Hawkins Electrical Guide, Number One Part 29

=Ques. What is the princ.i.p.al objection to the use of cast iron for core construction?=

Ans. Since its sectional area must be considerably more than wrought iron, a much greater quant.i.ty of copper is required for the magnetizing coils.

Copper is expensive, while cast iron cores are less expensive than equivalent ones of wrought iron; in this connection, it is interesting to observe how different designers aim at true economy in construction.

Steel is sometimes used in place of wrought iron, and though less efficient magnetically, it can be cast into the desired shape, thus avoiding the somewhat expensive processes of forging and machining, which are necessary in the case of wrought iron.

=Ques. What form of core requires the least amount of copper for the magnetizing coils, and why?=

Ans. The cylindrical core, because it has the shortest periphery or boundary for a given area enclosed.

[Ill.u.s.tration: FIGS. 222 to 225.--Several forms of pole piece. Where the extremities project as in figs. 222 and 223, they are called _horns_. The object of these is to reduce the reluctance of the air gap. The width of "fringe" of the magnetic field is influenced by the shape of the pole piece; the margin of fringe should be such that the flux density will vary from zero to a high value where the inductors enter.]

Figs. 216 to 221, show a series of cross sections, all of the same area. The number marked on each section indicates the length of the boundary line, that of the circle being taken for convenience as 100.

=Ques. What are the pole pieces?=

Ans. These are the end portions of the field magnets, joined to, or cast together with the core and placed adjacent to the armature.

The faces of the pole pieces are of circular shape, thus forming the sides of the so-called armature chamber within which the armature rotates.

[Ill.u.s.tration: FIG. 226.--Unsymmetrical pole piece introduced by Gravier to concentrate the magnetic field. When the dynamo is working at small loads, the flux in the gap is nearly uniform, but at heavy loads, the distortion due to the armature current forces the flux forward and saturates the forward horn, thus preventing much change in its flux density, on account of the saturation, and the diminishing area. Lundell combined the unsymmetrical and slotted forms of pole piece as shown in fig. 237.]

=Ques. Why are the pole faces made larger than the coils?=

Ans. In order to reduce the reluctance of the air gap between the face and the armature, thus enabling fewer magnetizing coils to be used.

[Ill.u.s.tration: FIG. 227.--Pole piece with oblique slots; a modification of Lundell's form of pole piece as suggested by Thompson. In operation, the neck of the casting becomes saturated and offers considerable reluctance, which tends to prevent distortion of the magnetic field.]

It is important that the field should be magnetically rigid, that is, not easily distorted. This stiffness of field can be partially secured by judicious shaping of the pole pieces. A few forms of pole piece are shown in figs. 222 to 231.

If the projecting tips of the pole pieces, or _horns_ as they are called, be widely separated, as in fig. 222, they are not always good, even though thin. It is better that they should be extended as in fig. 223 so that they may be saturated by the leakage field or else cut off as in fig. 224.

An extreme design, suggested by Dobrowolski, as shown in fig.

225, surrounds the armature with iron.

[Ill.u.s.tration: FIG. 228.--Non-concentric pole faces; one method of securing suitable magnetic "fringe" with fair magnetic rigidity of field.]

[Ill.u.s.tration: FIGS. 229 to 231.--Various shapes of pole piece for securing a gradual entrance of the armature inductors into the magnetic field.]

Another scheme, proposed by Gravier, employed the unsymmetrical form shown in fig. 226. In this pole piece the forward horn is elongated. The action due to this arrangement is such that when the machine is working at small loads, the field in the gap is nearly uniform, but at heavy loads with distorting reactions which have a tendency to drive the flux into the forward horn, the small section of the latter causes it to become saturated, thus reducing the distortion to a minimum.

=Eddy Currents; Laminated Fields.=--The field magnet cores and pole pieces, as well as the armature of a dynamo are subject to _eddy currents_, that is, induced electric currents occurring when a solid metallic ma.s.s is rotated in a magnetic field. These currents consume a large amount of energy and often occasion harmful rise in temperature.

This loss may be almost entirely avoided by laminating the pole piece, or both pole piece and core; in the latter case, both form one part without any joint.

[Ill.u.s.tration: FIG. 232.--Ill.u.s.trating the alteration of magnetic field due to movement of ma.s.s of iron in the armature. If the ma.s.ses of iron in the armature are so disposed that as it rotates, the distribution of the lines of force in the narrow field between the armature and the pole piece is being continually altered, then, even though the total amount of magnetism of the field magnet remain unchanged, eddy currents will be set up in the pole piece and will heat it. This is shown in the above figures, which represent the effect of a projecting tooth, such as that of a Pacinotti ring, in changing the distribution of magnetism in the pole piece.]

[Ill.u.s.tration: FIG. 233.--Eddy currents induced in pole pieces by movement of ma.s.ses of iron. These diagrams, which correspond to those of fig. 232, show the eddy currents grouped in pairs of vortices. The strongest current flows between the vortices and is situated just below the projecting tooth, where the magnetism is most intense; it moves onward following the tooth. At C is shown what occurs during the final retreat of the tooth from the pole piece. These eddy currents penetrate into the interior of the iron, although to no great depth. Clearly the greatest amount of such eddy currents will be generated at that part of the pole piece where the magnetic perturbations are greatest and most sudden. A glance at the figures shows that this should be at the forward horn of the pole piece.

