What is "Back EMF"?

In simple terms..

If you have current flowing through an inductor (coil, like in a motor) and you break the circuit in some way, the current will try to continue flowing, a large voltage will be generated which opposes to the original applied voltage.

E.G. If you take an old fashioned analogue meter (AVO) and set it to ohms and measure the resistance of a transformer primary, this passes a small current through the winding, as you break the test connection you have to be careful as the transformer winding can generate a high back EMF which can give you a little shock. This is an old trick that was often played on apprentices!

Try connecting two 12VDC toy motors together at their terminals. Spin the shaft on one. The shaft will spin on the other!

Back-EMF is the voltage that appears on a rotating machine's terminals as a result of its rotation. If the supply voltage is greater than the back-EMF, then the machine is motoring. If the supply voltage is less than the back-EMF, the machine is generating.

http://en.wikipedia.org/wiki/Back_emf explains more.

Yeh, I've done that with two of our pumps it freaks people out! Ghost pump..oooh spooky. I'd forgotten 'your' definition..I think both are correct usage.

(Terms and conditions apply...quite happy to be voted down and spanked if necessary)

Yes, DC and AC.

Answer # 2 by PWSlack is the best up to now.

A motor also acts as a generator. Back EMF is the voltage it generates because of its rotation. It is in opposite polarity to the voltage applied to the motor (hence the name). It applies to both DC and AC machines.

Back EMF is the result of the magnetic field collapsing, and the lines of flux pass thru the conductor in the oposite direction, causing a voltage to go oposite to the normal flow. This happens when the circuit is broken, opening a switch. Right hand rule for magnetism.

I'm not sure that spinning a motor, e.g. turning it into a generator, is an example of back emf. The discussions of breaking a curcuit quickly when there are reactive components involved is definately back emf. Of course if you stop a motor suddenly you'll get a nice reversed current. You can burn up a control circuit that way.

Most of the examples given here are referenced to motors and such, which is a good way to explain the phenomenon. Here's another. This may be beginners level/ Readers Digest for some members.....Another place the presence of Back-EMF is a design factor is in the Audio world.

Take speakers, for example. As your power amplifier delivers current to the speaker it causes the voicecoil to travel in the magnetic gap and voila! the speaker cone, it's attached to, moves. But....a speaker assembly is essentially a damped spring and will try to travel back to it's original zero position, outside of/and including, following the input signal, of course. But, as rudimentary electrical stuff, moving a coil through a magnetic field causes a corresponding current in the same coil...the speaker will generate a portion of Back-EMF that is superfluous to the business of creating low-distortion audio. This is why speakers and dynamic microphones really only differ in scale and materials.

This Back-EMF is reflected back to the output of the amplifier as an additional signal. The ability of the amplifier to "ignore" this Back-EMF is usually referred to as it's "damping factor" and shows up in most complete spec sheets as a number between say 50 and several thousand. This is an entirely complex discussion topic in itself, completely concerned with amp design, output impedance, speaker impedance curves and feedback, etc. The higher the number on this spec, the better, as a general rule, when shopping for power amps.

Nice one...I'd not considered back emf in a speaker!

If I learn 1 new thing a day...by the time I get to 60 I'll still be daft!

No, you will not, even to 99th birthday.

Keep your sense of humor, but sometime explain it to US rednecks. Have a nice weekend!

Would you care to explain that post?

I can't tell if it's a joke or what?

Which of my posts don't you get? I will happily explain...(The 'I'll still be daft at 60' is self deprocation...taking the mickey out of myself... something us Brits do...)

I believe all mine are straight honest replies.

I am somewhat bemused,

Del

So we have different kind of "sense of humor, or if you prefer humour".

That good. La vive this small difference according to Frenchwomen.

The English Channel (French: La Manche, "the sleeve") is not so wide to keep Ams (amperes) from Britons away, left or right? Of course we have OUR Atlantic Ocean, with or without EMF.

JUst...

Back EMF creation will vary wildly, of course, from speaker to speaker of all brands and types, contingent on many things, such as compliance of the spring (speaker suspension components), dimensions of the magnetic gap, strength of the magnet flux "motor', box venting, etc,etc,etc. That may be one of the many reasons, certain of the same speakers just seem to work great with some power amps with very similar specs, feel like a great match and then sound like shite on other amps with equally similar other specs. You can usually hear it as some vague or even obvious dynamic effect related to the bottom end, where the speakers impedance approaches zero. Lack of punchiness, sloppiness and lack of impact... poor impulse response, I suppose, without resorting to that "cork-sniffing-audiophile" hyperbole. It's very complex, even without dipping into psycho-acoustics.

Again, sort of a dog's breakfast, discussion-wise. I had better stop. This stuff really brings out the psuedo-science in certain types of folks.

Re: learning. I agree. The day you stop learning is time to open up a vein or take up golf. Just kidding. Too long spent in the proaudio electronics industry can breed a little sarcasm.

Dunno why everyone thinks I'm taking the mickey?

I was 100% genuine in my comment...

I am interested in speaker back EMF and I hadn't considered it before...

I used to sell Hi-Fi when I was first looking for a job out of college and was always interested in the Hi-Fi bull vs Hi-Fi science... The human ear certainly can hear differences that are hard to measure..unfortunately mine can't and I'm not at all musical.

I remember a guy bringing some speakers back to shop complaining of distortion..my boss wired them up to a stereo radio which could barely get enough signal strength to receive stereo... he listened intently from one speaker to the other ... thought carefully..scratched his head..spoke to the guy for a bit and then diagnosed...

'Listener Fatigue'

The guy went away happy and we all fell about laughing.

(This post contains no sarcasm, or attempted humour and one true story. Please don' t shoot I've had enough people on my case lately)

In a situation where you have a conductor and a magnetic field moving relative to each other, you will find a current induced in the conductor. Faster relative movement means higher current. Conversely, a current passed through a conductor will cause a magnetic field. Whereby the current comes from the voltage across the conductor.

The latter voltage, when applied to a motor, creates a current and magnetic field that causes the motor to turn. In doing so, the magnetic field of the armature/rotor which is now rotating, causes a current to be induced in the 'fixed' conductor windings, that in turn causes a voltage, that happens to be in the opposite direction to the supply voltage.

You can't measure it directly with a voltmeter as such, but it is there all the same, and it exists as an Electro Motive Force (EMF), and as it opposes the supply voltage, it is called a 'Back' EMF.

I hope this helps.

Another way of explanation what is EMF or "Back EMF"

Maybe it should need to be given a warning that in real world of Applied Voltage + Resistance + Inductance / = Impedance and resulting Current in these inductive circuits there are two phenomena:

1. The rotating electromagnetic field (in electrical machines) which is responsible of this E Motive F, which is analyzed / calculated (cannot be directly/ easily measured) mostly in steady state ( machine is running, i.e is in motion).

2. Transient state that last a limited time after dramatic change in the inductive circuit, as it happened just after interrupting its continuity. An example are rapid opening of CT circuit, could be the other circuit e.g. with huge loud speaker. Of course it is not called EMF.