Figure 3-31.Magnetization and permeability curves.
You have now seen how a change in the magnetizing force causes a change in permeability. The
next question is, how do you change the magnetizing force? Magnetizing force is a function of
AMPERE-TURNS. (An ampere-turn is the magnetomotive force developed by 1 ampere of current
flowing in a coil of one turn.) If you increase the ampere-turns of a coil, the magnetizing force increases.
Since it is not practical to increase the number of turns, the easiest way to accomplish this is to increase
the current through the coil.
If you increase the current through a coil, you increase the ampere-turns. By increasing the ampere-
turns you increase the magnetizing force. At some point, this causes a decrease in the permeability of the
core. With the permeability of the core decreased, the inductance of the coil decreases. As said before, a
decrease in the inductance causes an increase in power through the load. A device that uses this
arrangement is called a SATURABLE-CORE REACTOR or SATURABLE REACTOR.
A saturable-core reactor is a magnetic-core reactor (coil) whose reactance is controlled by changing
the permeability of the core. The permeability of the core is changed by varying a unidirectional flux (flux
in one direction) through the core.
Figure 3-32 shows a saturable-core reactor that is used to control the intensity of a lamp. Notice that
two coils are wound around a single core. The coil on the left is connected to a rheostat and a battery.
This coil is called the control coil because it is part of the control circuit. The coil on the right is
connected to a lamp (the load) and an a.c. source. This coil is called the load coil because it is part of the
As the wiper (the movable connection) of the rheostat is moved toward the right, there is less
resistance in the control circuit. With less resistance, the control-circuit current increases. This causes the
amount of magnetism in the core to increase and the inductance of the coil in the load circuit to decrease
(because the core is common to both coils). With less inductance in the load circuit, load current increases
and the lamp gets brighter.