Powering HMIs & Kinos with batteries for Night Shoot

[Isabelle Landers]

I am shooting a night scene on a back country road. We can't afford a movie blimped tow generator and I am afraid a portable generator will be too loud. What are my options for powering HMIs and Kinos on batteries. I have looked at a number of different types of inverters - from boat inverters to Dewalt construction inverters - but don't know which is right or what they can handle. Any tips would be greatly appreciated.

- Isabelle Landers,

 

[Colin Sheldon]

The Honda 2000 generator is pretty quiet and inexpensive. Put it in the ditch a hundred feet from set and you're good.

As for inverters, the type doesn't really matter, all you need to look at is how many watts it can handle. Most of them are in the 400 range which will run some kinos, but not a very large HMI. I have used 1200 watt inverters, but they are attached directly to a car battery and the vehicle must remain running.

Personally, I would just go with a generator.

 

[Steve G]

In terms of batts, you will find it hard to use anything bigger than a 400 watt hmi. You will also need a lot of batteries for a long night shoot. The small honda generators are pretty good for low budget shoots. You can think about long extension cables from the generator...you may get a bit of voltage drop, but should be fine. Do some tests with various lengths. Think about shooting with the genny noise, then switching it off and doing a voice only take straight after you have a take you are happy with visually. You can then drop it in during your edit. You would be surprised how often this can work for low budget films. For the wide shots, it is certainly no problem to do the dialogue this way. For the tight shots, you can also think about baffle cloths to help mask the generator noise. With careful placement, you can get the cloths in quite close without affecting the lighting. Your sound op can also get the mike in really close during the cu's.

 

[Mark Collins]

I've run into this scenario before, except that we were in a forest shooting at night. What we did was run as much AC as we could to put the generator far enough away. Then we put as many sound blankets as we could around it, leaving a hole at the top for exhaust. It worked fairly well and our audio was pretty clean. The generator was maybe 100ft away at most.

I've also used the Motomaster Eliminator to power lights before, and it works fine, so long as you aren't planning on running anything big. For lighting wide shots it's extremely impractical as you can't run anything big at all. For close ups it's just fine though.

The other thing you can do, if you have access to it is use the Rosco Pro Gaffer LED kit or something similar. They don't use a lot of energy yet throw a lot of light. I know the kit I have access to has batteries with it, which work well in situations like this.

 

[Isabelle Landers]

The other thing you can do, if you have access to it is use the Rosco Pro Gaffer LED kit or something similar. They don't use a lot of energy yet throw a lot of light. I know the kit I have access to has batteries with it, which work well in situations like this.

Thanks for the suggestions but I have to light the background in wide shots because a charater has to be seen moving in the woods along side the road. In my experience LEDs don't have that kind of punch. Someone suggested Day-for-night, but it is not an option because the car lights figure heavily into the lighting design and they do not read on day-for-night. We tried shooting some test footage with a Honda ES6500 and found that no matter where we put the generator we still heard it. Which brings me back to either renting a movie blimped tow genertor (not in the budget) or using inverters on batteries. I think I can get by with a 1200 Par HMI (a 2500 would be better) and the headlights for the background and a couple 4' 4 Bank Kinos for talent keys around the car. I would go with smaller Kinos but their light drops off so quickly that I found on the test shoot that only a 4' 4 bank gave us the level we needed. The largest True Sine Wave inverter I have found is an 1800W with a load capacity of 15amps. The problem is that all the 1200 Par Electronic ballasts I have looked at draw 18-19 Amps which is too much for the inverter and the car lighter socket. And I still need power for the Kinos. I have found larger modified square wave inverters but don't know if the HMI or Kinos will work on them. Any help would be greatly appreciated.

- Isabelle Landers

 

[Nick Gardner]

Remember that you can run any AC tungsten unit off of DC at the correct voltage (10 12v boat batteries equal 120v) and they make 12v par 36 bulbs for 9 lights and such. But when you start talking about pallets of batteries, you might as well just rent a generator.

Nick

 

[David W. Jones]

Thanks for the suggestions but I have to light the background in wide shots because a charater has to be seen moving in the woods along side the road. In my experience LEDs don't have that kind of punch. Someone suggested Day-for-night, but it is not an option because the car lights figure heavily into the lighting design and they do not read on day-for-night. We tried shooting some test footage with a Honda ES6500 and found that no matter where we put the generator we still heard it. Which brings me back to either renting a movie blimped tow genertor (not in the budget) or using inverters on batteries. I think I can get by with a 1200 Par HMI (a 2500 would be better) and the headlights for the background and a couple 4' 4 Bank Kinos for talent keys around the car. I would go with smaller Kinos but their light drops off so quickly that I found on the test shoot that only a 4' 4 bank gave us the level we needed. The largest True Sine Wave inverter I have found is an 1800W with a load capacity of 15amps. The problem is that all the 1200 Par Electronic ballasts I have looked at draw 18-19 Amps which is too much for the inverter and the car lighter socket. And I still need power for the Kinos. I have found larger modified square wave inverters but don't know if the HMI or Kinos will work on them. Any help would be greatly appreciated.

