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Spice modelling of back EMF

Started by Cursitor Doom March 22, 2016
Hi all,

I've been unable to get sensible results from simulating the voltage 
spike inductors generate when their magnetic fields collapse. The only 
possibility I can think of for this failure is that the standard inductor 
models have insufficient parameters for this purpose and require an 
extended parameter set. Like many of y'all I use LTspice, btw.
Any ideas?
On Tuesday, March 22, 2016 at 2:05:25 PM UTC-4, Cursitor Doom wrote:
> Hi all, > > I've been unable to get sensible results from simulating the voltage > spike inductors generate when their magnetic fields collapse. The only > possibility I can think of for this failure is that the standard inductor > models have insufficient parameters for this purpose and require an > extended parameter set. Like many of y'all I use LTspice, btw. > Any ideas?
you are doing something wrong in your simulation. THe spice stadard inductor model works fine for that. Connect a pulsed current source to an inductor.. the voltage will spike when the current shuts off Mark
On 22/03/16 19:02, Cursitor Doom wrote:
> Hi all, > > I've been unable to get sensible results from simulating the voltage > spike inductors generate when their magnetic fields collapse. The only > possibility I can think of for this failure is that the standard inductor > models have insufficient parameters for this purpose and require an > extended parameter set. Like many of y'all I use LTspice, btw. > Any ideas? >
Show your circuit. LTspice's inductor model is plenty good for this. Jeroen Belleman
On Tue, 22 Mar 2016 18:02:10 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

>Hi all, > >I've been unable to get sensible results from simulating the voltage >spike inductors generate when their magnetic fields collapse. The only >possibility I can think of for this failure is that the standard inductor >models have insufficient parameters for this purpose and require an >extended parameter set. Like many of y'all I use LTspice, btw. >Any ideas?
Add some shunt resistance and/or capacitance to avoid singularities. That's realistic anyhow. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Tue, 22 Mar 2016 18:02:10 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

>Hi all, > >I've been unable to get sensible results from simulating the voltage >spike inductors generate when their magnetic fields collapse. The only >possibility I can think of for this failure is that the standard inductor >models have insufficient parameters for this purpose and require an >extended parameter set. Like many of y'all I use LTspice, btw. >Any ideas?
The standard inductor model deals only with current and voltage. If you are seeking a model that reflects behavior when you force an externally-induced field you'll need to write your own subcircuit. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The touchstone of liberalism is intolerance
Okay, gentlemen, thanks for the suggestions. I've found the problem - 
seems to be the box running LTS on Linux under WINE. No probs on the 
Windows installations, so nothing you fine people can assist me with. 
I'll post to a more appropriate Linux/WINE forum see if anyone there can 
figure it out.
Thanks!
On 22/03/16 20:50, Cursitor Doom wrote:
> Okay, gentlemen, thanks for the suggestions. I've found the problem - > seems to be the box running LTS on Linux under WINE. No probs on the > Windows installations, so nothing you fine people can assist me with. > I'll post to a more appropriate Linux/WINE forum see if anyone there can > figure it out.
That's even more reason to post your circuit! I'm runnning LTSpice under Wine in Linux. I see inductive spikes just fine. Jeroen Belleman
On Tue, 22 Mar 2016 21:00:05 +0100, Jeroen Belleman wrote:

> That's even more reason to post your circuit! I'm runnning LTSpice under > Wine in Linux. I see inductive spikes just fine. > > Jeroen Belleman
I don't blame LT or Wine on reflection. I had an update for my Fedora installation a few days ago and something definitely got corrupted. I lost all my menus and have had to invoke applications from the command line in a terminal. I'd put money on these two things being connected, as various other things have been behaving rather oddly since as well.
<makolber@yahoo.com> wrote in message 
news:f8802a61-8e23-476f-a750-5eae5dc9fcb1@googlegroups.com...
> Connect a pulsed current source to an inductor.. > the voltage will spike when the current shuts off
Actually, it will always spike, going both up and down. If you set the risetime to zero, you'll even get a divide by zero error! (But then, SPICE doesn't *do* discontinuous functions, so that should be no surprise.) A real inductor has RLC elements around it, equivalent to loss and parasitic capacitance and stuff. This limits the voltage, even if an ideal CCS were applied. A real "current source" exhibits finite capacitance and resistance, and a finite compliance range, so tends to saturate for large voltage changes. The common case of e.g. a solenoid driver or flyback SMPS, is better seen as an impedance mismatch situation. If the drive impedance were always low, the voltage would always be well-defined (but the current will rise arbitrarily -- for a pure inductive load that is). If the drive impedance is switched*, then the voltage is well-defined when the switch is on (low resistance, load exposed to a constant voltage source), and only the current is well-defined when the switch is off (high resistance, load exposed to constant current source; that current being zero, usually). *A quite reasonable model of a switch, is something which goes from a high resistance state to a low resistance state fairly quickly. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
On Tue, 22 Mar 2016 18:04:17 -0500, Tim Williams wrote:

> A real inductor has RLC elements around it, equivalent to loss and > parasitic capacitance and stuff. This limits the voltage, even if an > ideal CCS were applied.
I've noticed the "parallel capacitance" of the inductor makes a BIG difference to the level of back emf produced; smaller = greater voltage. Is this "parallel capacitance" in LTS the same thing as *interwinding capacitance* in real life?