4
\$\begingroup\$

I learned that current is induced in a conductor by a changing magnetic field. According to this, it would follow that the secondary voltage (sinusoidal input) of a transformer should peak when the primary current is changing fastest — that is at the zero crossing. It doesn't.

My question is: Why aren't primary current and secondary voltage out of phase by about 90°?

\$\endgroup\$
  • 3
    \$\begingroup\$ Welcome to the site. I do appreciate that you mean well with a cheerful anecdotal style but it's quite difficult to find the question you want asked amongst it all. Can you trim it down and make it more direct? The better the quality of your question, the better the quality of the answers you will attract. Again, a very warm welcome to the site. \$\endgroup\$ – TonyM Apr 4 '18 at 20:41
  • \$\begingroup\$ The question seems pretty well stated, to me: "why are not input current and output voltage of a transformer shifted by 90° one with respect to the other?" (edit: I had forgot "current") \$\endgroup\$ – Sredni Vashtar Apr 4 '18 at 21:16
  • 5
    \$\begingroup\$ @SredniVashtar If one has to read 8 paragraphs to deduce this single-sentence question - then no, it is not stated well. \$\endgroup\$ – Eugene Sh. Apr 4 '18 at 21:42
  • \$\begingroup\$ @EugeneSh. c'mon... it's stated in the title. In. The. Title. (and in the very first paragraph). In the rest of the post he gave context about his education and level of expertise. Something actually needed to target an answer. \$\endgroup\$ – Sredni Vashtar Apr 4 '18 at 22:07
  • \$\begingroup\$ Transformer action is dominated by a feedback mechanism within the core. As the secondary Vout increases into a resistive load, the Iout increases and the core flux drops, instantaneously. Yet the core flux needs to maintained at a near constant level (due to the feedback) and more current is demanded from the primary. \$\endgroup\$ – analogsystemsrf Apr 5 '18 at 16:52
5
\$\begingroup\$

If you apply a sinusoidal voltage, the magnetization current (nothing to do with load currents) is formed from V = L di/dt (Faraday's Law). So, reversing the formula, you find that magnetization current is the integral of applied voltage. That shifts magnetization current by 90 degrees and also provides a core flux that is shifted 90 degrees. Then to the secondary...

The induced voltage in the secondary is subject to the formula V = N d(\$\Phi\$)/dt so the 90 degrees phase shifted flux produces a secondary voltage that is shifted by a further 90 degrees making 180 degrees in total.

it would follow that the secondary voltage (sinusoidal input) of a transformer should peak when the primary current is changing fastest — that is at the zero crossing. It doesn't.

With no secondary load current, the induced voltage in the secondary is shifted from the magnetization current by 90 degrees. If full load is applied the primary circuit contains both magnetization current and "primary referred secondary load current" this latter current is 180 degrees out of phase to the secondary load current. Here what it looks like for a simple step voltage applied: -

enter image description here

Picture and further reading source.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.