Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Two Phase Power

Two Phase Power

Two Phase Power


I apologize for the dumb question, but what the heck is two phase power? You see it sometimes in old electrical books, in the UGLY's handbook, and people sometimes refer 220/1 sinlge as 2 phase.

Is this just bad nomenclature, or is it something for real?



RE: Two Phase Power

Well, as you said it is old.  It is also called "V" phase.
Simply put, you are only using 2 out of the 3 phases of a 3 phase system.
It was very common in the oil fields.  They did it to save wire.  The 3 phase was "created" at the motor using capacitors.
If you had to string wire for 10 miles to get to a pump, saving the wire from one phase was a lot of money.

RE: Two Phase Power

Two-phase electric service is definitely for real.  See NEC Table 430-149 for 2ø motor ratings.  

The current iteration of the GE kV-series watthour meter is configurable for 2ø 5-wire service.

Philadelphia Electric Company lists 120/240V 2ø 5-wire as one type of electric service in their territory.  They also list 2400V 2ø 3-wire as available, but probably not for new work www.exeloncorp.com/peco/library/documents/1999tariff.doc Two-phase circuits properly rely on 90° phase displacement between conductors.  Amazing, but then think of the area as quite a hotbed of AC power delivery over the last century, and the desire for offering electric power in various forms to stimulate a regional economy.  

Charles F Scott’s invention of a workable three-phase to two-phase transformation was an important step in facilitating a practical power-distribution system.  See IEEE Industry Applications Magazine, Nov/Dec 2002  Pp.6-7  

RE: Two Phase Power

Never heard of that before. I suppose if your talking about two phase power on a three wire system (or four for that matter) then saying 2 phase is ridiculous.

Never new they had 5-wire 2Phase service before.

RE: Two Phase Power

Dear Buzzp,

Busbar is right about the two-phase system. This is a recognized system by NEC, ANSI/IEEE Std. C57, others standards and classical references.

Even though most power systems are configure either for single phase or three phase, there are options that include two-phase.

One way to obtain two-phase is connecting two transformers, one with a center tap called a main and one with an 86.6% tap called a teaser. The teaser is connected to the center tap of the main, and the start, finish of the main and 86.6% tap are all connected to the 3 wires of a 3 phase supply. The secondary on the teaser and the secondary on the main each produce a voltage, which is 90 degrees electrically from each other. These two voltages make up the two-phase supply.

This connection was named after its inventor, Charles F. Scott, the "Scott T" connection is a method used for changing 3 phase to 2 phase or vice versa. The Scott T connection is the most economical.

Check the enclose site for wiring diagrams options:

On October 9th, 1888, Nicola Tesla developed a two-winding two-phase (windings displaced 90 degrees) generator and motor. The same year Charles Scott joined Westinghouse Electric and Manufacturing Company in Pittsburgh, PA, and collaborated with Nicola Tesla.

One of the first practical applications of two-phase was the generator at a large hydroelectric plant at Niagara Falls, NY . Out of two phase from the hydro plant, C. Scott converted to three-phase power feeding a transmission line to Buffalo, NY, in 1896.

RE: Two Phase Power

Suggestion: There is a different two phase power application in the vector control of asynchronous drive for direct control of electrical induction motor torque  over the two-phase d-q transformation into d-q two-phase system, which in turn is converted into three phase system. Visit
etc. for more info

RE: Two Phase Power

Lewish, you may be describing a transformer arrangement also termed “open-wye primary/open-delta secondary.”  It is discussed on page 12 figure 21 of cooperpower.com/Library/pdf/R201902.pdf  Page 15 figure 27 illustrates a Scott-T connection.  Page 5 figure 7&8 display 2ø 4- and 3-wire connections.

RE: Two Phase Power

Is split phase, which is sometimes called two phase, 220V power in residential, taken from a center tap transformer which results in each leg 180 out from each other, i.e. inverted?

It can get confusing.

RE: Two Phase Power

Hi Chris,

I'm not going to directly answer your question - the US guys will, no doubt - but two-phase power as found in the UK has two phases in quadrature. You can't get that from any normal transformer, but there are arrangements such as the Scott (I wonder if he was a relation?) and LeBlanc connections which employ special connection and ratio arrangements of multiple transformers to convert 2-phase into 3-phase and vice-versa. This is, I suggest, a rather different animal from what you have heard described as two-phase power. Of course, terminology in the US often uses familiar words to mean something totally different, so that may be the case here!


