Can 2 6500w Genny's power a 4k?

[Jaden Scholes_65655]

Hey everyone!

 

I was thinking about this the other day and was wondering if anyone could confirm my theory (not really mine but its something I was thinking about before, I'm sure other people have either done it or thought of it before). I wondering if I can power a 4k off of two 110v 6500w Honda red genny's.

 

The way Im was thinking it could be done is if I run Joy (or Bate for you americans) from each of the genny's 30amp circuits and connect them at at the ballast with a male male to female adaptor and plug that into the ballast. In my head that gives me 220v at 60amps a total of around 6600w which should be enough even for the spike the light does when striking. Even giving my another 6400w (3200 in each Genny) to plug a couple other things into.

 

Im wondering firstly if that would work, and secondly if it would be safe to plug more lights into those genny's or if its better just to get another genny for the extra lights.

 

On a side note I was also wondering if a 4k could be powered off of two110v honda 3000w genny. The max I could pull out of them with Joy would be 40amps (20amps off each) at 220v a total of around 4400 watts. Would 4400w at 220v be enough to survive the spike when striking the light.

 

 

Let me know

 

J

 

[Jay Young]

A 4k HMI will draw about 34 Amps, so one of those 6500w hondas SHOULD be able to power it. However I understand they get a bit of flicker depending on line voltage.

[Edward Lawrence Conley III]

NO- and please do not try to ever use 2 generators to feed power like this.

 

Honda Inverter generators can be paralleled BUT you don't pull power from each separate generator. You use one receptacle on either generator or an Extra receptacle that comes in a kit that allows for more power through one receptacle.

 

In Montreal are your 4k's 240v?

 

The Honda 6500 watt generators are 120v/240v generators.

 

If your 4k's are the same as here with the 120v 60amp bates connectors then you'll need a generator that has been modified with a female bates connector on it.

[John E Clark]

There is a frequent poster here, Guy Holt, who has a 'paralleling' device to hook two generators of a specific brand and model to produce 2x the power. In particular the EU6500 and EU7000.

 

However, if one tries to do this without some serious electrical 'magic box', this spells disaster...

 

Here's the link, http://www.screenlightandgrip.com/html/slg_home.html

Edited September 30, 2015 by John E Clark

[JD Hartman]

There is a frequent poster here, Guy Holt, who has a 'paralleling' device to hook two generators of a specific brand and model to produce 2x the power. In particular the EU6500 and EU7000.

 

However, if one tries to do this without some serious electrical 'magic box', this spells disaster...

 

Here's the link, http://www.screenlightandgrip.com/html/slg_home.html

 

Just the facts, the device is a standard Honda product. No wizardry involved.

Generator output can be paralleled if you synchronize the output of one to the other, it's something power utilities do every day. Easy for them, no so much for you.

http://electrical-engineering-portal.com/preparing-to-synchronize-a-generator-to-the-grid

[Guy Holt]

 

... the device is a standard Honda product. No wizardry involved. Generator output can be paralleled if you synchronize the output of one to the other, it's something power utilities do every day. Easy for them, no so much for you. http://electrical-engineering-portal.com/preparing-to-synchronize-a-generator-to-the-grid

 

There is a bit of wizardry involved which is why Honda has yet to come out with a paralleling box for the EU7000. As the link JD provides above makes clear, to synchronize two power generation systems you must match the phase sequence, voltage magnitude, frequency, and phase angle of the two generators before bringing them together (use this link for more details.)

 

Our 100 Paralleling Control Box for the EU6500s provides 100A output 
and load sharing control to optimize the output.

 

For those interested, an article I wrote on paralleling Honda EU6500s and EU7000s will appear in the next issue of Protocol. If you are not familiar with Protocol, it is the quarterly publication of PLASA – an international organization working to raise standards, improve skills and strengthen the events, entertainment and installation industries. If you can’t find the print edition of the magazine, it will be available online on or about Oct. 9th at http://na.plasa.org/publications/protocol.html.

 

Guy Holt, Gaffer

ScreenLight & Grip

Lighting Rental & Sales in Boston

[JD Hartman]

Why would I want to parallel two screaming little Honda 6500's or 7000's when I can just use one tow behind 20Kw generator? More output, a heck of a lot quieter (unless the Honda is at idle), no transporting a quantity of highly flammable gasoline and less B.S. as well?.

[Edward Lawrence Conley III]

Very few people need the parallel capabilities of 2 Honda 6500/7000watt gensets. And as Guy pointed out Honda hasn't even released the special receptacle kits like they have for the small generators.

