wrote:
On Wed, 28 Oct 2009 13:03:03 -0600, bud--
wrote:


But here's my Q:
For a given voltage, what difference would the end user see in terms of a
delta or wye connection?
And why is 208 wye, and 240 V delta?

208V has 3 - 120V transformers. One endpoint of each transformer is
connected together to form a "neutral". You have 120V from each of the
phase conductors to the neutral. That is a major advantage when you are
supplying 120V loads (compare to 240V delta). The voltage between phase
conductors is 208V. A 3-phase motor would be 208V. A diagram of the
transformer connections looks like a Y-wye (or star).

This is a picture of one of those transformer arrays. The incoming
phases to the customer is black,red.blue

http://gfretwell.com/electrical/3_p_wye-wye.jpg

A picture is worth a thousand words.

Where do you get the wire with the neon insulation? (nice touch)


A 240V delta system starts out with a 120/240 transformer like is used
for a residential service. The center tap is the neutral, just like with
a residential service. For the 3-phase, 2 transformers are added, one
end of each connected to the ends of the original transformer and the
other end connected together to be the 3 phase "high leg". A diagram of
the transformer connections looks like a triangle or delta. The voltage
from the high leg to neutral is 208V. There may only be 2 transformers
(open delta). I suspect this system came from original 120/240V single
phase systems where some 3-phase load had to be added. You can add a
transformer, which can be much smaller than the original one (depending
on the 3 phase load).

This is a picture of that transformer array, the 120/240 phases are
typically black & red and the high leg is required to be orange by
code.


http://gfretwell.com/electrical/High...ransormers.jpg

Another nice pic. 2 transformers means open delta. Notice the right
transformer has 3 connections - the center one is the neutral. Left
transformer only has 2 connections.


There is another way you can see delta in a place that doesn't need
any 120v load. They make a standard delta with either 2 or 3
transformers and ground one phase leg AKA "corner grounded delta".
That will look a lot like single phase to someone who is not aware
since there will just be 2 ungrounded conductors and the 3d phase will
be white. The equipment will look like single phase with 2 pole
breakers.

That is one place where you will need those 2 pole "delta" rated
breakers.


I have never seen an installation with a delta breaker. Catalog pictures
what I remember is a delta breaker has 2 bus stabs and a wire. The wire
goes to the neutral bar?
Is this the only way delta breakers are used?


You can also have ungrounded delta but that will be in a special place
like a glass factory where the first fault to ground won't bring down
the power. There are special monitoring requirements for that.