David wrote:
On Mon, 24 Apr 2006 02:51:52 GMT, m II wrote:


David wrote:


Have you ever had to deal with a 3 wire 3 phase power drop (i.e.
without a neutral)? Not very common. Drives electricians nuts.


Why would it do that? A three wire feed implies a balanced load, such as
a three phase motor, charger or welder supply. Voltage drop is measured
line to line and line current is still measured the same way, as are
power factor numbers.

The only difference in a three phase motor connection is assuring
correct direction for motor rotation. Some things break in a dramatic
fashion if turned the wrong way.



Where does the 120 come from?


The cycles of each phase are out by that many electrical degrees in
relation to each other. I've never seen 120V three wire 3 phase. The
lowest voltage 3 phase 3 wire I've experienced is 208 volts.

If there is a 'wye' connection with a central white leg, (3 phase 4
wire) then you will see 120 volts from a hot leg to this white (neutral)
and 208 volts between any two hots.


The numerical stuff is covered he

http://encyclopedia.laborlawtalk.com/Three_phase

The really scary math happens with major short circuits on a leg causing
asymmetric fault currents.

Uneven power factor loads in the phases can make some interesting vector
diagrams..

it's been a while since school...sigh...

mike