The invention belongs to the technical field of electrical machines, in particular of the synchronous type, controlled in motor operation to generate a mechanical torque.

The invention is particularly advantageously applied to synchronous brushless machines, the position of the rotor of which is measured by magnetic sensors, such as Hall effect sensors.

STATE OF THE ART

A synchronous motor produces mechanical torque when stator windings are supplied with electric current and rotate magnetic elements of a rotor. In a popular configuration, the stator is a hollow cylinder and the magnetic elements of the rotor rotate inside the stator about its axis of rotation.

In a brushless or “self-driven” synchronous motor, the supply currents of the stator windings are controlled as a function of the angular position of the rotor around its axis.

The direction of the magnetic field produced by the rotor at a point varies depending on the angular position of the rotor.

Thus, in order to detect the angular position of the rotor, it is known to have, on the same section plane of the stator, magnetic sensors such as Hall effect sensors. These sensors can measure the direction of a normal component of the magnetic field.

Figure 1 shows, from top to bottom, position signals a1, a2, a3 acquired at the terminals of three Hall effect sensors separated by 60 °, as a function of the angular position Q of a given radius of the rotor, in a configuration with three stator windings.

In this figure, the square wave position signal at the terminals of a given sensor makes a transition during a change of direction of a component of the magnetic field which is normal to said sensor.

However, the measurement provided by a Hall effect probe according to the above principle may suffer from a certain uncertainty. This measurement uncertainty depends in particular on the size of the electric machine and on its operating speed, and can reach several degrees, for example up to 5 degrees.

However, when the rotor is near the polarity reversal position of a Hall effect sensor, the absolute values ​​of the magnetic field perceived by the sensor are close to zero compared to this uncertainty range.

Thus, in extreme cases of measurement error, the sensor can detect a change in polarity while the rotor has not passed the reverse polarity position, or vice versa.

The rotor position determination can then be erroneous, with the consequence of a non-optimal current control of the windings (which can create torque oscillations, that is to say non-negligible variations between the real value of the motor torque supplied and face value).

Another problem posed by the current use of Hall effect probes is a possible calibration error of the probes. For example, if the probes are assumed to have an angular difference of 60 ° two by two, a probe may be offset and have a difference other than 60 ° with neighboring probes.

However, the magnetic polarity measurement provided by Hall effect sensors is very sensitive to the inclination and to the position of the sensors.

Here again, the determination of the position of the rotor is erroneous. Motor torque oscillations are to be deplored, even in a hypothetical case where the measurement of magnetic polarity at the level of the sensors would not suffer from uncertainty.

A known solution for improving the timing of Hall effect probes consists in making probe housings in the stator and gluing each probe into a housing. However, this solution is not entirely satisfactory because the probe may shift if there is play in the housing. Moreover, this solution does not solve the

problem mentioned above of measurement uncertainty. The detection of the angular position of the rotor remains too imprecise.

We therefore always deplore a level of engine torque oscillations which is unacceptable for many practical applications.

GENERAL PRESENTATION OF THE INVENTION

There is a need for an electric motor for which the information supplied by magnetic rotor position sensors is reliable, so as not to generate motor torque oscillations.

We are looking for a solution that can be adapted to synchronous motors with Hall effect sensors for detecting the position of the rotor.

Preferably, the desired solution must have a low mass and small size, so as to be able to be used in many technical contexts and in particular in aeronautics.

As such, the invention relates to an electric machine according to claim 1.

The electric machine of the invention has several advantages.

The part added to the stator creates an amplification zone of the normal component of the magnetic field, in particular of a gradient of the normal component of the magnetic field, in the vicinity of the detection surface of the magnetic sensor. Thus, for the same angular displacement of the rotor, the variation of said normal component of the field is greater. The measurement uncertainty provided by the sensor therefore has a lower impact on the detection of the reverse polarity position.

This increases the precision of the detection of the position of reverse polarity of the rotor, without changing the magnetic sensor.

In addition, the addition of the magnetic field amplifying part does not require any in-depth modification of the architecture of the electric machine. The field amplifying part can for example be simply inserted between the magnetic sensor and the stator.

A simple solution is thus available for improving the electromagnetic performance of the electric machine, without greatly encumbering or weighing down the system.

In addition, the metal magnetic field amplifying part, which is in one piece, ensures that the angular difference between the two magnetic sensors is compliant. It is possible to perform the wedging of a single amplifying piece, instead of the wedging of several separate sensors. In addition, the presence of a third zone of reduced radial thickness makes it possible to prevent the magnetic field from saturating at the level of the first zone and the second zone of the part, the first zone and the second zone being located in the vicinity. magnetic sensors.

