Chapter XVII: How Electricity Is Generated by Dynamos

=126. The Dynamo=, _Dynamo-Electric Machine_ or _Generator_, is a machine for converting mechanical energy into an electric current, through electromagnetic induction. The dynamo is a machine that will convert steam power, for example, into an electric current. Strictly speaking, a dynamo creates electrical pressure, or electromotive force, and not electricity, just as a force-pump creates water-pressure, and not water. They are generally run by steam or water power.

=127. Induced Currents.= We have already spoken about currents being induced by moving a coil of wire in a magnetic field. We shall now see how this principle is used in the dynamo which is a generator of induced currents.

Fig. 137 shows how a current can be generated by a bar magnet and a coil of wire. Fig. 138 shows how a current can be generated by a horseshoe magnet and a coil of wire having an iron core. The ends of the coil are to be connected to an astatic galvanoscope; this forms a closed circuit. The coil may be moved past the magnet, or the magnet past the coil.

Fig. 139 shows how a current can be generated by two coils, H being connected to an astatic galvanoscope and E to a battery. By suddenly bringing E toward H or the core of E past that of H, a current is produced. We have in this arrangement the main features of a dynamo. We can reverse the operation, holding E in one position and moving H rapidly toward it. In this case H would represent the armature and E the field-magnet. When H is moved toward E, the induced current in H flows in one direction, and when H is suddenly withdrawn from E the current is reversed in H. (See "Study," Chapter XXV., for experiments.)

=128. Induced Currents by Rotary Motion.= The motions of the coils in straight lines are not suitable for producing currents strong enough for commercial purposes. In order to generate currents of considerable strength and pressure, the coils of wire have to be pushed past magnets, or electromagnets, with great speed. In the dynamo the coils are so wound that they can be given a rapid rotary motion as they fly past strong electromagnets. In this way the coil can keep on passing the same magnets, in the same direction, as long as force is applied to the shaft that carries them.

=129. Field-Magnets; Armature; Commutator.= What we need then, to produce an induced current by a rotary motion, is a strong magnetic field, a rotating coil of wire properly placed in the field, and some means of leading the current from the machine.

If a loop of wire, Fig. 140, be so arranged on bearings at its ends that it can be made to revolve, a current will flow through it in one direction during one-half of the revolution, and in the opposite direction during the other half, it being insulated from all external conductors. This agrees with the experiments suggested in § 127, when the current generated in a coil passed in one direction during its motion _toward_ the strongest part of the field, and in the opposite direction when the coil passed _out_ of it. A coil must be cut by lines of force to generate a current. A current inside of the machine, as in Fig. 140, would be of no value; it must be led out to external conductors where it can do work. Some sort of sliding contact is necessary to connect a revolving conductor with outside stationary ones. The magnet, called the _field-magnet_, is merely to furnish lines of magnetic force. The one turn of wire represents the simplest form of _armature_.

Fig. 141 shows the ends of a coil joined to two rings, X, Y, insulated from each other, and rotating with the coil. The two stationary pieces of carbon, A, B, called _brushes_, press against the rings, and to these are joined wires, which complete the circuit, and which lead out where the current can do work. The arrows show the direction of the current during one-half of a revolution. The rings form a _collector_, and this arrangement gives an _alternating current_.

In Fig. 142 the ends of the coil are joined to the two halves of a cylinder. These halves, X and Y, are insulated from each other, and from the axis. The current flows from X onto the brush A, through some external circuit, to do the work, and thence back through brush B onto Y. By the time that Y gets around to A, the direction of the current in the loop has reversed, so that it passes toward Y, but it still enters the outside circuit through A, because Y is then in contact with A. This device is called a _commutator_, and it allows a constant or _direct current_ to leave the machine.

In regular machines, the field-magnets are electromagnets, the whole or a part of the current from the dynamo passing around them on its way out, to excite them and make a powerful field between the poles. To lessen the resistance to the lines of force on their way from the N to the S pole of the field-magnets, the armature coils are wound on an iron core; this greatly increases the strength of the field, as the lines of force have to jump across but two small air-gaps. There are many loops of wire on regular armatures, and many segments to the commutator, carefully insulated from each other, each getting its current from the coil attached to it.

=130. Types of Dynamos.= While there is an almost endless number of different makes and shapes of dynamos, they may be divided into two great types; the _continuous_ or _direct current_, and the _alternating current_ dynamo. Direct current machines give out a current which constantly flows in one direction, and this is because a commutator is used. Alternating currents come from collectors or rings, as shown in Fig. 141; and as an alternating current cannot be used to excite the fields, an outside current from a small direct current machine must be used. These are called exciters.

In direct current machines enough residual magnetism is left in the field to induce a slight current in the armature when the machine is started. This immediately adds strength to the field-magnets, which, in turn, induce a stronger current in the armature.

=131. Winding of Dynamos.= There are several ways of winding dynamos, depending upon the special uses to be made of the current.

The _series wound_ dynamo, Fig. 143, is so arranged that the entire current passes around the field-magnet cores on its way from the machine. In the _shunt wound_ dynamo, Fig. 144, a part, only, of the current from the machine is carried around the field-magnet cores through many turns of fine wire. The _compound wound_ dynamo is really a combination of the two methods just given. In _separately-excited_ dynamos, the current from a separate machine is used to excite the field-magnets.

=132. Various Machines.= Fig. 145 shows a hand power dynamo which produces a current for experimental work. Fig. 146 shows a magneto-electrical generator which produces a current for medical use. Figs. 147, 148 show forms of dynamos, and Fig. 149 shows how arc lamps are connected in series to dynamos.