In alt.engineering.electrical Benj wrote:

| Since I'm posting from GoogleGroups I can't respond to Phil, but the
| rest of you can be enlightened.

Actually, I do see the ones the respond to my own posts. I think the reader
does that to keep the threading intact. New posts I won't see. And that is
what most of the spam is (I've seen some spammers that do followups to other
posts).


| In 120/240 or similar systems there is not the freedom to choose this
| ratio. The wiring of the source transformer determines it. As others
| have noted, in the "Edison" U.S. system the source is a center tapped
| transformer with the center tap grounded. This makes a two phase
| system with each 120v "leg" 180 degrees out of phase with the other
| one. The ratio of the high voltage (240v) and the low voltage (120v)
| is always therefore 2:1.
|
| In a three phase system there will be three transformers with
| secondaries (one for each phase) wired in a "star" or "Y"
| configuration. This is necessary because you need the center point of
| the "star" or "Y" to be ground for each low voltage phase. If you wire
| with a "delta" configuration there is no central grounding point
| available for the individual phases. IN three phase circuits the
| relationship between that individual phases to ground (say 120v) and
| the voltage measured between phases is not arbitrary. It is always
| determined by the square root of 3. Hence the between phase voltages
| being sqrt 3 x 120 = 208V. Just like the two phase system these
| ratios are determined by physics and can't be arbitrarily set.

There is no more or less option to choose once you have either system. The
choice you have is between the systems. If you have single phase, you only
get 2.0 as a ratio. If you have three phase, you only get 1.7320508 as a
ratio.


| Of course there is the issue that electric companies often will name a
| voltage one thing while actually supplying an other for small
| variations about the "standard" voltage.

They call it 208 volts, but it's closer to 207.8460969 :-)

Precise voltage is not really practical. The voltage standard is a target to
stay near.

--
|WARNING: Due to extreme spam, googlegroups.com is blocked. Due to ignorance |
| by the abuse department, bellsouth.net is blocked. If you post to |
| Usenet from these places, find another Usenet provider ASAP. |
| Phil Howard KA9WGN (email for humans: first name in lower case at ipal.net) |