Rotating machine, such as motor vehicle alternator

Details Rotating machine, such as motor vehicle alternator Rotating machine, such as motor vehicle alternator

2000-02-25

2001-08-07

Nguyen, Tran (Department: 2834)

Electrical generator or motor structure

Dynamoelectric

Rotary

C310S156030, C310S261100, C310S269000

Reexamination Certificate

active

06271613

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to rotary machines such as alternators for motor vehicles.
The single-phase or multi-phase generator constituted by a conventional motor vehicle alternator generally comprises a stator inside which there rotates a rotor provided with an excitation winding. The winding is powered by brushes in contact with two slip rings provided on a projecting portion of the shaft of the rotor.
Rotary machines are already known, in particular from EP-A-0 707 374, in which, in particular to increase their efficiency, the rotor excitation field is provided both by permanent magnets and by windings (this may be referred to as “combined” excitation), and in which the current delivered by the secondary is controlled by commutator means at the excitation windings, which commutator means make it possible selectively to reverse the excitation direction so as to reduce the flux from the permanent magnets or even make it substantially zero.
The need to reverse the excitation current direction makes it necessary to use an H-shaped semiconductor switching bridge whose cost is high and which thus pushes up the cost of the machine.
Electric motor structures are also known, in particular from FIG. 19 of Patent Application WO96/30992, in which the rotor is provided with:
at least two successive permanent excitation magnets which, in the rotor, generate two magnetic fluxes having components extending tangentially around the rotor structure in opposite directions in the direction of rotor movement; and
an even number of slots between the excitation magnets, together with windings that are wound in said slots and that are suitable for being powered in a full excitation direction or in a reverse direction so as to define alternating poles between said slots.
Those machines are not fully satisfactory insofar as slots between the magnets receive legs from two successive windings, each winding occupying only one half of the volume of a slot.
The fact that each slot between the magnets receives legs from two successive windings makes it necessary for the slots directly adjacent to the magnets to be filled only half-full. The ampère-turn distribution is thus asymmetrical in the slots of the wound poles; the slots directly adjacent to the magnets receive, in absolute terms, only one half of the absolute value of the ampère-turns received by the other slots.
That magnetic asymmetry modifies the usual distribution of the magnetic field lines (compared with wound machines having projecting poles, the machines have magnets only), thereby giving rise to premature saturation in the yoke or requiring the yoke to be over-dimensioned.
That is what is illustrated in
FIG. 1
a
which shows a machine of the type proposed in Patent Application WO96/30992, together with the lines on which the magnetic fluxes are looped when the windings in the slots of the rotor are powered in their full excitation direction.
BRIEF SUMMARY OF THE INVENTION
An object of the invention is to mitigate that drawback.
In the Applicant's Patent Application FR 98 02886 (which, it should be noted, is not part of the state of the art to be taken into account when determining the inventive step of the invention), the Applicant has already proposed an alternator whose secondary current is regulated by acting on the excitation of powered rotor coils.
The structure described in that patent application is of the type shown in
FIGS. 1
b
,
2
a
, and
2
b
. It comprises a stator
1
and a rotor
2
.
The stator
1
has a frame
12
defining an annular structure whose inside periphery is provided with a plurality of slots
13
which are defined between poles
15
of the rotor and which receive secondary winding legs
14
.
Throughout the present text the term “leg” is used to designate an assembly comprising the various portions of the turns of the same winding that are received together in the same slot.
The rotor
2
has a frame
22
in which a plurality of slots
23
are also provided, which slots define poles
25
and receive both permanent magnets
27
whose flux lines are oriented tangentially around the rotor structure, and excitation winding legs
26
.
The following are thus found in succession in the adjacent slots
23
of the rotor:
a magnet
27
having a first polarity;
two legs of a winding
26
wound on a pole
25
and received in two adjacent slots;
a second magnet
27
having a second polarity opposite from the first polarity; and
two legs of a winding
26
that are suitable for passing currents in directions opposite to the directions of the currents flowing through the legs of the preceding winding
26
, etc.
For example, the rotor and the stator may have the same number of poles, e.g.
12
.
With such a structure, in the absence of current in the legs
26
, the rotor adopts a polarity pattern such as . . . S-N-N-N-S-S-S-N-N-N, etc. (
FIG. 2
a
).
Thus, in the absence of current through the windings
26
, the N-S polarity pattern has a pitch that is equal to three times the pitch of the teeth (poles
25
) of the rotor and the energy transferred between the rotor and the stator is at a minimum.
In contrast, when a current flows through the windings of the rotor, the polarities of said rotor then become . . . N-S-N-S-N-S, etc. with a pitch that corresponds to the pitch of the teeth of the rotor; the magnetic energy that is transferred between the stator and the rotor is higher, and it increases with increasing excitation current.
FIGS. 1
b
and
2
b
show the lines along which the flux loops when the legs are powered in the excitation direction. As shown clearly in these figures, each of the flux lines loops through two successive poles.
FIG. 2
a
shows flux lines in the absence of excitation current.
In order to limit magnetic flux leakage and in order to prevent the flux of the magnets from looping along the shaft of the machine, FR 98 02 886 proposes making the shaft of the rotor of a non-magnetic material and shaping said shaft and the laminations of the rotor so as to keep the thickness of the frame beneath the magnets as small as possible.
Unfortunately, the use of non-magnetic materials (e.g. titanium) can give rise to large extra manufacturing costs.
Furthermore, mechanical strength constraints prevent the thickness of the frame between the magnets and the shaft of the rotor from being kept as small as would be desirable.
Another object of the invention is to mitigate those drawbacks.
The invention provides an electric machine comprising a stator and a rotor, the stator having at least one secondary winding received in at least one pair of slots, the rotor having at least two successive excitation magnets which generate two magnetic fluxes in the rotor and having components in opposite tangential directions around the structure of the rotor, said rotor having an even number of slots between said two magnets, which slots define projecting poles between them and receive winding legs that are suitable for being powered to co-operate with the magnets to define alternating poles, said electric machine being characterized in that each of the slots that receives the winding legs is occupied only by a single leg or by a plurality of legs wound around the same projecting pole, and in that the shaft of the rotor is made of a magnetic material, said machine including at least one non-magnetic zone in the vicinity of said shaft and in the vicinities of said magnets, which non-magnetic zone minimizes or eliminates flux leakage towards the shaft of the rotor.
In particular, in a preferred embodiment, the frame of the rotor is provided with a plurality of holes which extend in the vicinity of the shaft of the rotor, and which are distributed between the magnets, said holes constituting non-magnetic zones which define constriction areas on either side of each of the magnets, thereby limiting flux leakage to the shaft.
In yet another embodiment, the shaft has a non-magnetic covering which constitutes a spacer separating it from the magnets.
Advantageously, each of the laminations that makes

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