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Working with a 2.5kw HMI


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#1 Ashley Barron

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Posted 24 September 2007 - 11:04 PM

Hello,
I'm going to be shooting a night scene this weekend using a 2.5kw HMI. I have never used one before and am wondering what I should know about working with one. I've heard something about a certain amount of time to start up and that fact that it's quite heavy etc. Anyone have any tips on working with them? Any good/bad experiences?
Thank you muchly,
Ashley.
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#2 Jonathan Bowerbank

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Posted 25 September 2007 - 01:46 AM

Starting one up isn't an issue, unless you're talking about restriking. Whenever you turn it off, you have to wait a few minutes before you can restrike the light. Plus you do have to wait just a bit for it to reach its correct color temperature.
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#3 Xavier Plaza

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Posted 25 September 2007 - 11:47 AM

Ashley you have to keep in mind HMI they have ¨more useful light¨ than tungtens lights, you can check it the values in footcandles at arri website, use the photometric calculator & good luck

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#4 Paul Sallent

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Posted 25 September 2007 - 01:11 PM

operating one of those is actually easy. There are a couple of things though that you always need to remember.

1) Never move the light before it goes a little cold after you turn it off. (you may damage the bulb)
2) Always operate the light from the ballast and avoid switching on from the head.(After the half life of the bulb, in the first five minutes of the strike, it might explode sending hot glass all over the place.)
3) Inform everyone on the set that you're striking the light for safety reasons.

(4 but rare. If you need to change the lamp for any reason do no touch it with bare hands. The gas in it reacts to the temperature of our hands as well as the oil coating of the lamp, thus creating a weak spot on the glass. You better use a pair of gloves.)

I'll be happy to answer more of your questions if there are any! Have a nice shooting!



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#5 Michael Nash

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Posted 25 September 2007 - 04:34 PM

In addition:

Plug the header cable in at both ends before plugging in the ballast, for safety (the connection on the ballast could be "hot" other wise). Never plug or unplug the header cable when the ballast is "on." It's also common practice to tie the header cable to the bail, as a strain relief on the head. The header cable usually has some sash cord taped to one end for wrapping the cable; you can use that.

And as noted, just give it a couple minutes to warm up after striking, until it comes up to full color temperature and brightness. If power get cuts accidentally or if you have to power down the unit to move it, it make take a few minutes for the unit to cool down before it will restrike. HMI's don't like to "hot re-strike;" you can't turn them on and off rapidly.
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#6 Hans Engstrom

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Posted 25 September 2007 - 06:02 PM

If power get cuts accidentally or if you have to power down the unit to move it, it make take a few minutes for the unit to cool down before it will restrike. HMI's don't like to "hot re-strike;" you can't turn them on and off rapidly.


If you are quick you can make a quick restrike within some seconds from cutting the power but if that fails you have to wait for the bulb to cool down. I have done this when there´s no time spare and you must add another 32a cable. Keep in mind that you should however avoid "hot re-strikes".

Another reason not to use the switch on the head is because you´ll forget about it and start wondering whats wrong when the HMI wont strike and it´s 4m up in the air.
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#7 Barry Cheong

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Posted 25 September 2007 - 06:35 PM

Is it a joy connector on a 2500 ballast?
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#8 Ashley Barron

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Posted 25 September 2007 - 10:05 PM

Thanks to all for your help, it's muchly appreciated!
But my questions is..what is a header cable and ballast?
:)
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#9 Michael Nash

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Posted 25 September 2007 - 11:44 PM

HMI's require a ballast (power converter) to work. Magnetic ballasts are quite heavy; electronic or "flicker-free" ballasts are much lighter.

Posted Image

The header cable is the cable that goes between the ballast and the "head" (the unit itself). It's a multi-pin connection with a twist-lock. There's usually a keyway to help guide the pins into the right orientation, and then you twist and slide the collar to lock it in place. Don't force anything if it doesn't go in easily; sometimes the collars get bent out of round and can be a tight fit, but you don't want to bend the keyway or the pins. The header cable will come separate from the head and the ballast, so don't forget it during pickup.

Par lamps will also have a separate case for the lenses and scrims, although if you're using a 2.5K it will likely be a fresnel anyway. Take care when opening and closing the latch on the lamp to add/change the lenses; when the lamp is hot it you need extra good gloves not to burn yourself, and failure to close the latch may result in a scorching hot lens falling on someone and breaking (and you don't want to pay to replace it). And although most HMI units should have a safety glass to protect against UV radiation, it's good practice to always make sure a lens is in place before striking the unit.

You, or whoever will be setting up the light, should really have the rental house give you a quick run-through on how to set it up when you pick it up. There's no shame in learning a piece of gear for the first time, and the rental house will appreciate you knowing how to use their gear the right way.

