GB2331859A

GB2331859A - Magnetic cores for rotating electric machines

Info

Publication number: GB2331859A
Application number: GB9725310A
Authority: GB
Inventors: Lars Gertmar
Original assignee: Asea Brown Boveri AB
Current assignee: ABB AB
Priority date: 1997-11-28
Filing date: 1997-11-28
Publication date: 1999-06-02
Also published as: GB9725310D0

Abstract

A method of manufacturing a core shape for a rotating electric machine comprising assembling together a plurality of shaped laminations (1) of magnetic core material to form a temporary core structure and adding moulded magnetic composite material to the temporary core structure to produce the final shape of the core structure.

Description

2331859 Magnetic Cores for Rotating Electric Machines

Technical Field

This invention relates to magnetic cores or flux carriers for rotating electric machines and to a method of manufacturing such cores or parts of such cores. The invention is intended to embrace the construction and manufacture of magnetic cores of both stators and rotors of rotating electric machines. The invention also relates to rotating electric machines and their manufacture, such electric machines including motors and/or generators, machines which can be connected directly to a single or multiphase network with fixed frequency, and machines which can be connected via a static converter to such a network. Machines according to the invention may have a power range of from a hundred watts up to many hundreds of megawatts. Machines according to the invention can also be used as exciters for other larger (synchronous) machines.

Back-qround of the Invention A magnetic core of a rotating electric machine serves as a magnetic flux carrier to lead the magnetic flux to and from an air gap of the electrical machine. For a stator, the core provides flux pathways around the conductors of the stator. For a rotor, the core provides flux pathways around the rotor conductors in the case of rotors with windings and, in the case of non-wound rotors, provides flux pathways around permanent magnets in the rotor and/or establishes salient poles in the rotor. For small rotating machines salient poles can even be produced in the stator.

Almost all known magnetic cores for rotating electric machines are made of laminated iron often alloyed with silicon. Laminations are used to keep down eddy current losses in parts of the magnetic circuit where the flux rotates. The sheet thickness of such laminates is generally from 0.2 to 1.0 mm, and typically from 0.5 to 0.7 mm for a normally wound, 50 or 60 Hz machine in the above indicated power range.

In the lower power range, e.g. less than 1 MW, rotating electric machines are mainly asynchronous whereas in the higher power range, e.g. greater than 1 MW, they are mainly synchronous with the rotor typically made of solid wrought iron. The number of poles is normally 2, 4, 6 or 8. Larger numbers of poles are seldom used, because then mechanical gears are often preferred.

The investment costs of a motor represent a very small part of its lifecycle-costs or "LCC". Typically, investment and installation costs represent only some 5% of the LCC, maintenance costs represents another 5% of the LCC and the remaining 90% of the LCC represents energy costs. Legislation is being introduced around the world requiring high-performance electric machines to be energy efficient. Thus motor efficiency is increasingly becoming a more important aspect of machine design than production costs and sale prices. Even though efforts have been made in the past to reduce production costs, production methods are still fairly conservative with few industrial motor manufacturers having automated production facilities as is common with manufacturers of small motors, e.g. for household appliances. indeed with large motors, the production and assembly of windings is still a procedure for craftsmen.

Recent proposals for motor design have suggested the use of amorphous sheet and soft magnetic electrically insulated mouldable magnetic composite materials as magnetic materials for flux carrying parts and cores in rotating electrical machines and transformers. Hereinafter the term umagnetic composite material" is intended to include magnetic material including particulate, e.g. powdered, flaked or short lengths of cut wire or the like (which may also serve as reinforcemnt for the composite material) magnetic material and such magnetic composite material - 3 normally includes other materials such as lubricators and binders. In particular the "particulate" magnetic material may comprise waste material, such as amorphous iron or iron from laminated core stamping. Such magnetic composite materials are intended to be easily shaped before solidifying to a desired shape or form. The term magnetic composite material is also intended to include polymer matrices formed of magnetic polymers or a non- magnetic polymer with a filler of particulate magnetic material.

Advantages of cores made from laminated dynamo sheet are that they have a relatively high flux density saturation of well over 2 T. Advantages of cores made of amorphous sheet are that they have negligible hysteresis losses and low eddy current losses. However amorphous sheet is hard and expensive to punch or stamp, has low flux density saturation of around 1.5 T, and is expensive to purchase. Laminations of generally circular shape result in much wastage of the sheet material from which the laminations are stamped and thus most practical laminations have at least some straight edges to reduce wastage resulting from the stamping and punching process. However cores formed from straight-edged laminations result in compression of the magnetic lines of force in use of the core.

