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Author Topic: What is the correct way to do this outdoor event  (Read 7277 times)

Guy Holt

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Re: What is the correct way to do this outdoor event
« Reply #20 on: May 25, 2017, 05:59:12 pm »

The voltage drop over 200' of 6/4 awg cable is pretty significant, IIRC.

Another approach would be to use a small boost step-down transformer  at the end of the cable run to compensate for the voltage drop.  We use 7.5kVA full power transformer/distros all the time on our modified 7500W Honda EU6500is generators to compensate for the voltage drop caused by the long cable runs to set in motion picture lighting and the voltage drop on the EU6500s under load.  Our full power transformer/distros are equipped with three switch selected taps to increase voltage on the secondary by 0, 5, or 10 percent.

Guy Holt, Gaffer
ScreenLight & Grip
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Cailen Waddell

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Re: What is the correct way to do this outdoor event
« Reply #21 on: May 25, 2017, 06:02:14 pm »

A few of our regular corporate and municipal gigs can put any kind of service on their department credit card up to a certain dollar amount without any bidding process. But go one penny over the threshold $ number and a whole lotta paperwork starts. So find out what that dollar about is and you might be able to make it happen.

Yes.   Luckily my bid threshold on construction is into the $100's of thousands.  For equipment purchasing I have $90k bid threshold, and 3 quotes below. 

Yeah - if you are luckily in your municipality you have an amazing purchasing staff that help you follow the letter of the law .  I am that lucky.


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Ed Hall

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Re: What is the correct way to do this outdoor event
« Reply #22 on: May 25, 2017, 09:58:52 pm »

Another approach would be to use a small boost step-down transformer  at the end of the cable run to compensate for the voltage drop.  We use 7.5kVA full power transformer/distros all the time on our modified 7500W Honda EU6500is generators to compensate for the voltage drop caused by the long cable runs to set in motion picture lighting and the voltage drop on the EU6500s under load.  Our full power transformer/distros are equipped with three switch selected taps to increase voltage on the secondary by 0, 5, or 10 percent.

Guy Holt, Gaffer
ScreenLight & Grip

When using the boost setting on the transformer to compensate for the voltage drop, I'm assuming you get a reduction in current when boosting the voltage?
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Mike Sokol

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Re: What is the correct way to do this outdoor event
« Reply #23 on: May 26, 2017, 07:15:28 am »

When using the boost setting on the transformer to compensate for the voltage drop, I'm assuming you get a reduction in current when boosting the voltage?

While voltage boost taps on transformers are useful for constant loads such as lighting, they're of limited value with typical audio loads that have a huge crest factor. That being said, if you could do tricks like step up the voltage to 480 volts on the feeder side then step it back down to 240/120 volts on the load side, you would reduce the voltage drop on the load side significantly. So yes, you get a reduction in the current required in the long feeder run, and that's what reduces the voltage drop. But this is a bit of a trick question with switching power supplies in modern pro-sound gear because they're not linear loads. You still have to consider the load's power factor and crest factor to predict total voltage drop in the wiring. 
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Mike Sokol
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Guy Holt

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Re: What is the correct way to do this outdoor event
« Reply #24 on: May 26, 2017, 08:57:45 pm »

So yes, you get a reduction in the current required in the long feeder run, and that's what reduces the voltage drop. But this is a bit of a trick question with switching power supplies in modern pro-sound gear because they're not linear loads. You still have to consider the load's power factor and crest factor to predict total voltage drop in the wiring.

