About 3-phase AC

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

Reply to
Jack// ani
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Hi.

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

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

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--Larry Brasfield
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Reply to
Larry Brasfield

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

Reply to
Jack// ani

I'll go with that set of alternatives.

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.

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

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--Larry Brasfield
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Reply to
Larry Brasfield

Here in bulgaria a 3 phase AC is 220 V phase-ground and 380 V phase-prase

Reply to
svetoslav belchev

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.

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Peter Bennett VE7CEI 
email: peterbb4 (at) interchange.ubc.ca        
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Reply to
Peter Bennett

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.

Reply to
Lord Garth

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

Reply to
Andrew Holme

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

Reply to
John Larkin

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

Reply to
Bob Eldred

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

Reply to
Bob Eldred

What I described is this:

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

John

Reply to
John Larkin

Google "three phase stinger" or some such.

John

Reply to
John Larkin

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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

Reply to
wade_h

240

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transformer.

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

Reply to
Bob Eldred

Around here, all delta circuits are being replaced with a wye configuration. The utility has standardized on this...then all the other power utilities came on the scene so who knows !

Reply to
Lord Garth

configuration.

Well, that certainly makes more sense than some sort of a nutty asymetrical grounding scheme. Oh well, based on the above comments, I guess anything is possible. What does the NEC have to say about it? Bob

Reply to
Bob Eldred

Your two single phase 110 sources in series will give 220V or 0 depending on connection. This is the Edison or 3 wire single phase system in the 220V case (and something useless in the other case) which is a common North American configuration. In Europe, where it is not used, it is called a 2 phase system and if one defines an phase system as having n voltages to neutralThere are two basic forms of a 3 phase connection -star in which there is a common neutral or delta where there is no neutral. .You mention a voltage of 110V to neutral so the corresponding voltage between lines is

110*root(3)=190V.

In a star connected 3 phase system the phase and line currents are the same but line to line voltages are greater than phase to neutral voltages by the factor of root(3). In a delta the phase and line to line voltages are the same but the line (external) currents are root(3) times the phase (internal) currents.

There are advantages and disadvantages to both and advantages for 3 phase over single phase (2 or 3 wire), particularly in rotating machines and transformers.

--
Don Kelly
dhky@peeshaw.ca
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Reply to
Don Kelly

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