why 3-phase power?

[BFoelsch]

"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote in message
news:xij5OLAs1w9$EwJS@jmwa.demon.co.uk...

I read in sci.engr.electrical.compliance that BFoelsch
BFoelsch@snet.ditch.this.net> wrote (in <-qGdnRVNMqMRmmqiRVn-
vg@giganews.com&gt about 'why 3-phase power?', on Sat, 3 Jan 2004:

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.

Yes indeed.

Many of its critics fail to understand that the "center-tapped" system we
use in the US was developed in the early 1900s as a way to provide the
benefits of 230 volts to residences which had already been wired for 115
while maintaining perfect backward compatibility. The lower voltage to
ground may or may not be relevant as far as safety goes.

 

[BFoelsch]

"John Larkin" <jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote in
message news:1s3evv0l7eu2493tlr0r78ai24iln433ep@4ax.com...

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?

Nope. Draw a picture of a Delta, center-tap each winding, and the three
center taps form another delta at half the voltage. Used a perfectly
standard (for 1922) 3 phase, 3 wire, 110v, 25Hz motor.

The 110 volt service made no connection to the corners of the 220 delta. The
110 delta was fed only from the center taps.

 

[John Larkin]

On Sat, 3 Jan 2004 14:10:54 -0500, "BFoelsch"
<BFoelsch@snet.ditch.this.net> wrote:

"John Larkin" <jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote in
message news:1s3evv0l7eu2493tlr0r78ai24iln433ep@4ax.com...
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?

Nope. Draw a picture of a Delta, center-tap each winding, and the three
center taps form another delta at half the voltage. Used a perfectly
standard (for 1922) 3 phase, 3 wire, 110v, 25Hz motor.

Oh, yeah, OK. New Orleans is mostly below sea level, and the
Mississippi runs along most of it and peaks way above in the spring.
Up to fairly recently, New Orleans was drained by ancient pumps with
ancient switchgear, running on 25 Hz, and all the generators that made
the 25 Hz were (of course) in the city, below sea level too.

John

 

[BFoelsch]

Clever, clever, delta, New Orleans.........................

And who said there's no wit here!

Yeah, 25 Hz has kind of pooped out recently. It is still generated on the
Canadian side of Niagara Falls, but it is all converted to 60.

Across the river in Buffalo, the utility still provides 25 Hz to about 50
customers, but they have announced its termination as of 2007. There are a
few isolated paper mills that make their own, I believe that Con Edison
makes a little, and there is one isolated pocket in Iowa, but that's all I
know of that's left.

I remember when 60, 50 and 25 cycle motors were all stocked items!

Thanks in advance.

"John Larkin" <jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote in
message news:ke7evvo4mvalsj6km5ja2u4di666ge0g49@4ax.com...

On Sat, 3 Jan 2004 14:10:54 -0500, "BFoelsch"
BFoelsch@snet.ditch.this.net> wrote:


"John Larkin" <jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote in
message news:1s3evv0l7eu2493tlr0r78ai24iln433ep@4ax.com...
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?

Nope. Draw a picture of a Delta, center-tap each winding, and the three
center taps form another delta at half the voltage. Used a perfectly
standard (for 1922) 3 phase, 3 wire, 110v, 25Hz motor.


Oh, yeah, OK. New Orleans is mostly below sea level, and the
Mississippi runs along most of it and peaks way above in the spring.
Up to fairly recently, New Orleans was drained by ancient pumps with
ancient switchgear, running on 25 Hz, and all the generators that made
the 25 Hz were (of course) in the city, below sea level too.

John

 

[John Woodgate]

I read in sci.electronics.design that John Larkin <jjlarkin@highlandSNIP
techTHISnologyPLEASE.com> wrote (in <ke7evvo4mvalsj6km5ja2u4di666ge0g49@
4ax.com&gt about 'why 3-phase power?', on Sat, 3 Jan 2004:

Up to fairly recently, New Orleans was drained by ancient pumps with
ancient switchgear, running on 25 Hz, and all the generators that made
the 25 Hz were (of course) in the city, below sea level too.

Naturally: 25 Hz is an unusually LOW frequency.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

 

[Bill]

Rob Paisley wrote:

Bill <bb@syix.com> wrote in message news:<1073076554.884290@jaguar.syix.com>...

phil-news-nospam@ipal.net wrote:


In sci.electronics.basics Rob Paisley <rpaisley4@cogeco.ca> wrote:

| 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

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.
The motor I replaced was on an air conditioning blower and would rotate

if given a boost before turning on the power, but as you said not with
enough power. It just would not start without the third phase. The fix
was to replace a 3/4 horse motor with a 1 horse motor in the same NEMA
frame size. It was a 480 volt unit so we did not play much with it and
just tossed it. Seems us engineer types prefer 5 volt stuff.
Bill Baka

 

In sci.electronics.basics Spehro Pefhany <speffSNIP@interlogdotyou.knowwhat> wrote:
| On Sat, 3 Jan 2004 09:30:19 -0500, the renowned "BFoelsch"
| <BFoelsch@snet.ditch.this.net> wrote:
|
|>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.

