Harper's Electricity Book for Boys

by Joseph H. Adams

In the [chapter] on Dynamos and Motors we learned how to make and use those machines. Let us review, very briefly, just what happens in the double transformation—of mechanical energy into electricity and then back again at the end of a line of wires—that we call electric-power transmission. In the dynamo, the power of the water-wheel, or whatever other prime mover is used, is exerted in generating electricity by forcing the electric conductors of the machine through a magnetic field. The electricity is led away to a distance—a hundred miles, perhaps—by wires and allowed to enter another machine similar to the dynamo, but operating as a motor. Here the first process is reversed: the electricity passing through the conductors of the motor reacts upon its magnetic field, causing the machine to revolve and thus generating mechanical power again. The line-wires carry the power just as positively as though a long shaft ran from the prime mover to the receiving end of the line, and much more economically. The action that goes on is similar to the operation of the telephone—which is indeed a special case of electric-power transmission—as already explained in a former chapter: the sound of the voice being transformed, at the telephone-transmitter, into electrical energy in the form of alternating currents, then carried as such over the line and finally reproduced as sound again at the receiver.

Power from Water-wheels

“Hydro-electric” transmissions—i. e., electric transmissions of power from a water-wheel as prime mover—are the most important because they bring into use cheap water-power that formerly ran to waste. There are many hydro-electric transmissions in this country, Mexico, and Canada, some of them utilizing the power of waterfalls or rapids located in mountainous and inaccessible parts. The alternating current is nearly always used because by it men can much more easily and safely generate, transmit, and receive the high voltages that have to be used than by the continuous current. The machinery at the “main generating station” consists of big alternating-current dynamos, which sometimes have vertical shafts instead of horizontal ones, so that they may be driven directly by turbines. The current is generated at a moderate potential, which is then “stepped-up,” by “static transformers,” to the comparatively high-line voltage that is required in long-distance transmissions.

Fig. 8

Transformers

[Fig. 8] is a view of a very large transformer of over 2500 electrical horse-power capacity. In the picture the containing-tank is represented as transparent, so as to show the transformer proper inside. The latter is really a special kind of induction-coil, with primary and secondary windings, and a core, weighing many tons, built up of thin sheets of steel. In this kind of transformer, the tank is filled with oil, to keep the transformer cool in operation, and to help insulate it against the high potential to which it is subjected. At the receiving end, or “sub-station,” the high-voltage electric power enters a set of “step-down” transformers, from which it is delivered again, at moderate potential, to the motors.

Sometimes power is distributed from a single great generating station to several sub-stations. In the Necaxa transmission, in Mexico, over 35,000 horse-power is taken from a waterfall in the mountains and transmitted at 60,000 volts potential to Mexico City, 100 miles away, and to the mining town of El Oro, seventy-four miles farther on.

Several kinds of motors are used at the receiving end of electric-power transmission-lines, according to the work that they are called upon to do. For “stationary” work, like driving the machines in mills and factories, two principal kinds of alternating-current motors are employed—synchronous and induction motors. The former are built just like alternating-current dynamos, and when they are running they keep “in step” with the dynamo at the other end of the line; i. e., the motion of their field windings relatively to their armatures keeps exact pace with the same motion at the dynamo, just as though a long shaft ran from one machine to the other instead of the electric wires of the transmission-line. A motor of this type, at work driving an air-compressor, is shown in [Fig. 9]. The induction-motor is really a sort of transformer, the primary winding of which is the fixed part, or field, and the secondary winding the rotating armature. It does not keep in step with the dynamo, like the synchronous motor, but adapts its speed to the “load,” or amount of work that it is called upon to do, like a continuous-current motor.