DYNAM 0, DYNAMO-ELECTRIC MACHINE, or GENERATOR, a ma chine for transforming mechanical into electrical energy, and depending for its operation on the electromotive force de veloped in any conductor moved trans versely through the lines of force in a magnetic field. The manner in which the energy transformation is effected distinguishes the dynamo from the old frictional electric machine, and deter mines its general plan of construction.
Any dynamo must consist of at least two parts, the field-magnets which create the magnetic field, and the arma ture which comprises the conducting system which moves relative to the field. To these may be added the commutator, a device necessary to secure uniform di rection in the case of direct current ma chines. Dynamos for direct current are designed invariably with moving arma ture and fixed fields. In the case of ma chines of comparatively small capacity, thy field magnet may be bipolar, i. e., having one pair of poles, as in the horseshoe magnet. In fact, permanent steel magnets of this form constitute the fields of the little machines known as magneto-electric machines, or magnetos, such as are used in automobiles, in tele phony, etc. Bipolar machines may be either of the overtype or undertype pattern, according as the space between the poles is arranged above or below the yoke, which corresponds to the bend of the horseshoe. The former arrange ment is practicable only with smaller sizes, as it necessitates longer pedestals for supporting the bearings of the ar mature shaft, leading to excessive vi bration. The undertype has the dis advantage that the field magnets must be supported clear of the iron bed-plate by brackets of non-magnetic material, such as brass or gun-metal, in order to prevent passage of the field through the bed-plate in preference to the armature.
In any dynamo, the current passing through the moving armature causes a distortion and weakening of the mag netic field. This effect becomes very apparent in bipolar machines at high loads, and the serious disadvantage may be largely avoided by increasing the number of pairs of poles in the field magnet. Multipolar machines for con tinuous current may have as many as twelve pairs of poles. This form of
construction has, moreover, the advan tage of enabling material to be more economically arranged, thus securing relatively lighter weight; and, in addi tion, the speed of rotation may be re duced in inverse proportion to the num ber of pairs of poles for a given E.M.F. developed. These considerations have established the practice of constructing all machines for more than 150 kilo watt output of the multipolar type, and machines of considerably smaller capa city than this are regularly built with three or four pairs of poles.
In all cases the poles of the field magnet are fashioned so as to embrace as large a portion of the armature cir cumference, with as small an air-gap, as may be practicable. To effect this, soft-iron cheeks or pole-pieces are com monly fixed to the shanks which carry the magnetizing coils. Cross-magneti zation of the poles due to armature re action is often prevented by a deep nar row slot across the curved face of each pole-piece, paralleling the direction of the lines of force in the field.
There are several methods of arrang ing for the magnetization of the fields, which it will be convenient to defer until there has been given some description of the armature and its construction.
The principle underlying this may best be understood by considering what takes place when a single rectangular frame or loop conductor is revolved about the longer diameter in a magnetic field of parallel lines of force. We may imagine the axis or shaft, about which the rectangular frame is rotated, to cross the field at right angles, so that the two long sides of the rectangle parallel to the axis are continually cutting lines of force as the frame revolves. The two short sides do not cross the lines of force at all, but simply slide through them, and have, therefore, no E.M.F. actuaily induced in them. They serve only to co:nplete the electrical system, so enabling any E.M.F. induced in the active (long) sides to produce a cur rent in the system. The conductor must be imagined as insulated from the shaft.