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Why do we
need ballasts?
For many users,
ballasts are a mystery. Electrical distribution systems deliver
fixed AC voltage (50 or 60 Hz) and expect connected electrical loads
to limit the current drawn from the source. Low pressure and high
pressure arc discharge lamps exhibit "negative impedance."
Without a ballast, the arc will extinguish or draw increasing current
until some circuit element burns up. Ballasts provide system stability
by limiting the current that can be drawn. Ballasts use inductive
and capacitive components because they impede alternating current
with little power consumption. Resistive components generate high
loss and are usually avoided. This is true of conventional electromagnetic
ballasts as well as electronic ballasts.
HID ballasts
perform the following functions:
• Provide
voltage to breakdown the gas between the electrodes of arc lamps
and initiate starting.
• Provide
voltage and current to heat the electrodes to allow a low voltage,
high current arc mode to develop (referred to as glow-to-arc transition,
GAT).
• Provide
enough current to heat and evaporate the light emitting components
after an arc has been established. Provide enough sustaining voltage
(see Vss) to maintain the arc during warm-up and operation.
• Set lamp
current once all the evaporable materials have reached thermal equilibrium.
Breakdown
vs. Glow-to-Arc Transition (GAT)
Traditional
metal halide lamps (also called "probe start"), and high-pressure
mercury vapor (HPMV) lamps utilize an auxiliary electrode to facilitate
starting. These lamps are filled with a relatively low pressure
of argon gas. Breakdown occurs when several hundreds of volts are
applied. The lower the fill pressure, the lower the breakdown voltage
and less electrode heating occurs in the subsequent glow mode. Without
enough electrode heat the arc mode will not develop. There is a
trade off of breakdown voltage and GAT with fill pressure for these
lamps. For most mercury vapor lamps sinusoidal output voltages around
220 Vrms suffice. For most metal halide lamps, highly peaked (distorted)
output voltages around 300 Vrms suffice. Failing to attain a GAT
will destroy lamp electrodes in less than 100 hours.
Uni-Form®
pulse start metal halide and high pressure sodium (HPS) lamps dispense
with the auxiliary electrode, but have breakdown voltage requirements
in the range of several thousand volts. An "ignitor" adds
a narrow (µsec wide) pulse near the peak of the output voltage waveform.
Some lamps require more than one pulse per half cycle. The minimum
output voltage requirement (min. OCV) assures that a GAT will occur.
At room temperature, mercury interacts with argon to reduce breakdown
voltage. In cold weather or refrigerated spaces, the breakdown voltage
requirement goes up. Standard metal halide and mercury vapor ballasts
have to supply sufficient output voltage for low temperature starting.
This effect is not present in pulse start metal halide and HPS lamps.
The pulse voltage
requirement for pulse start lamps assures low temperature starting.
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Warm-up
Unlike
low pressure lamps, HID lamps have a low initial arc voltage following
GAT and warm up over several minutes to final operating voltage.
In HPMV lamps this involves the evaporation of a fixed amount of
mercury. In traditional metal halide and Uni-Form pulse start lamps,
a fixed amount of mercury evaporates and the metal halide salts
partially evaporate. For most HPS lamps, this involves the partial
evaporation of mercury and sodium as the lamp reaches thermal equilibrium.
Traditional and pulse start metal halide lamps have sustaining voltage
requirements after GAT to assure the lamp will continue to operate.
HPS lamps have a lamp power vs. lamp voltage space (see trapezoid)
that has been defined to assure stable warm-up and operation.
Operation
The
ballast determines the lamp current in normal operation. by providing
the impedance. The combination of lamp current and voltage determines
the power consumed by the lamp. The lamp power, in turn, determines
light output and color. For example if a 320 watt lamp is accidentally
operated on a 350 watt ballast, the lamp will run over wattage at
350 watts because the nominal lamp voltage is the same for both
lamps and the ballast delivers the current required for a 350 watt
lamp. Color will be warmer, light output will be higher and lamp
life will be shorter.
In stable operation,
lamp power varies with supply voltage and lamp voltage. Electronic
ballasts can be designed to minimize both sources of power variation.
On lag and HX ballasts, lamp power varies about 2% for each 1% of
line variation. On CWA and CWI ballasts, lamp power varies about
1% for 1% of line variation. These ballasts amplify lamp voltage
variations into power variations while lag and HX ballasts minimize
the same.
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Ballast
history
Most of the
world uses "lag" type ballasts for the operation of high
intensity discharge (HID) lamps. Another common name for the simplest
type of lag ballast is "reactor". These ballasts are constructed
from steel laminations and wire coils. The term "lag"
derives from the inductive nature of the ballast; the input current
lags the input voltage by up to 90 electrical degrees. Several input
taps may be provided to accommodate small local variations in nominal
voltage. Reactor ballasts provide outstanding lamp performance,
with excellent efficiency, at the lowest possible cost.
Lag ballasts
that can accommodate a wide range of input voltages are made using
an autotransformer stage in front of an inductive element. These
use two coils and are referred to as HX or high leakage reactance
autotransformers. The losses and material content are higher resulting
in higher operating and initial costs. The lamp performance benefits
are retained.
The CWA, or
constant wattage autotransformer ballast, became popular in North
America for mercury vapor lamps after World War II. The primary
application was roadway lighting. The circuit delivers relatively
constant lamp current, which, in turn, translates to relatively
constant lamp power as long as lamp voltage does not vary with power
input during life. This is a good assumption for mercury vapor lamps.
It allowed utilities to start a roadway circuit with as much as
+13% input voltage at the beginning of a string of lights and allow
for sag to —13% at the end of the string. The resulting lamp
power variation was an acceptable ±15%. A small "peaking"
capacitor across the lamp terminals provided enough voltage to start
lamps outdoors with modest OCV. The strategy had little to do with
temporal variations in line voltage, but rather addressed the economics
of lighting circuits along long stretches of road.
When HPS lamps
were introduced, they were incompatible with CWA ballasts because
they required a high starting voltage. The constant current characteristic
led to unstable operation. Lag and HX ballasts with electronic ignitors
became the preferred circuit types. Later, CWA circuits were developed
for HPS lamps that depart from a constant current characteristic
and incorporate ignitors.
Metal halide
lamps were introduced in the 1960’s. They required a higher
peak starting voltage than mercury vapor lamps, but were incompatible
with "peaking capacitors." The lamps would start and promptly
"drop out." By adding saturable elements to the magnetic
circuit of the ballast, the OCV could be "peaked" to start
the lamps. Probe start metal halide lamps and "peaked lead"
ballasts launched metal halide lighting in North America. Internationally,
the same lamps operated on lag ballasts by adding simple low cost
ignitors. Multiple input voltage taps for CWA ballasts were readily
accommodated. More ballasts could be operated on a circuit than
lag or HX ballasts of the same wattage. However, the current wave
shape left little margin for input voltage fluctuations during starting,
had poor energy efficiency and provided poor regulation of lamp
power with respect to lamp voltage. Evidence suggests that maintained
lumens of most metal halide lamps operated on CWA ballasts are worse
than those operated on lag circuits.
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