beginner 3 phase question

[fletcher]

i am taking a basic electrical class. the book mentions that a 3 phase delta system has no neutral.

my quesion is...how is this possible, i thought all circuits had to have a neutral to work.

also, what are common uses of this type of wiring system??

 

[augie47]

Re: beginner 3 phase question

thought all circuits had to have a neutral to work.

nope! simply a path for the current to flow. Car battery have a neutral???

common uses of this type of wiring system??

from my experience most common in industrial applications with large 3 phase loads and "0" "neutral" load.

 

[iwire]

Re: beginner 3 phase question

Originally posted by fletcher:
i thought all circuits had to have a neutral to work.

No you do not always need a neutral, a common item in a home that is straight 240 volt would be an electric water heater.

Originally posted by fletcher:
also, what are common uses of this type of wiring system??

Many of the buildings I work in have a 480Y/277 (3 phase w/neutral) service. The major loads like Air Conditioners, fans, pumps, any electric heat will be straight 3 phase loads (no neutral)

We also bring 3 phase no neutral to transformers that create a new voltage system with a neutral 208Y/120.

 

[fletcher]

Re: beginner 3 phase question

so, if i understand correctly, you need to bring in 3 phase to a building if you want to achieve anything other than 220???? i mean i thought 2 110 lines came into a house (for instance) so you had 110 and 220 and that is it!!!!! i am not getting how an electrician achieves 208, 480 or 277????

can i read more about this online somewhere because my book is not clear????

thanks for the help.

 

[augie47]

Re: beginner 3 phase question

you might want to look at some of the books offered by Mike Holt (see "Products")

in a nutshell, utility distribution is normally 3 phase..so you can obtain 3 phase power from them(480, 240, 208, etc. with or without neutral depending on your need and the transformer(s) selected).

 

[rattus]

Re: beginner 3 phase question

Fletcher,

The neutral is that point which is equally distant in voltage from the phase voltages, e.g., the wye configuration. (Or from L1 and L2 in a 120/240V service.)

The 2-wire 120V service does not have a neutral, only a grounded conductor. Neither does a delta. You can ground the corner of a delta secondary, but that is not a neutral.

You might also check your library for books on this subject.

[ November 15, 2005, 09:33 AM: Message edited by: rattus ]

 

[PlywoodSparky]

Re: beginner 3 phase question

The reason that there is no neutral in the above discussion is because of the fact that it is 3 phase. The three phases A B and C are 120 degrees out of phase with each other. For example When one phase is at peak the other two are at a voltage that is lower. The result is that the other phase(S) become the return path for the circuit. This means that there is not a requirement for a neutral.

 

[rattus]

Re: beginner 3 phase question

Sparky, I would add that there is no way to connect a neutral to a delta system.

 

[royal]

Re: beginner 3 phase question

Fletcher,
Try this site:
http://www.ibiblio.org/obp/electricCircuits/AC/index.html

Tim

 

[George Stolz]

Re: beginner 3 phase question

Originally posted by rattus:
Sparky, I would add that there is no way to connect a neutral to a delta system.

Huh? Then why do we have codes regarding marking the high leg of a midpoint-grounded delta?

 

[iwire]

Re: beginner 3 phase question

Originally posted by georgestolz:

Originally posted by rattus:
Sparky, I would add that there is no way to connect a neutral to a delta system.

Huh? Then why do we have codes regarding marking the high leg of a midpoint-grounded delta?

A grounded conductor is not always a neutral.

 

[George Stolz]

Re: beginner 3 phase question

But in this example there is--we're not talking about a grounded B phase or something. All this conductor is is a neutral, right? I believe it's even called a neutral in the code sections relating to it, but I'm also too lazy to look.

 

[iwire]

Re: beginner 3 phase question

Now I am confused

 

[roger]

Re: beginner 3 phase question

George, although the applicable graphic has been lost, this thread may be of some interest.

Roger

 

[rattus]

Re: beginner 3 phase question

George,

I believe you are speaking of the 120/240V neutral. That is not a 3-phase neutral. The neutral point must be at the center of the phasor diagram, not on one of the legs.

Ain't this better than playing golf?

Rattus

 

[russ]

Re: beginner 3 phase question

I think an important part of the explanation should be that the "neutral" or grounded conductor, is just a connection to the transformer winding, just like the "hot" or ungrounded phases.
A "neutral" is a connection on the winding that is midpoint usually between two points(hots) sometimes three points, with more separate windings used. But joined at the ends

[ November 19, 2005, 01:20 PM: Message edited by: russ ]

 

[George Stolz]

Re: beginner 3 phase question

I believe I will need to have morning caffeine in me to sort this out.

 

[stevearne]

Re: beginner 3 phase question

The NEC does not define a neutral, just a grounded conductor. The IEEE definition of a neutral includes the stipulation that there must be equal voltage between the neutral and all of the ungrounded conductors of that system - and it is part of a 3 wire or 4 wire system. So a delta 3 phase 4 wire system has a grounded condutor that has 120 volts potential from line 1 and line 3, but it has a different voltage (208 approx) to the wild leg, line 3, so it is a grounded conductor according to the NEC but not a neutral based on IEEE definition.
There is good coverage of this in several of our books, including Understanding the National Electrical Code vol 1. I will try to insert a couple of graphics here from Understanding NEC Vol 1. Article 200