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balancing a generator


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#1 Colleen Marshall

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Posted 21 December 2009 - 06:06 PM

I was recently asked if I knew how to balance a generator, and the answer was "no" because I've never really worked with a generator before. (I'm still a film student)

Could someone give me advice on how to go about doing this?

Thanks!
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#2 John Sprung

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Posted 21 December 2009 - 08:51 PM

Balancing means keeping as close as you can to the same load on all the hots. So, if you have a typical single phase generator with two hots and you're running a 10K, a 5K, and two 2K's, you'd put the 10K on one side, and the rest, which add up to 9K on the other. With a three phase generator, you'd try to get things divided up into three equal groups. The hots are usually color coded black and red for single phase, and black, red, and blue for three phase.



-- J.S.
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#3 Kyle Reid

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Posted 22 December 2009 - 12:36 AM

I'm a graduate film student at FSU and have acted as BBE and here we balance our genny truck (3 legs) as described above with no issue.
Other practical advice:
1) Stay informed about how many lights are going up and how many amps ( paper amps are fine) they draw.
2) Be aware of when a light or lights are being turned off and will not be used as expected.
3) If you cannot balance the load on all the hot legs with the lights that are being used, you will have to "ghost" a light, which is what we call plugging in an extra light on the necessary leg in order to balance the load and aiming it away from the set into a safe place.
4) You can use an amp meter to make sure the loads are reasonably balanced (no more than ~10 amps of inequality).
5) Attach the ground and neutral legs of the cam lock first and unattached them last.

There's a lot more about safety with the genny than balancing the legs, be wary and read up on it, or ask a professor I suppose.

Edited by Kyle Reid, 22 December 2009 - 12:41 AM.

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#4 Brian McGee

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Posted 23 December 2009 - 09:09 AM

I'm a graduate film student at FSU and have acted as BBE and here we balance our genny truck (3 legs) as described above with no issue.
Other practical advice:
1) Stay informed about how many lights are going up and how many amps ( paper amps are fine) they draw.
2) Be aware of when a light or lights are being turned off and will not be used as expected.
3) If you cannot balance the load on all the hot legs with the lights that are being used, you will have to "ghost" a light, which is what we call plugging in an extra light on the necessary leg in order to balance the load and aiming it away from the set into a safe place.
4) You can use an amp meter to make sure the loads are reasonably balanced (no more than ~10 amps of inequality).
5) Attach the ground and neutral legs of the cam lock first and unattached them last.

There's a lot more about safety with the genny than balancing the legs, be wary and read up on it, or ask a professor I suppose.

Good advice Kyle, Balance is actually 20% of Total load used, if you have 100 amps on L1 then L2 and L3 need to be 80 to 120 amps each. Ground it correctly.
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#5 Kyle Reid

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Posted 23 December 2009 - 03:00 PM

Good advice Kyle, Balance is actually 20% of Total load used, if you have 100 amps on L1 then L2 and L3 need to be 80 to 120 amps each. Ground it correctly.

Oh cool, I didn't know it worked out to be 20% like that.
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#6 JD Hartman

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Posted 23 December 2009 - 03:26 PM

Grounding requiments vary from state to state and even city to city. Most (tow behind) generators are insulated from the earth and are designed to operate properly that way. If the generator must be earthed, a ground rod (one or more) one would have to be driven, which would meet the local requirements.
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#7 John Sprung

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Posted 23 December 2009 - 06:16 PM

If the generator must be earthed, a ground rod (one or more) one would have to be driven, which would meet the local requirements.


Required or not, it's always better to ground portable generators. Bond them to the utility ground if there is one, and if you're out in the wild, drive a piece of re-bar into the ground. This prevents the mild shocks you sometimes get from capacitance coupling the hot of a 120 volt run to ground.




-- J.S.
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#8 Colleen Marshall

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Posted 26 December 2009 - 04:39 AM

Thanks so much to everyone who responded, I really appreciate it.

If you fail to balance the generator correctly, what are the negative side effects? Also, when you are striking on or saving the lights, would it be best to do it simultaneously at the source, or turn it off at the generator?

Edited by Colleen Marshall, 26 December 2009 - 04:39 AM.

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#9 Matthew Parnell

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Posted 26 December 2009 - 08:21 AM

If you are running a generator the best thing to do is be vigilant, and check, communicate and check again.

