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A.
Characterization of HID
Lamps A brief
characterization of HID lamps (HPS and MH
lamps) and the related ballast
requirements are summarized in the
following points. 1.
Ignition.
HID lamps need an appropriate voltage
across the electrodes to initiate and
mantain glow discharge. Furthermore the
ballast should provide sufficient current
at glow discharge voltage(appr. 90V for
HPS and 180V for MH) forcing the
glow-to-arc transition. Therefore, the
ballast should provide increased open
circuit voltage (>600V) for MH(Type I,
2+1 electrodes) lamps and high voltage
pulses (2000 - 3000V, 1µs) for MH
(Type II, 2 electrodes) and HPS(2
electrodes) lamps. 2.
Warm up time.
The warm up time for HID lamps is several
minutes (shorter for MH and longer for HPS
lamps). In this period the resistance of
the lamp (measured by applying square wave
current) continuously increases from a low
value [6W
(400W, MH)] to an essentially higher
nominal value [40W
(400W, MH)]. Therefore, the ballast
should act as a nearly constant current
source providing sufficient increasing
(nearly linear) power for the
lamp. 3.
Lamp Voltage Rise.
HPS lamps in particular, have an excessive
rise in lamp voltage during their life
time. This voltage rise can achieve
approximately one hundred seventy percent
(170%) of the one hundred hour operation
value. Therefore, a ballast should keep
the lamp power within an acceptable power
range derived from the ballast
curve. 4.
I-V
Characteristics.
If the lamp current is forced to change
with a certain value (DI)
the lamps can respond in two different
ways as it is shown in Fig 1. If the current is
changed slowly, (i.e. within a minute),
and with a certain value
(DI)
the lamp voltage changes only with a small
value . In this case the lamp acts like a
non-ideal bidirectional Zener diode.
Furthermore, if the change is fast (<
1s) a decreased lamp voltage is produced
by the increased lamp current and vice
versa.Therefore, if a lamp is connected
directly to a voltage source, a highly
unstable state can be resulted. Any small
current fluctuation can cause extinction
or a very fast current increase, which can
damage the lamp resulting a practically
short circuited voltage source. Evidently,
a ballast should act as a current source
allowing the lamp to determine its
voltage. 5.
Acoustic Resonance.
At high frequency (f > 4 kHz) operation
of HID lamps, standing pressure waves
(acoustic resonances) can occur in the
discharge tube. This phenomenon may lead
to visible arc distortions, resulting in
decreased lamp life time and, in some
cases, cracking of the discharge tubes.
Acoustic resonances are driven by periodic
instantaneous lamp power. In conclusion it
may be stated that the occurrence of
acoustic resonances at high frequency can
be considered as a limitation factor for a
wide and reliable application of high
frequency (< 60kHz) electronic ballasts
supplying HID lamps. 6.
Cataphoretic
phenomenon.
Cataphoretic effects may result when a
lamp is operated with DC current. Such
operation results in demixing of the
gas-filling as the sodium is transported
toward the cathode side of the tube,
making the lamp inadequate for lighting
purposes. Therefore, the polarity of the
lamp current should be periodically
changed by the ballast (i.e. every 10 ms)
providing an axially homogeneous
discharge. An approximately zero DC
component is recommended. Obviously the
situation is different for special HID
lamps designed for DC
operation. B.
Definition of Ballast According to the
particular features of HID lamps described
previously, a ballast, as it is shown in
Fig. 2, having an input which is connected
to a given (usually 50/60 Hz sinusoidal)
voltage source, can be considered as an
HID ballast if the output connected to a
HID lamp acts: b)
nearly constant effective power
equal to the nominal lamp power
between the minimum and maximum lamp
voltage; and 2.
it includes an appropriate
ignitor for starting
purpose. According to the
definition of a ballast for HID lamps, the
lamp current (I) vs. lamp voltage (V) and
the lamp power (P) vs. lamp voltage
V(ballast curve) diagrams are illustrated
in Fig.2. All values should be interpreted
as effective values.The lamp voltage(arc
discharge voltage!) at cold start is
approximately 20V(30V). In the definition,
for simplicity, zero(short circuit) value
was used as minimum output voltage. The
current in the range of 0 <
Vout< 20V can be lowered but
it should be sufficiently high forcing the
transition from glow discharge to arc
discharge at a certain glow discharge
voltage determined by the lamp. With the temperature
modulation depth in the central discharge
channel (flickering, reignition peak),
maximum current density in the electrodes,
and acoustic resonances, the frequency and
the crest factor of the lamp current (or
power) can be considered the logical
starting points for a simple
classification method of ballasts. From
the ballast perspective, the efficiency
(power loss) can be considered as a basic
parameter, directly affecting the
temperature rise. The ambient temperature
surrounding the electronic ballast will
affect the reliability and, necessarily,
the expected product lifetime.
