why 3-phase power?

[Spehro Pefhany]

Rich Grise wrote


Not as long as SIN X = - SIN ( X + 180 )

There is no difference with a sine wave.

Well, shoot. There's the difference, and it was in front of us all the
time. We use *cosines* in North America. Hope this helps.

Best regards,
Spehro Pefhany

 

[Rich Grise]

No, it can't work. Take a careful look at the magnet flux
path in any ordinary 3-phase transformer. They're all
in parallel, if you think of the top bar as one pole
of a bar magnet that's split into 3 things, and the
bar at the other end is the other pole. So all the
fields oppose, and energy gets to go to the secondary.
With your toroid, the magnetic paths are in series,
which is pretty much like a short circuit, so, as
John Larkin noted, "It would blow up instantly."

Cheers!
Rich

In sci.electronics.basics John Larkin

| On 1 Jan 2004 16:57:29 GMT, [email protected] wrote:
|
|>Having noted that utility distribution 3 phase transformers have a core
|>that is really one big core for all three (though spatially separated a
|>bit), I have been wondering what would happen in the magnetic field if
|>one were to take a toroid core, put the three phase primaries on it at
|>120 degree positions covering only 1/3 of the toroid each, and then have
|>a "rolling secondary" that slides around the toroid (mechanical support
|>for the mass would then be done with wheels holding up the 2/3 not being
|>used for the secondary).
|
| It would blow up instantly.

OK, take the rolling secondary back off. Just have 3 primaries covering
1/6 of the toroid each, and 3 secondaries covering the remaining gaps
between those secondaries.
http://ka9wgn.ham.org/ |
--------------------------------------------------------------------------

---

 

[Jonathan Barnes]

Yes. Use a phasor diagram.

I don't have a phasor diagram in my tool box, this is bit of theory not a
test.

See if you can find graph paper with polar and rectangular
coordinates, and draw directed segments [0,0][1,0] and [0,0]
[-1,0]. They're clearly inverses. Now, transform them both
to polar coordinates. Now it's vector 1.0 < 0deg and 1.0 <
180deg.

Overlie the two and they cover one another, it's simply two ways to say the
same thing.

I think thinking of it as just going up and down is one way,
and thinking of it as going around circles is another.

Exactly my point, half way round is the same as across.

And just another grain of sand - isn't there something about
reactance having to be involved?

Not as far as I am concirned

 

[Jonathan Barnes]

Well, shoot. There's the difference, and it was in front of us all the
time. We use *cosines* in North America. Hope this helps.

Best regards,
Spehro Pefhany

Well COS X = - COS ( X + 180 ) if your crossing my argument at 90 degrees


Regards
Jonathan

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

To reply remove AT

 

[John Woodgate]

I read in sci.engr.electrical.compliance that Rich Grise

No, it can't work. Take a careful look at the magnet flux
path in any ordinary 3-phase transformer. They're all
in parallel, if you think of the top bar as one pole
of a bar magnet that's split into 3 things, and the
bar at the other end is the other pole. So all the
fields oppose, and energy gets to go to the secondary.
With your toroid, the magnetic paths are in series,
which is pretty much like a short circuit, so, as
John Larkin noted, "It would blow up instantly."

It would be nice to have a more scientific explanation. The rot set in
with 'It would blow up instantly' - no explanation of what would cause
an uncontrolled energy release.

The point is that with the three voltage phasors at 120 degrees, it can
be seen by resolving the resulting inductions into orthogonal components
that both components sum to zero. No induction is created in the core,
so there is no transformer action, and the input current is limited only
by the winding resistances.

Two phases at 90 degrees would be a different story, but maybe of no
practical use.

 

[John Larkin]

Two phases at 90 degrees would be a different story, but maybe of no
practical use.

Would blow up instantly.

John

 

[Bill]

| For larger horsepower motors, Three Phases means that they will
| start with no external mechanical assistance.

And, as I understand it, they can then "stand on one leg" although they
will be a bit "shaky".

You can also reverse the rotation by swapping two of the phases. If one
phase is blown you can start it by giving it a spin in the right
direction, 'without' getting your hand caught. I had to replace one at
work once on a rooftop air conditioner, even though I was an electronics
engineer I was the only one who volunteered. It was only about 130 on
the black tar roof. I came down a hero to the assemblers who were under
it but had to leave and take a shower after that one, seems no one would
get near me for some reason. One leg may be possible if there is a
center return wire but I didn't try it.
Bill Baka

 

[[email protected]]

| On 1 Jan 2004 23:59:44 GMT, [email protected] wrote:
|
|>| On 1 Jan 2004 16:57:29 GMT, [email protected] wrote:
|>|
|>|>Having noted that utility distribution 3 phase transformers have a core
|>|>that is really one big core for all three (though spatially separated a
|>|>bit), I have been wondering what would happen in the magnetic field if
|>|>one were to take a toroid core, put the three phase primaries on it at
|>|>120 degree positions covering only 1/3 of the toroid each, and then have
|>|>a "rolling secondary" that slides around the toroid (mechanical support
|>|>for the mass would then be done with wheels holding up the 2/3 not being
|>|>used for the secondary).
|>|
|>| It would blow up instantly.
|>
|>OK, take the rolling secondary back off. Just have 3 primaries covering
|>1/6 of the toroid each, and 3 secondaries covering the remaining gaps
|>between those secondaries.
|
| Still blows up. The primary phases are fighting each other. A
| three-phase torroidal transformer needs three cores.

