# Spice modelling of back EMF

Started by March 22, 2016
```On 3/24/2016 12:39 PM, John Larkin wrote:
> On Wed, 23 Mar 2016 14:57:21 -0500, "Tim Williams"
> <tiwill@seventransistorlabs.com> wrote:
>
>> "John Larkin" <jjlarkin@highlandtechnology.com> wrote in message
>> news:tgu3fb53rh5a5pb60qqkuvumnm276rtghg@4ax.com...
>>> Not to be too pickey, but we don't consider inductors to have "back
>>> emf". That term is usually reserved for motors.
>>
>> In the fundamental inductor equation:
>> V = L * dI/dt
>>
>> V is defined as the EMF.
>
> We call it "voltage". Voltage isn't a force.
>

Actually the unit of EMF is Volt just as the unit of current is Ampere.

(snip)
```
```On 3/24/2016 2:43 PM, John Larkin wrote:
> On Thu, 24 Mar 2016 19:30:14 +0000, John Devereux
> <john@devereux.me.uk> wrote:
>
>> John Larkin <jjlarkin@highlandtechnology.com> writes:
>>
>>> On Thu, 24 Mar 2016 18:00:32 +0000, John Devereux
>>> <john@devereux.me.uk> wrote:
>>>
>>>> John Larkin <jjlarkin@highlandtechnology.com> writes:
>>>>
>>>>> On Wed, 23 Mar 2016 14:57:21 -0500, "Tim Williams"
>>>>> <tiwill@seventransistorlabs.com> wrote:
>>>>>
>>>>>> "John Larkin" <jjlarkin@highlandtechnology.com> wrote in message
>>>>>> news:tgu3fb53rh5a5pb60qqkuvumnm276rtghg@4ax.com...
>>>>>>> Not to be too pickey, but we don't consider inductors to have "back
>>>>>>> emf". That term is usually reserved for motors.
>>>>>>
>>>>>> In the fundamental inductor equation:
>>>>>> V = L * dI/dt
>>>>>>
>>>>>> V is defined as the EMF.
>>>>>
>>>>> We call it "voltage". Voltage isn't a force.
>>>>>
>>>>>>
>>>>>> (I don't have a curly 'E' so I put 'V' there.)
>>>>>>
>>>>>> Faraday's law says it doesn't matter if the dPhi/dt is from relative motion
>>>>>> (as in a motor) or varying magnitude (as in a transformer); indeed,
>>>>>> Relativity itself says the two are perfectly equivalent.
>>>>>>
>>>>>> A motor is just a spinning transformer!
>>>>>
>>>>> With one winding?
>>>>
>>>>
>>>> I think it's something like this
>>>>
>>>>      2.65Ohm  12,000uF    340uH
>>>> o------/\/\-----||--------//////----o
>>>>             '        '
>>>>             `--/\/\--'
>>>>            variable Load
>>>>             ~ 3-260Ohm
>>>>
>>>> I was trying to model a 6V Maxon F2140 motor
>>>>
>>>> Voltage on the C is proportional to speed. Current proportional to
>>>> torque.
>>>
>>> Right. The voltage in the cap is "back EMF" which decays slowly as the
>>> motor spins down.
>>
>> That's the 260Ohms, when the motor is just free-spinning. It's 3Ohms at
>> full load so it stops quickly then.
>
> It's fun to spin up a really good PM motor and then short the leads.
>
>
As long as it is mounted to some immoveable object.
```
```On Thu, 24 Mar 2016 10:39:04 -0700, John Larkin wrote:

> We call it "voltage". Voltage isn't a force.

So are you saying the term "EMF" is a misnomer? Or is this just an
argument about semantics?
```
```On Fri, 25 Mar 2016 13:39:38 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

>On Thu, 24 Mar 2016 10:39:04 -0700, John Larkin wrote:
>
>> We call it "voltage". Voltage isn't a force.
>
>So are you saying the term "EMF" is a misnomer? Or is this just an
>argument about semantics?

All I'm saying is that we don't use the term "EMF" when we mean
voltage, and we don't use "back EMF" to refer to the voltage across an
inductor's terminals. I suppose some people do. DVMs aren't called
DEMFMs. Too hard to pronounce.

We do use "potential" occasionally, generally to refer to fields, in
electrostatics, things you can't measure with a voltmeter probe. It
sort of means "what the voltage would be here if you could measure
it." Even that is kind of tricky.

Working engineers have their own language, and I'm sure engineers in
other countries or industries have their own dialects. Just words. But
electronics engineers in the USA have a pretty consistent communal
vocabulary, and we can spot amateurs and outsiders quickly by the
terms they use.

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics

