Figure 2-13. Direct current--(A) pulsating DC (full-wave); (B) uniform DC.
2-23. ADVANTAGES OF ALTERNATING CURRENT
a. Alternating current is a much more suitable and versatile source of power
than direct current, which is limited in number and kind of applications to power
machinery and electronic circuits
b. AC is required for the operation of high-voltage transformers used in industry,
as well as in the production of x-rays. An AC voltage may be readily changed by
c. Power generated at a power plant may be sent across great distances with
relatively small loss if the current is kept low. When it reaches its destination, the high
voltage (high-tension) AC is reduced in value, ordinarily to 110 or 220 volts. This
reduction is accomplished by means of a transformer. A transformer cannot be used
with DC, which is one of the chief reasons why AC is more widely used.
2-24. THE ELECTRIC MOTOR
a. Motors, like generators, are a means of transforming energy or power. The
electric motor is a device that converts electrical energy into mechanical work. The
simple electric motor does not differ essentially in construction from the generator or
dynamo, but it operates exactly the reverse of an electric generator.
b. When an electric current flows through the armature of an electric motor, it
sets up a magnetic field that is opposed by the field about the permanent magnet of the
motor. The current is supplied to the armature in such a manner that when the rotating
armature achieves a position (a half- turn) in which the magnetic fields are no longer