240V from a 3 phase main ?

Roy Smith wrote:
> In article <[email protected]>,
> [email protected] (e) wrote:
>> A note on DC- As of teo years ago, New York transit was still running
>> DC rotarary converters that are about 100 years old-AC in, DC out,
>> but not quite a motor generator like a welder. I believe the last
>> went out of service recently, but I may be wrong. The hookups use
>> have been thyristor converters (all electronic) since the 50's.
>
> If anybody's interested in this stuff, take a look at
>
> http://www.nycsubway.org/tech/power/rotary.html

Also, "Networks of Power"

http://www.amazon.com/exec/obidos/tg/detail/-/0801846145/102-8284350-1739318?v=glance

"If you're a history buff, and appreciate the technology that surrounds us
all, you'll love reading "Networks of Power: Electrification in Western
Society, 1880-1930" by Tom Hughes. Hughes takes us back to the days of
fierce rivalry between Edison and Westinghouse; the early era of electric
power generation and consumption where the battle of DC vs. AC consumer
power was born and decided.

Hughes doesn't stop there. Also included in this well-footnoted edition are
in-depth narratives of the evolution of commercial power systems in England
and Germany through 1930. A well written, readable snapshot in time.

Compelling historical reading for the non-technologist as well as the
student of electrical power commercialization."

Don Murray wrote:
> PJ wrote:
> There is also 4 phase power.
>
>
> I don't know about 4-phase power, but I know there is 4 wire primary.
> 3 hots and a primary neutral.

At the transformer, as per the diagram in that excellent link. When you do
the actual service lateral, you pull three primary cables and each one has
an aluminum (usually) center conductor and a copper concentric grounded
conductor. The grounded conductors are joined at the transformer and also
bonded to the transformer case and ground rods.

Everett M. Greene wrote:
> [email protected] (e) writes:
>> [email protected] (Everett M. Greene) wrote
>>> George <[email protected]> writes:
>>>> "AL" <[email protected]> wrote:
>>>>
>>>>> I'm looking to rent industrial space for hobby use and have
>>>>> looked at a few buildings. I was pleasantly surprised to find
>>>>> that 3 phase power was very common. But several of my tools only
>>>>> run on 240V, and not 208V.
>>>
>>> What kind of "240V" motor won't run on 208V?
>>
>> It isn't that the motor won't run, but that a) the efficiency and
>> life of the motor will likely be reduced because b) the current draw
>> will go up at the lower voltage.
>>
>> The speed of induction and synchronous motors (which most 240V tool
>> motors will be, single or three phase) is determined primarily by the
>> line frequency. The power needed by the motor is determined by the
>> load-the motor doesn't care how much current it draws... it draws
>> what it need to to meet the power demand of the load at the run
>> speed. At the lower voltage (about 12%) the current will be higher
>> (again, about 12%), leading to greater heat production in the motor
>> and greater I^2R losses in the motor and supply wiring. If the load
>> is near the 240V rating of the motor(compressor motor, large power
>> tool, etc) then at 208V, the motor will like lose some or all of
>> it's magic smoke and cease to function, especially if that 208V
>> supply is really only 200V (5% either way is very common with system
>> load variation, 10% not unusual, especially in the summer when lots
>> of AC units are on).
>>
>> Motors are generally not conservatively rated-it isn't economical to
>> overrate. You get the 10% or so maximum supply variation built in,
>> and that's it. Go below that, and you need to begin derating the
>> motor rapidly, go above that, and the likelyhood of insulation
>> failure goes way up.
>
> I agree that power = I * V * cos(pf), so the current will be greater
> for a lower voltage at a constant power draw. I also agree that a
> fixed load device such as an air compressor could have trouble with
> a lower supply voltage, but variable load devices will simply have
> less power available -- if your table saw is a little weaker due to
> the voltage being somewhat low, just don't push the work through so
> fast.
>
I think that would be hard for the user to judge. You would need a motor
starter with properly sized heaters to ensure you're not drawing too much
current.

"Roy Smith" <[email protected]> wrote in message
news:[email protected]...
> In article <[email protected]>,
> >
> For what it's worth, I haven't studied this stuff since college
> 20-mumble years ago. I found the diagrams perfectly readable and the
> explanation quite clear.

Damn! I am just a lowly HVAC tech, and even I understood everythiing he
posted too!
(Must not have my mind muddied up with all the ed-gication stuff!)
Greg

"Doug Miller" <[email protected]> wrote in message news:7FxLb.23545$P%> >
> >In all probability the service is from a center tapped delta
> >transformer. Put a voltmeter on each leg and you should read
> >120/120/208.
>
> Wrong.
>
> >Run your 2 phase stuff across the two 120 legs and you
> >will get the 240 that you want.
>
> NO YOU WON'T.
>
> In 208V 3-phase service, the phase-to-phase voltage is 208V, and the
> phase-to-neutral voltage is 120V.
>
> >If it's a wye transformer then all three legs will give you the same
> >reading. However, that's unlikely unless you are looking at strictly
> >warehouse space.
>
> Delta or wye, it doesn't matter. You won't get 240V anywhere in a 208V
3-phase
> service.
>
> --
> Doug Miller (alphageek at milmac dot com)
>
>

I don't know about everywhere else, but I rented a shop that had 208-3ph. It
was just as Doug described.
Phase to phase = 208 volt, any phase to neutral was 120 volt.
Greg

What newsreader are you using? Maybe I need to change readers. They were
scrambled on this end.

Bob

"Roy Smith" <[email protected]> wrote in message
news:[email protected]...
> In article <[email protected]>,
> "Bob Davis" <[email protected]> wrote:
>
> > I appreciate the tedious effort you attempted to draw and explain
> > everything. Ascii text drawings are virtually useless in this medium
> > because you never know if the reader will see it in fixed width or
> > proportional fonts. The only way to consistently do ascii text drawings
is
> > to use fixed width fonts, space, and no tabs. Even then, word wrapping
my
> > get you anyway.
> >
> > Bottom line is that I tried to follow everything you said and it was
just
> > intelligible (not your fault). I'm an electrical engineer with extra
> > coursework in power and probably have a better chance than average in
trying
> > to follow what you were saying, but couldn't.
>
> For what it's worth, I haven't studied this stuff since college
> 20-mumble years ago. I found the diagrams perfectly readable and the
> explanation quite clear.

George <[email protected]> writes:
> "AL" <[email protected]> wrote:
>
> >I'm looking to rent industrial space for hobby use and have looked at a few
> >buildings. I was pleasantly surprised to find that 3 phase power was very
> >common. But several of my tools only run on 240V, and not 208V.

What kind of "240V" motor won't run on 208V?

> Is it
> >possible to get 240V when a building is wired for 3 phase power? Does the
> >power company typically bother to provide a split phase main (like in a
> >house) when they already provide 3 phase?
>
> In all probability the service is from a center-tapped delta
> transformer.

Not very likely. What center is there to tap?

> Put a voltmeter on each leg and you should read 120/120/208.

On a delta-connected transformer, you'll get random
values WRT ground. Line-to-line you'll get the same
reading on all three pairs.

> Run your 2 phase stuff across the two 120 legs and you
> will get the 240 that you want.
>
> If it's a wye transformer then all three legs will give you the same
> reading. However, that's unlikely unless you are looking at strictly
> warehouse space.

You are not likely to see anything but Y-connected trans-
formers at the load end of the line. The supply end may
well be delta-connected.

[email protected] (e) writes:
> [email protected] (Everett M. Greene) wrote
> > George <[email protected]> writes:
> > > "AL" <[email protected]> wrote:
> > >
> > > >I'm looking to rent industrial space for hobby use and have looked at a few
> > > >buildings. I was pleasantly surprised to find that 3 phase power was very
> > > >common. But several of my tools only run on 240V, and not 208V.
> >
> > What kind of "240V" motor won't run on 208V?
>
> It isn't that the motor won't run, but that a) the efficiency and life
> of the motor will likely be reduced because b) the current draw will
> go up at the lower voltage.
>
> The speed of induction and synchronous motors (which most 240V tool
> motors will be, single or three phase) is determined primarily by the
> line frequency. The power needed by the motor is determined by the
> load-the motor doesn't care how much current it draws... it draws what
> it need to to meet the power demand of the load at the run speed. At
> the lower voltage (about 12%) the current will be higher (again, about
> 12%), leading to greater heat production in the motor and greater I^2R
> losses in the motor and supply wiring. If the load is near the 240V
> rating of the motor(compressor motor, large power tool, etc) then at
> 208V, the motor will like lose some or all of it's magic smoke and
> cease to function, especially if that 208V supply is really only 200V
> (5% either way is very common with system load variation, 10% not
> unusual, especially in the summer when lots of AC units are on).
>
> Motors are generally not conservatively rated-it isn't economical to
> overrate. You get the 10% or so maximum supply variation built in, and
> that's it. Go below that, and you need to begin derating the motor
> rapidly, go above that, and the likelyhood of insulation failure goes
> way up.

I agree that power = I * V * cos(pf), so the current will be greater
for a lower voltage at a constant power draw. I also agree that a
fixed load device such as an air compressor could have trouble with
a lower supply voltage, but variable load devices will simply have
less power available -- if your table saw is a little weaker due to
the voltage being somewhat low, just don't push the work through so
fast.

