Run a 3 phase lathe coolant pump on single phase.

[Ed.]

[/quote]you would need a neutral though one of the problems of doing that is you can overload the neutral in the run from the CU to socket l[/QUOTE]

I have tapped the neutral as well as the active and also the earth, overloading the wires is not something that I worry about as the wires from the house pole are 25mm to the main circuit box, the wires from the house circuit box to the shed's sub-board are (about 9m apart) 16mm copper, and from the sheds sub-board to each 3 phase wall socket is 10mm copper and lastly from the 3 phase socket to the 15A 240V GPO's are 6mm cables. There is also another independent and separate 240V sub-board in the shed running 2.5mm copper to other power points. That 240V sub-board was installed first before I put in the 3 phase to the house and the shed, and is 10mm to the main house circuit box. So not much chance of anything overloading the circuits, even my 3 phase MIG welder which can draw 27KVA runs quite happily on a 6mm 18m extension lead.

 

[knighty]

you would need a neutral though one of the problems of doing that is you can overload the neutral in the run from the CU to socket l

that doesn't happen, as you load up each of the phases from the 3 phase the negatives essentially cancel each other out - so if you have a 2kw heater attached to each phase, it;'s just the same as having one 6lw 3phase heater - negative isn't even needed

of course you'll never have perfect balance in real life... but they'll cancel out enough that you're in no danger of overloading the negative

 

[knighty]

I thought that it was in reference to taking 240V off one of the 3 phases only?

I'm no expert on those 3 phase converters etc.. but as I understand it... each of the phases is 230v to the neutral, but not 415v to each other

so it's not really 3 phase... it''s single phase with a bit of a delay on two of the phases so the sine wave is a bit out of sync

you can get away with that in delta because it's just like running three single phase motors a little bit out of sync with each other

 

[normspanners]

that doesn't happen, as you load up each of the phases from the 3 phase the negatives essentially cancel each other out

HOW ??? if you have several single phase items running off a 3phase socket each phase draws on the neutral, it has to, how can it cancel out ?

 

[knighty]

if you load each of the 3 phases evenly there'll be no current draw on the neutral at all

the 3 separate phases aren't really positive.... they're sine wave so they're constantly going positive/negative/positive/negative

when one is 240v positive the other two add up to 240v negative


I'm struggling to explain this, I can picture it in my head no problem, typing it out is a bit harder


(it would be easier, but the peak of the sine wave is 320v not 240 :-s )

 

[daleyd]

We do a lot of 3 phase heating stuff in work - not many actually have a neutral, though there's all sorts of different configurations you can get.

 

[Pete.]

if you load each of the 3 phases evenly there'll be no current draw on the neutral at all

the 3 separate phases aren't really positive.... they're sine wave so they're constantly going positive/negative/positive/negative

when one is 240v positive the other two add up to 240v negative


I'm struggling to explain this, I can picture it in my head no problem, typing it out is a bit harder


(it would be easier, but the peak of the sine wave is 320v not 240 :-s )

I've always wondered why the neutral conductor isn't larger than the phase conductors and your explanation clears it up. Wonder no more

 

[WorkshopChris]

if you load each of the 3 phases evenly there'll be no current draw on the neutral at all

I think that is one of those theories that doesn't work in practice.
Have been involved with distribution in sound and lighting set up, and the neutrals were often overrated, dude to lots of single phase supplies take of 3 phase outlets.

 

[normspanners]

if you load each of the 3 phases evenly there'll be no current draw on the neutral at all

I do understand the theory, btw it is because of the star point that is why the neutral on overheads is the same diameter.
But as per Chris I have seen oversized neutrals used

negative isn't even needed

It has to be because if 2 of the item are switched off the third wont work----- it cant.

 

[knighty]

I think that is one of those theories that doesn't work in practice.
Have been involved with distribution in sound and lighting set up, and the neutrals were often overrated, dude to lots of single phase supplies take of 3 phase outlets.

that's weird.... even if you load one phase to the max on its own... as you start to put a load on any of the other two the load no the negative starts to drop :-o

maybe it's something to do with the sound equipment and interference ?

 

[knighty]

that doesn't happen, as you load up each of the phases from the 3 phase the negatives essentially cancel each other out - so if you have a 2kw heater attached to each phase, it;'s just the same as having one 6lw 3phase heater - negative isn't even needed

of course you'll never have perfect balance in real life... but they'll cancel out enough that you're in no danger of overloading the negative

It has to be because if 2 of the item are switched off the third wont work----- it cant.

I only meant that in the example I gave

 

[knighty]

HOW ??? if you have several single phase items running off a 3phase socket each phase draws on the neutral, it has to, how can it cancel out ?

I do understand the theory, btw it is because of the star point that is why the neutral on overheads is the same diameter.

you've lost me there, you don't understand how it works and then you say you do

I think we've got our wires crossed somewhere (no pun intended)

 

[normspanners]

I know three phase cookers use the same 5 core equal size cable the neural prevents a 'floating' star
but running loads of seperate single phase things off one three phase cable is not really a good idea but at least if on a 3ph mcb at least if one overloads it trips the lot when fuses are used you have a problem with borrowed neutrals

 

[knighty]

but running loads of seperate single phase things off one three phase cable is not really a good idea

it doesn't matter at all

every single phase house supply is essentially a single phase of the 3 phase they run along the street

and everyone with 3phase... what about if they're running loads of single phase loads ? all those loads run back to the fuse box and different phases of the 3phase?

that's no different to running single phase things from a 3 phase socket (or multiple single phase things)(and we'll just accept you have the right gauge wire/fuses etc.)

 

[normspanners]

I think we've got our wires crossed somewhere (no pun intended)

Sorry Yes I think we have , I meant how could it work with no neutral on separate single phase items off one three phase socket, it has to have a neutral,
but I now understand your idea of three equal heaters creating there own artificial neutral star point, by being interconnected and not needing a neutral.
In the same way that the supply has a three phase supply only, to 3 ph tranny and the neutral 'appears' at the star point.

 

[WorkshopChris]

Am trying to find the info I was researching for a project a while back.
https://www.tlc-direct.co.uk/Book/4.3.12.htm
This one is us based but starts to explain harmonics and the effects.
http://www.schneider-electric.co.uk/en/faqs/FA212226/

 

[normspanners]

the problem you have if the neutral becomes disconnected or has bad connection,and one item is still plugged in and one is unplugged but left switched on the connectors of the separate unplugged plug will now become live will they not.

 

[Seadog]

As knighty explained, the nett current in a balanced 3-phase system is zero. This diagram perhaps makes it easier to follow.

When Phase 1 is at its peak value then both of the other two phases are at half-peak value but of the opposite polarity. The vector addition of the three phases is zero, therefore there is no nett current.

 

[normspanners]

I do understand the theory, btw it is because of the star point that is why the neutral on overheads is the same diameter.
But as per Chris I have seen oversized neutrals used

just to clarify
People ask because they are visible why the overhead cables all have the same size cabling, even the neutral.

 

[Pete.]

I never wondered about the overheads but I hve pulled enough redundant cable out of demo buildings. The four conductors are always the same size.