How to investigate CEMF in an inductor ?

[makerman]

I'm investigating the stored energy in an inductor, the standard equation says it's : W=1/2LI^2, implying that the two ways of increasing the stored energy are by increasing either the inductance or the current, i'm looking into what happens when we have two inductors, both with the same resistance but one with significantly higher inductance, will the latter store more energy given the same power input?

The maximum transient over-voltage reading for the Rigol 1054Z is 1000v p2p, when i test either of the inductors many of the CEMF spikes are over this limit, what i need to do is calculate the average of all of the spikes, is this possible and are the voltage spikes bad for the scope ?


Thanks, Gary.

[makerman]

No worries got it, this scope is amazing

[Gyro]

Yes, you are risking front-end damage to your scope, It's really not that amazing!

Also the x10 probes won't be rated for anything like 1kV so will probably flash-over the internal 9M resistor, probably allowing more energy through to the scope (and damaging the probes).

[makerman]

Yes, you are risking front-end damage to your scope, It's really not that amazing!

Also the x10 probes won't be rated for anything like 1kV so will probably flash-over the internal 9M resistor, probably allowing more energy through to the scope (and damaging the probes).

Thanks Gyro, i've ordered one of these :

https://www.ebay.co.uk/itm/100X-P4060-High-Voltage-2KV-2000V-Oscilloscope-Scope-Passive-Clip-Probe-60MHz-UK/332873577200?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

[Gyro]

Yes, a high voltage x100 is what you want. 

[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]

Technically, an inductor should be stored open for the same reason

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.

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/