240VAC single phase from three-phase?

[amdx]

mike

Where did ya go?
Was my argument persausive or did you just give up trying to
persaude me.
Mike (amdx)

 

[You]

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.

The above is EXACTLY why you don't understand what is REALLY GOING ON...
If you had just spent a little time listening in EE 101, instead of
sleeping, you would know just where your assumptions above, are leading
you down a Blind Alley, and why thew rest of us are laughing at your
ignorance.....

 

[daestrom]

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!

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

 

[Johnny B Good]

The message <[email protected]>
from "Josepi" <[email protected]> contains these words:

====snip====

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-------------------

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.

 

[Ulysses]

I finally got to the vacuum press and got it wired and everything is OAK :-D
The pump is 1/3 HP and runs from 120 VAC single phase so that was easy. I
never did check the voltage at the 240 outlet in the unit I moved from but
there is very little doubt that the power there was also three-phase so the
press was almost certainly running from 208 volts there. In any case we
are not noticing any difference in the operation of the pump or heating
elements.

Thanks everyone for all the help!

 

[Ulysses]

I finally (yesterday) got the vacuum press connected and everything is AOK
:-D The pump is 1/3 HP single phase 120 VAC so that was no problem. The
elements seem to heat up at the same rate they did before. I did not check
the voltage in my other unit but there is very little doubt that it was also
three-phase so it was probably running from 208 volts before. Now for the
neon signs....

Thanks everyone for all the help!

P.S. I tried to post this from my laptop and it wouldn't send so I hope this
isn't a duplicate.