Electrical machine with dual excitation, especially a motor vehicle alternator

A rotary electrical machine has at least one stator and a rotor. The stator includes at least one armature winding received in at least one pair of slots. The rotor includes flux switching arrangement, which is adapted for selectively establishing closed magnetic circuits which pass around the turns of the armature winding. The arrangement for switching the flux comprises at least one permanent magnet adapted to establish a magnetic flux which is looped on itself in one circumferential direction of the rotor, together with at least one excitation winding for establishing an adjustable magnetic flux locally in a circumferential direction opposite to the direction of the flux produced by the magnets.

FIELD OF THE INVENTION 
The present invention relates in general terms to rotary electrical 
machines such as alternators for motor vehicles. 
BACKGROUND OF THE INVENTION 
The single phase or multi-phase generator that constitutes a conventional 
alternator for a motor vehicle generally comprises a stator within which a 
rotor, which carries an excitation winding, rotates. The excitation 
winding is energized through brushes in contact with two slip rings 
arranged on a projecting portion of the shaft of the rotor. 
Various rotary machines are known, in particular from European patent 
specification No. EP 0 707 374, in which the windings of the stator are 
excited at the same time by permanent magnets and by excitation windings, 
and in which the current delivered by the armature to the excitation 
windings is controlled by switching, with inversion of the excitation 
current serving to reduce the flux of the magnets at high speeds. 
However, since the excitation current is, by nature in this type of 
machine, a two-way alternating current, such switching makes it necessary 
to have recourse to an H-shaped switching bridge, which is an expensive 
item. 
DISCUSSION OF THE INVENTION 
The present invention aims to overcome these drawbacks and to propose a 
rotary machine, in particular an alternator, in which regulation of the 
current delivered can be achieved by acting on the excitation, and in 
which, despite the use of permanent magnets to effect this excitation, the 
latter can be varied between a zero or essentially zero value and a 
maximum value of excitation. 
Another object of the invention is to provide a machine in which the 
excitation varies within such limits as a result of the application of an 
excitation current which itself also varies between zero value and a 
maximum value. 
A further object of the invention is to propose a structure for a rotary 
machine with a double air gap and a single rotor, which is compact in the 
axial direction and which has an increased power-to-weight ratio. 
According to the invention in a first aspect, an electrical machine having 
at least one stator and a rotor, the stator having at least one armature 
winding mounted in at least one pair of slots, the rotor including means 
which are adapted for selectively establishing closed magnetic circuits 
passing around the turns of the armature winding or windings, wherein in 
that the means include at least one permanent magnet adapted to set up a 
magnetic flux which is looped on itself in a circumferential direction of 
the rotor, and at least one excitation winding for establishing an 
adjustable local magnetic flux in a circumferential direction inverse to 
that of the flux produced by the magnet or magnets. 
Such a regulating structure permits the use of a conventional regulator as 
an interrupter for regulating the excitation, which divides by four the 
cost arising from the interrupters by comparison with machines working 
with two-way current. 
Preferably, the rotor includes along its periphery a plurality of permanent 
magnets arranged alternately with excitation windings. 
Preferably, in particular, the rotor has a plurality of permanent magnets 
arranged alternately with H-shaped structures which define slots in which 
the excitation windings are received. These H-shaped elements enable the 
winding heads to be reduced in length, which reduces the resistances and 
therefore the excitation energy losses. Output is therefore improved. 
In preferred embodiments, the machine has both an inner stator and an outer 
stator. One of the advantages of this arrangement as compared with a 
conventional rotary machine of the kind having double rotors arises from 
the fact that the rotating inertia is reduced. 
Such an arrangement is preferably used as a combined alternator and starter 
motor with a high power-to-weight ratio, given that it has a short axial 
length and has to deliver high output power on the two stators. 
According to the invention in a second aspect, there is provided a rotor 
for a hybrid electrical machine with double excitation, as defined above.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
Reference is first made to FIG. 1, which shows a stator 1 and a rotor 5 for 
a single-phase electrical rotary machine in a first embodiment of the 
invention. The stator 1 comprises an annular stator body 2 which is in two 
parts defining an endless annular structure. The inner periphery of this 
structure is formed with a set of stator slots 3 which receive the turns 
of an armature winding 4. 
These slots 3 are arranged in an even number, and are spaced apart at 
regular intervals on the periphery of the stator 1. 
