EP1579549A1

EP1579549A1 - Rotor for an electric motor

Info

Publication number: EP1579549A1
Application number: EP03779720A
Authority: EP
Inventors: Niels Christian Weihrauch
Original assignee: Danfoss Compressors GmbH
Current assignee: Danfoss Deutschland GmbH
Priority date: 2002-12-19
Filing date: 2003-12-12
Publication date: 2005-09-28
Also published as: AU2003287882A1; CN100423407C; DE10261763A1; DE10261763B4; US7247965B2; CN1726627A; WO2004057727A1; US20060145557A1

Abstract

The invention relates to a rotor for an electric motor, especially an electric line-start motor, comprising spaces (4 to 7) which receive permanent magnets (10 to 13) and extend in an axial direction, and spaces (20 to 25) that accommodate conductor rods and extend in an axial direction. In order for the rotor to run as regularly as possible, the spaces (20 to 25) accommodating the conductor rods are provided with a substantially elongate cross section in at least one sector of the rotor while being embodied in a curved manner along the longitudinal axis thereof in said sector when viewed from a cross-sectional perspective.

Description

Rotor for an electric motor

The invention relates to a rotor for an electric motor, in particular a line-start electric motor with receiving spaces for permanent magnets that run in the axial direction and with receiving spaces for conductor bars that run in the axial direction.

Hybrid three-phase motors that represent a combination of a three-phase asynchronous motor with a three-phase synchronous motor are referred to as line-start electric motors. Such a line-start electric motor comprises a stator, which is also referred to as a stator, with a plurality of stator or stator windings. The stator windings generate a rotating field that generates a voltage in a rotor or rotor, which causes the rotor to rotate. The rotor of a line-start electric motor has features of the rotor of a three-phase asynchronous motor as well as features of the rotor of a three-phase synchronous motor. Line start motors can also be designed for single-phase mains supply, possibly with an operating capacitor.

In the rotor of a three-phase asynchronous motor, which is also referred to as an induction motor, conductor bars, for example made of aluminum or copper, are arranged essentially in the axial direction. The conductor bars can be connected by short-circuit rings on the end faces of the rotor. The conductor bars together with the short-circuit rings form the rotor winding and can have the shape of a cage, which is why such a rotor is also referred to as a squirrel-cage rotor. In operation, the rotating field of the stator winding causes a change in the flux in the conductor loops of the rotor, which is initially stationary. The flow rate of change is proportional to the rotating field speed. The induced voltage causes current to flow in the rotor conductor bars connected by the short-circuit rings. The magnetic field generated by the rotor current causes a torque that rotates the rotor in the direction of rotation of the stator rotating field. If the rotor reached the speed of the stator rotating field, the flux change in the conductor loop under consideration would be zero and thus the torque causing the rotation. The rotor speed is therefore always lower than the rotating field speed in three-phase asynchronous motors. The rotor therefore does not run mechanically in synchronism with the rotating field speed. Permanent magnets can be arranged in the rotor of a three-phase synchronous motor, for example, which generate a rotating magnetic field during operation. When the stator winding is supplied with three-phase current, the poles of the rotor are attracted by the opposite poles of the stator rotating field and shortly afterwards repelled by its similar poles. Due to its inertia, the rotor cannot immediately follow the stator speed. If, however, the rotor has approximately reached the speed of the stator rotating field, then the rotor is, so to speak, drawn into the stator rotating field speed and continues to run with it. This means that after the rotor starts up, it rotates synchronously with 0 of the stator rotating field speed.

The rotor of a line start electric motor includes both permanent magnets and conductor bars. The conductor bars form a starting aid for the rotor. When the speed of the stator rotating field has approximately been reached, the permanent magnets are effective. The line-start electric motor 5 combines the good starting properties of an asynchronous motor, i.e. the large starting torque, with the high efficiency of the synchronous motor. When the motor starts, the conductor bars develop their effect, whereas the permanent magnets actually only have a disruptive role when the motor starts. During synchronous operation, for example at 50 Hz or 3000 rpm, the permanent magnets, on the other hand, develop their effect, whereas the conductor bars then no longer contribute to the generation of torque, since no voltage is induced in the conductor bars during synchronous operation.

