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Spice modelling of back EMF

This was purely a thought experiment, realised with LTSpice.

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. >

--
Bill Sloman, Sydney
Reply to
bill.sloman
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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
Reply to
John Devereux

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

With one winding?

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

There is a shorted secondary in the rotor of squirrel-cage motors.

The current in the rotor is what makes the rotor to chase the rotating magnetic field of the stator.

--

-TV
Reply to
Tauno Voipio

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.

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John Devereux
Reply to
John Devereux

Right. The voltage in the cap is "back EMF" which decays slowly as the motor spins down.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

That's the 260Ohms, when the motor is just free-spinning. It's 3Ohms at full load so it stops quickly then.

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John Devereux
Reply to
John Devereux

It's fun to spin up a really good PM motor and then short the leads.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

Yes I have done that. In the ideal case of zero winding resistance and inductance, there would be an infinite torque (current), and the speed (voltage on the "C") goes instantly to zero.

Similarly when open circuit it spins forever (the "C" above stays charged up) if there is no friction.

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John Devereux
Reply to
John Devereux

And the motor jumps off the table.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

Apparently, interwinding capacitance determines the upper frequency limit where Litz wire has no advantage.This article says the upper limit is about

5 MHz.

formatting link

"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."

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Reply to
Bill Bowden

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
Reply to
bill.sloman

Yup, or if it's just a fixed magnet (permanent or otherwise), it works just as well. :)

Of course, with an electromagnet, if it's not rotating, it's just a regular transformer.

Indeed, a motor is even more rich than the earlier simplification! It is a multiplying device, between mechanical motion and magnetic field. There exists no linear circuit which can convert, say, 50Hz to 60Hz mains power, but a motor-generator (with suitable gearing) can do the job with lockstep precision.

The reason a "universal" motor is, well, as its name suggests, is because it mixes the input with itself, giving a square-law response. The applied voltage becomes DC and even harmonics; the harmonics become torque ripple, while the DC makes it spin in a consistent direction.

Likewise, torque is proportional to current squared, and is inverse with RPM, making universal / series-wound motors extremely effective for high impact and high power density applications.

(Traction applications tend to favor induction motors because of their simplicity and efficiency, absorbing the complexity into a VFD instead. Or PM AC (synchronous) motors, because of their low-end torque and synchronous operation, without wasting the power of an electromagnet winding.)

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

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

(snip)

Reply to
John S

As long as it is mounted to some immoveable object.

Reply to
John S

So are you saying the term "EMF" is a misnomer? Or is this just an argument about semantics?

Reply to
Cursitor Doom

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
Reply to
John Larkin

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

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

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?

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.

Reply to
John S

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.

Reply to
Cursitor Doom

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
Reply to
John Larkin

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