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[Frank DeWitt]

Green venues bring people from far and wide so you will be limited on the size of the solar array because of the size of the parking lot you will need.  Figure extra spaces for the Hybrid only parking.  Parking spaces for vehicles like the Chevrolet Tahoe Hybrid need to be large.

[Mike Sokol]

Sure. What's the battery budget.... Hope you have deep pockets....
I participated in an experiment a few years back. Before LED lighting really took hold.

My point is that as newer low-energy products such as LED lighting and switched power supply amps get more common, solar panels become more affordable, and battery technology improves, how close are we to doing something like this without breaking the bank. Sure, it was next to impossible 20 years ago, and probably stinkin' expensive 10 years ago, but what would the numbers look like now? I have a solar cell manufacturer talking to me about building an RV camper totally "green" so you don't have to listen the the generator in the woods (without the rooftop air conditioner running, of course). So how long will it be for this sort of music production can happen "in a field"? Is it now, or 5 years, or 10 years, or what? I did a little play over the winter with a bunch of LED lighting fixtures (maybe 16 total) powered by a single 15-amp outlet. That was pretty impressive since I didn't have to run feeder cable from a genny in the parking lot due to the fact that the electrical system in the building was really old and limited.

[Tim McCulloch]

It depends on artist requirements and audience expectations.  I think we could do Traveling Shakespeare In the Park on a 20 amp circuit for both sound and lights (LED lighting).  We used to do it with 3 circuits with lighting on all 3 and sound on the least-loaded circuit; the lighting was pretty dismal.

I don't think we'd get away with that for the Husker Du reunion... Oh, wait, Bob Mould said that would require lawyers on stage... never mind.

[Guy Holt, Gaffer]

… choices are needed. Power source  Inverter generator,  Conventional generator,  Batteries …

The primary factors limiting the use of non-linear loads on portable generators are their inefficient use of power and the harmonic currents they generate. Because of their radically different designs, inverter generators, like the EU6500is, and conventional AVR generators, like the ES6500, react very differently to these factors. The harmonic currents generated by non-linear loads, in fact, have less of an adverse effect on inverter generators than they do on conventional AVR generators and so do not require de-rating as conventional AVR generators do.

Harmonic currents cause two major problems in conventional AVR generators: heat and voltage waveform distortion.  The first problem is that harmonic currents generate heat in the windings, core, and in the electromagnets of the rotor of conventional AVR generators. Since generator ratings are limited by allowable temperature rise, harmonics act as derating factors. In derating, the magnitude of the current is of obvious importance, because losses are proportional to the square of the current. Increased frequency causes increased core losses and increased copper loss from skin effect. 5th and 7th harmonics are the offenders here because they are in the 600 Hz range.

The second difficulty caused by harmonic currents is voltage waveform distortion. According to Ohm’s law, as each harmonic current encounters the impedance of the power distribution system, it will cause a voltage drop at the same harmonic voltage. Because, the capacitors of switch mode power supplies only draw current at the peak of the voltage waveform, this voltage drop occurs only at the peak of the voltage - leading to a flat topping of the voltage waveform.

The more harmonic content in the current, the more voltage distortion occurs throughout the distribution system. This includes the output terminals of the generator where the generator’s source impedance (particularly the subtransient reactance or “Xd”) will create the greatest voltage drops. If the flat topping distortion at the generator’s output terminals is severe, it can cause voltage regulator sensing problems (Self-Excitation or SE) and inaccurate instrument readings.

The effect that harmonic currents have on the generators is factored into the rating limits given them. How rating limits are affected by load can be illustrated in a “Limit Characteristic” graph that plots kVA and kW versus Power Factor. The fluctuations in the kVA line in the illustration below represent the generator’s operating limits depending on whether its load has a leading or lagging Power Factor. It is important to note that a generator’s Limit Characteristic graph will vary by the type of generator. The illustration below (courtesy of Caterpillar) is for a conventional AVR generator. Since the power quality of an AVR generator is intractably linked to its' engine - the effect of harmonics on the engine's governing system is the primary limiting factor. How the engine and its' governing systems are affected by lagging and leading power factor loads is illustrated by the engine kW limit line below.

A Limit Characteristic graph for a generator illustrates the effect of leading
or lagging Power Factor on the generator's output.
 