The magnetic field existing in the operation of the line-start electric motor in an air gap between the rotor and stator comprises two components. The first 5 components of the resulting field are caused by the stator windings. This is also called a rotating field. The second component of the resulting field is caused by the permanent magnets, which can also be referred to as permanent magnets. In the operation of conventional line-start electric motors, as are known for example from WO 01 / 06624A1, torque fluctuations can occur which are undesirable.

The object of the invention is therefore to provide a rotor according to the preamble of claim 1, in particular for an electric motor according to the preamble of the Proverb 14 to create that makes the magnetic field approximately sinusoidal during synchronous operation.

The object is achieved by a rotor which has the features of patent claim 1. Preferred embodiments of the invention are disclosed in the subclaims.

It is desirable that the field strength of the magnetic field existing between the rotor and the stator be approximately sinusoidal during synchronous operation. This is counteracted by the permanent magnets in the rotor, which lead to an angular course in conventional line-start electric motors. The desired sinusoidal rotating field is thus distorted by the conventional permanent magnets and thus contributes to torque fluctuations or torque pulsations during synchronous operation. This undesirable distortion during synchronous operation is generated in that the field strength of the permanent magnet is distributed unattenuated over the rotor surface. In the direction of the magnetic axis, the permanent magnets mainly determine the field. In conventional line-start electric motors, the rotor is only partially permeable to the magnetic field from the stator in the direction of the neutral axis, but not in the direction of the magnetic axis.

The rotor according to the invention is preferably a rotor for an electric motor, in particular a line-start electric motor, with receiving spaces for permanent magnets running in the axial direction and with receiving spaces for conductor bars running in the axial direction. The receiving spaces for the conductor bars have an essentially elongated cross section in at least one sector of the rotor. Viewed in cross section, the receiving spaces for the conductor bars in this sector are curved along their longitudinal axis. In tests carried out within the scope of the present invention, it has been found that the torque fluctuations in conventional line-start electric motors are due to the fact that the course of the field strength of the resulting magnetic field in the air gap between the stator and rotor is not sinusoidal over the angle of rotation, but, at least partially, is angular. Due to the inventive design and arrangement of the receiving spaces for the conductor bars, an approximately sinusoidal course can be achieved during operation. A preferred exemplary embodiment of the rotor is characterized in that a plurality of permanent magnets, in particular four permanent magnets, are arranged such that they generate a magnetic field with a neutral axis and a magnetic axis which is arranged perpendicular to the neutral axis. 5 The radii of curvature of the receiving spaces for the conductor bars decrease from the neutral axis to the magnet axis, that is to say the radii of curvature are smaller in the vicinity of the magnet axis than in the vicinity of the neutral axis. The neutral axis runs where the permanent magnets do not generate a magnetic field. The magnetic axis runs where the magnetic field generated by the permanent magnets is strongest. The field strength of the permanent magnetic field can be, for example, 1.5 Tesla. The magnetic field generated by the stator or stator windings runs from the stator through the rotor and back into the stator. By reducing the radii of curvature from the neutral axis to the magnetic axis, it is achieved that the magnetic field lines from the stator that occur during startup can penetrate the rotor well.

Another preferred exemplary embodiment of the rotor is characterized in that the distance between the receiving spaces for the conductor bars is constant. This arrangement has proven to be particularly advantageous in tests carried out within the scope of the present invention.

Another preferred exemplary embodiment of the rotor is characterized in that the receiving spaces for the conductor bars, viewed in cross section, are designed and arranged so as to be curved along their longitudinal axis such that the distance of the receiving spaces for the conductor bars from the axis of rotation of the rotor, in cross section through the rotor considered, increases from the neutral axis to the 5 magnetic axis. This creates freedom in which the

Field lines of the magnetic field generated by the stator windings can penetrate the rotor.

A further preferred exemplary embodiment of the rotor is characterized in that the longitudinal axes of the receiving spaces for the conductor bars, viewed in cross-section through the rotor, are essentially radially oriented in the vicinity of the neutral axis, apart from the curvature and relative to the rotor, and in that the longitudinal axes of the receiving spaces for the conductor bars, viewed in cross section through the rotor, point toward the magnetic axis are arranged rotated that the radially outer ends of the receiving spaces for the conductor bars, viewed in cross section through the rotor, are arranged at a smaller distance from the magnetic axis than in a radial alignment. It follows from this that the longitudinal axes of the receiving spaces for the conductor bars in the vicinity of the magnetic axis of the permanent magnets run essentially parallel to the latter. This in turn means that the magnetic field generated by the permanent magnets, even in the vicinity of the magnetic axis, can penetrate unhindered between the receiving spaces for the conductor bars.

Another preferred exemplary embodiment of the rotor is characterized in that the receiving spaces for the conductor bars, viewed in cross section, each have two side walls which are curved to different extents. This gives the receiving spaces for the conductor bars an essentially crescent shape.

Another preferred exemplary embodiment of the rotor is characterized in that the radii of curvature of the side walls of the receiving spaces for the conductor bars decrease from the neutral axis to the magnet axis. The smaller the radius of curvature of the side walls, the smaller the length of the receiving space enclosed between the side walls. This has proven to be advantageous in measurements carried out within the scope of the present invention with regard to the efficiency of the electric motor.

Another preferred exemplary embodiment of the rotor is characterized in that the two side walls of the receiving spaces for the conductor bars, viewed in cross section through the rotor, are connected at their inwardly facing ends by a rounded connecting wall. This has proven to be particularly advantageous from a manufacturing and functional point of view.

Another preferred exemplary embodiment of the rotor is characterized in that the connecting walls of all the receiving spaces for the conductor bars have the same radius. It follows from this that the distance between the side walls radially on the inside is also constant.

Another preferred exemplary embodiment of the rotor is characterized in that the receiving spaces for the permanent magnets are curved in this way formed and arranged around the axis of rotation of the rotor that the distance between the receiving spaces for the permanent magnets and the receiving spaces for the conductor bars, viewed in cross section through the rotor, is greater in the area of the magnet axis than in the area of the neutral axis. This creates sufficient space for the magnetic field lines of the magnetic field generated by the stator.

A further preferred exemplary embodiment of the rotor is characterized in that the receiving spaces for the permanent magnets, viewed in cross section through the rotor, have the shape of arcs which are arranged in the form of an ellipse, the main axis of which with the neutral axis and the secondary axis with the magnetic axis coincides. This arrangement has proven to be particularly advantageous with regard to the distribution of the magnetic field lines during operation of the device according to the invention.

Another preferred exemplary embodiment of the rotor is characterized in that the rotor has at least one transition zone in which the receiving spaces for the conductor bars are not curved. The rotor can be formed from a laminated core applied to a shaft. Laminated sheets can be arranged in the transition zone, which have no curved receiving spaces for the conductor bars. The transition zone serves to achieve a so-called slot bevel, that is to say that a conductor bar in a first end of the rotor is offset in comparison to the conductor bar in the other end of the rotor. The offset, for example between 10 and 20 mechanical degrees, is achieved in the transition zone in that the conductor bar does not run parallel to the axis of rotation of the rotor, but is tapered to the side. The amplitude of magnetic harmonics disturbing in the rotating field is desirably greatly reduced by the slot bevel. The transition zone consists, for example, of 10 to 20 sheets, the receiving spaces of which are offset from one another.

Another preferred exemplary embodiment of the rotor is characterized in that the receiving spaces for the conductor bars are closed radially on the outside. The receiving spaces for the conductor bars are preferably arranged on the outer circumference of the rotor and, although they have a closed cross section, can also be referred to as grooves. Because the Recording rooms are closed, it is achieved that high-frequency components in the magnetic field do not induce leakage currents in the conductor bars.

^" In the case of an electric motor, in particular a line-start electric motor, with a stator that comprises a plurality of windings, the above-mentioned tasks are solved in that a previously described rotor is rotatably arranged within the stator. The rotor according to the invention When starting the electric motor, the magnetic field can be controlled in such a way that gaps in the magnetic field of the permanent magnets can be filled in. Efficiencies of more than 90% can be achieved due to the almost sinusoidal course of the magnetic field or the magnetic field strength or the magnetic flux density.

A preferred exemplary embodiment of the electric motor is characterized in that short-circuit rings which connect the conductor bars to one another are arranged on the end faces of the rotor. The short-circuit rings and the conductor bars form a cage in which a voltage is induced.

Further advantages, features and details of the invention result from the following description, in which an exemplary embodiment is described in detail with reference to the drawing. Show it:

Figure 1 shows a cross section through a rotor;

Figure 2 is a reduced view of the rotor of Figure 1 with field lines of the magnetic field generated by the permanent magnets and

3 shows the rotor from FIG. 2 with field lines of the magnetic field generated by a stator.

A rotor 1 of a line start electric motor is shown in cross section in FIG. The rotor 1 has a central through hole 2, which serves to receive a shaft (not shown), via which a torque generated by the electric motor can be output.

Around the through hole 2 around four receiving spaces 4, 5, 6 and 7 for permanent magnets 10, 11, 12 and 13 are arranged. The receiving spaces 4 to 7 extend in the axial direction at least over part of the length of the Rotors 1. Viewed in cross section, the four receiving spaces 4 to 7 for the permanent magnets 10 to 13 are arranged in the shape of an ellipse. The poles of the permanent magnets 10 to 13 are each designated by the capital letters N for the north pole and S for the south pole. The arrangement of the permanent magnets 10 to 13 shown leads to the formation of a magnetic field, the field strength of which is zero along a neutral axis 16 and greatest along a magnetic axis 17. The neutral axis 16 is also referred to as the neutral axis.

To the outside, the rotor 1 is delimited by a circular cylinder surface, on the circumference of which a multiplicity of receiving spaces 20 to 25 and 28, 29 for 0 conductor bars are arranged. The receiving spaces for conductor bars (not shown) extend in the axial direction over the entire length of the rotor 1. The rotor 1 is symmetrical in relation to the neutral axis 16 and the magnetic axis 17. For reasons of clarity, therefore, only the receiving spaces 20 to 25 and 28, 29 for the conductor bars are provided with reference numerals.

Each receiving space for a conductor bar, which can also be referred to as a receiving space for a conductor winding, comprises two side walls 31 and 32, which are connected by a semicircular connecting wall 34. At the other end, the elongated receiving spaces for the conductor bars are tapered or tapered. The distances 35 to 39 between the outwardly facing ends of the receiving spaces for the conductor bars are constant.

In Figure 1 it can be seen that the two side walls of the receiving space 28 are concave. In contrast, the two side walls of the receiving space 29 are convex. The receiving space 28 is divided into two equal halves by the 5 neutral axis 16 and the receiving space 29 by the magnetic axis 17. The receiving spaces 20 to 25 arranged between the receiving spaces 28 and 29 and thus between the neutral axis 16 and the magnetic axis 17 each have a convex and a concave side wall. The radius of curvature of the receiving spaces 20 to 25 decreases from the neutral axis 16 to the magnetic axis 17. This means that the receiving space 20 has the largest and the receiving space 25 the smallest radii of curvature. 2 shows the magnetic field generated by the permanent magnets 10 to 13 in the form of magnetic field lines.

FIG. 3 shows the magnetic field generated by a stator (not shown) during the asynchronous starting of the rotor in the form of magnetic field lines 5. The magnetic flux through the rotor 1 is indicated in FIG. 3 by arrows 48 and 49.

The curved receiving spaces for the conductor bars, which can also be referred to as grooves, provide the advantage that the magnetic field generated by the permanent magnets is passed through the rotor 1 in a controlled manner during operation of the line start electric motor (not shown). As a result, an approximately sinusoidal course of the field strength of the resulting magnetic field can be generated in the air gap between the stator and rotor during operation of the electric motor.

The curvature of the grooves or receiving spaces for the conductor bars 5 has the primary function during the synchronous operation of the electric motor to distribute the magnetic field generated by the permanent magnets sinusoidally in the air gap between the rotor and the stator. As a result, the magnetic field will be weakest in the area of the neutral axis and strongest in the area of the magnetic axis.

o In addition, the curved design of the receiving spaces for the conductor bars and the special arrangement of the conductor bars when the electric motor starts up creates a lot of space for the magnetic field of the stator which penetrates the rotor. As can be seen in FIG. 3, there is sufficient space between the receiving spaces 24 and 25 for the conductor bars and the permanent magnet 11 for the passage of the magnetic field lines. This avoids magnetic bottlenecks that could lead to undesired saturation of the rotor plate. The special arrangement of the permanent magnets increases the available space.