```
```On 3/25/2016 11:08 AM, John Larkin wrote:
> On Fri, 25 Mar 2016 13:39:38 -0000 (UTC), Cursitor Doom
> <curd@notformail.com> wrote:
>
>> On Thu, 24 Mar 2016 10:39:04 -0700, John Larkin wrote:
>>
>>> We call it "voltage". Voltage isn't a force.

There is a force between two objects caused by the voltage (EMF)
difference between them.

>> So are you saying the term "EMF" is a misnomer? Or is this just an
>> argument about semantics?
>
> All I'm saying is that we don't use the term "EMF" when we mean
> voltage, and we don't use "back EMF" to refer to the voltage across an
> inductor's terminals. I suppose some people do. DVMs aren't called
> DEMFMs. Too hard to pronounce.

You are correct. The common usage these days is to refer to EMF as
voltage. However, Voltage is the *units* of EMF.

> We do use "potential" occasionally, generally to refer to fields, in
> electrostatics, things you can't measure with a voltmeter probe. It
> sort of means "what the voltage would be here if you could measure
> it." Even that is kind of tricky.

Not really. Electrostatic meters do that quite well. Note that they use
force for the indicator positioning.

So, if "potential" (energy) isn't a force, what is it?

> Working engineers have their own language, and I'm sure engineers in
> other countries or industries have their own dialects. Just words. But
> electronics engineers in the USA have a pretty consistent communal
> vocabulary, and we can spot amateurs and outsiders quickly by the
> terms they use.

Why are you interested in spotting amateurs and outsiders? Why not try
to understand and work with them?

This whole thing is prompted by your statement that "Voltage isn't a
force." You are wrong and you should have your wrists slapped with a ruler.

```
```On Fri, 25 Mar 2016 09:08:50 -0700, John Larkin wrote:

> On Fri, 25 Mar 2016 13:39:38 -0000 (UTC), Cursitor Doom
> <curd@notformail.com> wrote:
>
>>On Thu, 24 Mar 2016 10:39:04 -0700, John Larkin wrote:
>>
>>> We call it "voltage". Voltage isn't a force.
>>
>>So are you saying the term "EMF" is a misnomer? Or is this just an
>>argument about semantics?
>
> All I'm saying is that we don't use the term "EMF" when we mean voltage,
> and we don't use "back EMF" to refer to the voltage across an inductor's
> terminals. I suppose some people do. DVMs aren't called DEMFMs. Too hard
> to pronounce.
>
> We do use "potential" occasionally, generally to refer to fields, in
> electrostatics, things you can't measure with a voltmeter probe. It sort
> of means "what the voltage would be here if you could measure it." Even
> that is kind of tricky.
>
> Working engineers have their own language, and I'm sure engineers in
> other countries or industries have their own dialects. Just words. But
> electronics engineers in the USA have a pretty consistent communal
> vocabulary, and we can spot amateurs and outsiders quickly by the terms
> they use.

OK, note to self about the terminology being different in America. All
the old standard textbooks by English authors use the term EMF
interchangeably with voltage and I've never encountered your
interpretation of it before.
```
```On Fri, 25 Mar 2016 12:00:53 -0500, John S <Sophi.2@invalid.org>
wrote:

>On 3/25/2016 11:08 AM, John Larkin wrote:
>> On Fri, 25 Mar 2016 13:39:38 -0000 (UTC), Cursitor Doom
>> <curd@notformail.com> wrote:
>>
>>> On Thu, 24 Mar 2016 10:39:04 -0700, John Larkin wrote:
>>>
>>>> We call it "voltage". Voltage isn't a force.
>
>There is a force between two objects caused by the voltage (EMF)
>difference between them.
>
>>> So are you saying the term "EMF" is a misnomer? Or is this just an
>>> argument about semantics?
>>
>> All I'm saying is that we don't use the term "EMF" when we mean
>> voltage, and we don't use "back EMF" to refer to the voltage across an
>> inductor's terminals. I suppose some people do. DVMs aren't called
>> DEMFMs. Too hard to pronounce.
>
>You are correct. The common usage these days is to refer to EMF as
>voltage. However, Voltage is the *units* of EMF.
>
>> We do use "potential" occasionally, generally to refer to fields, in
>> electrostatics, things you can't measure with a voltmeter probe. It
>> sort of means "what the voltage would be here if you could measure
>> it." Even that is kind of tricky.
>
>Not really. Electrostatic meters do that quite well. Note that they use
>force for the indicator positioning.
>
>So, if "potential" (energy) isn't a force, what is it?
>
>> Working engineers have their own language, and I'm sure engineers in
>> other countries or industries have their own dialects. Just words. But
>> electronics engineers in the USA have a pretty consistent communal
>> vocabulary, and we can spot amateurs and outsiders quickly by the
>> terms they use.
>
>Why are you interested in spotting amateurs and outsiders? Why not try
>to understand and work with them?
>
>This whole thing is prompted by your statement that "Voltage isn't a
>force." You are wrong and you should have your wrists slapped with a ruler.

Volts are measured in volts. Force is measured in newtons.

--

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

