Chapter XVI: Field Magnets

The object of the field magnet is to produce an intense magnetic field within which the armature revolves. It is constructed in various forms, due in a large measure to considerations of economy, and also to the special conditions under which the machine is required to work.

Electromagnets are generally used in place of permanent magnets on account of: 1, the greater magnetic effect obtained, and 2, the ability to regulate the strength of the magnetic field by suitably adjusting the strength of the magnetizing current flowing through the magnet coils.

The field magnet, in addition to furnishing the magnetic field, has to do duty as a framework which often involves considerations other than those respecting maximum economy.

=The Make Up of a Field Magnet.=--In construction, the electromagnet, used for creating a field in which the armature of a dynamo revolves, consists of four parts:

1. Yoke;
2. Cores;
3. Pole pieces;
4. Coils.

These are shown assembled in figs. 201 to 204.

=Ques. What is the object of the yoke?=

Ans. The yoke serves to connect the two “limbs,” that is, the cores and pole pieces, and thus provide a continuous metallic circuit up to the faces of the pole pieces.

=Ques. How is the yoke constructed?=

Ans. It usually forms the frame of the dynamo as shown in figs. 205 and 206.

=Ques. What may be said of the cores?=

Ans. The cores, which are usually of circular form, carry the coils of insulated wire used to excite the magnets.

=Classes of Field Magnet.=--Although numerous forms of field magnet have been devised, they can be classed into two groups according to the type of pole, as:

1. Salient pole;
2. Consequent pole.

The distinction between these two types of pole is shown in
figs. 201 to 203. By inspection of the figures, it will be seen
that the term _salient_ applies to poles produced when the pole
pieces form the _ends_ of the magnet, as distinguished from
_consequent_ poles, or those formed by coils wound on a
continuous metal ring or equivalent.

In the salient pole bipolar magnet, the winding may be either
upon the limbs, M M fig. 202, or upon the yoke, Y as shown in
fig. 201. The magnetic circuit of salient and consequent poles
is indicated in the figures by the dotted lines.

=Multi-Polar Field Magnets.=--In the multi-polar machine, the subdivision of the magnetic flux reduces the amount of material of both magnet and armature. Moreover, there is less heating on account of the greater capability of dissipating the heat, offered by the increased area of surface per unit of volume in each magnet pole and winding.

There may be four, six, eight, or more poles, arranged in
alternate order around the armature. Fig. 204 shows a four pole
field magnet having a common yoke or iron ring, with four pole
pieces projecting inwardly, and over which the exciting coils
are slipped.

In the larger machines the yoke is made in two parts bolted
together as shown in fig. 206, so that the upper portion may be
lifted off for examination of the armature.

=Ques. Can the number of poles in a multi-polar machine be advantageously increased to 16, 32, or more?=

Ans. A large number of poles is not advisable except in very large machines, since it involves an increase in the expense of machine work, fittings, etc., somewhat out of proportion to the reduction in cost of material and increase in efficiency.

=Ques. What materials are generally used for field magnets?=

Ans. Wrought iron, steel and copper.

There are a number of considerations which govern the selection
of the materials to be used in a particular machine, such as
initial cost, weight, efficiency, etc.

=Ques. In the construction of field magnets, what governs the choice of materials?=

Ans. For cores, wrought iron is most desirable, as requiring the smallest amount of material for a given flux. There is a saving in copper due to using wrought iron for the core since, on account of its small size, the length of each turn of the magnetizing coil is reduced. For heavy yokes, where lightness is not essential, but very often the reverse, cast iron is used, as its cross section can be made larger than that of the cores, this increase in area serving to give strength and rigidity to the machine. Cast steel occupies a place intermediate between cast iron and wrought iron both in cost and magnetic properties.

=Ques. Name two forms of yoke in general use.=

Ans. The solid, and divided types as shown in figs. 205 and 206.

=Ques. What is the object of dividing a yoke?=

Ans. To permit access to the armature, where the construction does not admit of removal of the latter from the side.

=Ques. How is the yoke usually divided?=

Ans. Across its horizontal diameter into an upper and lower half, as shown in fig. 206, the lower half being seated on, or more frequently cast in one piece with the bed plate.

