If the value of inductance is decreased to 11.7 millihenries, the formulas yield the
X L = 2π fL
X L = (2) (3.14)(60 Hz)(11.7 mH)
X L = 4.41Ω (roundedoff)
Z = XL + R 2
Z = (4.41Ω2 + (5Ω)2
Z = 44.4481Ω
Z = 6.67Ω (roundedoff)
I = 15 A (roundedoff)
True Power = I 2 R
True Power = (15 A)2 x (5Ω)
True Power = 1125W
So, a decrease in inductance of 11.3 millihenries (23 mH - 11.7 mH) causes an increase in
power to the load (true power) of 625 watts (1125 W - 500 W). If it took 1 watt of power to
change the inductance by 11.3 millihenries (by some electrical or mechanical means),
Figure 7-26 would represent a power amplifier with a gain of 625.
METHODS OF CHANGING INDUCTANCE
7-105. Changing the inductance of a coil enables the control of power to a load. There are
methods that are available to change the inductance.
7-106. Permeability was defined as the measure of the ability of a material to act as a path
for additional magnetic lines of force. Soft iron was presented as having high permeability
compared with air. In fact, the permeability of unmagnetized iron is 5,000 while air has a
permeability of 1. A nonmagnetized piece of iron has high permeability because the tiny
molecular magnets (Weber's Theory) or the directions of electron spin (Domain Theory)
can be aligned by a magnetic field. As they align, they act as a path for the magnetic lines
7-107. The inductance of a coil increases directly as the permeability of the core material
increases. If a coil is wound around an iron core, the permeability of the core is 5,000. If
the iron is pulled part way out of the coil of wire, the core is part iron and part air.
Therefore, the permeability of the core decreases. As the permeability of the core
decreases, the inductance of the coil decreases. This increases the power delivered to the
load (true power). This relationship is shown in Figure 7-27.
23 June 2005