Bob S. wrote:

So if I use a MG unit running on the single phase coming into my

house

to create polyphase power you are saying it is still really single

phase

since that is what it came into my house as? Polyphase can be

created

from single phase and vice versa. This has been done for years in

both

power systems and electronic systems.

Yes, the convention in the power industry is to call two-phase only

that

which has two sinusoids separated by a phase angle of 90 degrees,
however, that is a definition used only by the power industry. Any

time

two phases are present, it can rightfully be called "two phase" no
matter what the phase angle separation may be. Again, the power
industry refers to three phase only when the separation angle is 120
degrees (which gives no current in the neutral with a balanced load -


which has some advantages), however, the broader definition covers

any

system with three distinct phases.


Matt


Ok, I followed the whole argument up to "polyphase". Care to explain
that to a semi-novice?

Polyphase simply means more than one phase. It is a generic term.


On a related topic: In rural Canada there are thousands of homes fed by
a single transmission line. No neutral/ground. This line goes into a
transformer which then feeds the home & outbuildings. What's happening
here? I assume the transformer is supplying 110/220 single phase, but
if Canada can do it with one wire, why does the USA do it with two?
Does that factor into the one phase/two phase argument?

I'm not a power transmission expert so I'm not 100% sure on this. They
are obviously using the ground as the "neutral." In the US typically a
separate neutral line is run, but it is also grounded at each service
entrance to ensure that no potential exists between the neutral and the
grounding conductor.

The only reason I can think of for two wires is safety. In a single
wire system, a short to ground (earth, the real ground) could be very
hard to detect. With a two wire system, you could monitor the current
in both the hot and neutral wires and detect if the hot was shorted to
earth by the imbalance. Since I've seen very few power lines that
de-energize when a short occurs, I doubt this is being done. The other
reason for two wires is to keep the grouding conductor dedicated for
handling safety issues such as shorts. In a one-wire system, the ground
rods and ground conductors have to be carrying current all of the time.
This can cause a wide range of problems such as varying voltages as the
resistance of the ground connection varies over time (soil conditions,
etc.). Again, I'm not a power distribution engineer, my EE is in
digital systems, and I learned just enough to pass the PE exam, however,
basic circuit theory tells you the above.

There must be good reason for the two conductor distribution system or
the power companies would not invest the added cost.


Matt