# What is Back emf: Definition and 76 Discussions

Counter-electromotive force (counter EMF, CEMF), also known as back electromotive force (back EMF), is the electromotive force (voltage) that opposes the change in current which induced it. CEMF is the EMF caused by magnetic induction (see Faraday's law of induction, electromagnetic induction, Lenz's law).

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1. ### Back EMF cancelation question with multiple coils wound on the same core

While thinking about back EMF I stumbled upon a curious question. We know that whenever a coil/conductor that passes current is switched OFF and the current is stopped it never stops immediately but instead has a type of "inertia" where the magnetic field created by the current as it passed now...
2. ### I Does an inductor always create a back EMF?

Let’s use a solenoid for example. Let’s say you have a current that passes through a solenoid clockwise. Due to Faraday’s law of induction, the induced current from the magnetic field from an increasing current opposes the initial direction of current causing resistance. If the initial current...
3. ### Back EMF vs Reverse Currents: What's the Difference?

Is it Back EMF, Reverse Current, Reverse Saturation current, Backfeeding? If not, tell the difference.
4. ### Is it possible to reduce back EMF in DC motors using shorting rings?

Shorting rings (also called Faraday rings) are commonly used in loudspeakers to reduce the back emf that is induced in their voice coils. Could something similar, i.e. shorted turns of wire that are placed in the same plane as each of the windings in a DC motor, be used to reduce the back emf...
5. ### Question about a simple model for for back EMF

Summary:: I've found an exercise about sparks caused by back emf. The exercise is based on a mathematical model of the current flowing in the circuit around the time when the switch is closed. I'm wondering to what extend that model is realistic The exercise I'm referring to proposes this...
6. ### Checking the Back EMF of a Motor

I want to check the back emf of motor as shown in the article below My question is does a normal probe will be sufficient or do I need a differential probe to check the back - EMF waveform?
7. ### Universal motor with commutator, recovering the back EMF?

So while thinking about motors this suddenly struck me, So as the universal series wound motor is spinning there is always some arcing going on around the place where the brushes contact the copper segments that slide past them, I assume this is at least partly because as each coil pair of the...
8. ### Planning to measure voltage (back EMF) caused by opening a switch in series with an inductor

i am planning to measure the back emf produced by inductor when you open a switch. i know it is very hard to predict the voltage. but is there any way to narrow the possibilities?
9. ### How to detect an inductor's back EMF?

how do i detect an inductor's back emf on a breadboard? explain in detail. i am planning to connect an inductor to a switch. i have read when you open a switch, the inductor creates a back emf. how do i detect this back emf, when the switch is opened? explain in detail.
10. ### B What would happen if there were no back EMF in an electric motor?

I know that as per Lenz's Law, back EMF moves in a direction so as to counter the flow of the changing magnetic flux which induced it in the first place. And it does so because if it flowed in the same direction it would contravene the law of conservation of energy as more and more energy would...
11. ### Computing Alternator Back EMF as a function of load current

I am attempting to make a basic model of an alternator and determine terminal voltage levels at several different current levels. The alternator design looks like this: 900 RPM fixed, 60hz, single phase 2000 gauss permanent magnet Flux Density, Magnet circumferential velocity 9.58 m/sec, 4...
12. V

### Calculating the RMS Value of Sinewave Voltage

Homework Statement I keep telling myself that this problem should be easy, but I keep getting the wrong answer. Any help is appreciated. A generator has an armature with 500 turns, which cut a flux of 8.00 mWb during each rotation. Compute the back emf it develops when it runs as a motor at...
13. ### Dynamo design with regenerative back mmf

In the above presented dynamo design, three snapshots taken during three instances during the rotor running. In fig 1 the approaching magnet builds an induced voltage across the coil, however the circuit is still open, hence no current flows. In fig 2 when the rotor reaches TDC (top...
14. ### Is this a valid way to measure the Back EMF of a DC motor?

While the motor is spinning, I measured the voltage (Vrm) across the motor by connecting the two probes from the multimeter to the two terminals of the DC motor via alligator to alligator banana cables. Since the current was fairly constant, I could ignore the inductance. So, no voltage across...
15. ### Back EMF Equation and Calculating A/C Starter Current | Homework Example

Homework Statement An A/C starter has a 12V supply & a back EMF of 6V. If the resistance is 0.1 ohms, calculate current during start-up & while in operation Homework Equations E=IR Not sure what else? Sorry I am stuck. The Attempt at a Solution Start up current E=IR 12V=I x .01 I = 120A...
16. ### Why Does Back EMF Decrease as Current Derivative Decreases in an LR Circuit?

Good evening everyone. I have a misconception regarding self inductance. I know that back emf induced in an inductor starts with the same value of the original voltage source and then keeps decreasing to reach zero. I also know how this is related to the derivative of current with respect to...
17. ### Average back emf induced in coil

Homework Statement The coil in a loudspeaker has an inductance of L = 56uH (or 5.6 x 10^-5 H). To produce a sound frequency of 20 kHz, the current must oscillate between peak values of +2.2 A and -2.2 A in one half of a period. What average back emf is induced in the coil during this variation...
18. ### Understanding Back EMF: What is it & What Does it Do?

