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).
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...
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...
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...
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...
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?
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...
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?
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.
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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##)...
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...
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...
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...
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...
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
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...
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...
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?
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...
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...
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...
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
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...
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...
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?
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...
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...
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...
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...
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...
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...