Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Resonant transformer - sign of the voltage reinduced in the primary winding

  1. Jul 18, 2012 #1

    htg

    User Avatar

    Consider a biresonant transformer with a small coupling coefficient k << 1.
    Let the voltage applied to the primary LC circuit be U1(t)=U*sin(2*Pi*f*t), then also the current I1 will be sinusoidal. The voltage induced in the secondary winding and the current in the secondary LC circuit will be a positive multiple of -cos(2*Pi*f*t). Then the voltage reinduced in the primary winding will be a positive multiple of -sin(2*Pi*f*t), so it will tend to decrease the current in the primary winding. As far as I know, drawing current from the secondary winding INCREASES the current through the primary winding.
    What is the explanation?
     
  2. jcsd
  3. Jul 20, 2012 #2

    NascentOxygen

    User Avatar

    Staff: Mentor

    See what you think of this:

    Apply a voltage sin(ωt) and a current sin(ωt) flows, setting up a magnetic flux sin(ωt)
    The voltage induced in the secondary by that flux is dɸ/dt, viz., cos(ωt)
    and causing a secondary current of cos(ωt)
    This secondary current produces a flux of cos(ωt), and the derivative of that, dɸ/dt, inducing a component in the primary of –sin(ωt)

    The increased difference between the source emf and the coil´s emf causes increased primary current.

    (I leave you to prepend an amplitude co-efficients to each time-varying term.)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook