A Simple A.C. Generator

Author: J.B. Hoag

When a conductor moves through a magnetic field, an e.m.f. is produced between its ends. This can drive a current through the wire, provided a complete circuit exists.

Fig. 4 A. An a.c. generator

In Fig. 4 A, a loop of wire is rotated in a magnetic field. The ends of the loop are fastened to circular metal rings with brushes so that, as the coil rotates, the generated current can flow by way of the brushes to a load resistance R and a meter i. When the coil is in the vertical plane, its wires will be moving parallel to the magnetic lines of force and there will be zero e.m.f. and no current, as indicated at 0, Fig. 4B.

Fig. 4 B. A sine curve

When the coil is passing through the horizontal plane its wires are moving directly across the magnetic field, the e.m.f. will be a maximum, and so will the current in the resistance R. This is shown at I_m, Fig. 4 B. It occurs at a time (T/4), which is one-quarter of the total time T taken by the loop for a complete rotation. When wire A, Fig. 4 A, moves up, and B moves down, continuing their rotation in a counter-clockwise direction, the e.m.f. and current decrease until they are zero. This occurs when the loop is again in a vertical plane with A at the top and B at the bottom. The coil has now rotated one-half of one revolution or 180°, or, in radian measure, through an angle of π radians (π = 3.1416, 1 radian = 57.3 degrees).

As the coil continues its rotation, B now moves up and to the right, while A goes down and to the left; A and B of Fig. 4 A are now interchanged; the e.m.f. and the current are reversed in direction. This is shown in Fig. 4 B, from T/2 to T. This is called the " negative half-cycle " of the current. Each time the coil rotates once, a complete cycle is generated; the current, starting from zero, rises to a maximum in one direction, drops to zero, reverses its direction, rises to a maximum, then drops to zero, ready to repeat the cycle again. This is the kind of current we have in our ordinary lighting circuits.

The customary symbol for an alternating current generator is shown near R in Fig. 4 A.