Although motors are sized to meet their (presumed) load, motors
are built in discrete sizes -- 1/2, 3/4, 1, 2, 5,...HP (or the
equivalent in watts). If a load is 7/16 HP, the manufacturer is
going to use a 1/2 HP motor which is then going to have >10%
reserve. Motors are also designed to work in a particular voltage
/class/, so they'll produce their rated power output at the minimum
supply voltage without damage (or at least without catching fire).
If the voltage is higher than the minimum, a motor can produce
more than its rated output without damage. It would be a violation
of various standards (UL, etc.) to manufacture a device that relies
on a more than minimum supply voltage to safely produce the necessary
power.

Industrial installations are another matter, but this discussion
started as question about a "home" woodworking shop.

RB <[email protected]> writes:
> The thing to remember here is that with induction motors (as opposed to
> smaller permanent magnet rotor motors) the torque is proportional to the
> cross product of the stator and rotor currents. To a first
> approximation, if the supply voltage is 86.6% (208/240) then the torque
> will be 75% of that at rated voltage.

Your math escapes me. If the supply voltage is lower, the
current will be higher for the same power output, thus the
torque would be higher, not lower, by your statement.

RB <[email protected]> writes:
> Everett M. Greene wrote:
> > RB <[email protected]> writes:
> >
> >>The thing to remember here is that with induction motors (as opposed to
> >>smaller permanent magnet rotor motors) the torque is proportional to the
> >>cross product of the stator and rotor currents. To a first
> >>approximation, if the supply voltage is 86.6% (208/240) then the torque
> >>will be 75% of that at rated voltage.
> >
> > Your math escapes me. If the supply voltage is lower, the
> > current will be higher for the same power output, thus the
> > torque would be higher, not lower, by your statement.

> It doesn't work that way. The impedance of the motor's winding doesn't
> change just because you have a lower voltage. To get the same torque
> you need a higher current but you won't get it.
>
> If as the voltage is lowered the current rises then when the voltage is
> zero (as in a short across the input to the motor) the current will be
> how much?
>
> If the voltage is 86% the torque developed will be 0.86 x 0.86 or 75% of
> that at rated voltage (240 volts.)

torque != power

We're talking about two different things

Gary Coffman <[email protected]> writes:
> [snip]
> The fly in this ointment is that winding *resistance* doesn't change.
> Energy dissipated in the windings is a function of the square of current
> and the winding resistance (P=I^2 * R). So the windings heat more
> rapidly at a lower line voltage, but speed is constant, so the amount
> of cooling air remains constant. That causes winding temperature to
> rise, leading ultimately to insulation failure, and all the magic smoke
> is let out of the motor.

But, again, we're talking about shop tools, most of which are
going to be hand fed their work and whose motors are going to
be lightly loaded most of the time. If the voltage is less, the
available power will be less so you have to feed the work to
the tool a little bit more slowly. In other words, you won't
be pushing the motor load to the limit and causing the smoke
to start rising. Up to a point, you can even overload a motor
for a brief period without causing any harm.

Gary Coffman <[email protected]> writes:
> [email protected] (Everett M. Greene) wrote:
> >Gary Coffman <[email protected]> writes:
> >> [snip]
> >> The fly in this ointment is that winding *resistance* doesn't change.
> >> Energy dissipated in the windings is a function of the square of current
> >> and the winding resistance (P=I^2 * R). So the windings heat more
> >> rapidly at a lower line voltage, but speed is constant, so the amount
> >> of cooling air remains constant. That causes winding temperature to
> >> rise, leading ultimately to insulation failure, and all the magic smoke
> >> is let out of the motor.
> >
> >But, again, we're talking about shop tools, most of which are
> >going to be hand fed their work and whose motors are going to
> >be lightly loaded most of the time. If the voltage is less, the
> >available power will be less so you have to feed the work to
> >the tool a little bit more slowly. In other words, you won't
> >be pushing the motor load to the limit and causing the smoke
> >to start rising. Up to a point, you can even overload a motor
> >for a brief period without causing any harm.
>
> The available power will not be less. Remember, increasing slip
> decreases effective reactance in the motor, so at a given load,
> current will automatically increase to satisfy load demand when
> voltage decreases. Since P = I * E, power can remain the same
> when voltage is decreased. It is how electric motors work. So
> you will not have any sensible feedback telling you to slow down.

P != I * E

You're saying the utilities cannot reduce their demand load
by reducing voltage because the motor loads will just draw
increased current?

> Now for some power tools, such as a table saw, you can consciously
> and deliberately reduce the load by decreasing feed rate so that
> power demanded decreases to match decreased voltage, and then
> current will not increase. But you can't do that by noting the motor
> is bogging, it won't. You'd have to continuously monitor motor current
> to stay within the safe area. The tool itself won't give you any feedback
> telling you to slow down, until you note the smoke coming from the
> motor.
>
> For some motor operated loads, such as an air compressor, you
> have to change pulley ratios to reduce the load. While this will
> reduce running current, it may make the compressor hard to start,
> and possibly damage motor, contactors, or capacitors anyway.

I would think you'd reduce the size of the driving pulley
so reduce the motor load, thus making it easier to start.

> In short, you can run a lightly loaded motor on reduced voltage,
> but you need some way to monitor current to ensure you really
> are loading it lightly enough to keep it from overheating. And you
> have to be wary of other factors which may come into play, such
> as excessive current draws required to come up to speed on
> reduced voltage.

And, again, we're talking about voltages that are within
the motors' voltage rating class.

"Martin H. Eastburn" <[email protected]> writes:
> Everett M. Greene wrote:

> > P != I * E
>
> Power does = E * I for D.C.
> Power does = E * I * Cos(theta) for A.C. theta is the phase angle
> between E and I.
> Sometimes Cosine equals 1. :-)
> Those are the facts.

Quite true. The subject line says 240V, 3-phase, so we have
to presume AC is being discussed.

> > You're saying the utilities cannot reduce their demand load
> > by reducing voltage because the motor loads will just draw
> > increased current?
> The swinging transformers move taps raising and lowing voltage.
> These are massive horz. transformers in substations.
>
> They often drop voltages to shed load. Many motors and compressors
> don't start. Those that do will draw more but with the drop off's
> and the resistive load loss it is a win.
>
> They often run this valley on between 68 and 93 volt,
> not the normal 120-140v.

If they do this very often, you have grounds for some loud
complaints to your state utility commission. Electric
utilities don't guarantee much of anything, but they do
have to stay within shouting distance of the nominal
voltage of the service class. 68V isn't even close.

[snip]

In article <[email protected]>,
Bob Davis <[email protected]> wrote:
>What newsreader are you using? Maybe I need to change readers. They were
>scrambled on this end.

If you're using a MS-Windows based reader, make _sure_ that you've
selected a *fixed*pitch* font ("fixedsys" is one, so is anything with
'courier' in the name).

>
>Bob
>
>"Roy Smith" <[email protected]> wrote in message
>news:[email protected]...
>> In article <[email protected]>,
>> "Bob Davis" <[email protected]> wrote:
>>
>> > I appreciate the tedious effort you attempted to draw and explain
>> > everything. Ascii text drawings are virtually useless in this medium
>> > because you never know if the reader will see it in fixed width or
>> > proportional fonts. The only way to consistently do ascii text drawings
>is
>> > to use fixed width fonts, space, and no tabs. Even then, word wrapping
>my
>> > get you anyway.
>> >
>> > Bottom line is that I tried to follow everything you said and it was
>just
>> > intelligible (not your fault). I'm an electrical engineer with extra
>> > coursework in power and probably have a better chance than average in
>trying
>> > to follow what you were saying, but couldn't.
>>
>> For what it's worth, I haven't studied this stuff since college
>> 20-mumble years ago. I found the diagrams perfectly readable and the
>> explanation quite clear.
>
>

AL wrote:
> I'm looking to rent industrial space for hobby use and have looked at a few
> buildings. I was pleasantly surprised to find that 3 phase power was very
> common. But several of my tools only run on 240V, and not 208V. Is it
> possible to get 240V when a building is wired for 3 phase power? Does the
> power company typically bother to provide a split phase main (like in a
> house) when they already provide 3 phase?
>
>

What you need is a buck/boost transformer. These are often used in buildings
with 120/208 V service to run 240 V air conditioners, etc. They are
autotransformers, and quite small for the power they provide, as they
only have to handle the difference in voltage.

Jon

NO!!!

You will have 120 volts from each leg to NEUTRAL, but from leg to leg,
you have (120 + 120 volts) x sin 120 degrees = 208 volts. There is
no such thing as "2-phase stuff". It's either single-phase, 240-volt
(not 220), or it is 208-volt 3-phase with 208 volts between adjacent
phases.

What you need is a single-phase 208-to-240-volt step-up autotransformer
with a high-enough KVA rating to carry the load you are anticipating.
The primary is 208 volts, and the secondary is 240 volts, but if it is
an autotransformer, you won't have a center-tap that connects to a neutral,
but most 240-volt welders and motors don't need a neutral; just a
non-current-carrying safety ground (green wire).

