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bobthenormal
bobthenormal is offline
#1
Dec1-08, 05:04 AM
P: 17
Hello, I am having some difficulty that I thought some of you might be able to help with.

In the LRC circuit models, the phasor method is used to find the instantaneous values of various properties. I've been having a lot of trouble picturing how this theory actually works in reality.

I apologize if the questions seem too easy, I've been struggling with the physical concepts and working through them, so I might be missing something obvious just from the fatigue.

Usually the situations I don't understand have to do with the inductance part of the circuit. Inductors seem to get treated more mathematically than physically.

For example, a problem with an AC voltage source (Vmax*Sin(omega*t)), an inductor and a resistor, all in parallel.

When I think about how this would work out, I end up going in circles. Here's why:

1) The voltage in the circuit doesn't seem to be constant. It would be instantaneously, but as soon as things begin to move, the EMF induced by the inductor will cause the effective voltage to be lower, even across the resistor, correct?

2) Shouldn't the current in the circuit skyrocket to infinity?

3) The inductor provides a back-EMF, but how long does that back-emf last? It's not really clear from formulas how quickly a magnetic field, once created, re-establishes a current.

4) Shouldn't the inductor cause a spike in current on the 2nd quarter-cycle of the AC input voltage source? That is, in the first quarter-cycle, the voltage is increasing, which presumably is met with a large back-EMF created by the inductor... when it reaches the voltage maximum and the 2nd quarter-cycle begins, shouldn't the current be at a maximum? The input voltage is peaking, so the inductor should be just leveling off because there is no longer (for a short time) any attempt to change the rate of the current.

5) After the situation in (4), I would expect the current to fall slowly or flatten off? Because, as the voltage falls, the current should also be trying to fall, but that fall is resisted by the inductor providing a now forward-EMF, effectively increasing the voltage difference (which I would expect to look like an almost flat line, the inductor and the input voltage working together to provide a constant current as long as the inductor can maintain it).

Being able to picture this has really been holding me back... and it's not the phasor diagrams or math that I'm not getting, so please don't derive the inductor's current from Kirchoff's loop rule. I know you can derive the 90 degree lag by math, I'm trying to get a feel for why THAT graph is not the one I intuitively expect.

--Bob
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