Hello all, I have got a hard time thinking of what back E.M.F is. Does it appear only at the moment of switching and closing the circuit ? why it's even there and what is its relation with E.M.F . If some one can provide me with a brief and good source that elegantly explains the whole thing I...
19. ### Average back emf induced in coil

Homework Statement hey! I saw you've answered this question before... Hi, I would appreciate any help with the following: The coil in a loudspeaker has an inductance of L = 112 microH. To produce a sound of frequency 40kHz, the current must oscillate between peak values of + and - 4.4 A in...
20. ### Why does Back EMF produce so much voltage?

I apologize if this question is a bit confusing, but I am studying electrical engineering and what keeps baffling me is why when you run current through an inductor then place an open in the circuit how come the Back EMF current is able to cause a spark across the open. I get that the inductor...
21. ### Polarity of back EMF in an inductor....

Hello Forum, the law of electromagnetic induction states that a time-changing current causes a back (counter) emf in an inductor. The back emf opposes the change in current by generating an induced current I_ind. If dI/dt>0, the induced current is opposite in direction to the main changing...
22. ### Induced EMF due to changing current in straight wire

I see a lot of problems on constant current inducing a current in another wire due to its associated magnetic field, but not so much a single wire's induced electric field. If the current along an infinitely long wire is flowing out of the page and inducing a counter-clockwise spinning magnetic...
23. ### Net force between two Electromagnets considering Back EMF

First this Q might be trivial thus apologies. Consider two bar electromagnets that can simulate bar magnets M1 and M2, placed end to end at distance D apart (North Pole of M1 facing South Pole of M2). Magnet M1 is 'fixed' to a base. Assuming both electromagnets have the Current I1 and Voltage...
24. ### Help understanding 'back emf'

Homework Statement A series dc motor:[/B] A dc motor with its rotor and field coils connected in series has an internal resistance of 2.00 Ω. When running at full load on a 120-V line, it draws a current of 4.00 A. a) What is the emf in the rotor? b) What is the power delivered to the motor...
25. ### Back emf in a single loop of wire?

I have been thinking about coils of wire carrying alternatingcurrent. From what I understand, a back emf is induced in the coil because there is a changing magnetic field around each individual loop of wire, and therefore there is a changing flux through each loop because the changing field...
26. ### Do back force and back emf come together?

i have heard it said that back force occurs in generators and back emf occurs in motors, however I was considering a motor and I thought that it seems to me that as the coil is rotating and it has the changing flux linkage, and thus a back end is induced in it by Lenz's law, this reduces the...
27. ### Back EMF in Average Loudspeaker Coil: Compare to 18V

Homework Statement The coil in a loudspeaker has an inductance of L = 56uH (or 5.6 x 10^-5 H). To produce a sound frequency of 20 kHz, the current must oscillate between peak values of +2.2 A and -2.2 A in one half of a period. What average back emf is induced in the coil during this variation...
28. ### Two electromotive forces existing at the same time how?

Something a bit boggling to think, which I accepted as a fact in nature, but might anyone explain as how is it possible to have to electromotive forces(EMF) at the same time in the same conductor? Assume a circuit placed in a magnetic field, that is connected to a voltage source(##\epsilon_1##)...
29. ### Magnetic fields in transformers and back emf?

i am having some trouble with understanding the magnetic fields inside transformers and how they relate to back emf. In particular, I am considering the two cases of current passing through the primary coil but not the secondary coil, and then with the secondary coil as part of a complete...
30. ### Does a CFL Pose a Fire Risk Due to Inductive Load Characteristics?

As far as I know, Compact Fluorescent Lamp (CFL) is an inductive load. I understand that inductive load doesn't allow an immediate change in current. When the circuit was initially open (through open switch) then became close, the inductive load was energized. So opening again the circuit after...
31. ### How Do You Calculate Current from a Faraday Disc?

Hi, How do you calculate the current from a faraday disc? I have found calculations for the voltage, but what about the current? I think it is a bit more complicated due to back emf, but if you know what motor you are using (the torque it can generate), then can you work it out? Thanks for any...
32. ### How Does Back EMF Cause a Voltage Drop in Household Circuits?

Hi pf, I have been reading about back EMF and just had a few questions. I found this article on wiki (http://en.wikipedia.org/wiki/Counter-electromotive_force) and the last paragraph talks about the effects of back emf when you plug a vacuum cleaner into your wall. It says that you can notice an...
33. ### Power Input for Motor and Back EMF

Hello everyone, I'm a bit confused about the power drawn by a motor. Since back emf reduces current when the motor is rotating at high speed, is it true that back emf reduces the power required at high speed? THX :D
34. ### Understanding Back EMF: How It Affects Motor Performance and Power Consumption"

I need help in understanding back emf, I'm a bit confused here. Example(for understanding): Let's assume we have a 12 V DC motor, that draws 20Amps when it starts, as it reaches maximum speeds, current drops due to back emf, let's assume the resistance is 0.6 ohms, and b.emf = 10V...
35. ### Understanding Back EMF and Inductors: A Closer Look at KVL and Induced Voltage

Hi guys, I'm pretty sure this has been answered somewhere around here. So, consider a pure inductance (zero resistance) connected in series to a battery. We know that the magnitude of the induced voltage across the inductor is given by E=L\normalsize\frac{di}{dt}. According to KVL...
36. ### Is back EMF equal to the induced EMF in self-induction?

