240V from a 3 phase main ?

What you need is a buck/boost transformer. These are often used in buildings with 120/208 V service to run 240 V air conditioners, etc. They are autotransformers, and quite small for the power they provide, as they only have to handle the difference in voltage.

Jon

Consult your electric company. They are more than happy to help and advise. Their advice should be free and it will result in the safest of installations for you.

Generally, utility companies prefer not to let their customers get hurt.

In all probability the service is from a center tapped delta transformer. Put a voltmeter on each leg and you should read

120/120/208. Run your 2 phase stuff across the two 120 legs and you will get the 240 that you want.

If it's a wye transformer then all three legs will give you the same reading. However, that's unlikely unless you are looking at strictly warehouse space.

George.

NO!!!

You will have 120 volts from each leg to NEUTRAL, but from leg to leg, you have (120 + 120 volts) x sin 120 degrees = 208 volts. There is no such thing as "2-phase stuff". It's either single-phase, 240-volt (not 220), or it is 208-volt 3-phase with 208 volts between adjacent phases.

What you need is a s>

Wrong.

NO YOU WON'T.

In 208V 3-phase service, the phase-to-phase voltage is 208V, and the phase-to-neutral voltage is 120V.

Delta or wye, it doesn't matter. You won't get 240V anywhere in a 208V 3-phase service.

-- Doug Miller (alphageek at milmac dot com)

How come we choose from just two people to run for president and 50 for Miss America?

Bingo.

The above is very straight forward, low cost and SAFE.

Any decent industrial electrical distributor will have B-B x'fmrs in stock and the necessary documentation to help select the correct size unit complete with the wiring diagram.

Buck-boost transformer, as Jon Elson said (below) in an earlier post.

"What you need is a buck/boost transformer. These are often used in buildings with 120/208 V service to run 240 V air conditioners, etc. They are autotransformers, and quite small for the power they provide, as they only have to handle the difference in voltage."

Bob Swinney

There are several common 3phase hookups:

If the 3 phase is specified as 208V, then it is a 120/208Y setup. This is a four wire (wye-type) connection, with the threee phase legs and a neutral

B A \ / \ / \ / * buildings. I was pleasantly surprised to find that 3 phase power was very

E, I followed you, and thought I had it until you said the BOOM part. If you wrote what you wanted to say, how are you going to get 208V from that 120/240 volt setup? The way I see it, A to B is 240V and either A or B to G/C is 120V (each out of phase by 180 degrees). A or B to ground (G/C) is 120V and G/C to earth is 0 volts.

Al, you stated it right when you said "split phase" instead of the mis-named 2 phase. That subject caused a lot of noses to curl when I used it here for loss of a proper term. Gary C. explained many months back that 2 phase does or did exist but is rare. Just filter & follow the good advise here and with the Power Co. to be safe when you play with your landlord's power. New environments often bring surprises with them.

What kind of "240V" motor won't run on 208V?

Not very likely. What center is there to tap?

On a delta-connected transformer, you'll get random values WRT ground. Line-to-line you'll get the same reading on all three pairs.

You are not likely to see anything but Y-connected trans- formers at the load end of the line. The supply end may well be delta-connected.

For some reason, electrical matters always bring out lots of posts in this group by those who know and those who think they know (but don't understand that three phase power is a different animal from the simple DC they had in

9th grade shop class)

I believe you are one of the guys who knows. Thanks for the definitive post.

Bob

Actually, he was correct. The voltages are proportional to the distances between points, and the phase angles are the same as the angles as measured on diagram (presuming equilateral triangles) in the following:

In the following, A, B, and C are the phase legs, G is the grounded current carrying conductor (often a neutral, not always) N is the neutral conductor.