The machine of the invention can have the following additional, non-limiting characteristics, taken alone or in any of the technically possible combinations: the machine comprises a first magnetic sensor and a second magnetic sensor arranged in the same orthogonal section plane to an axis of rotation of the rotor of the machine, the part being in one piece and comprising a first amplification zone extending in a first angular sector of the machine in which the first sensor is arranged as well as a second amplification zone extending in a second angular sector of the machine in which the second sensor is placed.

An advantage of this variant is that both the impact of the measurement uncertainty of the sensors and the probability of calibration errors of one of the sensors are limited. The magnetic field amplifying part, which is in one piece, acts as a magnetic field amplification for several sensors. This ensures that the angular difference between the two sensors is compliant. It is possible to perform the wedging of a single amplifying piece, instead of the wedging of several separate sensors. With this variant, the errors in detecting the angular position of the rotor are further reduced;

- in this last variant, a third zone of the part extends from the first zone to the second zone, a maximum radial thickness of the third zone in the orthogonal section plane being less than a minimum radial thickness of the first zone and at a minimum thickness of the second zone in the orthogonal section plane.

An advantage of this additional characteristic is to prevent the magnetic field from saturating the first zone and the second zone of the part;

the first angular sector over which the first amplification zone extends has an angular amplitude of between 10 ° and 30 °, preferably 20 °;

- the part has the shape of a ring or the shape of a ring sector;

the machine comprises magnetic sensors each distributed in the vicinity of a field amplification zone and at angular positions regularly spaced along a machine perimeter;

- the machine includes six magnetic sensors spaced two by two by 60 °;

- the magnetic sensors are interposed between the part and the rotor and are mounted fixed on the part;

- a magnetic sensor includes a Hall effect probe;

- a magnetic element of the rotor comprises a permanent magnet or an electromagnet; - the electric machine further comprises a device for controlling an electric current flowing in the windings, the control device being configured to control a frequency of an electric current within one of the coils as a function of a polarity change signal transmitted by a magnetic sensor;

- in this last variant, the machine comprises three windings configured to operate with a three-phase current supply, the control device being configured to generate, via the windings, an electromotive force of the trapezoidal type in order to generate a rotation of the rotor relative to the stator.

GENERAL PRESENTATION OF FIGURES

Other characteristics, aims and advantages of the invention will emerge from the following description, which is purely illustrative and non-limiting, accompanied by FIG. 1 already commented on above as well as by the other appended drawings, among which:

Figure 2 is a perspective view of a synchronous machine of the prior art comprising a stator with three windings and a rotor with permanent magnets;

Figure 3 is a partial sectional diagram of a synchronous machine according to a first embodiment of the invention, with a section plane perpendicular to the axis of rotation of the machine;

Figure 4 schematically shows a Hall effect sensor;

Figure 5 is a magnetic field map at the interface between the rotor and the stator in a machine not provided with a magnetic field amplifying part;

Figure 6 is another magnetic field map at the interface between the rotor and the stator in a machine having a magnetic field amplifying part;

FIG. 7 is a partial sectional diagram of a synchronous machine according to a second embodiment of the invention, with a section plane perpendicular to the axis of rotation of the machine.

DETAILED DESCRIPTION OF EMBODIMENTS

In the description which follows and in the appended figures, similar elements are associated with the same reference numerals.

The term “magnetic elements” is understood to mean the elements responsible for setting the rotor in rotation in motor operation, by interaction with the magnetic field of the stator; the magnetic elements can in particular be permanent magnets or electromagnets supplied with direct current.

Furthermore, in what follows, particular embodiments of the invention will be described in the case of a synchronous machine in motor operation, the speed of rotation of the rotor of which depends on the frequency of the electric current applied to the windings. The stator here extends outside the rotor.

However, the invention applies with the same advantages for another synchronous machine architecture, for a synchronous machine in generator operation, or for any other type of electrical machine in which the angular position of the magnetic elements must be measured.

FIG. 2 shows a synchronous machine 1 according to one embodiment, which can be used in an aircraft engine.

The machine 1 comprises a rotor 2 with permanent magnets and a stator 3 in the form of a hollow cylinder extending around the rotor 2.

The rotor 2 comprises magnetic elements, here four permanent magnets 4 spaced 90 ° two by two, including two face-to-face magnets having a "North" polarity and two face-to-face magnets having a "South" polarity. Alternatively, one could provide a different number of magnetic elements. The rotor 2 has a degree of freedom in rotation about a rotor shaft 8 along an axis A.

