How Do You Solve a DC+AC Transformer Circuit Problem?

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Discussion Overview

The discussion revolves around solving a circuit problem involving both DC and AC components in a transformer circuit. Participants explore the implications of ideal transformers and the behavior of circuit elements at steady state, particularly focusing on voltage relationships and the effects of DC current.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents a circuit problem involving transformers and seeks to find the output voltage, expressing uncertainty about handling the DC component alongside the AC component.
  • Another participant asserts that transformers do not couple DC across windings, explaining that at steady state, there is no change in flux due to DC current, which means no induced current in the secondary winding.
  • A follow-up question challenges the previous assertion by referencing the behavior of a capacitor and inductor in the circuit, questioning how a capacitor can have no charge buildup if the inductor is said to have zero voltage across it.
  • Another participant clarifies that at DC steady state, an ideal inductor behaves like a short circuit, leading to zero voltage across it, and suggests that the capacitor effectively disappears from the circuit.

Areas of Agreement / Disagreement

Participants express differing views on the behavior of circuit elements in the presence of DC current, particularly regarding the role of the capacitor and inductor at steady state. The discussion remains unresolved, with competing interpretations of the circuit dynamics.

Contextual Notes

Participants reference KVL and the transformer equation, but there are unresolved assumptions about the behavior of circuit elements under DC conditions and the implications for voltage across components.

ZetaOfThree
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Homework Statement


Find ##V_{out}## for the following circuit:
m7dhn7.jpg

We are given that the transformers are ideal and ##\frac{n_1}{n_2}=a>1## where ##n_1## is the number of windings on the coil on the left and ##n_2## is the number of windings on the left.

Homework Equations


KVL
Transformer equation: ##\frac{n_1}{n_2}=\frac{V_1}{V_2}## where the V's are the voltages in the transformers.

The Attempt at a Solution


Just use superposition of the voltage sources. I've got the AC part down, but I'm not so sure how to effectively handle the DC part. We can use KVL to write down a system of differential equations that relate the currents that flow through the three loops in the circuit. I was able to solve it using Maple, but the solution is pretty messy and Maple was unable to evaluate the solutions in the limit as ##t \rightarrow \infty##. Is there any way to easily see a steady state solution for the DC part of this problem, without using software as a crutch?

Thanks in advance!
 
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Transformers do not couple DC across windings; at steady state there's no change in flux from DC current in an inductor. No change in flux means no changing flux through the other windings to induce a current there.
 
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Thank you for the response!
The only problem I have with that is what's going on in the rightmost loop in the circuit on the left (the one with the capacitor and the inductor). KVL gives ##V_C + V_L = 0##, but if there is not change in current, then ##V_L=0##, so ##V_C=0##. How can it be that there is no charge buildup on the capacitor?
 
At DC steady state an ideal inductor looks like a short circuit, so zero volts potential across it. The capacitor is effectively shorted out and "disappears" from the circuit.
 

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