 

I can't see someone outside of Motion Picture industry or a very specialized need for this option- general power needs can be handled by one 12,000watt genset for construction jobs.

 

Even in those situations where more power is need they pick up a small Tow plant for the site and run out a Spider box.

[Albion Hockney]

you can run a 4k hmi off 1 6500w honda. it needs to be wired in a special way which I'm not sure the specifics of but I have it done on jobs all the time.

[Stephen Whitehead]

One will power a 4k just fine, it's done all the time. You need a few things to do it though. First of all you need an electronic ballast for your 4k capable of 220v power, but most are. You then need a breakaway cable that adapts the 220v twist lock plug on the generator to cam lock, then run two hots into a bates line, and a ground instead of hot, neutral, ground. You run the light in 220v power. You won't have flicker issues with an electronic ballast, although the system sometimes is a little unstable and can shut down from time to time. It's a common setup though, often used in remote locations where it's hard to get full size generators.

Edited October 2, 2015 by Stephen Whitehead

[JD Hartman]

Very few people need the parallel capabilities of 2 Honda 6500/7000watt gensets. And as Guy pointed out Honda hasn't even released the special receptacle kits like they have for the small generators.

 

I can't see someone outside of Motion Picture industry or a very specialized need for this option- general power needs can be handled by one 12,000watt genset for construction jobs.

 

Even in those situations where more power is need they pick up a small Tow plant for the site and run out a Spider box.

 

Spider box isn't going to solve the need for a single high amperage circuit. Something like a 6506G isn't going to do it for a 4K: http://www.cepnow.com/temporary-power-boxes

[Guy Holt]

Why would I want to parallel two screaming little Honda 6500's or 7000's when I can just use one tow behind 20Kw generator? More output, a heck of a lot quieter (unless the Honda is at idle), no transporting a quantity of highly flammable gasoline and less B.S. as well?.

 

I can think of four good reasons: noise, cost, permitting, and power quality. Here are details on each:

 

Noise:

 

There are no, at least in this market, movie blimped generators smaller than 60kVA (500A.) MultiQuip makes a 25 kVA (180A) Whisper Watt, but they are very rare (there are none in the Boston market.) What’s available in the 20-25 kVA range are construction generators which are considerably louder than a pair of EU6500/7000s operating in parallel. The next step up in terms of noise is typically a 60 kVA movie blimped tow plant. A generator of that size is not only expensive to rent, but comes with a number of hidden costs.

 

Cost:

 

The hidden costs start with the fact that Ryder and Penske box trucks (the preferred mode of transportation for low budget productions) like are not equipped to tow, so you have to pay to have the generator delivered and picked up – usually $125 each way. Since many film productions change locations frequently, sometimes within the same day, delivery and pick-up of diesel generators is generally not an option. For these reasons the only option for production is to hire the rental house’s grip truck to tow them. And, since most rental houses require that one of their employees drive their trucks for insurance reasons, they also have to hire a driver at roughly $575/10hrs – which is probably more than anyone else on a typical low budget production is getting paid these days. In the past, even if an low budget production could afford to rent the big light, the cost of powering it made it prohibitively expensive. The advantage to paralleling Hondas is that they are easily transported in SUVs, Cargo Vans, or rental box trucks. Because no special tow equipment is required, production is not required to rent a grip truck with the added expense of a rental house driver/grip. Using a big light is now much more affordable.

 

Permitting:

 

Adding to the cost of a diesel tow plant is the fact that in most municipalities the use of a generator over 10kw requires an electrical permit, which can only be pulled by a licensed electrician. Because the electrician must be bonded and is putting their license on the line, they are not cheap. In the Boston market there are only a handful of licensed electricians that will pull a permit for a production and they charge from $600-$1000 for the service. An additional cost to tow plants is that under many IA contracts only a member of the electrical department can operate generators over 10kW. And, since power is needed whenever there is work of any kind on set, they are usually the first in and last out and clock serious overtime.

 

These costs to operating tow plants, even small ones, can add up to the point where they are a consideration for even major motion pictures. Which is why the caterers on the Sandra Bullock & Melissa McCarthy blockbuster “The Heat” used four Hondas to power their mobile kitchen, refrigerator trailer, and food truck. Caterers are usually the first in and last out on movies. The cost and logistics to pull permits and schedule inspections for generators on multiple locations over a three-month shoot was daunting even for a studio production. On top of that they would have had to pay the overtime of a union electrician to come in to power up the generator and stay late to shut it down – racking up serious overtime. To make their job easier and save money in rentals, inspections, and over-time, we set them up with a trailer of three Honda EU6500s and a EU3000. We used 60A Full Power Tranaformer/Distros to step down the 240V output of the EU6500s to 120v to supply the mobile kitchen and food truck.

