WO2016165759A1

WO2016165759A1 - Rotating electric machine

Info

Publication number: WO2016165759A1
Application number: PCT/EP2015/058176
Authority: WO
Inventors: Eino Silander
Original assignee: Abb Technology Ag
Priority date: 2015-04-15
Filing date: 2015-04-15
Publication date: 2016-10-20

Abstract

A rotating electric machine comprising a magnetic core, which comprises a center leg (1) comprising a winding (6) for magnetizing the magnetic core, multiple of side legs (2) each comprising a winding (7) wound on the side legs, the side legs being connected to the center leg for providing magnetic flux paths for the magnetization provided by the winding wound on the center leg, wherein a rotor is arranged in each of the flux paths for providing variable magnetic reluctance to the flux paths.

Description

ROTATING ELECTRIC MACHINE

FIELD OF THE INVENTION

The present invention relates to rotating electric machines, and particularly to rotating electric machines having multiple rotating axis. BACKGROUND OF THE INVENTION

Rotating electric machines, such as generators and motors, are typically formed of a rotor which rotates inside a stator. In a generator, the mechanical energy provided to the rotor is transformed into electrical energy when a magnetized rotor is rotated and the rotating magnetic field produces a current in the stator windings which surround the rotor.

The stator windings of an AC generator are typically composed of copper bars or wires that are inserted in stator slots in certain order for achieving the desired voltage and current properties. The stator slots are oriented axially in the stator and the windings are connected from one slot to another at the end of the stator. At the end of the stator coils the insulation of the coils is very important. The coils of the windings cross each other at a close proximity and the coils may belong to different phase windings, and therefore a high potential difference may exist between the coils. Further as the coils have to be led from one slot to another in the end of the stator, the amount of coil material is large and the winding ends do not produce any working power to the machine. Additionally undesired copper losses are produced in the overhang of windings.

Another drawback in electric machines with conventional design is the hysteresis losses of the stator core. The magnetic flux in the stator of the machine changes its direction in a known manner. The stator core is further dimensioned such that the magnetic flux density reaches a desired value near the saturation point. As the flux direction is changed, the area of the hysteresis loop is maximal and therefore the hysteresis losses are significant.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is to provide a rotating electric machine so as to alleviate the above disadvantages. The objects of the invention are achieved by a rotating electric machine which is characterized by what is stated in the independent claim. The preferred embodiments of the invention are disclosed in the dependent claims. The invention is based on the idea of employing a transformer-like structure in which multiple of rotating rotors are used. The rotors are arranged in the side legs of the core structure and a field winding in a center leg produces magnetization to the core. Each side leg comprises phase windings to which output voltage is induced.

The rotors of the structure are rotating in a magnetic field produced by the field winding arranged in the center leg. The rotors are formed such that depending on their rotation angle the reluctance of the magnetic path through the legs is changed. Once the core of the machine is magnetized, a torque depending on the magnetization is required to rotate the rotors, that is, work is required when the reluctance of the flux path through one side leg is increased. As the reluctance is changed, the magnetic flux through the side leg is changed and voltage is induced to the phase winding. Energy is required for increasing the reluctance of the side legs when the rotors are rotating. Despite the reluctance variations in the side legs the total reluctance through the whole magnetic circuit keeps constant. Thus the flux through the center leg is unchanged and depends only on the magnetization. The possible tiny flux fluctuation in the center leg is additionally damped by the rings on the both ends of the center leg. The damping rings are of copper, for example.

The rotors of the structure of the machine are synchronized such that reluctances of the parallel magnetic paths are not equal at the same time. That is to say that a phase difference is set between the rotors. In an embodiment of the invention, the rotors are mechanically coupled to planet gears of a planetary gearing which is preferably employed in wind turbine installation. When arranged in a wind turbine, the rotating electric machine of the invention is a wind power generator in having multiple of rotors connected to planet gears of planetary gearing. The planetary gearing provides input power to the generator and rotates the rotors with the same speed and with a set phase difference.

With the rotating electric machine of the present disclosure the losses of a generator system can be reduced as the magnetic flux in the core structure does not change its polarity. As a result of the homopolar change of flux, the hysteresis loop is reduced, and therefore the hysteresis losses are also reduced.

Further, the rotors of the machine are simple in structure comparatively small and lightweight and without any windings. As the field and phase windings are located substantially remote from the rotor, the windings do not have an effect on the operation of the rotor. In the invention, the operation principle is not based on rotating magnetic field, and therefore the field or/and the phase windings are not surrounding rotors. The windings are wound on stationary legs as in a transformer and the wire for the windings can be made of flat wires or foils without placing the wires in slots. The absence of slots saves the production costs of the rotating machine.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail by means of preferred embodiments with reference to the attached [accompanying] drawings, in which

Figure 1 is a perspective view of the core structure of the machine of an embodiment of the invention;

Figure 2 shows a longitudinal intersection of the core structure of an embodiment of the invention, and

Figure 3 shows a view of the core structure from one side of the structure.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows a perspective view of the core structure of the rotat- ing electric machine of an embodiment of the invention. In the example of Figure 1 , three side legs 2 are attached to a center leg 1 . The center leg is provided with a field winding (not shown) for producing DC excitation to the core, i.e. a magnetic field is produced with a DC current flowing in the field winding.

The side legs 2 of the core are attached to the center leg such that center leg 1 is in the middle of the side legs 2. Each of the side legs produces a magnetic path for the magnetic field produced by the field winding. As seen in Figure 1 , gaps 3 are formed between the ends of the side legs and the center leg such that rotors or rotatable magnetic elements can be inserted to the gaps.

The side legs 2 comprise phase windings (not shown) wound around the side legs. The phase windings produce the output electric power from the rotating electronic machine when the machine acts as a generator.

The material of the core structure is preferably laminated steel. Correspondingly, the rotors are preferably of laminated steel. Laminated steel is used for minimizing the core losses. Further, as the core structure is somewhat similar to that of a transformer, the same materials as in transformers may be applied. Other possible materials include sintered or composite materials. An example of composite material is marketed by Hoganas AB under trade name Somaloy®.

Figure 2 shows a longitudinal intersection of the core structure with side legs 2 on the both sides of the center leg 1 . Only two side legs are shown as other side legs are positioned correspondingly to surround the center leg 1 . For example, when the number of legs is three, the side legs are situated e.g. at an angle of 120 degrees from each other surrounding the center leg when the side legs are placed symmetrically. In case of Figure 2, the number of side legs can be six, for example, as two side legs 2 of the same phase are opposite to each other when an even number of side legs are symmetrically placed around the center leg 1 . In the shown examples the side legs are placed symmetrically with respect to the center leg. However, the symmetry is not required for the operation of the device.

Specifically Figure 2 illustrates the placement of the rotors 4 with respect to the core structure. As shown, the rotors 4 are placed in gaps 3 between side leg 2 and the center leg 1 such that a magnetic circuit or a flux path is produced through the core and the rotor. The core structure produces paral- lei flux paths that are joined in the center leg. Figure 2 further shows the axles 5 attached to the rotors. As seen, the direction of the axles corresponds substantially to the direction of the center leg 1 of the core. The other axles attached to other rotors have the same direction as the ones shown.

Figure 2 also shows the field winding 6 and the phase windings 7. As the magnetic flux flows in a path defined by the core structure, the windings may be of overlapping design. The windings may have multiple layers and may be formed of a foil-like or a flat conductor for minimizing the losses in the windings. The windings are similar to those used in transformers. Figure 2 shows also damping rings 8. The damping rings 8 are wound around the center leg 1 of the core structure and located at the both ends of the center leg and are used for dampening possible small variations in flux of the center leg 1 .

Figure 3 shows the simplified structure of Figure 2 when seen from another side of the core. The direction of the view of Figure 3 is from the driving end of the machine i.e. in the direction of the axles. In the example of Fig- ure 3, the side legs 2 surround the center leg 1 in a symmetric manner. As seen from the example of Figure 3, the center leg 1 has a larger cross-section than the individual side legs 2. If a three-phase output is desired from the structure of Figure 3, then it is preferable that the phase windings of the side legs that are opposite to each other are connected to form a single phase output. For example windings U1 and U2 shown in Figure 3 form phase U output, windings V1 and V2 form phase V output and windings W1 and W2 form phase W output. The separate windings are connected preferably in series for forming the outputs U, V and W.