240 delta: (not as common a hookup as it used to be, many northeast power co's are trying to get rid of it)

B / \ / \ / \ C---G---A

G is the grounded conductor (not neccisarily a neutral in this hookup, NOT the safety ground) A-B, B-C and C-A are all 240V, 120 degree phase difference, G-C and G-A are both 120, opposite phase, and G-B is 208V, 90 degrees behind G-A. Occationally seen with A, B, or C grounded rather than the phase A-C being split--there is no 120V supply in that case. Also seen as the open delta (two transformer) hookup (supplies about 58% of the power the three transformer hook will, and the reading across the 'missing' transformer may be a tad on the wild side) and as the two transformer T hookup, with a transformer from A to C and from G to B. Measurements A-B and A-C tend to be a bit off as the 120V loads change. Sometimes supplied for a wye feed with only two power legs and the neutral, which derates the capacity, but greatly cheapens the hookup. The third phase is created by the load transformers (My local utility used to do this and not even run the neutral-- just two cables for phase A and B, no conduit, the generally wet swamp mud ground being used as the neutral. Leads to some really bizarre voltage and phase swings, especially during a drout.)

120/208Y and 277/480Y:

A | | N / \ / \ B C

a) 120V across N-A, N-B and N-C, all 120deg out of phase. 208V across A-B, A-C, and B-C, all 120 degrees out of phase, 60 degrees out with the 120V hookups. This is a real common hookup in small industrial.

b) 277V across N-A, N-B, and N-C; 480V across A-B, B-C and C-A. This is why there are 277V fluorescent and HID fixtures so cheap. common in heavier industry, large schools, shiopping centers, etc. where long electrical runs and/or high power loads are common. Can use lighter gauge wiring at the reduced current and save across the board: cheaper wire, cheaper breakers, lower heat loss, lower line drop, etc.

120-120 two phase:

A | | | B----C

A-B 120V, B-C 120V, 90 degree phase difference. B-C is 172V at 45 degrees (not used on its own generally) This is outmoded by many tears, but is still seen in some older industrial plants, and is still supplied in many areas. It can be derived from three phase using the T hookup for the primary side. and L for the secondary, taking care to use the proper transformer ratios. When needed, it is usually derived in plant anymore, but is still provided in a few places by the utility (I think Niagra may still generate 2 phase on the 25Hz side)

3phase 2phase side side A A' | | | | | | B--N--C B'----C'

I appreciate the tedious effort you attempted to draw and explain everything. Ascii text drawings are virtually useless in this medium because you never know if the reader will see it in fixed width or proportional fonts. The only way to consistently do ascii text drawings is to use fixed width fonts, space, and no tabs. Even then, word wrapping my get you anyway.

Bottom line is that I tried to follow everything you said and it was just intelligible (not your fault). I'm an electrical engineer with extra coursework in power and probably have a better chance than average in trying to follow what you were saying, but couldn't.

Bob

For what it's worth, I haven't studied this stuff since college

20-mumble years ago. I found the diagrams perfectly readable and the explanation quite clear.

Just a note, I could read "e" 's post perfectly in fixed space ascii. I use OE6 and went to the 'View' menu, 'Text Size' menu and then clicked on 'Fixed'. By the way, I feel that "e" 's explanation is Right On and very accurate information.

Someone else commented in the thread that 2 phase power didn't exist. It does, as has been explained by "e". There is also 4 phase power. Matter of fact there is also DC power still being generated for public use in a very small area of lower Manhattan. (That is a hold over from the 'Edison' years. Tesla gave us our current power system.).

3 cheers.! PJ

Etc.... was snipped by Bob..

Hey Bob,

His suggestion for ASCII art made me cringe too, but it actually came out perfect on my computer.

Bit I also had some problem "following" along.

Take care.

Brian Laws>I appreciate the tedious effort you attempted to draw and explain

Clarke, you must have never heard of Delta Power. In that system, Two of the three phase are 120 to ground. The third phase is 208 to ground. between any two phases is 240 volts. Really.

With Delta Power, you have 120 volts from two of the three phases, and you have 240 volts three phase.

Pete

PJ wrote: There is also 4 phase power.

I don't know about 4-phase power, but I know there is 4 wire primary. 3 hots and a primary neutral. There's also 4 wire 2-phase and 5 wire

2-phase. Here's a link that shows the connections that e was discussing, along with the 3 wire 2-phase that he explained and the 4 and 5 wire that I just spoke of. These pages were copied from a General Electric Distribution Transformer Manual.

Don