Forums

Spice modelling of back EMF

Started by Cursitor Doom March 22, 2016
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?
Not to be too pickey, but we don't consider inductors to have "back emf". That term is usually reserved for motors. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Wednesday, March 23, 2016 at 10:55:08 AM UTC+11, Cursitor Doom wrote:
> 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?
Correct. You can estimate it tolerably well by treating you windings as layers of wire with capacitance between the layers. Single layer windings have much lower parallel capacitance - typically around 1pF - which can be estimated fairly accurately if you are good enough at calculus. -- Bill Sloman, Sydney
On Wednesday, March 23, 2016 at 1:08:10 PM UTC+11, John Larkin wrote:
> On Tue, 22 Mar 2016 18:02:10 -0000 (UTC), Cursitor Doom > <curd@notformail.com> wrote: >=20 > >Hi all, > > > >I've been unable to get sensible results from simulating the voltage=20 > >spike inductors generate when their magnetic fields collapse. The only=
=20
> >possibility I can think of for this failure is that the standard inducto=
r=20
> >models have insufficient parameters for this purpose and require an=20 > >extended parameter set. Like many of y'all I use LTspice, btw. > >Any ideas? >=20 > Not to be too picky, but we don't consider inductors to have "back > emf". That term is usually reserved for motors.
You might reserve it for motors. I've always understood it to be the voltag= e generated by an inductor that acts against a reduction of the current thr= ough an inductor. It's electronics for dummies, but I did first year physic= s in the class that wasn't expected to have master calculus, which was pure= anti-Tasmanian prejudice, and we got all sorts of over-simplified explanat= ions. Tulane might have had other problems. --=20 Bill Sloman, Sydney
On Tue, 22 Mar 2016 19:08:14 -0700, John Larkin wrote:

> Not to be too pickey, but we don't consider inductors to have "back > emf". That term is usually reserved for motors.
So what's your preferred term for the same effect in a solenoid, then? I'm guessing "flyback" perhaps, although this seems to be more of an American English term than anything else.
On Wed, 23 Mar 2016 09:57:40 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

