The New Student's Reference Work/Dynamo-Electric Machine
Dynamo-Electric Machine or, simply, dynamo, a machine for generating an electric current in a conductor by moving the conductor in the magnetic field of a magnet or system of magnets. The motion is relative, that is, either the conductor or the magnet may be the moving part. All dynamos are based upon the fundamental discovery made by Michael Faraday in 1831. Faraday discovered that, if a wire forming a part of a circuit is moved across the space (called a magnetic field) in front of the pole of a magnet, an electric current will be set up in the wire and circuit during time of motion. The modern dynamo consists of two essential parts: (1) an electromagnet or system of electro-magnets called the field-magnets and (2) a system of copper wires, usually wound on a laminated iron core. This system of conductors is called the armature, and is the part in which the current is generated. In most varieties of dynamos the armature revolves in the magnetic field of the field-magnets, but in many large alternating-current dynamos the field-magnets form the revolving part.
|
| Simple Direct Current Dynamo |
|
| Simple Alternating Current Dynamo |
Dynamos are divided into two general lands: direct current (D. C.) and alternating-current dynamos (A. C.) or simply alternators. The current generated in the circuit of a D. C. dynamo is constant in direction and magnitude. The current produced by an alternator reverses direction at regular intervals. All commercial D. C. dynamos have a commutator. This is a metal cylinder built up of a number of insulated sectors or bars, which revolves with the shaft on the armature. Fixed conducting-brushes rest against this commutator and make the connections with the external circuit. The armature-coils are connected with the commutator-bars, so that the connections with the external circuit are reversed as often as the current in the armature is reversed. In this way the current in the external circuit is kept constant in direction. The collecting device for a simple A. C. dynamo consists of two insulated copper-rings on which the brushes rest. The current in the external circuit is thus reversed as often as the current in the armature-coil is reversed. The common number of alternations or reversals (often called cycles) in commercial alternators is either 125 or 60, but as low as 25 are used in the Niagara Falls electric plant.
Alternators are often built to generate two or more alternating currents, which differ in “phase.” Such machines are called two- or three-phase alternators. Thus a two-phase machine is one which generates two separate alternating currents, the two currents differing in that the first reaches its maximum when the second is at zero, and when the second reaches a maximum the first is passing through its zero, and so on. Apparatus generating and using two or more phases is called polyphase apparatus.
The use of alternating currents has increased largely within the last 15 years, because of the inventions of transformers and of the induction-motor. The polyphase alternating-current system is the only one commercially feasible for long-distance transmission of electric energy (see Transformers and Transmission of Power), and even for many shorter lines there are engineering advantages in its use. The field-magnets of some of the earlier dynamos were permanent steel magnets, and such machines were called magneto-electric machines or magnetos. The little dynamos used in telephone call-boxes are magnetos. The field-magnets of all modern machines are electro-magnets. The machine is spoken of as bipolar or multipolar, according as the field-magnet system has two or more poles. The field-magnets of most alternators are separately excited by an electric current from a small auxiliary dynamo called the exciter. The field-magnets of most D. C. dynamos are excited by the current generated by the dynamo itself. If all the current of the armature passes through the field-coils, the dynamo is said to be a series-wound machine; if only a fraction of the armature-current goes through the field-coil, the dynamo is a shunt-wound machine. When there are two sets of field-coils, a series-set and a shunt-set, the dynamo is said to be compound-wound. The choice of the kind of field-windings is a question of regulation of the current-output with different loads.
Dynamos are rated in kilowatts as steam-engines are rated in horse-power. A kilowatt (K. W.) is almost exactly equal to one and one third H. P. Thus a 75 K. W. dynamo has the same work-capacity as a 100 H. P. steam-engine. To get the K. W. of a dynamo, divide the product of the volts and amperes by 1,000. The use of electrical machinery for power-purposes has made a demand for large dynamos, often reaching several thousand K. W. for one machine. Thus in the Niagara Falls plant there are ten machines, each of 5,000 K. W. output.
Faraday and others saw immediately that his discovery of electromagnetic induction (1831) could be used for the continuous production of electric currents, but the modern machine was evolved slowly and represents the added ideas of more than a generation of inventors. The first machine had permanent magnets. In 1832 Pixii devised a commutator. In 1848 Brett suggested the self-exciting electromagnet for the field. The name dynamo-electric machine was first given by Dr. Werner Siemens in a paper read before the Berlin academy in 1867. In 1870 Gramme invented the ring-armature, and three years later Hefner-Alteneck improved the early form of Siemens armature and gave us the present drum-armature. The ring- and drum-armatures have been used more than any others in D. C. dynamos. The theory of the dynamo-machine has been studied carefully by Hopkinson, Frolich, Kapp, S. P. Thompson and others, so that the designing of a dynamo for any specific purpose has been reduced to a question of exact calculation. The efficiency of modern dynamos is often 96 to 98 per cent. The largest makers of dynamo-electric machinery in the United States are the General Electric Company, the Westinghouse Electric and Manufacturing Co. and the Stanley Electric Manufacturing Co. The shops of these companies are among the largest manufacturing-plants of any kind in the world.