Phase question

[mivey]

I believe that the 'battle of the phases' disagreement is not quite applicable to the original poster's question, however during that discussion we raised many relevant points.

The 'battle of the phases' disagreement was about the meaning/value/interpretation of a '180 degree phase difference'.

However it sure sounds to me as though the original poster is discussion a situation where there is a 120 degree phase difference. "how he defined 2 phase alternating current as taking two phases from three phase a/c transformers ".

In this case, there are clearly _two_ separate phases present, and you clearly do have a polyphase system

Having said that, describing a supply with two separate phases 120 degrees apart is not one of the classic usages of the term 'two phase power'. All of the classic usages involve a 90 degree phase difference. So while there are two phases present, using the term 'two phase power' would be misleading.

If it is appropriate to identify this as a subset of a three phase system, then it should not be called 'two phase', but should instead be described in some fashion that names the three phase system. For example 'open wye'. But often it is not necessary to identify this as a polyphase system. If you are talking about service into a home, with two hots and a neutral from a _wye_ secondary, that service is _called_ a single phase service. (The name does not describe the phase angle differences present...but it does describe the usage!)

-Jon

One prevalent term for this type system is "network". I like the way you noted the "classic usage" for 2-phase. This is what Jim means by keeping in context. In that context, we should all know what is meant by 2-phase.

What if our forefathers had used an 85 degree phase angle difference? Some would be sold out on an 85-degree 2-phase. I just think it is possible for other systems to exist other than the classic 1, 2, or 3 phase systems we know. As a matter of fact, I believe winnie has proven that to be the case.

The classic 2-phase was called 2-phase because there were two phases, not because it fit the strict pattern of a polyphase system. If they would have had 3 phases with a 90 and 135 degree phase difference, we would be calling that a classic 3-phase system.

 

[jim dungar]

First there was single phase (at 133Hz) with its 120/240 version. Then Tesla created a two phase induction (asynchronous) motor, using a 4-wire system. Then three phase was created as an improvement over two phase. Then people discover you could have even phases.

It is not possible to study the history of AC power without understanding why 2-phase was such an important discovery versus the 3-wire single phase system which was competing with Edison's DC offering.