-Check your oil and coolant levels before starting the generator.

-Check your oil pressure and water temperature regularly and be aware of what is the acceptable range for the generator.

-Check the load regularly.

-Communicate to the crew and keep them up to date with what phases a heavy and what phases are light, so the next lamp to go in, goes in on your lighter phase and helps keep everything in balance.

-Check your line voltages and frequencies, make sure all your phases are operating within range.

-If you can, throw a meter on your Neutral and keep and eye on it too.

-It might sound weird, but use your nose and ears around your generator, if it doesn't sound right, if it doesn't smell right, check it out.


When your aware that your going to have a load that is not going to be balanced, and it does happen, I have always been taught that if you have a single heavy load, load up B phase, if you have two heavy loads, load up A and C phases.

Some generators can handle off balance loads better than others, but its always best to be safe, a generator being out of balance can cause serious issues. They can range from plug burnouts from overloaded neutral pins, to your genset going up in flames and many more issues in between.
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#10 John Sprung

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Posted 26 December 2009 - 06:57 PM

If you fail to balance the generator correctly, what are the negative side effects?


Mechanically, having everything on one leg sets up vibration, more so on some makes/models than others. You can hear it in some cases. That puts extra wear on bearings, gears, etc. Electrically, it puts all the current, and therefore all the heat, on one set of windings. That cooks the insulation quicker on those wires than the rest. Both are long-term problems, so being way out of ballance for a few seconds as you turn things on or off isn't anything to worry about. The longer the duration and the heavier the load, the more important balance is.




-- J.S.
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#11 David McLean

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Posted 27 December 2009 - 12:02 PM

So here's my question. What if you have a lot of 220v stuff? We had a thing happen a couple of years ago in that we had a couple of 12Ks, a couple of 18Ks and a couple of 4Ks. The way the balance worked out, we put the 4Ks on one leg and the load was nice and even across the board. BUT, we ended up burning the neutral up because it was taking so much back because of the 4Ks (we think). So here's the thing: do you have to pay special attention to not just balancing the load, but also balancing the stuff that has a neutral? And is one more important than the other?

- dave@electricandgrip.com
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#12 John Sprung

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Posted 27 December 2009 - 02:04 PM

Hmmm -- that doesn't make sense. Balance is equivalent to minimizing the current in the neutral. On a 120/240 volt single phase system, the neutral current equals the difference between the hots. 240 volt loads on such a system are naturally balanced, since they put equal and opposite current in each hot and don't even touch the neutral. Given that the neutral fried, my best guess is that what you had really wasn't balanced at all. Did you check with one of those clamp-around ammeters from Fluke, Sperry, Simpson, etc.?




-- J.S.
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#13 Frank Barrera

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Posted 27 December 2009 - 09:21 PM

Balancing the legs is not just for generator power sources but also when using a tie-in from the local electric company. For example if you are working with 120V 3 phase @ something like 100AMP per leg you need to balance the load to efficiently use each leg. Each leg is often fused and blowing a fuse can cause a great deal of delay during production. Trust me, I blew a 200 amp fuse once and it was not pretty. We lost two hours until we could get access to the mains.

Of course keeping balance when using local power is great practice for when you do need to balance a generator.
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#14 Hal Smith

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Posted 27 December 2009 - 11:30 PM

So here's my question. What if you have a lot of 220v stuff? We had a thing happen a couple of years ago in that we had a couple of 12Ks, a couple of 18Ks and a couple of 4Ks. The way the balance worked out, we put the 4Ks on one leg and the load was nice and even across the board. BUT, we ended up burning the neutral up because it was taking so much back because of the 4Ks (we think). So here's the thing: do you have to pay special attention to not just balancing the load, but also balancing the stuff that has a neutral? And is one more important than the other?

- dave@electricandgrip.com


What was the service and how were the loads connected? If it was a three-phase circuit (I'm guessing this might have been the case from your saying "we put the 4K's on one leg") and all the loads were 208/240 volt then there normally wouldn't have been any neutral current. If that's true then I suspect one of your lights or cables had a short from one leg to neutral.
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#15 John Sprung

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Posted 28 December 2009 - 03:01 AM

If it's three phase, we also need to know if it's 120/208 Wye connected, or 120/240 Delta connected. Another possibility, do we know that this question is from here in the U.S., or could it be from elsewhere, where they have 240 volt branch circuits?




