3phase and Neutral

[snowplough]

Hi Guys,

I am fairly new on here,

I am just studying my first ear on the 2330 Electrical course and i have tonne of stuff to ask you all which doesn't seem to make sense to me yet.

Well here goes with the first thing regarding (3 load phase and the neutral wire).

I think i understand that a 3 phase wire system is 3 wire containing 3 separate voltages at 120 apart but produced by the same generator by spinning 3 coils of wire in a magnet , or is it a magnet spinning inside a coil of wire?.

Secondandly it says that if the 3 phases run into a balanced load from the generator does this mean that it is connected in the generator into this balanced delta or star connection and then the 3 phases are taken off this star or delta connection at the generator and run into another star or delta connection at transformer where it goes into this balanced load, and it says that the 3 voltages add up to zero which i dont understand?.

Thirdly It says the Neutral wire is for the out of balanced current?
I Can only understand at this point that if you take a line from one of the phases and into a home say then that single supply needs a return path to its source so it can complete its circuit , hence the so called neutral wire .Is this what it means by being out of balance, the fact that the return current flow doesnt go back down the other 2 phases , but down a seperate return wire( called the neutra wire).

And fourthly, if the neutral wire is connected to earth ,and is said to be at earth potential (zero),
what happens to the current flow in the A.C domestic circuit as it flows firstly through line in the home and out of the neutral, and then through the neutral, back in the home and out of the line at 50 times a second.
If the neutral is at zero potential connected to earth at the star point when the current flow comes in through the neutral does this mean that the current just as no potential as it comes in on that side.

I hope you can understand this little lot for starters and thank you for your patience .

Many Thanks

Snowplough

 

[raylewis]

Hope I can clarify for you

Firstly all power stations generators(altenators) are in delta configeration
Magnet(dc excited windings) rotating inside stator windings.
Transformers are normaly wound delta/delta for three phase systems
Three phase motors etc are all balanced loads
Remember you can out of balance 3 phase loads as well
Single phase systems are inherantly the unbalanced loads eg if you connect 3 houses to the same star transformer each one a seperate phase and neutral, not all will take the same load at the same time.
Transformers for single phase systems are wound delta/star(you need neutral from star point)+(415v divided by square root of 3)
Zero potential between neutral and earth not the line

Hope I have managed to explain

Ray

 

[Darkwood]

You wrote an essay of questions but you seem to be half right as to how it works.

At the power station the electricity is generated over 3phases as you say at 120 degrees to each other although im wont go too deep into it all it is then transformed with step up transformers to reach high voltage in order to carry it long distances with minimal loss, when it approaches its destination it will be stepped down in various stages to bring the voltage down to a useable level with the last transformer been star wound and usually 400v at the local substation. Up to this point neutral dosent exist and is thus made by tapping the center point of the star transformer at this same point the earth is also taken or in pme it is the same wire.
As an example phase '1' might do street (a)
phase'2' might be street (b) etc...

when they arrive at you house you normally have line(phase) neutral and earth, N&E because they are from the same origin are at the same potential so touching neutral will not give a p.d. between you and earth hence you can usually get away with touching it.
Yes neutral is the outbalanced return but in this example its forced into the system, if for example you had three phase in a factory and put an identical row of lights on each phase it would be balanced so if you clamped tested neutral on the incomming you would see no current flow but 3 identical readings on the phases.

 

[snowplough]

Many Thanks to you Ray Lewis and Darkwood,

Darkwood , when you said the neutral in the domestic situation was forced into the system, did you mean its there as a return path for the single phase wire into the home so ou can get 230 volt supply or its there for the out of balanced 3 phase load in the factory situation as ou said earlier, or is it there for both domestic and industrial needs equal?.

When you said that if you had 3 equal loads say in the lights example you gave me in the factory, you said there would be no return current through the neutral, so can you give me an example of a out of balance load situation in the same example briefly explaining to me with figures current flowing down the neutral so i can maybe see whats happening.

Oh , and lastly can you explain the bit at the bottom of my first question regarding A.C current running through the line and then back through the neutral and vise versa as i am still unclear as to the answer to that question.

Many thanks in advance for your patience and taking time to answer my questions,

Snow Plough

 

[Darkwood]

Ok snowplough, Forced might be the wrong word which i used but more or less yes its what you say its the return path for the phase, it is there for any installation that requires single phases and with the exception of aged factory boards nearly all supplies now have neutrals, as a factory requires 230v to run office sockets, lighting etc, but the neutral to a factory is shared amongst the 3ph so as a designer or installer you will try balance the different 1ph loads over different phases to try balance the incomming supply the better the balance the lower the neutral current on the incomming cable thus the more efficient and cheaper to run.
Your last question you are correct up to the point where you said the neutral has no potential difference, well crucial bit here is p.d. between neutral and what?
Well between Neutral and Earth it is Zero as they are strapped together but it still has 230v to phase even though for that split second its going the other way.
Hypothetical if we had american system where 3ph is 220v between phases we could take over one of our 230v equipment and use it on 2 phases without the need for a neutral but note this isnt wise as the equipment isnt designed to do this safely but because your not using a neutral with the same potential as earth you will get 220v to earth off both sides of the circuit. (Note this is their 3ph voltage 220v and not the single phase which they use about 115v.)

