Two-phase motor, particularly a time piece motor or a motor for driving the hand of a display

A two-phase motor formed by a stator part excited by electric coils and a magnetized rotor. The rotor has N pairs of poles which are radially magnetized in alternate directions, N being equal to 3 or 5. The stator part has at least two W-shaped circuits comprising each an electric coil surrounding the central leg. The "W" circuits are so arranged that when one of the central legs faces a magnetic transition, the other central leg faces a magnetic pole. The polar opening outs of the legs of a "W" circuit are angularly spaced by .pi./4. The polar opening outs of the central legs of the two "W" circuits belonging to different phases are angularly spaced apart by an angle which is substantially equal to .pi./2.+-.k..pi./N, where N is the number of pairs of magnetic poles, that is 3 or 5, and k is equal to 0, 1 or 2.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The present invention relates to a two-phase electric motor, especially a 
motor for a counter mechanism or a motor for driving an indicator pointer. 
SUMMARY OF THE INVENTION 
Such motors are described, for example, in European Patent EP 587,685 of 
the Applicant. The object of the invention is to improve such motors so as 
to increase the torque and eliminate the magnetic shunt without losing the 
advantage of simplicity of manufacture and the possibilities of 
integrating associated mechanisms such as step-down gears. 
To this end the invention relates to a two-phase motor formed by a stator 
part excited by electric coils and by a magnetized rotor provided with N 
pairs of poles magnetized radially in alternate directions, N being equal 
to 3 or to 5. The stator part is provided with at least two W-shaped 
circuits, each having an electric coil surrounding the central leg. The W 
circuits are disposed such that when one of the central legs is facing a 
magnetic transition, the other central leg is facing a magnetic pole. The 
pole shoes of the legs of a W circuit being spaced angularly by .pi./4 and 
the pole shoes of the central legs of two W circuits belonging to 
different phases being spaced angularly by an angle substantially equal to 
.pi./2.+-.k. .pi./N, where N is the number of pairs of magnetic poles, in 
this case 3 or 5, and k is equal to 0, 1 or 2. 
Such a motor meets the objectives of the invention by virtue of an 
increased number of active teeth (6 instead of 4) and by virtue of a 
shorter path of the lines of flux in air. The "W" shape of the stator 
circuit ensures closing of the lines of flux between the central pole 
accommodating the coil and the two adjacent poles. 
Advantageously, the two "W" circuits have a common leg. The stator can 
therefore be constructed as a single piece, thus eliminating any problem 
of angular positioning that is found with stators formed from a plurality 
of pieces fixed on one support. 
In a preferred embodiment, the invention relates to a motor of low assembly 
cost and very small thickness. 
According to a first embodiment, the outer legs of the two "W" circuits are 
connected by a yoke shaped like the arc of a circle, the yoke 
advantageously being provided with stator teeth having a spacing of .pi./4 
degrees. This embodiment permits construction of a motor with a smaller 
number of components, namely a rotor, a stator formed by a single piece 
supporting only two electric coils. 
The prior-art step motors also exhibit jerky displacement, due to the fact 
that a magnetized pole of the rotor assumes a preferential equilibrium 
position when it is placed facing a stator tooth or when a transition 
between two magnetic poles is facing a stator tooth. Consequently the 
relaxing torque constitutes a periodic function of the angular position, 
with frequency depending on the number of magnetic poles and on the number 
of stator poles. Attempts have been made in the prior art to "smooth" this 
function by increasing the number of poles. However, this entails greater 
mechanical complexity and high manufacturing and assembly cost. 
A second object of the present invention is to remedy this drawback by 
proposing a construction that permits substantial limitation of the 
relaxing torque with a reasonable number of magnetic and stator poles. 
To this end, the invention relates to a preferred embodiment using four "W" 
circuits, the adjacent legs of which are common, thus forming an 8-pole 
stator. The central legs of the diametrically opposite "W" circuits 
support coils of like phase. 
This structure permits considerable reduction of the relaxing torque by 
canceling out harmonics 2 and 4 of the function (relaxing torque, angular 
position). The motor according to the invention therefore exhibits smooth 
displacement despite the reduced number of magnetic or stator poles. 
Advantageously, the stator comprises a single piece cut such that it has 
two pairs of diametrically opposite "W" circuits, the pole ends of which 
are angularly equidistant. Advantageously, the legs accommodating the 
electric coils extend into a hollow central part, the dimensions of which 
make it possible to introduce an electric coil into the plane of the legs 
and then move it sideways to position it around the corresponding leg. 
The coils can therefore be manufactured separately by processes more 
economical than winding around a complex stator part, and can then be 
slipped onto the corresponding legs. 
The stator can be manufactured by simple and inexpensive processes. The 
fact that it is formed by a cutting as a single piece circumvents the 
problems of assembly precision and loss of adjustment. 
Preferably, the legs supporting the electric coils extend radially. 
According to a specific embodiment, the stator comprises a stack of thin 
laminations. This embodiment makes it possible to use inexpensive 
machining techniques for cutting the stator and to improve the magnetic 
performances of the stator. 
Preferably, the front end of the pole shoe is situated in the plane of the 
inner front face of the coil. 
According to a specific embodiment, the rotor is formed by a magnet 
magnetized in a direction perpendicular to the plane of the stator and 
placed between two slotted disks having offset teeth. 
According to a specific embodiment, the two-phase motor is provided with an 
auxiliary magnetic shunt, advantageously a magnetic shunt of soft material 
coaxial with the rotor. 
According to a first embodiment, the magnetic shunt is a cylindrical piece 
in the form of a ring. This ring can be fixed relative to the rotor or can 
be integral with the magnet. 
