CH675810B5 - - Google Patents

Abstract

The motor includes two stator pole pieces (2a, 2b) surrounding a rotor (3). A coil former (4) links the two pole pieces together, and a coil is wound on this former. The pole pieces have a degree of asymmetry w.r.t. the axis (BB') passing through the rotor, in order to provide favourable starting conditions. The core of the coil is made from a semi-hard ferromagnetic material having a coercive field between 150 and 450/m. The permanent magnet rotor has a coercive field of less than 300 kA/m. ADVANTAGE - Has more efficienct operation.

Description

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CH 675 81OG A3

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Description

The present invention relates to a stepping motor for horological application, making it possible to produce an economical motor with good performance.

The current stepping motors used in quartz watches with analog display have a SmCos rotor, stator pole pieces and a coil core made of ferromagnetic material with high permeability and low coercive field (for example FeNi alloys), a coil of several thousand turns being wound around the coil core. Figure 1 illustrates an engine of this type. In these motors, the rotor is subjected to two types of couples: the positioning torque Ca due to the interaction between the magnet and the stator reluctant structure, the mutual torque Cab due to the interaction between the field created by the current flowing in the coil and the field of the magnet. These 2 couples are shown in Figure 2.

In the absence of current, the rotor is in the stable equilibrium position Si (a = 0). When a voltage pulse is applied to the motor coil, under the effect of the current flowing in the coil, the Cab torque appears and moves the rotor in the direction S2. The voltage pulse is generally interrupted when the rotor exceeds the unstable equilibrium position I, the rest of the path to S2 being provided by the torque Ca becoming positive and the kinetic energy stored in the rotor.

It can be seen that in such an engine, the duration of the supply pulse depends on the amplitude of the mutual torque Cab. In fact, the greater the Cab torque, the shorter the time required for the rotor to travel between Si and I.

The amplitude of the mutual torque Cab depends, on the one hand, on the intensity of the field created by the coil, therefore the intensity of the current and, on the other hand, on the intensity of the field created by the magnet. To obtain a low current consumption, magnets with a very high energy product are used in these motors such as SmCos magnets whose energy product is around 170 kJ / m. The rotor magnet in SmCos is an expensive element compared to the manufacturing cost of the motor.

The use of cheaper magnets such as ferrites, whose energy product is between 4 and 28 kJ / m3 in these types of motors unfortunately leads to unacceptable current consumption for the life of the battery.

The object of the invention is to remedy the aforementioned drawback by proposing a solution in which the use of a low energy product magnet is possible. The invention will be better understood on reading the description which follows, with reference to the appended drawing in which:

- Figure 1 shows a conventional stepper motor;

- Figure 2 shows the static torques in the motor shown in Figure 1:

- Figure 3 shows a first embodiment of the engine according to the invention;

- Figure 4 shows a second embodiment of the engine according to the invention;

- Figure 5 shows a third embodiment of the engine according to the invention.

FIG. 3 represents a first example of execution of the engine according to the invention. This motor comprises 2 stator pole pieces 2a and 2b, the rotor 3, the coil core 4 and the coil 5. The stator pole pieces 2a and 2b have an asymmetry with respect to the axis BB ', in order to create a position of favorable start for the rotor. The pole pieces 2a and 2b are made of a soft ferromagnetic material with a low coercive field, while the coil core 4 is made of a semi-hard magnetic material. Systematic measurements have shown that for horological applications, when the maximum current delivered by the integrated circuit is of the order of 1 mA, the optimal coercive field of the semi-hard magnetic material is between 150 and 450 A / m , depending on the size of the engine.

The use of a semi-hard material for the coil core allows short pulses to be applied. Indeed, the duration of the pulse in this case is related only to the time necessary to reverse the operating point in the hysteresis cycle constituting the coil core and not to the movement of the rotor.

In French patent application 7 234 667, the applicants of which are DE VALROGER and LAVET, it is described on page 21, line 34 - »page 24, 25 a multipolar motor for large volume clocks whose pole pieces 107 of Figure 23 are made of semi-hard magnetic material.

The use of pole pieces of semi-hard material in place of the coil core requires a larger current to reverse the operating point in the hysteresis cycle, as shown in Figure 3. We see that the flux created by the coil is composed of a leakage flow <î> o and the main field flow oh. The coil core is crossed by all 2 fluxes, while the pole pieces are crossed only by © h. In addition, the geometry of the motor shown in Figure 3 implies a smaller core section than that of the pole pieces. Under these conditions, the induction in the coil core is much greater than that in the pole pieces, allowing an easier reversal from the operating point of view.

This characteristic, which is not very important in motors for high-volume clocks with high volumes and energy sources, becomes essential for small timepieces in which there is not always the field necessary to reverse the operating point in the hysteresis cycle of the semi-hard magnetic material.

Thanks to the particular location of this semi-hard magnetic material at the level of the coil core and the choice of this material with a coercive field between 150 and 450 A / m, it is possible to envisage the use of the motor according to the invention in the

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CH 675 81OG A3

small timepieces such as wristwatches.

FIG. 4 represents a second exemplary embodiment of the motor according to the invention, in which the 2 pole pieces 6a and 6b are connected together by the saturable zones and the favorable starting position of the rotor is created by the cells 7a and 7b.

FIG. 5 shows a third embodiment of the motor according to the invention, in which the coil core and one of the 2 pole pieces form a single piece of semi-hard magnetic material. For reasons mentioned above, the process of reversing the operating point in the hysteresis cycle takes place essentially at the level of the coil core, the pole piece forming a single piece with the coil core mainly serving to channel the flow. up to the rotor level.

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Claims (2)

Claims 1. Stepper motor for timepieces comprising a permanent magnet rotor, stator pole pieces made of ferromagnetic material, a coil core and a coil, characterized in that said coil core is made of semi-hard ferromagnetic material with a coercive field between 150 and 450 A / m. 2. Motor according to claim 1, characterized in that the coercive field of the rotor magnet is less than 300 kA / m. 35 40 45 50 55 60 65 4