Among advantages of cores made from mouldable 25 magnetic composite materials, such as soft magnetic powder iron, are that they can be formed with little wastage of material, they can be shaped to optimum designs so that compression of the magnetic lines of force are avoided and they can be used to provide transitions to better magnetically conducting material. However, mouldable magnetic composite materials have a limited flux density saturation, compared to sheet material, of around 1.7 T, have high hysteresis losses and, if made of soft magnetic powder iron alone, have high no-load currents and thus corresponding high motor losses.

In a thesis by Mats Alakiila entitled "on the Control of Saturated Synchronous Machines", Lund University, 1993, CODEN: LUTEDX/(TEIE-1005)/1144/(1993), there is a disclosure of how the performance of a small synchronous rotating electric machine (approximately 5 kVA) is improved by modifying the stator of the machine. The stator of the machine was originally formed in a conventional manner from laminations having generally circular or arcuate outer peripheries but with segments missing therefrom. In the finished stator this resulted in four flattened segmental regions around the periphery of the core. Alakiila took the old stator from the synchronous machine and modified it by filling in the flattened segmental regions with powdered iron and noticed an improvement in the output of the machine. AlakUla was interested in attempting to linearise the machine by obtaining a circular backing or yoke for the stator of the machine and was not concerned with increasing the efficiency of the machine.

A book entitled "Energy Efficient Improvements in Electric Motors and Drivesn by A. de Almeida and W. Leonard, published in 1997 (ISBN 3-540-63068-6) describes the art of energy efficient technology for the production of costeffective energy saving motors. There are contributions on the latest research from relevant large-scale manufacturers as well as established institutions around the world but there is no disclosure of being able to produce magetic cores for rotating electric machines using a combination of laminated core material and mouldable magnetic composite material.

Summary of the Invention

An aim of the present invention is to provide an improved construction and method of manufacture of a stator and/or rotor for a rotating electric machine. It is also an aim of the invention to provide a core made of laminated material and magnetic composite material.

According to one aspect of the present invention there is provided a method of manufacturing a core of a given shape for a rotating electrical machine, the method being as claimed in the ensuing claim 1.

According to another aspect of the present invention there is provided a method of manufacturing wound core for a rotating electrical machine, the method being as claimed in the ensuing claim 4.

According to a still further aspect of the present invention, there is provided a rotating electric machine, e.g an induction machine, as claimed in the ensuing claim 11.

The invention enables the rotor and/or stator of a rotating electric machine, such as an induction motor, to be made and/or to operate in a more efficient manner. To achieve maximum efficiency in a rotating electric machine, there should be a constant thickness air gap between the outer surface of the rotor and the confronting inner surface of the stator. This is achieved in practice by the confronting surfaces being arcuate or circular in shape.

The present invention allows rotor and/or stator cores to be constructed from laminations or stampings having one or more flattened sides and, when these laminations are assembled together, the resulting temporary core structure has corresponding flattened regions along its length. In accordance with the invention, these flattened regions can be filled in by a mouldable magnetic composite material which is bonded to the core structure. Such a composite core is made from laminations having straight or flattened sides and thus the stamping operation can be performed with minimum wastage of the laminate sheet material from which the stampings are made. By rounding off the flattened regions of the temporary core structure with the mouldable magnetic composite material, the compression of magnetic - 6 lines of force in the completed core is avoided and core losses are minimised.

The stator and rotor cores of a rotating electric machine act as flux carriers for conducting the magnetic flux to and from the air gap of the machine. To a large extent the stator and rotor cores shape the flux paths respectively around the stator conductors and around the rotor conductors (for wound rotors) or around permanent magnets (for non-wound rotors), and/or establish salient poles in the rotor. The inclusion of a magnetic composite material in a stator core enables the symmetry of a stator core to be broken up, e.g. by the provision of slits or the like, resulting in moving or damping of the resonance in the electric machine.

is A conventional stator core is assembled from laminations with slots therein which are aligned to create an annular core having inwardly opening, closed, lengthwise extending winding-receiving grooves or slots. in order to arrange winding conductors in these grooves or slots, the 20 winding operation is performed from inside the stator core. In a preferred design of stator according to the present invention, a temporary stator core structure is provided with winding slots which open both radially inwardly and radially outwardly. Windings can thus be arranged in the winding slots from radially outside the stator core, e.g. using a conventional d.c. armature winding machine, thereby simplifying the winding process. The radially outer portions of the slots are closed in the completed core by the magnetic composite material, either in the form of a 30 surrounding yoke or fillers in the slots.