Figure 3 also reveals the structure of the rotors 4. The rotors are formed such that the reluctances in the flux paths vary when the angle of the rotor is changed. In the example of Figure 3, the rotor minimizes and maximizes the air gap in the core structure four times per one rotation cycle of the rotor. The rotor may also have any other even number of projections or poles than four.

As shown in Figure 3, the magnetic path of the core structure is closed via the rotor such that when the reluctance is at the minimum, two of the four projections of the rotor are towards center leg 1 and other two are towards side leg 2 such that the convex surface of the rotor is close to concave surface of the legs. Figure 3 illustrates a possibility in which both the center leg and the side leg comprise two convex surfaces such that the reluctance is min- imized in the structure. The shape of the rotor is preferably such that the magnetic flux in the core varies sinusoidally during the rotation of the rotor.

When the rotating electric machine is used as a generator, the rotors 4 are preferably attached to planet gears of a planetary gearing. The planet gears of a planetary gearing are rotated in synchronism and thus the rotors rotate with a same rotational speed. The synchronism enables to form a multiphase system such that the reluctance of the flux paths is minimized and maximized in certain order. Further, when a symmetrical multi-phase system is formed, the magnetic flux density stays substantially constant in the center leg.

The structure of the rotating electric machine suitable to be used with planetary gearing as the rotors are situated in a circumference of a circle similarly as the planet gears of a planetary gearing. In a planetary gearing input power is provided to the main shaft and the rotating motion is led to the planet gears in a known manner as such. The planetary gearing used in connection with the invention may comprise one or multiple planetary stages.

The rotating electric machine can also be used as a motor with multiple rotors. In that case the rotors of the machine are rotated by supplying electrical power to the phase windings. As the electric machine comprises multiple axles, the axles may be connected to a single load through a gearing system, for example, such that mechanical power is inputted to the load through a single axis.

In a preferred embodiment, the rotating electric machine acting as a motor drives a drum or similar structure. In such a case a single drum is driven with each of the rotors. The drum may be equipped with teeth in the outer or inner circumference of the edge of the drum and rotor shafts of the machine comprise gears. The teeth of the gears fit to the teeth of the drum and thus the drum is rotated with the rotation of the rotors. An example of the motor of an embodiment is a mill motor.

The operation of the rotating electric machine is described in the following in connection with operation as a generator connected to the planetary gearing. A direct current is fed to the winding on the center core to provide magnetization of the core. Magnetic flux is formed in the core such that flux flows from the center leg to each of the side legs. The rotors act as magnetic valves and depending on their angle the reluctances of the air gaps and flux paths changes. The magnetic flux is higher in those side legs in which the reluctance is smaller. When the rotors are rotated, the reluctances change and thus the magnetic flux changes in the side legs.

The side legs comprise windings from which the electrical output power is obtained. As mentioned, the magnetic flux in the side legs changes when the rotors are rotated. The changing magnetic flux induces electrical voltage to the windings. If the number of rotors and side legs is three, a sym- metrical three-phase voltage is obtained if a corresponding phase difference is provided between the rotors. When the number of side legs and rotors is six, a three-phase output voltage can be generated by having three pairs of rotors operating without phase difference. This is to say, that for example opposite legs of the core structure has rotors at the same phase, such that magnetic reluctances of the opposite legs are equal. The windings of the opposite legs can then be connected in series. Other two opposite pairs are arranged similarly such that a three-phase voltage is generated.

The generated flux in the side legs does not change polarity and flows only in one direction. The frequency of the output voltage is proportional to the rotational speed of the rotors. It is generally advisable to have rotational speed of the rotors which produces voltage having a frequency higher than the typical network frequency. The output frequency may be in the range of 70 to 120 Hz, for example. The high frequency of magnetic flux is preferable, as the voltage generated to the output windings is proportional to the time derivative of the magnetic flux. Even higher frequencies may be profitable in some appli- cations.

