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Name:  Ben
Status: student
Age: 17
Location: N/A
Country: N/A
Date: 2000-2001


Question:
I am confused about the concept of 'back emf'. Have you got a clear, concise and logical explanation of just what it is?


Replies:
Hi, Ben !!

Well, as you probably already heard before, the back-emf is the same as counter emf (cemf). It is a voltage produced in a conductor that tends to neutralize the present voltage. It is a phenomenon that always tends towards the contrary of what happens!! Lets suppose: if lines of a magnetic field are cut by a conductor, than a voltage is generated in this conductor, which causes a current of electrons to flow in one direction. At the same time, like trying to avoid this, ANOTHER voltage is created that tends to neutralize this effect, and it forces the electrons to flow in the contrary direction. As you know, the back-emf is directly pro- portional to the velocity of the magnetic field. It is proportional to the relative motion between them.

I am sure that you know all of this. But, to give you an example, think of a wire with a switch. Suppose the electrons are running from a direction into another and suddenly, you open the switch. What happens?? Well, when electrons run, they give rise to a magnetic field. When you open the switch, the electrons should stop their flow. But - as a kind of inertial action - they try to continue the flow, and ionize the air, in an electric arc to find its way. Or let it be, the originally initial magnetic field, in a circumference around the wire, changes its sense, and try to avoid stopping the original flow. It is possible to see this in an oscilloscope, as a negative peak in certain experiences where you make use of a coil and - abruptely - stops the electron flow.

I know you know all of this. But, that is it!!! It is the way things are. Just try to answer this question: what is electrical charge? Why is that we have positive and negative charges?? Answer : nobody knows it!!! We must accept this condition!!! It belongs to the laws of the universe, since the big bang created it like as it is.

Just to speak a little bit more about this subject, you know that when an electrical induction "squirrel" motor runs, you have a magnetic field that circulates around the squirrel. There is a speed difference between them, what in turn generates a voltage and a force appears, which drives the "squirrel". A back-emf also appears, which lowers the net electric electron flow. The current through a rotating electric motor is greatly reduced because of this back-emf and if you avoid the moviment of the rotor, the current should be so large that could damage the equipment.

And last but least, back-emf corresponds in a way to inertial forces in the movement of a mass through space.

Inertia is a mass and has no intrinsic force. The electron has a mass ( ca. 1/1840 of the mass of a proton ). Just to compare, the Newtons first law states that a mass will maintain its linear direction of movement, unless the resultant of the forces applied on it be different from zero. When you interrupt a current, the flow of electron tend to continue, like having inertia (no voltage difference, no electrical pressure and the flow should stop). Nevertheless, magnetic fields appear alongside the current which act like forcing electrons keep on their movement.

Alcir Grohmann
Beschaffung SAM


Dear Ben,

The short answer to your question is "No", but some words about it may help. The universe is put together in such a way that whenever the magnetic field through a loop changes, a voltage or emf is induced in that loop. Now, in principle that emf could be in such a direction as to oppose the change that produced it or, in the opposite direction around that loop, to aid and increase the change that produced it.

Lenz's Law says the induced emf always opposes the change producing it, leading to such terminology as "back emf". The argument, by the way, is very simple. If the current aided the change that produced it, the induced current would change the field more, producing more emf, thereby producing more field, and so on to infinity, thereby blowing up the universe. This does not happen!

To take a practical example, an electric motor running under no load uses very little power. If the motor were frictionless and superconducting, it would use no power. This is because the back emf opposes the imposed voltage. As the load on the motor increases, the back emf decreases as must be the case; the motor must be given energy to produce energy. To see why the back emf decreases under load is often complicated, having to do with the angle between coils in the motor and the rotating magnetic field which is driving the coils.

Best, Dick Plano



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