Facing the magnets 4 of the rotor, the stator 3 has, on an inner surface, stator windings 5. The windings 5 ​​are suitable for being supplied with an electric current and for driving the rotor in rotation with respect to the stator, due to the interaction between the magnetic field of the windings and the magnetic field of the magnetic elements of the rotor. Here, the machine 1 comprises three coils 5 spaced 120 ° two by two around the axis A. Each coil is wound around a magnetic pole of the stator and is limited inwardly by a surface 50 facing the rotor.

In motor operation, the power supply to the stator windings creates a magnetic flux which interacts with the magnetic field of the rotor magnets and turns the rotor. A frequency of the current supplied to the windings determines a speed of rotation of the rotor shaft 8.

In generator operation, an external mechanical torque is applied to the rotor. The magnetic flux varies within the coils and induces an electric current in the coils which can be recovered.

In the example which follows, we go to engine operation.

The machine 1 has three windings 5 ​​supplied with three-phase current by an electrical supply control device (not shown in the figures).

The electromotive force at the level of a given winding depends on the voltage at its terminals. It is sought to obtain, at the level of the shaft 8 of the rotor, a mechanical torque corresponding to a predetermined setpoint, for example a constant torque. To do this, the direction of the electric current is switched across the terminals of each of the windings in a manner synchronized with the rotation of the rotor.

For a given winding, when the rotor exceeds a predetermined angular position, it is known that the magnetic field within the winding has changed (for example, if the interface between a "North" magnet and a "South" magnet has passed in front of a winding axis) and that it is necessary to switch the direction of the current at the terminals of said winding.

It is therefore necessary to provide the power supply control device with very precise information on the position of the rotor.

Indeed, if the position information supplied to the power supply control device is imprecise, the voltages of the windings are switched at the wrong time. The stator and the rotor are no longer synchronous and motor torque oscillations are generated.

In order to measure the position of the rotor, it is possible to place, on a plane offset axially with respect to the surfaces 50 of the stator, magnetic rotor position sensors (not shown in FIG. 2). Said axial offset is understood to be along the axis A of the electric motor.

Typically, the rotor 2 extends over an axial extension beyond the coils 5 and the surfaces 50 (this extension not being illustrated in FIG. 1). The magnetic rotor position sensors are positioned facing said axial extension, and are spaced axially with respect to the surfaces 50.

In a preferred embodiment, the magnetic rotor position sensors are Hall effect sensors.

Depending on the direction of the component of the magnetic field normal to the sensor, the potential difference across the sensor is positive or negative. By “normal component” is meant the projection of the total magnetic field detected by the sensor onto an axis of the sensor orthogonal to the sensor plate. The sensor plate forms a detection surface for the normal component of the magnetic field. The sensor can therefore detect a change in the position of the rotor which causes a change in direction of the normal component.

As an alternative, other types of magnetic sensors can be used, for example inductive sensors.

The power supply control device controls the displacement of the rotor along its period. The power supply control device can be configured to generate at the stator windings a trapezoidal type electromotive force.

There is schematically shown in Figure 3 a half of a synchronous machine according to one embodiment of the invention, seen in section along a sectional plane orthogonal to the axis A of the rotor 2. The rotor 2 comprises magnetic elements, here four permanent magnets 4 on the periphery of a non-magnetized zone 40.

The cutting plane passes through three Hall effect sensors 6a, 6b and 6c placed on respective surfaces 60 facing the rotor.

Figure 4 shows the Hall effect sensor 6b. The Hall effect sensor comprises a plate 61 to the terminals of which an Ex voltage is applied. The wafer 61 forms a surface for detecting the normal component of the magnetic field exerted on the sensor.

When the sensor 6b is subjected to a magnetic field having a component normal to the detection surface, bearing the reference B in the figure, a potential difference Ey appears at the terminals of the sensor. The sign of the potential difference Ey corresponds to the direction of the normal component of the field with respect to the detection surface.

Returning to Figure 3, each of the Hall effect sensors must be oriented so that a change in direction of the normal component across a sensor corresponds to a passage of an interface between a north magnet of the rotor (bearing the reference N in the figure) and a south magnet of the rotor (bearing the reference S).

The angular difference i2 between the sensors 6a and 6b is 60 °. Likewise, the angular difference between the sensors 6b and 6c is 60 °.

In Figure 3, the rotor is in an angular position where the sensor 6a and the sensor 6c do not perceive a change in direction of the normal component of the field.

On the other hand, the normal component of the magnetic field on the sensor 6b is canceled out. In the state represented in FIG. 3, the sensor 6b must therefore perceive a reversal of direction of the normal component of the field.

