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#128608 - 07/16/03 05:41 AM three phase distribution
james S Offline
Member

Registered: 05/13/03
Posts: 102
Loc: West England
one thing i can't understand about three phase generation,is that in single phase motors, pumps, general lighting and power etc there is a neutral to provide the difference in potential to enable voltage to flow, but in three phase equipment where is the difference?
I know there is 0 voltage within the neutral in a fully balanced 3 phase delta setup,so how does the current return back to the transformer to complete the circuit?

does it return through one of the two other phases when at 0 potential due to the phase displacement of 120 degrees?

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#128609 - 07/16/03 01:21 PM Re: three phase distribution
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
James,

Yes, it is the 120 degree difference between phases which results in a potential difference between those phases. You'll get current to flow any time you connect a load between two points at different potentials --One of those points does not have to be a neutral.

It's that 120-degree difference which gives you the 415V between phases that you'll be used to seeing in the U.K.

If it helps to visualize the situation with a single-phase system, think about the 3-wure system used in America. 240V heating elements are connected directly across the two hot legs, no neutral needed. I that case, it is a 180 degree difference which results in double the line-to-neutral voltage.

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#128610 - 07/16/03 03:35 PM Re: three phase distribution
james S Offline
Member

Registered: 05/13/03
Posts: 102
Loc: West England
say the red phase and yellow phase are connected to the element, the red phase current would run down the yellow phase when at 0 volts potential (at mid point of cycle)and the other way around when yellow phase is pulling current?

p.s sorry about the location of topic

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#128611 - 07/16/03 04:50 PM Re: three phase distribution
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
James,

Adding to Paul's excellent response, here's a little more info + some graphics to use as teaching aids (graphics found in the Technical Reference Area - either directly, or through the "Menu").

The easiest thing to visualize first off is how a "Wiggy" works when connected across two lines (phases) of a polyphase system - in this case, the polyphase system will be 3Ø.

If the "Black" lead of the Wiggy is connected to Line A (ØA), and the "Red" lead is connected to Line B (ØB), there will be a voltage reading. Same for connections between A & C + B & C. If the system is Grounded, there will typically be a reading from one Line (phase) to Ground, which is NEAR ½ the Line-to-Line voltage.
Keep these key points in mind, as they cover the basic idea overall.

Basic flow - part 1:

In the most simplest idea, a Polyphase Transformer (or Generator) has two, or more windings, connected in some fashion. Each "Winding" is a simple coil of wire, which has two ends (told you it's really a simple idea!).
Lets call the end to the left "X", and the other end "Y".
Induce an Alternating Current into this winding, and you have a Single Phase 2 wire source.
At a certain time, current flows out of the X side, and returns at the Y side. The flow reverses direction for a certain time, then the whole thing starts over again.

Keep this "Single Coil Flow" principle in mind also, which explains what's going on in a stand-alone coil winding on a polyphase system.

For reference, view this image:


BTW: Split coils are used here, but think of the windings as singluar units which are continuous.

As you can see, there is a Ground connection on one of the Secondary lines - yet this does not make that circuit conductor a "Neutral"! It is a Grounded Conductor, but it is not "Neutral". Remove the ground connection and the system functions normally! Grounding is a Safety Issue, not something to enhance normal operation (in a nutshell!).

On this system, voltage readings can be made from "A" to "B", and "B" to ground. Without the ground connection, only solid voltage reading will be found between "A" and "B".

This next image shows a single phase 3 wire system, which has a Grounded "Neutral" conductor:



Voltage readings may be taken from A-N, N-B and A-B. Also, with the system grounded, voltage to ground can solidly be measured from A to ground, and also B to ground. Remove the grounding connection, and there is no SOLID voltage reading to ground.

(PS: I refer to "Solid" readings as being something easilly readable, or readable from a low Impedance meter. High input Impedance meters can read voltages on ungrounded systems, but this is beyond the scope of this discussion).

Advanced flow - part 1:

Taking the principle ideas of the single phase coils shown above, here's how to apply them to a polyphase system.

Take three of the single phase 2 wire setups (like the one shown in the first image), and connect them together in a Triangle Fashion - so that "A" on one coil connects to "B" on another coil.
(PS - remove all the grounding connections!!!).