 

[johnstantine]

Hi
There are some good books available that make understanding 3-phase a bit easier as they have great diagrams and may offer explanations that you miss at college. I've picked up a nunber of such books from places like Waterstone, Ottakars etc, or you can try Amazon. It's very useful to have stuff that supplements what you're learning in class.
An example of some books I would recommend are-
Newnes pocket electrical
Electrical installations nvq and technical certificate books 1 and 2. By Heinemann.
Guide to the wiring regulations. By Darrell Locke

 

[snowplough]

Many Thanks Darkwood,

When you said that by having 3 idententical lights in the factory scenario when discussing 3 phase , you said that there would be no current measured on the neutral conductor because they were balanced, did you mean that these lights would be connected to a 3 phase supply like in delta or star, where you dont need neutral if it is balanced , or did you mean the 3 sets of lights were connected individualy, so they are on there own individual phase ,like one from brown , one from black etc.

Secondly im not sure what you meant when you said neutral on the incoming ?

And thirdly ,If for instance say you take 3 separate single phase cables and connect them to 3 identical loads but they are all seperate then the 3 sepearate currents just flow back through neutral ,which to me means nothing is balaced if im correct.
It sas in my text book that if you measure the current in an of the phases ,ou get a high reading , but if ou measure the current in the neutral ,iou get a low reading, when it says phases does it mean the ain incoming 3 phases as ie brown ,black or grey as surley the current in the line just before the load is gunna be the same going into the neutral at the other side of the load .

sorry if im missing something obvious ,just trying to understand this balancing thing.

Many Thanks

Snow plough

 

[Darkwood]

will try answer all in one:-
If an industrial unit has a 3ph incomming supply then it will comprise of 3ph/N/E, the 3phases share a common neutral which when made off into a dist' board will be the common neutral bar. Usually 3 ph boards are split into single phase ways alternating through the phases as you go along.
If you take a single phase way off brown and wire say a fixed 10amp load and repeated with black ph and grey then you would have a balanced load situation. If your measure each of the 3 phase wires going out to single phase circuits each would show 10amps and if you were to measure each of the returning neutrals again you will get 10amps as you expect, but the situation isnt as it seems once back at the board as you have 3 returning neutrals from each single ph circuit you would expect the supply neutral to be an accumulative value hence 30amps? but no you would be wrong as if you measured the load on the incomming neutral to the board it will be zero because the 3 neutrals cancel each other out as the loads of each ph are perfectly balanced.
This is why it is common to find no neutral is needed in 3ph motors as each phase to the motor is equally balanced although a neutral may be supplied for the control side of the motor but not in every case.

 

[snowplough]

many Thanks Darkwood ,

I thought that ou could only get a balanced load say in a motor where all the phases are connected together so each phase current returns down the other two phases like in a delta connection, i didnt think that if you have 3 single like you mentioned that you can get zero current reading in the neutral as i assumed the 10amp current from each single phase load can only return back down the neutral.

I clearly need to do alot more tudying and im sure at some point it will click.

In the mean time many thanks and ill still keep picking your brains

Kind Regards

Snow plough

 

[Darkwood]

No worries snow, just remember the above example relies on the fact you have no other loads on the phases and the 3 loads you used are exact in all respects then the result would be as above, but in reality its unlikely to achieve zero in neutral with all the different single phase loads and out of balance 3ph loads on a standard industrial/commercial installation but understanding the background will always aid your experiences.
Just study at a good pace for yourself and try not to run before you walk it all just falls into place with experience and one day the lightbulb will appear over your head and you will think 'Oh! its quite simple when you think about it.' this is what i found anyway as i had similar confusions in my early years.
As Johnstantine has mentioned the Newnes set of books are excellent choice to help you understand better, keep at it and you will be fine.

 

[snowplough]

Hi Guys,
Wonder if any of you can just clarify a few questions i have on testing as i have started to do it at college.

Firstly , when ou do the continousy test for the cpc , i was told to loop the earth and line at the consumer unit and then test at the last ceiling rose for a lighting circuit , i am a bit confused, by doing this , are you testing the continuity of the line as well as the cpc ,or does the line cable just substitute the long lead for the cpc, or is the line cable classed as a cpc as well as the earth when connected to earth. I now our suppose to connect it like this when you do an r1 and r2 reading , is his the reason why it is connected as above so you can do an earth continuity and an r1 and r2 reading at the same time.

Secondly , when ou test for polarity on the lighting circuit , i no its to nmake sure the line cable is in the correct terminals ,if ou have light bulbs in , b flicking the switch , if the bulb illuminates that to me says the line is in the correct terminal, but if like us our practising on a circuit board without power ou can just put our probe in the line and earth in the ceiling rose and flick the switch to get a reading confirming polarity. abaut in a proper dometic situation the switch might be to far to reach from the ceiling rose so surel b putting our probes in the light switch line (at load side ) and earth with the switch on this would prove polarity of the line in the switch terminal .

Im not ure if i am correct but i will appreciate our clarification and also tell me if ou would also put your probes in other switches ,say a two wa and intermidite ,and which terminals you would put them in as there are strappers invoved.

Sorr if i am not very clear ,

Many thanks

Snowplough

 

[sparks1234]

You should be able to gain R1 R2 from this and it will also prove the polarity