Description

The application refers to an electric machine comprising a stator, a rotor rotatable relative to the stator about an axis, the rotor having permanent magnets, at least one short circuit ring axially shiftable over a distance D between a first position and a second position, wherein the short circuit ring is more distant from the rotor in the first position than in the second position, and wherein an induced magnetic flux of the permanent magnets is reduced when the at least one short circuit ring is shifted towards the second position, and at least one actuator assembly actuating the at least one short circuit ring to shift from the first position to the second position.

Permanent magnet electric machines are known to provide high torque density and efficiency. The induced back electromotive force of permanent magnet electric machines increases with speed, generally reaching at a base speed the voltage limit of the inverter. To maintain a constant back electromotive force, it is known to weaken the magnetic flux linkage from the permanent magnets, which can be achieved, for example, by adjusting a relative position of active and/or passive parts of the electric machine.

In US 8 390 232 B2, a permanent-magnet electrical machine is disclosed in which the rotor or stator have at least one movable iron segment. A magnetic field of the electric machine is weakened when the movable iron segment is moved a position away from the rotor or stator, respectively. When the movable iron segment is in a first position, such as in contact with the rotor or stator, the field strength is high. When the movable iron segment is in a second position in which the movable iron segment is displaced away from the rotor or stator, the field strength is low. The ability to weaken the field strength causes the constant-power speed ratio to be increased and thereby increases the utility of the electric machine for applications in which a wide speed range is desired. The electric machine may be used as both a permanent-magnet motor and generator.

In CN 10217021 1 B, a variable excitation permanent magnet synchronous motor is disclosed. The variable excitation permanent magnet synchronous motor comprises a stator and a rotor arranged on a motor shaft, wherein the two sides of the rotor are provided with a set of flux-weakening regulation mechanism respectively; the fluxweakening regulation mechanism comprises soft iron plates which are arranged on the two sides of the rotor and coaxial with the rotor; a spring is arranged between the rotor and the soft iron plate; and connecting rod counterweight mechanisms are arranged corresponding to each soft iron plate. The variable excitation permanent magnet synchronous motor has the advantages of automatically and correspondingly regulating distances between the rotor and the soft iron plates along with the change of rotating speed, thereby changing a magnetic flux path, fulfilling the aim of controlling reduction in motor excitation intensity along with the increasing of the rotating speed and further increasing the highest running speed of the motor.

In JP- H-1 1275787 A, an electric motor that has high torque and a large constant output operation range is disclosed. A rotor is provided with a rotor body, a flux barrier, a permanent magnet that is buried into the flux barrier, a slit part that extends from an area between the end parts of adjacent flux barriers to an area near the end face of the rotor, a magnetic flux passage member which is arranged in the slit part, and a spring for moving the magnetic flux passage member that is provided at the slit part. The magnetic flux passage member is movable in the slit part, is at a gap position where no magnetic flux flows to the magnetic flux passage member when the rotor is rotating at a low speed, the magnetic flux passage member projects from the gap part when the rotor rotates at high speed, and a magnetic flux short-circuiting part is formed at the slit part, thus obtaining a weak field effect and a high torque regardless of highspeed rotation.

An objective can be to provide an electric motor with a less complex adjusting mechanism, which is at least as reliable and durable as the state of the art. The objective is met by an electric machine comprising a stator, a rotor rotatable relative to the stator about an axis, the rotor having permanent magnets, and at least one short circuit ring axially shiftable over a distance D between a first position and a second position, wherein the short circuit ring is more distant from the rotor in the first position than in the second position. An induced magnetic flux of the permanent magnets is reduced when the at least one short circuit ring is shifted towards the second position. The at least one actuator assembly hydraulically actuating the at least one short circuit ring to shift from the first position to the second position depends on a rotation speed of the rotor.

As an advantage the electric machine allows to weaken the magnetic field, thus increasing the motor speed for a given voltage after a maximum power of the electric machine is accessed. The weakening of the magnetic field allows the electric machine to reach a higher speed and lower torque at the maximum power of the electric machine.

The electric machine may also be referred to as permanent magnet electric machine or permanent magnet electric motor, which may also be applied as a generator. According to an embodiment, two axially shiftable short circuit rings are provided, one at each axial end of the rotor, with the respective first position at an axial distance D from the respective second position. Both short circuit rings may be hydraulically actuated by a common actuator assembly. According to a further embodiment, each of the two short circuit rings has one actuator assembly allocated to it for hydraulic actuation. The distance D from the first position to the second position may be between one and ten percent of an axial length or a diameter of the rotor, and shifting the at least one short circuit ring from the first position to the second position reduces the induced magnetic flux of the permanent magnets by at least 5 percent. According to a further embodiment, the at least one short circuit ring extends to cover a magnet section of the rotor, where the permanent magnets are located. The at least one short circuit ring can be made of a magnetisable material.