Here's a graphic image of the result:


Now you have a 3 phase 3 wire Delta system! Visualize currents flowing in one coil at a time, and you can see how the complete polyphase flow works!
Keep in mind that the Generating source will "Activate" a current flow in each coil, which occurs at 1/3 of the complete rotation of its Armature. Even though the time currents flo in a given coil is 120° ahead or behind another coil, the current flow characteristics in any coil at any time will always resemble the basic single phase model.

A 4 wire Delta will have one coil "Center Tapped", which is equal to replacing one of the coils from the first single phase example image, with one from the second single phase sample image.

Here's an image of a Wye system:


Same principles apply here!

Hope this is effective.

Scott35

BTW: Take a look at items posted in the Technical Reference section, and also you placed this topic in the correct place / area!
If there are no topics shown, drop down the dialog box which says "View Topics From Last X Days", then select "View All Topics" and press " Go"

Scott35
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#128612 - 07/17/03 03:30 AM Re: three phase distribution
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Scott's given an excellent and thorough description there. I'd just add that as you're in the U.K., we don't have delta supplies at low-voltage. Motors and other loads may be wired in 415V delta configuration across the supply as you know, but the actual source in our public supply system is always a 4-wire wye system with solidly grounded (earthed) neutral.

 Quote:
say the red phase and yellow phase are connected to the element, the red phase current would run down the yellow phase when at 0 volts potential (at mid point of cycle)and the other way around when yellow phase is pulling current?


If one phase is at the zero point, the direction of current flow depends upon the polarity of the other phase at that instant.

Let's look at it from a slightly different angle and analyze the potential with reference to ground/neutral on each phase.

If you plotted the voltage on phase A (red in Britain) and that on the phase B (yellow) on the same axes, you have two sinewaves each of the same amplitude (240V RMS, or about 340V peak) but with a 120-degree phase shift between them. (Any of our friends here with fancy CAD software have an image they could provide?)

I'll assume conventional current flow from positive to negative in the following, although electron flow is, of course, negative to positive.

If you place a ruler vertically on the graph and move it slowly allow the horizontal axis, you'll see that at any given moment you can have any of the following state of affairs:

1. One phase positive, the other negative. Current flows from positive phase to negative phase.

2. One phase at zero, the other positive. Current flows from positive phase to zero phase.

3. One phase at zero, the other negative. Current flows from zero phase to negative phase.

4. Both phases positive. Current flows from whichever phase has the higher positive potential to the other phase.

5. Both phases negative. Current flows from whichever phase has the lower negative potential to the other phase.

Note that in all cases, current will flow from whichever phase is the more positive to whichever is the more negative.

So to go back to your 415V element connected across two phases, if phase B (yellow) is at the zero crossing, current direction will depend upon the polarity of phase A (red) at that instant:

If A is positive, current flows A to B.
If A is negative, current flows B to A.

If you moved your ruler slowly along the two sinewaves and plotted a third wave which at any point is the instantaneous sum of the two phases A and B, the result would be another pure sinewave of 415V RMS amplitude.

So as far as that element is concerned, it is just receiving single-phase power at 415V.


[This message has been edited by pauluk (edited 07-17-2003).]

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#128613 - 07/19/03 05:03 AM Re: three phase distribution
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
James,
In a 3P Star system, the Neutral, merely carries the "out of Balance" current, this is normally quite small, unless the phases are way out of balance.
With respect to Delta systems, a Neutral is not required, as all of the loads are interconnected and equalise themselves.
Delta loads are normally, pre-balanced, with respect to motors, transformers and the like.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#128614 - 07/19/03 01:23 PM Re: three phase distribution
james S Offline
Member

Registered: 05/13/03
Posts: 102
Loc: West England
I see you have taken time out on explaining answers to my problems poorly understood by myself thanks for your time paul uk,scott35 and trumphy!!
somethings can be made much more simple when explained in diffrent ways /situations.


ps where would a four wire delta setup be used?would it be correct to say in maybe a motor configured in delta but may need a single phase voltage for say a control circuit, and the other two phases not being used for single phase are compensated for by a slight decrease in impeadance to keep all phases balanced?

or am i just being really silly?