George wrote:
>
> "AL" <[email protected]> wrote:
>
> >I'm looking to rent industrial space for hobby use and have looked at a few
> >buildings. I was pleasantly surprised to find that 3 phase power was very
> >common. But several of my tools only run on 240V, and not 208V. Is it
> >possible to get 240V when a building is wired for 3 phase power? Does the
> >power company typically bother to provide a split phase main (like in a
> >house) when they already provide 3 phase?
>
> In all probability the service is from a center tapped delta
> transformer. Put a voltmeter on each leg and you should read
> 120/120/208. Run your 2 phase stuff across the two 120 legs and you
> will get the 240 that you want.
>
> If it's a wye transformer then all three legs will give you the same
> reading. However, that's unlikely unless you are looking at strictly
> warehouse space.
>
> George.

PJ wrote:
There is also 4 phase power.


I don't know about 4-phase power, but I know there is 4 wire primary. 3
hots and a primary neutral. There's also 4 wire 2-phase and 5 wire
2-phase. Here's a link that shows the connections that e was discussing,
along with the 3 wire 2-phase that he explained and the 4 and 5 wire
that I just spoke of. These pages were copied from a General Electric
Distribution Transformer Manual.

http://murrayranch.com/Electricity.htm

Don

On Fri, 09 Jan 2004 21:31:11 -0500, Roy Smith <[email protected]> wrote:

>In article <[email protected]>,
> [email protected] (e) wrote:
>> A note on DC- As of teo years ago, New York transit was still running
>> DC rotarary converters that are about 100 years old-AC in, DC out, but
>> not quite a motor generator like a welder. I believe the last went out
>> of service recently, but I may be wrong. The hookups use have been
>> thyristor converters (all electronic) since the 50's.
>
>If anybody's interested in this stuff, take a look at
>
>http://www.nycsubway.org/tech/power/rotary.html

And anyone in Southern California that wants to see a working 2400
VAC 60Hz 6-phase to 600 VDC Rotary Converter station only has to go to
the Orange Empire Railway Museum on an operating weekend.
http://www.oerm.org There's a nice little GE station there, with a
completely restored and re-wound rotary converter (ask about how that
restoration came about, it's interesting). And it's all self-starting
and self-shutdown, done with a big drum controller - just hit the
button and watch it come up.

Looks like something Rube Goldberg designed, complete with a
ball-and-worm armature shaft wiggler to keep the brushes wearing
evenly, and big live-front contactors with open arc chutes...

They now have a solid-state converter for everyday use, but they can
still fire up the rotary - for demonstrations, if they're working on
the other power plant, or if they're going to be running a lot of
rolling stock at once.

--<< Bruce >>--
--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
5737 Kanan Rd. #359, Agoura CA 91301 (818) 889-9545
Spamtrapped address: Remove the python and the invalid, and use a net.

ATP wrote:
>
> Don Murray wrote:
> > PJ wrote:
> > There is also 4 phase power.
> >
> >
> > I don't know about 4-phase power, but I know there is 4 wire primary.
> > 3 hots and a primary neutral.
>
> At the transformer, as per the diagram in that excellent link. When you do
> the actual service lateral, you pull three primary cables and each one has
> an aluminum (usually) center conductor and a copper concentric grounded
> conductor. The grounded conductors are joined at the transformer and also
> bonded to the transformer case and ground rods.


The concentric on the primary cable along with the transformer grounds
are not always a metallic return to the substation. Primary neutral and
common neutral (common to primary and secondary, run in the secondary
position) are metallic returns to the substation. You find primary
neutrals in the older 4KV where they've used 2400V transformers phase to
neutral (star or wye). We also run a common neutral system in our 21KV
that we use 12000V transformers on.

A side note to the underground cable, is they are not getting the life
expectancy out of it that they thought when they put it in. One of the
problems that we've run into is some soil conditions dissolve the copper
neutral on the older installations where it was direct buried. We've
since gone to a jacketted cable and require everything to be in conduit
now. Another problem they find is when the cable is faulting it's
through hairline cracks through the poly. We've replaced a considerable
amount of this at great expense, and have recently been working with a
company that injects silicone in the end of the strands that fills these
hairline cracks. At 5 dollars a foot, it's about half the price of cable
replacement. This year we have some 40000' coming up for cable
replacement and another 20000' that we are going to do the silicone
injection on.

Don

ATP wrote:
>
> Roy Smith wrote:
> > In article <[email protected]>,
> > [email protected] (e) wrote:
> >> A note on DC- As of teo years ago, New York transit was still running
> >> DC rotarary converters that are about 100 years old-AC in, DC out,
> >> but not quite a motor generator like a welder. I believe the last
> >> went out of service recently, but I may be wrong. The hookups use
> >> have been thyristor converters (all electronic) since the 50's.
> >
> > If anybody's interested in this stuff, take a look at
> >
> > http://www.nycsubway.org/tech/power/rotary.html
>
> Also, "Networks of Power"
>
> http://www.amazon.com/exec/obidos/tg/detail/-/0801846145/102-8284350-1739318?v=glance
>
> "If you're a history buff, and appreciate the technology that surrounds us
> all, you'll love reading "Networks of Power: Electrification in Western
> Society, 1880-1930" by Tom Hughes. Hughes takes us back to the days of
> fierce rivalry between Edison and Westinghouse; the early era of electric
> power generation and consumption where the battle of DC vs. AC consumer
> power was born and decided.
>
> Hughes doesn't stop there. Also included in this well-footnoted edition are
> in-depth narratives of the evolution of commercial power systems in England
> and Germany through 1930. A well written, readable snapshot in time.
>
> Compelling historical reading for the non-technologist as well as the
> student of electrical power commercialization."


Do they mention anything in the book, "Networks of Power" about the
Folsom Powerhouse? I was privileged enough in my career to work with a
man that operated it. He operated it from 1948 til they closed it in
1955. I worked with him in the '70's when he was Chief Dispatcher with
the company I work for. I took a tour of the powerhouse with him and I
took my video camera. I have him telling a lot of wonderful stories of
things like the collectors flying off the generators, sparks everywhere
and him diving out the window.

http://www.parks.ca.gov/default.asp?page_id=1335


Today when you talk about a network in the electrical utility industry
it usually refers to a system in the downtown area of large cities. This
is different from the transformers in the suburbs, in that, the
secondaries are all in parallel. So if you lose a transformer you don't
drop any customers, you just lose KVA. But you have to run your network
with more reserve than the largest transformer.

Don

"Allan Hessenflow" <[email protected]> wrote in message
news:[email protected]...
snip---.

For what
> it's worth, before this thread I had no idea that anyone provided
> delta power.

That's an interesting observation. I've had three phase power in three
different structures, all of them in residential areas, and it's been no
problem at all to get three phase delta. The one problem, however, is
getting them to run it in underground. I have been told by more than one
EE that there are fire problems associated with service of that nature.
Not sure I understand it, but I've learned to live with the three
transformers overhead.

Harold

ATP wrote:
>
>
> >
> You're way above my head, are you talking about upstream of the
> customer-owned cable? In the typical case, the concentric grounds are the
> only return, right?
> >
> >
I can't speak to what direction you are referring to about customer
cable. Around here, it varies so much. We have some customer owned
primary and secondary. But when I'm talking about the grounds not being
a return to the sub, say you have a 3-phase primary (just 3 wires, no
neutral) going out from the sub a couple of miles and now you install an
pad-mount transformer (or string of transformers) being fed from a riser
off of this 3-phase primary. Your grounds on all these transformers are
tied together, like you say. But there is no metallic return to the sub
on them.



>
> They inject silicone in the center strand which fills the gaps in the white
> poly insulation between the center strand and the semiconducting layer?

Yes


> That's a pretty neat trick. Is it only practical for utilities? Do they have
> to redo the potheads/elbows?


I can't say if it's practical for other than utilities, you'd have to
weigh the costs. And yes, they have to redo the potheads, and elbows.
Around here we don't use that many potheads, we use 3M termination kits.

Don

The thing to remember here is that with induction motors (as opposed to
smaller permanent magnet rotor motors) the torque is proportional to the
cross product of the stator and rotor currents. To a first
approximation, if the supply voltage is 86.6% (208/240) then the torque
will be 75% of that at rated voltage.

