US2492016A - Magnet frame for dynamo-electric machines - Google Patents

Description

Dec. 20, 1949 H. E. STOKES 2,4929% MAGNET FRAME FOR DYNAMOELECTRIC MACHINES Filed July 20. 1946 2/ j, 5 7 M 1141 MW WITNESSES: INVENTOR 54W Hare/d E 5/0 K65 ATTORNEY Patented Dec. 20, 1949 Harold E. Stokes, Murraysville, Pa, assignor to- Westinghouse Electric Corporation, EastPittsburgh, Pa., a corporationof. Pennsylvania Application July 20 1946-, Serial NO; 685061 My invention relates to high-resistance steel magnet-frames for dynamo-electric machines and other electrical apparatus, in which it is not generally economically feasible to laminate the flux-carrying part to eliminate or reduce eddy-currents.

An important object of my invention is to produce a variable-load direct-current commutator typedynamo-electric machine, having laminated. steel. interpoles and a solid highresistance field-frame to prevent or reduce the spitting and flashing which. has heretofore occurred at the brushes as a result of rapid changes of load-current, as will subsequently be explained in: detail.

More generally stated, it is an object ofmy invention to provide a solid high-resistance steel core for any electrical apparatus having electricwinding==means for causing a variable-magnitude magnetic. flux to flow through said core, as willsubsequently be explained.

By solid. I mean not physically subdivided in a way to minimize eddy-currents. A fluxcarrying magnetizable' member is physically subdivided when its material is either comminuted or laminated, regardless of Whether the subdivided parts' or pieces are left separated, in the final product, or whether they are joined by a binder, or by sintering, or by any other process of adhesion or cohesion. The magnetizable memher need not be all in one piece, so long as the pieces into which it is subdivided are not of such small dimension, in any direction, as to have a pronounced eddy-current-reducing effect because of that small dimension.

An illustrative application or embodiment of my invention is shown in the accompanying drawing, wherein Figure 1 is a diagrammatic end-view or symbolic showing of an interpole direct-currentmotor or generator to which my invention is applied, and Figs. 2 and 3 are sectional views through an interpole and a main pole, respectively, on the section-planes II-II and III-III of Fig. 1.

The dynamo-electric machine shown in Fig. 1 is a large-power, variable-load, interpole-type, direct-current motor or generator, of such large size that the field-magnet frame 4 is conveniently made in two separate halves, joined together as shown at 5 The bottom half has steel feet 6 connected thereto, on which the machine is supported. The field-frame 4 is in the form of a cylindrically shaped yoke, to the inside of which are attached a plurality of inwardly projecting main pole-pieces l, and a plurality of 4 Claims. (Cl. 171-252) l4 supports the armature.

inwardly projecting interpolar pole-pieces 8. The machine also has an armature member. ID, rotatably supported within the field member, and separated from the pole- pieces 1 and 8 by an air-gap H. The armature member Hl carries a commutator, which is diagrammatically indicated by the same circle l2 which indicates the periphery of the armature-core. Brushes l3 bear on the commutator. Avariable-l'oad shaft The main poles l are excited by main-pole windings l5, which are shown as separately excited froma direct-current line l6 through a rheostat IT. The interpoles 8 are excited by interpole field-windings l8,

-Which are serially included in a variable-load circuit l9 which is connected to the brushes l3.

Variable-load, compensated, directcurrent machines have heretofore been subject to excessive momentary spitting or flashing, at the brushes, whenever the load: changes rapidly. I- do not believe that it has been generally under-- stood, heretofore, that this spitting and flashing at the brushes is caused by a flux-lag of the interpole or commutating-pole flux, behind the load-current,- thus causing the machine to be undercompensated during quick build-up of the load-current, and overcompensated during quick build-down of the load-current.

During a change of load-current, the commutati'n'g-pole coil I8 sets up a changing flux in the commutating pole-piece 8, and this flux cuts the inside layer 2i of the magnet-steel 4, in order to get into the magnet-steel or frame 4; inducing a voltage in this inside layer in an axial direction, and causing circulating currents to flow,

