WO2014048452A1

WO2014048452A1 - Electric motor

Info

Publication number: WO2014048452A1
Application number: PCT/EP2012/068869
Authority: WO
Inventors: Pierrick Maze; Sylvain Chaussat
Original assignee: Aktiebolaget Skf
Priority date: 2012-09-25
Filing date: 2012-09-25
Publication date: 2014-04-03

Abstract

This electric motor (M) includes a stator (S) comprising a housing (2) in which stator coils (22) are arranged, and a non-rotatable portion (4) fixed in rotation with respect to the stator coils (22), a rotor (R) including a rotor shaft (6) and a magnetized ring (14) adapted to rotate with respect to the non-rotatable portion (4) of the stator (S), a bearing (12) mounted between the rotor (R) and the stator (S), comprising an inner ring (120) and an outer ring (122), and a sensor unit (16) comprising an encoder (160) fast in rotation to the rotor (R) and a sensing element (162) fixed with respect to the stator (S). The encoder (160) belongs to the rotor (R).

Description

ELECTRIC MOTOR

TECHNICAL FIELD OF THE INVENTION

The invention relates to an electric motor.

BACKGROUND OF THE INVENTION

Electric motors equipped with position sensors have to be calibrated prior to operation in order to correctly set the origin of the angular position of their rotors. To this end, one must align the electrical origin of the signals delivered by the sensor with the electrical origin of the magnetic flux generated by the stator coils and the rotor. This is done in order to improve the accuracy of the angular measurement.

It is known to perform a calibration of an electric motor at the end of an assembly line, by measuring the electromotive force of the motor and the sensor output signals when the motor is rotating. To align the sensor and the motor, an electronic phase offset is added in the electronic controller of the electric motor to perform the alignment. It is also known to roughly align, by mechanical positioning, the sensor and the rotor during assembly, then to perform an electronic alignment as described here-above.

These solutions are not satisfying because they can lead to a loss of the angular position if the motor controller is damaged, and because of the supplementary production cost of the electronic calibration step.

It is also known to assemble the motor with a special shape of an encoder, which belongs to the sensor unit, in order to align it with the rotor shaft, or to assemble the electric motor with the sensor unit using a specific process. These solutions imply complex production operations. SUMMARY OF THE INVENTION

The aim of the invention is to provide a new electrical motor in which the alignment between the electrical origin of the encoder of the sensor unit and the electrical origin of the rotor is obtained without specific mechanical or electronic supplementary production steps.

To this end, the invention concerns an electric motor including a stator comprising a housing in which stator coils are arranged, and a non-rotatable portion fixed with respect to the stator coils, a rotor including a rotor shaft and a magnetized ring adapted to rotate with respect to the non-rotatable portion of the stator, a bearing mounted between the rotor and the stator, comprising an inner ring and an outer ring, and a sensor unit comprising an encoder fast in rotation to the rotor and a sensing element fixed with respect to the stator. This electric motor is characterized in that the encoder belongs to the rotor.

Thanks to the invention, there is no need to perform an specific post-assembly alignment operation between the encoder and the rotor, as the encoder belongs to the rotor. This permits to produce the rotor with a directly properly aligned encoder.

According to further aspects of the invention which are advantageous but not compulsory, such an electric motor may include one or several of the following features.

- The encoder is a portion of the rotor.

- The encoder is a portion of the magnetized ring, which extends radially towards the sensing element along an axial surface of a tubular portion of the rotor.

- The encoder is a portion of the magnetized ring, which extends radially towards the sensing element against a thinned-down portion of a tubular portion of the rotor, whereas said thinned-down portion is made of a non-magnetic material.

- The encoder is a part fixed to the rotor.

- The encoder is fixed in an annular cavity provided on the internal side of a tubular portion of the rotor. - The sensing element reads the encoder along a radial outwards direction with respect to the rotation axis of the rotor.

The sensing element is fixed on the inner ring of the bearing.