=Ques. What is the objection to dividing a yoke?=

Ans. The joints introduced, even if carefully faced and well bolted together, add a little reluctance to the magnetic circuit.

=Ques. How does this affect the poles adjacent to the points, and what provision is made?=

Ans. It weakens them, and in order to overcome this, the coils of these poles are given a few extra turns.

=Ques. How is the reluctance of a yoke joint reduced?=

Ans. By enlarging the area of contact; the flange for the bolts furnishes the necessary increase.

=Ques. What determines chiefly the cost of field magnets?=

Ans. The material used in making the cores and their shape.

=Ques. How does this affect the cost?=

Ans. Since considerable cross sectional area of core is required, the problem confronting the designer is to design the core by judicious selection of material and shape, that the required number of turns in the magnetizing coil is obtained with the shortest length of wire.

=Ques. What is the principal 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 quantity 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.

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.

=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.

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.

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 mass 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.

=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 assembled 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.

=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.

The frame end of the core as shown in the illustrations 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.

=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 brass, or of sheet iron with brass flanges, or of very thin cast iron. Some builders use sheet metal with a flange 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.

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

Ans. Former wound coils are wound upon a block of wood having temporary flanges 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.

=Ques. What may be said with respect to the coil ends?=

Ans. Several methods of bringing out the ends of coils are shown in figs. 238 to 241. In fig. 239 copper strip, laid in behind an end sheet of insulating material, makes connection to the inner end, as shown in the right side of the figure, while another strip, shown on the left side similarly inlaid, serves as a mechanical and electrical attachment for the outer end of the winding.

Two other methods are shown in figs. 240 and 241. A simple
device for securing the outer end is to fashion a terminal piece
so that it can be laid upon the winding, the last three or four
turns of which are wound over its base, and after winding, are
bared at the place and securely soldered.

=Ques. How are the coils insulated?=

Ans. The spools upon which the coils are wound are usually insulated with several layers of paper preparations; a thickness of one-tenth of an inch made up of several superposed layers is generally sufficient. Varnished canvas is useful as an underlay, and vulcanized fibre for lining the flanges. It is important to protect the joint between the cylindrical part and the flanges. A core paper may be laid upon every four layers of winding. Between series and shunt coils, in compound wound machines there should be an insulation as efficient as that on the cores. When the winding is completed, two layers of pressed board or equivalent are laid over and bound with an external winding of hard rope or tape. This protective external lagging covering the outer surface of the completed coils is not altogether a benefit for it tends to prevent dissipation of heat.

=Ques. How are the coils attached?=

Ans. Where the pole pieces are simply extensions of the cores without enlargement, the coils can be slipped over the ends, but some kind of clamping device is necessary to hold them in place, as for instance, the method shown in fig. 244.

In case the pole piece be made larger than the core and separate
therefrom, it is put into position after the coils are in place,
thus serving the double purpose of pole piece and clamp.

=Ques. Describe the coil connections.=

Ans. Coils are generally united in series so that the same magnetizing current may flow through all of them. The coils should be so connected that they produce alternate north and south poles.

If all the coils be similarly wound with respect to the
terminals, and similarly placed; that is, so placed that the
winding, considered from the coil terminal nearest the pole
face, starts in all the coils in the same direction, then the
connections will come at the north end and at the south end of
the spools.

=Heating.=--The heat generated in the magnetizing coils is dissipated in three ways; by:

1. Induction;
2. Radiation;
3. Convection.

In the first instance, it passes through the copper and the insulation, either to the external surface, whence it passes off by radiation and convection into the air, or to the magnet core and yoke, which in turn conduct it away. In large multi-polar machines the masses of metal in the pole cores and frame are more efficient in dissipating heat than the external surface of the coil.

=Ventilation.=--Sometimes provision is made for ventilation of the field magnet coils as shown in fig. 246. Here the series and shunt coils are wound side by side, ample ventilation being provided lengthwise through and between the coils.

