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wrote in message
...
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.
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Just a bitch that we have dealt with befo
Phil- please realize that 207.846096....... is meaningless except that it is
"about 208". 208V is correct to 3 significant figures which is actually
better than one can assume to be true in practice. If the voltage line to
neutral is actually 120.V (note the decimal) then we have 3 significant
digits implying something between 119.5 Vand 120.5.V
Then all you can truly claim is 208.V
If it is 120.0V then there is reason to assume 208.0 V but no more decimals
than that.
If you have a meter which gives you 120.000000V with less than 1 part in 120
million error then you can claim 207.846097V for line to line voltage Do
you have such a meter?
Engineering and physics students who ignore the principle of "significant
digits" lose marks for this "decimal inflation".
Sure- you can let the calculator carry the extra digits (as it will do
internally) but accepting these as gospel truth to the limit of the
calculator or computer display is simply not on as you can't get better
accuracy from a calculation than the accuracy of the original data (actually
you will lose a bit). All that you get rid of is round off errors in
calculations.
Since, as you say, precise voltage is not really practical, then
multi-decimal point numbers are meaningless. If we say 120V +/-10% then we
are talking about 108-132V which for line to line becomes 187-229V (average
208V) and any extra decimal points don't mean anything.
Don Kelly
remove the X to answer