Page:The New International Encyclopædia 1st ed. v. 06.djvu/646

* DYNAMO-ELECTBIC MACHINERY. 504 DYNAMO-ELECTRIC MACHINERY. Dieclianical or electrical energy and whether it is, therefore, giving out electrical or mechanical cncrg)', respectively. In an electric (/oicrulor mechanical energy is converted into electrical energy by means of continuous relative motion between electrical conductors and a magnetic field, or fields, such motion causing the conduc- tors to cut or traverse the lines of force of the fields. In an electric motor electrical energy is transformed into mechanical energy by means of continuously supplying a system of electrical conductors with an electric current, which causes a magnetic force to act between the conductors carrying it and the magnetic field, or fields, thereby producing continuous relative motion be- tween the conductors and the magnetic fields. The preceding dolinitions are general and nec- essarily technical. To comprehend them fully a knowledge of the fundamental princiiilcs in- volved in the operation of dynamo-electric macliines is necessary. They may, however, be in a measure elucidated by a descriptive defini- tion as follows: All generators consist essentially of one or more electromagnets between the poles of which an armature, consisting of a soft iron core wound with coils of insulated copper wire, is made to revolve very rapidly by means of a steam-engine, a water-wheel, or other ])rime mover. In the transmission of energ}' by elec- tricity the current produced by the generator is made to pass through another machine, similar and often identical in construction, and there •causes the armature to revolve, and this revolu- tion may be employed to do any kind of me- chanical work. This second machine, working in reverse order from the first, is an electric motor. This description makes it clear, as do the pre- ceding definitions, that to understand thorough- ly the dynamo-electric machine requires a knowledge of three great branches of science — magnetism, electricity, and mechanics. The necessity for this special knowledge makes the subject of dynamo-electric machines a difficult one to discuss in universally familiar terms. At best, therefore, only an indication of the structural details and operating principles in- volved is possible when the demonstrator is lim- ited by such restrictions. FiND.MENT.i. Pri.N'CIPLes. To Understand the fundamental principles involved in the opera- tion of dynamo-electric machines, consider first two magnetic poles, N and S, of opposite polarity placed near to each other, as in Fig. 1. Between the poles N and S is a field of magnetic force in the conductor depends upon the rate at which the lines of force are cut. Due to this electro- motive force, one end of the conductor is raised to a higher electric potential than the other, in consequence of which there is a tendency for electricity to fiow along the conductor, and if its two ends are electrically connected exterior to the magnetic field so as to make a closed circuit, a current will flow through this circuit. If the gap between the poles X and S were infinite in extent in the direction C D, and if the conductor C in its motion along this infinite path were sup]died with some sort of sliding contacts by which current could be taken to one end and from the other end, the device would possess all the essential features of a generator or of a motor. An equivalent condition is secured ( 1 ) if the poles N and S arc made annular in cross- section; (2) if the conductor C is arranged so as to rotate as a radius on the common axis of N and S; and (3) if sliding contacts are pro- vided at the centre of rotation and at the ex- tremity of the conductor outside of the polar forces ; the device then becomes a homopolar gen- erator or motor. The form of machines just described is note- worthy chiefly because of its simplicity and the fact that it was the first form to be invented. All commercial machines now in use operate in such a manner that the conductor moves alter- nately forward and backward through the field of force. To illustrate, if the conductor C Pio. 1. composed of so-called lines of magnetic force which may be pictorially indicated by parallel lines, as is done in the illustration. If a con- ductor, for example a round' copper bar C, is placed in the magnetic field, with its axis hori- zontal and perpendicular to the lines of force, and is raised and lowered along the path C D so as to cut the lines of force, an electro-motive force is set up or induced in the conductor. The magnitude of the electro-motive force produced Fig. 2. (Fig. 2) be attached to the circumference of a cylinder or other round body, which rotates about an axis A perpendicular to the paper, it will cut the lines of force by alternate down- ward and upward motions. An electro-motive force in one direction will be generated in the conductor C as it is going up and in the oppo- site direction as it is coming down. If the ends of C are brought down to the shaft A and formed into rings around it, brushes bearing on these rings, if connected to an external circuit, will receive an alternating current, that is. one which flows first in one direction, then in the opposite direction. JIachines of this descj-ip- tion are called aUernators; the rotating part is called the armature: the pole pieces. X and S, with the remaining magnetic circuit, are called the field, and the rings around the shaft arc called collector rmgs. If the current collected is to flow always in the same direction in the circuit which is external to the machine, some device must he provided to change autoniatically the connection between the armature circuit and the external circuit. Such a device is called a conunutator. Its operation may be explained as follows: If in Fig. 2 the ends of the conductor C are brought down to the shaft A and ex- tended across to the opposite side of the shaft, and then parallel to it so as to make the com- plete loop shown by Fig. 3. it will be obvious that the electro-motive forces generated in the