Don't think you'll have any concern about running out of juice. Stay safe, work smart, not fast.
You won’t run out of juice, but there are a few traps that you can easily get caught in if you are not careful.
First, magnetic HMI ballasts will draw more current than you would think. Because of something called Power Factor (PF) a 12kW magnetic ballast will draw more than the 50A that you calculated using Ohm’s Law (W=VA.) In realty, a 12kw magnetic ballast may draw up to 130-140 Amps (65-70 Amps per leg at 208V.) In magnetic HMI ballasts, through a phenomenon called Inductive Reactance, the multiple fine windings of the ballast transformer induces considerable current, called Reactive Power, that is in opposition to the primary current, causing the primary current to lag behind voltage, a reduction of current flow, and an inefficiency in the use of power. Put simply, the ballast draws more power than it uses to create light.
If, in this situation, you were to measure the current (using a Amp Meter) and voltage (using a Volt Meter) traveling through the cable supplying the magnetic HMI ballast and multiply them according to Ohm’s Law (W=VA) you would get the “Apparent Power” of the ballast (expressed as KVA.) But, if you were to instead, use a wattmeter to measure the actual amount of energy being converted into real work (light) by the ballast, after the applied voltage overcomes the induced voltage, you would get the “True Power” of the ballast (expressed as KW.) The difference between Apparent Power and True Power, or the additional power required for the applied voltage to overcome the induced voltage, is the Reactive Power (expressed as KVAR.) The ratio of True Power to Apparent Power is called the “Power Factor” of the ballast.
The favorite analogy electricians like to use to explain these terms is that if Apparent Power is a glass of beer, Reactive Power is the foam that prevents you from filling it up all the way, so that you are left with less beer or Ture Power. In other words, the thirst-quenching portion of your beer is represented by KW in the figure above. The foam is represented by KVAR. The total contents of your mug, KVA, is this summation of KW (the beer) and KVAR (the foam). In our beer mug analogy, Power Factor (P.F.) is then the ratio of Beer (True Power) to the entire volume of the mug (beer plus foam or Apparent Power.)
Thus, for a given KVA: the more foam you have (the higher the percentage of KVAR), the lower your ratio of KW (beer) to KVA (beer plus foam). Thus, the lower your power factor. Or, the less foam you have (the lower the percentage of KVAR), the higher your ratio of KW (beer) to KVA (beer plus foam). In fact, as your foam (or KVAR) approaches zero, your power factor approaches 1.0. When lights with a low power factor are used, a generator must be sized to supply the apparent power (beer plus foam), even though only the beer (true power) counts as far as how much actual drinking is possible.
Our beer mug analogy is a bit simplistic. In reality, when we calculate KVA, we must determine the “vectorial summation” of KVAR and KW. Therefore, we must go one step further and look at the angle between these vectors.
To understand this concept let’s use the analogy of a man dragging a heavy load as illustrated above. The man’s Working Power (or True Power) in the forward direction, where he most wants his load to travel, is KW. Unfortunately, the man can’t drag his load on a perfect horizontal (he would get a tremendous backache), so his shoulder height adds a little Reactive Power, or KVAR. The Apparent Power the man is dragging, KVA, is this “vectorial summation” of KVAR and KW. The “Power Triangle” below illustrates this relationship between KW, KVA, KVAR, and Power Factor:
In an ideal world (one without gravity), the man wouldn’t have to waste any power along his body height and so the KVAR would be very small (approaching zero.) KW and KVA would be almost equal and so the angle (formed between KW and KVA) would approach zero and the Cosine would then approach one. Power Factor would then approach one. For a light to be considered “efficient”, the Power Factor should be as close to 1.0 as possible. Where a typical 12kW magnetic HMI ballast draws 130-140 Amps to generate 12000 Watts of light (KW), the Power Factor is .74 (PF = KW/KVA=12000W/16200W= .74). In other words, a 12kW magnetic ballast wastes roughly 25% of the power that it uses in Inductive Reactance.
A second 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, which is different than their higher reactive load discussed above. 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.”
Users of 2.5kw HMIs on portable generators constantly make this mistake. Even though a 2.5kw magnetic ballast draws approximately 26 amps they find that they do not run reliably on the 30A/120V twist-lock receptacle on putt-putt generators. 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 2.5kw magnetic ballast will climb to upward of 30 Amps. Add to that the higher striking load and you begin to understand why the breaker trips. To make matters worse, 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 except on the generator’s 240V outlet using a 240V-to-120V step-down transformer/distro like the one we make for the Honda EU6500s & EU7000s.
For these reasons, I would not put any other loads on the two power legs supplying the 12kw ballast. If you have additional lighting loads on the same leg, and no head-room, the higher striking load of a 12kW magnetic ballast may push the entire load over the breaker’s threshold. If you must add additional lighting loads to the same legs, always strike the 12kW magnetic ballast first and wait until it settles down before turning on any other lights on those two legs.
Finally, magnetic ballasts in general 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.
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 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 refined square-wave signal of an electronic ballast (left) creates virtually even light output (right)(Illustrations courtesy of Harry Box
So the last downside to using magnetic ballasts is that you are restricted to using only certain safe frame rates and shutter angles (use this link to tables of safe speeds.) 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.
For more detailed information on using magnetic HMI ballasts on portable generators, 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.)
Harry Box, author of The Set Lighting Technician’s Handbook has cited my article in the 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 original white paper is still available online for free at http://www.screenlig...generators.html.
- Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rental & Sales in Boston