Re: Explain 3 Phase

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <jlnance-at-intrex-dot-net>

On Sun, Apr 22, 2001 at 08:07:47AM -0600, Tesla list wrote:
> Original poster: "Matt Skidmore by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <fox-at-woozle-dot-org>
> 
> Hi all,
> Could someone please explain 3 phase power to me and the hooks up
> configurations that go with that? i've heard of it here on the ttesla list
> and also been looking at some substations that has labels on them
> indicating 3 phase power. looking over some of the pictures of coilers
> using pigs i have seen 3 hookups on the low voltage side of the pig. i was
> wondering how these are hooked up. is the middle post ground?

Three phase power is used primarily when you are driving large motors, and it
uses 3 non-ground (and possiably 1 grounded) conductors to deliver it.  If
transformers are involved there will typically be 3 of them, though you can
get by with 2 if you do some magic.  Three phase power is not something you
are likely to find in your house, at least not in the united states.

> also, 240 runs into the home, how do we derive only 120 from that? ive
> noticed that the 240 outlet is 4 pronged. i think i once came accross a
> post that described how to get 240 using 2 120 outlets. could i get an
> explination on how to do that?

You mentioned seeing 3 hookups on the low voltage side of the pig.  You are
correct about the center one being ground.  Typically there is 120V from
either of the other hookups to the center, and 240 volts between the outside
posts.  When your house is wired, half of your 120 volts outlets will be
hooked up between one outside post and ground, and the other half between the
other outside post and ground.  If you find two outlets which are connected
to opposite outside posts, there will be 240V between the "hot" sides of
the outlets.  If the outlets are connected to the same side, there will be
0V between the two hot sides.

> lastly, when a person gets 3 phase power to their house, doesnt most
> equipment run on single phase? im just not fully clear on it. is there a
> second phase also? thanks

The single phase power you have at your house is actually 1 phase of the
three phase power that runs down the street.  Assuming that you have above
ground power lines, go outside and look at the wires on the pole that has
your house transformer on it.  You should see 3 high voltage wires on the
pole, and perhaps a fourth ground wire that runs along the top of the pole.
If you look at your transformer, it will either be connected between 2 of the
3 high voltage wires, or between 1 high voltage wire and the ground wire.  If
you wanted three phase power in your house, you would have to hange two
additional transformers on the pole and run two additional wires into your
house.  A machine that requires three phase power will have a plug that
has 3 prongs for "hot" connections and another prong for ground.  The last
time I looked at one, the 3 hot prongs were aranged in a circle and the ground
was in the center.  The plug sort of rotated and locked into the socket.
Anyway, if you have 3 phase power and you need to power something that just
needs single phase power, you would connect it between one of the hot wires
and ground.

There is such a thing as two phase power where you have two sine waves 90
degrees out of phase.  You can actually have as many phases as you want,
however three phase power and single phase power are all that you will ever
find used.  Single phase power is used when the power requirements are low
and the cost of the extra transformers and wiring would be significant.
Three phase power is used when large motors are involved, when a lot of
power is required, or when power must be transmitted over long distances.

Hope this helps,

Jim