Furthermore, the energy saving is also
directly determined by the
efficiency. 1.
Frequency.
From practical viewpoint the following
frequency ranges can be taken into
consideration. 2.
Crest Factors.
The lamp current and lamp power are
fluctuated periodically where frequency of
the instantaneous power is twice of the
lamp current frequency with the exception
of the square wave operation where the
instantaneous power is constant. The
fluctuation can be characterized by crest
factors as it will be shown in the
following part. Power crest
factor: Cp
=Pm/Pe
(Cp > 1), where
Pm is the maximum
instantaneous power and Pe
is the effective power . If the lamp
resistance is nearly constant in a
period time, then Cp is
approximately equal to
Ci2. In the case
of a square wave lamp current,
Cp = Ci = 1.
Furthermore if Cp > 1
acoustic resonances can occur at high
frequency operation. Using the frequency
and current crest factors a simple
classification of HID ballasts is shown in
Fig.3. The current pulse operation (
Ci >> 1 ) has some
specific features such as decreased light
output, with a slightly increased color
temperature at low frequency operation,
stronger acoustic resonance problems and
practical circuit difficulties at high
frequency operation. At square wave
operation there are no flickering,
reignition peaks and acoustic resonance
related problems, but the ballast circuit
is more complex and more
expensive. 3.
Efficiency.
The efficiency and the closely related
energy savings, ambient temperature
handling capability and reliability can be
considered as a crucial factor according
to the practical application of ballasts.
Therefore the following sub-classification
of ballasts with respect to the efficiency
may be justified: 2.
Electronic The average
temperature inside an electronic ballast
(this is a very global approach, separate
temperature measurments are recommended
for crucial components) depends on the
external ambient temperature (which can be
high as 50°C for industrial HID
applications) and the temperature rise
which is directly related to the power
loss of the ballast. Therefore the
efficiency of an electronic ballast for
HID lamps (especially at high lamp power
range) can be a crucial limitation factor
according to the applications. 4.
Power Factor.
High power factor ballast are recommended
especially in the high power range(>
150W). Low power factor
equipments can result an increased
harmonic distortion and effective value of
the current in the power line. On the
other side an extra unit (power factor
preregulator) is required decreasing the
efficiency and reliability. The cost of
ballast can be approximately increased by
30%. Bibliography Further
readings: 1. The high
pressure sodium lamp, J.J de Groot,
J.A.J.M. van Vilet, 1986
MacMillan. 2., The need for
high-pressure sodium ballast
classification, M.C. Unglert,,
Lighting Design and Application, March
1982. 3. An elementary
arc model of the high pressure sodium
lamp, J.F. Waymouth, Journal of
IES/April 1977. 4. Ballast Curves
for HPS Lamps Operating on High
Frequency, J. Melis, IAS 1992
Technical Conference,
Houston,Texas. 5.A power
controlled current source, circuit and
analysis, J. Melis, APEC' 94, IEEE
Technical Conference, Orlando,
Florida. Some
HID lamp related technical
papers: 7. A theoretical
investigation of the pulsed high-pressure
sodium arc, C.L. Chalek and
R.E.Kinsinger, J.Appl. Phys. February
1981. 8. Study of HID
lamps with reduced acoustic
resonances, S. Wada, A. Okada, S.
Moori, JOURNAL of the Illuminating
Engineering Society, Winter
1987. 9. Characteristic
of Radiation-Dominated Electric Arc,
J. J. Lowke, J.Appl. Phys. May
1970 10.
High-Intensity Sodium Lamp Design Data
for Various Sizes, W. C. Louden, W. C.
Matz, LIGHT SOURCES II preprint no.
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