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FOOTNOTES:

[1] NOTE.--In 1749, Benjamin Franklin, observing lightning to possess almost all the properties observable in electric sparks, suggested that the electric action of points, which was discovered by him, might be tried on thunderclouds, and so draw from them a charge of electricity. He proposed, therefore, to fix a pointed iron rod to a high tower, but shortly after succeeded in another way. He sent up a kite during the passing of a storm, and found the wetted string to conduct the electricity to the earth, and to yield abundance of sparks. These he drew from a key tied to the string, a silk ribbon being interposed between his hand and the key for safety. Leyden jars could be charged, and all other electrical effects produced, by the sparks furnished from the clouds. The proof of the identity was complete. The kite experiment was repeated by Romas, who drew from a metallic string sparks 9 feet long. In 1753, Richmann, of St. Petersburg, who was experimenting with a similar apparatus, was struck by a sudden discharge and killed.

[2] NOTE.--Suppose that the conditions are as in the fig. 34, that is, the segment A_1 is positive and the segment B_1 negative. Now, as A_1 moves to the left and B_1 to the right, their potentials will rise on account of the work done in separating them against attraction. When A_1 comes opposite the segment B_2 of the B plate, which is now in contact with the brush Y, it will be at a high positive potential, and will therefore cause a displacement of electricity along the conductor between Y and Y_1, bringing a large negative charge on B_2 and sending a positive charge to the segment touching Y_1.

As A_1 moves on, it passes near the brush Z and is partially discharged into the external circuit. It then passes on until, on touching the brush X, it is put in connection with X, and has a new charge, this time negative, driven into it by induction from B_2. Positive electricity, then, being carried by the conducting patches from right to left on the upper half of the A plate, and negative from left to right on its lower half.

A similar process is taking place on the B plate, but in this case the negative electricity is going from left to right above, and the positive from right to left below. On the whole, therefore, positive electricity is being supplied to the left hand main conductor Z by both upper and lower plates, and negative to Z_1.

[3] NOTE.--The discovery of this property of matter is due to Stephen Gray, who, in 1729, found that a cork, inserted into the end of a rubbed glass tube, and even a rod of wood stuck into the cork, possessed the power of attracting light bodies. He found, similarly, that metallic wire and pack thread conducted electricity, while silk did not.

Gray even succeeded in transmitting a charge of electricity through a hempen thread over 700 feet long, suspended on silken loops. A little later, Du Fay succeeded in sending electricity to no less a distance than 1,256 feet through a moistened thread, thus proving the conducting power of moisture. From that time the classification of bodies into conductors and insulators has been observed.

[4] NOTE.--Copper is pre-eminently the metal used for electric conduction, being among the best conductors, it is excelled by one or more of the other metals, but no other approaches it in the average of all qualities.

[5] NOTE.--A current of electricity always flows in a conducting circuit when its ends are kept at different potentials, in the same way that a current of water flows in a pipe when a certain pressure is supplied. The same electrical pressure does not, however, always produce a current of electricity of the same strength, nor does a certain pressure of water always produce a current of water of the same volume or quantity. In both cases the strength or volume of the currents is dependent not only upon the pressure applied, but also upon the _resistance_ which the conducting circuit offers to the flow in the case of electricity, and on the friction (which may be expressed as resistance) which the pipe offers to the flow in the case of water.

[6] NOTE.--The prefixes “meg” and “micro” denote million and millionth. For example, a megohm equals 1,000,000 ohms, a microhm equals 1/1,000,000 of an ohm.

[7] NOTE.--The reciprocal of a number is equal to 1 ÷ the number; for instance, the reciprocal of 3/20 = 1 ÷ 3/20 = 20/3 = 6-2/3

[8] NOTE.--A writer in the _New Science Review_ undertakes to answer the question: “What is electricity?” In order to lead the reader up to the main question, he first considers the natural forces, gravitation and heat. Examples are given of how these forces are manifested and how energy is changed from one form to another. Every form of force, the author says, should be regarded as a different method in which energy makes itself known to the senses. He calls particular attention to the important fact that the “resistance of one kind or another is always the agent that acts to alter energy from one form to another,” and suggests that electricity is simply a form or manifestation that energy may assume under given conditions, and generally is a mere transitory stage between the mechanical form and the heat form. “In most operations,” he continues, “mechanical force passes to the heat form without passing through the electric form; but whenever magnetism is brought into play as a resistance that must be overcome, then mechanical power applied to overcome this resistance always becomes electricity, if only momentarily in its passage from the mechanical to the heat form.” In conclusion, he asks if the question: “What is electricity?” cannot be answered in a fairly satisfactory way by saying that it is simply a form that energy may assume while undergoing transformation from the mechanical or the chemical form to the heat form or the reverse.

[9] NOTE.--The cathode is the conductor by which current flows away as distinguished from the _anode_ or conductor through which the current enters. The terms usually apply to conductors leading the current through a liquid or gas, as an electrolytic cell, or vacuum tube.

[10] NOTE.--The name _voltameter_ was given by Faraday to an electrolytic cell employed as a means of measuring an electric current by the amount of chemical decomposition the current effects in passing through the cell.

[11] NOTE.--Faraday’s own description of his discovery is as follows: “Two hundred and three feet of copper wire in one length were coiled round a large block of wood; another two hundred and three feet of similar wire were introposed as a spiral between the turns of the first coil, and metallic contact everywhere prevented by twine. One of these helices was connected with a galvanometer, and the other with a battery of one hundred pairs of plates, four inches square, with double coppers, and well charged. When the contact was made there was a sudden and very slight effect at the galvanometer, and there was also a similar slight effect when the contact with the battery was broken.”

[12] NOTE.--In reality it would be impossible to have a magnetic field exactly like fig. 129, for in the less dense part, the magnetic lines would be of curved complex form.

[13] NOTE.--These values are correct for effective sinusoidal voltages.

[14] NOTE.--It should be understood that a dynamo does not generate electricity, for if it were only the quantity of electricity that is desired, it would be of no use, as the earth may be regarded as a vast reservoir of electricity. However, electricity without pressure is incapable of doing work, hence a dynamo, or so-called “generator,” is necessary to create an electromotive force by electromagnetic induction in order to cause the current to flow against the resistance of the circuit and do useful work.

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TRANSCRIBER’S AMENDMENTS

Transcriber’s Note: Blank pages have been deleted. Some illustrations may have been moved. Notes at the bottom of pages in the text were converted to footnotes and footnote tags were added to the text itself. The footnotes are now located prior to this section. When the author’s preference can be determined, we have rendered consistent on a per-word-pair basis the hyphenation or spacing of such pairs when repeated in the same grammatical context. The publisher’s inadvertent omissions of important punctuation have been corrected. Duplicative front matter has been removed.

The following list indicates any additional changes. The page number represents that of the original publication and applies in this etext except for footnotes and illustrations since they may have been moved.

TOC = Table of Contents

TOC [Added INTRODUCTORY CHAPTER and SIGNS AND SYMBOLS]
TOC between electric and netic[magnetic] circuits
TOC action of Toepler-Holz[Holtz] machine
4 species inhabiting the Mediteranean[Mediterranean]
14 would fly from it without any elecrical[electrical]
16 pith balls [on] strips of paper C, D, E, as shown.
28 between the source and rerminal[terminal].
39 a certain anount[amount] of work
43 When metals differeing[differing] from each other
61 Various zincs; fig. 58 Fuller; fig. 59 Daniel[Daniell]
62 Various carbons; fig. 61 Cylindrical from[form],
63 fig. 68 Lockwood; fig. 69 fire alram[alarm].
66 A paralled[paralleled] or multiple connection
70 but no other aproaches[approaches]it
93 With this prelimary[preliminary] caution,
96 If a current of 10 amperes flow[flows] in a wire
98 as an electrotylic[electrolytic] cell,
99 rising from the kathode[cathode] P′ is hydrogen
100 hydrogen atoms in their journey towards b[B] meet
100 at the electrodues[electrodes] and not
133 VI.[6.] _The approach and recession of a conductor
142 3[2]. Vibrator coils;
142 2[3]. Condenser coils.
143 soaked in shellac dissolved in alchool[alcohol]
151 the hammer or piece of of[2nd ‘of’ del] iron
170 FIG.[FIGS.] 169 to 173.-- The sine curve
209 An extreme design, suggested by Dobrowolsky[Dobrowolski],

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