RB

Everett M. Greene wrote:
> [email protected] (e) writes:
>
>>[email protected] (Everett M. Greene) wrote
>>
>>>George <[email protected]> writes:
>>>
>>>>"AL" <[email protected]> wrote:
>>>>
>>>>
>>>>>I'm looking to rent industrial space for hobby use and have looked at a few
>>>>>buildings. I was pleasantly surprised to find that 3 phase power was very
>>>>>common. But several of my tools only run on 240V, and not 208V.
>>>>
>>>What kind of "240V" motor won't run on 208V?
>>
>>It isn't that the motor won't run, but that a) the efficiency and life
>>of the motor will likely be reduced because b) the current draw will
>>go up at the lower voltage.
>>
>>The speed of induction and synchronous motors (which most 240V tool
>>motors will be, single or three phase) is determined primarily by the
>>line frequency. The power needed by the motor is determined by the
>>load-the motor doesn't care how much current it draws... it draws what
>>it need to to meet the power demand of the load at the run speed. At
>>the lower voltage (about 12%) the current will be higher (again, about
>>12%), leading to greater heat production in the motor and greater I^2R
>>losses in the motor and supply wiring. If the load is near the 240V
>>rating of the motor(compressor motor, large power tool, etc) then at
>>208V, the motor will like lose some or all of it's magic smoke and
>>cease to function, especially if that 208V supply is really only 200V
>>(5% either way is very common with system load variation, 10% not
>>unusual, especially in the summer when lots of AC units are on).
>>
>>Motors are generally not conservatively rated-it isn't economical to
>>overrate. You get the 10% or so maximum supply variation built in, and
>>that's it. Go below that, and you need to begin derating the motor
>>rapidly, go above that, and the likelyhood of insulation failure goes
>>way up.
>
>
> I agree that power = I * V * cos(pf), so the current will be greater
> for a lower voltage at a constant power draw. I also agree that a
> fixed load device such as an air compressor could have trouble with
> a lower supply voltage, but variable load devices will simply have
> less power available -- if your table saw is a little weaker due to
> the voltage being somewhat low, just don't push the work through so
> fast.
>
> Although motors are sized to meet their (presumed) load, motors
> are built in discrete sizes -- 1/2, 3/4, 1, 2, 5,...HP (or the
> equivalent in watts). If a load is 7/16 HP, the manufacturer is
> going to use a 1/2 HP motor which is then going to have >10%
> reserve. Motors are also designed to work in a particular voltage
> /class/, so they'll produce their rated power output at the minimum
> supply voltage without damage (or at least without catching fire).
> If the voltage is higher than the minimum, a motor can produce
> more than its rated output without damage. It would be a violation
> of various standards (UL, etc.) to manufacture a device that relies
> on a more than minimum supply voltage to safely produce the necessary
> power.
>
> Industrial installations are another matter, but this discussion
> started as question about a "home" woodworking shop.

It doesn't work that way. The impedance of the motor's winding doesn't
change just because you have a lower voltage. To get the same torque
you need a higher current but you won't get it.

If as the voltage is lowered the current rises then when the voltage is
zero (as in a short across the input to the motor) the current will be
how much?

If the voltage is 86% the torque developed will be 0.86 x 0.86 or 75% of
that at rated voltage (240 volts.)

RB

Everett M. Greene wrote:
> RB <[email protected]> writes:
>
>>The thing to remember here is that with induction motors (as opposed to
>>smaller permanent magnet rotor motors) the torque is proportional to the
>>cross product of the stator and rotor currents. To a first
>>approximation, if the supply voltage is 86.6% (208/240) then the torque
>>will be 75% of that at rated voltage.
>
>
> Your math escapes me. If the supply voltage is lower, the
> current will be higher for the same power output, thus the
> torque would be higher, not lower, by your statement.

Everett M. Greene wrote:
> RB <[email protected]> writes:
>
>>Everett M. Greene wrote:
>>
>>>RB <[email protected]> writes:
>>>
>>>
>>>>The thing to remember here is that with induction motors (as opposed to
>>>>smaller permanent magnet rotor motors) the torque is proportional to the
>>>>cross product of the stator and rotor currents. To a first
>>>>approximation, if the supply voltage is 86.6% (208/240) then the torque
>>>>will be 75% of that at rated voltage.
>>>
>>>Your math escapes me. If the supply voltage is lower, the
>>>current will be higher for the same power output, thus the
>>>torque would be higher, not lower, by your statement.
>>
>
>>It doesn't work that way. The impedance of the motor's winding doesn't
>>change just because you have a lower voltage. To get the same torque
>>you need a higher current but you won't get it.
>>
>>If as the voltage is lowered the current rises then when the voltage is
>>zero (as in a short across the input to the motor) the current will be
>>how much?
>>
>>If the voltage is 86% the torque developed will be 0.86 x 0.86 or 75% of
>>that at rated voltage (240 volts.)
>
>
> torque != power
>
> We're talking about two different things

I understand that, but for constant speed torque and power are directly
proportional. The motors used for most tools are synchronous so the
useful rotation speed will be close to constant.
Torque=k*2*pi*r*(rotation speed)*Power.

Unlike older motors, what is being produced today has little design
margin. If you try to raise the current above the nameplate levels as
the NI product goes up flux saturtion is likely to occur. Not a good,
or useful point to be operating at.

RB

For some reason, electrical matters always bring out lots of posts in this
group by those who know and those who think they know (but don't understand
that three phase power is a different animal from the simple DC they had in
9th grade shop class)

I believe you are one of the guys who knows. Thanks for the definitive
post.

Bob

"Clarke Echols" <[email protected]> wrote in message
news:[email protected]...
> NO!!!
>
> You will have 120 volts from each leg to NEUTRAL, but from leg to leg,
> you have (120 + 120 volts) x sin 120 degrees = 208 volts. There is
> no such thing as "2-phase stuff". It's either single-phase, 240-volt
> (not 220), or it is 208-volt 3-phase with 208 volts between adjacent
> phases.

Clarke Echols <[email protected]> wrote in message news:<[email protected]>...
> NO!!!
>
> You will have 120 volts from each leg to NEUTRAL, but from leg to leg,
> you have (120 + 120 volts) x sin 120 degrees = 208 volts. There is
> no such thing as "2-phase stuff". It's either single-phase, 240-volt
> (not 220), or it is 208-volt 3-phase with 208 volts between adjacent
> phases.
>
> What you need is a single-phase 208-to-240-volt step-up autotransformer
> with a high-enough KVA rating to carry the load you are anticipating.
> The primary is 208 volts, and the secondary is 240 volts, but if it is
> an autotransformer, you won't have a center-tap that connects to a neutral,
> but most 240-volt welders and motors don't need a neutral; just a
> non-current-carrying safety ground (green wire).
>
>

Clarke, you must have never heard of Delta Power.
In that system, Two of the three phase are 120 to ground.
The third phase is 208 to ground.
between any two phases is 240 volts. Really.

With Delta Power, you have 120 volts from two of the three
phases, and you have 240 volts three phase.

Pete

Don Murray wrote:
> ATP wrote:
>>
>> Don Murray wrote:
>>> PJ wrote:
>>> There is also 4 phase power.
>>>
>>>
>>> I don't know about 4-phase power, but I know there is 4 wire
>>> primary. 3 hots and a primary neutral.
>>
>> At the transformer, as per the diagram in that excellent link. When
>> you do the actual service lateral, you pull three primary cables and
>> each one has an aluminum (usually) center conductor and a copper
>> concentric grounded conductor. The grounded conductors are joined at
>> the transformer and also bonded to the transformer case and ground
>> rods.
>
>
> The concentric on the primary cable along with the transformer grounds
> are not always a metallic return to the substation. Primary neutral
> and common neutral (common to primary and secondary, run in the
> secondary position) are metallic returns to the substation. You find
> primary neutrals in the older 4KV where they've used 2400V
> transformers phase to neutral (star or wye). We also run a common
> neutral system in our 21KV that we use 12000V transformers on.

You're way above my head, are you talking about upstream of the
customer-owned cable? In the typical case, the concentric grounds are the
only return, right?
>
> A side note to the underground cable, is they are not getting the life
> expectancy out of it that they thought when they put it in. One of the
> problems that we've run into is some soil conditions dissolve the
> copper neutral on the older installations where it was direct buried.
> We've since gone to a jacketted cable and require everything to be in
> conduit now.

That's probably the way to go- I once pulled an old lead-jacketed primary
run through about 400' of conduit from a man-hole cover to an underground
vault. It was a real PITA but pulling the new poly-jacketed stuff in was
easy and saved a lot of trenching and restoration. Still, the majority of
installations in this area are direct-bury. The only advantage is that
sometimes you can Biddle the cable and make a quick repair without pulling
the whole line out.


Another problem they find is when the cable is faulting
> it's through hairline cracks through the poly. We've replaced a
> considerable amount of this at great expense, and have recently been
> working with a company that injects silicone in the end of the
> strands that fills these hairline cracks. At 5 dollars a foot, it's
> about half the price of cable replacement. This year we have some
> 40000' coming up for cable replacement and another 20000' that we are
> going to do the silicone injection on.
>
> Don

They inject silicone in the center strand which fills the gaps in the white
poly insulation between the center strand and the semiconducting layer?
That's a pretty neat trick. Is it only practical for utilities? Do they have
to redo the potheads/elbows?

In article <[email protected]>, [email protected] wrote:
>"AL" <[email protected]> wrote:
>
>>I'm looking to rent industrial space for hobby use and have looked at a few
>>buildings. I was pleasantly surprised to find that 3 phase power was very
>>common. But several of my tools only run on 240V, and not 208V. Is it
>>possible to get 240V when a building is wired for 3 phase power? Does the
>>power company typically bother to provide a split phase main (like in a
>>house) when they already provide 3 phase?
>
>In all probability the service is from a center tapped delta
>transformer. Put a voltmeter on each leg and you should read
>120/120/208.

Wrong.

>Run your 2 phase stuff across the two 120 legs and you
>will get the 240 that you want.

NO YOU WON'T.

In 208V 3-phase service, the phase-to-phase voltage is 208V, and the
phase-to-neutral voltage is 120V.

>If it's a wye transformer then all three legs will give you the same
>reading. However, that's unlikely unless you are looking at strictly
>warehouse space.