The sensing element is fixed to the housing of the stator.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in correspondence with the annexed figures, as an illustrative example. In the annexed figures:

- figure 1 is a longitudinal sectional view of an electric motor according to a first embodiment of the invention;

- figure 2 is a view, at a larger scale, of detail II on figure 1 ;

- figures 3 to 5 are views similar to figure 2, of electric motors according to a second, a third and a fourth embodiments of the invention.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

Each electric motor M represented on one of figures 1 to 5 comprises a stator S including a housing 2 in which stator coils 22 are arranged. Stator S also includes a non- rotatable shaft 4 mounted in a hole 24 of housing 2. Non-rotatable shaft 4 defines a longitudinal axis X4.

Electric motor M also includes a rotor R which comprises a rotor shaft 6 adapted to rotate around non-rotatable shaft 4 around axis X4. Rotor R also comprises a tubular portion 8 which extends on the opposite side of rotor shaft 6 with respect to an end 62 which extends outside a casing 10 of electric motor M. Tubular portion 8 is centred around axis X4 and has a diameter superior to the diameter of rotor shaft 6.

Rotor R defines an annular internal volume V8 which extends axially from an internal surface of housing 2 to a bottom surface 82 of tubular portion 8. Internal volume V8 extends radially from a cylindrical outer peripheral surface 42 of shaft 4 to an inner cylindrical surface 84 of tubular portion 8 centred around axis X4. The adjectives "axially" and "radially" are defined with respect to rotation axis X4 of electric motor M. An axial direction denotes a direction parallel to axis X4, while a radial direction defines a direction perpendicular to axis X4.

A rolling bearing 12 is mounted in internal volume V8 to permit the rotation between rotor R and a stator S. Rolling bearing 12 comprises an inner ring 120 fixed on non- rotatable shaft 4, an outer ring 122 fast in rotation with rotor R and one row of balls 124 arranged between inner ring 120 and outer ring 122. According to a non-shown embodiment of the invention, rolling bearing 12 may comprise another type of rolling elements, such as rollers or needles. The bearing 12 may also comprise several inner rings and/or several outer rings, as well as a plurality of rows of rolling element. In particular, the bearing may comprise one inner ring , one outer ring, and two rows of balls.

Rotor R further includes a magnetized ring 14 mounted on an outer peripheral surface 86 of tubular portion 8, so as to radially face stator coils 22. The rotation of the rotor R is driven by the magnetic attraction between stator coils 22 and magnetized ring 14, this magnetic attraction being created by stator coils 22 which are fed with electrical current.

Electric motor M also comprises a sensor unit 16 adapted to sense the angular position or the rotational speed of rotor R with respect to stator S. Sensor unit 16 comprises an encoder ring 160 fast in rotation with rotor R, and a sensing element 162. Encoder ring 160 produces magnetic field variations which are detected by a sensing element 162, which is fixed with respect to stator S. Sensing element 162 is housed in a sensor body 164 which includes data processing means and a connection area in which a connector can be plugged.

A first embodiment of the invention is represented on figures 1 and 2. In this embodiment, magnetized ring 14 comprises a radially inwards extending portion 140, which forms the encoder 160 of sensor unit 16 and which is oriented radially towards axis X4. Tubular portion 8 has an axial end surface 88 oriented towards housing 2, which is aligned with the respective lateral axial surfaces 120a and 122a of inner ring 120 and outer ring 122. Radial portion 140 extends along axial surface 88 of tubular portion 8, between this surface and housing 2, and comprises an inner cylindrical surface 142 which radially delimits internal volume V8.

Sensing element 162 is mounted in annular volume V8 next to rolling bearing 12 so as to detect the magnetic field generated by the rotation of portion 140 of magnetized ring 14 along a radial direction. Sensing element 162 is fixed to inner ring 120 thanks to fastening means comprising an axially extending member 162a having a terminal bulge 162b, adapted to be received in a circular groove 120b provided on the inner side of inner ring 120 with respect to axis X4.

This structure permits to use a single part which provides a magnetized ring 14 for the operation of the electric motor M and the encoder ring 160 for the operation of sensor unit 16. This single part can be produced with the right alignment between the encoder 160 and the rotor R. This avoids costly post-assembly calibration operations.

A second, a third and a fourth embodiments of the invention are represented on figures 3 to 5. In these embodiments, elements similar to the first embodiment have the same references and work in the same way.

