FIELD MAGNETS.

present between the pole pieces of a field magnet; hence, when the armature is rotated in the armature chamber, its conductors cut the lines of force contained in this magnetic field, and a small E. M. F. is set up in the armature in consequence.

The ends of the magnetizing coils being suitably connected to the brushes, if theşe latter are in contact with the commutator, and a closed circuit through the field magnet windings is formed, this small E. M. F. immediately sends a minute current through the exciting coils; this immediately increases the strength of the magnetic field, and as a consequence an increased E. M. F. is induced in the armature. This results in a stronger current being sent through the exciting coils, and the increase of magnetism which follows results in an increased E. M. F. in the armature.

Thus the process goes on, until eventually, for a given speed of rotation of the armature, the E. M. F. reaches a maximum value, beyond which it will not increase without a further increase in the speed of rotation; the exciting current has then arrived at a constant value, and the magnetization of the machine will remain at a constant strength, and maintain the E. M. F. so long as the armature rotates at its normal speed. If the armature ceases to revolve, the field magnets will of course be deprived of their exciting current, and will therefore lose their magnetism; the iron will, however, retain a sufficient amount of residual magnetism to again start the process when the machine is again started.

FIELD MAGNETS.

Whilst the construction of the armatures of different dynamos may be said to be very much the same, differing in small details only, and being confined to two types, viz., the ring and drum respectively, the construction of the field magnets varies greatly, almost every manufacturer having his own particular form and arrangement. This great variety in the form of the field magnets of different dynamos, is due in a large measure to consideration of economy involved in the manufacture by different makers, and also, to a less extent, to the different conditions under which a machine is required to work. For example, it is sometimes necessary for a machine to give a maximum of output with a minimum of weight, and under such circumstances the field magnet is constructed wholly of wrought-iron, and this necessarily entails an entirely different method of construction and arrangement than if cast-iron were employed.

Again, as a rule, the direct coupling of the armature to the engine-shaft involves a different form of field magnet than would be the case if the armature were belt driven.

Owing to difficulties of construction, and other considerations, field magnets are not in practice usually constructed out of a single-piece of iron, but are usually built up of a combination of parts, and composed either wholly of wrought or cast iron, or of a combination of both.

The construction of a typical field magnet is illustrated in Fig. 75, from which it will be seen that it may be divided into five parts, viz.:-the two limbs of cores M M, upon which

FIG. 75.

Y

FIELD MAGNETS.

the exciting coils C are wound; the two end portions P P, called the "pole pieces," which are bored out so as to form the "armature chamber" within which the armature revolves; and the yoke, Y, which serves to connect the two limbs together, and thus complete the magnetic circuit. The permeability of wrought-iron being very much greater than cast-iron, the portions M M of the field magnet, upon which the exciting coils are wound, are frequently constructed of this material; these portions are also usually constructed of a circular section, and thus the amount of wire required for exciting the field magnet is economized to the ut

TYPICAL FIELD

MAGNET.

most extent. The pole pieces P P and the

yoke Y are in many cases of cast-iron, bolted on to the wrought-iron limbs.

Although innumer

able forms of field magnets have been devised, they can all be arranged into two groups, viz., those in

[FIG. 76.

SALIENT POLE FIELD MAGNET.

which the poles are "salient," and those in which the poles

are

FIELD MAGNETS.

consequent." A salient pole is the term applied to poles which are produced at the ends of a bar of iron, in distinction to consequent poles, which are produced in a continuous ring of iron.

The salient pole form of field magnet, being least costly to construct, is most frequently met with in practice. Fig. 76 shows its simplest form; in this arrangement only one magnetizing coil is required, this being wound upon the yoke

FIELD MAGNETS.

which is usually of wrought iron, let into and bolted to the cast iron pole pieces N S. The paths and directions of the lines of force, with the magnetizing current flowing in the

FIG. 78.

S

THE UNDERTYPE FIELD MAGNET.

direction shown, is indicated by the dotted arrow heads and lines. Another form of field magnet which is very extensively used, and in which two exciting coils are required, is