Is it me or do some authors get back EMF er backwards? - Page 1
Forum section: electronics / beginners

author: Quantumplate
author_rank: Regular Contributor
timestamp: March 31, 2022, 12:36:34 pm
content: Here's a quote from a nameless book introducing the subject of inductors:"As current flows into an inductor, a magnetic field is created around the coil. When the current stops, the magnetic field collapses, generating an induced current flow in the coil in the opposite direction to the original current."er...I thought the inductor opposes any change, so if the current is suddenly decreasing (switch is opened), the inductor tries to keep it going IN THE SAME DIRECTION, which equals sparking at the switch contacts. That also means the inductor emf generated at this point is not in the reverse direction - as back EMF implies - but in the same direction as the voltage source (a battery say). A back EMF is generated ONLY when the switch is closed on a simple battery-switch-inductor circuit, as the inductor tries to oppose the sudden increase in current.So is it me?

author: Circlotron
author_rank: Super Contributor
timestamp: March 31, 2022, 12:46:38 pm
content: The current will indeed try to keep going in the same direction.What is perhaps confusing is if you apply neg and pos to the left and right ends of an inductor respectively so that electrons go in the left end and come out the right, then remove the power, electrons will try to exit the right end as before and try to enter left end as before. That means the right end is now negative and the left end is positive so the voltage polarity that exists at each end of the inductor is now opposite to what it was when it was connected to an external supply.

author: Berni
author_rank: Super Contributor
timestamp: March 31, 2022, 12:50:53 pm
content: The reason for the flip is that the inductor is acting like a load when it is "charged up" but then once you try to stop the current the inductor starts acting like a power source to try continue pushing the current in the same direction.We tend to think of 'forward' on EMF as being what we see as a voltage drop across a resistor as this is the direction of current flow. But inside a battery the reverse is happening, since it is outputting power and so that EMF is reverse of the current.The mathy reason for this is the Kirchhoff voltage loop rule that states the sum of voltages in a circuit loop is 0. So in order to have a 'voltage drop' across a resistor you need to have a 'negative voltage drop' somewhere else.

author: CaptDon
author_rank: Super Contributor
timestamp: March 31, 2022, 01:04:06 pm
content: I can see your logic here. Lets say for instance we are looking at a high side fet pushing current into an inductor and we are seeing a positive voltage at that inductor/fet junction. Now if we turn off the high side fet the collapsing field will attempt to keep pushing current in the same direction which will result in a voltage attempting to go negative while it searches for a current path, usually the low side fet or a catch diode, the catch diode could even be the body diode within the fet. Meanwhile at the load side of the inductor the current will continue in its original positive direction (assuming the input side of the inductor found a return current path. You have made a point to ponder. It kind of reminds me of a strange use of language here in the Great Lakes region that goes like this...."We have lots of water in our yard from all the rain"...the expected reply would be "So do we"...but often here in this region the reply will be "So don't we" but indicating yes they do have water in their yard also. If you were looking at the fet side of the inductor in my above example the current was being pushed forward into the inductor while the high side fet was on but when the fet was cut off the current at that node would appear to be flowing backward while at the same time the current at the load end would appear to continue in a forward direction. I guess the statement in the book needs to be qualified with a circuit example?

author: Quantumplate
author_rank: Regular Contributor
timestamp: March 31, 2022, 02:24:04 pm
content: Thanks for all your inputs - good to read!That bit about the current flowing in the opposite direction in the opening quote I gave is wrong and very confusing then. It does make more sense if the inductor was in parallel with say a resistor. So a simple battery, switch L & R in parallel circuit, but the reversal of current would be through the resistor NOT the coil.Here's another example of this writers' phenomenon from another source - a very popular fix-it manual:"When a coil generates a magnetic field and then the direction of applied current reverses, the field collapses on the coil and generates current in it in the opposite direction to the current that created the field. Essentially, the coil stores some energy in the magnetic field and then puts it back into the circuit, but going the other way."So yes, the last sentence is right if we consider other components in parallel with the inductor, but again the first sentence is completely wrong.It seems there's a lot of this confusion about.

author: Capernicus
author_rank: Frequent Contributor
timestamp: March 31, 2022, 02:31:26 pm
content: I haven't experimented with inductors much, but I have a little, the discharge current is in the opposite direction as far as I know, but the thing I thought was wrong was it said it stopped passing current when the inductor was full charged, its actually the opposite, it conducts more when its full charged, its wierd that way, U explain it by knowing when the inductor is empty its going to suck up infinite charge in it, and when its full it wont take any more so it passes through, I'm not an expert at it, I think caps are easier.

author: Benta
author_rank: Super Contributor
timestamp: March 31, 2022, 05:19:56 pm
content: The quote in the first post is complete gobbledygook. Who wrote that? Never mind...Leaving resonant and filter circuits aside, the relevant two rules for reactors in DC circuits are:1: a capacitor will fight to keep a constant voltage across it.2: an inductior will fight to keep a constant current running through it.ad1: a charged capacitor that is short-circuited will deliver an immense current spike.ad2: a current-carrying inductor that is open-circuited will deliver an immense voltage spike.There is 100% inverse symmetry between the two types of reactive components.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: March 31, 2022, 05:29:17 pm
content: Isn't this the reason for 'flyback diodes' when using relays?

