Page:Popular Science Monthly Volume 88.djvu/652

 Construction of Unipolar Dynamos

��THE direct-current dynamo, as pic- tured by almost everyone, is a complicated machine having many poles and an iron armature which is wrapped up with many turns of copper wire and which has at one end a huge copper commutator on which copper or carbon brushes bear gently, to conduct the energy to distributing wires and ca- bles. Very few, however, realize that there is another type of direct-current machine which, although suitable, as yet, only for some special uses, may eventual- ly earn an important place for itself. This machine is the unipolar dynamo.

In the old style dynamo, the current

���Fig. 1. Barlow wheel Fig. 2. Two wheels acting as a current that revolve in op- ^ generator posite directions.

set up in the armature windings is alter- nating, because the conductors, as they revolve, pass successively under a mag- netic north pole and then under a south pole. In order that direct current may be delivered to the line, an expensive and delicate commutator is required, which reverses the connections with the line every time the current begins to flow in a direction opposite to that in which it was flowing before.

If arranged so that the armature con- ductors, as they revolve, cut across a magnetic field always in the same direc- tion, the current generated will always flow in the same direction, and no com- mutator will be required. This arrange- ment has received the name of unipolar dynamo.

The most practical form of unipolar generator in use is, to a certain extent, a reproduction of the apparatus known as the Barlow Wheel (Fig. i). It consists of a metal disk mounted so that it projects between the poles of a

��magnet. Connections are made to the shaft of the wheel and to the periphery of the disk by means of sliding-contacts. These contacts can be compared, in some respects, with the brushes of multi- polar dynamos. If, now, the disk is rotated, the lines of force passing through it from pole to pole will be cut, and if the sliding-contacts are connected together, an electric current will flow in the circuit so formed. The disk is equivalent, electrically, to a large num- ber of radial conductors connected in parallel, and hence, the voltage of the machine is the same as that obtained from a single conductor only; however, on account of the very large cross-sec- tion of the disk, the machine can supply a very large amount of current. It is evident that in the construction just de- scribed, the disk always cuts the lines of force of the magnet in the same di- rection, and hence the current supplied by the machine is direct and absolutely continuous, showing no pulsating effects. It is known that in order to induce a tension of one volt in a conductor mov- ing across a magnetic field, the conductor must cut one hundred million lines of force per second, and from this, it is evident that in order to have a unipo- lar dynamo delivering current at a high ten- sion, it is necessary ei- ther to use a very large disk and magnet, or to rotate the disk at an ab- normally high speed. Two or more disks, con- nected in series, can be used also, but in that case, adjacent disks must either be ro- tated in opposite directions, as shown in Fig. 2, or insulated from the shaft and connected by means of sliding-contacts. Adjacent disks may also be. connected with the shaft, revolved in opposite mag- netic fields (Fig. 5), and connected to- gether by sliding-contacts on their pe- riphery ; for, if the conductors connect- ing the disks were revolved with them, an electric force would be induced in

���Fig. 3. Uni- polar dynamo with one disk

��624

�� �