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Author Topic: Specifying Generators in Kva  (Read 13685 times)

Shane Ervin

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Re: electricity, energy, work, etc.
« Reply #50 on: October 19, 2005, 11:24:33 am »

Hi Ryan,

Hi Phill,

Great topic I've been watching unfold.  Kudos to Phill as usual for kind and thoughtful explanations.  Kudos to Ryan for applying healthy scrutiny.

As an electrical engineer with a background in power distribution and who paid his way through school in the engine room of oil tankers (that have diesel gennies), I thought I'd try to mediate (wrong word?).

I'm very much a close enough for rock'n'roll kind of guy, though, and that usually makes me want to skim over lower level details and retain just the broad strokes necessary for good, safe operation of gear so the beer can continue to be sold and audience members get laid after the show.

Power Factor, as a concept so named, can serve the very purpose of simplifying things for guys like me who just want to have an easy, yet solid engineering basis for safe and effective operation of distribution systems and the rotary machines feeding them (and loading them).  As such, and in this light, the good advice might be to learn the material in the earlier chapters of the EE textbook prior to delving (publicly at least) into the subtler details of subsequent chapters.

There's no harm in bringing calculus or dot product into the explanations on a forum like this, especially when it's Phill doing the talking and he simultaneously offers "theory lite" in follow-on sentences as he does for broader reader appeal (and accessibility I guess, too).

Equally there's no harm in responding with questions aimed at steering the thread towards basic application of theory-lite for the rock'n'roll purposes we're here for.

So all is well.

OK, then.  We're talking real power, apparent power, reactive power, power company pricing policies, genny rentals, cable sizes, resistive losses, mechanical torque applied to the prime mover's crankshaft to keep the thing spinning at the desired speed for 60 Hz or so, and uh... what was the original poster's question again...?  ( Hi Bennett! )

Ryan, Phill's just asking to you to consider that the prime mover only needs to overcome rotation friction and supply sufficient torque for real power to remain at the desired RPM.  He describes how purely reactive loads can be fed from a DG up to the nameplate rating with surprisingly little mechanical torque, since the only real power loading down the DG comes from I2R losses in the connecting cable in such a case.

He's also correct in describing the rationale power companies use in establishing the terms and conditions of industrial-grade service to factories with lots of squirrel cage motors.  Power factor most certainly is a concern for precisely the reasons he cites (and also explains why synchronous capacitors are used local to the inductively reactive load of a typical industrial plant).

But to properly size a rented DG, isn't it clouding the picture to bring Hydro accounting into the analytical framework for selecting the nameplate rating of a standalone machine?  There are better ways to arrive at Theory-Lite aimed at pragmatic application to Bennett's show-biz operations.
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Shane

Phillip_Graham

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Re: electricity, energy, work, etc.
« Reply #51 on: October 19, 2005, 01:41:53 pm »

JR [John Roberts

 wrote on Wed, 19 October 2005 10:25]
While to the limits of my understanding everything stated is accurate we seem to be missing how this impacts small generator systems. There has been nothing but silence to my query about what kind of reactive PFs could we expect from significant power consuming gear other than "lamps and amps" if any?


Yeah JR, I have no idea of the practical power factor numbers here.  Langston? Smile

Quote:


The one issue that I can speculate on is that most power amps and electronic gear draw current in two large bites per cycle at the mains voltage peaks. While still technically resistive (in phase with voltage) these concentrated current pulses will cause larger IR wiring caused voltage sag than would be predicted from the average current draw. Another possible parameter I have no idea about is the source impedance of the generator. Probably not as stiff as Con Edison but low enough to "git er done".


I think you are exactly right on the real issues here.  Wish Gene Pink was around, he might know something on this practically.  

Lee Patzius, do you know anything about this?

So yeah, what JR said in terms of the relevant issues for small PD systems.
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Phillip_Graham

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Re: electricity, energy, work, etc.
« Reply #52 on: October 19, 2005, 01:49:01 pm »

Shane Ervin wrote on Wed, 19 October 2005 11:24


But to properly size a rented DG, isn't it clouding the picture to bring Hydro accounting into the analytical framework for selecting the nameplate rating of a standalone machine?  There are better ways to arrive at Theory-Lite aimed at pragmatic application to Bennett's show-biz operations.