Pictured above, the three EU6500is and EU3000is provide a total of 200 Amps of power for the base camp catering trucks (pictured above right.) Two EU6500s with 60A Transformer/Distros power the refrigerators, freezers, and industrial exhaust fans of the set mobile kitchen. Right: the EU3000is powers a walk-in refrigerator trailer (located behind the van.) While, the 3rd EU6500is powers the catering truck, and the food buffet toasters and heating lamps (right of frame) through a 60A Transformer/Distro (pictured center on a handtruck with 20A break-out boxes.)

 

With this approach, the caterers were totally self-sufficient and didn’t have to worry about electricians pulling permits to operate generators because the generators were under 10’000 watts each.

 

Power Quality

 

After a hellish night lighting a set with a 25kVA MulitQuip generator for a PBS American Experience production, I learned the hard way that power quality can be an issue with diesel construction generators. The problem was the breakers on our HMIs kept tripping intermittently.

 

At the time, I thought the generator was more than adequately sized for the lighting package, which consisted of nothing more than a 2500, three 1200s, and several 1ks. To trouble shoot the problem we put a handheld scope meter on the power line and found the voltage waveform was severely distorted by the harmonics drawn by the magnetic and non-power factor corrected electronic ballasts we were using.

 

Under the circumstances (a remote location at night) all we could do was pare down our lighting package, so throughout the night, each time an HMI went out unexpectedly in the middle of a shot, we eliminated one in our set up until we found a happy medium of a couple of babies, a 1200 Par and a 2500 Fresnel – much less than I would have expected a generator of that size could run reliably. I learned that hellish night that small construction generators are very prone to voltage waveform distortion when powering non-linear loads which is why the general rule of thumb is to oversize your generator by a factor of two.

(AC voltage "flat topped" by harmonics drawn by 2.5kW HMI with non-power factor corrected ballast running on a 45kVA industrial generator)

 

We double the size of construction generators when powering non-pfc HMI ballasts because it is the only way to mitigate the voltage “Flat Topping” (pictured above) created by the harmonic currents they draw. What causes flat-topped voltage? According to Ohm’s Law current reacts with impedance to cause voltage drop. For example, when encountering the high impedance of an industrial generator, a 3rd harmonic current drawn by the load will produce a voltage drop at a 3rd harmonic voltage. Likewise a 5th harmonic current will produce a voltage drop at a 5th harmonic voltage, etc.

 

(harmonic currents drawn by 2.5kW HMI with non-power factor corrected ballast)

 

Each harmonic current drawn by a non-power factor corrected HMI ballast flows through the system impedance resulting in a voltage drop at that harmonic frequency. In other words, where a distorted current waveform is made up of the fundamental plus one or more harmonic currents (see FFT for 2.5kw non-pfc ballast above), each of these currents flowing into an impedance will according to Ohm’s Law, result in a voltage drop resulting in voltage harmonics appearing at the load bus. And since, non-pfc electronic HMI ballasts only draw power during the peak of the supply voltage to recharge their smoothing capacitor, this voltage drop only occurs at the peak of the voltage waveform, which accounts for the voltage waveform being flat topped like that exhibited in the power quality meter screen shot above.

(AC voltage waveform generated by 2.5kW HMI with non-power factor corrected ballast on grid power)

 

This pattern does not appear in the voltage waveform of the same light on grid power above because of its’ much lower impedance. The impedance of the grid is so low, the distortion of the original applied power waveform so small (less than 3%), and the power plant generating capacity so large by comparison to the load, that harmonic currents will not affect the voltage at a grid load bus.

 

However, it is an all-together different situation when plugging these same loads into a small portable generator that is not specifically designed to remediate the effects of harmonics. Given the comparatively large sub-transient impedance of industrial generators, and the THD value of their inherent power waveform, you have a situation where even a small amount of harmonics being fed back into the power stream will result in a harmonic distortion in its’ voltage. The impedance of a generator is not an easily known quantity. Depending on its’ size and design, the impedance of a generator will be 5 to 100 times that of a utility transformer and it will change as the load changes.

