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Author Topic: When to 3 phase?  (Read 2232 times)

Dave Guilford

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When to 3 phase?
« on: June 26, 2018, 11:14:13 am »

Been renting small MQ15kVA generators for a few years.  Got lined up at another rental co and the smallest they offer is a 25kVA. Same price too, and about 30 mins closer to their shop.

Their 25kVA has 3-phase option.  When / why would I do that?  What benefits do I get from 3 phase versus regular ass 1 phase. 

And what about 240 vs 120.  I think virtually all my gear is “universal power supply” type where it’ll take 100-250v or whatever. 

Teach me.
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TJ (Tom) Cornish

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Re: When to 3 phase?
« Reply #1 on: June 26, 2018, 11:17:05 am »

Been renting small MQ15kVA generators for a few years.  Got lined up at another rental co and the smallest they offer is a 25kVA. Same price too, and about 30 mins closer to their shop.

Their 25kVA has 3-phase option.  When / why would I do that?  What benefits do I get from 3 phase versus regular ass 1 phase. 

And what about 240 vs 120.  I think virtually all my gear is “universal power supply” type where it’ll take 100-250v or whatever. 

Teach me.
What did Guitar Center say?
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John Daniluk (JD)

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Re: When to 3 phase?
« Reply #2 on: June 26, 2018, 12:23:19 pm »

the 25kw will give you more amperage in single phase than you have now.   3 phase gives you 1/3 more amperage to use. 

Basically if a generator is 3 phase  the KW power is rated for 3 phase.   When you set it for single phase you loose one leg of power or 1/3  of your available amps.

For example a generator rated for 25kw at 3 phase is a basically 70 amps per phase giving you 210 total amps.  when you change the generator to single phase you have around 140 amps available for use.   ALL of this is depends on how the generator monitors the legs for over current and voltage.

If you change your equipment to 208v I think you will be changing them back and forth for smaller shows.   Leave them at 120v and get a AC distro.  If you get a 3 phase distro, mark which outlets are on each phase.  This helps when you can only get single phase.

I have I did not confuse you....

jd
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Dave Guilford

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Re: When to 3 phase?
« Reply #3 on: June 26, 2018, 07:29:34 pm »

Great reply and thank you.  The last thing you mentioned was building distro for 3phase. 

But when I use it for single phase, as you mention, what happens?  “When you can only get single phase power”. Does that mean 1/3 if my outlets won’t work?
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Dave Guilford

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Re: When to 3 phase?
« Reply #4 on: June 26, 2018, 07:31:50 pm »

But basically the answer to “when to 3-phase” is “whenever you can”.

I’ve been renting from UR and Sunbelt and the like.  They provide those ugly yellow spider boxes.  Those probably don’t denote which circuits are on which legs, do you suspect?

Usually it’s california type 50a feeder
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David Sturzenbecher

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Re: When to 3 phase?
« Reply #5 on: June 26, 2018, 07:40:14 pm »

But basically the answer to “when to 3-phase” is “whenever you can”.

I’ve been renting from UR and Sunbelt and the like.  They provide those ugly yellow spider boxes.  Those probably don’t denote which circuits are on which legs, do you suspect?

Usually it’s california type 50a feeder

Don't use three phase unless you have a three phase distro.
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Erik Jerde

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Re: When to 3 phase?
« Reply #6 on: June 26, 2018, 11:19:31 pm »

Don't use three phase unless you have a three phase distro.

Yep, this.  Unless you have the distro for it 3p is useless to you.
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Tom Bourke

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Re: When to 3 phase?
« Reply #7 on: June 27, 2018, 01:44:17 am »

the 25kw will give you more amperage in single phase than you have now.   3 phase gives you 1/3 more amperage to use.

Basically if a generator is 3 phase  the KW power is rated for 3 phase.   When you set it for single phase you loose one leg of power or 1/3  of your available amps.

You don't normally loose power that way on a truly switchable generator.  Typically they have some multiple of 6 poles and maybe some center taps to switch around depending on the voltage and phase  settings.