2 phase would not be the result of a failure of power source, but it could
happen if the internal windings/circuit on one of the phases opened up.
I've never seen that happen but I can't say it's impossible.

--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
-----------------------------------------------------------------------------

 

In sci.electronics.basics BFoelsch <BFoelsch@snet.ditch.this.net> wrote:

| 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.

I do remember in the 1960's seeing a catalog of window mount air conditioners
that included a 3 phase power option on a few larger models. And these were
home style units. My grandfather did have 3 phase power to his home for the
purpose of feeding motors in his wood shop (but at the time I never found out
which of the machines actually used it; several had the rotating/locking plug
style, but NEMA defines them for both 1 phase and 3 phase services).

I ran across a web page a couple weeks ago for an electric power coop that
listed the categories of service available. For residence, they would provide
3 phase "where available" (which I interpret to mean that if there are just 1
phase wires running by, it's not available unless you want to spring for the
costs of bringing new primary service in) if there was a need (and they cited
motors over 5 HP as an example).

--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
-----------------------------------------------------------------------------

 

In sci.electronics.basics John Woodgate <jmw@jmwa.demon.contraspam.yuk> wrote:
| I read in sci.engr.electrical.compliance that BFoelsch
| <BFoelsch@snet.ditch.this.net> wrote (in <-qGdnRVNMqMRmmqiRVn-
| vg@giganews.com&gt about 'why 3-phase power?', on Sat, 3 Jan 2004:
|
|>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.

Is that 230 V hot-to-neutral with 400 V hot-to-hot?

Personally, I'd rather have a higher voltage, up to not more than 600 volts.
I guess it just depends on your perspective. But clearly many applicances
will have voltage issues with respect to safety. Some of the internal wiring
in cheap power supplies and some wall warts clearly look to me like 120 volts
is too much for them. Maybe the US needs 3 phase 120Y/69 for them

--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
-----------------------------------------------------------------------------

 

[John Woodgate]

I read in sci.engr.electrical.compliance that phil-news-nospam@ipal.net
wrote (in <bt7q0j21oja@enews4.newsguy.com&gt about 'why 3-phase power?',
on Sun, 4 Jan 2004:

Is that 230 V hot-to-neutral with 400 V hot-to-hot?

Yes.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

 

[Jonathan Barnes]

<phil-news-nospam@ipal.net> wrote in message

Is that 230 V hot-to-neutral with 400 V hot-to-hot?

Yes

Personally, I'd rather have a higher voltage, up to not more than 600
volts.
I guess it just depends on your perspective. But clearly many applicances
will have voltage issues with respect to safety. Some of the internal
wiring
in cheap power supplies and some wall warts clearly look to me like 120
volts
is too much for them. Maybe the US needs 3 phase 120Y/69 for them

You have it the wrong way round, use 1200 Y / 690 and let Darwin sort the
problem out.

--
| Phil Howard

--
Jonathan

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

To reply remove AT

 

[Rob Paisley]

ggober@alltel.net (Phideaux) wrote in message news:<fd732a29.0401030655.54992bda@posting.google.com>...

alanh_27@yahoo.com (Alan Horowitz) wrote in message news:<1e3670a7.0312300921.599776f8@posting.google.com>...
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

Single phase motors can rotate in either directions simply by
reversing one of the windings at the connection terminals, usually the
starting winding. - No double stock is required.

Which direction it runs depends on which way the armature is kicked
when power is applied. When the motor reaches a certain speed the
starting winding cuts out.

Rob.

 

In sci.electronics.basics John Woodgate <jmw@jmwa.demon.contraspam.yuk> wrote:
| I read in sci.engr.electrical.compliance that phil-news-nospam@ipal.net
| wrote (in <bt7q0j21oja@enews4.newsguy.com&gt about 'why 3-phase power?',
| on Sun, 4 Jan 2004:
|>Is that 230 V hot-to-neutral with 400 V hot-to-hot?
|
| Yes.