>On Tue, 22 Mar 2016 19:08:14 -0700, John Larkin wrote: > >> Not to be too pickey, but we don't consider inductors to have "back >> emf". That term is usually reserved for motors. > >So what's your preferred term for the same effect in a solenoid, then? >I'm guessing "flyback" perhaps, although this seems to be more of an >American English term than anything else.
Flyback, kickback, elldeeeyedeetee maybe. I just don't hear "back emf" much around here. A solenoid is interesting because the armature moves and modulates the value of L as it does. Relays, too. No big deal, as Humpty Dumpty said. -- John Larkin Highland Technology, Inc lunatic fringe electronics
<bill.sloman@ieee.org> wrote in message 
news:d0553769-2367-43ce-a720-93a5b528e1e3@googlegroups.com...
> On Wednesday, March 23, 2016 at 10:55:08 AM UTC+11, Cursitor Doom wrote: >> 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? > > Correct. You can estimate it tolerably well by treating you windings as > layers of wire with capacitance between the layers. > > Single layer windings have much lower parallel capacitance - typically > around 1pF - which can be estimated fairly accurately if you are good > enough at calculus. > > -- > Bill Sloman, Sydney
Interwinding-capacitance causes eddy currents in the wire and lowers the Q of the inductor. This is why Litz wire is used to wind antenna loopsticks.for higher Qs and narrower bandwidths. LTS apparently considers the capacitance of the inductor to be entirely external, and not between the windings. I tried it both ways and noticed no difference. . --- news://freenews.netfront.net/ - complaints: news@netfront.net ---
On Thursday, March 24, 2016 at 9:28:48 AM UTC+11, Bill Bowden wrote:
> <bill.sloman@ieee.org> wrote in message=20 > news:d0553769-2367-43ce-a720-93a5b528e1e3@googlegroups.com... > > On Wednesday, March 23, 2016 at 10:55:08 AM UTC+11, Cursitor Doom wrote=
:
> >> 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 a=
n
> >> > 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 =3D greater volt=
age.
> >> Is this "parallel capacitance" in LTS the same thing as *interwinding > >> capacitance* in real life? > > > > Correct. You can estimate it tolerably well by treating you windings as=
=20
> > layers of wire with capacitance between the layers. > > > > Single layer windings have much lower parallel capacitance - typically=
=20
> > around 1pF - which can be estimated fairly accurately if you are good=
=20
> > enough at calculus. >=20 > Interwinding-capacitance causes eddy currents in the wire and lowers the =
Q=20
> of the inductor.=20
The current that charges the inter-winding capacitances runs through the wi= ndings. If you bank wound an inductor, so it had lower interwinding capacit= ance it would be self-resonant at a higher frequency, so the inductive reac= tance would be higher and the winding resistance less important.
> This is why Litz wire is used to wind antenna=20 > loopsticks.for higher Qs and narrower bandwidths.
Wrong. Litz wire is better because at higher frequencies, current tends to = flow on the surface of a wire. Litz wire has a lot more surface area spread= over a number of parallel windings. If you look at the manufacturers notes= on Litz wire, you will find that progessively finer-filament Litz wire is = recommended for progressively higher frequency applications. Interwinding capacitance doesn't come into it.
> LTS apparently considers the capacitance of the inductor to be entirely=
=20
> external, and not between the windings. I tried it both ways and noticed=
no > difference. I've split an inductor into close-coupled halves, each half having a quarte= r of the original inductance and twice the parallel capacitance, and it did= make very little difference. Treating the interwinding capacitance as exte= rnal parallel capacitance does seem to an adequate approximation at frequen= cies where the single loop inside the inductive core is resistive enough no= t to matter. --=20 Bill Sloman, Sydney
On Thursday, March 24, 2016 at 2:42:58 AM UTC+11, John Larkin wrote:
> On Wed, 23 Mar 2016 09:57:40 -0000 (UTC), Cursitor Doom > <curd@notformail.com> wrote: > > >On Tue, 22 Mar 2016 19:08:14 -0700, John Larkin wrote: > > > >> Not to be too pickey, but we don't consider inductors to have "back > >> emf". That term is usually reserved for motors. > > > >So what's your preferred term for the same effect in a solenoid, then? > >I'm guessing "flyback" perhaps, although this seems to be more of an > >American English term than anything else. > > Flyback, kickback, elldeeeyedeetee maybe. I just don't hear "back emf" > much around here.
Nor any other evidence that a comprehensive and detailed education in electronic engineering is valued. It's well-known that it's unwise to sound more sophisticated than the boss, and John Larkin went to Tulane, and didn't pay all that much attention to what his lecturers were telling him. <snip> -- Bill Sloman, Sydney
On Thursday, March 24, 2016 at 2:44:49 PM UTC+11, Clifford Heath wrote:
> On 24/03/16 14:30, bill.sloman@ieee.org wrote: > > I've split an inductor into close-coupled halves, > > each half having a quarter of the original inductance > > and twice the parallel capacitance, >=20 > Is twice really the operable factor though? > You'd have removed additional capacitance between > the windings of the first and second coils, so > I thought you'd get a smaller increase. >=20 > Perhaps this depends on the insulation thickness?
This was purely a thought experiment, realised with LTSpice.
> If you have thin insulation, most of the capacitance > will be from wires touching, not indirectly to the > next layer - whereas thicker insulation will mean > that the proportion of capacitance from close contact > is less.
It isn't. Single layer windings typically have a parallel capacitance of ab= out 1pF. Layer to layer capacitance is more importance because the voltage differenc= e between successive layers is higher. The wire to wire capacitance isn't h= igher - and in fact may be less if you use a layer or two of transformer ta= pe (typically 60 micron thick Mylar) between each layer - but the current f= lowing through the layer-to-layer capacitance is a whole lot higher, which = is why the self-resonance frequency of a layered inductor is a whole lot lo= wer than that of a similar banked inductor. =20 >=20
> >and it did make very little difference.
> Your thoughts?
<snip> --=20 Bill Sloman, Sydney
John Larkin <jjlarkin@highlandtechnology.com> writes:

> 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? > > Not to be too pickey, but we don't consider inductors to have "back > emf". That term is usually reserved for motors.
I have mainly heard it used in the context of relay coils. Better put a diode across that coil otherwise the back-emf will blow up the transistor. Etc. DC motors mainly look like a big capacitor I think, in that the angular momentum of the armature tries to keep the voltage across them constant. Then there is a series inductance and resistance from their coils. And I guess a parallel resistance across the "capacitor" which is the mechanical load / friction. That is my "mental model" anyway. A small motor I used was modelled as 2.65 Ohms in series with 12,000uF. You can add parts for winding inductance, external mechanical load and inertia. -- John Devereux