Delta or wye, it doesn't matter. You won't get 240V anywhere in a 208V 3-phase
service.

--
Doug Miller (alphageek at milmac dot com)

How come we choose from just two people to run for president and 50 for Miss America?

"Jon Elson" writes:

> What you need is a buck/boost transformer. These are often used in
buildings
> with 120/208 V service to run 240 V air conditioners, etc. They are
> autotransformers, and quite small for the power they provide, as they
> only have to handle the difference in voltage.

Bingo.

The above is very straight forward, low cost and SAFE.

Any decent industrial electrical distributor will have B-B x'fmrs in stock
and the necessary documentation to help select the correct size unit
complete with the wiring diagram.


--
Lew

S/A: Challenge, The Bullet Proof Boat, (Under Construction in the Southland)
Visit: <http://home.earthlink.net/~lewhodgett> for Pictures

> What you need is a single-phase 208-to-240-volt step-up autotransformer
> with a high-enough KVA rating to carry the load you are anticipating.
> The primary is 208 volts, and the secondary is 240 volts, but if it is
> an autotransformer, you won't have a center-tap that connects to a
neutral,
> but most 240-volt welders and motors don't need a neutral; just a
> non-current-carrying safety ground (green wire).

Buck-boost transformer, as Jon Elson said (below) in an earlier post.

"What you need is a buck/boost transformer. These are often used in
buildings
with 120/208 V service to run 240 V air conditioners, etc. They are
autotransformers, and quite small for the power they provide, as they
only have to handle the difference in voltage."

Bob Swinney


"Clarke Echols" <[email protected]> wrote in message
news:[email protected]...
> NO!!!
>
> You will have 120 volts from each leg to NEUTRAL, but from leg to leg,
> you have (120 + 120 volts) x sin 120 degrees = 208 volts. There is
> no such thing as "2-phase stuff". It's either single-phase, 240-volt
> (not 220), or it is 208-volt 3-phase with 208 volts between adjacent
> phases.
>
>
> George wrote:
> >
> > "AL" <[email protected]> wrote:
> >
> > >I'm looking to rent industrial space for hobby use and have looked at a
few
> > >buildings. I was pleasantly surprised to find that 3 phase power was
very
> > >common. But several of my tools only run on 240V, and not 208V. Is it
> > >possible to get 240V when a building is wired for 3 phase power? Does
the
> > >power company typically bother to provide a split phase main (like in a
> > >house) when they already provide 3 phase?
> >
> > In all probability the service is from a center tapped delta
> > transformer. Put a voltmeter on each leg and you should read
> > 120/120/208. Run your 2 phase stuff across the two 120 legs and you
> > will get the 240 that you want.
> >
> > If it's a wye transformer then all three legs will give you the same
> > reading. However, that's unlikely unless you are looking at strictly
> > warehouse space.
> >
> > George.

Just a note,
I could read "e" 's post perfectly in fixed space ascii. I use OE6 and
went to the 'View' menu, 'Text Size' menu and then clicked on
'Fixed'. By the way, I feel that "e" 's explanation is Right On and very
accurate information.

Someone else commented in the thread that 2 phase power didn't exist.
It does, as has been explained by "e". There is also 4 phase power.
Matter of fact there is also DC power still being generated for public
use in a very small area of lower Manhattan. (That is a hold over
from the 'Edison' years. Tesla gave us our current power system.).
3 cheers.!
PJ

"Bob Davis" <> wrote in message ...
> I appreciate the tedious effort you attempted to draw and explain
> everything. Ascii text drawings are virtually useless in this medium
> because you never know if the reader will see it in fixed width or
> proportional fonts. The only way to consistently do ascii text drawings is
> to use fixed width fonts, space, and no tabs. Even then, word wrapping my
> get you anyway.
>
> Bottom line is that I tried to follow everything you said and it was just
> intelligible (not your fault). I'm an electrical engineer with extra
> coursework in power and probably have a better chance than average in trying
> to follow what you were saying, but couldn't.
>
> Bob
>
> "e" <> wrote in message...
> > In the following, A, B, and C are the phase legs, G is the grounded
> > current carrying conductor (often a neutral, not always) N is the
> > neutral conductor.
Etc.... was snipped by Bob..

I appreciate the tedious effort you attempted to draw and explain
everything. Ascii text drawings are virtually useless in this medium
because you never know if the reader will see it in fixed width or
proportional fonts. The only way to consistently do ascii text drawings is
to use fixed width fonts, space, and no tabs. Even then, word wrapping my
get you anyway.

Bottom line is that I tried to follow everything you said and it was just
intelligible (not your fault). I'm an electrical engineer with extra
coursework in power and probably have a better chance than average in trying
to follow what you were saying, but couldn't.

Bob

"e" <[email protected]> wrote in message
news:[email protected]...
> In the following, A, B, and C are the phase legs, G is the grounded
> current carrying conductor (often a neutral, not always) N is the
> neutral conductor.

Harold & Susan Vordos wrote:

> "Allan Hessenflow" <[email protected]> wrote in message
> news:[email protected]...
> snip---.
>
> For what
>
>>it's worth, before this thread I had no idea that anyone provided
>>delta power.
>
>
> That's an interesting observation. I've had three phase power in three
> different structures, all of them in residential areas, and it's been no
> problem at all to get three phase delta. The one problem, however, is
> getting them to run it in underground. I have been told by more than one
> EE that there are fire problems associated with service of that nature.
> Not sure I understand it, but I've learned to live with the three
> transformers overhead.
>
> Harold
>
>
Fire nature is based on the IR drop. You draw the current through the
smaller than needed current and drop some voltage. I^2R or E^2/R that is power.
The heated wire doesn't dissapage heat except down the wire. Some place gets
to hot.

There is times when water leaks in and there is more IR drop - mostly to water.

Such is life.

Martin


--
Martin Eastburn, Barbara Eastburn
@ home at Lion's Lair with our computer [email protected]
NRA LOH, NRA Life
NRA Second Amendment Task Force Charter Founder

Don,

That is great.. Thanks for the URL. Much appreciated. I went
through NY Trade School in 1957 and have been trying my
very best to remember all of the 'stuff' I learned back then.
Everything from motor winding to pulling and hooking up
raw power. Now, at a retarded (eh.. retired?) state of being,
it is fun, but sometimes hard to remember. It's kind of like the
ole' dog, you know, he chases cars but cannot figure out what
to do with them once caught? lol..

PJ

Oh - 4 phase is nothing more than 2 phase with a center tap on
the generator winding. (Plus a polarity change) Each phase
is a 90º displacement on the rotor/stator. (3 phase is 120º)


"Don Murray" <> wrote in message ...
> > PJ wrote:
> > There is also 4 phase power.
>
> I don't know about 4-phase power, but I know there is 4 wire primary. 3
> hots and a primary neutral. There's also 4 wire 2-phase and 5 wire
> 2-phase. Here's a link that shows the connections that e was discussing,
> along with the 3 wire 2-phase that he explained and the 4 and 5 wire
> that I just spoke of. These pages were copied from a General Electric
> Distribution Transformer Manual.
>
> http://murrayranch.com/Electricity.htm
>
> Don

page 758
http://www.mcmaster.com/

Jon Elson wrote:

>
>
> AL wrote:
>
>> I'm looking to rent industrial space for hobby use and have looked at
>> a few
>> buildings. I was pleasantly surprised to find that 3 phase power was
>> very
>> common. But several of my tools only run on 240V, and not 208V. Is it
>> possible to get 240V when a building is wired for 3 phase power? Does
>> the
>> power company typically bother to provide a split phase main (like in a
>> house) when they already provide 3 phase?
>>
>>
>
> What you need is a buck/boost transformer. These are often used in
> buildings
> with 120/208 V service to run 240 V air conditioners, etc. They are
> autotransformers, and quite small for the power they provide, as they
> only have to handle the difference in voltage.
>
> Jon
>

Don Murray wrote:
> ATP wrote:
>>
>>
>>>
>> You're way above my head, are you talking about upstream of the
>> customer-owned cable? In the typical case, the concentric grounds
>> are the only return, right?
>>>
>>>
> I can't speak to what direction you are referring to about customer
> cable. Around here, it varies so much. We have some customer owned
> primary and secondary. But when I'm talking about the grounds not
> being a return to the sub, say you have a 3-phase primary (just 3
> wires, no neutral) going out from the sub a couple of miles and now
> you install an pad-mount transformer (or string of transformers)
> being fed from a riser off of this 3-phase primary. Your grounds on
> all these transformers are tied together, like you say. But there is
> no metallic return to the sub on them.

OK.
>
>
>>
>> They inject silicone in the center strand which fills the gaps in
>> the white poly insulation between the center strand and the
>> semiconducting layer?
>
> Yes
>
>
>> That's a pretty neat trick. Is it only practical for utilities? Do
>> they have to redo the potheads/elbows?
>
>
> I can't say if it's practical for other than utilities, you'd have to
> weigh the costs. And yes, they have to redo the potheads, and elbows.
> Around here we don't use that many potheads, we use 3M termination
> kits.
>
> Don

I'm referring to the legacy potheads in the underground transformer vaults
which the utility desperately wants to phase out in favor of pad mounts.