What this generator’s Limit Characteristic graph tells us is that, operating a capacitive non-linear load (the leading power factor quadrant right of the Unity Power Factor center line), this AVR generator first reaches a thermal limit as a consequence of heat generation in the generator's rotor from harmonic currents. And since, conventional AVR generators regulate voltage by means of a power feedback loop from the generator Stator (via the Sensor Coil), through the Exciter (Voltage Regulator), to electromagnets in the Rotor, Armature flux generated by harmonic currents in the Stator leads to erroneous Self-Excitation (SE) and therefore voltage. Put simply, lower power factor loads cause instability of the generator’s voltage output. Finally, since there comes a point as the Power Factor of the load decreases, when harmonics inhibit the successful operation of the generator’s Automatic Voltage Regulator all together, and hence the generator’s capacity to generate any power at all, the kW output eventually drops to zero. Since the voltage instability in conventional AVR generators is a function of the non-linear loads they power, the conventional wisdom is to limit the amount of non-linear loads it can power by roughly half of the generators capacity.

The Limit Characteristic graph for an Inverter generator.
Note the negligible effect that leading Power Factor loads have on the generator's power capacity.
 

As the Limit Characteristic graph for an inverter generator above illustrates, it is a completely different situation with inverter generators. Because the speed of the motor is always changing, inverter generators cannot maintain voltage output by the conventional means of regulating the excitation current in Rotor electromagnets. Instead, inverter generators use permanent magnets in place of electromagnets. 
A permanent Magnet is an object made from a material that is naturally magnetized (Neodymium in this case) and hence creates its own persistent magnetic field. Since permanent magnets do not require an excitation circuit, armature flux created by harmonic currents will not cause voltage instability as it does in conventional AVR generators.

Left: Conventional AVR Generator w/1200W non-pfc electronic ballast. Right: Inverter Generator w/1200W non-pfc electronic ballast.

And since the generator’s inverter completely processes the raw power generated by the permanent magnet (converting it to DC before converting it back to AC by means of a micro-processor), the AC power it generates is completely independent of the engine. By switching IGBTs according to Pulse Width Modulation control logic, an inverter generator is much better able to sustain output voltage against transient loads and, therefore, it has a much lower internal reactance compared to conventional AVR machines. Finally, since the Impedance encountered by harmonic currents that causes voltage waveform distortion is a function of the internal reactance of the generator’s engine to changes in load, a second benefit to using permanent magnets in place of electromagnets in the generator’s Rotor is that as is evident in the oscilloscope shots above, inverter generators consequently are much less susceptible to voltage waveform distortion.

 

TABLE COURTESY OF KIRK KLEINSCHMIDT.

The end result is that leading power factor loads do not cause voltage regulation errors in inverter generators as they do in conventional AVR generators. Inverter generators are able to hold their voltage stable within ±1% of the mean voltage, as opposed to the ±3% of conventional generators using analogue AVRs and are much less susceptible to voltage drop and AC Frequency (Hz) as a function of load (see table above.) 

The rock solid power and low sub-transient impedance of inverter generators enable you to operate larger non-linear loads on them than can be operated on conventional AVR generators. For instance, we have struck 6kw HMI Pars on a modified Honda EU6500is inverter generator without problem.

These power quality issues have been vexing film electricians for years, to learn more about how we have learned to remediate the adverse effects of harmonics read a white I have written on the use of portable generators in motion picture production available at http://www.screenlightandgrip.com/html/emailnewsletter_generators.html.

Guy Holt, Gaffer
ScreenLight & Grip
Lighting Rental and Sales in Boston
Cell 617-224-85634
[email protected]

[Art Welter]

I'm going to up the challenge a bit and suggest that you also need to consider stage lighting as well for your OTG (Off The Grid) system. So let's say you wanted to create a weekly "green" outside concert series that was only powered from the solar cells you use for recharging your batteries during the week. Just how many solar cells  and batteries would you need to have a decent concert on Saturday night? Limiting the musicians to low power backline gear such as Line-6 guitar and bass pedals would be a start. And certainly LED lighting for a 3-hour show wouldn't draw too much power. Efficient speaker and amplifier selection would be very important as well. Could you get a decent 100 dB SPL concert mix for a few thousand people in a "Green Shed" powered by nothing but sunlight?

The intensity level of lighting, SPL desired and efficiency of both lighting and sound are primary considerations.
If lighting were done at a "moderate" level, say using thirty 25 watt RGB LED lights, only about 1050 watts would be required, assuming it takes 35 watts per 25 watt unit (which is what the RGB LED lights I use draw).
Sound and stage power could be run using less than 2000 watts average with an efficient sound system and moderate stage gear and achieve over 100 dBC for a few thousand people. I found out that for fact when a miswired guitar amp tripped a breaker, the single breaker served four separate quad receptacles which I had assumed (duh) were on separate breakers, killing the sound system as well as the stage power  .