- Isabelle Landers

Purchasing an inverter/battery setup to provide enough amp hours for your lighting requirements is going to be much more expensive than simply renting a proper generator or two.
My suggestion, and I know you don't want to hear this, is to rethink the scene location. Move the scene to a place where proper power is available.
Honestly, how much of the background surroundings are you going to be able to see lit with a 1200w par at night.

 

[Steve G]

If you don't want to go down the road of wild tracking the dialogue, then all you can do is hire a quiet genny or find a location with mains power. Batts, as mentioned above, will be more expensive than a film genny.

If you are looking to backlight your scene, a 1.2 hmi will not be enough. I used a 12k par on a shoot recently, and even that was barely enough for a large area.

 

[David W. Jones]

Steve is spot on with his reply.

 

[Isabelle Landers]

Powering HMIs & Kinos with batteries for Night Shoot

If you don't want to go down the road of wild tracking the dialogue, then all you can do is hire a quiet genny or find a location with mains power. Batts, as mentioned above, will be more expensive than a film genny.

If you are looking to backlight your scene, a 1.2 hmi will not be enough. I used a 12k par on a shoot recently, and even that was barely enough for a large area.

We had originally planned for an 18k but the project funding did not come through, and we are pushing ahead with what we can afford. We had originally planned to rent a tow generator, but now can't afford the additional costs involved in renting towing it. Since Ryder, Budget, U-Haul, and Penske don't equip their trucks to tow we have no alternative but to get the rental house to tow it on one of their grip trucks. So rather than renting a box truck and shelving it out, we have to rent the rental house grip truck. And instead of driving the truck myself, we have to hire the company grip/driver to drive the truck for insurance reasons. At the day rate they want, the grip/driver would be the highest paid member of the crew. Where the cost of renting a movie tow plant is much more than just the cost of the generator we are forced to scale down.

I realize a 1200 Par is not going to go far so I had planned to use the car headlights to work the background as well. A 2500 Par would be better but I don't see how we can power it. I don't see how we can power a 1200. Moving to a location with a power drop is unfortunately not an option because the whole reason we are shooting there is a natural rock formation that is part of the story.

- Isabelle Landers

 

[Matt W.]

The honda inverter generators are cheap and surprisingly quiet. I've used the 6500 and 3500 and they're great and reliable, though the 6500 gets louder at full load. We've run hmis (magnetic ballasts and electronic ballasts), tungsten lights, and kinos reliably off the one generator so the sine wave is good. You can rent a U-Haul filled with blankets or something and run the generator inside the van with one door open facing away from camera as a sort of "blimp" and as a way to move the generator. Rental rates really vary with these things, so shop around. You can run a 2.5k and a 1.2k plus more, or a 4k and a little more off a single 6500.

You need more than one HMI to light a night ext.--I'd say one background light, one key light, and fill at least. Car headlights are useless, particularly due to color temperature and the fact that they'll run your car battery down. I've never heard of running anything except a "sun gun" off batteries.

I've had limited experience with them, but I would rent a dSLR for night scenes--but test first. The 7D looks nice and seems useable to 3200ISO. Red is like 320ISO and so with a 7D you've gained gained 3+ stops; now your 1.2k is almost a 12k. Shoot at f2 you'll be okay. Just be careful with exposure since you're not shooting raw anymore.

Try to rent at least a 3500 generator, maybe two 1.2k pars (one to 3/4 backlight talent the other to sidelight the background through a camo net), a kino four bank, and whatever else you think you might need. (Do you have flashlights/torches? Then some tungsten lights.) Many rental places do three-day weeks so that might be a $1,500 rental, and you can try to schedule your feature around one week of night ext. and then return the gear before the rest of the shoot. Other cheap/nice daylight fixtures include the blue photofloods, which are 4800K and great in china lanterns to light the foreground (with a 1.2k or brighter as your background light), but they can explode.

 

[Steve G]

We had originally planned for an 18k but the project funding did not come through, and we are pushing ahead with what we can afford. We had originally planned to rent a tow generator, but now can't afford the additional costs involved in renting towing it. Since Ryder, Budget, U-Haul, and Penske don't equip their trucks to tow we have no alternative but to get the rental house to tow it on one of their grip trucks. So rather than renting a box truck and shelving it out, we have to rent the rental house grip truck. And instead of driving the truck myself, we have to hire the company grip/driver to drive the truck for insurance reasons. At the day rate they want, the grip/driver would be the highest paid member of the crew. Where the cost of renting a movie tow plant is much more than just the cost of the generator we are forced to scale down.

I realize a 1200 Par is not going to go far so I had planned to use the car headlights to work the background as well. A 2500 Par would be better but I don't see how we can power it. I don't see how we can power a 1200. Moving to a location with a power drop is unfortunately not an option because the whole reason we are shooting there is a natural rock formation that is part of the story.

- Isabelle Landers

Well, you are in the position of having to say to the director that the location can't work with the resources that you have access too. This is not being negative. It is just part of the job of being a DoP.

 

[Guy Bryan Holt]

Powering HMIs & Kinos with batteries for Night Shoot

I've never heard of running anything except a "sun gun" off batteries.