The rotor 5 is defined by a succession of H-shaped elements 6, which are 
arranged alternately with seatings 5 in which permanent magnets 8 are 
received. The H-shaped elements 6 are defined by outer slots 6a and inner 
slots 6b. The rotor includes a rotor winding or excitation winding, the 
turns 9 of which are received in the outer and inner rotor slots 6a and 
6b. The rotor winding 9 is connected to suitable means represented in FIG. 
1 at 10, for supplying the rotor winding with an adjustable excitation 
current. 
The H-shaped elements 6 and the permanent magnets 8 are spaced apart at 
regular intervals on the rotor 5. The number of magnets 8 and the number 
of H-shaped elements 6 both correspond to the number of pairs of poles in 
the rotary machine, that is to say one half the number of stator slots 3. 
More precisely, the magnets 8 and the H-shaped elements 6 are distributed 
in such a way that, when an outer slot 6a is in line with a first slot 3 
of the stator, the magnets 8 on either side of this H-shaped element are 
in line with those stator slots 3 which are on either side of this first 
slot 3. 
The north/south orientation of the permanent magnets 8 is such that in the 
absence of any current in the rotor windings 9, they set up an essentially 
circumferential magnetic flux on the rotor 5 in the direction of the 
arrows indicated in FIG. 1. Thus, in the absence of any excitation 
current, there will be no field flowing through the stator 1 (or at most a 
negligible part). This will be so regardless of the angular position of 
the rotor 5, and no current (or at most a negligible current) will 
therefore be produced in the armature winding 4. The excitation of the 
machine is therefore zero. 
If now a unidirectional current is caused to flow in the excitation winding 
9 in such a way as to produce a magnetic flux in the opposite direction 
from that created by the magnets 8, this opposition forces the flux from 
the magnets 5 and the excitation winding 9 to cross the air gap and to 
extend into the stator 1 where the excitation current is sufficiently 
large. 
As a result, an electromotive force will be produced in the armature. It 
will be understood that the value of this electromotive force depends on 
the amplitude of the magnetic field created by the excitation winding 9, 
so as to vary between a substantially zero value and a maximum value. 
The magnets 8 and the excitation winding 9 are so dimensioned that the 
fluxes from the excitation winding are of the same order of magnitude as 
those from the magnets, for reasonable intensities of the excitation 
current. Thus, by regulating the variations of current in the winding 9, 
not only the flux of the excitation winding 9 is regulated, but so also is 
the flux of the magnets 8 in the stator. As a consequence, with the 
structure which has just been described, the operation of the electrical 
machine is regulated using a simple unidirectional current. This 
regulation may for example be obtained by means of a single transistor 
controlled by a regulating microprocessor. 
With reference now to FIG. 2, one example of a polyphase machine in 
accordance with the present invention will now be described. In FIG. 2, 
those elements of the rotary machine of FIG. 1 which are repeated in FIG. 
2 are given the same reference numerals with 100 added. 
The machine represented in FIG. 2 is a three-phase machine and comprises a 
rotor 105 which is identical to that in FIG. 1, and which includes a 
plurality of H-shaped elements 106 disposed alternately with permanent 
magnets 108, the openings defined by the H-shaped elements receiving the 
turns of an excitation winding 109. The permanents magnets 108 are 
received in seatings 107, the depth of which is slightly smaller than the 
radial thickness of the rotor 105. 
The stator 121 is continuous, i.e. endless, in the circumferential sense 
and is of conventional form comprising a stack of laminations. The stator 
121 has six stator slots 103 per pair of rotor poles, these various slots 
103 being defined by inner teeth 122 spaced apart angularly in a 
substantially regular manner. The slots 103 receive the distributed turns 
of the various phase windings 104a, 104b and 104c. Three successive slots 
103 at the periphery of the stator contain respective turns of the 
three-phase windings. 
The machine represented in FIG. 2 works on the same principle as that in 
FIG. 1. In particular, when no excitation is applied to the excitation 
winding 109, the permanent magnets 108 cooperate so as to produce a 
magnetic flux which flows almost entirely within the rotor, and the phase 
windings of the stator therefore deliver a substantially zero current. 
By contrast, when an excitation is set up in the inverse sense of the field 
created by the permanent magnets, this gives rise to phenomena of looping 
of the fields around the various turns of the phase windings 104a, 104b 
and 104c, in such a way that, during rotation of the rotor at a given 
speed, the machine sets up a three-phase electromotive force which varies 
substantially between zero and a maximum value when the excitation current 
is varied between zero and a maximum excitation value. 
In the examples shown in FIGS. 1 and 2, the rotors 5 and 105 are fitted on 
shafts not shown, which are of non-magnetic materials, or which are 
insulated by non-magnetic materials from the assemblies consisting of the 
permanent magnets, the H-shaped elements and the excitation windings. 
