Interior Lighting for Designers

at which it is designed to operate, not the
wattage at which it will operate. Therefore, it
is impossible to reduce the wattage of a dis-
charge system simply by changing the watt-
age of the lamps. The ballasts must also be
changed.
Two-lampfluorescent ballasts are used
frequently to reduce ballast cost and instal-
lation cost per unit of light. They are avail-
able for two-lamp series operation or two-
lamp parallel operation.
Two-lampseriesdesigns are more com-
mon because they offer the lowest cost and
the minimum size and weight. Only two lamp
leads are supplied from the ballast. Both
lamps go out when one lamp fails; the good
lamp remains undamaged.
Two-lamp parallel designs have two
independent ballast circuits and therefore
are more expensive than two-lamp series
designs. Three or six lamp leads are supplied
from the ballast. Failure of one lamp leaves
performance of the second lamp unaffected.

Electromagnetic Ballasts
Until the 1980s, all discharge ballasts were
of the electromagnetic kind. The electro-
magnetic ballast consists of two copper or
aluminum wire coils around a common core
of steel laminations. This assembly converts
electrical power into a form appropriate to
start and regulate the lamp.

The ballast usually consists of a trans-

former, inductance coils, and a capacitor.

The transformer converts the service voltage

of the lighting circuit to the starting voltage

required for the lamp. Theinductance coil

limits the current that can be drawn by the

lamp, but in so doing introducesinductive

reactance into the circuit. The capacitor

realigns the phase relationship between volt-

age and current; it is a controlling device that

consumes no electrical power.

The capacitor improves the ballast’s

power factorso that it uses energy more effi-

ciently. An electromagnetic ballast that is

equipped with a capacitor is called ahigh-

power-factorballast.

Power Factor

Thepower factorof a ballast is the measure-

ment of how effectively the ballast converts

the voltage and current supplied by the elec-

trical distribution system to the power deliv-

ered by the ballast to the lamp. Perfect

phase relationship would result in a power

factor of 100 percent.

Power factor

Input watts (W)

Line volts (V) Line

=

× amps (A)

The power factor of an inductive circuit

is lagging (figure 8.2) and that of a capaci-

tive circuit is leading. When discharge lamps

are operated in conjunction with simple

AUXILIARY EQUIPMENT

Figure 8.2Power factor.