From: dhky@shaw.ca   
      
   On 18/09/2014 5:59 AM, tctomcosby@hotmail.com wrote:   
   > On Sunday, September 7, 2014 4:51:44 AM UTC-4, Daniel wrote:   
      
   > Tom Tech,   
   >   I somewhat stumbled upon this interesting discussion regarding the power   
   transfer in iron core transformers via leakage flux or core flux. I have given   
   it MUCH thought and experimentation and research thru many college and   
   manufacturers text books    
   regarding transformer theory and construction. Until I took a much deeper look   
   into the transfer mechanism, I too was convinced thru what I thought was a   
   common view that energy is transferred via the core from one winding to   
   another. I do not hold that    
   view today. Several things can shed light on the somewhat hard to see transfer   
   mechanism. Yes, alternating flux in the iron core is responsible for the   
   induced EMF we see in any winding around the core, however when secondary load   
   current flows (assume a    
   resistive load), the secondary H field tries to reduce the core flux, and does   
   for a very brief period, reducing the induced  "Back EMF" seen in the primary.   
   More current flows in the primary to maintain the core flux, however, regardle   
   ss of how high the core permeability is,"leakage Flux" will be produced. The   
   term "leakage flux" is perhaps a misnomer, as it implies a fault in materials.   
   Under load conditions, the "Leakage" flux will always be produced. This H   
   "leakage" flux is in    
   phase with the primary voltage and in phase with the induced EMF in the   
   secondary winding for "Real Power". Note that the core flux does not change,   
   at least not much, maybe a little due to IR drops in the primary winding. If   
   you work out the Poynting    
   Theorem, it will show the induced EMF normal to the H flux from primary   
   current to represent real power flow into the secondary winding. Remember core   
   flux is lagging the primary voltage 90 degrees or so. Another insight is that   
   under loaded conditions,    
   there is MUCH MORE energy in the "Leakage" H flux than in the core flux. The   
   "leakage" flux has a very high field intensity, that, when crossed with   
   induced EMF is real power flow. If one examines the signal times thru for   
   instance, an audio   
    interstage transformer, you will see the time required to pass thru the   
   transformer is much faster than it would be if it had passed thru the high   
   permeability iron core region. I do admit applying the Poynting theorem is   
   somewhat diffucult and    
   transformer manufacturers do not refer to it at all, but then the makers of   
   tennis rackets do not seem to refer to the molecular makeup of nylon in their   
   wares either, but the info is there for those who wish to dig for it.....   
   >   
   I am having problems in that what you write appears as a single line   
   longer than my screen so that it is easier to list this as a quote   
      
   -------------------------   
   QUOTE   
   "Tech,   
     I somewhat stumbled upon this interesting discussion regarding the   
   power transfer in iron core transformers via leakage flux or core flux.   
   I have given it MUCH thought and experimentation and research thru many   
   college and manufacturers text books regarding transformer theory and   
   construction. Until I took a much deeper look into the transfer   
   mechanism, I too was convinced thru what I thought was a common view   
   that energy is transferred via the core from one winding to another. I   
   do not hold that view today. Several things can shed light on the   
   somewhat hard to see transfer mechanism. Yes, alternating flux in the   
   iron core is responsible for the induced EMF we see in any winding   
   around the core, however when secondary load current flows (assume a   
   resistive load), the secondary H field tries to reduce the core flux,   
   and does for a very brief period, reducing the induced  "Back EMF" seen   
   in the primary. "   
   UNQUOTE   
   --------------   
   Close: but it is not for a "brief period"   
   For AC there is another relationship independent of the core material.   
   This is easily derived from Faraday'Law  and that is that the average   
   half cycle voltage depends on the peak flx (all symmetrical flux waves)   
   and in the sinusoidal case results in Vrms =4.44FN(phimax) where phimax   
   is the flux enclosed by the winding. In other words for a given Vrms   
   and number of turns and frequency, there is a given maximum flux. Core   
   permeability doesn't change this. Using an iron core :   
   a) reduces  reduces the current required to produce this peak flux   
   b)provides a good magnetic circuit involving the secondary -BUT as you   
   indicate below -the world is not perfect so some of the flux produced by   
   the primary doesn't link the secondary (think of a copper wire loop   
   insulated with wet toilet paper). This is the leakage flux.   
   ----------------   
   QUOTE   
   "More current flows in the primary to maintain the core flux, however,   
   regardless of how high the core permeability is,"leakage Flux" will be   
   produced.   
   The term "leakage flux" is perhaps a misnomer, as it implies a fault in   
   materials. Under load conditions, the "Leakage" flux will always be   
   produced."   
   UNQUOTE   
   ---------   
   It is a fault of real life- in the case of an unloaded secondary, there   
   will still be some primary flux that doesn't couple the secondary so   
   that leakage flux does occur at no load. The exciting current will   
   consist, in practice of core loss current and magnetizing current.   
   ---------------------------   
   QUOTE   
     'This H "leakage" flux is in phase with the primary voltage and in   
   phase with the induced EMF in the secondary winding for "Real Power".   
   UNQUOTE   
   --------------   
   THAT is wrong!   
   --------------------   
   QUOTE   
   Note that the core flux does not change, at least not much, maybe a   
   little due to IR drops in the primary winding. "   
   UNQUOTE   
   ------------------------   
   Yes, core flux doesn't change much but it is due to an IZ drop in the   
   primary and Z includes the effect of leakage flux   
   -----------------------------   
   QUOTE   
   If you work out the Poynting Theorem, it will show the induced EMF   
   normal to the H flux from primary current to represent real power flow   
   into the secondary winding. Remember core flux is lagging the primary   
   voltage 90 degrees or so.   
     Another insight is that under loaded conditions, there is MUCH MORE   
   energy in the "Leakage" H flux than in the core flux. The "leakage" flux   
   has a very high field intensity, that, when crossed with induced EMF is   
   real power flow. If one examines the signal times thru for instance, an   
   audio interstage transformer, you will see the time required to pass   
   thru the transformer is much faster than it would be if it had passed   
   thru the high permeability iron core region. I do admit applying the   
   Poynting theorem is somewhat diffucult and transformer manufacturers do   
   not refer to it at all, but then the makers of tennis rackets do not   
      
   [continued in next message]   
      
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    * Origin: you cannot sedate... all the things you hate (1:229/2)