From: Daniel47@teranews.com   
      
   Lieutenant Scott wrote:   
   > On Tue, 27 Nov 2012 14:49:56 -0000, Daniel47@teranews.com   
   >  wrote:   
   >   
   >> Lieutenant Scott wrote:   
      
      
      
   >>   
   >> Back to the beginning, forget that you have a Variac, just think of a   
   >> transformer with four connection points. The Primary winding is   
   >> connected to the mains and the secondary winding is connected to the   
   >> load.   
   >>   
   >> Current flows from the wall socket, into one terminal of the Primary   
   >> winding, flows through the Primary, and current then flows back to the   
   >> wall socket. No if's, no maybe's.   
   >>   
   >> The alternating current flowing in the Primary produces a fluctuating   
   >> magnetic field around the Primary winding. This fluctuating magnetic   
   >> field also cuts the windings of the Secondary winding. This induces a   
   >> voltage into the Secondary winding. If there is no load connected, there   
   >> is no load current flowing, just voltage at the Secondary winding   
   >> terminals. Because the is no load, load current will be zero!   
   >>   
   >> Now when you connect a load to the Secondary winding, the voltage   
   >> induced into the Secondary winding causes a current to flow from one end   
   >> of the Secondary winding, out through the load and back in the other end   
   >> of the secondary winding. Totally separate from the Primary.   
   >>   
   >> The Primary current has not gone anywhere near the secondary winding,   
   >> but the Primary current has caused the magnetic field which then caused   
   >> the Secondary current.   
   >>   
   >> Daniel   
   >   
   > Yes I understand a normal transformer.  But... in your example above, is   
   > the current flowing in the same direction in the primary and secondary?   
   > I know it's AC, but think of the first half of the sine wave.   
   >   
   > Now think of the Variac.  The "secondary" is actually half of the   
   > primary.  One half of the coil is just a bit of primary.  The other half   
   > of the coil is BOTH half the primary and all the secondary.  So this   
   > part of the winding carries TWO currents.  It matters if the currents   
   > are in the same direction, as they might either add or subtract from   
   > each other.   
   >   
      
   Not possible, Scotty, you cannot have both Primary and Secondary   
   currents flowing in one part of the (primary & Secondary) winding and   
   just Primary current flowing in the remainder of the (primary) winding.   
      
   Cannot happen!!   
      
   As for the phase relationships between totally separated Primary and   
   Secondary windings, this can depend on the load connected to the   
   Secondary (i.e. is the load purely resistive or capacitively reactive or   
   inductively reactive. And the phase relationship would also depend if   
   the "top" or the "bottom" of the Secondary is connected to the bottom of   
   the Primary winding!!   
      
   Forget a transformer.....think of two series resistors with a centre   
   take-off point, say a nine ohm resistor and a one ohm resistor with ten   
   volts applied across the combination. One amp of current would be   
   flowing through the two resistors, with nine volts developed across the   
   nine ohm resister and one volt across the one ohm resistor.   
      
   Now, if you connect another resistor across the one ohm resistor, you   
   don't get an increase of current flowing through the one ohm resistor to   
   provide the current that would flow through the additional resistor.   
      
   Daniel   
      
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