Fig. 1,395.—Western Electric stationary armature and frame of engine driven alternator. It is of cast iron and surrounds the laminated iron core in which the armature windings are embedded. Heavy steel clamping fingers hold the core punchings in place and numerous ventilating ducts are provided in the core at frequent intervals to allow free circulation of cool air. The armature coils are form wound, insulated, and retained in the core slots by means of wedges.
Inductor Alternators.—In this class of alternator both armature and field magnets are stationary, a current being induced in the armature winding by the action of a so called inductor in moving through the magnetic field so as to periodically vary its intensity.
Figs. 1,396 and 1,397.—Elementary inductor alternator; diagram showing principle of operation. It consists of a field magnet, at the polar extremities of which is an armature winding both being stationary as shown. Inductors consisting of iron discs are arranged on a shaft to rotate through the air gap of the magnet poles. Now in the rotation of the inductors, when any one of them passes through the air gap as in fig. 1,396, the reluctance or magnetic resistance of the air gap is greatly reduced, which causes a corresponding increase in the number of magnetic lines passing through the armature winding. Again as an inductor passes out of the air gap as in fig. 1,397, the number of magnetic lines is greatly reduced; that is, when an inductor is in the air gap, the magnetic field is dense, and when no inductor is in the gap, the field is weak; a variable flux is thus made to pass through the armature winding, inducing current therein. The essential feature of the inductor alternator is that iron only is revolving, and as the design is usually homopolar, the magnetic flux in its field coils is not alternating, but undulating in character. Thus, with a given maximum flux through each polar mass, the total number of armature turns required to produce a given voltage is just twice that which is required in an alternator having an alternating instead of an undulating flux through its field windings. The above and the one shown in figs. 1,398 and 1,399 are examples of real inductor alternators, those shown in the other cuts are simply so called inductor alternators, the distinction being that, as above, the inductor constitutes no part of the field magnet.
Ques. What influence have the inductors on the field flux?
Ans. They cause it to undulate; that is, the flux rises to a maximum and falls to a minimum value, but does not reverse.
Ques. How does this affect the design of the machine as compared with other types of alternator?
Ans. With a given maximum magnetic flux through each polar mass, the total number of armature turns necessary to produce a given pressure is twice that which is required in an alternator having an alternating flux through its armature windings.