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Flickering with HMI and generator


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#1 Thomas Worth

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Posted 24 March 2013 - 02:33 PM

I've got a 1200w HMI par with a magnetic ballast and I want to run it off a cheap Coleman 1800w generator. I've tried it and it works, but there's a constant flicker. What are my options? Is there anything I can do to remedy this? I was running the ballast at 60 Hz. I didn't try switching to 50 Hz mode, if that matters. Is there a device I can put inline between the generator and the ballast to filter the power?


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#2 timHealy

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Posted 30 March 2013 - 01:32 PM

I would get a square wave ballast or a genny that is crystal sync. Are you sure the genny was running at 60 hertz? Did you have a meter that measures hertz? The safe hertz shooting range is approx. 59.80 to 60.20 if I recall correctly.

 

I assume you were shooting at 24 frames per second.

 

If not, what are you doing? and in what part of the world?

 

Thanks


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#3 Guy Holt

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Posted 26 April 2013 - 06:19 PM



I've got a 1200w HMI par with a magnetic ballast and I want to run it off a cheap Coleman 1800w generator. I've tried it and it works, but there's a constant flicker. What are my options? Is there anything I can do to remedy this? 

 

 

There is a third option Tim hasn’t mentioned – inverter generators. To power older magnetic HMI ballasts without getting “flicker” the generator must put out a true sine wave with low distortion and stable frequency in addition to stable voltage. Honda's sine-wave inverter generator provide smoother, higher quality power than conventional (non-inverter) generators. With an unparalleled waveform distortion factor of less than 2.5%, the power generated by Honda’s EU series of generators is quite often better than what you get out of the wall outlet. Because it passes the power its generates through an inverter, the Honda EU6500is is rock solid and its’ frequency varies only hundreths of a cycle which eliminates the need for costly crystal governors.

 

There is a popular misconception that you should only use electronic ballasts with portable generators.  Where that is true with conventional generators without crystal governors, it is not true of inverter generators for the reasons above.  In fact, 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 (once they have come up to speed) than the commonly available electronic equivalents. Operating at 120V, a 1.2kw HMI with non-power factor corrected electronic ballast will draw 18-19 amps  verses the 13.5 amps of a magnetic ballast.

 

The downside to magnetic ballasts is when you have little head room - i.e. striking a 1200Watt HMI on a EU2000is with other things plugged into the generator as well.  A magnetic ballasts 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 ballasts “ramps up.”  That is, its’ current draw gradually builds until it “tops off” -  but it “tops off” with a considerably greater draw than a magnetic ballast “settles down” to. A Honda EU2000is will power either.  A magnetic ballast offers the slight advantage that you can power another tungsten or fluorescent lights light on the generator, but only after you have already struck your HMI.For more details see an article I wrote for my company newsletter on the use of portable generators in motion picture lighting. It is available at www.screenlightandgrip.com/html/emailnewsletter_generators.html.

 

Guy Holt, Gaffer, New England Studios, Lighting & Grip Equipment Rental & Sales in Boston


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#4 James Daggy

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Posted 22 September 2013 - 01:20 PM

hi guys

 

I can't really find a solid answer anywhere. I am in similar situation.

 

I have built a 1kw metal halide light with digital ballast. The type used for hydro. etc.

 

Are there any cheap generators that will work with this setup and future setups like this?

 

We haven't got the coin for a honda yet.

I am looking on ebay for something used - http://www.ebay.co.u...etrol generator

 

 

thanks!


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#5 JD Hartman

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Posted 24 September 2013 - 08:45 AM

Problem with the small, cheap generators is voltage and frequency control.  To keep the costs down, you typically won't find any electronic monitoring and feedback control of either, usually a simple mechanical throttle governor will attempt to keep both voltage and frequency within an acceptable range as load increases. As load increases toward generator maximum, both values will suffer, that's probably why you're see flicker from the HMI.  A DMM that measures frequency will prove this.

   Is the Colman rated at 1800w continuous or surge?  A larger generator, a 3K or so may be the answer.  Magnetic ballasts are usually pretty forgiving of voltage and frequency.  What you are feeding it must be very out of range. 