You can run a 1200 Par along with a couple of Kinos on a "True Sine Wave" 1800W inverter if the ballasts (both HMI & Kinos) are Power Factor Corrected. Not just any DC-to-AC inverter will do when powering fluorescent and HMI fixtures. You have to be really careful when choosing a inverter for film production because there are three basic types of inverters and not all of them are suitable for production applications. Since a blanket assertion like this would not satisfy me either, I will explain in detail why it matters that you use only "True Sine Wave" Inverters and Power Factor Corrected ballasts.

But first I should mentions that, my recommendations are based upon extensive research I have done on the use of portable gas generators in motion picture production. For this research, I ran a series of tests in order to analyze the interaction of the different type of portable †generators with the prevalent light sources available today. Since inverter generators use the same three types of inverters, the findings of my research are applicable to stand alone DC-to-AC inverters designed for use with batteries as well. The results of my tests are going to be cited †in the upcoming 4th edition of the "Set Lighting Technicianís Handbook." I have also compiled the results in an article for my company newsletter and it is available on our website.

Waveform of power output by conventional generator. Note the frequency error and noticeable distortion (ILLUSTRATION COURTESY OF KIRK KLEINSCHMIDT]​

To understand how the application is the same it may help to review why inverters are used in generators in the first place. Even though a conventional generator makes a pretty decent AC power sine wave, it is considered “dirty” power. Measured on an oscilloscope (pictured above), its’ sine wave appears jagged. Those small spikes in the sine wave indicate harmonic distortion that can cause problems for sophisticated electronics, like video cameras, monitors, computers, and hard drives that need a clean sine wave to operate. With the increasing use of personal computers and microprocessor-controlled recording equipment in motion picture production, there is a greater demand for clean, reliable power on sets.

Step 1: Rectifier Bridge converts multi-phase AC power to rectified sine wave. Step 2: rectified sine wave is flattened to DC. Step 3: micro processor switching alternates wave polarity creating a modified square wave. (ILLUSTRATION COURTESY OF HARRY BOX]​

Inverter generators meet this demand for cleaner power by adding an additional step that completely processes the “dirty” AC power from the generator’s alternator. An inverter module takes the raw power produced by the generator's alternator and passes it through a microprocessor controlled multi-step process to condition it. But, rather than using the simple two pole cores of conventional genertors, 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 an inverter. In the process the inverter cleans and stabilizes the power.

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. By using a microprocessor to convert AC power to DC, and then an inverter module to convert the DC back to AC, inverter generators generate power that is independent of engine speed and rock solid with a frequency variance of only hundredths of a cycle - which eliminates the need for costly crystal governors. In this limited sense, it can be said that inverters, whether for batteries or built into a generator, are "actually better for use with the HMI electronics." In fact, only one of the three types of inverters are suitable to power HMI lights.

The three types of inverters are, "true sine wave", "modified square wave" (also known as "modified sine wave"), and "square wave." One might wonder why there are so many types of inverters. As John De Armond, explains in his informative article "The “Hows” and “Whys” of Inverters and Inverter Generators" the primary reason is cost. To paraphrase John's article, to make a nice sine wave from DC power (whether generator power or battery power) is expensive. There is a trade-off between cost and waveform purity. An approximation of a sine wave may be created by outputting one or more stepped square waves with the amplitudes chosen to approximate a sine (a "modified square wave" inverter). The more steps, the more like a sine wave the output (a "true sine wave" inverter). However, each of the voltage steps requires its own voltage supply, its own transistor switch, plus the necessary control circuitry. The bottom line is that the more steps, the more expensive the inverter. The two go hand in hand.

Ideal Sine Wave (black), Single Step Square Wave (blue), Three Step Square Wave (red)( ILLUSTRATION COURTESY OF JOHN DE ARMOND)​

Take a look at the figure above. The black trace is a pure AC sine wave. The blue wave is a single step approximation or square wave. The red wave is a three step wave or simple modified square wave. As is intuitive, the three step wave produces a closer approximation of a sine wave and thus will satisfactorily operate more devices than the single step one. The trade-off is cost and complexity

Switch sequence of three step output stage of a modified square wave inverter. ( ILLUSTRATION COURTESY OF JOHN DE ARMOND)​

The figure above is a line drawing of a typical three step output stage of a simple modified square wave inverter. The voltages V1 through V3 are increasingly higher DC voltages. A microprocessor generates this pseudo sine wave by sequentially switching S1 through S3 on, S3 through S1 off, S4 through S6 on, S6 through S4 off. It repeats this 60 times a second. Where each of the voltage steps requires its own voltage supply, its own transistor switch, plus the necessary control circuitry, one can intuit that the more steps in the modified square wave, the more complicated and thus more expensive the inverter. Where it is less expensive to make a modified square wave that will satisfactorily operate most construction equipment and RV appliances, than it is to make a true sine wave there is not the cost/benefit return to warrant the incorporation of more switches in inverters manufactured for these markets. This is why there are still three types of inverters available on the market to this day for use with batteries and built into generators.

Square wave (SW) inverters will run simple things like tools with universal motors with no problem, but not much else. For this reason, SW inverters are now found only in the construction trades, where they offer the benefit of being cheaper. For the reasons detailed below, SW inverters have no application in motion picture production.