However, other forms of construction are also possible. In particular, as 
shown in FIGS. 3 and 4 to which reference is now made, it is possible to 
have the rotor mounted between an outer stator and an inner stator. 
In FIGS. 3 and 4, those elements of the version shown in FIG. 1 that are 
repeated in FIGS. 3 and 4 are given the same reference numerals with 200 
added. Thus in FIGS. 3 and 4 the machine comprises an outer stator 201 and 
a rotor 205, which are identical to the stator 1 and rotor 5, 
respectively, of FIG. 1. 
In particular, the rotor 205 consists of a succession of H-shaped members 
206 arranged alternately with permanent magnets 208, with the slots 
defined by the H-shaped members containing the turns of an excitation 
winding 209. As to the outer stator 201, this consists of a stack of 
laminations and it has at its inner periphery a succession of slots 203 
receiving an armature winding 204. 
The rotor 205 rotates between the outer stator 201 and an inner stator 211. 
The inner stator 211 again consists of a stack of laminations, and has 
along its inner periphery a succession of slots 213 which again receive an 
armature winding, 214. The number of slots 213 is identical to the number 
of the slots 203, with the slots 213 and 203 being aligned with each other 
as can be seen in FIGS. 3 and 4. 
The machine with the structure shown in FIGS. 3 and 4 operates in a similar 
way to those shown in FIGS. 1 and 2. In the absence of any excitation 
current, the magnetic field from the permanent magnets 208 rotates in the 
rotor 205. No current is generated in the armature windings 204 and 214 
during rotation of the rotor 205. 
By contrast, when an excitation current is generated, the magnetic flux of 
the magnets 208 is forced to extend through the two air gaps into the 
inner and outer stators 201 and 211, and an electromotive force is 
generated in the armature windings 204 and 214. 
It will be noted that the armature windings of the two stators 201 and 211 
are independent, so that this structure has the advantage that it lends 
itself well to the construction of a dual voltage three-phase machine, in 
which the first stator can work at 14 volts and the second at 24 volts or 
48 or 96 volts, according to the requirements of the circuitry of the 
vehicle. 
Various structures of the same type as those which have been described 
above may of course be connected in parallel or in series with each other. 
In addition, it will be observed that the machines in FIGS. 1 to 4 can if 
necessary be subjected to an initial magnetization or to re-magnetization. 
More precisely, if the magnets are mounted in the stator without having 
first been magnetized, it is sufficient to energise the excitation 
windings with a current which is inverse to the normal excitation current. 
A circumferential magnetic field is set up in the rotor in the same 
direction as that which is produced by the permanent magnets, and this 
field is capable of magnetizing or re-magnetizing the magnets. This 
establishment of an inverse magnetizing or re-magnetizing field can for 
example be carried out using semiconductor switching means which a person 
familiar with this technical field will be able to achieve without any 
difficulty. 
This operation may be carried out either on board the vehicle with the 
electrical energy supplied by the battery of the vehicle, or in the garage 
during servicing of the vehicle. 
In addition, it will be noted that the fitting of the windings on the 
H-shaped elements or members is facilitated by the form of these 
structures and can be carried out separately from the fitting of the 
permanent magnets in place. In addition, when the magnets are magnetized, 
the assembly of the rotor on the stator or stators is made even easier due 
to the fact that the flux of the permanent magnets is closed in the rotor. 
There is thus no risk that the structure of the rotor will stick on the 
ferromagnetic components of, in particular, the inner and outer stators. 
The novel structures provided by the invention give an increase of 10 to 
20% in output as compared with those in the prior art. 
The present invention is of course not limited to the embodiments described 
above and shown in the drawings. In particular, machines can be made in 
accordance with the features described above which work as generators, as 
well as motors, and also machines having any number of phases whatever. In 
addition, the construction of the excitation winding around the H-shaped 
elements (i.e. in the form of a toroidal winding) naturally improves 
resistance to centrifugal force, by contrast with the winding in machines 
with projecting poles. 
It is known that the main disadvantage of permanent magnet alternators used 
in the automotive field arises from the permanent presence of a useful 
flux generated by the magnets, from which there arises a risk of loss of 
regulation. This can give rise to excessive voltages, leading to total or 
partial destruction of the components of the electrical network of the 
vehicle. The proposed structure eliminates this problem because, in the 
absence of any excitation, the flux of the magnets is neutralized because 
it cannot become closed through the stator.