Dual excitation electrical machine, and especially motor vehicle alternator

A flux commutating electrical machine includes a stator and a rotor. The rotor selectively establishes closed magnetic circuits around sections of the armature coils. Excitation permanent magnets establish a magnetic flux closing on itself in a circumferential direction and excitation coils establish a localized variable magnetic flux in an opposite direction to that of the flux produced by the magnets. Each magnet is housed in a first rotor part defining a first pair of rotor poles, and the coils are disposed around a second rotor part ends of which define a second pair of poles. The rotor has between the adjacent first and second rotor parts third rotor parts forming with the first and second parts a circumferential magnetic conduction path.

FIELD OF THE INVENTION 
This invention relates to rotating machines such as motor vehicle 
alternators. 
BACKGROUND OF THE INVENTION 
The single-phase or multiphase generator constituting the conventional 
motor vehicle alternator generally has a stator within which rotates a 
rotor carrying an excitation coil. The coil is connected to brushes in 
contact with two slip rings on a projecting part of the rotor shaft. 
EP-A-0 707 374 discloses rotating machines in which, for improved 
efficiency, the rotor excitation field is produced by permanent magnets 
and by coils (the expression "mixed excitation" is generally used), and in 
which the current delivered by the armature is controlled by excitation 
coil switching means which selectively reverse the excitation direction to 
reduce or even substantially eliminate the flux from the magnets. 
This need to reverse the direction of the excitation current imposes the 
use of a so-called "H" semiconductor switching bridge, which is costly and 
adds to the unit cost of the machine. 
DISCUSSION OF THE INVENTION 
An object of the invention is to overcome the above drawbacks and to 
propose a machine, in particular a rotating machine such as an alternator, 
with mixed excitation and in which the output current can be regulated by 
varying unidirectional excitation by coils, and in particular by varying 
excitation by coils between an essentially zero value and a maximum value 
to deliver an essentially zero energy and a maximum energy, respectively. 
Another object of the invention, in a machine of the above kind, is to 
reduce the number of magnets required for a given number of rotor poles 
without leading to any imbalance between the level of excitation by 
magnets and the level of excitation by coils. 
Accordingly the invention proposes a flux commutating electrical machine 
including a stator and a rotor wherein the stator includes at least one 
armature coil housed in at least one pair of notches, the rotor includes 
means for selectively establishing closed magnetic circuits around 
sections of the armature coil(s) including at least one excitation 
permanent magnet adapted to establish a magnetic flux closing on itself in 
a circumferential direction of the rotor and at least one excitation coil 
adapted to establish a localized variable magnetic flux in an opposite 
circumferential direction to that of the flux produced by the magnets, the 
magnet or each magnet is housed in a first rotor part defining a first 
pair of rotor poles, the coil is or the coils are disposed around a second 
rotor part ends of which define a second pair of rotor poles and the rotor 
has between the adjacent first and second rotor parts third rotor parts 
forming with the first and second parts a circumferential magnetic 
conduction path. 
The machine in accordance with the invention has the following preferred 
but non-limiting features: 
the second rotor part or each second rotor part has two excitation coils 
adapted to create magnetic fluxes one of which is directed towards the 
interior of the rotor and the other of which is directed towards the 
exterior of the rotor. 
the rotor has along its periphery an alternating series of first parts and 
second parts. 
the second rotor part or each second rotor part is generally U-shape and 
receives an excitation coil on each of its two branches. 
the third rotor parts are at a distance from the rotor poles and extend a 
radial distance significantly less than the radius of the rotor. 
the first, second and third rotor parts are defined by a single core. 
the rotor is formed by at least two separate yoke elements between 
respective pairs of magnets and joined together by the magnets. 
The invention further proposes a machine as defined hereinabove 
constituting a motor vehicle alternator. 
Other aspects, aims and advantages of the invention will appear more 
clearly on reading the following detailed description of preferred 
embodiments of the invention which is given by way of non-limiting example 
only and with reference to the accompanying drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
Referring first to FIGS. 1a and 1b, there is shown diagrammatically and in 
developed form part of a stator 1 and the corresponding part of a rotor 2 
of a single-phase or multiphase electrical machine in accordance with the 
invention, such as an alternator. 