“Modified Sine Wave”, “Psuedo Sine Wave”, and “Cycloconverter” are all sales terms used for a modified square wave type of AC power generated by inverters with more switching capacity. Modified square wave (MSW) inverters are low in cost and will satisfactorily operate almost all common household appliances and power tools. For this reason, MSW inverters are the most prevalent and the ones typically used in the economy RV/Residential Standby and Industrial lines of inverter generators. Unfortunately, they also are not suitable for use in motion picture production except for the powering of quartz lights. Since I am out of space, I will have to explain in my next post why MSW Battery Inverters and Inverter Generators are not suitable for HD digital cinema productions.


-Guy Holt, Gaffer, ScreenLight & Grip , Boston

 

[Guy Bryan Holt]

I've never heard of running anything except a "sun gun" off batteries.

Cont. from above

MSW Battery Inverters and Inverter Generators are not suitable for HD digital cinema productions because a modified square wave will cause sensitive electronic equipment (computers, hard drives, video cameras) to overheat, and will cause electronic HMI and Fluorescent light ballasts to kick harmonic noise back into the power stream. Furthermore, electrical components that depend on the peak value of the voltage waveform to operate effectively (the diode-capacitor power supplies in computers, hard drives, and electronic HMI ballasts, as well as the bridge rectifiers of battery chargers) will not operate effectively on a modified square wave. In his article "The “Hows” and “Whys” of Inverters and Inverter Generators," John De Armond explians why using the example of a battery charger powered by a rudimentary MSW inverter generator - a Honda EX350.

Output waveform of a Honda EX350 square wave inverter generator ( ILLUSTRATION COURTESY OF JOHN DE ARMOND)​

The illustration above is of an oscilloscope shot of the modified square wave of a Honda EX350 inverter generator. The output of a MSW battery inverter would be almost identical. Notice the Root Mean Square (RMS) voltage indication (on the right side) is 120 volts even though the peak voltage is only 142 volts. A true sine wave with a RMS voltage of 120 would have a peak voltage of 169 volts (120 x 1.414 = 169 volts.) This difference in peak voltages is what makes MSW inverters unsuitable for motion picture production applications. This becomes apparent when we compare the operation of a battery charger on the true sine wave of grid power to its operation on a modified square wave of an MSW inverter.

A battery charger typically consists of a step down transformer, a rectifier and support electronics like charge control circuitry. On each half-cycle of the 60 hz line voltage, the voltage first increases and then decreases in the shape of a sine. The transformer secondary of the battery charger follows this voltage. Connected to the secondary is the rectifier that converts the AC to DC for battery charging. Only when the instantaneous AC voltage exceeds the battery voltage plus the 0.7 voltage drop of the rectifier does current flow to charge the batteries. Photo 5 illustrates this effect. The two lines at “1” and “2” mark on the voltage sine wave where the rectifier starts conducting and causing current to flow.

( ILLUSTRATION COURTESY OF JOHN DE ARMOND)​

Problems arise when a charger of this type is powered by a modified square wave regardless if it is generated by a battery inverter or inverter generator. Recall from the first photo above that the peak voltage of a modified square wave does not rise as high as a sine wave (142 volts verses the 169 volts of a true sine wave.) The horizontal line in the photo above shows about where the modified square wave would reach. In this particular case, the modified square wave would never reach a voltage sufficient to make the rectifier conduct and so the battery would never charge even though power is connected, the LED indicators light up, and a true RMS voltmeter would indicate about 120 volts. Problems also arise when a modified square wave is used to power the electronic ballasts of HMI and fluorescent lights. This becomes apparent when we compare the power waveform induced by the operation of a Quartz Incandescent Light (an Arri 300) to that of a fluorescent fixture that uses Compact Fluorescent Lamps (CFL) operating on MSW inverter (the same Honda EX350.).

Voltage and the current output waveforms of a Honda EX350 square wave inverter generator powering 300W incandescent light ( ILLUSTRATION COURTESY OF JOHN DE ARMOND)​

The illustration above is of a scope shot of both the voltage and the current output of the Honda EX350 generator driving a 300 watt Quartz Incandescent light (a resistive load.) As you see, a modified square wave works well for a resistive load like an incandescent light. The current waveform rises and falls with the voltage waveform (they are in phase) and the inherent distortion of the modified square wave does not induce distortion of the current. Things get a whole lot more interesting when we connect a Compact Fluorescent Lamp (CFL) to the modified square wave. As you can see in illustration below the fluorescent lamp creates all kinds of current oscillation that slightly distorts the voltage waveform (creates a spike).

Voltage and the current output waveforms of a Honda EX350 square wave inverter generator powering fluorescent light ( ILLUSTRATION COURTESY OF JOHN DE ARMOND)​

Electronic ballasts (both fluorescent & HMI) utilize solid state electronic components (rectifiers, capacitors, and IGBTS) that use only portions of the power waveform - they place all their load on the peak values of the voltage waveform. These devices then return the unused portions to the distribution system as a harmonic current. These harmonic currents stack on top of one another in the distribution system creating harmonic distortion that pulls the voltage and current out of phase. Under certain conditions, these harmonic currents that are kicked back into the power stream by the electronics of the ballast can cause a distortion of the voltage waveform (the spike in the waveform above) that manifests itself as a noticeable audio buzz or equipment to malfunction. For a detailed explanation for why harmonic currents cause voltage waveform distortion see my article on the use of portable generators in motion picture production available on our website.