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#128615 - 07/20/03 05:48 AM Re: three phase distribution
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
James,
From our persepctive on this side of the Atlantic, the 4-wire delta is a peculiarly American concept. America seems to have favored delta supply arrangements for LV in the past whereas Britain has always been pretty much a wye country.

Here's the 4-wire delta configuration:


It starts out as a basic 240V delta, such as might be required for light commercial service.

As you know, however, most general lighting and small aplliances in the U.S. run on 120V, so by placing a center-tap to ground on one of the delta xfmr windings and bringing that tap into the building as a neutral you have a source of 120V.

In the diagram above, 120V loads can be connected ph-A to neutral and ph-B to neutral while 3-ph loads are connected A-B-C as usual. So the 4-wire delta supply provides for small 120V loads without having to run a separate service. The xfmr in the bank providing the 120V on A and B can be made a little larger than the A-C and B-C xfmrs which are contributing only to the 3-ph loads. I'm sure one of our friends who works with this stuff regularly will be able to tell you a typical kVa differential between the xfmrs employed (<-Hint! .

A consequence of the arrangement is that ph-C ends up at 208V to ground. This phase is freqeuently referred to as the high leg or wild phase. As you might see from looking through posts in the general area, the unsuspecting person may sometimes attach a breaker onto that phase and inadvertantly feed 208V into 120V loads.

Another confusing aspect seems to be that the phases change their designation between service equipment and internal wiring, so that in most panels it is phase B which ends up being the wild leg. I think that's right, anyway.

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#128616 - 07/20/03 10:18 AM Re: three phase distribution
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
One simplified way to look at 3ø versus 1ø system is to compare the voltage relationships of 3-wire circuits. [For illustration, ignore voltages with respect to ground.] In a 120/240V 3-wire arrangement, you can identify each wire, as for instance “A” – “N” – “C” where 120V can be measured A-N and N-C, and the arithmetic sum of 240V from C-A. This is the alternating-current version of what Thomas Edison’s DC generators originally provided for his neighborhoods of electric lights.

In a 240V 3-wire configuration, each wire is identified as “A” – “B” – “C”, where A – B is 240V, B – C is 240V, and also {note a major difference} C – A is 240V, which is not the sum of the shorter lines like in the 1ø case.

This ingenious arrangement looks simple but has major advantages—it turns out 3ø alternating current is a much more efficient way to get power from one point to another. Visualize 120/240V 1ø 3-wire voltages as two short lines of the same length placed end-to-end. Then, visualize 240V 3ø 3-wire as three lines, but placed end-to-end they make a “loop” or equilateral triangle. The simplified but real-world voltages: A – B is 240V, B – C is 240V, and C – A is 240V for the 3ø system.

For equal current {for instance 100 amperes} and voltage {240 volts} the power delivered in a 3-wire, 120/240V 1ø circuit is 24,000 watts. For effectively the same 3 wires, 100 amperes and 240 volts, the power delivered in a 3-wire 240V 3ø circuit is 41,500 watts. This is a very simplified example, but a valid comparison.




[This message has been edited by Bjarney (edited 07-20-2003).]

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#128617 - 07/22/03 09:52 PM Re: three phase distribution
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Just following up on Paul's mentioning of the 4 wire Delta's KVA ratings per Transformer:

On Open Delta setups (two Transformers), the one used for 1Ø 3W service is typically no less than 150% larger than the "Kicker" Transformer. Normally it's 250% larger.
Example:
25 KVA Open Delta 4 wire setup.
Main Transformer = 20 KVA, Kicker = 10 KVA.

3Ø 3W Open Deltas normally use equal size pots on both sides.

For a 3Ø 4W Closed Delta (three Transformers), the "Center" pot (used for 1Ø 3W service) is typically 150% larger than one of the "Outer" pots.
Example:
45 KVA Closed 4 wire Delta:
Center pot = 25 KVA, Outer pots (2) = 10 KVA each.

These are rough guesstimates, just for discussion purposes only.

Scott35

BTW: Great job by all that replied in this thread! Very impressive knowledge base here!
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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