So getting 3 phase mains power there is not as troubling as it is in North
America. The basic choices we have are either 120 V hot-to-neutral with
208 V hot-to-hot, and 277 V hot-to-neutral with 480 V hot-to-hot (in Canada
this would be 347 V hot-to-neutral with 600 V hot-to-hot, and I've seen this
available in Texas, too). Normally we have in single phase 120/240 V center
tapped neutral so larger appliances use 240 volts. But with 3 phase, they
get hooked up to the 208 volt connection, which tends to be a bit low and
I have seen motors burn out early because of this. I suspect you don't have
much need for appliances going above the 230 volt level. But it seems they
have a more interesting compromise in at least some parts of Mexico: 127 V
hot-to-neutral with 220 V hot-to-hot.

The usual designations for these voltage configurations are:
120/240, 208Y/120, 220Y/127, 480Y/277, 600Y/347
with sometimes the 3 phase values written the other way and sometimes the
347 volt value written as 346. Just about every combination of voltage to
voltage conversion is available from transformer manufacturers, surplus
resellers (Google for voltage designations), and on EBay.

--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
-----------------------------------------------------------------------------

 

In sci.electronics.basics Jonathan Barnes <jbarnes@atnetcomuk.co.uk> wrote:
|
| <phil-news-nospam@ipal.net> wrote in message
|
|> Is that 230 V hot-to-neutral with 400 V hot-to-hot?
|
| Yes
|>
|> Personally, I'd rather have a higher voltage, up to not more than 600
| volts.
|> I guess it just depends on your perspective. But clearly many applicances
|> will have voltage issues with respect to safety. Some of the internal
| wiring
|> in cheap power supplies and some wall warts clearly look to me like 120
| volts
|> is too much for them. Maybe the US needs 3 phase 120Y/69 for them
|
| You have it the wrong way round, use 1200 Y / 690 and let Darwin sort the
| problem out.

Actually, the real voltage levels are more like 4160Y/2400 and 12470Y/7200.
For Darwin in action, see: http://www.cdc.gov/niosh/injury/traumaelface.html

--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
-----------------------------------------------------------------------------

 

[John Woodgate]

I read in sci.engr.electrical.compliance that phil-news-nospam@ipal.net
wrote (in <btau25018fp@enews4.newsguy.com&gt about 'why 3-phase power?',
on Mon, 5 Jan 2004:

In sci.electronics.basics John Woodgate <jmw@jmwa.demon.contraspam.yuk> wrote:
| I read in sci.engr.electrical.compliance that phil-news-nospam@ipal.net
| wrote (in <bt7q0j21oja@enews4.newsguy.com&gt about 'why 3-phase power?',
| on Sun, 4 Jan 2004:
|>Is that 230 V hot-to-neutral with 400 V hot-to-hot?
|
| Yes.

So getting 3 phase mains power there is not as troubling as it is in North
America. The basic choices we have are either 120 V hot-to-neutral with
208 V hot-to-hot, and 277 V hot-to-neutral with 480 V hot-to-hot (in Canada
this would be 347 V hot-to-neutral with 600 V hot-to-hot, and I've seen this
available in Texas, too).

Those voltages mean that equipment needs more insulation and less
copper. I wonder whether that works out cheaper. It's certainly less
safe.

Normally we have in single phase 120/240 V center
tapped neutral so larger appliances use 240 volts. But with 3 phase, they
get hooked up to the 208 volt connection, which tends to be a bit low and
I have seen motors burn out early because of this.

No doubt. You could have three phases of 120-0-120, giving 480 V between
phases. The problem with that is where to ground the system! With a star
secondary and the star point grounded, you would get 0-120-240, not
120-0-120.

I suspect you don't have
much need for appliances going above the 230 volt level.

Not in UK, because our network impedance is generally quite low. We can
use instant-heat water heaters up to 7 or even 10 kW in most places. It
is possible to have a 3-phase supply, although the extra cost of
providing it is not usually carried by the supplier. On the Continent,
3-phase is now normally supplied to larger houses.

But it seems they
have a more interesting compromise in at least some parts of Mexico: 127 V
hot-to-neutral with 220 V hot-to-hot.

127 +/-20%? (;-)
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

 

[Phideaux]

rpaisley4@cogeco.ca (Rob Paisley) wrote in message news:<bb39a577.0401040927.78fd8647@posting.google.com>...

ggober@alltel.net (Phideaux) wrote in message news:<fd732a29.0401030655.54992bda@posting.google.com>...
alanh_27@yahoo.com (Alan Horowitz) wrote in message news:<1e3670a7.0312300921.599776f8@posting.google.com>...
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


Single phase motors can rotate in either directions simply by
reversing one of the windings at the connection terminals, usually the
starting winding. - No double stock is required.

Which direction it runs depends on which way the armature is kicked
when power is applied. When the motor reaches a certain speed the
starting winding cuts out.

Rob.