The embodiment of figure 3 differs from the first embodiment by the fact that portion

140 of magnetized ring 14 extends radially towards sensing element 162 against a thinned-down portion 90 of tubular portion 8. The thickness of tubular portion in thinned- down portion 90 is inferior to the thickness of tubular portion in an area axially aligned with rolling bearing 12. The total thickness of magnetized ring 14 including its radially extending portion 140 is inferior to its thickness in the third embodiment. This structure permits to more easily realize the magnetization operation of ring 14. In order for sensing element 62 to detect the magnetic field generated by the rotation of magnetized ring 14, thinned-down portion 90 is made of a non-magnetic material.

A third embodiment of the invention is represented on figure 4. In this embodiment, encoder 160 is fixed in an annular cavity 92 which forms a shoulder on the inner surface 84 of tubular portion 8.

In this embodiment, encoder ring 160 is aligned, on the side of rolling bearing 12, with lateral surfaces 120a and 122a of inner ring 120 and outer ring 122, and with the lateral end surface 144 of magnetized ring 14 on the side of housing 2. Sensing element 162 is mounted in internal annular volume V8 so as to detect the magnetic field generated by encoder ring 160 along a radial direction.

Encoder ring 160 may be fixed to tubular portion 8 by press-fitting, gluing, crimping or any other convenient means.

A fourth embodiment of the invention is represented on figure 5. This embodiment differs from the embodiments of figures 1 to 4 by the fact that sensing element 162 is fixed to housing 2, instead of being fixed to inner ring 120 of bearing 12. To this end, housing 2 comprises, on its internal surface 26, an enlarged cavity 26a adapted to receive sensing element 162. Housing 2 and sensing element 162 comprise non-shown fastening means. Sensing element 162 may also be integrated in housing 2 as a single specifically produced part including also data processing means and connectors. The sensing element integration described in this embodiment can be combined with each of the previously described embodiments.

The features of the previously described embodiments can be combined within the scope of the invention.

In the embodiments described above, the magnetized ring 14 is preferably obtained from the magnetization of a ring previously overmoulded onto the shaft. Alternatively, the magnetized ring 14 may be obtained by inserting a plurality of permanent magnets in slots arranged on the shaft, or by attaching the permanent magnets directly onto the shaft, preferably by gluing. Such permanent magnets can advantageously be made from rare earth materials.

Claims

1 .- Electric motor (M) including:

- a stator (S) comprising a housing (2) in which stator coils (22) are arranged, and a non-rotatable portion (4) fixed in rotation with respect to the stator coils (22);

- a rotor (R) including a rotor shaft (6) and a magnetized ring (14) adapted to rotate with respect to the non-rotatable portion (4) of the stator (S);

- a bearing (12) mounted between the rotor (R) and the stator (S), comprising an inner ring (120) and an outer ring (122);

- a sensor unit (16) comprising an encoder (160) fast in rotation to the rotor (R) and a sensing element (162) fixed to the stator (S),

wherein the encoder (160) belongs to the rotor (R).

2.- Electric motor according to claim 1 , wherein the encoder (160) is a portion

(140) of the rotor (R).

3. - Electric motor according to claim 2, wherein the encoder (160) is a portion (140) of the magnetized ring (14), which extends radially towards the sensing element (162) along an axial surface (88) of a tubular portion (8) of the rotor (R).

4. - Electric motor according to claim 2, wherein the encoder (160) is a portion (140) of the magnetized ring (14), which extends radially towards the sensing element (162) against a thinned-down portion (90) of a tubular portion (8) of the rotor (R), and wherein said thinned-down portion (90) is made of a non-magnetic material.

5. - Electric motor according to claim 1 , wherein the encoder is a part (160) fixed to the rotor (R).

6. - Electric motor according to claim 5, wherein the encoder (160) is fixed in an annular cavity (92) provided on the internal side (84) of a tubular portion (8) of the rotor

(R).

7. - Electric motor according to any preceding claim, wherein the sensing element (162) reads the encoder (160) along a radial outwards direction with respect to the rotation axis (X4) of the rotor (R).

8.- Electric motor according to any preceding claim, wherein the sensing element (162) is fixed on the inner ring (120) of the bearing (12).

9.- Electric motor according to one of claims 1 to 7, wherein the sensing element

(162) is fixed to the housing (2) of the stator (S).