However, the measurement of the Hall effect sensor 6b is affected by an uncertainty. The sensor 6b is therefore likely to perceive a change in polarity at a position of the rotor slightly offset, to the right or to the left, with respect to the position shown in FIG. 3.

According to the invention, the electrical machine comprises a part 7, integral with the stator 3. The part 7 comprises an area 7b configured to amplify the normal component of the magnetic field at the level of the magnetic sensor 6b, in particular the gradient of the normal component. of the magnetic field.

Preferably, part 7 faces an axial extension of rotor 2, said extension extending beyond the coils of stator 3. Thus, part 7 is axially spaced from the coils of stator 3, said axial spacing s' hearing in relation to the motor axis.

Zone 7b is typically a metal cylinder sector, for example iron. The zone 7b is here of thickness eb. The thickness eb is for example several millimeters, preferably between 1 and 20 millimeters, even more preferably between 5 and 10 millimeters. The zone 7b is placed behind the surface 60 which carries the sensor 6b, integrally with the stator.

Thus, for the same position of the rotor, the normal component of the field perceived by the sensor 6b is increased in absolute value.

The impact of the uncertainty range of the sensor 6b on the detection of a change in polarity is therefore less.

For example, if the uncertainty range of the sensor 6b is 0.05 Tesla, the sensor 6b can detect a change in direction of the normal component of the field, while the normal component of the magnetic field does not change. meaning and is between -0.05 Tesla and 0.05 Tesla.

In the presence of zone 7b, the normal component of the magnetic field is amplified. The same angular displacement of the rotor therefore causes a greater variation of the normal component of the field at the level of the sensor. Thus, the same uncertainty for the change in polarity (from -0.05 Tesla to 0.05 Tesla) gives a small range of rotor position uncertainty (e.g. from -0.1 ° to 0.1 °).

The amplification of the field by the zone 7b of the part 7 therefore improves the synchronization of the switching of the current in the windings. The induced rotational movement of the rotor is better controlled.

It will be noted that thanks to this solution, the measurement precision of the Hall effect sensors is improved, without modifying either the sensors or the electronics for controlling the current in the stator windings.

Not necessarily but advantageously, part 7 comprises two zones 7a and 7b of field amplification corresponding to two sensors 6a and 6b. A zone 7c of the room extends from zone 7a to zone 7b. The part 7 is in one piece between the zones 7a and 7b.

The fact of having two amplification zones corresponding to two distinct Hall effect probes, with a connection within the part 7 between the two zones, facilitates the calibration of the sensors. Instead of separately adjusting the position of two sensors, the angular position of a single element (part 7) is adjusted relative to the rotor.

This configuration of the part 7 therefore makes it possible to guarantee a better orientation and a correct angular difference between the two sensors (here 60 °). The accuracy of the rotor position information provided by the Hall effect sensors is further increased.

It will be understood that it is even more advantageous to have on the same part 7 three magnetic field amplification zones corresponding to three sensors, as in FIG. 3. A correct angular difference between three sensors is thus guaranteed.

In this example, the part 7 is in the form of a ring sector and covers the three magnetic sensors 6a, 6b and 6c. An angular amplitude i3 of part 7 is greater than 120 °, here about 140 °.

But as an alternative, part 7 could correspond to a single Hall effect sensor, for example by being limited to zone 7b.

Preferably, a maximum radial thickness ec of zone 7c, along the section plane of FIG. 3, is less than a minimum radial thickness ea of ​​zone 7a as well as a minimum radial thickness eb of zone 7b.

Preferably, the thickness ec is less than 50% of the thickness 7a and of the thickness 7b (for example between 20% and 40%).

For example, the thickness ec is between 1 and 3 millimeters.

The "finer" zone 7c saturates more easily, in contrast to the zones 7a and 7b which saturate little.

Zone 7c channels the magnetic field produced by magnets 4. The magnetic field is therefore more intense at zone 7c.

One advantage of this configuration is that it reduces the risk of saturation of the areas located behind the sensors (here areas 7a and 7b). The saturation of these zones could disturb the interaction between the magnetic fields of the rotor and the stator, and disturb the angular movement of the rotor.

In the example of FIG. 3, the angular sector 11 over which the zone 7a of high thickness extends has an amplitude of between 10 ° and 30 °, preferably 20 °. Likewise, zone 7b has an angular amplitude close to 20 °. The remaining angular sector (approximately 40 °) is occupied by a zone 7c of low thickness.