Each of the rotors of the generator is preferably connected to a planet gear of a planetary gearing such that planet gears rotate the rotors with a common rotational speed in the magnetic field. This field is achieved when the magnetic core is magnetized. Torque is required to increase the reluctance by rotating the rotor. The sum of torques of all the rotors of the machine is breaking. The mechanical work is converted to electrical energy in the side leg windings which are connected to load. In order to compensate armature reaction in the side legs magnetizing current is added when loading is increased.

The generator is preferably a wind power generator and the me- chanical power driving the planetary gearing is obtained from the rotor of a wind generator system. The planetary gearing is used for changing the rotational speed and torque of the rotor of the wind generation system.

The machine of the invention is preferably a megawatt class machine. Such a machine is suitable to be used as a wind power generator.

The magnetization to the center core of the machine is provided by a suitable energy source. The magnetization is preferably controllable such that different operating situations can be taken into account. When magnetization is increased, more energy is obtained from the rotation of the rotors as the torque required to rotate the rotors increases.

According to a preferred embodiment, a frequency converter is electrically connected to the windings arranged on the side legs. In generator operation, the frequency converter receives alternating voltage with a changing frequency. The frequency converter is operated to convert its input voltage to an output voltage with a set frequency. The output voltage of the generator varies according to the rotational speed of rotors.

When the machine is operated as a motor, the frequency converter is used for driving the motor by feeding an AC voltage to the windings of the side legs.

According to an embodiment, the winding of the center leg and/or the windings of the side legs are formed as superconducting windings. For that matter, the structure comprises the necessary equipment for providing the su- perconducting operation. The superconducting winding is advantageous as windings are stationary and therefore hollow shafts or other special structural modifications are not required.

The drawings show few possible implementations of the core struc- ture. As is understood, the side legs can have other structures than shown. For example, the side legs may have substantially same direction as the center leg. Further, it is understood that both the side legs and the center leg may have protrusions to close the magnetic path in desired manner. For example, it is shown in Figures 1 and 2 that the ends of the side legs extend towards the center leg and similarly the end of the center leg extends outwards towards the side legs.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims

1 . A rotating electric machine comprising a magnetic core, which comprises

a center leg (1 ) comprising a winding (6) for magnetizing the mag- netic core,

multiple of side legs (2) each comprising a winding (7) wound on the side legs, the side legs being connected to the center leg for providing magnetic flux paths for the magnetization provided by the winding wound on the center leg,

wherein a rotor is arranged in each of the flux paths for providing variable magnetic reluctance to the flux paths.

2. A rotating electric machine according to claim 1 , wherein each rotor is arranged to provide a variable air gap to the side legs.

3. A rotating electric machine according to claim 1 or 2, wherein the magnetic core and the rotors are formed of laminated steel.

4. A rotating electric machine according to claim 1 , 2 or 3, wherein the side legs are arranged symmetrically around the center leg.

5. A rotating electric machine according to any one of the claims 1 to

4, wherein the rotors are arranged to provide symmetrical distribution of mag- netic fluxes between the side legs.

6. A rotating electric machine according to any one of the claims 1 to

5, wherein a frequency converter is connected electrically to the windings of the side legs.

7. A rotating electric machine according to any one of the claims 1 to 6, wherein one of the windings is a superconducting winding.

8. A rotating electric machine according to any one of the claims 1 to 7, wherein the rotating electric machine is a generator.

9. A rotating electric machine according to claim 8, wherein the shafts of the rotors are connected to planet gears of a planetary gearing for driving the rotors.

10. A rotating electric machine according to claim 8 or 9, wherein the generator is arranged to be driven by wind power.

11 . A rotating electric machine according to claim 8 or 9, wherein the frequency converter is adapted to convert alternating voltage of the windings of the side legs to an alternating voltage with a constant frequency.

12. A rotating electric machine according to any one of the claims 1 to 7, wherein the rotating electric machine is a motor.

13. A rotating electric machine according to claim 12, wherein the rotors of the motor are adapted to drive a common load either directly or through a gearing system.

14. A rotating electric machine according to claim 12 or 13, wherein the frequency converter is adapted to control the motor by supplying alternating voltage to the windings of the side legs.

15. A rotating electric machine according to any one of the claims 1 to 14, wherein a damping ring 8 is arranged to surround the center leg.