 

Now we can understand why we would want to use a Honda inverter generator to power small lighting packages instead. As demonstrated above harmonic currents react with impedance to cause voltage waveform distortion. The magnitude of the voltage waveform distortion is a function of the source impedance. Which means that the generator with the lowest internal reactance to an instantaneous current change at a given load will typically have the lowest value of total harmonic distortion under nonlinear load conditions. This is one of the great benefits to using inverter generators over industrial generators: inverter generators have much lower internal reactance and so they are less prone to voltage waveform distortion caused by the harmonic currents drawn by switching power supplies.

 

(Left: A 1200W non-linear Load powered by Grid Power. Center: A 1200W non-linear Load powered by an industrial generator. Right: A 1200W non-linear Load powered by Inverter Power.)

 

As can be seen in the oscilloscope shots above, inverter generators are less prone to voltage waveform distortion because the inverters completely process the raw power generated by its alternator (converting it to DC before converting it back to AC) – making the AC power it generates completely independent of the engine. In fact, its’ microprocessor controller can vary the engine speed without affecting the voltage or frequency of the power the inverter module puts out. Now that the internal reactance of the engine is separated from the power output, harmonic currents encounter very little impedance; and, as is evident in the oscilloscope shot above right, there is considerably less voltage distortion at the load bus of inverter generators than there are industrial generators. The net benefit is that non-linear loads, like non-pfc HMI ballasts, do not adversely affect the power of inverter generators as they do the power of industrial generators. Which means that non-linear loads will operate more reliably on inverter generators.

 

For a more detailed explanation of what causes voltage waveform distortion and how to mitigate it, see my white paper on the use of portable generators in motion picture production that is available at http://www.screenlightandgrip.com/html/emailnewsletter_generators.html.

 

Guy Holt, Gaffer

ScreenLight & Grip

www.screenlightandgrip.com

Edited October 2, 2015 by Guy Holt

[Edward Lawrence Conley III]

 

Spider box isn't going to solve the need for a single high amperage circuit. Something like a 6506G isn't going to do it for a 4K: http://www.cepnow.com/temporary-power-boxes

 

I know- That's why I stated construction sites :)

 

I've run the 4k on the 6500 with the 120v 60amp bates conversion and with an adapter to run it off the 240v twist lock receptacle @240v

[JD Hartman]

Such gloom and doom Guy Holt. How many years ago was your "hellish night" and how many of the loads didn't have PFC? Contrast that to today

 

Availability of a 20 or 25kW "silenced" diesel generator, I have three dealers within a short distance, four if count Sunbelt's Brooklyn, NYC location. Two carry MQ Whisperwatt the third a blimped Baldor. Hire a truck and driver to deliver and relocate the genny? If it's not a union shoot, likely there is a spark, grip or PA with a pickup, SUV or van that can tow 2000lbs and would like to make some extras cash. I can tow the 35KW behind my 1/2 ton pickup with no problem.

 

Permitting? Personally haven't run into that issue in NJ or NYC. Perhaps others here can comment on this. Boston must be an very unfriendly place to shoot.

 

I see at lest four, five gallon gas cans sitting by the trailer in the fourth image, but no fire extinguisher. Where was the local Fire Marchall/Inspector? Diesel fuel, extremely safe by comparison.

 

How many other rental companies, aside from yours, outside of the Boston area have adopted your solution to location power?

[Guy Holt]

How many years ago was your "hellish night" and how many of the loads didn't have PFC? Contrast that to today.

 

It wasn’t that long ago and things have not changed that much since power factor correction (pfc) is still not mandatory in the U.S., as it is in the E.U. To this day a surprising number of motion picture lights are not power factor corrected. For instance, even a lot of LED AC power supplies distributed in this country are not pfc, making them much less efficient than tungsten lights of the same wattage that have unity power. With power factors as low as .45, LEDs can draw twice the current than a tungsten light of the same wattage. If you don’t take into account the extra current they will draw and the harmonic currents they will generate, you may find breakers tripping and portable generators running erratically (use this link to see what other LED fixtures are not power factor corrected.)

(The Litepanel Sola 4 has a pf of .60 and Total Harmonic Distortion of 77.7%)

 

Until the introduction of their new DMX compatible Universal ballast this year, all Kino Flo ballasts in this country driving fixtures with T12 tubes were not pfc (the industry workhorses 4, 2, and Single Bank fixtures as well as the Wall-o-Light, Flat Head 80, Image 80 and Image 85 fixtures.)