To illustrate the amplification of the magnetic field by part 7 and the phenomenon of saturation, there is shown in Figure 5 a magnetic field map in the vicinity of the sensor 6b without the part 7, when the rotor is in the angular position shown in Figure 3. There is shown in Figure 6 a magnetic field map in the vicinity of the sensor 6b with the addition of the part 7.

In these two figures, the North magnet is at the bottom right and the South magnet is at the bottom left. Thus, the magnetic field B in the air gap is oriented from right to left. The area 60 of the bearing surface of the Hall effect sensor is shown in the two figures.

It can be seen that the magnetic field is deformed vertically at the level of the zone 60 in FIG. 6. The presence of the part 7 and in particular of the amplification zone 7a deforms the magnetic field lines upwards.

Thanks to the amplification zone of the normal component of the magnetic field, in particular the gradient of the normal component of the magnetic field, the change in direction of the magnetic field lines (i.e. the reversal of polarity of the field magnetic) is more concentrated at the center of the Hall effect probe. The gradient of the normal component of the magnetic field is increased in area 60 in Figure 6, compared to the configuration of Figure 5 without an amplifying part.

A synchronous machine according to an alternative embodiment is shown in FIG. 7, in section along a section plane orthogonal to the axis A of the rotor.

Here, the machine comprises six Hall effect sensors 6 distributed over the entire periphery of the rotor. The sensors 6 are spaced two by two by 60 ° and cover the entire range of angular displacement of the rotor.

In this mode, a ring 7 'integral with the stator produces a localized amplification of the magnetic field, as described previously in relation to part 7 of FIG. 3.

The ring 7 'is in the form of a sector of a metal cylinder, for example made of iron. As for the machine of FIG. 3, each magnetic sensor 6 is associated with a zone of high radial thickness of the ring 7 '. Two consecutive zones of high thickness are connected by a zone of low thickness.

The ring 7 ′ makes it possible to reduce the uncertainty of the detection of the change in direction of the normal component of the magnetic field, at the level of each of the magnetic sensors 6. The shape of the ring 7 ′ allows good calibration of the sensors 6 and increases again the precision of detection of the angular position of the rotor.

This embodiment is advantageous because the detection of the position of the rotor is improved over the entire range of angular displacement of the rotor.
CLAIMS

1. Electric machine (1) comprising a rotor (2) and a stator (3), the rotor (2) being movable in rotation relative to the stator (3) about an axis of rotation (A) and comprising a plurality permanent magnets (4) or a plurality of electromagnets, the stator (3) comprising a plurality of coils (5) integral with the stator (3) and suitable for being supplied with an electric current so as to drive the rotor in rotation (2) relative to the stator (3),

the stator comprising a first magnetic sensor (6a) and a second magnetic sensor (6b) each comprising a detection surface, the first magnetic sensor and the second magnetic sensor being arranged on the same section plane orthogonal to the axis of rotation ( A), the machine comprising a first angular sector along which the first magnetic sensor is arranged and comprising a second angular sector along which the second magnetic sensor is arranged,

said magnetic sensors (6) being configured to detect a change of direction of a normal component of a magnetic field generated by the permanent magnets (4) or electromagnets, the normal component being orthogonal to the detection surface,

the electric machine being characterized in that it comprises a metal part (7) integral with the stator (3), said part comprising:

- a first amplification zone (7a) extending in the first angular sector,

- a second amplification zone (7b) extending in the second angular sector,

- a third zone (7c) extending from the first zone to the second zone, a maximum radial thickness (ec) of the third zone (7c) in the orthogonal section plane being less than a minimum radial thickness (ea ) of the first zone (7a) and to a

minimum thickness (eb) of the second zone (7b) in the section plane.

2. Electric machine according to claim 1, wherein the first angular sector (11) in which the first amplification zone (7a) extends has an angular amplitude of between 10 degrees and 30 degrees, preferably 20 degrees.

3. Electric machine according to any one of claims 1 or 2, wherein the part has a ring shape (7 ') or a ring sector shape (7).

4. Electric machine according to any one of claims 1 to 3, comprising magnetic sensors (6) each distributed in the vicinity of a field amplification zone and at angular positions regularly spaced along an entire perimeter of the machine.

5. Electric machine according to claim 4, comprising six magnetic sensors (6) spaced two to two by 60 degrees.

6. Electrical machine according to any one of claims 1 to 5, wherein a magnetic sensor comprises a Hall effect probe (6).

7. Electric machine according to any one of claims 1 to 6, wherein the part (7) is configured to amplify, at one of the detection surfaces, a gradient of a component orthogonal to said detection surface. magnetic field produced by permanent magnets or electromagnets.