Fluorescent lights are a good choice for operation on battery inverters in the limited sense that they use a quarter of the power of a comparable tungsten light. However, the ballasts of the many fluorescent fixtures, like the Kino Flo fixtures that use the T-12 tubes (the Single, Double, and 4 Bank Fixtures, the Wall-o-Lite, Flathead 80, and the Image 20, 40, & 80 fixtures) and fluorescent fixtures that use Compact Fluorescent Lamps (CFLs) return harmonic currents into the power stream. For this reason, Kino Flo cautions users, on their website: "Kino Flo ballasts ... will draw double the current on the neutral from what is being drawn on the two hot legs. On large installations it may be necessary to double your neutral run so as not to exceed your cable capacity."( FAQ "Why is the neutral drawing more than the hot leg" at http://www.kinoflo.com/FYI/FAQs.htm#2) For a detailed explanation for why harmonic currents cause unusually high neutral returns see my article on the use of portable generators in motion picture production available on our website.

When you plug a single 4' - 4 Bank Kino into a wall outlet you need not be concerned about harmonic currents causing voltage waveform distortion. The impedance of the electrical path from the power plant is so low, the distortion of the original voltage waveform so small (1-3%), and the plant capacity so large in comparison to the load of the one light, that the inherently noisy load of the 4'- 4 Bank Kino will not affect the voltage at the distribution bus. Since I am out of space, I will have to explain in my next post how it is a different situation when plugging fluorescent fixtures into the inherently distorted voltage waveform of MSW inverters or conventional portable generators.


Guy Holt, Gaffer, ScreenLight & Grip , Boston

 

[Guy Bryan Holt]

I've never heard of running anything except a "sun gun" off batteries.

Cont. from above

When you plug a single 4' - 4 Bank Kino into a wall outlet you need not be concerned about harmonic currents causing voltage waveform distortion. It is, however, an all together different situation when plugging fluorescent fixtures into the inherently distorted voltage waveform of MSW inverters (as seen above) or conventional portable generators (as we will see below.) If we compare the oscilloscope shots below of the voltage waveform induced at the power bus by a Kino Flo Wall-0-Lite (with 10 - 4' T-12 Tubes) operating on grid power (left), power generated by a conventional generator (middle), and power generated by a Pulse Width Modulated (PWM) Inverter generator to the same power sources without a load, we see clearly that the degree of voltage waveform distortion caused by the return of harmonic currents by the Kino Flo T-12 ballasts is directly proportional to the degree of distortion of the inherent power waveform.

Left: Grid Power w/ no load and a THD of less then 3%. Center: Conventional Generator w/ no load and a THD of 17-19%. Right: Inverter Generator w/ no load and a THD of 2.5%.​

The fact that the original supply voltage waveform of conventional generators is appreciably distorted (a THD of 17-19%) to begin with (see middle frame above) causes harmonic currents returned by an electronic Kino ballasts to induce significant waveform distortion of the voltage in the distribution system. The magnitude of current and voltage waveform distortion depends upon the quality of the original applied power waveform and the relative size of the electronic load with respect to the source impedance and capacity of the power system. That is, the amount of voltage distortion increases as distortion of the applied waveform increases and the percentage of electronic ballasts taking up the total capacity of the power system increases.

Left: Grid Power w/ Kino Flo Wall-o-Lite. Center: Conventional AVR Power w/ Kino Flo Wall-o-Lite. Right: Inverter Power w/ Kino Flo Wall-o-Lite.​

This is evident in the oscilloscope shots below of an Arri 1200 HMI Par Plus with standard Arri electronic ballasts. The adverse effects of the severe voltage waveform distortion exhibited in the power of the conventional generator below, can take the form of overheating and failing equipment, efficiency losses, and circuit breaker trips. This is because, as we saw above, circuits that depend on the peak value of the voltage waveform to operate effectively (the diode-capacitor power supplies in computers, hard drives, and electronic ballasts), will work sporadically, if at all, on a squared off wave form whether it is created by harmonic currents distorting the voltage waveform (above) or created by the switching of a MSW battery inverter. Common symptoms are computers locking up and other operational malfunctions that are unexplainable.

Left: Grid Power w/ 1.2Kw Arri non-PFC Elec. Ballast. Center: Conventional AVR Power w/ 1.2Kw Arri non-PFC Elec. Ballast. Right: Inverter Power w/ 1.2Kw Arri non-PFC Elec. Ballast.​

On top of that, the excess part of the wave (the shaded area in the diagram below) must be dissipated somehow. This comes in the form of heat. The bigger the current draw from the unit, the more it produces excess heat within the unit that was not factored for in its' original design. Extended exposure to square wave power supplies, and the heat it generates, may eventually cause premature component level failures within the unit. Harmonic distortion of the magnitude above can also create ground loops and radio frequency (RF) interference. For a detailed explanation for why this is, see my article on the use of portable generators in motion picture production available on our website.