Every time you use house power, you operate the switching power supplies  of your modern pro-sound gear on a transformer - usually a 480/277 to 208/120 Delta--to-Wye utility transformer.  While there are legitimate concerns to operating non-linear loads that will draw harmonic currents on a transformer (excessive heat generation being the primary), a step-down transformer will not distort the voltage waveform.  In fact, because of their low impedance, you will get a cleaner voltage waveform on a step-down transformer operating on utility power than you will get from a generator - even the Honda Inverter generators. In addition to the voltage flat topping generated when the harmonics drawn by a non-linear load encounter the relatively higher impedance of the Honda Inverter generators, the Hondas will also drop up to 10V under load. The inverters drop voltage under load because they must use a droop voltage regulation because its permanent magnets can't be regulated by an excitation current as is the case with the electro-magnetics of an AVR system.  Without a doubt,  a step-down transformer operating on utility power will provide a higher quality of  power,  more stable and less distorted,  than that generated by an inverter generator. Add the reduced current draw  and subsequent voltage drop, Mike mentioned, and you can see that there are real benefits to using a small step-down transformer with boost taps - which is why we use them with our Honda generators. With proper sizing and design (K Factor Rating) to reduce heat generation, a step-down transformer will always supply better quality power with non-linear loads than a generator, which is the difference between hard and soft power. For more details on how the relatively high impedance of portable generators can lead to severe voltage distortion when powering non-linear loads,  see a three part series on production power on a budget I just completed for Protocol Magazine (available at http://www.screenlightandgrip.com/html/hd_plug-n-play_pkg.html.)

Guy Holt, Gaffer
ScreenLight & Grip
« Last Edit: May 27, 2017, 07:16:23 pm by Guy Holt »
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eric lenasbunt

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Re: What is the correct way to do this outdoor event
« Reply #25 on: May 26, 2017, 09:42:02 pm »

As others have said, just get them to install a proper 50amp outlet. I've had these put in in a pile of venues, both municipal and private and it has never cost more than about $200 (of course I'm sure NYC is more money). It's a pretty negligible expense and it really is the correct and safe way to do it.

Plus then you don't need to leave $600+ of feeder in a room for months at a time.


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Stephen Swaffer

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Re: What is the correct way to do this outdoor event
« Reply #26 on: May 26, 2017, 11:36:42 pm »

Mike's reference on the transformer is primarily directed at the crest factor in audio-not running on a transformer per se.  If you adjust for the peak voltage drop that happens on a hard drum hit or whatever, then you will likely be running a higher voltage on your gear than you really want when it is "idling".  Your average voltage will be more to your liking, but you will still have larger swings in the voltage.  The best way to minimize the voltage variation on a dynamic load is to have more copper in the feeder.  Buck-boost transformers certainly have there place-and I have tweaked taps on step down transformers in buildings to compensate for long feeder or branch circuit runs-but it is not always the best or even a good option.
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Steve Swaffer

Mike Sokol

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Re: What is the correct way to do this outdoor event
« Reply #27 on: May 27, 2017, 07:37:23 am »

Mike's reference on the transformer is primarily directed at the crest factor in audio-not running on a transformer per se. 

Interestingly, the Furman M-8X AR is actually a set of buck/boost transformers with Triac switching, so it will automatically ratchet the AC voltage up or down to get it within a few volts of the desired level. Of course, it can only change the taps on the zero crossover of the line frequency, so it's not an actual regulator. But for brownout situations it could be useful.
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Mike Sokol
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Stephen Swaffer

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Re: What is the correct way to do this outdoor event
« Reply #28 on: May 27, 2017, 01:30:17 pm »

I knew there were "active" buck-boost devices-didn't know the manufacturers/models.  Do they respond quick enough to deal with a few cycle brown out?  I would be curious how fast they react.
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Steve Swaffer

Mike Sokol

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Re: What is the correct way to do this outdoor event
« Reply #29 on: May 27, 2017, 03:03:31 pm »

I knew there were "active" buck-boost devices-didn't know the manufacturers/models.  Do they respond quick enough to deal with a few cycle brown out?  I would be curious how fast they react.

I've never played with one, but from what I remember these are Triac switched taps and should respond with one line cycle.
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Mike Sokol
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ProSoundWeb Community

Re: What is the correct way to do this outdoor event
« Reply #29 on: May 27, 2017, 03:03:31 pm »


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