returning at the outer periphery of the frame, and damping the change in flux in the commutating pole; Thus, to choose a very simple case which will sufiice to illustrate the principle, without giving actual working flux-densities, let us suppose that the main poles I are excited so as to have six flux-lines in each main pole-piece, thus producing three flux-lines in the yoke or frame 4, between two successive main poles 1 and 'I", as shown by the flux-line arrows 22 in Fig. 1. Let us suppose that the load-current, and hence the interpole-fiux, is at first zero, but that itsuddenly changes to a value yielding four flux-lines in each interpole 8, as shown by the flux-line arrows 23 in Fig. 1. particular interpole 8' between the main poles 'I" and 1", it will be seen that the field-frame flux is suddenly reduced fromthree lines to one, on one side-of the interpole 8', say between said interpole and the main pole 1", thus causing" Referring to the eddy-currents 24 to flow in the magnet-frame 4, in a direction to produce a flux 25 opposing said change. On the other side of the interpole 8', say between said interpole and the main pole I, in Fig. 1, the field-frame flux will he suddenly increased from three lines to five lines, causing eddy-currents 26 to flow in the magnet-frame 4, in a direction to produce a flux 21 opposed to said change.

It has been known, heretofore, in designing direct-current machines having rapidly changing fluxes, either in the main poles, or the interpoles, or both, to laminate both the frame or yoke member and the variable-flux pole-pieces, in order to reduce eddy-currents, so as to permit the flux to change rapidly. Thus, a remedy is provided, at the expense of the extra cost of laminating. My present invention is applicable, however, to a machine which would involve exorbitant or unwarranted extra expense to laminate the magnet-frame 4, because of the costly dies which would be required, together with the expense of securing the split-frame joint and the feet 6. And yet the brush-spitting or flashing has been excessive, with laminated pole-pieces and a solid-steel magnet-frame of ordinary design.

And yet, it is recalled that, in former years, when the magnet-frames were made of cast-iron rather than mild steel, as in the more recent designs preceding my present invention, the brushspitting trouble was not, in general, a source of trouble, in variable-load machines having laminated steel interpoles. I believe that this freedom from spitting-troubles, in the old-fashioned machines with cast-iron frames or yokes, was due largely to the relatively poor permeability of the cast iron, which necessitated working it at a low working-flux density in order to obtain as high a permeability as possible. This resulted in a large field-frame, and hence a long path for the circulating eddy-currents, which, in turn, resulted in a small value of the eddy-currents. In modern machines, such large field frames could not be tolerated, and hence mild steel has come to be the accepted standard frame-material, prior to my present invention, in machines of the types and sizes to which my invention particularly applies.

According to my present invention, I make the field-magnet frame or yoke-member 4 of a solid piece or pieces of a special high-resistance steel or other magnetizable material having substan tially as good permeability as the previously used mild-steel frames; and I continue to make the main and interpole pole-pieces I and 8 of the laminated mild steel. The extra resistivity of the frame-material makes the eddy-currents 24 and 26 smaller in magnitude, and produces, at least in part, some of the advantages of laminating, in reducing the eddy-current damping or lagging of the changed-flux effects, in response to sudden changes in the exciting-currents.

One difierence in the steels is that ordinary mild steel, for magnetic purposes, has no more than a trace of silicon, whereas a preferred highresistance steel, which I use for the magnetframe, is obtained by the addition of silicon, to obtain the necessary electrical resistivity per centimeter cube, plus enough additional manganese to impart machinability and also to impart the ductility necessary for satisfactorily roll- (ill 4. as for modern, laminated pole-pieces, and such as I still use for the laminated pole-pieces I and 8, has the following chemical content:

Per cent Carbon 0.15 to 0.25 Manganese .3 to .7 Phosphorus maximum .04 Sulphur .05 Iron substantially the balance Its characteristics are described as a commercial grade of soft, open-hearth, rolled-steel plates, suitable for welding and bending. The magnetizing force required, at 70,000 lines per square inch, is about 18.5 ampere-turns per inch; and at 85,000 lines per square inch it is about 31.8 ampere-turns per inch. These values of flux-density are about the working-limits for a machine working at a constant field, but the flux-density may go to nearly zero for a variable-voltage machine, which is necessarily a variable-flux machine. The electrical resistivity of this standard mild steel is about 13.3 microhms per centimeter cube, at 20 C.

The high-resistance steel, which I now use for the magnet-frame 4, should not require a much greater magnetizing force than the mild soft steel just discussed, generally not more than 30 ampere-turns per inch at a Working density of 70,000 lines per square inch, and not more than ampere-turns per inch at a working density of 85,000 lines per square inch. It should have an electrical resistivity at least twice as great, and preferably three or more times as great, as the aforesaid mild soft steel. When its added resistivity is obtained by the addition of silicon, the iron molecules are apparently coated with silicon, or a silicon compound or alloy, thus giving the material more resistivity, at the rate of an extra 13.3 microhms per centimeter cube (more or less) for every one percent of silicon which is added to the steel. I am not limited to any particular resistivity, or any particular chemical composition of the high-resistance magnetic frame-material, other than as heretofore indicated. A typical preferred resistivity is 45 microhms, or more, per centimeter cube at 20 C. A typical preferred chemical content is:

Per cent Carbon .07. Manganese .45 Phosphorus -04 Sulphur .035 Silicon 2.5 to 3.0 Iron substantially the balance.