According to a second embodiment, the magnetic shunt is a cylindrical piece 
in the form of a ring having grooves forming teeth facing the stator teeth 
.

DISCUSSION OF THE PREFERRED EMBODIMENTS 
FIGS. 1 and 2 respectively represent top and cross-sectional views of a 
motor according to the invention. The motor comprises a stator (1) and a 
rotor (2). 
The stator (1) is cut from sheet metal with a thickness of 2 millimeters, 
or is made by stacking a plurality of metal laminations of magnetically 
soft material. The stator is provided with two "W" circuits (3, 4), each 
having a central leg (5, 6) and two side legs (7, 8 and 9, 10). The 
central legs (5, 6) are each terminated by respective pole shoes (11, 12). 
The side legs are also provided with pole shoes (14 to 16), the two legs 
(8, 9) belonging respectively to the first "W" circuit (3) and to the 
second "W" circuit (4) being provided with a common pole shoe in the 
practical example described here. 
The stator (1) is also provided with holes (17, 18, 19), through which a 
means such as screws or rivets for fixation to a support can be passed. 
The central legs (5, 6) accommodate electric coils (20, 21) respectively. 
These two legs (5, 6) are oriented radially. 
The rotor (2) is provided with 5 pairs of magnetic poles. It is formed by a 
cylindrical piece (25), magnetized radially to present alternating SOUTH 
and NORTH poles at the outer surface (26). The magnetized part can be 
formed by a combination of thin magnets in the form of tiles bonded to a 
cylindrical core or by magnetization of tubular sectors. 
Yet another embodiment consists in magnetizing a cylindrical piece in 
scallop-shaped profiles at the surface, such that two opposite poles are 
formed by adjacent elements of peripheral surface, connected by a zone in 
the form of an arc of a circle extending to the inside of the cylindrical 
element. 
The rotor (2) is supported in known manner by pivots (28, 27) as shown in 
FIG. 2. 
The central legs (5, 6) of the two opposite phases supporting the electric 
coils (20, 21) are oriented such that, when a magnetic pole (30) is facing 
one of the pole shoes (12), the other pole shoe (11) is facing a 
transition (31). 
The rotor (2) is provided with a slotted ring (32) suitable for driving a 
gear train (33 to 35). This gear train transmits the rotary movement to an 
output shaft (36). 
In the example described, the stator (1) is housed in a case formed from 
two complementary shells (37, 38), which also provide support for the 
shafts of the gear train (32 to 35). 
FIG. 3 represents an embodiment different from that described in the 
foregoing in that the pole shoes (14, 16) forming the ends of the outer 
legs (7, 10) of the two circuits are joined by a yoke (40) provided with 
polar teeth (41 to 43). The complete unit formed by the "W" circuits (3, 
4) and by the yoke (40) is cut from a single piece of magnetically soft 
material or from a plurality of laminations forming a stack. 
The central part (45) of the stator (1) is hollow. The electric coils (20, 
21) are introduced by moving them first in a direction perpendicular to 
the stator plane, which corresponds to the plane of FIG. 3, then by moving 
them in radial direction in order to slip the coil around one of the 
central legs (5, 6). Of course, this operation is completed before the 
rotor (2) is installed. The coil can be formed on a core (46, 47), the 
cross section of which is complementary to that of the legs (5, 6), the 
said core having a front face (48, 49) flush with the end of the pole shoe 
(11, 12) of the corresponding leg (5, 6). 
FIG. 4 represents a top view of a third embodiment different from the 
foregoing embodiments in that the "W" circuits (3, 4) are separate. They 
are positioned such that, when the pole shoe (11) of the central leg (5) 
of one of the "W" circuits (3) is facing a magnetic transition (31), the 
pole shoe (12) of the central leg (6) of the other "W" circuit (4) is 
facing a magnetic pole (50). 
FIG. 5 shows a top view of a fourth embodiment different from the foregoing 
embodiments in that it has four "W" circuits (51, 52, 53, 54). Two 
diametrically opposite "W" circuits have like phase. The central legs (55, 
56, 57, 58) each support an electric coil (59, 60, 61, 62). Two successive 
side legs (63, 64) , (66, 67), (69, 70), (72, 73) represent respective 
common pole shoes (65, 68, 71, 74). The successive pole shoes (65, 68, 71, 
74, 75 to 78) are angularly separated by .pi./4. 
The stator provided with the four "W" circuits (51 to 54) is cut from a 
single piece or from a stack of pieces, defining a central hollow (80) 
with dimensions and shape permitting coils (59 to 62) to be slipped onto 
the central legs (55 to 58). 
FIGS. 6 and 7 respectively represent cross-sectional and top views of a 
rotor for the motor according to the invention. The rotor (2) is formed by 
two slotted disks provided with offset teeth (81, 82), between which there 
is positioned a magnet (83) magnetized in a direction perpendicular to the 
central plane of the stator. The teeth (89 to 93) of one of the slotted 
disks (81) coincide with slots formed between two successive teeth (94 to 
98) of the other slotted disk (82). Each of the slotted disks (81, 82) is 
provided with 5 teeth separated by 5 slots. 
FIG. 8 represents a cross-sectional view of another rotor embodiment. The 
rotor has a ring (99) of magnetically soft material, provided with ten 
zones (100 to 109) magnetized in scallop-shaped profiles such that they 
exhibit alternating poles at the surface of the said ring (99). 
The description of the invention hereinabove is given by way of 
non-limitative examples. The person skilled in the art will be able to 
construct different stator or rotor forms in accordance with the 
invention.