You are correct, of course. However, in a plant maintenance
environment, you do not wish to be out in bad weather, tearing down
connection heads and reversing winding leads. I'd much rather do that
in a climate controled motor control center where the starters are.
Starting 15,000 hp motors "in the shop" to check the initial rotation
would not be a practical matter, in my opinion. Vacuum contactors are
not cheep, and to have one, and the feed to it, just so this could be
done before installation just doesn't seem to be the right way to go.
I guess the old addage "Things are the way they are, for good reason"
seems to apply to the use of 3 phase motors in higher horsepower
applications.

One other comment that should be made concerns the use of variable
frequency drives on these motors. The power loss (in torque) that
occurs when reducing the line frequency is much more pronounced on
single phase motors than on 3 phase motors due to the singles not
having any phase overlap. This makes them unsuitable in many variable
speed applications.

 

[Steve Nosko]

Why is three phase used?
Several people mentined the reason (motors) , but didn't go further. To
get a motor to turn you have to generate a magnetic field that "turns".
With three phase this can be done. The relative voltage/cirrent between two
phases (lines) can each drive a winding and the result is a rotating
magnetic field inside the motor. This will then "drag" the rotor around.
With the single phase motor, this rotation must be made some other way.
With the capacitor start/run motors, the cap provides some phase shift on a
second winding giving a pseudo two phase supply and therefore some rotation
in the field. The cap-start motors do then run on one phase when the cap is
switched out. Thus the comments on "running a three phase motor on 2 phases
(actually this is just one phase when one of the three lines is cut -- the
"three phases" needed for the motor are not the three lines, but the phase
between each TWO line-pairs...of which there are three pairs) .
Shaded pole motors use a part of the field core which is separated from
the main core.. You will see a single turn of large copper wire wound
around a small section of the core. This "shorted turn" causes a lag in the
field in that section of the core and a resulting rotation of the composite
field. These are usually in small blowers like typically found in bathroom
fans. Go to the hardweare store and look at one of these motors.

Steve K;9;D:C:I
..

 

[Smoke]

"Jonathan Barnes" <jbarnes@ATnetcomuk.co.uk> wrote in message
news:bsss8k$56l$1@taliesin2.netcom.net.uk...

"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> wrote in message

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


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

Perhaps so, but "2 phase" is wrong.

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.

I am a sucker.

Direct and Alternating Currents by Oppenheimer and Borchers says " Any
circuit with a single source of a-c is simply referred to as single-phase
a-c"

If you look on the at the transformer on the pole and look at the primary
connection there would most likely be a single source of a-c feeding the
transformer.hence single-phase. Transformers fed by a single source of a-c
do not create polyphase voltages.

Example 2: if one connects a train transformer to 110 a-c and the
transformer has many voltage taps you still have single phase voltages
because the transformer is fed by a single source of AC.

Rich.

 

[Steve Nosko]

"Smoke" <rsmokey@newsguy.com> wrote in message
news:btdfq501pv5@enews2.newsguy.com...

"Jonathan Barnes" <jbarnes@ATnetcomuk.co.uk> wrote in message
news:bsss8k$56l$1@taliesin2.netcom.net.uk...

"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> wrote in message
And in most places, "plain ole hot & neutral" comes in as 2
phase,
.....
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.

I am a sucker.
Direct and Alternating Currents by Oppenheimer and Borchers says .....
.........
Example 2: if one connects a train transformer to 110 a-c and the
transformer has many voltage taps you still have single phase voltages
because the transformer is fed by a single source of AC.
Rich.

This could be called nit picking, but if I have two which are 180 degrees
out of phase it does look like two phases, no? Any center tapped
transformer can do this and I think it is valid to call it two phase.
Connect across the two hots & you get twice the voltage, of course. Now
connect across two of the lines in a 3-phase system. You still get some
voltage ( don't know the numbers) , no? Why should 180 degrees not be
called two phases while 120 degrees is?
I agree that this can be called a "single phase system", but I still see
two phases for this special case. Symantics, perhaps as long as we all know
what is really happening, no?

Steve, K;9;D;C;I

 

[Smoke]

This could be called nit picking, but if I have two which are 180
degrees
out of phase it does look like two phases, no? Any center tapped
transformer can do this and I think it is valid to call it two phase.
Connect across the two hots & you get twice the voltage, of course. Now
connect across two of the lines in a 3-phase system. You still get some
voltage ( don't know the numbers) , no? Why should 180 degrees not be
called two phases while 120 degrees is?
I agree that this can be called a "single phase system", but I still see
two phases for this special case. Symantics, perhaps as long as we all
know
what is really happening, no?

Hi Steve,

It may be nit picking, but how would you describe a polyphase-3-wire-system,
one wire grounded, where voltages do not algebraically add?. (such as
supplied by two synchronous but out of phase generators sharing a common
neutral)


> Steve, K;9;D;C;I