The mouldable magnetic composite material preferably includes particulate, e.g. powdered, magnetic material. The particulate magnetic material preferably comprises iron, preferably soft iron, powder which may have two different size distributions. The magnetic composite material may also include lubricators for bringing together and binders 7 for holding together the particulate magnetic material together when the composite material is moulded or compacted into a desired shape. Additionally adhesion activators to provide good wettability and adhesion may be used to ensure adhesion of the compacted magnetic composite material to the laminated core structure. For example the temporary core structure may be coated with an adhesion activator and the finer particulate magnetic material spread over the core structure as required. The (stator) housing is wetted by an adhesion activator and heated to initiate a shrinking process. The coarser particulate magnetic material is then spread over the volume to be filled and is compacted and heat-treated to form its desired shape on the core structure. The use of finer size magnetic particles provides a better filling and a better contact between laminations having a typical thickness of 0.5 mm or 0.65 mm. The coarser magnetic particles are, however, cheaper.

Brief Description of the Drawings

Embodiments of the invention will now be described, by way of example only, with specific reference to the accompanying drawings, in which:

Figure I is a view of a lamination of a stator core of a small rotating electric machine; Figure 2 is a side view of a plurality of stator laminations of the type shown in Figure 1 assembled together to form a temporary core structure; Figure 3 is a schematic view illustrating the formation of one embodiment of a stator core formed from laminations and a moulded magnetic composite material; and Figure 4 is a schematic view illustrating the formation of another embodiment of a stator core - 8 formed from laminations composite material.

and moulded magnetic Figure 1 shows a lamination 1, stamped f rom iron sheet, for assembling together with other laminations to form a stator core structure 2 (see Figure 2) of a small rotating electric machine (less than 10 kW) having a typical IEC shaft height of form 63 - 180 mm. The lamination 1 has a generally circular outer periphery but without four segments so as to provide four flattened or straight regions 3. Slots 4 (only some of which are shown in Figure 1) open into the inner peripheral surface. In the stator core structure 2, the slots 4 are aligned to provide grooves or slots (not shown) for receiving stator conductors (not shown).

is The lamination 1 has a different height to its width.

Adjacent laminations 1 in the core structure 2 are turned through 451 as can be seen in Figure 2.

The core structure 2 is of only temporary form. in order to form its completed shape, mouldable magnetic composite material (e.g. particulate material including magnetic particles) is bonded to the core structure 2 to alter its shape, to improve its magnetic properties and to improve machine performance. Suitably the magnetic composite material is moulded to the laminated core structure 2 to form a rounded core having no sharp edges thereby avoiding compression of magnetic lines of force in use of the core. The mouldable magnetic composite material suitably comprises iron powder consisting of fine and coarse particles of soft iron or other magnetic material and lubricating and binding agents. The surfaces of the core structure 2 to which the magnetic composite material is bonded are wetted with an adhesive activator. The gaps between adjacent laminations are filled with the finer magnetic particles and larger volumes are filled with the coarser magnetic particles. The magnetic composite material - 9 is compacted or moulded to the desired shape. The magnetic composite material may include reinforcing wires.

There are several advantages in f orming the core f rom a combination of laminated material and the magnetic composite material By applying the composite material, magnetic lines of force are not compressed and the effective magnetic diameter of each lamination is increased. The mechanical rigidity of the laminations changes, especially if slits in the lamination are filled with iron powder, and this provides increased damping resulting in quieter motor operation.

Figures 3 and 4 illustrate two different methods of producing wound stator cores. In Figure 3 a band of lamination material 10 is punched to create winding receiving slots 11 in a core structure 12 which slots extend from the radially inner side to the radially outer side of the core structure 12. Winding conductors are positioned in the slots 11 from the radially outer side, e.g. using d.c. armature winding machinery (not shown). The radially outer ends of the slots 11 are closed in the completed core by a curved yoke 14 bonded to the core structure 12 and comprising a-moulded magnetic composite material. The yoke 14 may be moulded against the outside of the core structure 12 or may be pre-formed to the desired shape and then bonded to the core structure.