Edited by JD Hartman, 24 September 2013 - 08:46 AM.

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#6 James Daggy

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Posted 29 September 2013 - 08:53 AM

thanks. looks like testing is the only way.


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#7 Guy Holt

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Posted 02 October 2013 - 06:34 AM

A larger generator, a 3K or so may be the answer.  Magnetic ballasts are usually pretty forgiving of voltage and frequency. 

 

Magnetic ballasts are not forgiving when it comes to flicker. The problem with them is that the light intensity of a HMI powered by a magnetic ballast follows the waveform of the supply power and increases gradually until it peaks and then decreases. Since there are two peaks per cycle (+ & - ), the light pulses twice every AC cycle  or 120 times a second (see illustration below. )  This fluctuation in the light output is not visible to the eye but will be captured on film or video if the frequency (Hz) of the AC power is not precisely synchronized with the film frame rate or video scan rate.  If the AC Frequency of the power were to vary, a frame of film or video scan, would receive more or less exposure depending upon the exact correspondence of the film/video exposure interval to the cycling power waveform because the light intensity is pulsating at twice the AC frequency. 


 

HMIMagneticOutput.jpg

The sinusoidal 60Hz current of a magnetic ballast (left) creates a pulsating light output (right) 

 

Electronic square wave ballasts eliminate the potential for flicker by squaring off the curves of the AC sine wave supplying the globe. Squared off, the changeover period between cycles is so brief that the light no longer pulsates but is virtually continuous (see illustration below.) Even if the AC Frequency of the power were to vary, a frame of film or video scan, would receive the same exposure because the light intensity is now not pulsating but nearly constant. In other words, electronic ballasts are “flicker free” because they square off the power sine wave which causes an increase in the duration of the peak level of light output so that the light is on more than it is off.  Electronic HMI ballasts are also called “square wave” ballasts for this reason. The down side to electronic HMI ballasts is that because of their sophisticated electronics they are more expensive and more fragile than magnetic ballasts.

 

HMISquareWaveOutput.jpg

The refined square-wave signal of an electronic ballast (left) creates virtually even light output (right)(Illustrations courtesy of Harry Box

 

So it is not a matter of getting a larger generator, but of getting a different type of generator. Magnetic ballasts will operate reliably on the Honda EU series generators because Honda's sine-wave inverter technology provides much higher quality power than conventional (non-inverter) generators. With a waveform distortion factor of less than 2.5%, the power generated by Honda’s EU series of generators is quite often better than what you get out of the Crawford Studio Units. The power these machines generate is rock solid with a frequency variance of only hundredths of a cycle - which eliminates the need for costly crystal governors. The Honda EU series generators provide true sine wave power with enough frequency stability to power HMI's with magnetic ballasts without flicker at certain safe frame rates and shutters.  I won’t address the issue of flicker and frame rate/shutter angles because it is well established elsewhere in this forum that there are safe windows that are “flicker free” with magnetic ballasts as long as the power supply is stable. As long as you shoot at one of the many safe frame rates, magnetic ballasts are “flicker free” with the Honda inverter generators.

 

portableGenBook.jpg

 

For more detailed information on successfully using Honda portable generators in digital cinema productions, I would suggest you read a white paper I wrote on the use of portable generators in motion picture production that will be available soon as an e-book from the Academy of Production Technology Press (APT.)( http://store.aptxl.c...ctCode=BK-PGMPP)

 


BoxBookLinkGenSetSm.jpg

 

Harry Box, author of The Set Lighting Technician’s Handbook has cited my article in the just released 4th Edition of Harry Box's “Set Lighting Technician's Handbook” (http://www.screenlig...ml/BoxBook.html) and featured on the companion website “Box Book Extras."  (http://www.screenlightandgrip.com/html/BoxBookExtras.html)  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 original white paper is still available online for free at

 

http://www.screenlightandgrip.com/html/emailnewsletter_generators.html.

 

Guy Holt, Gaffer, ScreenLight & Grip, Lightng & Grip Rental in Boston


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