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Industrial Lighting - Mercury Lamps question

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Tony Wells

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#1
OK, I hope we have some knowledge here on the operation of multi vapor lamps, their respective ballasts and operating wattage compatibility.

I have a "light tower" that I am attempting to put back into service. It is older, and the repair parts I need are obsolete and NLA, even from the manufacturer (Magnum). The original equipment is 4 1000 watt (mh1000/U) lamps. The owner of the tower first wanted to convert to LED, but that conversion is prohibitively expensive. Runs about $1,700 per fixture (X4), so I am going another direction. He also has available some indoor (suitable for wet locations) complete fixtures that were taken down elsewhere. However, they are only 400 watt fixtures with all plastic housings and lenses. They are physically much larger than the aluminum original reflectors. I have serious doubts that 1600 watts will be satisfactory on the tower, which is intended to extend to about 30 feet vertically. That would be replacing 4,000 watts of light.

So, it was suggested that I simply do some fancy swapping and use the plastic housings and lenses on the 1,000 watt lamp assemblies. The 400 watt fixtures were meant for stationary installations, and have the ballasts built into a cast aluminum housing, making the whole fixture weigh about 10x what the original does. So the design weight capacity of the mast would definitely be out of a safe range, IMO. I could relocate the ballasts to where the present ballasts are, inside the cabinet of the trailer. That could turn into a lot of hassle and may not be all that easy. Not my first choice. All in all, my gut tells me that the plastic parts from the 400 watt fixtures, while I could build adapters and mount to the 1,000 watt receptacles, would not hold up to the 1,000 watt lamps due to the heat generated. Plus they do not function as reflectors. Plus they are much larger and would present a greater wind load, a danger when elevated. And as this is a trailer, I'm quite certain the vibration would crack them and eventually they would be lost while transporting.

That's the story; now my question. I know the ballasts are different for the two different lamps, even though they are the same style and type lamp. What I need to know is what could happen if I run the 400 watt lamps from the ballasts meant to power the 1,000 watt lamps? I am supposing the strike voltage should be less, as the 400 watt lamps have a shorter inner envelope where the vapor and arc actually are. But the strike voltage is only for a short duration. The actual arc voltage should be the same, maybe.....that's part of the question.

What I really want to do is get a glass company to cut me some circles of tempered glass that can handle the heat. I can fabricate some clamps to hold it in place. That seems to me to be the best way to get the full 4,000 watts of light out of this tower. I can stay with the aluminum reflectors that were designed to take the heat, reflect the light downward, and tolerate any vibration they may experience. Plus this leaves me out of messing with trying to wrangle the wrong ballasts into the machine. The glass pieces would be about 19" in diameter, so I am thinking 1/4" thick would be acceptable.

So what say ye? Any options I am overlooking?
 

Ulma Doctor

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#2
Hi Tony, interesting project!
i don't think you will experience problems using a 1000w fixture/ballast for a 400w bulb. logic tells me that the transformer will only need to be working less than 1/2 capacity.

the only possible problem i can see is if you were to try to replace a Metal Halide type bulb with a Sodium Type bulb- they require different ballasts
 

Tony Wells

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#3
Yes, I know that sodium lamps are a whole different animal. They don't put out the color of light intended to work under anyway, so they are not under consideration.

I did an experiment this evening. Since I have the 4 400 watt fixtures, I simply put one of those lamps in the 1000 watt socket and fired it up. It ran for about 2 hours with no problems. My thermal gun showed the aluminum reflector quite a bit cooler, as might be expected. But then it hit me that it was pointless. If I am going to leave the metal reflectors and original ballasts, I should stay with the 1,000 watt lamps and things will be as designed, except for however I end up closing the end of the reflector. I don't know of a plastic that would hold up to the heat, or I would just buy a sheet and make them myself. Has to be cheaper than having tempered glass cut and ground. Interestingly, I hung a small piece of 1/8" thick lexan in front of the 1,000 watt lamp yesterday and it did not melt or even get soft. The problem, unless is was already brittle from age or UV exposure, was that handling it, it didn't flex very well. Maybe it doesn't need too. And this was not a piece that would close off the reflector and trap the heat, increasing the temperature. I am tempted to just try one of the 4 and see just what happens. Might not need to be glass after all. I'd hate to be the cause of a hot string of plastic to come down on someone working under the light though. Maybe a circle of hardware cloth would prevent that.
 

Tony Wells

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#4
After some research and looking at a lot of technical papers on HID lamp power, the conclusion I have reached is that it is unwise to use a ballast larger than the lamp size. Not only is the strike voltage higher, but when they are lit and running, the operating voltage is higher due to the longer arc tube. This will usually cause the arc tube to overheat and at the very least shorten its life, and at most cause a catastrophic failure of the envelope, rupturing the outer containment globe. This potentially sends fragments of glass and quartz up to 1300°C (2372°F) outward. That is why most of the higher wattage luminaires are enclosed and use metal and tempered glass in their construction. Some even have metal screens to help contain the damage if there is an explosion. Several cases are documented where fires have been started due to different failures of HID lamps.

So it's unlikely I will find a suitable plastic, and I will call a couple of glass shops here to see if they can cut a good circle out of tempered glass for me.
 

jim18655

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#5
The ballast on any HID lamp is also a choke to limit the lamp current. Wrong ballast would equal the wrong current. A trouble shooting/test connection for these lamps is to short circuit the lamp wires and measure the current supplied and limited by the ballast.
Some lamps are rated for open fixtures and are coated to contain the bulb if it fails. Failure of the outer jacket can also expose people to ultra-violet rays from the arc tube.

Advance ballasts has a HID lamp troubleshooting guide available for download that explains all the test procedures in detail.
 

Tony Wells

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#6
The lamps I am working with are 1,000 watt. I don't find any designed for open fixtures that are also suitable for wet locations. Since these are outdoor, harsh condition lights, I am staying with OEM style if at all possible. Getting a single raindrop on the lamp I believe would be a bad thing. ;)

I did find a local glass shop that can do circles for me out of 1/8 or 1/4 in either annealed or tempered glass. I have some Lexan out on one of them with it closed up to see how hot it really gets. So far, not as hot as I thought. Of course, this particular ballast may be failing. The color doesn't look quite right. I may move it to another fixture on this rig that was definitely whiter.
 
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