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240VAC single phase from three-phase? - Page 6

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Posted by m II on April 7, 2010, 7:43 am
 



Before some nit picker finds my 'mistakes',

 > There is 120 degrees between the rms peaks of the sine waves.

'rms peaks' should have read 'peaks'.



 > 180^2 + 103.92^2 =  43009
 > square root of 14399.36  = 208 volts.


should have read:

square root of 43009   = 208 volts

Posted by Josepi on April 7, 2010, 11:59 am
 


Mike

The total output of the transformer is considered single phase but when you
tap the middle of the winding, ground it and it becomes a measurement
reference point for your phasors....

          you have two phases, 180 degrees apart.

Use your own battery analogy but correctly use the centre tap as the ground,
to match the ac circuitry.

     +/-12vdc.

Look! two different voltages to common!



This could go on forever, but it won't. This is really my last posting
on this topic.

Take a transformer secondary.

It makes 240 volts.

It has only ONE current flow in it.

Put a tap in the winding at exactly the mid point.

Measure the voltage in each half, moving the meter leads up in unison.

Both readings are identical and at the same phase angle.

Putting a centre tap on that winding has NOT reversed the current in
half the winding. The current is still the same as it was, at the SAME
angle all the way through the winding.

You are making things harder than they are. (-120 volts at 180 degrees)
***IS*** (120 volts at ZERO degrees). They are one and the same, now and
forever.

This is KEY:

******Any *two* wires in a single OR three phase circuit constitute a
SINGLE phase circuit. They have to. It's the LAW. Call your local
University. If you don't like their answer, Call God.******


Next, 208 volts phase to phase, in a three phase circuit, is obtained by
  the result of 120 volts at zero degrees and 120 volts at 120 degrees.

There is 120 degrees between the rms peaks of the sine waves.

Try it. Break down the voltage to it's rectangular components.

120 volts at zero degrees equals 120 v + j 0
120 volts at 120 degrees equals 120 * (cos 120) +j 120 * sin 120   =
(-60 volts + j 103.92)


So, we have:

  (120 + j 0)
-(-60 + j 103.92)
----------------

180+ j 103.92   Which is the resultant of the two vectors at 120 degrees
apart.

Now, re-convert back to polar notation. Pythagoras comes in handy.

180^2 + 103.92^2 =  43009

square root of 14399.36  = 208 volts.



Note that when the sine waves are the same peak value, 120 degrees
apart, multiplying the 120 line voltage by the square root of 3 also
gives you the 208 phase voltage

The newer 347 line voltage gives you 600 phase voltage. That voltage is
getting more popular for high rise building distributions. They now put
various small transformers in to feed office area 120 V needs.

In a Delta connection, the line voltage IS phase voltage and the root 3
is used to find current instead of voltage.

In a Delta connection, line current is root 3 times phase current.

Back to single phase. The centre tapped wire (neutral) in a three wire
system is NOT grounded because it's at zero potential. It is grounded
there in order to limit the maximum fault voltage to ground at the LINE
sides.

That screws up a lot of people. They mistakenly believe in erroneous and
  quite fictitious, non existent, phase angles.

The only phase angles that exist in single phase system are caused by
lagging or leading power factors and show the relationship between
voltage and current.

Wake up, people...it's called single phase for a reason. Two phase and
three phase systems are birds of a completely different colour. Do your
homework.




   mike



Posted by daestrom on April 7, 2010, 8:44 pm
 

Josepi wrote:

But there is no reason to suppose that the center tap is a reference.
As Mike said, if you measure from left side to center, then measure from
center to right side, you get exactly the same voltage and the two are
in exact phase.

You could just as easily measure from left side to middle and then
measure from left side to right side (using the left side as your
reference).  Two measurements, exactly in phase, one just happens to be
double of the other.

In the power industry 'two-phase' does not mean two voltages derived
from some single phase source that is center-tapped.

Two-phase is two AC voltages that are 90 degrees displaced in time.
Sometimes you connect a couple of transformers to a three phase supply
in what is referred to as a Scott-T connection to get two phase.  It's
use in power systems has all but died out though.  It *is* still used in
some servo-motor control systems, one phase is applied to windings as a
reference and the second winding is energized with an AC supply that is
either 90 degrees leading or 90 degrees lagging to make the servo spin
one direction or the other direction.

This argument about the three-wire 'Edison Connection' that is used here
in the US as being 'center-tapped single phase' or 'two phase 180
degrees apart' has been going on since they invented it.  Most power
EE's would side with Mike and call it 'center tapped single phase' and
reserve 'two-phase' for those rare situations where two single phase
supplies are locked 90-degrees out of phase with each other.

daestrom

Posted by Josepi on April 7, 2010, 9:49 pm
 

Things are a little contorted here. It seems you are mixing different
arguments going on in the same thread...LOL

The argument put forth in this leg of the thread was

The two "phases" of a 120/240vac supply are 180 degrees out of phase.  Mike
argues they are at 0 degrees "in phase" and not 180 degrees. He also used an
analogy of two 12volt batteries indicating they add up to 24 volts so they
have to be the same phase.

As a few, here,  have tried to point out, this is reference dependant, as
you seem to be aware of.

Voltage vectors and measurements, as most people know are "across two
points" with one used as a polarity and the other used a reference. When
people refer to a voltage without indicating a second point, it is implied
that ground or the common conductor is used.

Since the centre tap of a supply transformer from a utility is almost always
grounded (in N.America)  and used for the reference, by default the two 120
volt outside legs are 180 degrees out of phase by definition.