-- J.S.
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#16 Marc Roessler

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Posted 28 December 2009 - 07:48 AM

This also depends on the types of lights used...

The neutral wire is only for carrying "off balance" currents. Usually, with a perfectly balanced 3-phase-Y-circuit, neutral will not carry any current. Let's suppose you run a 2K tungsten between each of the three phases and ground, then neutral is current free. Only when the balance is off, e.g. because you just switched off one of the lights, neutral will start to carry the current difference.
It is important to note that this current difference (under "normal" circumstances) can not be higher than any of the three phase currents. This means, as long as the neutral wire is as heavy (AWG) as each of the phase wires, it can't burn up.

Well - as I wrote, "normal" circumstances...
Enter the "special" power consumers, known as: uncompensated electronic switchers, and magnetic ballasts of cheap design... in this case, current consumption either has a shifted phase or a differrent current draw pattern altogether (high frequency switched power supplies, having high frequency harmonics; magnetic ballasts going into saturation, thus generating harmonics). This means that the phases' currents won't align any more and thus the return currents in neutral won't cancel out each other any more - in part, they will ADD! And this is what will burn up your neutral wire. And the nasty thing is: you can't measure these currents with an average RMS current meter, because the currents are not 50/60 Hz, but much more complex.

The good solution is to use power factor compensated and sufficiently filtered devices (ballasts etc). This will spare you a lot of trouble and is the way to go. This is really up to the manufacturers of equipment to do. You can use external filters/compensation, but it gets clumsy quickly.

The "hack" solution is to use a larger genny/thicker cables. You will need to supply more power (thus spend more money) just to make sure your wires/genny don't burn up - despite that fact that you're not even really consuming that power... It works, but from a safety / economic viewpoint it is not desirable.

Regards,
Marc

Edited by Marc Roessler, 28 December 2009 - 07:50 AM.

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#17 Marc Roessler

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Posted 28 December 2009 - 09:22 AM

Let's suppose you run a 2K tungsten between each of the three phases and ground, then neutral is current free.


A minor but VERY important correction:
I meant to say "Let's suppose you run a 2K tungsten between each of the three phases and neutral, then neutral is current free.
Of course you never run anything between phase and ground :ph34r:
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#18 John Sprung

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Posted 28 December 2009 - 01:16 PM

Well, theoretically current free. Tungsten or quarts halogen lamps aren't high precision devices, so I'd expect a few hundred mA difference to be carried by the neutral. And perhaps even a detectable color temperature difference.




-- J.S.
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#19 Hal Smith

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Posted 28 December 2009 - 01:53 PM

Of course you never run anything between phase and ground :ph34r:


At the closest distribution transformer the neutral and ground are bonded together and earthed.

Separation of neutral and ground wires to the loads is a safety consideration. The separate ground wire is there solely to supply an earth ground to any and all accessible conductive surfaces. In the earlier post where Frank Barrera reported having burnt out a neutral that could have created an extremely dangerous situation if there wasn't a ground still being supplied to the equipment. Without the ground, my hypothesis about a short between on leg and neutral would have meant that after the burn out there easily could have been 208/240 volts between an equipment case and earth ground.
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#20 Guy Holt

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Posted 28 December 2009 - 05:25 PM

So here's my question. What if you have a lot of 220v stuff? We had a thing happen a couple of years ago in that we had a couple of 12Ks, a couple of 18Ks and a couple of 4Ks. The way the balance worked out, we put the 4Ks on one leg and the load was nice and even across the board. BUT, we ended up burning the neutral up because it was taking so much back because of the 4Ks (we think). So here's the thing: do you have to pay special attention to not just balancing the load, but also balancing the stuff that has a neutral? And is one more important than the other?- dave@electricandgrip.com


David McLean’s experience demonstrates, there is a lot more to operating a generator than just balancing its’ load. While there is good information in this thread on how to balance a generator, there has been very little discussion of why generators need to be balanced in the first place. Without an understanding of why generators should be balanced and the difference between lighting loads there is the possibility of creating a hazardous situation like David’s.
A little knowledge can be a dangerous thing – especially when it comes to handling electricity – so I would like to take this opportunity to explain in some detail why the neutral burned on David’s shoot.