The important thing to do is meter your outputs to make sure you're getting what you expect based on the connected distribution. I have seen over 170V on a 120V connection to a generator.  That is not counting the times a tech set the generator for 480 3 phase when we expected 120/240V single phase.
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Ray Aberle

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Re: When to 3 phase?
« Reply #8 on: June 27, 2018, 09:30:35 am »

Let's take a minute to step back and address something critical that's possibly being overlooked. My apologies if everyone knows this and is assuming as such, but I want to make certain the OP is aware of this when deciding whether to run single or three phase on the generator.

If you are renting diesel generators, ONLY in 1Ø mode are those 50A twist locks connections available. If you set the unit to 3Ø, those twist locks will NOT be functioning. So, 3Ø will not get you "more power" to the twist-locks, and "some outlets (on the spider boxes) will not be functioning" will not be the case.

The lugs (for attaching cams to) will work in either 1Ø or 3Ø. There's silk-screen graphics indicating as to which two lugs to connect to when only running 1Ø; you hit all three for 3Ø. If your rented unit has cam lugs bolted to the body (for ease of connections; often these are fed with internal cabling to the aforementioned lugs, and are usually after-market installs... not sure if Cat, MQ, etc, offer that as an option from the factory) then those cams are going to always be hot whether the generator is running 1Ø or 3Ø.

Breakers: Everything is still covered by the main breaker in the control panel. On a 25kW unit, its typically 60A. On a 45kW usually 110A. It's a three-pole breaker, but covers the system regardless of whether it's in 1Ø or 3Ø mode. So, JD was on the right track when he said that 3Ø would give you more amperage than you have now (technically a 50% increase from 120A to 180A; 120A IS a 1/3 hit from 180A but blah blah math and being pedantic ;) ) -- but again, when you are in 1Ø mode, that extra "power" for being in 3Ø mode just isn't available. This is why I mentioned the use of spider boxes- changing to 3Ø mode isn't going to free up that extra power for use with spider boxes. 1Ø is only pulling from the U & W legs (they're marked "U," "V" and "W.").

What's also important to keep in mind is that in 1Ø mode, when you're breakered at 60A (on that 25kW unit), that's covering BOTH 50A twist locks. So, even though in your mind you have 100A/220V (2-50A circuits) you are still going to be restricted by the 60A MAIN breaker. (60A per leg/220V.)

Finally, spider boxes normally come with an L6-30 30A/250V (H-H-G) as one of the circuits. Some rental shops converted those to L14-30, which can be useful for you. It's never a bad idea to ask if your rental shop has some of those converted boxes, and if they don't (and you know you'll be using them regularly) to make you one or two.

-Ray
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Kelcema Audio
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Ray Aberle

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Re: When to 3 phase?
« Reply #9 on: June 27, 2018, 09:43:18 am »

The important thing to do is meter your outputs to make sure you're getting what you expect based on the connected distribution. I have seen over 170V on a 120V connection to a generator.  That is not counting the times a tech set the generator for 480 3 phase when we expected 120/240V single phase.
"Gosh, Tom, how does the voltage get changed?"

PICTURE TIME! (I wish I had already gotten my generator wrapped with my company logo and such, so it'd look way cooler than it does.... alas.)

1) (45kW.jpg) Here's a typical 45kW WhisperWatt.

2) (LookHere.jpg) This is the side panel you'll probably need to open.

3) (Inside.jpg) Here's what it looks like inside.

4) (ModeSelect.jpg) This is the voltage selection switch. THE ENGINE MUST BE OFF WHEN CHANGING THIS SWITCH.

5) (lugs.jpg) The graphics on the lugs, showing you which lugs to connect to based on the output you want. We have cam tails attached to the generator permanently, so I don't have to futz with these anymore. When someone has 1Ø cams/distro (a distro with 1Ø cams) I attach to my black and blue cam tails, that are wired to "U" and "W" respectively.

-Ray
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Kelcema Audio
Regional - Serving Pacific Northwest (OR, WA, ID, BC)
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