From: dhky@shaw.ca   
      
   On 20/02/2014 5:54 PM, Tom Horne wrote:   
   > On Tuesday, December 3, 2013 12:07:05 PM UTC-5, bud-- wrote:   
   >> On 12/3/2013 8:25 AM, trader4 optonline.net wrote:   
   >>   
   >>>   
   >>   
   >>> And I'm still waiting for an answer why a system with   
      
   >>> two phases that differ by 90 deg is acknowledged by everyone to   
   >>> have two   
      
   >>> phases. If they differ by 240 deg, that's two phase right? If   
   >>> they   
      
   >>> differ by 170 deg, that must be two phases, right? So, what   
   >>> magically   
      
   >>> happens when they differ by 180 deg that suddenly there are no   
   >>> longer two phases? And how do the electrons know?   
      
   >> For a garden variety split-phase supply (240/120V from a   
   >> transformer   
     with a centertap) are there 2 "phases"?   
   >   
   > If you only have one phase supply to the transformer how can you get   
   > a two phase output.  If I put a multi tap transformer on that pole do   
   > I get a multiphase output?   
   >   
   > What about three phase, high leg delta were one of the transformers   
   > is up sized to carry the 120 volt loads and there are three   
   > ungrounded current carrying conductors off the secondary.  I suppose   
   > someone is going to argue that is four phase.   
   >   
   > -- Tom   
   >   
   There is a definition problem - In North America the usage for a single   
   phase center tapped system is simply "single phase center tapped" (or   
   the "Edison" system because it works for DC as well as AC and Edison   
   used it). However if you consider a balanced N phase system as having 2   
   voltages 360/N degrees apart it is technically  a balanced  2 phase (as   
   with equal loads on each leg, there is no neutral current) and is often   
   called that in places where it isn't used.   
     If you have  some other relationship such as a 60 or 90 degree shift   
   it is unbalanced 2 phase according to this as there will be neutral even   
   with balanced loads.   
    From a point of view of rotating  machines, the balanced system is one   
   that produces a single unidirectional  rotating magnetic field. A two   
   phase 90 degree phase shift system and a balanced 3 phase system do   
   this- but the 180 degree shift doesn't do this. Use of 2 phase in motors   
   is pretty limited- often for control motors but even there steppers have   
   taken over.   
   The high leg delta is, in operation, with single phase loads on one leg,   
     simply a delta with an unbalanced load. A Y can have single phase   
   loads as well - but they can be distributed to get a balanced or near   
   balanced load.   
      
      
      
   --   
   Don Kelly   
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