The stator 1 has a core 12 defining a continuous annular structure with a 
plurality of notches 13 on its inside periphery receiving sections of 
armature coils 14 in a manner that in itself is entirely conventional. 
There is an even number of notches 13 which are equi-angularly distributed, 
leaving poles 15 between them. 
The rotor 2 is defined by a succession of ferromagnetic material structures 
that are either separate or preferably in one piece. The structures are 
shown as individual structures in FIGS. 1a and 1b to simplify the 
explanation and comprise a U-shape first structure 21 with two branches 
211, 212 the free ends of which define two external projecting poles, the 
angular pitch of which is equal to that of the poles 15 of the stator in 
the case of a single-phase machine, and a base 213. 
A respective excitation coil 215, 216 is wound around each of the two 
branches 211, 212, the coils being interconnected to generate two magnetic 
fluxes in opposite directions, as described in more detail hereinafter. 
A plurality of equi-angularly spaced U-shape structures as described above 
is preferably provided. 
Interleaved between the U-shape structures 21 are the same number of 
permanent magnet structures 22 each comprising a permanent magnet 225 
trapped between two ferromagnetic members 221, 222 the faces of which that 
face towards the stator constitute poles, the poles of the U-shape 
structures 211 and those of the members 221, 222 being equi-angularly 
spaced around the outside periphery of the rotor. In the case of a 
three-phase machine the number of stator notches 13 is three times the 
number of rotor poles as defined hereinabove. 
The structures 21 and 22 are interconnected by ferromagnetic material 
intermediate members 23 forming magnetic connectors and occupying a radial 
dimension of the stator (which corresponds to its height in FIGS. 1a and 
1b) significantly less than the radial dimension of the structures 21 and 
22. 
As shown here, the connecting members 23 preferably extend essentially the 
height of the bases 213 of the U-shaped members 21, leaving above them the 
space needed for the outer sections of the two excitation coils 215, 216. 
Clearly, given the above explanations, the various components of the rotor 
can be provided in N sets disposed in successive groups, according to the 
required number of poles. 
The behavior of a rotating machine, in this instance an alternator, the 
operating principle of which is as described hereinabove will now be 
described with reference to FIGS. 1a and 1b. 
Consider first the situation in which there is no excitation current in the 
excitation coils 215, 216 (FIG. 1a). 
In this case the magnetic flux generated by the permanent magnets 225 
follows a closed circuit through the members 221, 222, the magnetic 
connectors 23 and the base part 213 of the U-shape members 21. 
As a result this flux, indicated by the arrows F1 in FIG. 1a, is not 
transmitted to the stator, except possibly to a negligible extent in the 
form of a leakage flux. The alternator is therefore in a non-excited 
condition and the stator coils deliver substantially no current. 
If current is fed to the excitation coils 215, 216 in a direction such that 
a downwardly directed magnetic flux is generated in the coil 215 at the 
upstream end (relative to the flux direction F1) and an upwardly directed 
magnetic flux is generated in the downstream coil 216, three main fluxes 
circulate between the rotor and the stator: 
a first flux indicated by the arrow F2 flows in the branches 211, 212 and 
the base 213 of the U-shape member 21 and between two adjacent stator 
poles, in the anticlockwise direction as shown in FIG. 1b; 
a second flux indicated by the arrow F3 flows clockwise via the left-hand 
branch 211 of the member 21, the magnetic connector part 23 and the member 
222 adjacent the magnet 225, and also via the stator between two adjacent 
poles thereof; 
the magnetic flux produced by the permanent magnet 225 encountering at the 
magnetic connector 23 a flux in the opposite direction (flux F3), this 
magnet flux circulates at least in part, as described in more detail 
hereinafter, through the two members 221, 222 adjacent the magnet and via 
two adjacent stator poles (anticlockwise flux F4); 
finally, a complementary homopolar magnetic flux F5 is produced by the 
magnet 225 and by the excitation coils 215, 216. 
