Residential Power

[SmokinJoeR]

There seems to be some controversy on the internet wherever I check on how 240 volts is brought into your house. Some sources say that the electrical output of the transformer on the power pole is a 3 wire single phase, or others say it is 2 single phase 120V that have 180 deg of phase difference. I know the transformer has a secondary that is center-tapped, but I could never get the clear answer whether or not its 240V/single phase and (2) 120V/single phase output or (2) 120V with 180 deg of difference between them.

Anyone know for sure?

Thanks!

 

[Marmite]

It's all a question of terminology.In the UK the two phase centre tap arrangement is called split phase. When I went to work in Bermuda, which applies North American standards I found it confusing when contractors would ask for 200A single phase because to me that meant 200A 120V. What they wanted was 2 phase 200A. Your three descriptions are all the same. You have a centre tapped transformer. The centre tap is neutral. Between either of the phases and neutral you get 120V. Between the two phases you get 240V. This is commonly referred to as single phase, but in my view is actually 2 phase.
Regards
Marmite

 

[davidbeach]

Marmite, sorry; it is not 2 phase. The 240 is a single phase. There is also a center tap that provides two 120V single phase connections phase-to-phase.

2 phase is something else all together and now quite rare. 2 phase could use as few as three conductors, two phases and a neutral but in that case there is a 90[°] phase difference between the two phase conductors, often referred to as a quadrature connection.

Think of it this way - a single phase circuit can not create a rotating magnetic field in a motor with out use of axillary connections such as capacitors or shaded poles while a 2 phase or 3 phase circuit can create a rotating magnetic field without the need for other connections. The 240V of a 120/240V circuit cannot create a rotating magnetic field any more than either of the 120V. Therefore it is single phase.

 

[Marmite]

David
I'm interested in how you would correctly distinguish in terminology between a 240-N single phase supply and a 120-0-120V single phase supply. ie if you ask for a 240V single phase supply how do you know which one you will get? In the UK single phase is single phase, the centre tapped arrangement is called split phase or split single phase, and three phase is obviously three phase.
Regards
Marmite

 

[davidbeach]

Nothing that will run on 240V will care or know which version of 240V, because there is no means of distinguishing them. If you throw in ground (earth) as a reference, then you could make a distinction, but what ever is using the 240V won't care.

 

[Marmite]

I went looking for some defintions and came up with this website, which explains the differences very well. It also has information on the historic 2 phase system.
Regards
Marmite

http://www.answers.com/topic/split-phase

 

[waross]

[rant/on]
In North America, A 120/240 volt supply is single phase. To call it two phase is fun with words and vectors. If you want to refer to the center tap as the origin, then consider one winding with the polarity reversed rather than rotated 180 degrees.
If you must call a center tapped supply two phase then I will feel justified in calling two 12 volt batteries in series with the center tap brought out a two phase DC system.
Please don't use the term "180 degree phase shift" to describe a reversal of polarity. You have not "shifted the phase", you have just changed your reference point.
[rant/off]
respectfully

 

[jghrist]

A google search of "electric '120/240' phase" finds many utility websites referring to single-phase service, but none calling it any other kind of phase:

PGE - "single-phase 120/240 V"
City of Concord - "Single Phase 120/240 volt"
City of Dowagiac - "single-phase, three wire, 60 Hertz, at approximately 120/240 volts"
Nipsco - "120/240 volt, 1 phase, 3 wire"
Magic Valley Electric Cooperative - "120/240 volts, 3 wire 1ø"
Cuero Development Corporation - "single phase, 120/240 volts"
City of Riverside - "single-phase service voltages are 120, 120/240, 120/208, 240 and 480 volts."

If you want to be understood by your electric utility, refer to it as single phase.

 

[stevenal]

To add to the confusion: Some single phase transformers have dual secondaries (four wires). The two windings can be connected in series to provide 120/240, or in parallel to provide full kVA capacity to a 120 V circuit.

 

[waross]

Hi stevenal;
Virtually all single phase distribution transformers have dual secondaries. A very few have 4 secondary bushings. The majority have the series or parallel connections made internally. It is common for standard distribution transformers to be opened and the normal series connection changed to parallel for 120/208 volt installations.
An exception would be 277 volt transformers for 277/480 service. These transformers have only one winding.
respectfully.

 

[jraef]

I know of no place in the US where there is a 240-N 1 phase distribution. That doesn't mean it doesn't exist, but that I believe it would be the exception. 1 phase is, as far as I know, always referring to 120/240 (nominal) 3 wire distribution system; 2 hots and a neutral. You have 120V to N on either side and 240V Line to Line. The 240V is the same phase, just opposite ends of the winding.