author: Quantumplate
author_rank: Regular Contributor
timestamp: March 31, 2022, 06:08:13 pm
content: Quote from: BILLPOD on March 31, 2022, 05:29:17 pmYes, otherwise typically the collector of a transistor is all of a sudden at 95v. The 'catch' diode sends it through the relay coil instead.

author: Quantumplate
author_rank: Regular Contributor
timestamp: March 31, 2022, 06:16:19 pm
content: Quote from: Benta on March 31, 2022, 05:19:56 pmOh it's so tempting. A lecturer at Stanford. The book's introduced me to the concept of otoacoustical emissions so I'll give bonus points for that.

author: Benta
author_rank: Super Contributor
timestamp: March 31, 2022, 06:36:34 pm
content: Quote from: BILLPOD on March 31, 2022, 05:29:17 pmYes, it's atandard practice for protecting the driver, but not always the right solution.It delays the off and fall time of the relay significantly (this can give contact burning).In certain applications it's better to use a Zener or Tranzorb and just allow the relay to spike to whatever the driver will tolerate.

author: Quantumplate
author_rank: Regular Contributor
timestamp: March 31, 2022, 09:15:36 pm
content: I think the problem here is that everyone traditionally calls the spike voltage due to the coil's collapsing magnetic field...back emf. But it's not!The coil tries to keep the current flowing in the direction it was going in - so it's a forward emf. This misunderstanding or misnaming leads prominent authors to then say that the current reverses direction through the coil as the field collapses!It's no wonder I dropped out of engineering many moons ago...

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 31, 2022, 09:25:24 pm
content: Or maybe back EMF is just aetherwind.

author: Benta
author_rank: Super Contributor
timestamp: March 31, 2022, 10:28:22 pm
content: Quote from: Quantumplate on March 31, 2022, 09:15:36 pmThe problem is that the term is used in this context at all.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 31, 2022, 10:36:53 pm
content: Phrases that have caused incalculable damage to science:"Follows path of least resistance""Collapse of magnetic field"etc.Magnetic field is just... magnetic field. It's there because current flow is there. Current flow depends on integral applied voltage (flux). Simple as that.The confusion is easiest seen when approaching the problem from a standpoint of linear time-invariant (LTI) systems. The justification for all this is rather high level (in short: the differential equations are almost trivially tractable*), but the fact is, these are conditions where the systems are very easy to work with, at least relatively speaking.*Which, might not sound like much when we're talking differential equations to start with, but these are a class of problems that range from "the solution is 'trivial'", to "a solution exists but it's very hard to write out", to "there is a solution but it cannot be written out", to "it is impossible to know if a solution exists". So... count your blessings.So, given an LTI system, what's that mean? It means, for the inductor, its voltage and current are well known as a function of time, applied voltage/current, etc. Everything is bounded and smoothly changing.And what's the disconnect? The problem is when you're "switching". If you consider touching a wire to / removing from a battery and inductor circuit, things are very different. When connected, the circuit has low resistance, and the inductor charges eventually to some current I = V/R, with an exponential curve having a time constant τ = L/R. When disconnected, the circuit has very high resistance, and the inductor discharges... instantly? But this violates a rule of reality itself: all quantities are continuous, smoothly changing from instant to instant. So we know it mustn't be instant. We are missing something, and our equivalent circuit must get much more complicated to express what's really going on. And if we go that way, we find grotesque horrors like spark discharge in air, propagating waves of EMP, and ringing and all manner of high-frequency phenomena our L-R circuit is wholly unequipped to express.When we lack these details -- we might fall back on imprecise language to merely hint that there's something going on, without really knowing what. I think this is the sense in which "collapse" is meant. It's hard to see what's actually going on there, it's just, a spark, some noise, and the magnetic field stops, y'know? This phrasing probably predates any instrument that could resolve such effects (as it turns out, sparks ignite in the sub-nanosecond regime, very fast phenomena indeed!), so I expect it is from this position of ignorance that it was created, and it's just stuck around.I guess I don't know if this is all that helpful of an explanation, either -- I've skimmed past undergraduate level EE/physics topics in the process, without having to delve into them, but just to say: there is depth here, which cannot possibly be captured by such a simple phrase; and which is most likely contradictory to the phrasing. And worst of all, I haven't really explained much in the process -- denying one inaccurate phrase, without even replacing it with something more reasonable.Well, the reasonable truth is, the equation of the inductor:V = L dI/dtThis is a calculus expression, so, if you know, you know; and if you don't, it's probably still not saying much. Well, I won't explain the minutiae of calculus here, there are many excellent intuitive explanations out there to look up -- especially 3blue1brown's introduction -- but suffice it to say, it relates one quantity, to thechangein another over time. Also, this is equivalent to the integral I mentioned above (and, an integral is just adding up little bits over time, and the fundamental theorem of calculus says that these are equivalent operations -- or rather, complementary or inverse).What this means in circuit is, current simply changes at the rate corresponding to applied voltage; if you suddenly increase the applied voltage (say by dramatically increasing resistance in circuit), current will change at a correspondingly accelerated rate. Or if you force current to be whatever, in such-and-such time, the voltage must jump around to suit. Effectively, a switch is suddenly changing the resistance in the circuit, and as it turns out (due to sparking), this happens in a very messy way. But by understanding that it's such a change (in resistance say), we can stick in the better-grounded framework, where change is gradual and continuous, and voltage and current are well-defined.As for "back EMF", think of it this way: consider a DC motor, at rest. It has some resistance between its terminals. We apply voltage, and current flows. Current creates magnetic field, and torque is produced. If we lock the rotor in place, then that's simply the relationship: I = V/R. If we allow it to spin, however, it spins up to some speed where applied voltage is balanced by back EMF. The motion of the rotor through the surrounding magnetic field, develops a voltage, which acts to oppose the applied voltage. It's givingbackcurrent to the source, reducing current draw. It's not like it's reversing (polarity or current direction), at least not just by spinning the motor by itself; but current flow has been opposed, the apparent resistance R' = V/I (for measured V and I while spinning) is much higher than the terminal resistance at rest.We model a motor as a Thevenin voltage source, where the series resistance is the same terminal resistance as usual, and the voltage source is the EMF.EMF also applies to inductors, in which case one might use the sense of "back EMF" as "reversed voltage" or "flyback voltage", when the rate of current is reversed -- and thus the terminal voltage of the inductor is reversed.They're ultimately equivalent senses, as there's a changing magnetic field underlying both examples; and the motor just produces a continuous (DC) voltage because of the synchronous switching effect of the commutator. (If you wired up an oscilloscope to the rotor itself, to fixed connection points, you'd see a sinusoidal wave of course.)Honestly, I don't use the term, in either sense, very often if at all; and, at the moment, I don't have a strong sense of which way is more correct. I'd just as well avoid using the term, and stick with the models instead -- but, again, I appreciate that models require time and thought to develop, something I've obviously done the work in, and so long ago at that, that I've lost some appreciation for what it's like without it...Tim