(The Kino Flo Image 85 has a pf of .63 and Total Harmonic Distortion of 77.2%)

 

But, by far, the largest source of harmonics in location lighting packages are older HMI fixtures. Because of the added cost, weight, and complexity of PFC circuitry, HMI distributors in the US only offer PFC circuitry as standard in luminaries larger than 6kw, and only then by necessity. The early line of Lightmaker electronic ballasts were nick named by film electricians “Troublemaker” ballasts because they were not power factor corrected and proved that pfc circuitry was absolutely necessary in large ballasts to reduce heat and return on the neutral, and to increase ballast reliability. Because of the added cost, HMI distributors in the US still only offer pfc circuitry as an option in medium-sized 2.5-4kw ballasts.

 

And, until very recently manufacturers did not offer pfc circuitry in HMI ballasts smaller than 2.5kw. Which means that almost all 575/1200 ballasts in this country are non-pfc because pfc circuitry added a $1000.00 (or 25%) to the cost of the ballast. For the same reason, the Joker 200, 400, and 800 ballasts sold in this country are not power factor corrected. What this means is that most of the lighting rental inventory in this country that would likely be used on a portable generator is not power factor corrected.

 

Availability of a 20 or 25kW "silenced" diesel generator, I have three dealers within a short distance, four if count Sunbelt's Brooklyn, NYC location. Two carry MQ Whisperwatt the third a blimped Baldor.

 

I do not include the MQ Whisperwatts in the category of blimped movie generators (neither does MQ for that matter.) They are a sound attenuated construction generator that has found some use in event staging. They are not as silent as the MQ studio units and do not use the same governors and voltage regulators. With 2/3 pitch windings, the MQ Studio (Crawford) generators are specifically designed to remediate the most troublesome of the harmonics generated by non-linear loads and as such have specifications for total harmonic distortion (THD) values of less than 7% under full linear load, and of not more than 3% of any given harmonic current. For this reason, and the fact that they offer a comparatively low sub-transient impedance value because they are typically oversized for the load, harmonic currents do not cause substantial voltage waveform distortion. It was a more blimped version of the Whisperwatt (MQ’s Silent Star now renamed Ultrasilent) that gave us such trouble and in the example I cite below I believe.

 

Hire a truck and driver to deliver and relocate the genny? If it's not a union shoot, likely there is a spark, grip or PA with a pickup, SUV or van that can tow 2000lbs and would like to make some extras cash. I can tow the 35KW behind my 1/2 ton pickup with no problem.

 

That’s my point: there are additional costs incurred in towing a diesel generator. You can try to minimize them by having one of your crew tow it, but you still have to give that person a bump for use of their truck, compensate them for their fuel, pay them something additional to pick it up the day before and drop it off the day after, as well as overtime because they will have to start before crew call to pick it up from the holding yard and work after wrap to return it to the holding yard. You incur none of these additional expenses with a pair of Hondas that you can throw in the back of the Ryder or Penske box that is serving as your grip truck as they did in the production of the indie feature “Tooth Brush” pictured below.

 

Left: Scene in parking lot. Center: Transformer/Distro hidden behind newspaper box (set 200' in distance.) Right: Generator baffled by truck (Transformer/Distro 300' in distance.)

 

Permitting? Personally haven't run into that issue in NJ or NYC. Perhaps others here can comment on this. Boston must be an very unfriendly place to shoot.

 

No more so than other cities. Perhaps you just haven’t gotten busted yet. You can’t count on flying under their radar. We were once met at an early Monday morning call by a city inspector who had learned of the production from the traffic department because the production had bought out parking spaces outside the location. Another time the inspector stumbled across the production because we happened to be shooting on his route home.

 

I see at lest four, five gallon gas cans sitting by the trailer in the fourth image, but no fire extinguisher. Where was the local Fire Marchall/Inspector? Diesel fuel, extremely safe by comparison.

 

Per code there were fire extinguishers in the mobile kitchen and catering truck. The Fire Marshall had no issue with the use of gasoline.

 

How many other rental companies, aside from yours, outside of the Boston area have adopted your solution to location power?

 

We have sold systems to companies in just about every major U.S. market and many provincial production markets as well as in Brazil. We have sold probably a dozen in the NYC area. They are available from both Available Light NYC and Scheimflug. Quite often they are used on shows that are also carrying a tow plant because they are quicker to set up. Here is what the best boy electric on the feature “Dangerous Words From The Fearless”, that was shot in probably the worst conditions for recording sound with a generator (a desert in Utah), had to say about our system in one of Scheimflug’s blog posts:

 

“… gear that took a beating and just kept on going is the 7500w Genny + Transformer combo. This great set up allowed the guys to run over 300′ of 5 wire in a moments notice. This is great not only for the quick set-up but also for allowing for a quiet set.”

