Electronics Forums > About 3-phase AC

Jack// ani
Guest
Posts: n/a

 04-07-2005, 06:03 PM
Hi there,

In 3-phase AC wiring, if phase to neutral voltage is 110V, then why is
phase to phase voltage 220? I know phase difference between any two
phases differ by 120 degree, so they should add up to give something
less than 220V! It should sum up to give 220V if the phase difference
were 0 degree or 360degree!

Thanks

Larry Brasfield
Guest
Posts: n/a

 04-07-2005, 06:20 PM
"Jack// ani" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> Hi there,

Hi.
> In 3-phase AC wiring, if phase to neutral voltage is 110V, then why is
> phase to phase voltage 220? I know phase difference between any two
> phases differ by 120 degree, so they should add up to give something
> less than 220V!

happen. What makes you think it does?

> It should sum up to give 220V if the phase difference
> were 0 degree or 360degree!

Many people do not distinguish 0 and 360 degrees
for continuous sinusoids.

--
--Larry Brasfield
email: (E-Mail Removed)
Above views may belong only to me.

Lord Garth
Guest
Posts: n/a

 04-07-2005, 06:25 PM

"Jack// ani" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> Hi there,
>
> In 3-phase AC wiring, if phase to neutral voltage is 110V, then why is
> phase to phase voltage 220? I know phase difference between any two
> phases differ by 120 degree, so they should add up to give something
> less than 220V! It should sum up to give 220V if the phase difference
> were 0 degree or 360degree!
>
> Thanks
>

The phase to phase voltage in the USA is 207 VAC which is phase to ground x
1.732
120 x 1.732 = 207.84 You can find this voltage supply running the light in
many
buildings.

Houses get one 240 volt phase which is transformed to provide 2 outputs of
120 volts
that are 180 degrees apart, with respect to ground.

Three 240 volt phases implies the phase to phase voltage from this circuit
is 415.68 volts.

Andrew Holme
Guest
Posts: n/a

 04-07-2005, 06:41 PM
Jack// ani wrote:
> Hi there,
>
> In 3-phase AC wiring, if phase to neutral voltage is 110V, then why is
> phase to phase voltage 220? I know phase difference between any two
> phases differ by 120 degree, so they should add up to give something
> less than 220V! It should sum up to give 220V if the phase difference
> were 0 degree or 360degree!
>
> Thanks

They would *add* to give 110V, but I suspect you're talking about the
potential difference between phases. Using the cosine rule, the PD between
phases is:

sqrt(110*110 + 110*110 - 2*110*110*cos(120)) = 190.5

Jack// ani
Guest
Posts: n/a

 04-07-2005, 07:12 PM
I think you got something wrong, or I didn't expressed it correclty!
Say you have two AC sources of 110V, now if I put them in series they
should add up to give 220V if their instantaneous phases are same(0 or
360) or if they are 180 phase out they should sum up to zero. I think
these two AC sources are just like two phases of 3-phase AC supply
which are 120degree phase apart. And they should give a voltage less
than 220V when summed up.

Any Help...Thanks

Larry Brasfield
Guest
Posts: n/a

 04-07-2005, 07:19 PM
"Jack// ani" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
>I think you got something wrong, or I didn't expressed it correclty!

I'll go with that set of alternatives.

> Say you have two AC sources of 110V, now if I put them in series they
> should add up to give 220V if their instantaneous phases are same(0 or
> 360) or if they are 180 phase out they should sum up to zero. I think
> these two AC sources are just like two phases of 3-phase AC supply
> which are 120degree phase apart. And they should give a voltage less
> than 220V when summed up.

Your above statements are consistent with phasor
arithmetic as I understand it, as long as "just like",
"less than", and "summed up" are interpreted in a
way most favorable to your understanding.

> Any Help...Thanks

You'll need to describe your issue more specifically
to get any help with it, I believe.

--
--Larry Brasfield
email: (E-Mail Removed)
Above views may belong only to me.

svetoslav belchev
Guest
Posts: n/a

 04-07-2005, 09:48 PM
Here in bulgaria a 3 phase AC is 220 V phase-ground and 380 V phase-prase

Peter Bennett
Guest
Posts: n/a

 04-07-2005, 10:22 PM
On 7 Apr 2005 11:03:49 -0700, "Jack// ani" <(E-Mail Removed)>
wrote:

>Hi there,
>
>In 3-phase AC wiring, if phase to neutral voltage is 110V, then why is
>phase to phase voltage 220? I know phase difference between any two
>phases differ by 120 degree, so they should add up to give something
>less than 220V! It should sum up to give 220V if the phase difference
>were 0 degree or 360degree!
>
>Thanks

In North America, line voltage is 120 volts to ground. With three
phase power this gives you 208 volts between phases.

If the phase difference between two circuits is 0 degrees, you will
measure zero volts between them.

In normal residential wiring, we have two wires that are 180 degrees
out of phase - this gives 240 volts between "phases" (some people
object to using the term "phase in this situation...)

Although the electrical distribution system as a whole is three-phase,
individual homes are fed from the secondary of a single phase
transformer. The secondary of the transformer is center-tapped, with
the tap grounded to form the neutral conductor. There is 240 volts
between the ends of the secondary.

--
Peter Bennett VE7CEI
email: peterbb4 (at) interchange.ubc.ca
GPS and NMEA info and programs: http://vancouver-webpages.com/peter/index.html
Newsgroup new user info: http://vancouver-webpages.com/nnq

Bob Eldred
Guest
Posts: n/a

 04-09-2005, 02:12 AM

"Jack// ani" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> Hi there,
>
> In 3-phase AC wiring, if phase to neutral voltage is 110V, then why is
> phase to phase voltage 220? I know phase difference between any two
> phases differ by 120 degree, so they should add up to give something
> less than 220V! It should sum up to give 220V if the phase difference
> were 0 degree or 360degree!
>
> Thanks

It's a very simple geometric relationship. If two lines or vectors are 120
degrees apart and are of equal length from their common, crossing end, the
distance between the tips of the lines is 2*sin (120/2). = 2*0.866 = 1.732.
Now, if the line lengths represents 120 volts from the center or crossing
point to the tip, the tips must be 120 * 1.732 apart = 207.8 Volts. OK
class, for homework, prove the geometric relationship. BTW it can be proven
without trigonometry.
Bob

John Larkin
Guest
Posts: n/a

 04-09-2005, 03:27 AM
On 7 Apr 2005 11:03:49 -0700, "Jack// ani" <(E-Mail Removed)>
wrote:

>Hi there,
>
>In 3-phase AC wiring, if phase to neutral voltage is 110V, then why is
>phase to phase voltage 220? I know phase difference between any two
>phases differ by 120 degree, so they should add up to give something
>less than 220V! It should sum up to give 220V if the phase difference
>were 0 degree or 360degree!
>
>Thanks

One common but bizarre US wiring practice is to have a 240 volt
line-to-line delta three-phase system in which one side of the
triangle is center-tapped and is neutral. So 120 single-phase is
available for regular outlets, 240 single-phase is available for
things that need it, and 240 line-to-line is available for three phase
loads. That's fairly common in small commercial buildings. The leg
opposite the neutral is call the "bitch leg" or the "stinger."

John

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are Off Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post zytrahus General Electronics Chat 8 05-05-2013 03:10 AM kingrosekhan123 Microcontrollers and Programming 9 04-26-2013 04:54 PM IrJoWo99 Electronics Repair 5 04-25-2013 03:08 AM quantumtangles General Electronics Chat 6 04-23-2013 07:50 PM lotec General Electronics Chat 7 04-16-2013 12:09 AM