 

[itsmoked]

Jeff you left out 120V service. We have a lot around here. Just 120V period.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rbulsara]

My two cents to affirm why a center tapped 240V is still a single phase system.

1. The two 120V measured from two lines to the center are in phase with each other and not so 180 out of phase. The vectors are ----->-------> and not <------.------->.

2. Also the such trasformer's primary are fed by single phase supply, or only two wires the very definition of a single phase circuit.

3. Speaking of the definition, 240V center-tapped supply can only feed loads with two wires either 240V or 120V. The third or neutral wire is only for 120V loads. There is no such thing as two phase 240V load.

4. To add to confusion (or explanation), many residences in USA are fed by two phases of a 208Y/120V 3 phase system and the neutral. Even there, the service at each house still called (and are) single phase 120/208V system, because they can only feed two wire circuits, (again the definition of single phase circuit), either 208V or 120V. There is no two phase 208V load or appliance.

 

[rbulsara]

To emphasize point 1 in my last post..the net voltage of two 120V, 180 deg out of phase voltages will be zero, which is not the case in a 240V/120V center-tapped system.

[I will stop my rant]

 

[sslobodan]

Well you are running out of the point
Its very simple we all have 3 phase system because it is the smallest phase number system that will allow motor rotation (copper savings in wires) it can be done with 4,5,6... phases cant be done with 1 or 2, but most equipment does not need three phase system and they are on voltage rating phase to neutral.

So you have 2 kinds of voltage ratings
phase to neutral and phase to phase. does not mater if your base rating(phase to neutral) 120V/60Hz (US) or 230V/50Hz (Europe) or some other voltage level principles are the same.
Difference between phase to neutral and phase to phase system is in the angle that two phases have between them selves If you take book of basics of electrotehnics you will find very nicely explained why the difference is exactly sqrt(3) between these two voltages.

So you calculate voltage between two phase and have 3 phase (3wires)or 3 phase and neutral system (4wires) or 3 phase neutral with grounding (5wires)
4 wire system is usually used in distribution systems to end consumers because of save of copper (4 instead of 5 wires) and you have your own grounding system that will make you 5 wire system in house. Distribution system that transmit power between two transformers is 3 wire because load is balanced (98% times at least)and you don't need 4th wire between two transformers.

 

[jghrist]

Its very simple we all have 3 phase system because it is the smallest phase number system that will allow motor rotation (copper savings in wires) it can be done with 4,5,6... phases cant be done with 1 or 2, but most equipment does not need three phase system and they are on voltage rating phase to neutral.

Great! Now we find out that there is no such thing as a single-phase motor.

 

[sslobodan]

Of course there is... don't be sarcastic but you know why people use 3 phase motor.... I was implying rotary field as a breakthrough in electro energetic, and I would love to see how would you make that with 1 phase system....

 

[davidbeach]

A two phase system, with the phases in quadrature, can produce a rotating magnetic field. Three phases are not the minimum required for a motor that does not require capacitors or other aux devices (shading poles, etc.). Three phases are, however, the minimum required such that the instantaneous power - and torque - is a constant rather than pulsating with each half cycle.

 

[rbulsara]

sslobadan:

I am trying to be patient here. Take a deeeep breath! The key is to understand that single phase circuit means that it uses only two wires, one supply and one return. The return does not have to be a neutral!

A circuit fed from two phases of a three phase system is still a single phase circuit. For example, a sigle phase pole mounted trannsfomer more street power, fed from two line wires of a 3 phase system on primary side.

Other way of looking at it, if you draw a phasor of voltage between the two points of a single phase source, it will be a single arrow or sum of two or more arrow in the same direction and angle.

A true two phase system will have "three" wires, but phasors of the two voltages will have some phase angle between them, as in davidbeach's example in his last post.

The "single phase" motors are called single phase as they require only two wire connections and it is a blackbox to the source. The motor gets its starting torque by splitting the single phase using a capacitor. Now that is a split phase case in point, and not a center-tapped transformer.

 

[sslobodan]

I know you are right (I have just read lecture of DR Gerhard Henneberger few days ago concerning that subject) But we strayed from subject, and such discussion is the reason why I haven't mentioned 2 phased and single phased engines. tread is about difference in interpreting system marking on internet. ("There seems to be some controversy on the internet wherever I check on how 240 volts is brought into your house....")
And making deep theory about engine types and their construction is a little bit out of subject... Don't you agree?