We have 440v and 220v 3 phase in our building. When we need 220v
single phase we just take any two lines from the 220v 3 phase. When
using plugs, we use the 3 phase plugs, but only use two of the taps
going to the load. Works on motors, welders, everything, everytime.
I never knew there was a problem until I read this thread. (this
ain't theory, we been doing this for years). Oh yes, when we need
110v we use one leg from the 220v 3 phase.
Paul

This reminds me of a class we had in engineer school. The object was
to calculate the horsepower required to move a quantity of dirt in a
scraper over a certain soil at a certain grade and speed. You did the
math and came up with the required horsepower. However, There you
are out in the field, you load your scraper and it either goes up the
hill or it doesn't. If it doesn't, you take off some of the load, or
go a different way. You don't go to your Company Commander and ask
for a bigger motor because your theta ain't cosigned with the delta
max.
Paul

Don Murray wrote:
> ATP wrote:
>>
>> Roy Smith wrote:
>>> In article <[email protected]>,
>>> [email protected] (e) wrote:
>>>> A note on DC- As of teo years ago, New York transit was still
>>>> running DC rotarary converters that are about 100 years old-AC in,
>>>> DC out, but not quite a motor generator like a welder. I believe
>>>> the last went out of service recently, but I may be wrong. The
>>>> hookups use have been thyristor converters (all electronic) since
>>>> the 50's.
>>>
>>> If anybody's interested in this stuff, take a look at
>>>
>>> http://www.nycsubway.org/tech/power/rotary.html
>>
>> Also, "Networks of Power"
>>
>>
http://www.amazon.com/exec/obidos/tg/detail/-/0801846145/102-8284350-1739318?v=glance
>>
>> "If you're a history buff, and appreciate the technology that
>> surrounds us all, you'll love reading "Networks of Power:
>> Electrification in Western Society, 1880-1930" by Tom Hughes. Hughes
>> takes us back to the days of fierce rivalry between Edison and
>> Westinghouse; the early era of electric power generation and
>> consumption where the battle of DC vs. AC consumer power was born
>> and decided.
>>
>> Hughes doesn't stop there. Also included in this well-footnoted
>> edition are in-depth narratives of the evolution of commercial power
>> systems in England and Germany through 1930. A well written,
>> readable snapshot in time.
>>
>> Compelling historical reading for the non-technologist as well as the
>> student of electrical power commercialization."
>
>
> Do they mention anything in the book, "Networks of Power" about the
> Folsom Powerhouse?

It's been a few years since I read it, but I don't remember that. A lot of
the book dealed with rationalization (standardization and the
rationalization movement) of the power industry, the balance between
competition and the need for a unified grid. On the technical side, much of
what we take for granted, such as the concept of a feed with two hots and a
neutral, (vs. one hot and one neutral) was once a bright new idea. With your
background, I think you would enjoy at least some of the book.

Actually, he was correct. The voltages are proportional to the
distances between points, and the phase angles are the same as the
angles as measured on diagram (presuming equilateral triangles) in the
following:

In the following, A, B, and C are the phase legs, G is the grounded
current carrying conductor (often a neutral, not always) N is the
neutral conductor.


240 delta: (not as common a hookup as it used to be, many northeast
power co's are trying to get rid of it)

B
/ \
/ \
/ \
C---G---A

G is the grounded conductor (not neccisarily a neutral in this hookup,
NOT the safety ground)
A-B, B-C and C-A are all 240V, 120 degree phase difference, G-C and
G-A are both 120, opposite phase, and G-B is 208V, 90 degrees behind
G-A. Occationally seen with A, B, or C grounded rather than the phase
A-C being split--there is no 120V supply in that case. Also seen as
the open delta (two transformer) hookup (supplies about 58% of the
power the three transformer hook will, and the reading across the
'missing' transformer may be a tad on the wild side) and as the two
transformer T hookup, with a transformer from A to C and from G to B.
Measurements A-B and A-C tend to be a bit off as the 120V loads
change.
Sometimes supplied for a wye feed with only two power legs and the
neutral, which derates the capacity, but greatly cheapens the hookup.
The third phase is created by the load transformers (My local utility
used to do this and not even run the neutral-- just two cables for
phase A and B, no conduit, the generally wet swamp mud ground being
used as the neutral. Leads to some really bizarre voltage and phase
swings, especially during a drout.)



120/208Y and 277/480Y:

A
|
|
N
/ \
/ \
B C

a) 120V across N-A, N-B and N-C, all 120deg out of phase. 208V across
A-B, A-C, and B-C, all 120 degrees out of phase, 60 degrees out with
the 120V hookups. This is a real common hookup in small industrial.

b) 277V across N-A, N-B, and N-C; 480V across A-B, B-C and C-A. This
is why there are 277V fluorescent and HID fixtures so cheap. common in
heavier industry, large schools, shiopping centers, etc. where long
electrical runs and/or high power loads are common. Can use lighter
gauge wiring at the reduced current and save across the board: cheaper
wire, cheaper breakers, lower heat loss, lower line drop, etc.



120-120 two phase:

A
|
|
|
B----C

A-B 120V, B-C 120V, 90 degree phase difference. B-C is 172V at 45
degrees (not used on its own generally) This is outmoded by many
tears, but is still seen in some older industrial plants, and is still
supplied in many areas. It can be derived from three phase using the T
hookup for the primary side. and L for the secondary, taking care to
use the proper transformer ratios. When needed, it is usually derived
in plant anymore, but is still provided in a few places by the utility
(I think Niagra may still generate 2 phase on the 25Hz side)

3phase 2phase
side side
A A'
| |
| |
| |
B--N--C B'----C'





Clarke Echols <[email protected]> wrote in message news:<[email protected]>...
> NO!!!
>
> You will have 120 volts from each leg to NEUTRAL, but from leg to leg,
> you have (120 + 120 volts) x sin 120 degrees = 208 volts. There is
> no such thing as "2-phase stuff". It's either single-phase, 240-volt
> (not 220), or it is 208-volt 3-phase with 208 volts between adjacent
> phases.
>
> What you need is a single-phase 208-to-240-volt step-up autotransformer
> with a high-enough KVA rating to carry the load you are anticipating.
> The primary is 208 volts, and the secondary is 240 volts, but if it is
> an autotransformer, you won't have a center-tap that connects to a neutral,
> but most 240-volt welders and motors don't need a neutral; just a
> non-current-carrying safety ground (green wire).
>
> George wrote:
> >
> > "AL" <[email protected]> wrote:
> >
> > >I'm looking to rent industrial space for hobby use and have looked at a few
> > >buildings. I was pleasantly surprised to find that 3 phase power was very
> > >common. But several of my tools only run on 240V, and not 208V. Is it
> > >possible to get 240V when a building is wired for 3 phase power? Does the
> > >power company typically bother to provide a split phase main (like in a
> > >house) when they already provide 3 phase?
> >
> > In all probability the service is from a center tapped delta
> > transformer. Put a voltmeter on each leg and you should read
> > 120/120/208. Run your 2 phase stuff across the two 120 legs and you
> > will get the 240 that you want.
> >
> > If it's a wye transformer then all three legs will give you the same
> > reading. However, that's unlikely unless you are looking at strictly
> > warehouse space.
> >
> > George.

"PJ" <[email protected]> wrote in message news:<[email protected]>...
> Just a note,
.
.
> It does, as has been explained by "e". There is also 4 phase power.
> Matter of fact there is also DC power still being generated for public
> use in a very small area of lower Manhattan. (That is a hold over
> from the 'Edison' years. Tesla gave us our current power system.).
> 3 cheers.!
.
.

A note on DC- As of teo years ago, New York transit was still running
DC rotarary converters that are about 100 years old-AC in, DC out, but
not quite a motor generator like a welder. I believe the last went out
of service recently, but I may be wrong. The hookups use have been
thyristor converters (all electronic) since the 50's.

I read recently that the last of the Edison DC service was finally
discontinued.

e

[email protected] (Everett M. Greene) wrote in message news:<[email protected]>...
> George <[email protected]> writes:
> > "AL" <[email protected]> wrote:
> >
> > >I'm looking to rent industrial space for hobby use and have looked at a few
> > >buildings. I was pleasantly surprised to find that 3 phase power was very
> > >common. But several of my tools only run on 240V, and not 208V.
>
> What kind of "240V" motor won't run on 208V?
>
>

It isn't that the motor won't run, but that a) the efficiency and life
of the motor will likely be reduced because b) the current draw will
go up at the lower voltage.

The speed of induction and synchronous motors (which most 240V tool
motors will be, single or three phase) is determined primarily by the
line frequency. The power needed by the motor is determined by the
load-the motor doesn't care how much current it draws... it draws what
it need to to meet the power demand of the load at the run speed. At
the lower voltage (about 12%) the current will be higher (again, about
12%), leading to greater heat production in the motor and greater I^2R
losses in the motor and supply wiring. If the load is near the 240V
rating of the motor(compressor motor, large power tool, etc) then at
208V, the motor will like lose some or all of it's magic smoke and
cease to function, especially if that 208V supply is really only 200V
(5% either way is very common with system load variation, 10% not
unusual, especially in the summer when lots of AC units are on).

