On 08/27/2015 04:27 AM, Peter Coghlan wrote:
The neutral is defined by being connected to the star
point on the transformer secondary as well as being
grounded. If it is not (at least intended to be) connected
to the star point, it's a ground, not a neutral. It is
possible to get a three phase power feed with no netural
provided but this is only suitable for use with balanced
three phase loads, not a typical building load which
includes single phase lighting and power outlets.
There have been a number of crazy systems used in the US,
generally referred to as "open delta".
One of them uses two single-phase transformers, and the
common connection between them is grounded.
This, you have two unbalanced 220 V circuits (on different
phases) with a common point. Any 3-phase motor or device is
perfectly happy with this, although one "corner" of the
three-phase triangle is grounded. A benefit is you can use
common split single-phase panels and double-pole breakers
for it. This system is called "corner grounded open delta".
Another system uses one center-tapped residential 120/240 V
transformer and one 240 transformer (or can use two split
transformers as long as the center tap of the 2nd
transformer is not used). The center tap of the split
transformer is grounded. This allows you to derive standard
120/240 V single-phase service AND 240 V 3-phase service
from only 2 transformers. But, it gets really confusing, as
the 3-phase service is grounded at the center between two of
the 3-phase lines. You have to use 3-pole breakers on this,
as none of the 3-phase lines is at neutral/ground
potential. This system is called "center-grounded open delta".
Then, the typical 3-phase service can be 120/208 V Wye
service, with the center tap of the Wye grounded. This
gives both 120 V for office equipment and 208 for industrial
loads.
Also, there is 240 V Delta service, where loads cannot be
placed between line and neutral. These transformers
generally have a small balance transformer that is just 3
windings with one end all tied together and grounded.
Often, they have a ground fault interrupter connected to the
the balance transformer. If any load draws more than a
small current to ground, the transformer is shut down. But,
you can run unbalanced loads from one line to another.
Welders and big computer gear were often set up this way.
Jon