According to a further embodiment, the at least one actuator assembly rotates together with the rotor about the axis. The actuator assembly can be firmly joined to the rotor. The at least one actuator assembly may thus advantageously be passively controlled by centrifugal force acting on the rotating actuator assembly.

According to a further embodiment, the at least one actuator assembly has at least one cavity and a fluid inside the cavity, the cavity being arranged such that the fluid is centrifuged radially outward from the axis by a centrifugal force of the rotating actuator assembly. At least one piston can be hydraulically connected to a radially outward end of the cavity. A hydraulic force resulting from the centrifugal force acting on the fluid is transmitted by the piston onto the short circuit ring. The piston and the short circuit ring may be formed integrally as a one-piece part.

According to a further embodiment, the at least one cavity forms a closed hydraulic system, a volume of the closed hydraulic system being varied by a movement of the piston, wherein the piston may be displaceable in axial direction. The at least one cavity may have a reservoir chamber arranged at a radially inward end of the at least one cavity. The actuator assembly may have only one cavity provided as an annular cavity, wherein the piston is an annular piston, and the annular cavity and the annular piston being arranged around the axis. The annular cavity and piston advantageously distribute the centrifugal force acting on the fluid evenly about the circumference of the short circuit ring.

According to a further embodiment, at least one resilient member is arranged between the rotor and the at least one short circuit ring. The resilient member exerts an axially biasing force onto the short circuit ring towards the first position, thus providing a restoring force to shift the short circuit ring back to its first position as the rotation speed of the rotor decreases. The at least one resilient member can be provided per short circuit ring as an annular spring or a stack of annular springs. The resilient member may also be formed by a plurality of coil springs arranged circumferentially about the axis. The coil springs may advantageously be arranged in recesses in the rotor or in recesses in the short circuit ring to allow the short circuit ring to abut against the rotor in the second position.

According to a further embodiment, a summarised force acting on the short circuit ring in axial direction towards the second position is composed of the hydraulic force of the fluid and a magnetic force exerted by the rotor. This summarized force exceeds the biasing force of the resilient member acting on the short circuit ring in axial direction towards the first position at a predetermined rotational speed of the rotor. The predetermined rotational speed may be in a range between 10,000 and 30,000 revolutions per minute.

Exemplary embodiments and further advantages of the electric machine will be illustrated as follows with reference to the accompanying drawings, wherein

Figure 1 shows a first exemplary embodiment of the electric machine in a perspective view;

Figure 2 the embodiment of Figure 1 in a longitudinal cut view;

Figure 3 a detail of Figure 2 in a first position;

Figure 4 the detail of Figure 3 in a second position;

Figure 5 a second exemplary embodiment of the electric machine;

Figure 6 the embodiment of Figure 5 in a longitudinal cut view;

Figure 7 a detail of Figure 6 in a first position;

Figure 8 the detail of Figure 7 in a second position

Figure 9 a schematic sectional view of a general electric machine.

In Figure 9, a schematic sectional view of a general electric machine is depicted for illustration purposes. The electric machine comprises a stator 10 and a rotor 1 rotatable relative to the stator 10 about an axis L, the rotor 1 having permanent magnets 1 1 . In this case, eight pairs of permanent magnets 1 1 pairwise arranged in a V-form are integrated into the rotor 1 . Slots 19 are provided in the stator 10 to receive a winding (not depicted). In Figures 1 to 8, the stator 10 (Figure 9) is omitted. In Figure 1 , an embodiment of the electric machine is shown in a perspective, partly exploded view. The electric machine comprises the stator 10 (not depicted, see Figure 9), the rotor 1 rotatable relative to the stator 10 about the axis L. The permanent magnets 1 1 are arranged in V-shaped recesses of the rotor 1 . Two short circuit rings 2 are arranged at two axially distanced ends of the rotor 1 and two actuator assemblies 3 hydraulically actuate the short circuit rings 2 to shift in axial direction along the axis L. Figure 2 shows the embodiment in a longitudinal cut view. A detail A is depicted in Figure 3 and in Figure 4, respectively, in different positions. The Figures 1 to 4 are described jointly.

The short circuit rings 2 are axially shiftable over a distance D as best seen in Figure

3 between a first position, which is depicted in Figure 3 and a second position, which is depicted in Figure 4. The short circuit rings 2 are more distant from the rotor 1 in the first position than in the second position. An induced magnetic flux of the permanent magnets 1 1 is reduced when the short circuit rings 2 are shifted towards the second position. The actuator assemblies 3 hydraulically actuate the short circuit rings 2 to shift from the first position to the second position depending on a rotation speed of the rotor 1 . The actuator assemblies 3 are passively controlled by centrifugal force, which is further illustrated now.

