How to investigate CEMF in an inductor ? - Page 1
Forum section: electronics / beginners

author: makerman
author_rank: Regular Contributor
timestamp: January 21, 2019, 05:34:24 pm
content: 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.

author: makerman
author_rank: Regular Contributor
timestamp: January 21, 2019, 06:24:14 pm
content: No worries got it, this scope is amazing

author: Gyro
author_rank: Super Contributor
timestamp: January 21, 2019, 06:44:57 pm
content: Yes, you are risking front-end damage to your scope, It's really notthatamazing!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).

author: makerman
author_rank: Regular Contributor
timestamp: January 21, 2019, 06:50:51 pm
content: Quote from: Gyro on January 21, 2019, 06:44:57 pmThanks 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

author: Gyro
author_rank: Super Contributor
timestamp: January 21, 2019, 06:52:40 pm
content: Yes, a high voltage x100 is what you want.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 21, 2019, 11:31:38 pm
content: 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

author: viperidae
author_rank: Frequent Contributor
timestamp: January 22, 2019, 02:13:50 am
content: Quote from: T3sl4co1l on January 21, 2019, 11:31:38 pmIf 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 22, 2019, 05:13:57 am
content: 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

author: rstofer
author_rank: Super Contributor
timestamp: January 22, 2019, 02:12:38 pm
content: 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-actionMy 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.

author: makerman
author_rank: Regular Contributor
timestamp: January 22, 2019, 10:11:00 pm
content: Thanks for info allI've started a thread for this project in the projects section :https://www.eevblog.com/forum/projects/investigations-into-the-stored-energy-of-inductors/