And how is that different when you take an iron core and wrap 3 coils around
it like:

_______________________
| |
| _____ _____ |
| | | | | |
/=======\ /=======\ /=======\
|=======| |=======| |=======|
|=======| |=======| |=======|
|=Phase=| |=Phase=| |=Phase=|
|===A===| |===B===| |===C===|
|=======| |=======| |=======|
|=======| |=======| |=======|
\=======/ \=======/ \=======/
| |_____| |_____| |
| |
|_______________________|

Certainly it is different in that they are not in "magentic series". Is that
what is relevant?

 

[[email protected]]

| No, it can't work. Take a careful look at the magnet flux
| path in any ordinary 3-phase transformer. They're all
| in parallel, if you think of the top bar as one pole
| of a bar magnet that's split into 3 things, and the
| bar at the other end is the other pole. So all the
| fields oppose, and energy gets to go to the secondary.
| With your toroid, the magnetic paths are in series,
| which is pretty much like a short circuit, so, as
| John Larkin noted, "It would blow up instantly."

Is there a way to mix phases in the magnetic domain much like they are
mixed in the electrical domain by connecting leg to leg in a 480Y/277
secondary to get the 480 volts which is 30 degrees different in phase
then the 2 277 volt secondary windings?

What I was trying to come up with was something that lets you adjust
the phase with a big knob much like a variac is used to adjust voltage.



| Cheers!
| Rich
|
| |> In sci.electronics.basics John Larkin
|> | On 1 Jan 2004 16:57:29 GMT, [email protected] wrote:
|> |
|> |>Having noted that utility distribution 3 phase transformers have a core
|> |>that is really one big core for all three (though spatially separated a
|> |>bit), I have been wondering what would happen in the magnetic field if
|> |>one were to take a toroid core, put the three phase primaries on it at
|> |>120 degree positions covering only 1/3 of the toroid each, and then have
|> |>a "rolling secondary" that slides around the toroid (mechanical support
|> |>for the mass would then be done with wheels holding up the 2/3 not being
|> |>used for the secondary).
|> |
|> | It would blow up instantly.
|>
|> OK, take the rolling secondary back off. Just have 3 primaries covering
|> 1/6 of the toroid each, and 3 secondaries covering the remaining gaps
|> between those secondaries.
|>
|> --
|> --------------------------------------------------------------------------
| ---
|> | Phil Howard KA9WGN | http://linuxhomepage.com/
| http://ham.org/ |
|> | (first name) at ipal.net | http://phil.ipal.org/
| http://ka9wgn.ham.org/ |
|> --------------------------------------------------------------------------
| ---
|
|

 

[Rob Paisley]

You can also reverse the rotation by swapping two of the phases. If one
phase is blown you can start it by giving it a spin in the right
direction, 'without' getting your hand caught

Bill Baka

I am not sure if a 3 phase motor would even rotate unloaded on 2
phases but it certainly would not develop a usable amount of
horsepower. I would suspect that running on 1 phase would be out of
the question.

I do know that if a loaded 3 phase motor blows a fuse it will almost
always blow a fuse in one of the other lines due to an overload. Due
to the way the control power for the motor is connected the starter
will then open the circuit if it did not already do so when the first
fuse failed.

Also, there is a number of 1.57 being bandied about in this thread
for the relative horse power of a 3 phase versus single motor for a
given current. This number should be 1.73 (The square root of 3.

Rob.

 

[John Woodgate]

I read in sci.engr.electrical.compliance that [email protected]
wrote (in <[email protected]>) about 'why 3-phase power?',

Certainly it is different in that they are not in "magentic series". Is that
what is relevant?

The inductions in B and C add up to minus(the induction in A) so there
is induction in the core.

 

[BFoelsch]

How would you hook up a 3-phase motor to run on 2 phases?

How would you hook up a 3 phase motor to run on one phase?

I don't care whether they work or not, I just want to see the wiring
diagram.

Thanks in advance.

 

[Phideaux]

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

Alan, from an industrial maintenance and engineering perspective, 3
phase motors can be reversed quite easily by switching any 2 of the 3
leads. This elliminates double stocking of single phase motors due to
them being directionally oriented in their rotation. Three phase
equipment is also less "noisy", electrically speaking, due to them not
having the zero voltage cross-over inherent in a single phase supply.

Happy New Year

 

[Spehro Pefhany]

How would you hook up a 3-phase motor to run on 2 phases?

A 3-phase motor has 3 wires. If you break one wire, it's running on
single phase, not two. Break two wires and it tends to just sit there.

How would you hook up a 3 phase motor to run on one phase?

I don't care whether they work or not, I just want to see the wiring
diagram.

Single phase applied to two of the three wires. It works better with
capacitors to the third (otherwise unconnected) wire. The motor won't
start like this, you either need to give it a spin or use another
(starting) capacitor and some switching.


