About 3-phase AC
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Jack// ani
Guest
Posted: Thu Apr 07, 2005 6:10 pm    Post subject: About 3-phase AC Reply with quote
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

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Larry Brasfield
Guest
Posted: Thu Apr 07, 2005 9:10 pm    Post subject: Re: About 3-phase AC Reply with quote
Quote:
I think you got something wrong, or I didn't expressed it correclty!

I'll go with that set of alternatives.

Quote:
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.

Quote:
Any Help...Thanks

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

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
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Jack// ani
Guest
Posted: Thu Apr 07, 2005 9:10 pm    Post subject: Re: About 3-phase AC Reply with quote
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

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Andrew Holme
Guest
Posted: Thu Apr 07, 2005 9:10 pm    Post subject: Re: About 3-phase AC Reply with quote
Jack// ani wrote:
Quote:
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
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Larry Brasfield
Guest
Posted: Thu Apr 07, 2005 9:10 pm    Post subject: Re: About 3-phase AC Reply with quote
Quote:
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!

The mathematical impossibility you ask about does not
happen. What makes you think it does?

Quote:
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: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
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Lord Garth
Guest
Posted: Thu Apr 07, 2005 9:10 pm    Post subject: Re: About 3-phase AC Reply with quote
Quote:
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.
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Peter Bennett
Guest
Posted: Fri Apr 08, 2005 12:10 am    Post subject: Re: About 3-phase AC Reply with quote
Quote:
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
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svetoslav belchev
Guest
Posted: Fri Apr 08, 2005 12:10 am    Post subject: Re: About 3-phase AC Reply with quote
Here in bulgaria a 3 phase AC is 220 V phase-ground and 380 V phase-prase
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Bob Eldred
Guest
Posted: Sat Apr 09, 2005 3:10 am    Post subject: Re: About 3-phase AC Reply with quote
Quote:
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
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Bob Eldred
Guest
Posted: Sat Apr 09, 2005 6:10 am    Post subject: Re: About 3-phase AC Reply with quote
Quote:
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

I can't see how that can be a delta. I think what you have described is a
six phase "Y", center neutral as usual. It's 120 Volts out any leg and 240
Volt center tapped any leg to it's stinger. And, 208 Volts leg to leg. It's
also 120 Volts from any leg to the adjacent stinger. It's not all that
bizarre and can be created from any three phase system with a transformer.
Bob
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John Larkin
Guest
Posted: Sat Apr 09, 2005 6:10 am    Post subject: Re: About 3-phase AC Reply with quote
Quote:
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
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John Larkin
Guest
Posted: Sat Apr 09, 2005 6:10 pm    Post subject: Re: About 3-phase AC Reply with quote
Quote:
I can't see how that can be a delta. I think what you have described is a
six phase "Y", center neutral as usual. It's 120 Volts out any leg and 240
Volt center tapped any leg to it's stinger. And, 208 Volts leg to leg. It's
also 120 Volts from any leg to the adjacent stinger. It's not all that
bizarre and can be created from any three phase system with a transformer.
Bob



What I described is this:


C
/ \
/ \
/ \
/ \
/ \
/ \
/ \
/ \
/ \
/ \
/ (gnd) \
/ | \
A------------N------------B
| | |
| | |
120 N 120



John
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Bob Eldred
Guest
Posted: Sat Apr 09, 2005 9:10 pm    Post subject: Re: About 3-phase AC Reply with quote
Why would anybody do that? What is the voltage from A to C? A to N?, What
is the relation of N to ground. Clearly this kind of a bastard connection
would be prone to gross unbalance or to parasitic voltages on the neutral.
If its 240V leg to leg, with N in the center of one leg, what is N to B for?
What is the voltage B to C? Furthermore if its 240 Volts three phase on all
legs, what equipment uses it, most are 208? Since 120 Volts is only
available from one leg with a neutral how is power distributed in a building
say to lighting or to other 120 Volt circuits maintaining some semblance of
balance between the phases. I'm not saying you have never seen this but I
never have and it can't be very common.
Bob
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John Larkin
Guest
Posted: Sat Apr 09, 2005 9:10 pm    Post subject: Re: About 3-phase AC Reply with quote
Google "three phase stinger" or some such.

John
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Guest
Posted: Sun Apr 10, 2005 12:10 am    Post subject: Re: About 3-phase AC Reply with quote
Quote:

Why would anybody do that? What is the voltage from A to C? A to N?,
What
is the relation of N to ground. Clearly this kind of a bastard
connection
would be prone to gross unbalance or to parasitic voltages on the
neutral.
If its 240V leg to leg, with N in the center of one leg, what is N to
B for?
What is the voltage B to C? Furthermore if its 240 Volts three phase
on all
legs, what equipment uses it, most are 208? Since 120 Volts is only
available from one leg with a neutral how is power distributed in a
building
say to lighting or to other 120 Volt circuits maintaining some
semblance of
balance between the phases. I'm not saying you have never seen this
but I
never have and it can't be very common.
Bob

It's still common in the US where houses are mixed with small motor
installations. That unexpected 208-to-neutral is(was) a problem on
water-well controls when you needed 120v for the control-circuit.
WAde H
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