Yes this is still single phase, and two phase is an incorrect reference, but
the "phases" ("legs" if you will?) are always 180 degrees out of phase ***
WITH RESPECT TO GROUND OR CENTRE TAP ***
---------- clarified the implied reference-------------------

We always called "3pH 4W with one leg out" (two phase) (120 deg.
potentials)(two phase), "Network" and you will find this nomenclature on
most kWh meter manufacturer's documentation and devices.

You will also find most EE people still talk in terms of 3ph 3W and are not
fluent with 4 wire systems. It was always a fight to get them to say
3EI...LOL Each field is developing their own lingo and it clashes sometimes.

I have only run into a few  Scott "T" transformers in my life. They were all
power tranformers (black box) in a box. (600//120/208 3ph 4w)  Strange
design but can be done economically, with only two cores. I have never heard
them called "two phase" before. Fun phasor training exercise at 90 degree
cores for the young'uns.

Ever run into 3phase 4wire delta? Engineering dicontinued them as only the
metering people could understand how to meter them correctly...LOL



But there is no reason to suppose that the center tap is a reference.
As Mike said, if you measure from left side to center, then measure from
center to right side, you get exactly the same voltage and the two are
in exact phase.

You could just as easily measure from left side to middle and then
measure from left side to right side (using the left side as your
reference).  Two measurements, exactly in phase, one just happens to be
double of the other.

In the power industry 'two-phase' does not mean two voltages derived
from some single phase source that is center-tapped.

Two-phase is two AC voltages that are 90 degrees displaced in time.
Sometimes you connect a couple of transformers to a three phase supply
in what is referred to as a Scott-T connection to get two phase.  It's
use in power systems has all but died out though.  It *is* still used in
some servo-motor control systems, one phase is applied to windings as a
reference and the second winding is energized with an AC supply that is
either 90 degrees leading or 90 degrees lagging to make the servo spin
one direction or the other direction.

This argument about the three-wire 'Edison Connection' that is used here
in the US as being 'center-tapped single phase' or 'two phase 180
degrees apart' has been going on since they invented it.  Most power
EE's would side with Mike and call it 'center tapped single phase' and
reserve 'two-phase' for those rare situations where two single phase
supplies are locked 90-degrees out of phase with each other.

daestrom


Josepi wrote:

But there is no reason to suppose that the center tap is a reference.
As Mike said, if you measure from left side to center, then measure from
center to right side, you get exactly the same voltage and the two are
in exact phase.

You could just as easily measure from left side to middle and then
measure from left side to right side (using the left side as your
reference).  Two measurements, exactly in phase, one just happens to be
double of the other.

In the power industry 'two-phase' does not mean two voltages derived
from some single phase source that is center-tapped.

Two-phase is two AC voltages that are 90 degrees displaced in time.
Sometimes you connect a couple of transformers to a three phase supply
in what is referred to as a Scott-T connection to get two phase.  It's
use in power systems has all but died out though.  It *is* still used in
some servo-motor control systems, one phase is applied to windings as a
reference and the second winding is energized with an AC supply that is
either 90 degrees leading or 90 degrees lagging to make the servo spin
one direction or the other direction.

This argument about the three-wire 'Edison Connection' that is used here
in the US as being 'center-tapped single phase' or 'two phase 180
degrees apart' has been going on since they invented it.  Most power
EE's would side with Mike and call it 'center tapped single phase' and
reserve 'two-phase' for those rare situations where two single phase
supplies are locked 90-degrees out of phase with each other.

daestrom



Posted by Johnny B Good on April 8, 2010, 2:06 am
 


====snip====


 I'm getting heartily sick of the confusion over what is normally
referred to as a "Bi-Phase" supply. In the UK such supplies are normally
only seen on building sites where a 55-0-55 vac supply is used to power
tools in order to limit the maximum rms voltage with respect to ground
to that of any of the two anti-phase lives (or hots) thus greatly
reducing the electrocution hazard.

 The tools are rated for 110 volt operation and receive exactly this by
using the two lives (hots). Effectively the supply is derived from a
centre tapped 110v secondary where the centre tap is earthed to prevent
an earth fault that would otherwise cause the voltage on the other leg
rising to 110v if the secondary were left 'floating'.

 The stateside domestic dwelling supply is simply the same scheme but
with double the voltage to feed high wattage loads and the centre tap
connected to the neutral return (which is tied to the local substation
earth). Lighting circuits and low wattage appliances make use of just
one of the bi-phase lives (hots) and the neutral.

 This has the advantage of a reduced electrocution risk and, as far as
GLS tungsten filament lamps are concerned, allow a more robust filament
for any given wattage to be used compared to the UK and european
standards of 240 and 220 volts respectively[1].

 Essentially, for a given wattage, a 115v lamp can use half the filament
length at twice the CSA compared to its 230v counterpart. This gives the
choice of either the same life with a higher efficiency or a longer life
at the same efficiency compared to a 230 volt lamp.

[1] Although the EC have 'Harmonised'(tm) the domestic PSU voltage
levels to a nominal "230" volt, the allowable tolerances in each country
of the EC have simply been adjusted to avoid the need to actually make
any change to the original 240 and 220 volt supplies.

 Most appliances designed for a nominal 230volt supply will work
perfectly fine on either voltage but tungsten filament lamps are an
exception to this rule since they are extremely sensitive to the effects
of voltage variation on their service life and efficiency and are
therefore designed for the voltage used in the region they are marketed
in. In this regard, the modern electronically ballasted CFL has the edge
over the traditional tungsten filament GLS lamp.

--
Regards, John.

 Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.


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