XPost: alt.home.repair   
   From: dhky@shaw.ca   
      
   On 03/12/2013 9:58 AM, Andrew Gabriel wrote:   
   > In article <529e1008$0$49510$c3e8da3$fdf4f6af@news.astraweb.com>,   
   > 	bud--  writes:   
   >> 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"?   
   >   
   > In maths/physics, you have two phases with a mathematical   
   > relationship of -x (or a 180 deg phase shift).   
   >   
   > In electrical distribution, two-phase is a specific jargon term   
   > which means something else.   
   >   
   > There's no one right answer - it depends on the context of the   
   > discussion.   
   >   
   Any supply with two voltages which are not in phase can be considered 2   
   phase  (3 or 4 wires).  "Balanced" 2 phase, mathematically speaking   
   (where for equal loads on each phase such that the sum of the currents   
   is 0), will have a phase difference of 180 degrees - as in  the 3 wire   
   120/240V system. This is not generally called 2 phase in North America   
   where it is commonly used.  "2 phase" generally refers to the  2 phase   
   90 degree shift system (sum of currents will not be 0) which, for   
   motors, has the advantage that, as for 3 phase, a rotating field with   
   ideally no pulsations is produced. Such motors are not common-no   
   advantage in general with respect to 3 phase motors. They have been used   
   as control motors in the 40's-80's.   
      
   With regard to comparative practices in distribution-this is a kettle of   
   fish with pros and cons either way which were determined by  (sometimes   
   poor) choices made over 100 years ago. Practice depends on the past as   
   well as factors such as housing styles, lot sizes, peak and average   
   demands, price of copper, etc with compatibility with the past being a   
   major  factor.   
   While, in many places 3 phases are run down each street, in many areas   
   only a single phase is used.   In my district a single HV phase is run   
   underground supplying several pad mounted transformers- each feeding a   
   few houses. Heavy air conditioning loads are not a concern.   
      
      
      
   --   
   Don Kelly   
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   --- SoupGate-Win32 v1.05   
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