332 Electrical Power Systems Technology
Incandescent lamps usual-
ly have a thin tungsten filament,
which is in the shape of a coiled
wire. This filament is connected
through the lamp base to a volt-
age source (usually 120 volts
AC). When an electric current
is passed through the filament,
the temperature of the filament
rises to between 3000° to 5000°
Fahrenheit. At this tempera-
ture range, the tungsten pro-
duces a high-intensity white
light. When the incandescent
lamp is manufactured, the air
is removed from the glass en-
velope to prevent the filament
from burning; also, an inert gas
is added.
Incandescent lamps are
purely resistive devices, and
thus have a power factor of 1.0.
As they deteriorate, their light
output is reduced. Typically, at
the time that an incandescent
lamp bums out, its light out-
put is less than 85 percent of its
original output. A decrease in
the voltage of the power system
also reduces the light output.
A 1 percent decrease in voltage
will cause a 3 percent decrease
in light output.
Fluorescent Lighting
Fluor escent lighting is used extensively today, particularly in in-
dustrial and commercial buildings. Fluorescent lamps are tubular bulbs
with a filament at each end. There is no electrical connection between the
two filaments. The operating principle of the fluorescent lamp is shown
Figure 13-3. The incandescent lamp: (A)
Construction, (B) Types