One of the two actuator assemblies 3 is depicted in an exploded illustration in Figure 1. The actuator assembly 3 has a first body part 14 and a second body part 15, both ring-shaped, which form an annular cavity 4 between them when mounted. A bodies O-ring 17 is provided for sealing the cavity 4. The short circuit ring 2 is connected to an annular piston 5, which cooperates with an annular piston chamber 18 formed between the first body part 14 and the second body part 15, which is best seen in Figure 4. The piston 5 has a notch with a piston O-ring 16. The piston chamber has also a notch formed at the second body part 15 with a piston O-ring 16. The piston O-rings 16 seal the cavity 4.

The actuator assemblies 3 rotate together with the rotor 1 about the axis L. In the depicted embodiment, the rotor 1 and both the first body parts 14 and the second body parts 15 of the actuator assemblies 3 are connected to a driveshaft 12. Inside the cavity

4 is a fluid, for example an oil. Due to the form of the cavity 4 the fluid is centrifuged radially outward from the axis L by a centrifugal force of the rotating actuator assembly 3. As piston 5 is hydraulically connected to a radially outward end of the cavity 4, a hydraulic force of the fluid acts on the piston 5 and on the short circuit ring 2, which is connected to the piston 5. The cavity 4 forms a closed hydraulic system, a volume of the closed hydraulic system being varied by a movement of the axially displaceable piston 5. The rotational force acting on the fluid centrifuges the fluid from a reservoir chamber 6 arranged at a radially inward end of the at least one cavity 4 towards the radially outward piston chamber 18.

A resilient member 7 is arranged between the rotor 1 and each short circuit ring 2, the resilient member 7 exerting an axially biasing force onto the respective circuit ring 2 towards the first position. In the depicted embodiment, the at least one resilient member 7 is an annular spring 7a, which may have a smaller outer diameter than an inner diameter of the short circuit ring 2, which allows the short circuit ring 2 to abut against the rotor 1 in the second position, as shown in figure 4.

In Figure 5, a second embodiment of the electric machine is shown in a perspective, partly exploded view. The electric machine comprises the stator 10 (not depicted, see Figure 9), the rotor 1 rotatable relative to the stator 10 about the axis L. The permanent magnets 1 1 are arranged in V-shaped recesses of the rotor 1 . Two short circuit rings

2 are arranged at two axially distanced ends of the rotor 1 and two actuator assemblies

3 hydraulically actuate the short circuit rings 2 to shift in axial direction along the axis L. In this regard the second embodiment is identical to the first described embodiment and will not be described in detail. Figure 6 shows the embodiment in a longitudinal cut view. A detail A is depicted in Figure 7 and in Figure 8, respectively, in different positions. The Figures 5 to 8 are described jointly.

In the second embodiment, the resilient member 7 is formed by a plurality of coil springs 7b distributed circumferentially about the axis L, instead of the annular spring 7a. The eight coil springs 7b are arranged in recesses 8 in the rotor 1 and in recesses 9 in the short circuit ring 2, as best seen in Figure 7, to allow the short circuit ring 2 to abut against the rotor 1 in the second position, which is shown in Figure 8. The short circuit rings 2 extend in a radial direction to overlap with an outer radial extension of the rotor 1 , preferably with a magnets section of the rotor 1 , where the permanent magnets are located. The short circuit rings 2 can be made of a magnetisable material. A summarised force in axial direction of the hydraulic force of the fluid and a magnetic force being exerted by the rotor 1 onto the short circuit ring 2 exceeds the biasing force of the resilient member 7 at a predetermined rotational speed of the rotor 1 , which predetermined rotational speed is for example in a range between 10,000 and 30,000 revolutions per minute. The distance D from the first position to the second position can be between one and ten percent of an axial length or a diameter of the rotor 1 . Shifting the at least one short circuit ring 2 from the first position to the second position reduces the induced magnetic flux of the permanent magnets 1 1 by at least 5%.

Reference Numerals

1 Rotor

2 Short circuit ring

3 Actuator assembly

4 Cavity

5 Piston

6 Reservoir chamber

7 Resilient member

7a Annular spring

7b Coil springs

8 Recess

9 Recess

10 Stator

1 1 Permanent magnets

12 Driveshaft

14 First body

15 Second body

16 Piston O-rings

17 Bodies O-ring

18 Piston chamber

19 Slots