As Marc Roessler correctly observed in his post the reason for David’s problem has to do with the vagaries associated with the load he put on the generator. If you haven't already, I would suggest you read the article I wrote for our company newsletter on the use of portable generators in motion picture lighting. In it I cover some of the basic electrical engineering principles behind harmonic distortion and how it can adversely effect generators. The article is available on our website.

As John Sprung correctly states one of the primary reasons to balance a generator is to minimize current on the neutral return. Where there are two (single phase) or three (three phase) hot wires to only one neutral wire, you must balance the load on the hot legs so that there is minimal return on the neutral after phase cancellation. If you don’t balance the load there is the possibility of overloading the neutral wire – a potentially hazardous situation.

If we draw equal current from each leg of a single phase generator with incandescent lights (a resistive or linear load), there will be no return current on the neutral. That is because with a linear load current & voltage remain in sync, and where the legs are 180 degrees out of phase, the current cancels out when combined on the neutral return. If we draw 100A on one leg and 140A on the other leg, we will have 40A on the neutral. With a linear load, the more balanced the load the less current on the neutral return. Things get a bit more complicated with inductive (magnetic HMI ballasts) and capacitive (electronic HMI & Kino ballasts) non-linear lighting loads, or the “special” consumers Marc refers to. Since non-linear loads cause current and voltage to be out of sync, the phase currents no longer entirely cancel when they return on the neutral. When using magnetic HMI ballasts, it is normal to have as much as 20-25% of the total amperage return on the neutral when the legs are evenly loaded.

Electronic square wave ballasts (both HMI & Kino), in addition to pulling the voltage and current out of phase, also create harmonic currents that can stack on top of one another, creating very high currents returning to the power source on the neutral wire. As David found out the hard way, if the neutral return path has not been oversized to accommodate additional current, these high currents can cause excessive heat on the neutral wire, and the neutral bus of the generator. Where the neutral of a distribution system is not fused, this excessive heat can lead to a possibly hazardous situation. Where high currents on the neutral can be hazardous, it is important to understand the root cause of these currents.

Electronic square wave HMI ballasts are a major source of harmonic currents. These currents are produced by the diode-capacitor section of the ballast. This is the part of the ballast that rectifies the AC input power into the DC power that is then used by the power module to create the square wave. The diode-capacitor section accomplishes this by first feeding the AC input current through a full wave bridge rectifier, which inverts the negative half of the AC sine wave and makes it positive. The rectified current then passes into a bank of capacitors which removes the 60 Hz rise and fall and flattens out the voltage-making it essentially DC. The DC is then fed from these capacitors to the power module. Since the rectifying circuit of the power supply only draws current from the AC line during the peaks of the supply voltage waveform, charging the capacitors to the peak of the line voltage, these power supplies draw current in high amplitude short pulses. The remaining unused current feeds back into the power stream as harmonic noise that distorts the voltage waveform at the distribution bus.

Of the harmonic currents that electronic ballasts generate, the odd harmonics (i.e. 3rd, 5th, 7th, 9th, etc.) are more of a concern because the even harmonics have a tendency to cancel out. Of these the 3rd harmonic, and odd multiples of the 3rd (9th, 15th, etc) are particularly troublesome. These harmonics are called the “triplens.” What makes them troublesome is that the triplen harmonics dumped back onto each phase of the distribution system are all in phase with each other. For this reason, rather than cancel each other out on the neutral conductor, as the out of phase fundamentals do, they instead add up. If the lighting package consists entirely of electronic HMI & Kino ballasts without power factor correction, about 80 percent of the current does not cancel out between legs, resulting in very high current on the neutral return.

