why the two out of phases is called single phase ??

[magdyfayad]

I have answered you for one month ago in PM and you wrote to me it is correct

I have not never seen these posts till today !!

 

[Alan64]

As far as I know in most of the High voltage sections the schema is tri phases without any neutral. The step down transformer is selected in accordance with the requirements (power , voltage, neutral)
The most common tranformer schema is called DyN11
DyN11 means :
D primary Delta
y Secondary Star
N Neutral wired (center tap)
11 330 Deg (11*[360/12]) phase shift between primary and Secondary

If the neutral is wired (center Tap)then it can be used for single phase.

 

[audi2000]

Relative phases

a single phase motor use 2 phases 120 degrees out of phase. A and B, measuring from A to B gives only one phase measurement. A 3 phase system uses 3 phases, A, B and C. Measuring phases to phases yields A-B, B-C, and C-A. 3 phase to phase measurements. The nomenclature is confusing. Hope this clears up the confusion.

 

[Sliver]

close, but not quite.

When considering single phase sources it is helpful to look at them relative to a reference usually a grounded nuetral. A 120 volt source would not be considered out of phase from the nuetral but is still a single phase source. A 240 volt source would consist of 2, 120 volt sine waves that are 180 degrees out of phase with each other (usually) and when referenced to each other create a larger single phase sine wave. I just wanted to clarify that the 120 degrees comes from the 3 phase sine wave phase shift. Otherwise spot on.

 

[audi2000]

oops

When considering single phase sources it is helpful to look at them relative to a reference usually a grounded nuetral. A 120 volt source would not be considered out of phase from the nuetral but is still a single phase source. A 240 volt source would consist of 2, 120 volt sine waves that are 180 degrees out of phase with each other (usually) and when referenced to each other create a larger single phase sine wave. I just wanted to clarify that the 120 degrees comes from the 3 phase sine wave phase shift. Otherwise spot on.

yes, change 120 to 180

 

[iant]

Guys well answered but this post is many years old

 

[JRB]

Guys well answered but this post is many years old

Actually, all 3 of the last posts are a little off.


"a single phase motor use 2 phases 120 degrees out of phase."

This statement is true, if the single phase motor is supplied by 2 phases of a 3 phase system.

This statement is NOT TRUE for a single phase source, 120 OR 180 degrees out of phase do not
apply, since there are NOT 2 sinusoidal AC voltage waveforms present in a single phase system.



"A 240 volt source would consist of 2, 120 volt sine waves that are 180 degrees out of phase with each other"


A 240 Vac single phase source DOES NOT consist of 2 sinusoidal waveforms, unless it is derived from 2 phases
of a 3 phase system, in which case 2 sine waveforms 120 electrical degrees apart could be viewed simultaniously
on a dual channel input oscilloscope screen.

At any point in time measurement, the polarity of line 1, is 180 degrees opposite line 2 of a single phase
transformer secondary. Single phase power consists of only one sinusoidal waveform. Two seperate phases are
NOT produced as a result of a transformer secondary center tapped neutral, and therefore the single phase
that exists, cannot be 120 or 180 degrees out of phase with another phase that does not exist.

Even when 2 phases of a 3 phase system are used to supply the primary of a single phase stepdown transfomer,
the resultant waveform induced into the secondary, will consist of a single waveform. Only one phase is ever
induced into the single coil of a single phase transformer secondary winding, and as such, only one sinusoidal
voltage waveform exists in that phase. The single applied sinusoidal waveform, is the phase. Thus, the term
SINGLE PHASE.

When connecting a 240Vac single phase motor to a 240/120Vac single phase system,
1 sinusoidal AC voltage waveform is being applied to the motor.

When connecting a 240Vac single phase motor to 2 phases of a 3 phase 240Vac system,
2 sinusoidal AC voltage waveforms 120 electrical degrees apart are being applied to
the motor.

 

[Tom Jenkins]

Most 240 Volt systems in the US, particularly household systems, are split phase single phase system.

If you connect across V1 and V2 you get 240 VAC and the waveforms are 180 degrees apart. If you connect across V1 or V2 to ground or neutral you get 120 VAC.

http://en.wikipedia.org/wiki/Split-phase_electric_power

 

[Lancie1]

If you connect across V1 and V2 you get 240 VAC and the waveforms are 180 degrees apart.