In the case of self induction, when the intensity increases gradually an emf is induced which generates a back current " equal in magnitude, opposite in direction " to oppose the increase, so if that is right shouldn't the intensity in the coil Become ZERO till the intensity becomes steady?
37. ### Maximizing Efficiency: Back EMF and Transformer Behavior in an Ideal Circuit

Hello , I have a question , assuming ideal devices here without losses for the sake of simplicity. Imagine I have a circuit like the one attached. Now if I would manage to flip either one of the switches fast enough that the switch is cut off right when the magnetic saturation of the...

Correct me if I am wrong. For a RL circuit, I know as a fact at t=0, there is a back emf that is almost equivalent to the original emf source. In order for the back emf to exist, there needs to be a magnetic field in the inductor. In order for there to be a magnetic field in the inductor...
39. ### Back EMF in Solenoid: Explaining Current Flow

Consider an ideal solenoid (no resistance, no leakage reactance etc.) connected across an AC supply. The back EMF induced in it will be exactly equal and in opposite direction to the source voltage (which means that when a certain terminal of the AC supply is positive, the side of solenoid...
40. ### Back Emf same as battery emf?

Homework Statement A coil of inductance 0.5 H is connected to a 18V battery. Calculate the rate of growth of current? Homework Equations E=-L(dI/dt) The Attempt at a Solution Actually I have the solution but the problem is that in the book they have assumed that the back emf that would develop...
41. ### Calculating Back EMF: 200V DC Motor w/ 20A, 0.3Ω Resistance

Homework Statement i have a 200v dc motor with 20A flowing through the armature with a resistance of 0.3Ω, how do i calculate the back emf? very confussed :( Homework Equations The Attempt at a Solution
42. ### Is there a delay of 1 or 2 seconds in generation of back emf in a dc motor

Does the back emf in a dc motor start to appear after a slight delay(say 1 or 2 seconds) after starting up the motor? I have had this simple doubt for a long time.According to the "generator principle" both applied emf and back emf will always exist simultaneously.Also it would be in violation...
43. ### How does back emf make energy conversion possible

I've come across one thing about energy conversions: "To make an energy conversion possible, it is important that the supplied form of energy must be opposed by another force, otherwise this energy cannot be converted." Agreed! Once the motor starts generating torque, a back emf is produced...
44. ### The difference of Back EMF waveform shape of BLDC and brushless PM generator

I am a bit confuse on the back EMF waveform shape of the brushless permanent magnet machine. As I know brushless DC permanent magnet motor has a trapezoidal shape back EMF waveform. If I use this motor as a generator, do I still get the same waveform shape or I will get a sinusoidal waveform?
45. ### Can Back EMF in Inductors and Motors be Eliminated or Reduced?

Well I have been researching a lot about inductors and motors and so on and I just had a question that I can't seem to answer. Is it possible to get rid of or reduce the back emf generated in say a inductor or motor. I know you can decrease the inductance of a inductor in many ways, but is there...
46. ### Confused about how inductors work and produce a back EMF?

Homework Statement A coil with zero resistance has its ends labeled a and b. The potential at a is higher than at b. Explain why the two following statements are consistent/correct with the situation. a) The current is increasing and is directed from a to b. b) The current is decreasing...
47. ### The back EMF of a DC motor spinning at full speed

Homework Statement The resistance of the armature windings of a DC motor is 5Ω. The motor is connected to a 120V line. When the motor is spinning at full speed, it draws a current of 4A. The back EMF at full speed is: Homework Equations Pav=\frac{1}{2}I2peakR Pav=\frac{1}{2}εpeakIpeak...
48. ### Average Back EMF Induced in Coil in Loudspeaker

Hi, I would appreciate any help with the following: The coil in a loudspeaker has an inductance of L = 112 microH. To produce a sound of frequency 40kHz, the current must oscillate between peak values of + and - 4.4 A in half a period. What average back emf is induced in the coil during this...
49. ### Back EMF: Explaining the Counter Voltage Induced by Change in Current

Hi, I have a question about back emf So, my understanding of back emf (\epsilon_b=-L\frac{dI}{dt})is that its the counter voltage that's induced by change in current, whether it be from a battery or magnetic field (\epsilon_0). We can model this as an LR circuit where \epsilon_0 +\epsilon_b=IR...
50. ### What is causing the input voltage to be attenuated on the oscilloscope?

Hello all, I'm not quite sure if this is a physics question or an EE question because it came up in an electronics design course for physicists with mostly engineering motivation, anyways here goes: I have an RLC band pass filter with a sinusoidal AC voltage input (Agilent function...