how 3 phase works.

[jim23]

wondered if anybody could answer my query.

was wondering how 3 phase works? if ive got a 3 phase + neutral supply at the mains intake and there's a 400 amp fuse in each phase does that mean it can take 400 amp down each phase before it blows the fuse or is it 400 amp total?

cheers.

 

[Adam_151]

was wondering how 3 phase works? if ive got a 3 phase + neutral supply at the mains intake and there's a 400 amp fuse in each phase does that mean it can take 400 amp down each phase before it blows the fuse

Yes

 

[jim23]

what about diversity for 3 phase? is it worked out using same principles as on site guide for single phase. does the equipment drag equal amounts of amps down each phase roughly then?

many thanks.

 

[Adam_151]

what about diversity for 3 phase? is it worked out using same principles as on site guide for single phase.

Yes, but

a) its only a guide, and it usually errs very much on the side of caution
b) 3 phase is very rarely used domestically, so chances are you'll be looking at the commerical sections of the table...

does the equipment drag equal amounts of amps down each phase roughly then?

Only if you balance your phases

, (put a 3 showers on red, and 3 lights on each of blue and yellow and it'll be completely out of balance, it doesn't automatically become the equivelent of 2 lights and a shower per phase... you have to do that )

May I ask why you are asking this?, I'd hate to see you end up six feet under...

 

[jim23]

its a question ive got in my college assignment, bit lazy i know but suppose thats wot the forums for!!! one last question, if the phases are out of balance is that a problem or not?

cheers.

 

[jim23]

sorry, meant to ask for an individual peice of equipment, say a motor or a machine? are the mps balanced out for this instance?

 

[Adam_151]

one last question, if the phases are out of balance is that a problem or not?

cheers.

Well there is an equal amount of current available on each of the phases, it'd be very wasteful to stick everything on red and size everything 3 times what would otherwise be required It also keeps down neutral currents, and so there is going to be less volt drop (3 conductors carrying 20A each will drop less than 2 carrying 60A each...)

So yes, perfect balence is something to strive for, not possibly to do it perfectly, due to loads changeing as different things are used, but correctly done things will even themselves out, etc (red might be loaded high on one board, and yellow loaded high on another)

 

[jim23]

thanks, all much appreciated!

 

[jim23]

hey, thanks again. forgot to ask, what about say a motor or a machine in a workshop. do they pull equal amps down each phase as they are working? thanks again, should get that pass grade now!

 

[RF Lighting]

Providing the motor is a properly balanced 3 phase motor then the load will be equally balanced across all the phases. This is why some industrial motor circuits are supplied with three phase only and do not require a neutral connection.

 

[plugwash]

sorry, meant to ask for an individual peice of equipment, say a motor or a machine? are the mps balanced out for this instance?

usually the motors will be 3 phase motors and hence balanced but there may be other loads (heaters, power supplies for control gear etc) that are not. single phase loads inside such equipment may also be connected phase-phase further complicating the calculations.

 

[ban-all-sheds]

its a question ive got in my college assignment, bit lazy i know but suppose thats wot the forums for!!!

Err - no, actually it's not....

 

[Bright_Spark]

3 phases, at 120 degrees each...why not have 4 phases at 90 degrees and call it a good do

 

[plugwash]

there would be two issues with having four at 90 degree seperation

1: you've got two mirror images waveforms there so from a motor and recifier smoothness point of view it would be worse than 3 not better.
2: you could no longer reaonablly assume that the neutral current would be as low as or lower than the highest phase current (yes some stuff breaks this assumption in 3 phase as well but it tends not to work out too bad in practice because the majority of big loads are simple loads)

 

[Stoday]

3-phase transformers are the most efficient - more efficient than single phase or any other number of phases.