Hey Shane, I totally agree with you.  I guess I went down the power factor trail because it is something I understand and could talk about in a half-way intelligent manner.

I think JR has brought up the salient points for the live sound setting.  I don't know the answers.  I would be very interested to hear your experiences on this from working with those ship generators.

This thread is now officially passed to those with practical experience on de-rating portable gennies  Very Happy
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Iain_Macdonald

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Re: Specifying Generators in Kva
« Reply #53 on: October 19, 2005, 05:37:21 pm »


I thought 0.85 was the standard figure used.

For a USA viewpoint http://www.abrconsulting.com/Conversions/elec-con.htm


Iain
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Peter Morris

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Re: electricity, energy, work, etc.
« Reply #54 on: October 19, 2005, 09:53:03 pm »

Absolutely!

One of the big issues with things like amplifiers is related to the effect the DC power supplies has on the supply.  The bridge rectifier / filter cap type supply tends only to draw current on the peak bits of the wave - to top up the filter caps so to speak --- it draws current only when the supply voltage is above the voltage stored in the filter cap (multiplied by the turns ratio of the TX etc.).

This causes distortion of the supply, flattening of the tops of the waveform.  Its one of the reasons for the push towards switch mode PSUs. There were millions and millions of computers and other similar equipment doing just that when they had those old power supplies. I believe it was becoming quite an issue for power authorities around the world.

Using a single generator I believe its possible to see a power factor as bad as 0.5 –0.6  in some situations when the load is highly inductive and unlike a supply utility you will not be able to add any correction. (capacitor/ inductor banks/ rotating synchronous condensers/ controllable static var sources or var generators)

I don’t know what to expect for PA equipment, but its generally combined with a lot of lighting equipment, which is probably the dominant part of the load with a power factor close to 1.

As a guess if you use 0.8 for your cals and conservatively estimate your load I would not expect too many problems.

This is not a bad link for everyone that explains this stuff - http://www.ibiblio.org/obp/electricCircuits/AC/AC_11.html

Peter
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John Roberts {JR}

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Re: electricity, energy, work, etc.
« Reply #55 on: October 19, 2005, 11:46:19 pm »

Peter wrote on Wed, 19 October 2005 20:53

Absolutely!

One of the big issues with things like amplifiers is related to the effect the DC power supplies has on the supply.  The bridge rectifier / filter cap type supply tends only to draw current on the peak bits of the wave - to top up the filter caps so to speak --- it draws current only when the supply voltage is above the voltage stored in the filter cap (multiplied by the turns ratio of the TX etc.).

This causes distortion of the supply, flattening of the tops of the waveform.  Its one of the reasons for the push towards switch mode PSUs. There were millions and millions of computers and other similar equipment doing just that when they had those old power supplies. I believe it was becoming quite an issue for power authorities around the world.

Using a single generator I believe its possible to see a power factor as bad as 0.5 –0.6  in some situations when the load is highly inductive and unlike a supply utility you will not be able to add any correction. (capacitor/ inductor banks/ rotating synchronous condensers/ controllable static var sources or var generators)

I don’t know what to expect for PA equipment, but its generally combined with a lot of lighting equipment, which is probably the dominant part of the load with a power factor close to 1.

As a guess if you use 0.8 for your cals and conservatively estimate your load I would not expect too many problems.

This is not a bad link for everyone that explains this stuff - http://www.ibiblio.org/obp/electricCircuits/AC/AC_11.html

Peter




The only thing I would expand upon is that most amplifiers with switch mode PS use a standard FW bridge in their front end so they too will present concentrated current pulses. In fact they will be slightly worse because their charging current is not limited by a transformer in series. Regarding consumer gear this is indeed an issue for marginal utility infrastructure and Europe is far ahead of the US in mandating PF consumer gear. While there are only a handful of PFC audio power amps in production, every serious amp manufacturer developed the technology to avoid getting locked out of the European marketplace. The effectivity date for the rule kept getting postponed and I am no longer in a loop that cares so I don't know what the current status is but expect the benefit to European power distribution systems to full (professional power amp) implementation would have been minimal while impact on small Euro power amp companies would have been significant.

JR
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Re: electricity, energy, work, etc.
« Reply #55 on: October 19, 2005, 11:46:19 pm »


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