Unuseable portion of distorted waveform (shaded) dissipated in heat.​

For these reasons, audio/video production equipment, computers, electronic ballasts, and battery chargers require a nearly pure (low distortion) true sine wave input. If these devices are to be run from a battery inverter or an inverter generator, then the inverter must supply a sine wave or something pretty close to it. As discussed at the outset, inverters of this sophistication are appreciatively more expensive - from 2 to 3 times - because of the number of and prohibitive cost of high power electronic switch devices and components required. However, recent rapid developments in the field of IGBT (Insulated Gate Bipolar Transistors) electronics and miniaturization/mass production of microprocessor based digital control systems have reached the stage that Pulse Width Modulation inverter modules are now economically viable and affordable. Since I am out of space, I will have to explain the benefits of Pulse Width Modulation in my next post.


Guy Holt, Gaffer, ScreenLight & Grip , Boston

 

[D Fuller]

I'm not sure if this would change the economics you described above, but does anyone on the crew have a pickup capable of pulling the generator?

 

[Matt W.]

This is really above my head since I'm not a gaffer and have no training as one, but it makes sense. I was on a set once where we couldn't get a kinoflo running off an inverter hooked up to a car battery (because it was sawtooth power) and I've noticed that kinos seem to draw more amperage on generators than from 110V AC wall power.

Out of curiosity, what kind of inverters do the honda generators have? I've successfully run jokers off them no problem, and also had no problem striking HMIs with magnetic ballasts. Can generators (specifically the honda inverter generators) damage magnetic ballasts (or possibly bulbs)? Or is the worst damage that they can do (assuming the light strikes) is to cause out-of-phase flicker? With an electronic ballast running kinos and electronic-ballasted hmis you might damage the ballasts simply due to the heat generated by the harmonic resonance they create in the sine wave? Is this correct? So for good inverter power a magnetic ballast might be best (second to a power-corrected electronic one) as it introduces less distortion into the sine wave?

Also, realistically, what damage can be done from not using power-corrected ballasts? If the camera is running off batteries and the distortion is minor (a 1200w magnetic hmi and a few kinos on a 6500w honda generator), what are the real-life repercussions of such a set up? Sorry for my ignorance.

Edit: The parabeam looks great, by the way.

 

[Guy Bryan Holt]

I've never heard of running anything except a "sun gun" off batteries.

Cont. from above

Pulse Width Modulation (PWM) inverters provide a more sinusoidal current and for that reason are commonly called True Sine Wave inverters. PWM inverters use extremely fast micro-processor control modules to switch Insulated Gate Bipolar Transistors (IGBTs) at extremely high speeds to produce AC power with a "true" sine wave (with full width and amplitude) from DC power (whether from batteries or the rectifier bridge of an inverter generator.)

Pulse Width Modulated (PWM) Voltage Steps(ILLUSTRATION COURTESY OF SIEMENS CORP) ​

In the case of "True Sine Wave" inverter generators, the PWM inverter module consists of a converter, DC link, control logic, and an inverter. The converter section consists of a fixed diode bridge rectifier which converts the more than 300 three phase AC sine waves generated by the multi-pole core of the generator's alternator to high voltage DC power. AC Output is then generated from the high voltage DC by the inverter section with voltage and frequency set by a PWM control logic. A highspeed microprocessor switches IGBTs on and off several thousand times a second according to the PWM control logic to create a variable voltage and frequency.

Waveform of power output of PWM inverter generator. Note there no discernable distortion or frequency error. (ILLUSTRATION COURTESY OF KIRK KLEINSCHMIDT]​

The "true" sine wave that PWM inverters deliver is more suitable for computers, solid-state equipment with built-in computer functions or microcomputer-controlled functions. Not as cheap as MSW inverters, generator manufacturers only put PWM inverter modules in their deluxe or Super Quiet product lines. For instance, the Honda super quiet EU series of generators employ Pulse Width Modulation (PWM) inverter modules with a waveform distortion factor of less than 2.5% - which is considerably better than conventional generators and quite often better than what you get out of the wall outlet.

Even though "True Sine Wave battery" inverters generate clean stable power, they have limited application in motion picture lighting because they are only capable of limited power output (1800W max.) The amount of power they can generate from a car battery through the lighter socket is further limited by the wiring of the 12V car lighter socket. That is because when voltage goes down, amperage goes up. Wire that carries 12V DC has to be sized considerably larger then wire that carries 120V AC for the same load (wattage.) For instance to supply 12 volts to a 1800W inverter requires 2 ought feeder cable to the car's battery. Also the car's alternator has to be large enough to take the load without burning out (most are not.) The wiring of a car cigarette lighter circuit is typically only sufficient to power a 300W inverter. This is enough power to operate a few fluorescent fixtures, but for the reasons cited above you would want to make sure to use a "True Sine Wave" Inverter.

Exo-skeletal pipe rig on shuttle to rig lights and mount 750W batt-verter on front (covered for rain protection) ​

A better alternative to using a car's lighter socket is a Battery/Inverter set up that is commonly called a "Batt-verter." A "Batt-verter" system consists of one or more deep cycle 12V batteries (usually Marine Cells), a 12V DC-to-120V AC "True Sine Wave" Power Inverter, and a 12V Battery Charger. Batt-verters can work great for traveling car shots but offer limited capacity and run time. Your run time will depend on your load and how many batteries you wire in paralell. Here are some production stills that show you two Batt-verter systems we built to run Kinos to light the inside of an airport shuttle bus for the feature "Shuttle." The first is a 750W "Batt-Verter" rig wired into in a Calzone case and mounted on an Exo-skeletal pipe rig that also held the Kino Flos.