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#10 timHealy

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Posted 26 September 2007 - 11:59 AM

1) Never move the light before it goes a little cold after you turn it off. (you may damage the bulb)
2) Always operate the light from the ballast and avoid switching on from the head.(After the half life of the bulb,


I disagree with these two statements.

When moving a tunsgsten light while it is on, it is important to avoid shocks. The tungsten filiment is very fragile while it is off and extremely fragile while it is on. A really large shock will break a bulb filiment while it is off and a minor shock with break it while it is on.

However HMI's do not have a filiment and are not as susceptable to shocks as tunsten. One should be careful nonetheless. In fact with some HMI that suffer from the hot restrike issue it is advisable to leave the light on while moving it short distance.

One can safely operate an HMI from the head or ballast. There is no difference. However when one turns on an HMI, always make sure no one is in front of the lamp when striking. Although rare, bulbs do explode once in a while.

Also, if I may add, never use an HMI without its safety glass and don't defeat the micro switch so one can operate the light with the door open (thus bypassing the safety glass). The safety glass protects people from UV light.

Best

Tim
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#11 Ryan Patrick OHara

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Posted 25 March 2011 - 06:48 PM

What does the yellow switch do next to the ballast red 'power' switch? The yellow one is a temporary toggle, meaning it won't seat in the opposite position, just temporary while you press it.
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#12 Guy Holt

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Posted 26 March 2011 - 04:32 PM

What does the yellow switch do next to the ballast red 'power' switch? The yellow one is a temporary toggle, meaning it won't seat in the opposite position, just temporary while you press it.


The yellow button is a ground test button – if it lights when you depress it you have a good ground. It is important to have a good ground with HMIs because a very high voltage is required to strike the globe. A HMI ballast creates a start-up charge for the head igniter circuit, rapidly increasing the potential between the electrodes of the head’s arc gap until an electrical arc jumps the gap and ignites an electrical arc between the lamp electrodes. The ballast then acts as a choke, regulating current to the lamp to maintain the pulsating arc once the light is burning.

I'm going to be shooting a night scene this weekend using a 2.5kw HMI. I have never used one before and am wondering what I should know about working with one.


Because of the constant improvement in HMI technology there are many 2.5 HMI ballast options available and if you are not careful how you power them you can get stuck. In ballast design you have a choice between magnetic and electronic ballasts; and to complicate matters even more, you have a choice between Power Factor Corrected electronic ballasts and non-Power Factor Corrected electronic ballasts.

Power Factor Correction (PFC) is fairly new in HMIs and so you will quite often come across non-PFC 2.5/4kw Ballasts in rental house inventories. In fact, I wouldn’t be surprised if you were not even familiar with Power Factor Correction in HMI ballasts. Since Power Factor Correction (PFC) is not mandated in this country, as it is in Europe for any electrical device that draws more than 75W, we are pretty much ignorant of Power Factor and the effect that poor Power Factor can have on a distribution system. However, any film technician familiar with large HMI heads will be quite familiar with Power Factor and Power Factor Correction (PFC) because after a false start back in the 90s, all major manufacturers now include PFC circuitry in HMI ballasts in the 6-18kw range. They do so by necessity. The early line of Lightmaker electronic ballasts were nick named by film electricians “Troublemaker” ballasts because they were not Power Factor Corrected and proved that PFC circuitry was absolutely necessary in large ballasts to reduce heat and returns on the neutral, and to increase ballast reliability (beware, some are still kicking around rental houses). But, because of the added cost, weight, and complexity of PFC circuitry, ballast manufacturers in the US still only offer PFC circuitry as an option in medium-sized ballasts (2.5-4kw). And, until very recently manufacturers did not offer PFC circuitry in HMI ballasts smaller than 2.5kw in the US (in the EU PFC circuitry in mandatory in all HMI ballasts sold.)

Part of the reason for the number of non-PFC ballasts in this country was that PFC circuitry does not offer a huge advantage when operating 2.5 HMIs on Crawford generators or tie-ins. A typical 2500W Power Factor Corrected electronic HMI ballast will draw 23 Amps at 120 Volts verses the 35 Amp draw of a non-PFC electronic ballast. Since neither ballast will operate on a standard 20A wall outlet, PFC did not offer a huge advantage when operating 2.5 HMIs on Crawfords or house power. However, the added efficiency of a PFC 2500 ballast can make a huge difference when powering a lighting package off of a portable generator. For example, when you consider that a Kino Flo Parabeam 400 draws only 2 amps, the 12 Amp difference between using a PFC 2500W electronic ballast and standard non-PFC 2500W electronic ballast, can mean the difference between running six additional Parabeam 400s on a portable generator or not – I think you would have to agree that is a major boost in production capability and pertinent to any one using a portable generator as their principle source of set power.