 

“… Fuel consumption from the tow behind generator was limited. In the smaller scenes that didn’t require more than 60 Amps – such as an M40, and a few Kinos or Jokers - we ran our tweaked out Honda 6500 with a transformer. The generator has been altered to be 7500w with 60amp output – enough to power a 6K with no transformer or distro. There were times when the genny was 300′ away from the scene, and the transformer was used to compensate for the line loss. The auxiliary fuel tank will allow the genny to run for a whole day without refueling during a take.“

 

 

“It was really nice to not have to run the tow behind all the time, as it consumed a good amount of fuel, and the 7500 is really quiet in comparison. I never had a sound guy up in my grill about how loud it was, and how I was ruining their lives. Having a Honda generator 300′ away, around a bend or behind a bunker, and with a sound blanket barrier, you really couldn’t even hear it at times.”

[Guy Holt]

Such gloom and doom Guy Holt.

 

Not gloom and doom, but just a realistic appraisal of the shortcomings of construction generators like the MQ Whisperwatts. Here is another example from an earlier post on these boards that also demonstrates that productions using small diesel tow plants are not immune from the effects of harmonic currents generated by non-power factor corrected Kino, HMI, and LED loads. The lighting set up consisted of the following: on each end of a football field the crew had rigged a 4k HMI Par and M18 into scissor lifts. Two additional M18s were rigged into the announcer’s booth at center field.

 

From a 500 Amp diesel generator that was dropped behind the bleachers at the 50 yard line, the electricians ran out 100ft of 2/0 to a main distro box at the 50 yard line. From there they ran 150' of banded #2 AWG cable in each direction down the field towards the end zones where the lifts were. They terminated the banded #2 runs into a 100A/120V snake bite, and from there they ran 100ft of #4 AWG to a 100A lunch box, and from the lunch box they stepped down to #6 AWG cable to power the 4k Par and a stinger to power the M18. This set-up was mirrored on either side of the field. The two additional M18s rigged into the announcers booth at center field were powered by a 50' run of #4 AWG to a 100A Lunch box underneath the booth. Load balance was almost dead even between the legs of the generator operating single phase.

 

Shortly into shooting, one of the 4k Pars went down. In addition, one of the lunch boxes started buzzing loudly. The crew replaced the bad ballast (a non-pfc Power Gems) with another and it too went down. Since they had oversized their cable runs by using #2 AWG rated for a 160A for a 70A Load (52A (4kW) + 18A (1.8kW) = 70A), they did not suspect the problem had to do with voltage drop, besides the M18s ran flawlessly. When the best boy electric checked the generator, the generator meters read 108v. The best boy was a bit flabbergasted why the voltage had dropped on the generator when no one had touched it. The answer to his questions has to do with the harmonic currents drawn by the non-pfc 4kW ballasts.

 

When powering lights that use non-pfc Switch Mode Power Supplies (HMIs, Kinos, CLF lamp banks, LEDs) you must take into account the harmonic currents these non-linear loads draw that can have a severe adverse effect on the power waveform of small diesel generators.

 

As is evident in the power quality meter reading of a non-pfc 2.5kW HMI I posted above, the high peaked pulsed current (lower waveform) drawn by its’ smoothing capacitors is a distorted waveform that does not resemble the sinusoid of AC voltage or the current drawn by an incandescent light. As such, the current drawn includes a number of harmonic currents in addition to the 60hz fundamental (see Fast Fourier Transformation of a non-pfc 2.5kW HMI ballast also posted above), which not only increases voltage drop but also causes the voltage “flat topping” discussed above.

 

 

Voltage flat topping, a particular form of voltage drop, is caused by harmonic currents interacting, according to Ohm’s Law, with the high impedance and soft power of a diesel generator. Since smoothing capacitors consume power only at the peak of the voltage waveform, voltage drop due to system impedance occurs only at the peak of the voltage waveform – causing the “Flat Topping” we see in the top waveform of the first power quality meter above that is characteristic of capacitive loads on generators. If the voltage waveform distortion is severe, it can cause voltage regulator sensing problems and inaccurate instrument readings in a generator’s control systems as well as ballast failure, which would explain the problems the crew experienced above.