750W "Batt-Verter" Rig wired into in Calzone case and tied into the Shuttle's alternator​

To maximize the running time on however many batteries you use, I suggest you use a "jumper cable" to attach the marine cells to the leads of the car's alternator. That way you can use the car alternator as a generator to run the lights during set up and rehearsals. When it comes time to shoot, shut off the engine and continue to run the lights on the silent Batt-verter alone. Running the vehicle engine between takes charges the batteries so that they will run longer.

1800W "Batt-Verter" Rig wired into the back of Shuttle​

The production stills above and below show you a more elaborate 1800W Batt-verter system that we built to run 16 - 4' Kino T-12 tubes inside the airport shuttle bus. Use this link - - for details on how we wired it. To supply 12 volts to the 1800W inverter we ran 2 ought feeder to the buses' alternator. And since the shuttle bus was equipped with a wheel chair lift and extensive inside lighting of it's own, it was equipped with an oversized alternator that could bear the 1800W DC load. Even then we were able to run only four 4' - 4 Bank Kinos on our 1800W rig.

SL&G's custom 1800W Batt-Verter powers 16 - 4' Kino Flo single tubes rigged in the interior and on the exterior of an Airport Shuttle​

As I said at the outset, you can run a 1200 Par along with a couple of Kinos on a "True Sine Wave" 1800W inverter, but the ballasts (both HMI & Kinos) have to be Power Factor Corrected. If I were to light that shuttle bus again, knowing what I know now, I would only use the Kino T-12 tubes where I absolutely had to because they were serving as practical lights in the shot. For off-camera lights I would instead use Parabeam or Diva lights because their ballasts are Power Factor Corrected and don’t dump harmonic currents into the power stream the way the T-12 ballasts do. For a detailed explanation for why the harmonic currents created by Kino T-12 ballasts can have an adverse effect see my article on the use of portable generators in motion picture production available on our [url="http://www.screenlightandgrip.com/html/emailnewsletter_generators.html"]website.

In Isabelle's case, I would recommend the Parabeam fixtures over the Divas. What distinguishes the Parabeam fixtures from the Diva fixtures (and all other fluorescent lights for that matter) is their throw, power efficiency, and the innovative accessories Kino Flo makes available for the fixtures that enhance their production capabilities. Accessories include barndoors, a gel frame, a diffusion panel, and Honeycomb Louvers. Honeycomb Louvers are available in 90, 60 and 45 degrees. Swapping louvers provides beam control similar to that of swapping lenses on an HMI Par. The Diffusion Panel, on the other hand, slightly softens the beam structure in the open face mode.

She would be best served by the ParaBeam 400 fixtures because they have computer aided designed (CAD) parabolic reflectors that focus the light output where it is needed most for lighting dramatic scenes - at a medium distance - making it a better key source than the Diva 400 or 4' 4 Bank Kinos. If you compare the photometric tables of the Parabeam 400 and the Diva 400 (which uses the same four lamps), you will notice that at 16' the Parabeam 400 puts out almost three times the light level (28FC) than the Diva 400 (10FC) even though they both use the same tubes. In fact a Parabeam 400 generates as much light at 16' as the 4' 8-Tube Kino Flathead 80 fixture, yet uses less than a quarter of the power. Kino Flo is able to obtain such efficiency in their Parabeam fixtures by incorporating Power Factor Correction circuitry into their ballasts. As it does in HMI ballasts, this advanced electronics contributes to a more economical use of power than Kino Flo's conventional electronic ballasts and reduces the return of harmonic currents into the power stream. With a Power Factor Rating of over .9, the Parabeam 400 fixtures are especially well suited for use on inverters.

If you need more than 1800 Watts you have no alternative but to use a generator.

Guy Holt, Gaffer, ScreenLight & Grip , Boston

 

[Eileen Ryan]

Out of curiosity, what kind of inverters do the honda generators have? … Can generators (specifically the honda inverter generators) damage magnetic ballasts (or possibly bulbs)? Or is the worst damage that they can do (assuming the light strikes) is to cause out-of-phase flicker? With an electronic ballast running kinos and electronic-ballasted hmis you might damage the ballasts simply due to the heat generated by the harmonic resonance they create in the sine wave? Is this correct? So for good inverter power a magnetic ballast might be best as it introduces less distortion into the sine wave?

Matt, I highly recommend that you read Guy's article. It answers many of your questions. I highly recommend that anyone using HMIs on a set read Guy’s article. While the HMI lighting package he has developed as a result of his tests is new (introduced Jan 09), the set power issues caused by HMI ballasts have been vexing electricians for years. His article explains the electrical engineering principles behind these issues, and how his lighting package resolves them, in a way I have not seen elsewhere.

Whats different about his article is that he undertakes a comprehensive survey of the prevalent lighting and portable power generation equipment and describes how he systematically tested how well they work together. One startling result of his tests, as Matt correctly sumizes above, was that if you don’t have access to the newest PFC electronic ballasts, you are better served by using the older magnetic ballasts on an inverter generator like the Honda EU 6500is over non-PFC electronic ballasts. I know this is contrary to the conventional wisdom, so I will quickly summarize his article because it is very long.