However, when a dynamo, with horned pole pieces, has been running for some time as a motor the forward horns are cool and the hindward horns hot.]

[Ill.u.s.tration: FIG. 234.--Fort Wayne laminated pole piece before being cast welded into frame. In the faces of solid pole pieces there exist minute electric currents called _eddy currents_ which cause heating of the iron and increase the energy required to maintain a magnetic circuit in much the same manner as does reluctance. This loss is reduced by dividing the magnetic circuit in the line of flux into numerous parallel paths separated by some material of relatively high resistance. In construction, the above core and pole piece is made up of sheets of annealed steel of two different widths a.s.sembled together to form proper size and shape. The minute s.p.a.cing between these laminations and the slight oxidizing on each surface is sufficient to reduce considerably the eddy currents. By cast welding the pole piece into the frame, a low reluctance is secured.]

=Ques. What is a laminated pole?=

Ans. One built up of layers of iron sheets, stamped from sheet metal and insulated, as shown in fig. 234.

=Ques. What may be said of this construction?=

Ans. It is a most approved method, and one frequently employed in the construction of cores and pole pieces.

Fig. 234 shows a combined core and pole piece made entirely of sheet iron punchings a.s.sembled and riveted together, and fig.

235, a core to be used with separate pole piece. It should be noted that in both cases there is a longitudinal slot extending from the end into the core. This was first suggested by Lundell, the object being to prevent, as far as possible, the distortion of the magnetic field due to armature reaction especially on heavy overloads.

[Ill.u.s.tration: FIG. 235.--Fort Wayne laminated core without pole piece, as used on large dynamos. It is constructed of punchings from sheet iron, and riveted under pressure. The alternate end projections and grooved base insure good mechanical union of metal in cast welding to magnet frame.

Reluctance between core and yoke is reduced to a minimum by cast welding.

The core is slotted parallel with the shaft to prevent, as far as possible, the distortion of the magnetic field, especially on heavy overloads.]

=Ques. What mode of construction is adopted to reduce the reluctance of the magnetic circuit when laminated poles are used?=

Ans. They are cast welded into the frame.

[Ill.u.s.tration: FIG. 236.--Fort Wayne one piece frame with cast welded combined cores and pole pieces. In any electrical apparatus a magnetic circuit of low reluctance requires less energy to maintain a given flux than one having a comparatively high reluctance. To reduce this to a minimum the pole pieces and cores are combined into one part and then cast welded into the yoke or frame. Thus the continuity of the magnetic circuit is practically unbroken save for the air gap.]

The frame end of the core as shown in the ill.u.s.trations has irregularities in the heights of the different sheets, as well as grooved undercut surfaces, in order to enable the molten metal of the frame to key well into the laminations of the core, making a good joint, both mechanically and electrically. By this construction, the continuity of the magnetic circuit is practically unbroken save for the air gap between the pole piece and armature.

Fig. 236 shows a one piece frame of a six pole dynamo having cast welded into it, combined cores and pole pieces.

=Ques. What is the disadvantage of laminating a core?=

Ans. It necessitates a nearly square or rectangular section, which requires more copper for the winding than the cylindrical form.

[Ill.u.s.tration: FIG. 237.--Lundell type of combined core and pole piece; a combination of Gravier's unsymmetrical horns and longitudinal slot designed to prevent distortion of field.]

=The Magnetizing Coils.=--The object of the magnetizing coils, is to provide, under the various conditions of operation, the number of _ampere turns_ of excitation required to give the proper flux through the armature to produce the desired electromotive force.

With respect to the manner in which magnetizing coils are wound they are said to be:

1. Spool wound; 2. Former wound.

=Ques. Describe the methods of constructing spool wound coils.=

Ans. The spool is made in various ways, sometimes entirely of bra.s.s, or of sheet iron with bra.s.s f.l.a.n.g.es, or of very thin cast iron. Some builders use sheet metal with a f.l.a.n.g.e of hardwood, such as teak. If a spool be simply put upon a lathe to be wound, the inner end of the wire, which must be properly secured, should be brought out in such a way that it cannot possibly make a short circuit with any of the wires in the upper layers.

To avoid this difficulty, the wire is sometimes wound on the spool in two separate halves, the two inner ends of which are united, so that both the working ends of the coil come to the outside as shown in fig. 238.

[Ill.u.s.tration: FIG. 238.--Method of winding magnet spool so that the two ends of the coil will come to the outside. This method has also been used for induction coils, where it is desirable to keep the ends of the wire away from the core and primary coil.]

=Ques. Describe the construction of former wound coils.=

Ans. Former wound coils are wound upon a block of wood having temporary f.l.a.n.g.es to hold the wire together during the winding. Such coils have pieces of strong tape inserted between the layers and lapped at intervals over the windings to bind them together. Coils are usually soaked with insulating varnish and stove dried.