TM 5-688
motor starters consisters of a coil, thermal overloads,
All types of lighting are sensitive to the supply voltage
and a set of contactors (contacts). The thermal over-
level and cannot be derated for voltage. For example,
loads, which are essentially circuit breakers, and the
subjecting a 120 V incandescent lamp to a 240 V
contactors are rated to handle a certain amount of cur-
source will result in the lamp burning twice as hot,
rent. Since at 50 Hz, a motor of a given horsepower
causing rapid lamp failure. Subjecting the iron core
rating will draw more current than an identically-rated
ballast use in many HID and fluorescent fixtures to
motor would draw at 60 Hz, the thermal overloads and
twice its rated voltage will saturate the ballast and will
the contactors must be sized accordingly.
subject the fixture to much more than its rated current.
As with transformers and motors, 60 Hz iron-core bal-
b. Computer power supplies include voltage regula-
lasts can also be saturated when operated at 50 Hz.
tors, isolation transformers, transient voltage suppressor
At first thought, frequency dependence may not be OS
transformers, computer regulator transformers, and
much of a problem with electronic ballasts since, in
power conditioning transformers. Computer power sup
most cases, the AC voltage source is first converted
plies are sensitive to both frequency and voltage level.
back to a high frequency AC source, and therefore,
the voltage source that is actually impressed across
the lamp is decoupled from the 60 Hz AC source.
age. Clocks rely on the frequency of the supply voltage
However, the power supply used to power the elec-
to keep correct time, so a clock designed for 60 Hz
tronics in these ballasts must be capable of 50 Hz
will not keep correct time at 50 Hz. The motor that runs
operation.
the clock is also sensitive to supply voltage level.
2-8. Other electrical equipment.
Therefore, a clock must either be purchased config-
ured for the supply voltage level, or a transformer must
Other electrical equipment includes motors, motor
be used to convert the supply voltage level to the
starters, computer power supplies, and clocks.
clock's rated voltage level. Clocks cannot be derated
for frequency, and therefore clocks designed for 50 Hz
a. Typically, motor starters are sensitive to both supply
must be purchased.
voltage level and frequency. The most commonly used
2-6
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