When electronic square wave HMI ballasts came on the market, they were at first thought to be the solution to all the problems inherent in running HMI lights on small portable generators. By eliminating the flicker problem associated with magnetic ballasts, they also eliminated the need for the expensive and ultimately unreliable AC governors required for flicker free filming with magnetic HMI ballasts and portable gas generators. 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. 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. Electronic square wave HMI ballasts allow you to film at any frame rate and even at a changing frame rate.

Since they are not frequency dependent, it was thought at first that electronic square wave ballasts would operate HMI more reliably on small portable generators – even those without frequency governors. For this reason, as soon as electronic square wave ballasts appeared on the market, many lighting rental houses replaced the more expensive crystal governed portable generators with less expensive non-synchronous portable generators. The theory was that an electronic square wave ballast would operate reliably on a non governed generator and allow filming at any frame rate, where as a magnetic HMI ballast operating on an unreliably AC governed generator allowed filming only at permitted frame rates.

In practice, electronic square wave ballasts turned out to be a mixed blessing. Part of the problem with operating electronic HMI ballasts on portable gas generators in the past has to do with the purity of the power waveform they generate. With an applied voltage waveform distortion of upwards of 19.5%, conventional generators do not interact well with the leading power factor (current leads voltage) of the capacitive reactance created by electronic square wave HMI ballasts. The net result is harmonic currents are thrown back into the power stream, which results in a further degradation of the voltage waveform and ultimately to equipment failure or damage (for the reasons discussed in my previous post.)

The oscilloscope shots of the power waveforms below is from the article mentioned above and is typical of what results from the operation of a 1200W HMI with non-power factor corrected ballast on grid power (left), on a conventional generator (middle), and inverter generator (right.) The adverse effects of the harmonic noise generated by non-PFC electronic ballasts and exhibited here in the middle shot, can take the form of overheating and failing equipment, circuit breaker trips, excessive current on the neutral wire, and instability of the generator’s voltage and frequency. Harmonic noise of this magnitude can also damage HD digital cinema production equipment, create ground loops, and possibly create radio frequency (RF) interference.

Left: Grid Power w/ 1.2Kw Arri non-PFC Elec. Ballast. Center: Conventional AVR Power w/ 1.2Kw Arri non-PFC Elec. Ballast. Right: Inverter Power w/ 1.2Kw Arri non-PFC Elec. Ballast.

As is evident in the oscilloscope shots below of a 1200W magnetic HMI ballasts on grid power, on power generated by a conventional Generator (Honda EX5500), and power generated by an inverter generator (Honda EU6500is), the lagging power factor caused by the inductive reactance of magnetic ballasts has by comparison only a moderately adverse effect on the power waveform. Outside of causing a voltage spike in the inverter power, magnetic ballasts actually show a positive effect on the already distorted power waveform of the Honda EX5500 conventional generator. For this reason magnetic ballasts work better on conventional generators with frequency governors than do non-PFC electronic square wave HMI ballasts.

Left: Grid Power w/ 1.2Kw Arri Magnetic Ballast. Center: Conventional AVR Power w/ 1.2Kw Arri Magnetic Ballast. Right: Inverter Power w/ 1.2Kw Arri Magnetic Ballast.

These oscilloscope shots show that if you don’t have access to the newest PFC electronic ballasts, the older magnetic ballasts are in fact cleaner running on portable gas generators than non-PFC electronic ballasts. And, where inverter generators like the Honda EU6500is do not require crystal governors to run at precisely 60Hz, you can operate magnetic HMI ballasts reliably on them. In addition, the smaller magnetic ballasts (575-2500W) offer the distinct advantage of being less expensive and draw less power (once they have come up to speed) than the commonly available non-PFC electronic equivalents (13.5A versus 19A for a 1.2kw.)

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 before the early 1990s was made this way, you realize it is not such a limitation. 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 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.”

For example, even though a 2.5 magnetic ballast draws approximately 26 amps you will not be able to run it reliably on the 30A/120V twist-lock receptacle on the generator’s power panel. As mentioned above, magnetic ballasts have a high front end striking load. For this reason, you must always leave “head room” on the generator for the strike. But, 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, 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. (for a more detailed explanation of why that is I, again, suggest you read SL&G’s newsletter article.) 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.

The only sure way to power a 120V 2.5kw (or even a 4kw) HMI magnetic ballast on a portable gas generator is from its 240V circuit through a 240v-to-120v step down transformer like the one SL&G manufactures for their modified Honda EU6500is. SL&G’s 60A Full Power Transformer/Distro will step down the 240V output of the generator to a single 60A 120V circuit that is capable of accommodating the high front end striking load, and even the voltage spikes, of either a 2.5kw or 4kw magnetic ballast at 120V. I highly recommend that anyone using HMIs on a set read Guy’s article. The article is available at www.screenlightandgrip.com/html/emailnewsletter_generators.html.

Eileen Ryan, Gaffer, Boston

 

[Matt W.]

Thanks for the article; I'll definitely check it out. That makes a lot of sense about magnetic ballasts and it's great to see they work well on inverter generators; we always strike them first to give them time to warm up and due to the voltage spike, but I didn't realize just how complicated all this gets. Very interesting about non-pfc kino ballasts, too... Out of curiosity, where do you learn this stuff? Purely on set?