2-19. MUTUAL INDUCTION
When the current flowing through a conductor is not uniform, there will be
corresponding changes in the magnetic field. If a second (secondary) coil which is part
of a closed circuit is placed near the first (primary) coil with its changing magnetic field,
an alternating EMF will be induced in the second coil, producing alternating current in
the second circuit. For mutual induction (inductance between two coils acting together)
to occur, the magnetic field must change in strength or direction, such as with AC.
Section V. ELECTRIC GENERATORS AND MOTORS
2-20. INTRODUCTION
An electric generator or dynamo is a device that changes mechanical energy into
electrical energy. Electric generators are based upon the principle that a voltage and
current are induced in a coil when it cuts magnetic lines of force. A generator has a
strong magnet to supply the necessary magnetic field and an armature, consisting of a
coil of insulated wire wound around an iron core that is mounted so that it can rotate
between the poles of the magnet. The mechanical energy required to rotate the
armature may be obtained by such means as waterpower, steam, or gasoline engines.
As the armature rotates, its coil cuts the magnetic lines of force and induces an EMF.
2-21. THE SIMPLE ALTERNATING CURRENT GENERATOR
a. The simplest form of an AC generator may be made by rotating a single loop
of wire placed between the north and south poles of a magnet. It is possible to analyze
what the characteristics of the induced current will be by studying the relation of the loop
of wire in the rotating armature to the magnetic field. In figure 2-12, the two ends of the
loop of wire (A and B) in the armature are separately connected to two metallic rings
(slip rings X and Y), which are insulated from each other and mounted on the same
shaft as the loop so that they rotate as the armature revolves on its axis. Two stationary
metal or carbon strips, called brushes, rest lightly on the slip rings as they revolve with
the loop. The ends of the wires of the external circuit (in this case, the meter) are
connected to the brushes. As the brushes rub against the rotating slip rings, they take
current from the armature and transmit it to the external circuit. The magnetic field
passes from the North (N) Pole to the South (S) Pole. As wires A and B pass from left
to right and right to left, respectively, through the magnetic field, currents induced flow in
the direction shown by arrows (II, figure 2-12); current leaves wire A through slip ring Y,
passes through the meter in the external circuit, and enters slip ring X, completing the
circuit. As the armature rotates, A and B each move to the other's original position (III,
figure 2-12); current leaves wire B through slip ring X, passes through the meter, and
enters slip ring Y (IV, figure 2-12). The current reverses direction with every half-turn of
the armature.
MD0950
2-18
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