In this way a succession of north poles and south poles is created on the 
rotor 2, enabling the stator coils to deliver a current. 
It is important to note here that the amplitude of the rotor current at the 
level of the rotor coils 215, 216 directly determines a general excitation 
level of the machine, which varies as a monotonous function of said 
current. 
To be more precise, when there is no rotor current (FIG. 1a) there is no 
excitation (see above). 
On the other hand, if the current in the coils 215, 216 is such that the 
absolute value of the flux flowing towards the left in the magnetic 
connectors 23 is equal to that which would be created by the adjacent 
permanent magnet in the absence of the rotor current, then all of the flux 
generated by said magnet is diverted towards the stator. The machine is 
then excited to the maximum and the means provided in the alternator for 
generating the variable rotor current are such that the current 
corresponding to the latter situation is a maximum current. 
This produces a machine in which, using a unidirectional excitation current 
varying between zero and a predefined maximum value, an excitation is 
obtained varying as a monotonous function of the current between zero 
excitation and a maximum excitation. 
It is therefore possible to dispense with any "H" electronic switching 
bridge or the like which, in the prior art mixed excitation machines, 
produces a bi-directional current according to the required excitation. 
This significantly reduces the cost of the switching means, which can 
comprise a single semiconductor switching device. 
Concrete embodiments of the rotor of a machine in accordance with the 
invention will now be described with reference to FIGS. 2 and 3. 
In both cases the ferromagnetic members described with reference to FIGS. 
1a and 1b are combined in a single core 20. FIG. 2 shows a rotor with 
eight poles, two diametrally opposed permanent magnets 225 and two pairs 
of excitation coils 215, 216. 
The ferromagnetic members corresponding to those from FIGS. 1a and 1b are 
designated by the same reference symbols. The part of the core 20 around a 
central bore 20a adapted to receive the rotor shaft defines the bases 213 
of the U-shaped members 21 and the magnetic connectors 23 which have a 
small radial dimension between said bore and the bottom of the notches 
that receive the outer sections of the coils 215, 216. 
If the FIG. 2 rotor is for a three-phase alternator the stator has 24, 
preferably equi-angularly spaced, notches in which the three-phase coils 
of the stator are fitted. 
FIG. 3 shows a rotor with 12 poles, three magnets 225 equi-angularly spaced 
at 120.degree. and three pairs of excitation coils 215, 216. In this case, 
for a three-phase machine, a stator with 36, preferably equi-angularly 
spaced, notches is provided. 
Variants of the rotor cores shown in FIGS. 2 and 3 will now be described 
with reference to FIGS. 4 and 5. 
FIG. 4 shows that the core 20 of the rotor is made as two sectors 20a, 20b 
designed to extend between the two magnets 225 and joined to them at 
assembly time in order to form a continuous cylindrical structure. 
In FIG. 5 there are three sectors 20a, 20b and 20c joined together with 
three magnets to form the rotor. 
This separation of the rotor into N sectors (N varying with the required 
number of poles) is advantageous in that it facilitates manufacture of the 
coils, the various sectors being easier to wind separately than a single 
cylindrical core. 
Note another advantage of this invention: because the magnetic flux from 
the magnets is closed on itself when there is no excitation current, there 
is no risk of the rotor sticking to ferromagnetic members that may happen 
to be on the assembly line during assembly of the machine. 
Of course, the present invention is not limited to the embodiments 
described and shown and the skilled person will know how to vary or modify 
them in any way within the spirit of the invention. 
In particular, any combination of magnet structures and coil structures can 
be provided in the rotor, for example two coil structures or more between 
each pair of magnet structures, or two magnet structures or more between 
each pair of coil structures. 
In this case the coils and magnets are designed so that the maximum flux of 
the coils can block most of the circumferential flux generated by the 
magnets in the absence of any excitation current. 
Each coil structure can include only one judiciously disposed coil. 
Each magnet structure can have two or more magnets, the fluxes from which 
combine to obtain the required effect of a circumferential flux in the 
rotor in the absence of any excitation by the coils.