I can take a buck boost transformer that has four taps on the secondary
and ground one of those taps. I will have three different voltages
relative to the grounded tap. Is it your position that I know have a
three phase transformer?
Why does a 240 volt motor that is connected to a center tapped 120/240
volt seccondary need a capacitor to make it turn?
again, are these voltages out of phase? if not, no you don't have different
phases, but if the voltages are not in phase, then they're different phases.
the 2 120V lines most people get in their residential units are 180 degres
out of phase, which is why together they can make 240V.
because them otor is being fed a single 240V phase and it needs at least 2
phases to run, the capacitor gives a 90 degree phase shift.
Make up my mind! Either the two conductors that supply the 240 are a
single phase or they are two phases. There is no difference between my
buck boost transformer and the utilities center tapped single winding
secondary service transformer except for the number of taps. They both
have a primary that is connected to one grounded conductor (the Multi
Grounded Neutral [MGN] in the case of the utility transformer) and a
single conductor that has a voltage relative to the grounded conductor.
They both have points on the single secondary winding that are tapped
so that different voltages can be supplied from the same transformer.
If the utility transformer is two phase why isn't my multi voltage
secondary with three voltages relative to the grounded tap three phase?
if you take a 3-phase 120V/phase Wye connection, line to line you get 208V
Are the 3 phases separate or the same phase?
If you take the line to line voltage, (using one phase as the neutral) do
you get another phase of 208V or is it the same phase?
I dunno, by the basic definition of "phase", since the 2 120V lines in the
house are 180 degrees out of phase, they are different phases. If they're
not in phase, they can't be the same phase...
On 11/2/03 10:45 PM, in article J3lpb.1067$ email@example.com,
JJ, look at it this way. Your second statement is true. I'm not sure
about the amount of degree shift. That means the first one can't be since
the 240 volt motor would run fine without a capacitor off residential
power. The residential power would supply the phase shift to start the
motor without the capacitor.
Power companies supply either single phase or three phase in my area.
This is in a rural area so the wires and transformers are up on power poles.
Three phase has 3 wires plus ground. Single phase has one wire plus ground.
The supply to the farm yards is generally single phase 240/120. This is a
really crude diagram of a transformer for the single phase 240/120 power.
The top line is the utility's primary high voltage connection.
The bottom line is the consumer's supply
secondary winding. Three wires. Two "hots" and the "neutral". This is a
single winding but it has a tap in the middle to provide the 120 voltage
line to neutral. There would be only 240 single phase without the center
tap. The secondary would be something like this:
Two wires, single phase just like the power company's supply voltage.
I did not include the equipment grounding conductor in either of the
The power company's supply voltage to the transformer is single phase so
the 120/240 to the consumer has to be single phase. There is nothing in the
transformer to create a time/degree shift.
actually when I was asked that question, I thought about why a motor
wouldn't work with the 2 phases in the house: since the phases are exactly
180 degrees out of phase, the resulting magnetic field in the motor just
pulsates, it doesn't rotate, and it's not enough to make the motor turn (at
least in an induction motor). a single phase induction motor will need a
capacitor to start because the capacitor gives it a 90 degreep hase shift,
and the resulting magnetic field will rotate.
but aren't the two hot wires out of phase by 180 degrees? (when one is at
positive peak, the other is at negative peak)
You have to change the way you define "phase" for yourself.
A phase, or multiple phases, are segments of a circle.
Multiple voltages, or "legs" derived from a single source, (only 1 phase
segment,) are multiple "taps" or "legs," not each an individual phase.
This is why you'll see most 3-phase equpiment labled A-B-C or Phase A - Phase B
- phase C , and most 240v volt equipment, especially residential, labled "L1 &
L2" meaning, leg (or line) 1 and leg 2.
On 11/3/03 5:59 PM, in article pZBpb.4511$ firstname.lastname@example.org,
No. What you actually have is 240 volts single phase. It's the tap
in the center that gives you the 2 120 volt hot legs. That tap could be
moved to change the output to, say, one line at 216 volts and the second
line at 24 volts. The total voltage would still be 240. The transformer is
just taking 7200 volts single phase and cutting it down to 240 single phase.
Here are a couple links that might help. There are 2 or 3 pages at the
first one. There's a next button way at the bottom to change pages.
The second one is an old military training manual. There are some
explanations for generators and transformers.
As someone mentioned the degrees out of phase refers to the rotation of
the generator making the power. One phase would be X degrees out of phase
with another. That's not possible if only one of the power company's lines
is being used. Look at the differences between a single phase generator and
a three phase generator at those links.
look, I know this already.
you have 3 wires, one neutral (the reference) and two phases or legs or
whatever you want to call them.
take an oscilloscope and measure the two voltages taken with respect to the
neutral. Are the two sinusoidal signals in phase or out of phase by 180?
They are "in-phase", only the _polarity_ has changed. Do they cross the
zero line at the same time? Do they peak at the same time? You're
referencing the _same_ waveform with respect to polarity _only_. How can a
single waveform be "out-of-phase" with itself?
well, I don't know what your definition of phase is.
where I live, the two waveforms are out of phase by 180.
in a shorted transmission line, for example, the reflecting wave is out of
phase by 180 while in an open transmission line, the reflection is in phase.
I didn't hear anyone say that the reflection in a shorted TL is in phase,
but reversed polarity.
Do they cross the
but there's 2 waveforms. I get 2 hot wires, and the waveform is not in
take a generator with 4 (separate) poles. the 4 waveforms are out of phase
by 90, so the third is out of phase by 180 with respect to the first, and
the fourth with respect to the second.
same goes for any even fraction of PI.
That's all fine and good. One doesn't need hands-on experience in power
distribution to comprehend, or rather in this case, *accept* that there's a
difference between a "phase" and a "leg", or that you don't have 2 "phases" in
a home just because you've been calling the legs phases for years.
Many electricians, even utility workers will use the term phase when making a
distinction between the 2 legs of a single-phase 240v service. The trouble
starts when one tries to apply practical electrical theory to this misnomer.
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