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Author Topic: Specifying Generators in Kva  (Read 13837 times)

Phillip_Graham

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Re: Specifying Generators in Kva
« Reply #10 on: October 12, 2005, 12:31:51 pm »

Bennett Prescott wrote on Wed, 12 October 2005 00:18

I would also be very appreciative if someone would explain to me what Kva is actually used for, and why they don't just spec the genny in, say, watts.


Power = voltage x current

KW is the same as KVA for purely resistive loads.

KVA is designed to be a reminder of the fact that you really need to do the phasor sum of the voltage and current, so break out your trig.

The component of the power where the voltage and current are in phase is effectively disspated in the load, performing work over time (power=energy/time or work/time)

The out of phase component is effectively "stored" in that it bounces back and forth to the load with each alternating cyle.

This return of energy is what is difficult for generators to handle, and the reason for power factor correction.

The power factor is the ratio of the dissipated power (in phase sum) to total power (Volt x Amps ignoring phase).

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Bob Healey

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Re: Specifying Generators in Kva
« Reply #11 on: October 12, 2005, 12:37:10 pm »

Condensed form of power calculations:

A/C power is a complex number of the form r + c*j.  A purely resistive load is a real number.  Power factor is the real power over the absolute value of the actual power.  Watts divided by power factor gives you KVa which is the actual power a generator can put out.

Its been a few years since I took a circuits course, but here are the notes I am working off of:
 http://www.ecse.rpi.edu/Courses/S03/ECSE-2010/lecture_materi als/Class%20slides%20(pdf)/class25S03.pdf

Edited to fix URL
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Bennett Prescott

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Re: Specifying Generators in Kva
« Reply #12 on: October 12, 2005, 06:06:05 pm »

I follow you until about halfway through there when you start talking about "stored" current and bouncing. Any chance I could get you to elaborate, and pretend I'm an idiot?
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-- Bennett Prescott
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RYAN LOUDMUSIC JENKINS

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Re: Specifying Generators in Kva
« Reply #13 on: October 12, 2005, 06:19:01 pm »

Hey Bennett,

I couldn't find the Power Factor for the GE generators but FYI the MQ Power generators that I spec for all of my events have a Power Factor of .8  The 25KVA gives 20kw output prime output and 22kw standby output.  I can't tell you exacty what "Standby" refers to but I go by the prime output rating.  I use them single phase 120/240 volts.  I have a friend that owns his own and he runs them as 3 phase.  In most instances I just use the 50 amp twistlock connectors and it works perfectly for me.

Hope that helps.  If I am not mistaken your rig is about 11,000 Watts F.O.H. and 5000 watts Monitors.  you could run you whole rig and backline off of one 50 amp connector and give the other to the light guy if two are available.  I assume your rig probably doesn't use any more than about 60 amps at 120 Volts? you'll be just fine.  Just watch out for the dimmer noise.
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Tim Duffin

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Re: Specifying Generators in Kva
« Reply #14 on: October 12, 2005, 07:58:35 pm »

theres also something wrong with that calculation, because a 6kva generator is also known as a 9000watt generator.  I just rented one last weekend.  I believe people spec KVA because with losses, one horsepower translates into about 1000 watts (not 746, thats only on paper).  So people with large motors which are 10Hp need 10KVA generators, etc.  

hope this helps

Tim

Bennett Prescott

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Re: Specifying Generators in Kva
« Reply #15 on: October 12, 2005, 08:54:53 pm »

RYAN LOUDMUSIC JENKINS wrote on Wed, 12 October 2005 18:19

Just watch out for the dimmer noise.


People keep saying this to me. Am I the only one with a properly grounded rig that uses equipment sans pin-1 problems? I have yet to have lampies add noise to my signal, although I've certainly seen the problem on others rigs but haven't had the time to troubleshoot to the point where I could definitively blame some SCR dimmers. Perhaps it's just that my cable runs are usually neatly contained and I'm therefore unlikely to have, say, and XLR connector sitting on top of a dimmer rack. Saved by my small size, perhaps?
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-- Bennett Prescott
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"Give me 6dB and I shall move the world." -Archimedes

Bob Healey

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Re: Specifying Generators in Kva
« Reply #16 on: October 12, 2005, 09:38:23 pm »

If I was actually using dimmers for this event, you'd probably have noise from these dimmers, especially the bigger ones.  I've found I generate the most noise when I have a large excess of feeder cable and no good place to put it.
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Rob Burgess

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Re: Specifying Generators in Kva
« Reply #17 on: October 12, 2005, 10:36:19 pm »

Bennett Prescott wrote on Wed, 12 October 2005 20:54

RYAN LOUDMUSIC JENKINS wrote on Wed, 12 October 2005 18:19

Just watch out for the dimmer noise.


People keep saying this to me. Am I the only one with a properly grounded rig that uses equipment sans pin-1 problems? I have yet to have lampies add noise to my signal, although I've certainly seen the problem on others rigs but haven't had the time to troubleshoot to the point where I could definitively blame some SCR dimmers. Perhaps it's just that my cable runs are usually neatly contained and I'm therefore unlikely to have, say, and XLR connector sitting on top of a dimmer rack. Saved by my small size, perhaps?




    Perhaps Smile  Some dimmers add gack to the AC, some not so much.  Some times I'm able to share power with the squints and other times It's been so bad that I had to rewire using wall outlets.  This is only a problem on medium sized gigs:  small gigs I can run off the wall easily and large gigs have a seperate feed.  It's the medium sized gigs that I worry about.

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Rob
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Rob Burgess

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Re: Specifying Generators in Kva
« Reply #18 on: October 12, 2005, 10:37:19 pm »

Bob Healey wrote on Wed, 12 October 2005 21:38

I've found I generate the most noise when I have a large excess of feeder cable and no good place to put it.


    You don't figure 8 the excess?

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Jim Cutshall

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Re: Specifying Generators in Kva
« Reply #19 on: October 12, 2005, 10:51:28 pm »

Aggreko and Cat 30KW gens are roughtly 100A / Leg 3 phase.  Depending on your needs for audio, the lighting rig described isn't even 20A / 3 phase at full output.  Generators are rated typically to run that full load 24/7 however, that load is usually static and doesn't flux like we make it do.  I wouldn't go over 70-75% capacity but that's me.

I've run audio and lighting off the same genset many times for fairly large festivals - the key is to make sure the gen is large enough to handle the surges of the lights "flashing".  It will also cause the Hz to flux so if you have any old organs (B3) it will go out of tune from the Hz not being correct but I've never had any noise from a properly setup rig.

FWIW,  

Jim
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Re: Specifying Generators in Kva
« Reply #19 on: October 12, 2005, 10:51:28 pm »


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