Re: Metal Halide Lamps
- From: don@xxxxxxxxxxxxxx (Don Klipstein)
- Date: Wed, 16 May 2007 00:53:15 +0000 (UTC)
In art. <4vmh43lsaglet0shiukd98tf3m2u2kiq3i@xxxxxxx>, Victor Roberts wrote:
I've had some tests run on metal halide lamps by a certified
independent laboratory (since I don't have a sphere.) The
main goal of this work was to quantify the high frequency
efficacy of metal halide lamps. The common wisdom is that
there is no high frequency efficacy gain, but there are one
or two papers that claim otherwise. This is my poster paper
for LS:11.
The results are in and they are a bit strange.
The tests were run on three 320-watt pulse start metal
halide lamps. there is more money from the sponsoring
organization to run 6 more 320-watt lamps if necessary.
As part of the test I measured lamp performance on five
types ballasts:
1) Standard reference ballast
2) Commercial linear inductor ballast
3) Commercial CWA ballast
4) Brand A 100 kHz electronic ballast
5) Brand B 100 KHz electronic ballast.
There were three of each type of commercial ballast.
Since I suspected that the commercial ballasts would not all
run the lamp at the same power, and since lamp efficacy is a
function of power, I had the lab operate the lamp on the
reference ballast at rated power, rated power + 15% and
rated power -15%, so I could construct a correction curve.
The really strange result is that lamp efficacy on both
types of commercial ballasts was lower than on the reference
ballast, even after correcting for the fact that the
commercial ballasts ran the lamp below rated power. The
corrected efficacy loss is about 6% to 7%. The only
explanation I have right now is that these lamps have lower
efficacy when operated with a high current crest factor
system. I don't see any other obvious difference between
lamp operation on the reference ballast and on the
commercial ballasts that operate the lamps at the same
frequency.
Any other suggestions for a cause?
I would blame/credit current crest factor and not yet expect lower
current crest factor to help overall luminous efficacy in general.
In a given metal halide lamp with a given vapor composition (from a
given temperature of cold spot of inner surface of arc tube), lower
instantaneous current favors radiation from elements whose radiation
varies less with electron temperature (such as sodium) and higher
instantaneous current favors radiation from elements whose radiation
increases more drastically with electron temperature (such as mercury,
among MH lamp ingredients).
If the value for RMS current is decreased for a given power input, then
sodium (or thallium) radiation could get favored more over mercury
radiation. If the percentage of each half-cycle of the power line
frequency that has significant lamp current conduction increases, then
average and RMS current as determined over the significantly conducting
portion of each half-cycle of power line frequency decreases, then sodium
(or thallium) radiation could get more favored over that of mercury.
Furthermore, lack of cool moments in the arc twice per cycle of power
line frequency could increase the arc tube temperature, and increase the
concentration of vapors of the halides.
So if more sodium (or thallium) radiation and less mercury radiation
means more efficacy, I suspect making the power input to the arc more
constant throughout each half-cycle of the power line frequency can
achieve this.
I would watch for any higher arc tube temperature and any ill effects
thereof.
I would also look for a way to obtain spectral power distribution
results, and look for shifting towards visible emission features favored
by higher arc tube temperature, and away from any favored by higher
electron temperature, as a result of changing to the electronic ballast.
Now for another matter: High frequency electronic ballasts in
comparison to line frequency iron core ones are known to increase the
efficacy of fluorescent lamps via the following mechanisms:
1. Avoids some "saturation" (or approaching such) of shortwave UV
production of a low pressure mercury discharge, that results from peak
power input (whether absolutely or long enough to be a major fraction of
"imprisonment time") approaching the ability of the discharge to radiate
such an amount of power from a given discharge surface area, available
bandwidth of the desired spectral feature, and available from the electron
temperature (which for that matter usually decreases when power input to
fluorescent lamps increases IIUC).
2. Avoids the "oscillatory anode fall" (approx. 2.5 volts
largely not useful-radiation-producing voltage drop) that most fluorescent
lamps have when fed either DC or AC of frequency of frequency around or
less than 1 KHz. I am aware of some fluorescent lamps having an electrode
structure that impairs this loss in DC and low frequency use, but for now
I forget about any examples other than US Patent 4,902,933.
This makes me wonder if MH lamps or the ones in question have any
electrode-related voltage drops that are nonlinear (in a way where losses
can be reduced by lower current crest factor) or time dependent/
oscillatory in a way that can be mitigated by use of high frequency AC.
- Don Klipstein (don@xxxxxxxxx)
.
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