In a high insolation area as we have in the summer here in the Santa Fe and Albuquerque New Mexico, allowing for loss in the inverters and battery storage, a system with around thirty 100 watt panels, about 235 square feet (only a fraction of the area of a 30' x 40' stage roof) could provide continuous daylight operation of both sound and light systems.

Depending on insolation and battery storage size, between one to several days of charge could provide all the power needs for a "Green" show. For shows using tons of video screens, incandescent lighting, a stage full of tube amps, and low efficiency main speakers the power demands would be several orders of magnitude greater.

[Mike Sokol]

Depending on insolation and battery storage size, between one to several days of charge could provide all the power needs for a "Green" show.

So just as a WAG for battery size, if we assume 4KW average use during a 3 hour show (your 2KW for FOH speakers, 1 KW for lighting, and maybe 1KW for losses, backline power and monitors) that could be 40 amps total (rounding up) at 120 volts, which would become 400 amps at 12 volts (yeah, there will be additional losses, but this is a WAG). So that's 1,200 amp/hours worth of 12-volt batteries at 100% discharge. I think the general rule of thumb is not to exceed 50% of battery amp/hour capacity, so that will require 2,400 amp/hours worth of 12-volt deep-cycle batteries.

Trojan makes a L16RE-A 325 AH Deep Cycle Battery at 6 volts, so that's 8 x 2 batteries needed (gotta series stack the 6 volt batteries) for a total of maybe 16 batteries at $300 each (total maybe $5K) to power a 3-hour show you described. Or you could just get one big fork truck battery and do the entire show. Your backup power could be a single EU6500 generator or maybe a pair of EU2000s which is certainly doable.

Also, I think at least some LED stage lights have a 12-volt DC option, so that could eliminate some of the inverter losses for lights, but maybe that would increase the line losses due to voltage drop. Argh...

So Frank's original idea isn't out of the question even after I added LED lights and backline power, and could be feasible as a promotion of battery, solar cell and inverter technology. Hey, we could even throw a wind turbine up on the roof to make the wind generator guys happy as well. 

Now I'm not seriously proposing this project. Just musing over how high efficiency sound and lighting technology could make such a thing possible.

[Josh Millward]

Frank, I remember hearing that one of the Danley Sound Labs passive loudspeakers (Jericho?) has been driven by only an iPod.  Can't seem to find it via a web search at the moment...

That was at InfoComm2013.

The loudspeaker was Caleb. Ten feet tall, four feet wide, and five feet deep I think... it may have been five feet wide and six feet deep... No details on the website about Caleb.

Anyway, yes, they took the mini-plug output from an iPhone, hardwired it into all the circuits on an NL-8 connector, and played music through the thing with no crossover filters or anything. It was an amazing exercise in efficiency.

[Barry Singleton]

  I think about this often since Craig Leerman brought it up in a post a while ago.

  I have several Middle Atlantic 2200VA two rack space UPS units that can be connected to external 48V battery packs to add capacity/run time to the internal batteries. I want to series four Optima's and see how long one of the smaller sound rigs would run on just that.

  I haven't done any measurement or math yet but plan on trying it when the weather breaks.

[Mike Sokol]

  I think about this often since Craig Leerman brought it up in a post a while ago.

  I have several Middle Atlantic 2200VA two rack space UPS units that can be connected to external 48V battery packs to add capacity/run time to the internal batteries. I want to series four Optima's and see how long one of the smaller sound rigs would run on just that.

  I haven't done any measurement or math yet but plan on trying it when the weather breaks.

Something like that would be very interesting. Of course, you could also stack eight of the 6-volt Trojan batteries for 48 volts. I often get requests for battery powered systems to do gigs like outside weddings in a park or a ground breaking ceremony. I have a Diehard 600-watt battery/inverter pack that does a reasonable job of powering a Fishman Line Array speaker for up to 2 hours, but there's a little buzz in the speaker from the inverter harmonics. That's works fine for what it is, but your setup might be able to power a small band in a remote area for an hour or two without a genny running.

[Frank DeWitt]

there's a little buzz in the speaker from the inverter harmonics.

I wonder if you ran the power through a isolation transformer. or even just hung the primary of a large transformer across the line, if that would kill the buzz.

[ProSoundWeb Community]