Posted 25 November 2010 - 05:28 PM
I had a bit of a problem with a 2.5kw HMI the other night. We were running it from a 4kw generator, 240v supply, and the HMI was all that was running from it. It worked fine for an hour and a half then started pulsing every second or so. After turning the light and ballast off for a few minutes and checking the connections we tried it again and it worked for a while but then we had the same problem.
It was fairly cold, probably around 1 or 2 degrees it was raining a little bit so we had the ballast undercover, with the vent clear.
After trying different cables with that light and having no luck we just packed it away and used other lights.
Any idea what could have caused this? Is it possible it could have been the weather or something we were doing wrong?
Posted 25 November 2010 - 07:15 PM
How tight was the rain cover? Could the ballast been too hot with no ventilation?
Did you check voltage off the genny? Hertz?
When you tried to use other lights do you mean other HMI's or you went to tungsten? Did another 2.5 work fine on the same genny?
Did you have another 2.5 that you could have changed one part at a time to see if that caused it? Change the header, then a ballast and then a head? Was the bulb connections tight? Was there a spare bulb around to try that? How did the original bulb look?
I think it was the light and the genny was not at fault, but without going through some on location trouble shooting steps to narrow down the issues, it will be impossible to figure out.
Edited by timHealy, 25 November 2010 - 07:17 PM.
Posted 26 November 2010 - 03:11 PM
Thanks for the reply.
We only had the one HMI on set so i was unable to try a different ballast, bulb or header.
I made sure the ballast had ventilation so i can't imagine it was overheating.
The other lights we had were all tungsten and we were running a 2kw fresnel and two 800w redheads off the Genny at one point so I would have thought it was at fault.
I guess i'll never know, i just thought maybe someone might have come across something similar.
Posted 26 November 2010 - 06:46 PM
Posted 29 November 2010 - 06:06 PM
HMI's can have so many issues.
No truer words have ever been spoken. Where the ballast did not shut down indicates that the problem was not related to low voltage. And, it is not likely to be related to hertz because we can’t see that type of flicker by eye, but only on a monitor. Given the generator size, it is safe to assume that Simon was using a conventional AVR generator without crystal governor. And, for that reason he was also probably using an electronic ballast - which is why he had problems. He overloaded the 4000W generator with his 2500W HMI.
When you use light sources like HMIs, Fluorescents, & CLF lamp banks, on generators it matters not only what type of generator you use but also what type of ballasts you use. The poor Power Factor and Harmonic Noise that non-Power Factor Corrected electronic ballasts (both HMI & Kino) kick back into the power stream can have a severe adverse effect on the power waveform of some generators, but not others. Under the best of circumstances a 2.5kw HMI will only draw 23 Amps and you will have no problem operating them on a portable generator. Under the worst of circumstances a 2.5kw HMI draw 35 Amps and you will have nothing but trouble operating them on the generator. Why the difference? Because it depends on whether the HMI ballast is Power Factor Corrected (PFC) and whether the generator is an inverter generator or a conventional AVR generator.
The poor Power Factor (PF) of lights that use Switch Mode Power Supplies (Electronic HMI, Fluorescent, & CFL ballasts) can cause them to use excessive amounts of power for the wattage of light they generate and to kick harmonic currents back into the power stream that can have a severe adverse effect on not only the generator, but also electronic equipment operating on the same power. Since PFC is not mandatory in this country as it is in Europe, you will encounter many non-PFC HMI, Kino, CFL, & LED power supplies. And, since the adverse effects caused by a poor Power Factor (PF) go beyond an inefficient use of power, it is well worth understanding PFC because it likely explains Simon Roger’s problem.
To understand PF lets first compare the PF of a CFL bulb, and its’ effect on the power supply, to that of an incandescent bulb. While not as sophisticated as an expensive 2.5kw HMI ballast, the ballasts of CFLs operate on the same basic principle. The AC power supply is first converted to DC by a diode-capacitor circuit and then back to AC by a switch mode converter. The only difference between an HMI ballast and a CFL is the type of AC power waveform the switch mode converter generates. An electronic HMI ballast generates a 60Hz square wave, while a CFL generates a high frequency sine wave. In contrast, an incandescent light is a simple resistive load. The high resistance of its tungsten filament creates heat until the filament glows - creating light. As we see in the oscilloscope shot below of a 25W incandescent bulb, the current is always proportional to the voltage (current is represented on the scope as the voltage drop on a 1 Ohm resistor.)
Current and Voltage Waveform of a ACEC 25W Incandescent bulb
If the applied voltage is sinusoidal, the current generated is also sinusoidal. That is, the current increases proportionately as the voltage increases and decreases proportionately as the voltage decreases. Since the peak of the voltage corresponds to the peak in current, the voltage and current are also in phase and so have a unity PF (PF of 1.)
