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130 High Intensity Discharge (HID) Lamp Operation
TECHNICAL
Metal Halide (MH)Lamps
Metal Halide lamps are high-pressure mercury
lamps with added metal iodides or iodides of
the rare earths – Dysprosium (Dy), Holmium (Ho)
and Thulium (Tm) – and complex compounds of
Caesium (Cs) and Tin Halide (Sn). The Halides
decompose in the core of the discharge arc
and the metals can be excited to emit light
with an intensity and spectral distribution that
depends on the vapour pressure of the metal
halides. The luminous efficacy and the colour
rendering properties of the Mercury discharge
are significantly improved and the gaps in the
mercury spectrum are closed. Metal Halide
(MH) lamps are similar in their construction
to mercury vapour lamps. They are popular
for their excellent colour rendering which is
achieved by the inclusion of rare earth elements
in the discharge tube.
Metal Halide lamps are temperature sensitive,
with optimal operation occurring at 100°C to
180°C ambient temperature. Luminaires should
be designed to provide suitable conditions.
Ballasts must operate lamps from the correct
supply voltage and their impedance should
not vary more than ±3% from the specified
value. The supply voltage should not deviate
for extended periods for more than 3% of the
nominal rated voltage of the ballast, and the
supply frequency should not deviate by more
than ±3%.
Metal Halide lamps require higher voltages for
starting than the usual supply voltages. Various
circuits are used to achieve the required starting
conditions.
Metal Halide lamps require leakage reactance
ballasts, constant wattage auto-transformer
control equipment or series reactor type
ballasts with ignitor. The latter is the most
economical solution, with the constant wattage
circuit being the best for electrical performance.
Electronic control gear for lower wattage MH
lamps is also available
Separate information and data is available for
the Hella range of constant wattage control
equipment.
Most Metal Halide lamps will require an ignitor
to start the discharge in the arc. The ignition
voltage is generally 4.0–5.0kV peak for a cold
start with the lamps requiring a cooling period
of between 10–15 minutes before they will
re-ignite. Certain lamps are capable of hot reignition,
in which case voltages of up to 35kV
peak are required. Some of the higher wattage
2000W and 3500W lamps can require hot,
instant re-strike voltages of up to 60kV peak.
NOTE: Not all makes and/or types of Metal
Halide lamps can be operated on ignitor circuits
– consult the lamp manufacturer or Hella for
information or lamp control gear compatibility.
High Pressure Sodium (HPS)
Lamps
High Pressure Sodium (HPS) lamps can operate
satisfactorily from –30°C to +100°C on series
reactor ballast circuits, but precautions against
increase arc tube voltage, due to thermal back
radiation when mounted in luminaires, should
be considered. Only a typical 10 volt increase
in lamp voltage is tolerable. The lamp power
and discharge tube temperature are highly
dependent upon the lamp voltage. It is therefore
important that the mains voltage does not
exceed 105% of the nominal rated voltage of
the choke for extended periods.
The ballast should exhibit a closely controlled
current/voltage characteristic, to prevent the
lamp from exceeding the maximum power limit
as it ages and the lamp voltage rises.
Standard lamps with standard ignitors
re-ignite after 60 to 90 seconds, requiring 2kV
to 5kV to ignite from a cold start, depending on
the lamp specifications. High pressure sodium
lamps can only be re-ignited hot if they are of
linear double-ended construction because hot
re-strike requires 18kV to 25kV peak starting
pulses.