Motors are generally not conservatively rated-it isn't economical to
overrate. You get the 10% or so maximum supply variation built in, and
that's it. Go below that, and you need to begin derating the motor
rapidly, go above that, and the likelyhood of insulation failure goes
way up.

Everett M. Greene wrote:
> Gary Coffman <[email protected]> writes:
>
>>[email protected] (Everett M. Greene) wrote:
>>
>>>Gary Coffman <[email protected]> writes:
>>>
>>>>[snip]
>>>>The fly in this ointment is that winding *resistance* doesn't change.
>>>>Energy dissipated in the windings is a function of the square of current
>>>>and the winding resistance (P=I^2 * R). So the windings heat more
>>>>rapidly at a lower line voltage, but speed is constant, so the amount
>>>>of cooling air remains constant. That causes winding temperature to
>>>>rise, leading ultimately to insulation failure, and all the magic smoke
>>>>is let out of the motor.
>>>
>>>But, again, we're talking about shop tools, most of which are
>>>going to be hand fed their work and whose motors are going to
>>>be lightly loaded most of the time. If the voltage is less, the
>>>available power will be less so you have to feed the work to
>>>the tool a little bit more slowly. In other words, you won't
>>>be pushing the motor load to the limit and causing the smoke
>>>to start rising. Up to a point, you can even overload a motor
>>>for a brief period without causing any harm.
>>
>>The available power will not be less. Remember, increasing slip
>>decreases effective reactance in the motor, so at a given load,
>>current will automatically increase to satisfy load demand when
>>voltage decreases. Since P = I * E, power can remain the same
>>when voltage is decreased. It is how electric motors work. So
>>you will not have any sensible feedback telling you to slow down.
>
>
> P != I * E

Power does = E * I for D.C.
Power does = E * I * Cos(theta) for A.C. theta is the phase angle
between E and I.
Sometimes Cosine equals 1. :-)
Those are the facts.

>
> You're saying the utilities cannot reduce their demand load
> by reducing voltage because the motor loads will just draw
> increased current?
The swinging transformers move taps raising and lowing voltage.
These are massive horz. transformers in substations.

They often drop voltages to shed load. Many motors and compressors
don't start. Those that do will draw more but with the drop off's
and the resistive load loss it is a win.

They often run this valley on between 68 and 93 volt not the normal 120-140v.

This is when storms take out a substation - they back-strap this valley
from another.

I caught them one Sunday a.m. - TV worked and most things - computer UPS didn't
like it one bit. I called it in - got service on the line - Naw that just can't be.
I asked for a service person to verify the substation as I have checked my house
from stem to stern. I gave the service person my number.

A super nice response engineer called from the substation. He was about to unlock,
but though to call first. I told him I used my Beckman and my Tektronix true RMS voltmeters.

Once he heard the true RMS - he knew I knew something. We talked as he into the station -
and found the main line and the back strap installed. That is when the fun came.

He had to undo a double hot backstop line voltage and heat. He asked me to stand by and
call in for him if he didn't come back. I did and he did. I verified 120V was on the lines.
We chatted as he locked up and off we both went. Him home, me computering.


>
>
>>Now for some power tools, such as a table saw, you can consciously
>>and deliberately reduce the load by decreasing feed rate so that
>>power demanded decreases to match decreased voltage, and then
>>current will not increase. But you can't do that by noting the motor
>>is bogging, it won't. You'd have to continuously monitor motor current
>>to stay within the safe area. The tool itself won't give you any feedback
>>telling you to slow down, until you note the smoke coming from the
>>motor.
>>
>>For some motor operated loads, such as an air compressor, you
>>have to change pulley ratios to reduce the load. While this will
>>reduce running current, it may make the compressor hard to start,
>>and possibly damage motor, contactors, or capacitors anyway.
>
>
> I would think you'd reduce the size of the driving pulley
> so reduce the motor load, thus making it easier to start.
>
>
>>In short, you can run a lightly loaded motor on reduced voltage,
>>but you need some way to monitor current to ensure you really
>>are loading it lightly enough to keep it from overheating. And you
>>have to be wary of other factors which may come into play, such
>>as excessive current draws required to come up to speed on
>>reduced voltage.
>
>
> And, again, we're talking about voltages that are within
> the motors' voltage rating class.


--
Martin Eastburn, Barbara Eastburn
@ home at Lion's Lair with our computer [email protected]
NRA LOH, NRA Life
NRA Second Amendment Task Force Charter Founder

On Mon, 19 Jan 2004 19:08:24 PST, [email protected] (Everett M. Greene) wrote:
>Gary Coffman <[email protected]> writes:
>> [snip]
>> The fly in this ointment is that winding *resistance* doesn't change.
>> Energy dissipated in the windings is a function of the square of current
>> and the winding resistance (P=I^2 * R). So the windings heat more
>> rapidly at a lower line voltage, but speed is constant, so the amount
>> of cooling air remains constant. That causes winding temperature to
>> rise, leading ultimately to insulation failure, and all the magic smoke
>> is let out of the motor.
>
>But, again, we're talking about shop tools, most of which are
>going to be hand fed their work and whose motors are going to
>be lightly loaded most of the time. If the voltage is less, the
>available power will be less so you have to feed the work to
>the tool a little bit more slowly. In other words, you won't
>be pushing the motor load to the limit and causing the smoke
>to start rising. Up to a point, you can even overload a motor
>for a brief period without causing any harm.

The available power will not be less. Remember, increasing slip
decreases effective reactance in the motor, so at a given load,
current will automatically increase to satisfy load demand when
voltage decreases. Since P = I * E, power can remain the same
when voltage is decreased. It is how electric motors work. So
you will not have any sensible feedback telling you to slow down.

Now for some power tools, such as a table saw, you can consciously
and deliberately reduce the load by decreasing feed rate so that
power demanded decreases to match decreased voltage, and then
current will not increase. But you can't do that by noting the motor
is bogging, it won't. You'd have to continuously monitor motor current
to stay within the safe area. The tool itself won't give you any feedback
telling you to slow down, until you note the smoke coming from the
motor.

For some motor operated loads, such as an air compressor, you
have to change pulley ratios to reduce the load. While this will
reduce running current, it may make the compressor hard to start,
and possibly damage motor, contactors, or capacitors anyway.

In short, you can run a lightly loaded motor on reduced voltage,
but you need some way to monitor current to ensure you really
are loading it lightly enough to keep it from overheating. And you
have to be wary of other factors which may come into play, such
as excessive current draws required to come up to speed on
reduced voltage.

Gary

On Sat, 17 Jan 2004 18:14:27 PST, [email protected] (Everett M. Greene) wrote:
>RB <[email protected]> writes:
>> Everett M. Greene wrote:
>> > RB <[email protected]> writes:
>> >
>> >>The thing to remember here is that with induction motors (as opposed to
>> >>smaller permanent magnet rotor motors) the torque is proportional to the
>> >>cross product of the stator and rotor currents. To a first
>> >>approximation, if the supply voltage is 86.6% (208/240) then the torque
>> >>will be 75% of that at rated voltage.
>> >
>> > Your math escapes me. If the supply voltage is lower, the
>> > current will be higher for the same power output, thus the
>> > torque would be higher, not lower, by your statement.
>
>> It doesn't work that way. The impedance of the motor's winding doesn't
>> change just because you have a lower voltage. To get the same torque
>> you need a higher current but you won't get it.
>>
>> If as the voltage is lowered the current rises then when the voltage is
>> zero (as in a short across the input to the motor) the current will be
>> how much?
>>
>> If the voltage is 86% the torque developed will be 0.86 x 0.86 or 75% of
>> that at rated voltage (240 volts.)
>
>torque != power
>
>We're talking about two different things

Power is a rate, so it requires a time element. Power is equal to the
product of torque and speed (time function). Since speed in an
induction motor is a function of the power line frequency, it doesn't
change as voltage changes. So power is proportional to torque.

However, I'd like to take exception to one thing that RB said. The
impedance of the motor windings is a function of load, speed, and
slip. As long as the speed, load, and slip are constant, impedance
is constant. So lowering the applied voltage does lower the current.
*But* an induction electric motor tries to compensate for this when
available power falls below load demand by increasing slip.

As slip increases, the winding impedance falls, and current can
increase, even at a lower supply voltage. Increased current yields
increased torque, and at the same speed, increased power.

The fly in this ointment is that winding *resistance* doesn't change.
Energy dissipated in the windings is a function of the square of current
and the winding resistance (P=I^2 * R). So the windings heat more
rapidly at a lower line voltage, but speed is constant, so the amount
of cooling air remains constant. That causes winding temperature to
rise, leading ultimately to insulation failure, and all the magic smoke
is let out of the motor.

Gary

On Fri, 09 Jan 2004 04:01:44 +0000, AL wrote:

> I'm looking to rent industrial space for hobby use and have looked at a few
> buildings. I was pleasantly surprised to find that 3 phase power was very
> common. But several of my tools only run on 240V, and not 208V. Is it
> possible to get 240V when a building is wired for 3 phase power? Does the
> power company typically bother to provide a split phase main (like in a
> house) when they already provide 3 phase?


Consult your electric company. They are more than happy to help and
advise. Their advice should be free and it will result in the safest of
installations for you.

