The Straight Dope

Go Back   Straight Dope Message Board > Main > General Questions

Reply
 
Thread Tools Display Modes
  #1  
Old 04-02-2007, 08:48 PM
gotpasswords gotpasswords is offline
Charter Member
 
Join Date: Mar 1999
Location: San Francisco area
Posts: 14,679
Converting 440 volt 3 phase to 120 volt?

We've got an option to obtain a rather hefty portable generator for somewhere between free and cheap - it's one of those trailer-mounted jobbies like you see at construction sites, but with one big fat gotcha.

It cranks out 44 kVA (roughly 44,000 watts) of 440 volt three-phase power.

My understanding is the thing does not have a means of configuring it to put out 120/240 household-style power, which is why it's something of an ugly duckling and available to us.

What do we need to convert 440 volt 3-phase to normal power? I did a bit of googling and found transfomers that can apparently take in the three phase juice and put out 120/240. These would seem to be a simple (yet heavy and expensive) solution.

Or are there other options short of shipping the generator off somewhere to be re-wound?
Reply With Quote
Advertisements  
  #2  
Old 04-02-2007, 09:08 PM
MrFloppy MrFloppy is offline
Guest
 
Join Date: Sep 2006
I would go with a 45KVA dry type transformer. They are available in the configuration that you are looking for - you would probably end up with a 480 delta to 240 delta with a 120V center tap.

A better way would be to get a single phase 480 to 240/120 dry type and simply use one phase of the generator's output.

Make sure you size your conductors and overcurrent protection. The genset probably has an output breaker. You would need one on the secondary side also.

Last edited by MrFloppy; 04-02-2007 at 09:09 PM..
Reply With Quote
  #3  
Old 04-03-2007, 01:20 AM
gotpasswords gotpasswords is offline
Charter Member
 
Join Date: Mar 1999
Location: San Francisco area
Posts: 14,679
How much power would we lose by using a single phase off the generator, and how is that better?

Just wondering as I found another supplier that provides more specs on the things, and they're describing 440 delta in, and 208Y or three x 120 out, which would fit our needs perfectly.

As for overcurrent protection, let's assume the generator has its own protection for now. To protect the 45 kVA transformer, if I've done the math right, a three-pole 120-amp breaker will be sufficient for the panel main, or should we de-rate it to 100 amps? Ultimately, we'll have a bunch of 15 or 20 amp branch circuits.
Reply With Quote
  #4  
Old 04-03-2007, 09:30 AM
Uncommon Sense Uncommon Sense is offline
Guest
 
Join Date: Oct 2002
Quote:
Originally Posted by gotpasswords
How much power would we lose by using a single phase off the generator, and how is that better?

Just wondering as I found another supplier that provides more specs on the things, and they're describing 440 delta in, and 208Y or three x 120 out, which would fit our needs perfectly.

As for overcurrent protection, let's assume the generator has its own protection for now. To protect the 45 kVA transformer, if I've done the math right, a three-pole 120-amp breaker will be sufficient for the panel main, or should we de-rate it to 100 amps? Ultimately, we'll have a bunch of 15 or 20 amp branch circuits.
Yes, at 100 amps (45,000/208/1.732 and then take 80% of that number and you come up with 99 amps you can use the next size breaker up the chart ie, 100 amp)
you will be sufficiently protecting the XFMR.
Reply With Quote
  #5  
Old 04-04-2007, 11:06 AM
gotpasswords gotpasswords is offline
Charter Member
 
Join Date: Mar 1999
Location: San Francisco area
Posts: 14,679
Cool beans! Now I just need to make myself known at local electrical suppliers and let them know we're looking for a used one on the cheap. (Such is life as a non-profit entity!)

I found a real heart-breaker of a deal on eBay - someone's got a new, but dented 300 kVA unit for $500, but the cross-country shipping for the 1000+ pound thing would consume any savings vs buying a new one locally. Not to mention that 45 kVA weighs about 600 pounds less, and we need this rig to be portable.

One other question - my gut feeling on derating appears to have been spot on, but whats the actual rationale and significance of the 1.732 factor? That number is all over electrical calculations, but where does it come from?
Reply With Quote
  #6  
Old 04-04-2007, 11:12 AM
Q.E.D. Q.E.D. is offline
Charter Member
 
Join Date: Jan 2003
Location: Richmond, VA
Posts: 22,536
Quote:
Originally Posted by gotpasswords
One other question - my gut feeling on derating appears to have been spot on, but whats the actual rationale and significance of the 1.732 factor? That number is all over electrical calculations, but where does it come from?
You'll see that a lot in three-phase work. The actual derivation is a bit beyond me, but it's the square root of 3. If you want to find the line-to-line voltage of a three-phase system given the line-to-neutral voltage, you simply multiply by 1.732; That's why residential drops which are two phases of a three-phase network are ~208 V (120 x 1.732).
Reply With Quote
  #7  
Old 04-04-2007, 11:43 AM
Uncommon Sense Uncommon Sense is offline
Guest
 
Join Date: Oct 2002
I don't fundamentally understand it either, but it has to do with the ratio between (of?) the peak to peak values and the RMS (root means squared) values.
And also, 277 volts X 1.732 will give you 480.
Reply With Quote
  #8  
Old 04-04-2007, 12:02 PM
Q.E.D. Q.E.D. is offline
Charter Member
 
Join Date: Jan 2003
Location: Richmond, VA
Posts: 22,536
Here is the clearest explanation of the SQRT(3) factor I think I've ever encountered. Kinda makes me feel silly for not having known it.
Reply With Quote
Reply



Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 10:44 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.

Send questions for Cecil Adams to: cecil@chicagoreader.com

Send comments about this website to: webmaster@straightdope.com

Terms of Use / Privacy Policy

Advertise on the Straight Dope!
(Your direct line to thousands of the smartest, hippest people on the planet, plus a few total dipsticks.)

Publishers - interested in subscribing to the Straight Dope?
Write to: sdsubscriptions@chicagoreader.com.

Copyright 2013 Sun-Times Media, LLC.