Posted Image
The Triplin Harmonics (shaded) add rather than cancel on the neutral return


For example, even if we draw a perfectly balanced load of 125A per leg on 2Awg banded feeder cable (rated for 190A) from a three phase generator (375A total), if our load consists predominantly of electronic HMI & Kino ballasts, we could potentially have upwards of 300A on the neutral wire. Since the neutral wire in banded feeder is also 2 Awg rated for 190A, return currents of this magnitude can cause sufficient heat to overload the neutral wire, and the neutral bus of the generator, leading to a possibly hazardous situation since the neutral return has no fused protection. Unfortunately, much of today's lighting technology is of a non-linear type because it relies on electronics such as DC rectifiers (electronic HMI ballasts), silicone-controlled rectifiers (SCRs), capacitors (magnetic & electronic HMI ballasts), and high-frequency switching power supplies (the IGBTs of electronic ballasts). And since this kind of load (non-linear) generates harmonic currents that can have undesirable effects like stacking on the neutral, it is no longer sufficient to just balance the load on a generator. One must also closely watch the current on the neutral return and take preventive measures when there is the possibility that it will be high.

On large film sets it is a standard practice when powering large numbers of electronic ballasts without Power Factor Correction to size the neutral feeder of the distribution system to carry the sum of the currents of the phase legs times 80 percent (.8). Likewise, the generator is typically oversized to handle the higher return current. On small film sets using small portable gas generators other measures must be taken. Other measures must be taken because the means by which the motion picture industry has more or less successfully dealt with harmonics - namely the over-sizing of generators, the over-sizing of neutrals, the incorporation of power factor correction circuitry in large HMI ballasts, and finally the use of generators, like the Crawford Studio Generators, with 2/3 pitch windings are generally not available to users of small portable generators as their primary source of power. The reason being, productions using portable gas generators are using them by necessity. For budgetary or logistical reasons, it is simply not an option to upscale their generator and customize their distribution package to accommodate a dirty load; and until very recently, power factor correction has not been available in HMI ballasts smaller than 4kw.

Not only do users of small portable gas generators have to find other means of remediating the adverse effects of harmonic distortion, but they also have to deal with much higher levels of Total Harmonic Distortion (THD.) It is a basic principle in electrical engineering that the magnitude of voltage waveform distortion is a function of the inherent harmonic distortion of the applied power waveform, the size of the source impedance, and the relative size of nonlinear loads with respect to the capacity of the power generating system. That is, the amount of voltage distortion increases as distortion of the applied waveform increases and the percentage of nonlinear loads taking up the total capacity of the power generating system increases.

As previously discussed, voltage waveform distortion as a result of harmonic currents is not a practical problem on large film sets because of remedial steps taken in the design of form specific generating and power distribution systems engineered to remediate the adverse effects of harmonic currents. With 2/3 pitch windings, MQ Power 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 and are typically oversized for the load, harmonic currents do not cause substantial voltage waveform distortion.

However, it is an all together different situation when plugging a couple of 1200W HMIs into a small portable generator that is not specifically designed to remediate the effects of harmonics. Given the comparatively large sub-transient impedance of conventional small gas generators, and the high THD value of their inherent power waveform (19.5% under full linear load), you have a situation where even a small amount of harmonics being fed back into the power stream will result in a large amount of harmonic distortion in its’ voltage. Making the matter worse is that, given the increasing prevalence of non-linear light sources in production, it is likely that the percentage of the generator’s capacity taken up by non-linear loads will be very high given its small size relative to the size of HMIs typically used on these generators (575-2500 Watts.) Conventional small gas generators present a perfect (electrical) storm where the return of any harmonic currents results in a very high degree of voltage waveform distortion.

For example, the power waveform below left (from my article) is typical of what results from the operation of a 2500W non-Power Factor Corrected load (electronic HMI & Kino ballasts) on a conventional portable generator (a Honda EX5500 with a Barber Coleman Governor.) Where the severe voltage waveform distortion exhibited here can cause overheating and failing equipment in addition to excessive current on the neutral return, it is even more imperative that users of small gas generators take preventive measures to minimize the voltage waveform distortion that can result from harmonic currents being dumped back into the power stream. Besides meticulously balancing the load (keeping legs within 20% simply won’t do it) users of small gas generators have two alternatives. The first is to de-rate the continuous load capacity of the generator so that the maximum load is small enough that the generator is able to accommodate the harmonic currents generated by the smaller load; or, alternatively eliminate harmonic currents being dumped back into the power stream by only using power factor corrected HMI and Kino electronic ballasts on inverter generators. For those of you not familiar with Power Factor Correction (PFC), a PFC circuit utilizes a RF Mains Filter to restrict the flow of harmonic currents back onto the supply service. In this fashion, the PFC circuit realigns voltage and current and induces a smoother power waveform at the distribution bus. Formerly only available in large HMI ballasts, this advanced electronics reduces voltage waveform distortion and contributes to a more economical use of power than typical HMI and fluorescent electronic ballasts.