The voltage and current waveforms below of a CFL bulb are very different from that of the incandescent light above. The most noticeable difference is that the current, generated by the CFL bulb, no longer proportionately follows the nice smooth sinusoidal voltage waveform supplied to it. Rather, it has been distorted by electrical components in the ballast of the CFL bulb so that it instead consists of high amplitude sharp spikes in current that quickly drop off. Also, the peak of the voltage no longer corresponds to the peak in current. The current now “leads” the voltage by 1.7 micro seconds. The voltage and current are no longer in phase as in the case of an incandescent bulb, but instead exhibits what is called a “Leading Power Factor.”
Current and Voltage Waveform of a Brelight 25W CFL Bulb
The distorted current waveform and leading PF exhibited here are also characteristic of non-PFC HMI ballasts because they operate on similar principles. When it converts the AC supply to DC, an electronic HMI ballast likewise uses only a portion of the voltage waveform, draws current in high amplitude quick bursts, and then returns the unused portions as harmonic currents that stack on top of one another, creating harmonic distortion similar to the CFL ( use this link for more details.) As such, the ballast draws more power than it uses to create light. For this reason, non-PFC electronic ballasts draw more current then quartz loads of the same wattage.
If, in the case of a non-PFC electronic HMI ballast, you were to measure the current (using a true RMS Amp Meter) and voltage (using a Volt Meter) traveling through the cable supplying the ballast and multiply them according to Ohm’s Law (W=VA) you would get its’ “Apparent Power.” It is important to understand that this greater Apparent Power consists not only of the high amplitude short pulses of current drawn by the ballast. A non-PFC electronic ballast also returns the unused portion of the voltage waveform into the distribution system as harmonic currents. That is, if you were to, use a wattmeter to measure the actual amount of energy being converted into real work (light) by the ballast, rather than into the generation of harmonic currents, you would get the “True Power” of the ballast. The ratio of “True Power” to “Apparent Power” is a measure of the “Power Factor” (PF) of the ballast. Where a typical 2500W non-PFC electronic HMI ballast takes 35 Amps at 120 Volts to generate 2500 Watts of light the PF is .54 (35A x 120V= 4200W, 2500W/4200W= .59). Which means the ballast has to draw 40 percent or more power than it uses.
The favorite analogy electricians like to use to explain PF is that if apparent power is a glass of beer, PF is the foam that prevents you from filling it up all the way. When using lights with a poor PF (HMIs, Kinos, CFLs, & LEDS), you must size your distribution system and generator to supply the Apparent Power (beer plus foam.) This is where Simon Rogers went wrong, the actual load on his 4000W generator was 4200W rather than the 2500W that he thought.
This greater Apparent Power does not completely explain Simon Roger’s problem because the generator continued to operate under the slightly larger load. The other half of the story is the harmonic currents generated by the ballast. In addition to drawing more power, electronic HMI ballasts also kick harmonic currents back into the distribution system, where they stack on top of one another and, when its’ power is generated by a conventional portable generator, they lead to severe distortion of the voltage waveform in the power distribution system (called “Harmonic Distortion” and illustrated below.)
For example, the power waveform below is typical of what results from the operation of a 2500W non-PFC load (electronic HMI & Kino ballasts) on a conventional portable generator (a Honda EX5500 with a Barber Coleman Governor.) This voltage waveform distortion can cause in explicable operational failures just like the one Simon Rogers experienced ( use this link for more details.)
The pulsed current consumed by electronic ballasts produce voltage distortion in the form of flat- topping.
There is a video on You-Tube by a Lighting Designer by the name of Kevan Shaw that illustrates just this. In his You-Tube Video, “Compact Fluorescent verses the generator,” (available at ) Kevan Shaw compares the effect of equal wattages of CFLs and Incandescent lights on a small portable generator. In his test, he first operates a 575W ETC Source Four Leko with Quartz Halogen bulb on an 850W two stroke conventional gas generator without problem. However, when he tries to operate an equivalent wattage of CFLs (30-18W bulbs) the generator goes berserk. Only after turning off half the CFL Bulbs does the generator operate normally with a remaining load of 15 - 18W CFLs (270 W.) What accounts for the erratic behavior of the generator in this video under a smaller load of CFLs? It is a combination of the poor Power Factor of the CFL bulbs and the harmonic currents they generate.
Even though the 15 CFL bulbs have a True Power of 270W (15 x 18W = 270W ), the Watt indicator on Kevan's generator indicates that they draw twice that in Apparent Power (535W), or have a Power Factor of .5 (270W/535W =.504.) The fact that CFL bulbs consume double the energy (Apparent Power) for the 18 Watts of light (True Power) they generate, is only half the story here also. Kevan Shaw’s video also clearly demonstrates the severe effect that poor power factor loads - like CFLs, HMIs, & Fluorescents - can have on the governing systems of conventional AVR generators.
