I'm looking at possibly picking up an old LTM 2.5kw Cinepar HMI to add to my kit. Was wondering if people could share their thoughts/experiences with them?
The LTM 2.5kw Par was a workhorse and extremely well made. Because of its longevity and the constant improvement in HMI ballast technology during that time there are many 2.5kw HMI ballast options available with used heads. If you are not careful you can get stuck. For ballasts for the LTM 2.5 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. 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.
Power Factor Correction (PFC) was fairly new in electronic ballasts when the LTM 2.5 was still being manufactured so you will more likely come across a non-PFC 2.5/4kw Ballast. 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 Tie-Ins. 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 2.5kw 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 2.5kw magnetic ballast draws approximately 26 amps at 120V 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 on the 30A/120V circuit.
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.
This article is cited in the 4th Edition of Harry Box's "Set Lighting Technician's Handbook." 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 for free online at http://www.screenlig...generators.html.
Guy Holt, Gaffer, ScreenLight & Grip, Lightng & Grip Rental & Sales in Boston