Connecting across V1 and V2 produces one voltage measurement and one waveform. Now if you meant connecting between V1 to Neutral and V2 to Neutral, then that is two measurements and two waveforms.

 

[Tom Jenkins]

Connecting across V1 and V2 produces one voltage measurement and one waveform. Now if you meant connecting between V1 to Neutral and V2 to Neutral, then that is two measurements and two waveforms.

Point taken. (Such a simple common application causes so much confusion!)

 

[JRB]

The following statement is absolutely not correct, nor was it correct in the previous posts;

"the waveforms are 180 degrees apart"

Only ONE sinusoidal AC voltage waveform is imposed on the secondary of a single phase transformer coil.
When the single coil of the transformer secondary is center tapped, a second sinusoidal waveform does
not magically appear. It is only the polarity of the winding measured at line 1 and line 2 that is 180
degrees opposite of the other line at any given point in time. It is not out of phase with a second
sinusoidal waveform, and cannot be, as only one waveform (phase) exists in the coil.

Two voltages are produced by center tapping a single phase transformer secondary coil. Two seperate sinusoidal waveforms
out of phase with each other (by any degree) ARE ABSOLUTELY NOT produced by center tapping the transformer
secondary coil. The single phase points of measurement differ by 180 degrees in polarity, not phase angle.
The phase winding is split, not the phase waveform, nor is a second waveform produced.

From Wikipedia; "The two halves are 180 degrees apart with respect to center point"

-this is a reference to the polarity of voltages produced at any given point in time,
not a phase relationship between two phases.

 

[jraef]

Zombie thread lives!

Although the OP is likely now retired because of how old this thread is, the answer I did not see (although I admittedly did not peruse all 9 pages) is that the SEMANTICS of referring to 2-out-of-three phases as "single phase" has also to do with the fact that here in the US, we have a few pockets of TRUE "two phase" power. Older industrial parts of Philadelphia PA and Buffalo NY still maintain old 2-phase system, true poly-phase using 4 wires, the two phases are 180 degrees apart.

http://www.3phasepower.org/2phasesystems.htm

Elsewhere in the world where this possibility is unknown to them, they do in fact call 2-out-of-three phases "two phase" power, which confuses people here in this country when they come across it in product literature.

 

[iant]

Most 240 Volt systems in the US, particularly household systems, are split phase single phase system.

If you connect across V1 and V2 you get 240 VAC and the waveforms are 180 degrees apart. If you connect across V1 or V2 to ground or neutral you get 120 VAC.

http://en.wikipedia.org/wiki/Split-phase_electric_power

not in Australia Tom

240v L-N
420V P-P

 

[rachadwicknmsu]

explaining one voltage over another

I wanted to answer this unanswered part:

I cannot explain the exact reason we chose one voltage over another, but I can tell you that using obviously using a lower voltage is much safer than using a higher voltage.

The reason 240 is used over 120, is to reduce the amount of current that must flow to achieve a given amount of power. A dryer operating at 120 Volts would have to run quite a bit of current to generate enough heat, making the wires huge and costly (it is current flow that determines wire thickness, high voltage and insulation aside),, not voltage. so you run higher voltage for large power devices (dryer, range, spa motor and heater) so as to avoid running very large wires.

That should explain why some devices use the higher voltage. It is also true that higher voltage is more dangerous, but both 120 and 240 volts at 60Hz can be fatal.

When you get to really high voltages, the insulation in the wire and gap distances become important, but at lower voltages of 120 and 240, it is wire diameter cross section current carrying capability that is of concern. Note I was also confused a lot about why 2 hot wire 240 house circuits are called single phase. But the diagram does explain it. The signals are of the same exact phase in degrees of sine wave, there is just 240 volts across them, rather than 120 which is the midpoint of the coil (secondary). That is a better explanation for the main question, I think.

Note I am an experienced electrical engineer, really an "electronics" engineer, but not a electrical power engineer, and power has often confused me! Motors, EMF, etc., are in the power domain and not my area of expertise.

 

[iant]

At the end of the day it doesn't matter, it just is.
Welcome to the sitehave to use a better estop circuit as to meet regulations anyway.

All devices have a resistance
All devices have a resultant load (current)
The PUSH is the Voltage

As far as Australia is concerned we are the same sized country as the mainland of USA.
But we only have 20 million people living here

This means we need to push power further between power stations, hence, we use a higher voltage

The reason we have a lower frequency . I don't care really