A definite relation exists between the direction of the field, the direction of the motion, and the
direction of current. These directions can be determined by applying Fleming's right-hand rule for
generators, and the left-hand rule for motors. Fleming's rules were originated by an English electrical
engineer, Sir John Ambrose Fleming, 1849-
1945. Look at the left-hand rule illustrated in
figure 1.14; the thumb points in the direction of
motion, the forefinger points in the direction of
the field, and the middle finger points in the
direction of the flow of current. The right-hand
rule for generators works the same way.
1.26. GENERATOR
The value of the electromotive force, or
emf, produced by a generator is related to the
amount of change in field and motion, or to the
rate at which lines of magnetic force are cut by
the armature. The emf in a single armature coil
varies even when the armature's speed of
Figure 1.14. Left-hand
rotation remains constant. Look at figure 1.15;
Motor Rule.
it shows how the armature cuts across more and
more lines of force as it approaches 90 degrees. At 20 degrees, for example, one line has been cut; at 40
degrees, in the same period, four more lines have been cut. The emf varies with the number of lines that
are being cut per unit of time and is zero at 0
degrees and a maximum at 90 degrees. At 180
degrees, the emf is again zero; between 180 and
360 degrees, emf goes in the opposite direction-
- giving it the name alternating current. This
describes a simple alternating-current (ac)
generator.
Brushes in a generator or motor should be set at
the point of zero emf, a point occurring
approximately halfway between the main poles.
Figure 1.15. Relation of Electromotive Force to
Unfortunately, a self-induced emf exists in the
Speed.
coil at the
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