```
```On Friday, March 25, 2016 at 6:42:55 AM UTC-7, Cursitor Doom wrote:
> On Thu, 24 Mar 2016 10:39:04 -0700, John Larkin wrote:
>
> > We call it "voltage". Voltage isn't a force.
>
> So are you saying the term "EMF" is a misnomer? Or is this just an
> argument about semantics?

"Voltage' is a coined word meaning electrical potential (sometimes called potential).
When the language was being worked out, electrostatic potential of Leyden jars was not known
to be the same as the move-magnets-in-coils induced potential, which was
'electromotive force'.   Yeah, potential isn't force, but it's how induction was
quantified, so 'voltage', 'electrical potential', 'electromotive force' are all words
for the same thing, but used in different contexts.
```
```On 3/25/2016 8:02 PM, John Larkin wrote:
> On Fri, 25 Mar 2016 12:00:53 -0500, John S <Sophi.2@invalid.org>
> wrote:
>
>> On 3/25/2016 11:08 AM, John Larkin wrote:
>>> On Fri, 25 Mar 2016 13:39:38 -0000 (UTC), Cursitor Doom
>>> <curd@notformail.com> wrote:
>>>
>>>> On Thu, 24 Mar 2016 10:39:04 -0700, John Larkin wrote:
>>>>
>>>>> We call it "voltage". Voltage isn't a force.
>>
>> There is a force between two objects caused by the voltage (EMF)
>> difference between them.
>>
>>>> So are you saying the term "EMF" is a misnomer? Or is this just an
>>>> argument about semantics?
>>>
>>> All I'm saying is that we don't use the term "EMF" when we mean
>>> voltage, and we don't use "back EMF" to refer to the voltage across an
>>> inductor's terminals. I suppose some people do. DVMs aren't called
>>> DEMFMs. Too hard to pronounce.
>>
>> You are correct. The common usage these days is to refer to EMF as
>> voltage. However, Voltage is the *units* of EMF.
>>
>>> We do use "potential" occasionally, generally to refer to fields, in
>>> electrostatics, things you can't measure with a voltmeter probe. It
>>> sort of means "what the voltage would be here if you could measure
>>> it." Even that is kind of tricky.
>>
>> Not really. Electrostatic meters do that quite well. Note that they use
>> force for the indicator positioning.
>>
>> So, if "potential" (energy) isn't a force, what is it?
>>
>>> Working engineers have their own language, and I'm sure engineers in
>>> other countries or industries have their own dialects. Just words. But
>>> electronics engineers in the USA have a pretty consistent communal
>>> vocabulary, and we can spot amateurs and outsiders quickly by the
>>> terms they use.
>>
>> Why are you interested in spotting amateurs and outsiders? Why not try
>> to understand and work with them?
>>
>> This whole thing is prompted by your statement that "Voltage isn't a
>> force." You are wrong and you should have your wrists slapped with a ruler.
>
> Volts are measured in volts. Force is measured in newtons.
>

And argumentativeness is measured in Larkins.

```
```<bill.sloman@ieee.org> wrote in message
news:55e6d941-777f-4c06-964f-579d049511c8@googlegroups.com...
> On Friday, March 25, 2016 at 10:15:31 AM UTC+11, Bill Bowden wrote:
>> <bill.sloman@ieee.org> wrote in message
>> news:0865199c-0749-46a3-b9c9-25ac8724fb30@googlegroups.com...
>> On Thursday, March 24, 2016 at 9:28:48 AM UTC+11, Bill Bowden wrote:
>> >> Interwinding-capacitance causes eddy currents in the wire and lowers
>> >> the
>> >> Q
>> >> of the inductor.
>>
>> >The current that charges the inter-winding capacitances runs through the
>> >windings. If you bank wound an inductor, so it had lower interwinding
>> >capacitance it >would be self-resonant at a higher frequency, so the
>> >inductive reactance would be higher and the winding resistance less
>> >important.
>>
>> >> This is why Litz wire is used to wind antenna
>> >> 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.
>>
>> Apparently, interwinding capacitance determines the upper frequency limit
>> where Litz wire has no advantage.This article says the upper limit is
>> about
>> 5 MHz.
>>
>> http://www.kerrigan-lewis.com/use-litz-wire/
>>
>> "At frequencies > 5 MHz, Litz wire becomes less practical. The
>> capacitance
>> effects between the strands are the reasons for this high-frequency limit
>> on
>> the use of Litz wire."
>>
>> <snip>
>
> This sounds like total nonsense. The skin depth for copper at 1MHz is 66
> micron, and 21 micron at 10MHz. You can't make wire strands  that fine,
> which is why Litz wire stops making things better at around 5MHz.
>
> The whole point about Litz wire is that there is no voltage difference
> between the strands in a bundle (and the strands have to be distributed
> rather carefully to make sure that this is true) so the capacitance
> between the strands should be a non-issue.
>
> --
> Bill Sloman, Sydney

I don't know if Litz wire is used for multi-layer inductors. If it is, the
voltage will be different between the layers and the skin effect is wiped
out by the capacitance between the layers. If you take things to the limit,
the surface area and capacitance becomes very large between layers.

--- news://freenews.netfront.net/ - complaints: news@netfront.net ---
```