The wound stator core 20 shown in Figure 4 differs from the core 10 in that the winding receiving slots 21 are closed by the magnetic composite material 22 filling and being bended or moulded into the radially outer end portions of the slots 21.

The invention can be applied to rotor and stator cores of small and large rotating electric machines. A rotor or stator core can be shaped so as to conduct the magnetic flux so that in every point in the core the magnetic field strength or H-field produced by the - 10 excitation current and/or permanent magnets, if used, gives rise to a magnetic flux density or B-field which is essentially parallel with the H-field.

The invention may be applied to both stator and rotor cores. For example an assembly of rotor laminations may be shaped by the addition thereto of magnetic composite material, for example to minimise the air gap between the stator and the rotor.

Although the magnetic composite material preferably comprises iron particles and lubricators and binders, it may also comprise a polymer matrix of magnetic polymers or a non-magnetic polymer with a filler of particulate magnetic material, e.g. soft iron powder.

Claims

- 11 CLAIMS

1. A method of manufacturing a core shape for a rotating electric machine comprising assembling together a plurality of shaped laminations of magnetic core material to form a core structure, characterised in that the core structure is of only a temporary shape and in that the core is formed into its final shape by the addition to the core structure of moulded magnetic composite material.

2. A method according to claim 1, characterised in that the said magnetic composite material is moulded directly to a part or parts of the core structure.

3. A method according to claim 1, characterised in that the magnetic composite material is moulded into a shaped member which is joined, e.g. bonded, to the core structure to form the final shape of the core.

4. A method according to claim 1, 2 or 3, characterised in that the core structure has at least one flattened peripheral region and in that the magnetic composite material is joined to the flattened region(s) to provide a substantially curved shape to the region or regions.

5. A method of manufacturing a wound core for a rotating electric machine, comprising the steps of assembling together a plurality of shaped laminations of magnetic core material to form a core structure having winding slots therein and arranging windings in the winding slots, characterised in that the core structure is of only a temporary shape and that the core is formed into its final shape by the addition to the core structure of moulded magnetic composite material.

6. A method according to claim 5, characterised in that core comprises a stator core.

7. A method according to claim 6, characterised in that the winding slots extend radially from the inside to the outside of the core structure, in that the windings are inserted radially into the slots from their radially outer sides and in that the radially outer ends of the winding slots are closed by said moulded magnetic composite material after the windings have been inserted in the winding slots.

8. A method according to claim 7, characterised in that the radially outer ends of the winding slots are closed by a stator back in the form of a yoke comprising said moulded magnetic composite material arranged radially outside the winding slots.

9. A method according to claim 7, characterised in that the magnetic composite material is moulded in the radially outermost portions of the winding slots to form stator backs for the winding slots.

10. A method according to claim 5, characterised in that core comprises a rotor core.

11. A rotating electric machine comprising a rotor having a first core with first slots therein and first windings arranged in the first slots and a second core with second slots therein and second windings arranged in the second slots, characterised in that at least one of the first and second cores comprises a combination of laminated magnetic material and moulded magnetic composite material.

12. A machine according to claim 11, characterised in that the magnetic composite material is moulded to a part or parts of the laminated magnetic material.

13. A machine according to claim 11 or 12, characterised in that the second core comprises first and second core means, the first core means being in the form of a ring, being made of said laminated magnetic material and having said second slots extending radially from inner to 13 outer sides thereof, the second core means comprising said magnetic composite material and being arranged to close the radially outer ends of the second slots.

14. A machine according to claim 13, characterised in that the second core means comprises a yoke arranged radially outside said first core means.

15. A machine according to claim 13, characterised in that the second core means are arranged in the radially outer portions of said second slots.

16. A machine according to any one of claims 11 to 15, characterised in that at least one of the first and second cores has slit means therein, e.g. for moving or damping the resonance of the machine.

17. A machine according to claim 16, characterised in that said slit means are filled with said moulded magnetic composite material.

18. A machine according to any one of claims 11 to 17, in which the machine is a rotating electric induction machine.