3 Phase for amatures....

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Chaps (& chapesses) seems quiet on here tonight so a question

Can you explain the basics of 3 phase to me please?

Now 'phase' = live, so do we have 3 'lives'? As it's AC are the phases the same or do we have them in different parts of the sine wave?

How many cores? Do we have 3 lives and 1 neutral?

Where do we get the 415V from? Can we run 'normal' appliences off 3 phase?

 

Flying Scotsman

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3 phase electrical supply is indeed 3 seperate lives within the supply, a transaformer has 3 coil 180 degrees out of 'phase' with each other which is what makes up the 3 phase supply to properties.

 
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3 phase electrical supply is indeed 3 seperate lives within the supply, a transaformer has 3 coil 180 degrees out of 'phase' with each other which is what makes up the 3 phase supply to properties.
How can you have 3 coils 180 degrees out of phase? Surly 2 would match up? Do you mean 120?

Is there a common neutral or do we have 3 of them?

 
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Here goes (for the second time - somehow deleted the first!!)

A 3 phase supply is taken from a generator with 3 coils (usually in a star configuration) all coils are connected in the centre which is the neutral/earth point. The coils are 120 degrees apart. To get 230v you connect across one of the coils (L1, L2 or L3 to N),to get 410v you connect across 2 phases (L1-L2, L1-L3 or L2-L3).

If you were to draw the 3 phases on a graph you would have 3 sine waves 120 degrees apart.

As for cabling, that depends on the application. Many motors only need the 3 phases and not a neutral, however you will find that some other equipment needs a neutral aswell.

Hope that helps

 

Mr Sworld

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The problem with 3-phase is when it's 'unbalanced'.

A three phase motor needs no neutral as each phase has the same current being drawn from it. A phasor diagram will return to its starting point hence no current flow out. The phases, being 120 degrees out from each other, balance themselves.

When a 3-phase supply is unbalanced it needs the neutral to return the current flow.

The problems occur when the phases are so unbalanced the neutral is returning more current than each phase is supplying. Say a 63A 3-phase supply is so unbalanced that the neutral is handling 80A. The neutral is the same size as the lives, so a cable the same size as the 63A cable is now handling 80A! Melted connections and a very hot cable! :_|

This is why in the street outside your house take the 1st house from red/brown phase, the 2nd blue/black phase and the 3rd house is yellow/grey. Then it repeats for the next three etc, etc. Although each house has different ampage loads over the whole area it balances out (or that the theory.)

 
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Just to add to the other guys posts..

The actual generation part of electrics.. which relies on the principal of a moving a coil of cable through a magnetic field.. inducing current into the coil.

(the same principal can be achieve either by moving a coil near a magnet.. or moving a magnet near a coil.)

the easiest way of creating constant movement is by mounting a coil or magnet on a spinning axis. and by mounting three things equidistant-distand apart they would be 120 degrees each around the axis..

{If you were doing four it would be 90 degrees apart..}

somewhere back in the e-ons of history three was deemed the best arrangement

so as our generator rotates around a

new phase of the electric current is induced per every 120 degrees of rotation.

the three phase have one end common end together at the neutral Star point.

If the load on the three phases is balanced the current just flows up & down the phase wires & the neutral current is zero.

But if the load on the phases are unbalanced some residual current is left flowing down the neutral cable

In the old Domestic property run off one of the supply phases...

the load taken by each house is different... so the three phases are unbalanced!! :)

 
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oh bum! :_|

looks like Mr S.. has explained wot I was typin slow..

better than what I said it? :eek:

Note to self

Too much bl00dyGuiness DrinkGuiness DrinkGuiness Drink

not enough A.B.C.D.E.F.G

or Q.W.E.R.T.Y.U.I.O.P.BlushingBlushing:p:pBlushingGuiness Drink

 

Mr Sworld

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No, motors use all three phases for each motor. However a factory floor will have it's lighting split over all three phases (30 lights on red, 30 on blue, 30 on yellow).

Each sub-main will have the load on that sub-main balanced out over the three available phases

 

Mr Sworld

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I'm afraid I'm sober tonight Spec Loc.

Pity..... 5000 ****ed freshers in the building for the student ball tonight.

 
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What you try and do in any 3 phase installation is to balance the loading across the phases.

E.G. say you had a shop with loads of lighting. You would arrange the lights in banks (with an equal(ish) number of lights on each phase) so that the current will be balanced. You would do the same with all the other circuits (especially fixed loads).

Motors tend to be either single or three phase, not 2.

So a 3 phase motor would be supplied by a 4 core cable (3 phases and earth) this will be fed from a 3 phase MCB which will disconnect all 3 phases at the same time in the event of a fault.

Now a single phase motor will be fed using a 3 core cable (Line, neutral an earth) and fed through a single phase MCB.

 
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So, for example, a farm of a factroy could have motors drawing 2 phases and use the other one for lighting etc?
As Mr S says your factories & agricultural equipment will most often be 3-phase eqipment...

But as another example a beauty salon had a 3-phase supply..

with all sorts of hair dryer (big-sit under jobbies),

heating, electric shower, sun bed, numerous socket for plug in curling tongs, hair straighten ners etc..

loads of lights.. ordinary office equipment..

etc..

etc..

the layout should be designed to balance the total load across the phases..

e.g. don't stick all the heating stuff on one phase and lights on another phase

split them equal across all three phases!

 

Admin1

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It's all making sense now :D Thanks everyone
Do I sense a "Career Change" coming on, Apahe, Sir?

]:)

:D

PS. I will move this great thread over to the "Student & Learning Zone", Gentlemen. :D

 

searley

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So a 3 phase motor would be supplied by a 4 core cable (3 phases and earth) this will be fed from a 3 phase MCB which will disconnect all 3 phases at the same time in the event of a fault.

quote from above

so if you run this in singles from the DB,L1 BROWN,L2 BLACK,L3 GREY, AND THEN A CPC, SO YOU DON`T RUN IN A NEUTRAL CABLE ?

does anybody have some pics of how to wire the starter and contacter.

 

searley

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andy if you get time could you do a quick drawing of a motor circuit for me think a have f--ed me AM2 UP.

IF POSSIBLE COULD YOU DRAW IT FROM THE 3 PHASE DB TO THE MOTOR STARTER YOU GET ON YOUR 2391 OR AM2 AND THROW THE CONTACTER TO THE MOTOR.

I RAN A NEUTRAL IN FROM THE DB TO THE MOTOR CONTROL AND I THINK I` LEFT IT IN A CHOCKA BLOCK AND PUT THE COVER BACK AS I WAS RUNNING OUT OF TIME.CHEERS

 
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