as in FIG. 4 with like polarized magnets 60 the magnets 61 will 
be firmly seated in the cut-outs by the radially-acting repelling force F. 
To avoid the magnets 61 jamming at their leading corners 80 (assuming 
clockwise rotation of ring 63 in FIG. 4) against the corners 81 of the 
fixed magnets 60, the circumferential gaps between the magnets 60 are 
filled with a plastics material 82. As can be seen from FIG. 5, the 
magnets 61 are constrained in an axial direction by the stator plate 20 
and the outer face of the pole pieces 65. 
Referring to FIGS. 8 and 9, the rotor 14 is a wave-wound disc rotor. A 
circular plate 44 is mounted on the rotor shaft, the periphery of the disc 
having 65 cut-outs 46. One or more rectangular stampings 48 of 
ferromagnetic sheet material are mounted in each cut-out such that the 
plane of the stamping is at right angles to the paper of FIG. 8. The 
stampings are preferably of mild steel, silicon steel, or soft iron and 
form the above-mentioned core 14'. Copper strip is then wound on the rotor 
around the stampings to provide the current carrying winding of the rotor. 
The winding lies generally in four parallel planes A,B,C,D the rotor planes 
A and D being adjacent opposite faces of the rotor, and the planes B and C 
being intermediate planes A and D. There are 130 section to the winding 
each pair of which sections progresses as follows: portion A1 in plane A 
extends from the periphery of the rotor to the stampings; thence portion 
A2 extends radially inwards between two stampings; thence portion A3 
extends to an inner limit of the winding where the section is bent and 
therefore integrally formed with bridging portion A-B into plane B; thence 
portion B1 extends to the stampings; thence portion B2 extends radially 
outward between two stampings, there being eight stampings between 
portions A2 and B2; thence portion B3 extends to a position B-C partway 
between the stampings and the outer periphery of the rotor, at which the 
winding section is soldered to the next winding section which extends as 
portion C1 in plane C to the stampings; thence portion C2 extends radially 
inwards between the same two stampings as portion A2, thence portion C3 
extends to the inner limit of the winding where the section is bent and 
therefore integrally formed with bridging portion C-D into plane D; thence 
portion D1 extends to the stampings; thence portion D2 extends radially 
outwardly between the same two stampings as portion B2; thence portion D3 
extends to the outer periphery of the rotor where it is soldered at 
position D-A to a portion A1 of a further pair of winding sections. The 
portion A2 of this further pair of winding sections extends radially 
inwardly between two stampings, there being sixteen stampings between this 
portion A2 and the portion A2 of the first pair of winding sections. 
Winding of the rotor continues in this manner so that in total there are 
65 pairs of winding sections, the portion D3 of the 65th pair being 
soldered at a position D-A to the portion A1 of the first pair. Thus, the 
winding forms one continuous loop. 
An important feature of the rotor is that the outwardly facing edges of the 
copper strip forming the winding portions A3 and D1 are directly engaged 
by the branches 28 and therefore provide two commutators of the rotor. The 
copper strip is relatively wide at the commutator in the axial direction 
of the rotor, and thus the commutator will have a long life. Furthermore, 
there are no soldered connections at an inner region of the rotor adjacent 
the commutator, these connections being made only at the outer periphery, 
where the cooling effect of the rotor movement is greatest, and thus any 
over-heating at the commutator is unlikely to cause failure of the rotor. 
Gaps G1 are provided between circumferentially adjacent A1 portions, B3 
portions, C1 portions and D3 portions, leading to corresponding gaps G2 
between D-A soldered connections and B-C soldered connections. These gaps 
enhance air-cooling at the outer parts of the winding segments, reducing 
even further the possibility of damage to the soldered connections by 
over-heating. The avoidance of the need for connections at the inner 
region of the rotor, and the provision of the gaps G2 at the periphery 
combine to serve the additional advantage of affording proper access to 
the ends to the winding sections for the making of the necessary soldered 
connections. 
The motor is fitted with brushes, each brush having a commutator engaging 
carbon sintered copper part 50 which is highly conductive and a 
lubricative carbon part 52 as seen in FIG. 1. The two parts may be bonded 
together or may be mounted independently but side-by-side in the brush 
holder. The carbon part acts not only as a lubricant for itself, but also 
for the copper-based part, and thus the life of the copper-based part is 
improved. 
Various modifications may be made of the machine described above. For 
example, instead of diverting the magnetic field by mechanical means, or 
by permanent magnets auxiliary field coils may be fitted to the stator to 
influence the magnetic circuits produced by the permanent magnets. 
Moreover, the particular construction of armature described above may be 
used with conventional stators. Also each circle of magnets (26 in FIG. 1; 
60 and 61 in FIG. 4) can be provided by a continuous ring of a material 
which can be locally magnetized at circumferentially spaced positions. 
This would be particularly advantageous in the embodiment of FIGS. 4 to 7, 
as the assembly of magnets 61 in the plastics retaining ring 63 would be 
replaced by a simpler and more reliable one-piece ring member.