Ask A Scientist=A9 Physics Archive
Question - I am confused about the concept of 'back emf'. Have you got a clear, concise and logical explanation of just what it is?
------------------------------------------------ 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!! Let=B4s 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.
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Which prooves the point that BOTH dc armature and ac rotor motors have back EMF!