 

Let’s look at the regular voltage drop component of this problem first. As you can see in the Fast Fourier Transformation of the 2.5kW HMI ballast above, non-pfc HMI ballasts draw a distorted current waveform that is rich in harmonics. The higher harmonic frequencies create what is known as "skin effect" in the cable. Skin effect is a phenomenon where the higher frequencies cause the electrons to flow toward the outer sides of a conductor. Since the flow of the electrons is no longer evenly distributed across the cross sectional diameter of the conductor, more electrons are flowing through less copper and the resistance of the conductor increases. Even though the cable was oversized in this case, this increase in resistance reduces the ability of the conductor to carry current, resulting in greater voltage drop over shorter distances.

The area of the cross sectional diameter of a conductor used by DC current (left), Low Frequency AC Current (center), High Frequency AC Currents (right).

But, voltage drop alone was not the cause of the problems the crew experienced. The other contributing factor was the voltage “Flat Topping” caused by the high impedance of their distribution system. Always remember, there are two components to the impedance of a distro systen: cable and generator. The skin effect caused by the harmonic currents generated by the non-pfc ballast increased the impedance of the cable. The second contributing factor was that they were operating single phase on a small generator. A 500A generator operating three-phase provides about 166A/leg. The same generator operating single phase does not provide 250A/leg, but rather the same 166A/leg but just single phase. The set up as described had about 100A on each leg. So, the generator was pretty well loaded and half that load (52A) was generating harmonic currents.

 

Under these circumstances one could expect much more severe voltage flat topping than that in the example of a 2.5kW HMI above, which would account for why the non-power factor corrected 4k ballasts were failing when the newer power factor corrected M18 ballasts were not. One adverse effect of flat-topped current is that it causes non-power factor corrected equipment to draw more current to maintain the power rating (watts) of the unit. Combined with the voltage drop caused by skin effect that would also cause the ballast to draw more current, the accumulative load can easily be enough to cause protective fuses on electrical boards of ballasts to blow.

 

Now for the reason the voltage output of the generator dropped even though no one touched its’ voltage regulator. The voltage output of the generator did not drop rather the flat-topped voltage caused an erroneous reading of the voltage by the meters. The flat-topped voltage manifests itself as low voltage on the generator’s meter because conventional electrical meters like those on construction generators are designed to read only sinusoidal waveforms accurately – not distorted waveforms. Flat-topped voltages introduce errors into the measurement circuits of these meters, which result in low readings. Since the consequences of under measurement can be significant - overloaded cables may go undetected, bus-bars and cables may overheat, fuses and circuit breakers will trip unexpectedly - it is important to understand why meters based on "true rms" techniques (like those found on the MQ Studio Units) should be used on power generation systems supplying harmonic generating loads.

 

Consider the square wave of voltage distortion generated by our 2.5kW HMI with non-power factor corrected HMI shown above. A peak reading meter would indicate an erroneous value of 96.52 RMS compared to the true RMS value of 117.7V. Why is this the case? The voltage drop that occurs when the harmonic currents generated by the ballast encounter the high impedance of the generator causes the voltage waveform to plateau well below the peak of the original sine wave (136.53V verses the 166.43V of a sine wave), which will cause meters that calculate RMS based upon the waveform peak to under measure the voltage at the generator’s load bus.

 

The table below demonstrates that the disparities in RMS values calculated by Peak and Averaging meters can be quite substantial when the waveform they are measuring has been distorted. Such errors in measurement can have catastrophic consequences because harmonic currents dumped back into a distribution system can quickly overload the neutral conductor, which has no overcurrent protection. So accurate measurement of distorted current is critical to safely distributing power to sets with a large number of non-linear loads.

 

Methods for Measuring Voltages and Currents with Multimeters

 

To accurately measure waveforms distorted by harmonics, a meter that will measure the true RMS value is required. True RMS meters overcome these problems by deriving the heating effect of the waveform to produce an accurate RMS value indication (use this link for more detailed information.)

 

For more detailed information on the source of harmonics and how to counteract their adverse effects use this link for a white paper I wrote on the use of portable generators in motion picture production.

 

 

This article is cited 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 article is available online at http://www.screenlightandgrip.com/html/emailnewsletter_generators.html.

 

Guy Holt, Gaffer, ScreenLight & Grip, Lighting & Rental and Sales in Boston.

[JD Hartman]

As a film set electrician, he or she should know that a generator will not deliver the full rated amps configured for single phase. Mistake number one.

Per your football example, you could play it safe and double up on your neutral. In theory, the neutral should be carrying no current, in reality, that is seldom the case. Yes, extra cost, I realize that.

 

Some of the issues you mention, while very real could have been mitigated to a degree. One could specify only HMI ballasts with PFC from the rental house. Same holds true for Kino ballasts. I realize that in the case of K5600 products, this would still be an issue. Maybe their newer products will be PFC, but inquires to them are left un-answered.