Best regards,
Spehro Pefhany

 

[BFoelsch]

Aww, you sprung my trap!

The previous poster was expostulating on running the motor on 3, 2 and 1
phase. I was waiting for him to show me the difference between the 2 phase
and single phase connections!

Same basic concept as phase rotation in a 3 phase system. You and I think
there are only two, but there are really many; ABC, BAC, CBA, ACB etc.etc.

(Facetious mode turned OFF)

I spent many years in an old manufacturing city in the Northeast, where we
had, in common use on a daily basis, AC (25 and 60 Hz), DC, every possible
permutation of 3 phase, 2 phase 3,4 and 5 wire, and everything in between.
My absolute favorite was the 6 phase double delta. Picture a straight 220
(YES, 220, not 240) delta connection. Now, center tap all the transformers,
and these midpoints give you another delta at 110 for the lighting load. Of
course, the whole thing is ungrounded. Yes, all the light switches in the
building were 2 pole. Yes, the small motors were 110 volt 3 phase. This
system gives you 2 three-phase voltages out of one set of transformers, The
power factor transforms at unity, unlike many connections. Only thing is, it
is very had to protect the transformers unless you use 6 primary fuses.

Years ago, maintenance electricians had to be aware of a lot of varied
systems. Today, life is a whole lot simpler.

Thanks in advance.

A 3-phase motor has 3 wires. If you break one wire, it's running on
single phase, not two. Break two wires and it tends to just sit there.


Single phase applied to two of the three wires. It works better with
capacitors to the third (otherwise unconnected) wire. The motor won't
start like this, you either need to give it a spin or use another
(starting) capacitor and some switching.


Best regards,
Spehro Pefhany

http://www.speff.com

 

[Mark Zenier]

What I was trying to come up with was something that lets you adjust
the phase with a big knob much like a variac is used to adjust voltage.

Goniometer. A transformer with two (in quadrature) primary windings and a
rotating core. (I see no reason why a 3 phase version isn't possible, too).

Synchros and Selsyns, too.

Mark Zenier [email protected] Washington State resident

 

[BFoelsch]

Oops, should have said that this connection needed 6 SECONDARY fuses.

Sorry about that.

 

[BFoelsch]

The most straightforward answer is that 3 phases provides the best
compromise between the benefits of polyphase transmission and the simplicity
of single phase transmission.

2 phase systems were indeed built in 3, 4 and 5 wire versions. The three
wire version uses the fewest wires, but it is not symmetrical, one of the
conductors must be larger. The 4 and 5 wire systems permit use of identical
wire sizes, but need 4 or 5 wires.

The 3 phase system needs three wires which may be equally sized, which is an
obvious benefit over any 2 phase system. Systems with more than three phases
may be built, but for the simple transmission of power they have no
advantage over three phase, and the disadvantage of needing more wires.

Single phase systems present difficulties as far as motors are concerned.
Single phase motors are not inherently self-starting, they all require some
additional mechanism which has costs either in complexity or efficiency.

There are additional issues, but the big benefit is in going from single
phase to polyphase. All the polyphase systems also provide the benefit of
requiring less copper per kW transmitted.

Why do we not use 3 phase in US residences? Because for the small electrical
loads served, the cost and complexity of having the extra wire outweighs the
benefits.

 

[John Woodgate]

I read in sci.engr.electrical.compliance that BFoelsch

Why do we not use 3 phase in US residences? Because for the small electrical
loads served, the cost and complexity of having the extra wire outweighs the
benefits.

The US system is a carefully-considered enginnering compromise.

The customer requirements are for the delivery of about 10 kW and a
voltage that is as non-hazardous as reasonably possible.

The solution, of a 120-0-120 system, appears to me to be quite clever.

In Europe, we have 3-phase 230 V supplies (if necessary) to residential
customers on the Continent, but in UK, residential loads normally have a
single-phase 230 V supply. This is not as safe as a 120 V supply, but is
more efficient.

 

[John Larkin]

My absolute favorite was the 6 phase double delta. Picture a straight 220
(YES, 220, not 240) delta connection. Now, center tap all the transformers,
and these midpoints give you another delta at 110 for the lighting load. Of
course, the whole thing is ungrounded. Yes, all the light switches in the
building were 2 pole. Yes, the small motors were 110 volt 3 phase. This
system gives you 2 three-phase voltages out of one set of transformers, The
power factor transforms at unity, unlike many connections. Only thing is, it
is very had to protect the transformers unless you use 6 primary fuses.

Wow. If it was delta, then 110 3-phase would require 6 wires (or five
if you cheat and share one corner) so the 110 3-phase motor would need
three isolated windings and most likely a 6-pole on/off switch. Was it
really like that? Might it have been wired wye with one central
common/neutral? That seems more logical, but then maybe it wasn't very
logical.

Years ago, maintenance electricians had to be aware of a lot of varied
systems. Today, life is a whole lot simpler.

The thing that always amazes me is that buildings are wired in the
most bizarre ways, and no documentation is left behind. If you call in
an electrician, they figure it out somehow, kluge it some more, and
leave.

John