author: Berni
author_rank: Super Contributor
timestamp: April 01, 2022, 05:39:13 am
content: The whole thing is confusing because what we tend to think of as 'forward EMF' is actually backwards.We can measure a voltage across a resistor with a volt meter. So we think of that as the resistor having a 'forward EMF' across it that is pushing electrons along (This is technically wrong!). However the resistor is actually trying to hold back those electrons from flowing. It is a resistor after all.So for the resistor to 'slow down' those electrons it actually has to push them backwards trough the circuit. This is the actual EMF of the resistor and it is pointing backwards of what the voltmeter across the resistor is showing. This is how the resistor determines how much current is allowed to flow trough it. The physics inside the resistor create more of this EMF the more current is flowing, so at some point the current becomes so high that enough EMF is created to match the EMF of the battery pushing the current. The sum of the resistors EMF and battery EMF is 0 now so the current stops increasing.So an inductor producing 'back EMF' upon being disconnected is technically correct if you assume the EMF across a resistor as being 'forwards'. But if you assume the battery EMF as being 'forward EMF' then the inductor is is also producing 'forward EMF' when disconnected, yet producing 'back EMF' when it is connected to a battery. It doesn't matter what convention you use, the math works either way since the loop sums to 0. The point is that it does flip. Just like the current direction flips on a capacitor when being charged/discharged.Conventional current direction versus electron flow direction is the same sort of confusion. The first scientists got the direction of it wrong, but once we figured out electrons it was too late to change, so we just stuck with it. The math again works exactly the same if you assume current flow is in the direction of electrons.

author: Quantumplate
author_rank: Regular Contributor
timestamp: April 01, 2022, 10:35:00 am
content: Quote from: Benta on March 31, 2022, 10:28:22 pmExactly. We need a new term, something like 'backed-up emf' should do it.

author: Benta
author_rank: Super Contributor
timestamp: April 01, 2022, 04:02:58 pm
content: No need. The term "stored energy" covers all aspects and can be converted to current or voltage or power as necessary."Back EMF" is a term normally reserved for DC motors.

author: Quantumplate
author_rank: Regular Contributor
timestamp: April 01, 2022, 05:51:21 pm
content: Quote from: Benta on April 01, 2022, 04:02:58 pmI think there is, as the term back EMF as applied to coils being cut-off has caused me loadsa mental pain over the past three years!Let's use the term for motors and the initialization of coils only. We need a specific term to replace so-called 'back EMF' in the cut-off case.I got some more ideas, the first one for lovers of the term 'back EMF' who see nothing hideously confusing about applying it everywhere..Let's call it high forward EMF, or HfeORBacked Up Magnetically Manifested Exponential Ramp voltage, that's BUMMER voltage for short. Backed up - meaning like cars at a level crossing.

author: Benta
author_rank: Super Contributor
timestamp: April 01, 2022, 06:53:45 pm
content: You can invent your own language if you like, if that helps your own understanding.But the existing terminology is completely sufficient.That you (and others) insist on calling the current-cutoff voltage spike from an inductor "back EMF" is your own (incorrect) thing.