With the invention it is achieved that the magnetic field is controlled when the electric motor starts so that gaps in the magnetic field that are caused by the permanent magnets are filled.

Claims

Expectations

1.Rotor for an electric motor, in particular a line-start electric motor, with receiving spaces (4 to 7) extending in the axial direction, for permanent magnets (10 to 13) and with receiving spaces (20 5 to 25) extending in the axial direction for conductor bars, characterized in that the receiving spaces (20 to 25) for the conductor bars in at least one sector of the rotor have a substantially elongated cross section, and in that the receiving spaces (20 to 25) for the conductor bars in this sector, viewed in cross section, along their longitudinal axis are curved.

2. Rotor according to claim 1, characterized in that a plurality of permanent magnets (10 to 13), in particular four permanent magnets, are arranged such that they generate a magnetic rotating field with a neutral axis (16) and a magnetic axis (17) that are perpendicular to the neutral axis (16) is arranged, the radii of curvature of the receiving spaces (20 to 25) for the conductor rods 5 decreasing from the neutral axis (16) to the magnetic axis (17).

3. Rotor according to one of the preceding claims, characterized in that the distance (35 to 39) between the receiving spaces for the conductor bars is constant.

4. Rotor according to one of the preceding claims, characterized in that the receiving spaces (20 to 25) for the conductor bars, viewed in cross section, are designed and arranged so as to be curved along their longitudinal axis such that the distance between the receiving spaces (20 to 25 ) for the conductor rods to the axis of rotation of the rotor, viewed in cross section through the rotor, increases from the neutral axis (16) to the magnet axis (17).

5 5. Rotor according to one of the preceding claims, characterized in that the longitudinal axes of the receiving spaces (20, 21) for the conductor bars, viewed in cross section through the rotor, in the vicinity of the neutral axis (16), apart from the curvature and based on the rotor, are oriented essentially radially, and in that the longitudinal axes of the receiving spaces o (24, 25) for the conductor bars, viewed in cross section through the rotor, are arranged rotated toward the magnet axis (17) such that the radially outer ends the receiving spaces (24, 25) for the conductor bars are arranged at a smaller distance from the magnetic axis (17) than in the case of a radial alignment.

6. Rotor according to one of the preceding claims, characterized in that the receiving spaces (20 to 25) for the conductor bars, in cross-section loading

5, each have two side walls (31, 32) which are curved to different degrees.

7. Rotor according to claim 6, characterized in that the radii of curvature of the side walls (31, 32) of the receiving spaces for the conductor bars decrease from the neutral axis (16) to the magnetic axis (17).

8. rotor according to claim 6 or 7, characterized in that the two side walls of the receiving spaces for the conductor bars, viewed in cross section through the rotor, are connected at their inwardly facing ends by a rounded connecting wall (34).

9. Rotor according to claim 8, characterized in that the connecting walls (34) of all the receiving spaces for the conductor bars have the same radius.

10. Rotor according to one of the preceding claims, characterized in that the receiving spaces (4 to 7) for the permanent magnets are so curved and arranged around the axis of rotation of the rotor that the distance between the receiving spaces (5) for the permanent magnets ( 11) and the receiving spaces (20 to 25) for the conductor bars, viewed in cross section through the rotor, is larger in the area of the magnetic axis (17) than in the area of the neutral axis (16).

11. A rotor according to claim 10, characterized in that the receiving spaces (4 to 7) for the permanent magnets (10 to 13) are viewed in cross section through the rotor, having the shape of arcs which are arranged in the form of an ellipse, whose main axis coincides with the neutral axis (16) and whose minor axis coincides with the magnetic axis (17).

12. Rotor according to one of the preceding claims, characterized gekennzeich- net that the rotor has at least one transition zone in which the receiving spaces for the conductor bars are not curved.

13. Rotor according to one of the preceding claims, characterized in that the receiving spaces (20 to 25, 28, 29) for the conductor bars are closed radially on the outside.

14. Electric motor, in particular line-start electric motor, with a stator which comprises 5 a plurality of windings, characterized in that a rotor (1) according to one of the preceding claims is rotatably arranged within the stator.

15. Electric motor according to claim 14, characterized in that short-circuit rings are arranged on the end faces of the rotor (1) which connect the conductor rods to one another.