120 V 2 phase system

[13392017]

All,
Please give me a shed of light on following things.

What does it mean 120V 2 phase system, is the 120 V is line voltage or line phase ? how we can have this system ?

If i have equipment required 120 V 1 phase system(with neutral, how do i connect to the power system above ?

Cheers,
RK

 

[itsmoked]

That's not a common description.

It could be two legs of 208V where each lead is one of the three phases. You would have to have the neutral to provide 120V. This would be a WYE setup and has become very common in recent years.


Completely different would be the classic center tapped 240V normally serving homes in the USA. Either leg to the center tap provides 120V. Some would argue that it's not two phases. I wouldn't. But to avoid arguments most would call this "240 with a neutral", or "center tapped 240", or some other more direct variant.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Where do you live?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MAGTiger]

There are 2 phase systems where the phases are separated by 90 degrees. Typically this would be 4 or 5 wire where the 5th wire would be a common point.

Neil

 

[X49]

The most common single phase service in North America is 120/240V. It is called a single phase service, but technically there are two phases whose waveforms are 180 degrees apart.

Here is a schematic diagram showing a 120/240V transformer:

http://laserpointerforums.com/attachments/f42/20284-creating-240v-two-120v-outlets-centertap.png

There is 120V between each "phase" and neutral, and 240V between the two "phases". To connect a 120V load, you would connect the load between one of the 120V legs and neutral. (Make sure that this is indeed the type of system you have first).

 

[jraef]

There are some semantics issues at play here.

Here in the US 9and maybe Canada?), there are some legacy "true 2 phase" systems in use near Philadelphia and Niagara Falls. This is likely why waross asked you where you live. But in the big scope of North American power distribution, the number of active true 2-phase system installations is infinitesimal, and shrinking because they are not installing new systems as far as I know. Anything new is 3 phase.

In other parts of the world, the term "2 phase" is used to describe "2-out-of-3" phases, in other words you are connecting to Phases A and B of a 3 phase distribution system. Here in North America we still call that "single phase". This is what Keith (itsmoked) was referring to when he mentioned 208/120V because in a 3 phase Wye system, the line-to-line voltage is higher than the line-to-neutral voltage by a factor of 1.732, so 120V x 1.732 = 208V. This is a common distribution system for light manufacturing and commercial installations.

To make matters just a tad more confusing, we have a somewhat unique (from the viewpoint of the rest of the world) residential distribution system where we take single phase power, as in 2 out of 3 phases, from the utility pole as the primary to a transformer and connect a center tapped secondary that provides 240V from line to line and 120V from line to neutral (the center tap point). We still call that "single phase" as well. This is what X49 is referring to, although I think most of us would agree that we generally do not refer to the two ends of that center tapped transformer as "phases" just because it confuses the masses.

But nowhere do he have a system that is 120V line-to-line, 2 phase. Someone is confused, it happens a lot with people unfamiliar with the quirks of our system.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[ScottyUK]

jraef,

We have, or at least had until fairly recently, some legacy 2-phase quadrature distribution out in northern England serving some of the remote farming communities. I suspect it was probably used in conjunction with a Scott transformer at the load end to allow 3-phase equipment to be used. Maybe it has all gone now, certainly there won't be much left.


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

If we learn from our mistakes I'm getting a great education!

 

[stevenal]

faq238-777