Posted Image
Left: Conventional generator power w/ pkg. of non-PFC Elec. HMI Ballasts & Kino Flo Wall-o-Lite. Right: Inverter generator power w/ Pkg. of PFC Elec. Ballasts & Kino Flo Parabeam 400.


By de-rating the load capacity of a generator, you can minimize the adverse effects of harmonic noise so that the generator and the load operate more reliably. The conventional wisdom in the past has been to not load a generator beyond 75% of its continuous load capacity for more than a short period (the maximum recommend continuous load on a 6500W generator, with a continuous load rating of 5500W, would be roughly 4000 watts.) However, this conventional wisdom no longer holds true of inverter generators when used with Power Factor Corrected (PFC) HMI & Kino ballasts. For example, the power waveform above on the right, is the same 2500W load but with power factor correction operating on our modified Honda EU6500is Inverter Generator. As you can see, the difference between the resulting waveforms is startling. Even though the load is the same, the fact that it is power factor corrected and the power is being generated by an inverter generator, results in virtually no power waveform distortion.

What this means is that an inverter generator can be loaded to capacity with PFC HMI and Kino Flo ballasts without any adverse effects. The substantial reduction in line noise that results from using PFC ballasts on the nearly pure power waveform of an inverter generator creates a new math when it comes to calculating the continuous load you can put on a portable gas generator. And if the inverter generator is one of our modified Honda EU6500is generators, you will be able to power a continuous load of 7500 Watts as long as your HMI and Kino ballasts are Power Factor Corrected.

Posted Image
Wide Shot of Night exterior scene lit with a pkg. consisting of PFC 2.5 & 1.2 HMI Pars, PFC 800w Joker HMI, Kino Flo Flat Head 80, 2 ParaBeam 400s, and a ParaBeam 200 powered by a modified Honda EU6500is.


We maximize the continuous load that can run off the enhanced output of our modified Honda EU6500is inverter generator, by operating HMI and Kino Flo lights with Power Factor Corrected ballasts through a 240V-to-120V step down transformer/distro that perfectly balances the load on the two legs of the generator.

Posted Image
The PFC 2.5 & 1.2 HMI Pars, PFC 800w Joker HMI, Kino Flo Flat Head 80, 2 ParaBeam 400s, and a ParaBeam 200 of our HD P&P Pkg. powered by our modified Honda EU6500is through our 60A Full Power Transformer/Distro


Where, in the past we had to de-rate portable generators because of the inherent short comings of conventional generators when dealing with the harmonic noise generated by non-PFC electronic ballasts; now you can load an inverter generator to capacity. And, the generator is able to handle the load more easily because the transformer/distro perfectly balances the load. According to this new math, when you add up the incremental savings in power to be gained by using only PFC HMI ballasts, add to it energy efficient sources like the Kino Flo Parabeam fixtures, and combine it with the pure waveform of inverter generators, you can run more HMI lights on a portable gas generator than has been possible before. For example, the 7500W capacity of our modified Honda EU6500is Inverter Generator can power a lighting package that consists of a PFC 2.5kw HMI Par, PFC 1200, & 800 HMI Pars, a couple of Kino Flo ParaBeam 400s, a couple of ParaBeam 200s, and a Flat Head 80. Given the light sensitivity of HD cameras, this is pretty much all the light you will need to light even night exteriors.

Posted Image
A Distro System consisting of a 60A Full Power Transformer/Distro, 2-60A GPC (Bates) Splitters, 2-60A Woodhead Box distributes power from a modified Honda EU6500is. Even though the generator is 100' away to reduce noise, plug-in points remain conveniently close to set.


It is beyond the scope of this post to go into more details about harmonic distortion and why it has an adverse effect on power from generators when it does not on power from the grid . Those interested should read the article I wrote for our company newsletter (mentioned above) on the use of portable generators in motion picture production. The article is available at www.screenlightandgrip.com/html/emailnewsletter_generators.html.)

Guy Holt, Gaffer, [url="http://www.screenlightandgrip.com""] ScreenLight & Grip [/url], Boston
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