At 03:19 PM 04/03/2010, you wrote:

>Dear friends,
>I'd like to get a better insight into inductors by asking two questions,
>hoping it may be of general interest:
>
>1) As we know very well, once we apply power to an inductor the current will
>build up till it saturates.

Sort-of correct. The inductance usually will drop smoothly as the current
increases, but it never gets to zero, because even an air-core inductor has
some inductance.

>At that point, the current we make flow into the
>inductor dissipates as heat by the well known joule-effect formula Q=I^2*R*T
>Now my first question is: BEFORE we reach coil saturation, does the above
>formula still apply, maybe substiting R with the instantaneous impedance,
>or part of the energy won't become heat but instead will be stored into
>magnetic energy? Apologies if I don't express myself like an engineer: I am
>not of course, but I am very passionated about the matter.

Yes, it will be stored. The energy stored in the inductance portion is i^2*L/2
at any given moment. If you think about putting a fixed voltage across an
ideal inductor, the current will ramp up linearly, so the power removed from
the supply is exactly that (and of course there is no energy loss in an ideal
inductor, so that is the amount stored in the inductor).


>2) If part of the energy is temporarily "lost" (stored!) into a 
>magnetic field,
>imagine if we disconnect completely the inductor in 0 time.. how long will the
>energy be retained, and what will estiguish it?
>An analogy with a capacitor is almost impossible, as (internal 
>losses set aside)
>it will retain the stored energy indefinitely. However an inductor 
>will lose it
>pretty quickly, that's my experience at least.

The analogy with capacitors is pretty easy. Imagine if we reduce the capacitor
voltage to zero in 0 time. How long will the energy be retained and what will
extinguish it?

They are both equally silly questions, because in reality you cannot do this
in zero time. With a real-ish part the time will not be zero, and the energy
(most of it anyway) gets dissipated during that non-zero time in the resistance
that you use to discharge the capacitor or the resistance you insert in series
with the inductor.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



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