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 a.s.sembled 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.

[Ill.u.s.tration: FIG. 201.--Salient pole, bipolar field magnet with single coil wound around the yoke.]

=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.

=Cla.s.ses of Field Magnet.=--Although numerous forms of field magnet have been devised, they can be cla.s.sed 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.

[Ill.u.s.tration: FIG. 202.--Salient pole, bipolar field magnet with two coils wound around the cores.]

[Ill.u.s.tration: FIG. 203.--Consequent pole, bipolar field magnet with two coils on the cores. This is known as the "Manchester" type in which the cores are connected at the ends by two yokes--so named from its original place of manufacture at Manchester, England.]

=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.

[Ill.u.s.tration: FIG. 204.--Modern dynamo with four consequent pole field magnets. In this construction the ring shaped yoke also serves as a frame; the circular form of yoke gives the least chance for magnetic leakage.]

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.

[Ill.u.s.tration: FIGS. 205 and 206.--Solid and split construction of yoke for multi-polar dynamos. In the latter type the yoke is in two halves joined along a horizontal diameter; while the upper half may be conveniently removed to give access to the armature, it has the disadvantage of the joint, which, no matter how well made, will add to the reluctance of the magnetic circuit. The figures also ill.u.s.trate the circular and segmental forms of yoke construction.]

=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.

[Ill.u.s.tration: FIGS. 207 to 209.--Various sections of cast iron yoke. In form, these yokes may be either circular or segmental as shown in figs.

205 and 206.]

[Ill.u.s.tration: FIGS. 210 to 212.--Various sections of cast steel yoke. The ribs shown in figs. 210 and 211 are provided to secure stiffness.]

=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.

[Ill.u.s.tration: FIGS. 213 to 215.--Some methods of attaching detachable cores. The core seat is machined to receive the core, it being necessary to secure good contact in order to avoid a large increase in the reluctance of 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 f.l.a.n.g.e 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.

[Ill.u.s.tration: FIGS. 216 to 221.--Comparison of field magnet core sections. The shorter the perimeter or outside boundary of the core for a given cross sectional area, the less will be the amount of copper required for the magnetizing coils. All the above sections are of equal area, and the figures marked on each represent relative values for the perimeters, the circle for convenience being taken at 100.]