You have several options when it comes to operating your 2.5kw HMI off of a 6500W generator depending on the type of ballast and generator you use. Where electronic HMI ballasts are typically auto-sensing multi-volt electronic ballasts (with an operating range of 90–125 & 180-250 Volts), you can plug it directly into the 240V 4 pin twist-lock receptacle on the generator and it will operate at 240 Volts (where 2.5 kw ballasts are typically wired with a 120V 60Amp Bates Plug (Stage Pin) you will need a 120V 60A Female Bates to 240V 4pin twist-lock adapter to plug a 4kw ballast directly into the generator. ) Or, if the electronic ballast is power factor corrected (draws 23 Amps) you can plug it into the 30A/120V twist-lock receptacle on the generator’s power panel. If the electronic ballast is not power factor corrected (draws 35 Amps) you will not be able to run it off of the 30A/120V twist-lock receptacle without tripping it’s fuse.

Even though a 2.5 magnetic ballast draws approximately 26 amps you will not be able to run it reliably on the 30A/120V twist-lock receptacle on the generator’s power panel. That is because even though the twist-lock receptacle is rated for 30 Amps conventional 6500W generators are only capable of sustaining a peak load of 27.5 Amps per leg for a short period of time. Their continuous load capacity (more than 30 minutes) is 23 Amps per leg. And if there is any line loss from a long cable run the draw of a 2.5 magnetic ballast will climb to upward of 30 Amps. To make matters worse magnetic ballasts have a high front end striking load. That is, a magnetic ballast draws more current during the striking phase and then they “settle down” and require less power to maintain the HMI Arc. By contrast, an electronic ballast “ramps up”. That is, its’ current draw gradually builds until it “tops off.” For this reason, you must always leave “head room” on the generator for the high front-end striking load of magnetic ballasts. And to complicate matters even more, the lagging power factor caused by the inductive reactance of the magnetic ballast kicking harmonic currents back into the power stream causes spikes in the supply voltage that can cause erratic tripping of the breakers on the generator or ballast. In my experience the load of a 2.5kw magnetic ballast is too near the operating threshold of a 6500W generator for it to operate reliably.

The only sure way to power a 120V 2.5kw (or even a 4kw) HMI magnetic ballast or non-PFC 120V electronic ballast on a portable gas generator is from its 240V circuit through a 240v-to-120v step-down transformer. A transformer will step down the 240V output of the generator to a single 120V circuit that is capable of accommodating the high front end striking load, and even the voltage spikes, of a 2.5kw magnetic ballast and the greater draw of a non-PFC electronic ballast. I won’t address the issue of flicker and frame rate/shutter angles with magnetic ballasts because it is well established elsewhere in this forum that there are safe windows that are “flicker free” as long as the power supply is stable – i.e. the generator has a “crystal” governor or is an inverter type. However it is something to be aware of when operating 2.5kw HMIs with magnetic ballasts on portable generators.

If you haven't already, I would suggest you read the article I wrote for our company newsletter on the use of portable generators in motion picture lighting. In it I cover some of the basic electrical engineering principles behind poor Power Factor, the harmonic distortion it can generate, and how it can adversely affect generators. These power generation issues have been vexing set electricians for years. Use this link for an informative newsletter article that explains the electrical engineering principles behind these issues and how to resolve them.

Posted Image


This article is cited in the just released 4th Edition of Harry Box's "Set Lighting Technician's Handbook" and featured on the companion website "Box Book Extras." Of the article Harry Box exclaims:

"Great work!... this is the kind of thing I think very few technician's ever get to see, and as a result many people have absolutely no idea why things stop working."


"Following the prescriptions contained in this article enables the operation of bigger lights, or more smaller lights, on portable generators than has ever been possible before."


The article is available online at http://www.screenlig...generators.html.

Guy Holt, Gaffer, ScreenLight & Grip, Lightng & Grip Rental in Boston
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#13 Stuart Brereton

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Posted 26 March 2011 - 08:04 PM

Thanks to all for your help, it's muchly appreciated!
But my questions is..what is a header cable and ballast?
:)


With respect, if you need to ask such basic questions about these lamps, perhaps you should have someone better qualified than yourself to operate them.
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#14 Phil Rhodes

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Posted 26 March 2011 - 09:38 PM

Y'know, every time I read something like this I reflect that it really is a lot simpler in the UK where everything has a ceeform on the end. And yes, all ballasts are PFC.

P
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#15 Adrian Sierkowski

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Posted 26 March 2011 - 10:02 PM

Quiet Phil, or we shall beat you in another war!

As for ballast and header cable

Big box and big cable which goes from big box to head proper.
If you're unsure how to operate it, please, for your safety, that of the crew, and the well-being of the lamp itself, get down to a rental house and familiarize yourself with it. It's a damned expensive light to risk damaging....
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