Electronics > Beginners

How to investigate CEMF in an inductor ?

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T3sl4co1l:
You don't need unclamped flyback (huge dI/dt) to investigate the energy storage of an inductor.

Inductors aren't dangerous at all, they deliver only as much voltage as you let them.  A shorted inductor delivers no voltage -- exactly complementary to the fact that an open-circuit capacitor delivers no current.  (A capacitor may be stored shorted for safety purposes -- to ensure a zero-energy state.  Technically, an inductor should be stored open for the same reason, not that we have any inductors which can store energy for nearly as long as capacitors can.)

Tim

viperidae:

--- Quote from: T3sl4co1l on January 21, 2019, 11:31:38 pm ---Technically, an inductor should be stored open for the same reason
--- End quote ---
If you were paranoid, would you not store inductors short-circuited? Stray changing magnetic fields would induce a current in the inductor. Left open, the voltage could reach quite high.
Shorted, it would do nothing more than generate heat.

T3sl4co1l:
I'd be rather more worried that said stray magnetic fields would crush me between the magnet producing them, and the inductor's iron core. ;D

Tim

rstofer:
Before you risk component or equipment damage, maybe you should model the setup in LTspice.  Or maybe just model the math with MATLAB or wxMaxima.

V = L * di/dt where di/dt is the rate of change of current.  Suddenly opening a switch will result in very high di/dt because dt (the switch clearing time) is very short, potentially microseconds.  Say you interrupt 1A in 1 us with a 1 Henry inductor.  This results in 1H * 1A / 1 us => 1 million volts.

As mentioned above the dielectric properties of the switch will be overcome at some point, you won't really get 1 megavolts.  But whatever it turns out to be, it will certainly damage circuit components and test equipment.

https://www.khanacademy.org/science/electrical-engineering/ee-circuit-analysis-topic/ee-natural-and-forced-response/a/wmc-inductor-in-action

My suggestion is to know what you're going to see on the scope before you start probing.  Sometimes it is better to just accept the math and move on.

makerman:
Thanks for info all :)

I've started a thread for this project in the projects section :

https://www.eevblog.com/forum/projects/investigations-into-the-stored-energy-of-inductors/

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