# why 3-phase power?

Discussion in 'Electronic Basics' started by Alan Horowitz, Dec 30, 2003.

1. ### Alan HorowitzGuest

what is the attraction of three-phase power? Why not 9 0r 317
phases? Why not plain ole hot & neutral?

2. ### ZakGuest

It is an efficient way to do the transmission, I understand - maybe the
most efficient?

It also needs just 3 wires. 90 degrees would need 4 wires or 3 wires
with 1 wire that is thicker than the other two.

Thomas

3. ### Spehro PefhanyGuest

It better uses transformer iron and makes better/cheaper induction
motors. For making DC it requires less in the way of filtering. It
would be nice if we had something like 6 phase power, we could get rid
of the input filter caps in most equipment, the only part that we
can't make smaller by increasing efficiency and frequency of SM power
supplies.

Best regards,
Spehro Pefhany

4. ### cpemmaGuest

The motor is effectively running on a much higher voltage than single phase,
so more power/weight, and to change direction just swap any 2 wires over.

5. ### Guest

Because the power lost in the transmission line resistance is proportional
to the square of the current, you want the current as small as possible.

Ways to do that are to increase the voltage and to increase the number
of phases.

While there is no theoretical limit to the number of phases you can have,
3 is the generally accepted trade off between efficiency and complexity.

And in most places, "plain ole hot & neutral" comes in as 2 phase, also
called "220".

6. ### Brenda AnnGuest

Actually, this is normally called "split phase" rather than 2 phase because
the the neutral 'splits' the single phase 240 volt drop.

7. ### Frank BemelmanGuest

9 phases.... 317 phases, have you ever wired a 317 prong plug?

8. ### Guest

Actually, what it's called is dependant on where and who you are.

Engineers and electricians, for example, have different terms for the same
things.

9. ### John WoodgateGuest

I read in sci.electronics.design that Brenda Ann <>
wrote (in <bssmvp\$e07\$>) about 'why 3-phase power?',
The question of whether 120-0-120 is split-phase or two phase is not
just a fighting issue in the US, it's a TOTAL ANNIHILATION issue. (;-)

Wholly incredible to this poor Brit.

10. ### Spehro PefhanyGuest

Actually, what it's called is dependant on where and who you are.

Engineers and electricians, for example, have different terms for the same
things.[/QUOTE]

Perhaps so, but "2 phase" is wrong.

Best regards,
Spehro Pefhany

11. ### John WoodgateGuest

I read in sci.electronics.design that Frank Bemelman
318; you forgot the ground prong.

12. ### Guest

How big would the sparks be if you crossed a British engineer with an
American electrician?

13. ### Frank WarnerGuest

320? Neutral...

14. ### Jonathan BarnesGuest

In England a 230 V single phase is just that, one wire is nutral, nominaly
earth but seperate from the true earth, and the other caries 230V ac.

I belive the American system is 2 x 110 V ac lines with respect to earth,
but at 180 degree phase angle, to give 220 V ac potential between them.

I would say it's fair to call this 2 phase.

--
Jonathan

Barnes's theorem; for every foolproof device
there is a fool greater than the proof.

15. ### Spehro PefhanyGuest

How big would the sparks be if you crossed a British engineer with an
American electrician?[/QUOTE]

Here's a picture of a suitable American electrician, let's see how the
Brits respond..

http://www.networkwomen.com//images/NOVEMBER_2000_Story_Photo.jpg

Best regards,
Spehro Pefhany

16. ### Spehro PefhanyGuest

Yes (or is it now 240 nominal there)?
120V, but close enough.
Not fair, just WRONG, though I can see that electricians who think of
"phase" as equivalent to "hot wire" might find it okay. True two phase
could be converted to 3, 6 or whatever phase using Scott-T
transformers. Not true for this, which is just a center-tapped 240V
line. You can go right or left on a one-dimensional line from zero,
but it's still in one dimension..

Best regards,
Spehro Pefhany

17. ### Frank BemelmanGuest

Yes, and even while being extremely careful, I swapped
phases 127 and 131, 177 and 178, 202 and 212, 214 and 215,
and 301 and 311.

I think a need phase tester too

18. ### Frank BemelmanGuest

With 3 phases you never run out of juice, even without a cap,
so why do you want 6 ?

19. ### BFoelschGuest

In many large solid-state rectifiers, (> 100kw) phase shifting transformers
are used to generate 12, 18, 24 or even 30 pulses of current per cycle.
Aside from simplifying the filtering, it simplifies the harmonic current
problem on the supply line.

But, no matter how hard you try, you can't do this with a single phase
source, 2 or 3 or 10 wire. If a transformer has a single primary winding, it
delivers single phase energy, no matter how the secondary is tapped.

Not sure that anybody mentioned that a single phase motor has a 4x/cycle
torque zero (2 voltage zeroes & 2 current zeroes). 3 phase eliminates these
zeroes.

20. ### Jonathan BarnesGuest

Well... the Europeans used 220V, the British used 240V, and we are all now
harmonising on a nominal 230V.
Ah... thats why some american equipment I had did not like running on a 110V
safty transformer I had for testing
3 phase supplies have a 120 degree phase angle between them.

Not true for this, which is just a center-tapped 240V
I would hate to have you navigate for me if you don't see a diference
between left and right...

I would sugest you would learn the difference between a single and two phase
system in a hurry in Russia, they earth the nutral leg to the frame on all
machines and then have a seperate earth bond. ( don't try to use an RCD
).
--
Jonathan

Barnes's theorem; for every foolproof device
there is a fool greater than the proof.