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

RE: Two Phase Power

My 0.02 Euro worth:

Nicola Tesla started with two phases. One sine and one cosine, i.e. 90 phase angle. The reason seems to be that he had learned in school that these two signals describe a circle, and hence a rotation, when plotted in an X/Y coordinate system. And from there he worked out that two electric signals in two coils would create a rotating magnetic field, which could be used to make a rotor with conductors on it go round. And that is how the asynchronous induction motor came about.

Westinghouse in USA and Wenstrom in Sweden figured out that a three-phase system would be more efficient and started production of three-phase generators, transformers and motors.

There are also other two-phase systems. On that has a 180 degree phase shift is used as a back-bone for electric railways. It has two wires which carry 65 kV to ground and having 180 degree phase shift means that the voltage between the lines is 130 kV. Since the overhead line cannot be anything else than single phase (OK. DC is also possible, but that is single phase, isn't it?) As said, since it can only be single phase, it needs power distribution on a single phase line - or a two phase line with 180 degrees phase shift.

Of course, a load that is connected between the two phases of a three phase system sometimes is referred to as a two phase load. But I wouldn't subscribe to that. It still is a single phase load.

So, in order not to be confused, it is best to have a look "behind the wall" and see how power is connected.

Gunnar Englund

RE: Two Phase Power

However, the center tapped primary on the 3-phase side of a Scott connected transformer has higher KVA than the secondary of that transformer that is on the 2-phase side. That is, 1 of the transformer winding operates at less than 100% internal power factor much like how an open delta vee transformer operates at 58% internal power factor. A better arrangement is to use a balance coil that provides what is effectively 4-wire delta 3-phase so that the 2 main transformers operate at 100% internal power factor.

At one place where I worked they had 2 induction melters for aluminum investment casting that had dedicated 3-phase 3-wire to 2-phase 4-wire transformers to run the induction coils.

Another way to get 2-phase power from a 3-phase wye system is to connect 1 transformer primary A to B on the 3-phase ystem and connect the other transformer C to neutral. For say a 7,200Y12,470 volt system you would use a 7,200 volt transformer and a 2 primary bushing 14,400 volt transformer which has ANSI taps for 13,800; 13,200; 12,500; and possibly also 12,000 and 11,500 volts.

The 2-phase system proved to be not as efficient because you either needed an extra wire or in the case of 2-phase 3-wire the neutral carries 141% of the phase current. Also turns out that iron nonlinearities in motors cause 2 phase motors to have multiple torque dips in the accelaration curve. These nonlinearities are much easier to balance out in a 3-phase motor.

Also, a lot of older railway power such as on the Reading Railroad ( which operates off of a hydroelectric plant ) was 2-phase 25 Hertz for a number of reasons. One of them was balnacing a 2-phase system is easier given that a mainline is 2 tracks or 4 tracks. Single phase generators over a certain size do not work because single phase power has pulsating instantaneous power and really does produce a counterrotating electromagnetic field in generators. The reasons for 25 Hertz are that larger series wound motors have a maximum frequency of 25 Hertz.

Actually, as machine size and torque go up and machine speed goes down, the optimal frequency for an induction motor is on the order of 15 to 30 Hertz. Steel mills produce their own 2-phase 4-wire low frequency power using cycloconverters for each large motor. Using a dedicated cycloconverter for each motor the speed of each motor on a rolling line can be tweaked for the required speed of the actual rolling dies. For larger drives a cycloconverter is a more efficient drive that a pulse width modulated variable frequency drive.

RE: Two Phase Power

Yes mc5W, the frequency of the railway system I described is 16.667 Hz - which fits nicely with your 15 - 30 Hz range.

Gunnar Englund

RE: Two Phase Power

Hi Mike,

I'm slightly confused by your term 'internal power factor'. Can you explain a little more? It's not a term I've come across before, although I'm quite familiar with the Scott-connected pair.


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

RE: Two Phase Power


Didn't Tesla also invent and patent all of the 3 phase technology for Westinghouse as well?

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close