 

I totally get your point and appreciate all the information you post. It's not a one size fits all world, I realize that too.

 

At the end of the day, if I have to get any on my hands, I prefer the smell of diesel fuel over gasoline. Gloves or not, I can get the stink of diesel off my skin.

[Guy Holt]

 

Per your football example, you could play it safe and double up on your neutral. In theory, the neutral should be carrying no current, in reality, that is seldom the case. Yes, extra cost, I realize that.

 

Doubling up on the neutral would not help here because in single phase distribution, as is the case here, harmonic currents in the neutral are minimal under balanced load conditions. It is only in three phase distribution systems that triplen harmonic currents are additive on the neutral and can result in the neutral overheating, which is another reason to use paralleled Honda EU7000s over a small diesel plant operating three phase.

 

To demonstrate this in a power quality workshop I developed for IATSE Local 481, we look at the harmonic currents contributed to the neutral return by each phase leg of a three phase system, and then a single phase system, by turning on a Kino Image 80s on each phase leg one at a time.

 

As you can see in the Power Point slide from the workshop above, Image 85s are rich in harmonics with a THD of 77% and a large triplen component (3rd, 9th, & 15th.) In the workshop I demonstrate how and why these non-linear lighting fixtures can lead to elevated neutral returns in 3-phase systems by watching what happens on a power quality meter as fixtures on separate legs of the service are turned on one at a time.

 

 

As you can see in the power quality readings above, absent phases B & C, the Image 85 on phase A returned 9A to the neutral (left picture.) When the Image 85 on phase B is switched on, and it returns another 9A to the neutral conductor, the current on the neutral climbs to 12.71A (center picture.) And, finally, when the Image 85 on phase C is switched on, and it returns another 9A to the neutral conductor, the current on the neutral climbs to 15.86A (right picture.) Even though the three phases are perfectly balanced (9A on each) the current on the neutral is 176% of any one of the individual phase legs. Clearly, there is some cancellation between the phase legs going on (otherwise the neutral would be carrying 27A), but it is not complete cancellation. Why?

 

When we dump return current from each leg into the “stew pot” of the common neutral, out of phase current cancels. The Fundamentals (A1,B1,C1 ) cancel each other out. The positive sequence harmonics (4th,7th, etc.) cancel each other out. The negative sequence harmonics (2nd, 5th etc.) cancel each other out. But, the zero sequence harmonics (3rd, 9th, 15th, etc.) do not cancel each other out. Instead they add. Why?

 

If, for a moment, we consider only the 3rd harmonic (180 Hz) of each phase as they return on the neutral, you will notice in the illustration below that in each positive half-cycle of any of the fundamental waveforms, there are exactly two positive half-cycles and one negative half-cycle of 3rd harmonic.

 

The net result, as illustrated above, is that the 3rd-harmonic waveforms of three 120 degree phase-shifted fundamental-frequency waveforms are actually in phase with each other and so stack on one another rather than cancel out as the fundamentals, positive, and negative sequence harmonics do. The phase shift figure of 120 degrees generally assumed in three-phase AC systems applies only to the fundamental frequencies, not to their harmonic multiples. A closer look at the harmonic currents making up the neutral return of our demonstration setup reveals that, though made up primarily of the third harmonics from each phase stacking one on another, the high neutral current also consists of the, 9th, and 15th harmonics from each phase stacking one on another as well.

 

As is evident in the illustration above, it is an altogether different situation in single-phase systems. Since the phase angle between legs is 180 degrees rather than 120 degrees, 3rd harmonic currents are also out of phase and will for the most part cancel just as the Fundamentals do. This is evident in the power quality meter reading below of the neutral of a single-phase system for first just one Image 85 on one leg (left) and then a 2nd Image 85 on the other leg which is 180 degrees out of phase (right.) As you can see here, when the two legs of a single-phase system are perfectly balanced (9 Amps on each) the triplen currents generated by a non-linear load will nearly cancel out – but not completely.

 

 

The issues described in my posts have to do with voltage waveform distortion caused by the harmonic currents non-linear loads draw on the hot legs and has nothing to do with the harmonic currents they return on the neutral. The voltage flat topping described in my posts above can also effect other equipment, such as monitors, battery chargers, and lap tops operating on the same power – but more on that latter.

 

 

Guy Holt, Gaffer

ScreeenLight & Grip

Lighting Rental & Sales in Boston