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Powering old HMIs on a small Honda Generator


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#1 Guillaume Cottin

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Posted 11 February 2012 - 08:29 AM

Hello,

I will be shooting a night scene and I will have a Honda EU 65 IS rented.
The rental house said it is crystal sync but I am not quite so sure. Anyway even if it's possible to power square wave HMI's on that genny, unfortunately I will be using old magnetic ballasts and I wonder if that may cause a problem since I think I've heard they have less tolerance regarding the quality of the current.

Thank you!
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#2 Guy Holt

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Posted 12 February 2012 - 09:28 PM

I will have a Honda EU 65 IS ... The rental house said it is crystal sync but I am not quite so sure.


The Honda EU65 is not crystal governed – it doesn’t have to be. Crystal engine governors are required in conventional AVR generators to eliminate flicker with magnetic HMI ballasts because the frequency of the AC power they generate is a function of the engine speed. Inverter generators like the EU65 break the link between engine rpm and Hz by taking the raw power produced by the alternator and passes it through a microprocessor controlled multi-step process to condition it. But, rather than using simple two pole cores, the alternators of inverter generators use multi-pole cores and small stators to produce a raw AC power that is multiphase (more than 300 overlapping sine waves), high frequency (up to 20’000 Hz), and upwards of 200 Volts. This high voltage AC power is then converted to DC. Finally the DC power is converted back to low voltage single phase AC power by the inverter. In the process the inverter cleans and stabilizes the power. The end result is AC power that is nearly a pure sine wave with a wave distortion of only 2.5% (which is as clean or cleaner than commercial power), a voltage stability within ± 1%, and frequency stability within ± 0.01 HZ. With power that stable, a crystal governor is simply not needed to eliminate flicker.

I will be using old magnetic ballasts and I wonder if that may cause a problem since I think I've heard they have less tolerance regarding the quality of the current.


Actually the opposite is true. If you don’t have access to the newest Power Factor Corrected (PFC) electronic ballasts, you are better served by using the older magnetic ballasts on an inverter generator (like the Honda EU6500is) over non-PFC electronic ballasts on conventional AVR generators (like the Honda EX5500 or ES6500) with crystal governors. Where this is contrary to the conventional wisdom, allow me to explain some of the advantages to operating magnetic ballasts on inverter generators.

With a frequency variance of only hundredths of a cycle, magnetic ballasts will operate “flicker free” on inverter generators, without the need for costly crystal governors, as long as you shoot at one of the many safe frame rates. Besides the extra bulk and weight of magnetic ballasts, the smaller magnetic ballasts (575-2500W) offer the distinct advantage of being less expensive and drawing less power (13.5A versus 19A for a 1.2kw) once they have come up to speed than the commonly available non-PFC electronic equivalents. Finally, magnetic ballasts will operate more reliably on inverter generators, than non-PFC electronic ballasts operate on AVR generators. The reason being the leading power factor caused by the capacitive reactance of non-PFC electronic ballasts have a more severe effect on the power waveform of conventional AVR generators than do magnetic ballasts on the power waveform of inverter generators.

Of course there are downsides to using magnetic ballasts. One down side is that you are restricted to using only the safe frame rates and shutter angles. But, when you consider that every film made up to the early 1990s were made with magnetic HMI ballasts you can see that being limited to the safe frame rates is not all that restrictive. Another downside to magnetic ballasts is that you can’t load the generator to full capacity because you must leave “head room” for their higher front end striking load. When choosing HMIs to run off portable generators, bear in mind that magnetic ballasts draw more current during the striking phase and then they “settle down” and require less power to maintain the HMI Arc. By contrast, an electronic ballasts “ramps up.” That is, its’ current draw gradually builds until it “tops off.”

For more details on how to operate magnetic HMI ballasts on portable gas generators see an article I wrote for our company newsletter called “Portable Generators in Motion Picture Production”.

Guy Holt, Gaffer, ScreenLight & Grip
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#3 Guillaume Cottin

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Posted 13 February 2012 - 02:37 PM

"Wow".
Thank you Guy for this great, thourough, very informative reply!
I couldn't dream of a better answer.


G.C.
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