In general, its silicon-content should be at least about two percent, and the amount of silicon could be considerably in excess of the three per-- cent maximum which is indicated in the sample specimen just cited. Its permeability is substantially the same as the mild soft steel, at the stated;

ratus, not necessarily dynamo-electric machines, having electric winding-means for causing a variable magnetic flux to flow through the core.

Important industrial applications of my invention include D. C; generators and motors for It is also applicable, in general, to the cores of any electrical appablooming-mills, steel mills, shovels, draglines, mine-hoists, and the like.

I claim as my invention:

1. A variable-load direct-current commutatortype dynamo-electric machine comprising a solid field-frame, main pole-pieces and interpolar polepieces extending inwardly therefrom, main fieldwindings for exciting the main field-poles, interpole field-windings for exciting the interpoles, an armature member rotatable within the fieldpole assembly, a commutator carried by the armature member, brushes bearing on the commutator, and a variable-load circuit connected to the brushes and serially including the interpole fieldwindings, characterized by a construction of at least the solid field-frame and the interpolar pole-pieces to avoid excessive eddy-current generation in opposition to flux-changes on sudden changes in the load-current of the load-circuit, at least said solid field-frame being essentially composed of one or a small number of molecularly integral and continuous pieces of magnetizable material requiring not more than 30 ampereturns per inch at a working density of 70,000 lines per square inch, and not more than 50 ampere-turns per inch at a working density of 85,000 lines per square inch, each piece being not physically subdivided in a way to minimize eddycurrents but having an electrical resistivity more than 35 microhms per cubic centimeter.

2. A variable-load direct-current commutatortype dynamo-electric machine comprising a solid field-frame, main pole-pieces and interpolar polepieces extending inwardly therefrom, main fieldwindings for exciting the main field-poles, interpole field-windings for exciting the interpoles, an armature member rotatable within the fieldpole assembly, a commutator carried by the armature member, brushes bearing on the commutator, and a variable-load circuit connected to the brushes and serially including the interpole fieldwindings, characterized by a construction of at least the solid field-frame and the interpolar pole-pieces to avoid excessive eddy-current generation in opposition to flux-changes on sudden changes in the load-current of the load-circuit, at least said solid field-frame being essentially composed of one or a small number of molecularly integral and continuous pieces of magnetizable material requiring not more than 30 ampereturns per inch at a working density of 70,000 lines per square inch, and not more than 50 ampere-turns per inch at a working density of 85,000 lines per square inch, each piece being not physically subdivided in a way to minimize eddycurrents but having an electrical resistivity of at least about 45 microhms per cubic centimeter.

3. A dynamo-electric machine comprising a statornnember and a rotor-member, separated by an air-gap, one of said members being a fieldmember, said field-member comprising a supporting-structure, a plurality of salient polepieces projecting therefrom toward the air-gap, and variable-current exciting-means including direct-current field-windings for exciting the pole-pieces, characterized by at least said supporting-structure being essentially composed of one or a small number of molecularly integral and continuous pieces of magnetizable material requiring not more than 30 ampere-turns per inch at a working density of 70,000 lines per.

square inch, and not more than ampere-turns per inch at a working density of 85,000 lines per square inch, each piece being not physically subdivided in a way to minimize eddy-currents but having an electrical resistivity more than 35 microhms per cubic centimeter.

4. A dynamo-electric machine comprising a stator-member and a rotor-member, separated by an air-gap, one of said members being a fieldmember, said field-member comprising a supporting-structure, a plurality of salient polepieces projecting therefrom toward the air-gap, and variable-current exciting-means including direct-current field-windings for exciting the pole-pieces, characterized by at least said supporting-structure being essentially composed of one or a small number of molecularly integral and continuous pieces of magnetizable material requiring not more than 30 ampere-turns per inch at a working density of 70,000 lines per square inch, and not more than 50 ampere-turns per inch at a working density of 85,000 lines per square inch, each piece being not physically subdivided in a way to minimize eddy-currents but having an electrical resistivity of at least about 45 microhms per cubic centimeter.

HAROLD E. STOKES.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number

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