Generally, utility companies prefer not to let their customers get hurt.

Roy Smith <[email protected]> wrote:

>In article <[email protected]>,
> "Bob Davis" <[email protected]> wrote:
>
>> I appreciate the tedious effort you attempted to draw and explain
>> everything. Ascii text drawings are virtually useless in this medium
>> because you never know if the reader will see it in fixed width or
>> proportional fonts. The only way to consistently do ascii text drawings is
>> to use fixed width fonts, space, and no tabs. Even then, word wrapping my
>> get you anyway.
>>
>> Bottom line is that I tried to follow everything you said and it was just
>> intelligible (not your fault). I'm an electrical engineer with extra
>> coursework in power and probably have a better chance than average in trying
>> to follow what you were saying, but couldn't.
>
>For what it's worth, I haven't studied this stuff since college
>20-mumble years ago. I found the diagrams perfectly readable and the
>explanation quite clear.


Yea me too!

I especially liked the part where he said "he [meaning me] was
correct."

Just thought I'd repeat it.

George.

In article <[email protected]>,
[email protected] (e) wrote:
> A note on DC- As of teo years ago, New York transit was still running
> DC rotarary converters that are about 100 years old-AC in, DC out, but
> not quite a motor generator like a welder. I believe the last went out
> of service recently, but I may be wrong. The hookups use have been
> thyristor converters (all electronic) since the 50's.

If anybody's interested in this stuff, take a look at

http://www.nycsubway.org/tech/power/rotary.html

In article <[email protected]>,
"Bob Davis" <[email protected]> wrote:

> What newsreader are you using? Maybe I need to change readers. They were
> scrambled on this end.

MT-NewsWatcher on a Macintosh.

I suspect the real problem is that your reader was trying to display the
text as HTML or in a proportional-width font. If you can somehow force
your reader to display the article as plain text in a fixed width font,
you should be fine.

In article <[email protected]>,
"Bob Davis" <[email protected]> wrote:

> I appreciate the tedious effort you attempted to draw and explain
> everything. Ascii text drawings are virtually useless in this medium
> because you never know if the reader will see it in fixed width or
> proportional fonts. The only way to consistently do ascii text drawings is
> to use fixed width fonts, space, and no tabs. Even then, word wrapping my
> get you anyway.
>
> Bottom line is that I tried to follow everything you said and it was just
> intelligible (not your fault). I'm an electrical engineer with extra
> coursework in power and probably have a better chance than average in trying
> to follow what you were saying, but couldn't.

For what it's worth, I haven't studied this stuff since college
20-mumble years ago. I found the diagrams perfectly readable and the
explanation quite clear.

RB <[email protected]> wrote:
> I understand that, but for constant speed torque and power are directly
> proportional. The motors used for most tools are synchronous

Are you sure about that? I would have thought most machines would have
induction motors. Synchronous machines keep absolutely constant RPM as
load increases, then suddenly stall (sometimes with disasterous results)
when the phase angle hits 90 degrees. Induction machines gradually slow
down as they get overloaded and have rather more benign stall behavior.

For example,

http://www.1-home-improvement.com/specialty-tools/Baldor-WWL3606-3-B00002
23WG.html

is almost certainly a 2-pole induction motor. The 3450 RPM rating is
the giveaway, and it only runs at that speed at its rated load. Under
no load, it'll run at close to (but not quite) 3600 RPM, and under
overload, it'll run slower. The motors you commonly see that are rated
at 1725 (or thereabouts) RPM are 4-pole induction motors (twice the
torque but half the speed for a given size/weight/power).

A 2-pole 60 Hz syncronous motor would have a nameplate speed of 3600 RPM.

"Greg O" <[email protected]> wrote:

>
>"Doug Miller" <[email protected]> wrote in message news:7FxLb.23545$P%> >
>> >In all probability the service is from a center tapped delta
>> >transformer. Put a voltmeter on each leg and you should read
>> >120/120/208.
>>
>> Wrong.
>>
>> >Run your 2 phase stuff across the two 120 legs and you
>> >will get the 240 that you want.
>>
>> NO YOU WON'T.
>>
>> In 208V 3-phase service, the phase-to-phase voltage is 208V, and the
>> phase-to-neutral voltage is 120V.
>>
>> >If it's a wye transformer then all three legs will give you the same
>> >reading. However, that's unlikely unless you are looking at strictly
>> >warehouse space.
>>
>> Delta or wye, it doesn't matter. You won't get 240V anywhere in a 208V
>3-phase
>> service.
>>
>> --
>> Doug Miller (alphageek at milmac dot com)
>>
>>
>
>I don't know about everywhere else, but I rented a shop that had 208-3ph. It
>was just as Doug described.
>Phase to phase = 208 volt, any phase to neutral was 120 volt.

It was probably set up as a warehouse originally. This was often done
because all that was needed was power for lights. The 208 wye service
has the added advantage that your load is more balanced since you have
three 120 circuits to use not just two. Plus you can run just about
any 3-phase power machinery.

The only drawback to the 208 wye service is that any 240 2-phase stuff
that you use won't get the power for which it was designed. But then,
for just about anything that you find in a shop, that's not a problem
anyhow. For instance, if you've got a 240 2-phase air compressor it
will run happily on 2 legs of the 208 service. The power output might
be a bit less, but you'll never notice it - plus the manufacturer
overstated it in the first place anyhow.

Oh, and one more thing: Where Doug was saying "Wrong" and "NO.." up
there. You now know that the correct answers are "Right" and "YES.."
don't you?

George.

> We have 440v and 220v 3 phase in our building. When we need 220v
> single phase we just take any two lines from the 220v 3 phase. When
> using plugs, we use the 3 phase plugs, but only use two of the taps
> going to the load. Works on motors, welders, everything, everytime.
> I never knew there was a problem until I read this thread. (this
> ain't theory, we been doing this for years).

This much shouldn't be a problem for either of the 3 phase setups
that have been discussed here - the voltage across any two legs will
be the same. Of course if the equipment needs a neutral as well and
you have delta power then it will matter which legs you use.

> Oh yes, when we need
> 110v we use one leg from the 220v 3 phase.

For 240V delta power, two of the legs will give you 120V compared to
neutral. For Y 208V power, any of the legs will be 120V.

Since you haven't had any problems you probably have a 208V Y
configuration (or you just happened to use the right legs). For what
it's worth, before this thread I had no idea that anyone provided
delta power. I knew what it was (although not the name) because
that's what you get from a phase converter where you're using two
legs as they are and generating the third. But every industrial
building I've worked in around here (near San Jose, CA) had 208V Y
power. Sometimes they'll have some higher voltage as well, but since
that tends to just go to the HVAC systems I've never paid any
attention to its configuration.

allan

--
Allan N. Hessenflow [email protected]

Hey Bob,

His suggestion for ASCII art made me cringe too, but it actually came
out perfect on my computer.

Bit I also had some problem "following" along.

Take care.

Brian Lawson,
Bothwell, Ontario.
XXXXXXXXXXXXXXXXXXXXX
On Fri, 09 Jan 2004 19:43:45 GMT, "Bob Davis"
<[email protected]> wrote:

>I appreciate the tedious effort you attempted to draw and explain
>everything. Ascii text drawings are virtually useless in this medium
>because you never know if the reader will see it in fixed width or
>proportional fonts. The only way to consistently do ascii text drawings is
>to use fixed width fonts, space, and no tabs. Even then, word wrapping my
>get you anyway.
>
>Bottom line is that I tried to follow everything you said and it was just
>intelligible (not your fault). I'm an electrical engineer with extra
>coursework in power and probably have a better chance than average in trying
>to follow what you were saying, but couldn't.
>
>Bob
>
>"e" <[email protected]> wrote in message
>news:[email protected]...
>> In the following, A, B, and C are the phase legs, G is the grounded
>> current carrying conductor (often a neutral, not always) N is the
>> neutral conductor.
>

"AL" <[email protected]> wrote:

>I'm looking to rent industrial space for hobby use and have looked at a few
>buildings. I was pleasantly surprised to find that 3 phase power was very
>common. But several of my tools only run on 240V, and not 208V. Is it
>possible to get 240V when a building is wired for 3 phase power? Does the
>power company typically bother to provide a split phase main (like in a
>house) when they already provide 3 phase?

In all probability the service is from a center tapped delta
transformer. Put a voltmeter on each leg and you should read
120/120/208. Run your 2 phase stuff across the two 120 legs and you
will get the 240 that you want.

If it's a wye transformer then all three legs will give you the same
reading. However, that's unlikely unless you are looking at strictly
warehouse space.

George.

Good information, Don. Thanks!

Bob Swinney
"Don Murray" <[email protected]> wrote in message
news:[email protected]...
>
>
> PJ wrote:
> There is also 4 phase power.
>
>
> I don't know about 4-phase power, but I know there is 4 wire primary. 3
> hots and a primary neutral. There's also 4 wire 2-phase and 5 wire
> 2-phase. Here's a link that shows the connections that e was discussing,
> along with the 3 wire 2-phase that he explained and the 4 and 5 wire
> that I just spoke of. These pages were copied from a General Electric
> Distribution Transformer Manual.
>
> http://murrayranch.com/Electricity.htm
>
> Don