When Kevan turns off the 18W CFL bulbs one at a time until the generator stabilizes, he is not only demonstrating that 15 – 18W CFL bulbs has roughly the same Apparent Power (535W), according to the generator’s Watt meter, as a 575W incandescent light; but, also that the maximum Leading Power Factor load a 850W conventional generator can operate satisfactorily is 270 Watts (15 – 18W CFL bulbs). Looked at from another angle, 576 Watts of Apparent Power with a Leading Power Factor (16 - 18W CFL bulbs) overloaded the generator, while 575 Watts of Apparent Power with a Unity Power Factor (the 575W Quartz Leko) did not. What accounts for this difference? Since the load is almost the same (576 & 575 Watts of Apparent Power respectively), the only factor that can account for the generator going berserk with the equivalent load of CFL lights is the harmonic currents that they generate, that the Quartz Leko does not. Without a doubt, Kevan Shaw’s video is a clear demonstration of the adverse effect that harmonic currents have on the governing systems of conventional AVR generators.
But that is not all. An even closer analysis of the video also shows that the voltage waveform distortion created by the harmonic currents also affects electronic equipment operating on it. For instance, after turning off 18W CFL bulbs until the generator stabilized, Kevan still does "not get.. all the lamps to illuminate properly." What accounts for the bulbs not illuminating properly even though the generator has stabilized? While the Harmonic Distortion generated by the remaining CFLs is not sufficient to affect the generator governor, it is clearly affecting the CFLs themselves - an indication that, short of affecting the generator's governing system, the voltage waveform distortion generated by harmonic currents will adversely effect electronic equipment operating on the distorted power ( use this link for more details.)
For the same reason that Kevan Shaw was not able to fully operate 270 Watts of CFL bulbs (15–18W bulbs) on his little 850W generator, Simon Rogers was not be able to operate a 2500W HMIs on a conventional 4000W AVR generator. The adverse effects of the harmonic currents that non PFC ballasts generate, so graphically demonstrated in Kevan’s video, limits the total amount of Leading Power Factor loads, as compared to Unity Power Factor loads, that can be reliably operated on conventional AVR generators.
For this reason, the conventional wisdom in the past has been to not load the generator beyond 75% for more than a short period when using a lighting package with low PF (like the pkg. of non-PFC electronic HMI & Kino ballasts depicted above). Where the maximum recommend continuous load on a 4000W generator is 3250W, the de-rated continuous load rating would be roughly 2438 watts. By de-rating the load capacity in this fashion, a Gaffer minimizes the adverse effects of high Harmonic Distortion experienced by Simon Rogers so that both the generator and the loads placed upon it operate more reliably. However, this conventional wisdom no longer holds true if the ballasts are Power Factor Corrected (PFC) and powered by an inverter generator.
Newer electronic HMI ballasts, like the new Arri EB1200/1800 for the 1800W “Baby Max,” incorporate PFC circuitry that increases the PF of a ballast to .98, making it a near linear load. As a result, the ballast uses power more efficiently (which accounts for why the 1800W Baby Max can operate on a 20A circuit), minimizes return current and line noise, and also reduces heat, thereby increasing its’ reliability. For the sake of comparison, a PFC 2.5kw electronic HMI ballast has an Apparent Power of 2760 Watts which means that it would draw approximately 23 Amps (as opposed to 35A) at 120V.
Again, the lower Apparent Power of PFC ballasts is only half the reason they operate more reliably on portable generators. The other half is that PFC circuitry realigns voltage and current and induces a smoother power waveform at the distribution bus.
Left: Conventional generator power w/ pkg. of non-PFC Elec. HMI Ballasts & Kino Flo Wall-o-Lite. Right: Inverter generator power w/ Pkg. of PFC Elec. Ballasts & Kino Flo Parabeam 400.
For example, the power waveform above on the right, is the same 2500W load but with PFC operating on our modified Honda EU6500is Inverter Generator. As you can see, the difference between the resulting waveforms is startling. Even though the load is the same, the fact that it is Power Factor Corrected, and power is being generated by an inverter generator, results in virtually no voltage waveform distortion. What this means is that an inverter generator can be loaded to capacity with PFC HMI and Kino Flo ballasts. The substantial reduction in line noise that results from using PFC ballasts on the nearly pure power waveform of an inverter generator creates a new math when it comes to calculating the continuous load you can put on a portable gas generator. And if the generator is one of our modified Honda EU6500is inverter generators, you will be able to run a continuous load of up to 7500W as long as your HMI and Kino ballasts are Power Factor Corrected.
The Power Factor of electronic ballasts (HMIs & Fluoros) have been vexing set electricians for years. For more detailed information on Power Factor and Harmonic Distortion, I would suggest you read an article I wrote on the use of portable generators in motion picture production. Harry Box, author of “The Set Lighting Technician’s Handbook” has cited my article in the just released Fourth Edition of the handbook. In addition, he has established a link to it from the companion website for the Fourth Edition of the Handbook, called “Box Book Extras.”
If you haven't yet read the article, or looked at it in a while, it is worth reading. I have greatly expanded it to be the definitive resource on portable power generation for motion picture production. Of the article Harry Box states:
"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."
You can log onto the Box Book Extras site at http://booksite.focalpress.com/box/setlighting/ with our pass-code "setlighting." Use this link for my news letter article on the use of portable gas generators in motion picture production.
Guy Holt, Gaffer, SceenLight & Grip, Lighting and Grip Rental in Boston.
Posted 05 December 2010 - 05:28 PM
I couldn't read he titles either.