# What is back-emf: counter-electromotive force?

Could someone knowledgeable on the subject explain what exactly is this back emf?

How is it caused in a motor/generator, which components and which effects determines it?

Inductive components like motor winding resist sudden changes in current. That's because the magnetic field caused by the current needs time to build up or decrease. That means that when current is flowing and this is suddenly cut off, the winding will try to maintain that current, and becomes a power source generating a voltage to be able to do so. It gets its power from the built up magnetic field.
Since the winding is now a power source instead of a consumer the voltage is reversed for the same current flow direction. That also explains how the voltage on a coil can become higher than the power supply: instead of subtracting the voltage over it you add it to the power supply. That's why you need a flyback diode on for instance a relay coil: the diode will allow the back emf to flow back to the power supply without damaging the switching transistor.

• i would say that's a great definition of 'inductive kick' Commented Aug 20, 2011 at 13:16
• "the voltage is reversed for the same current flow direction"-Let's say that I connect a power source to the inductor and mark its ends with A and B and current flows from A to B.When I take the source out and connect the winding to a non-supplied circuit,current will keep flowing in the same direction,but the voltage across AB is now -Vs.Am I right? Commented Aug 11, 2016 at 13:09

Motors and generators are somewhat interchangeable things. If you spin a motor, it can generate voltage - even if you spin it by electrical means. Back emf is the voltage produced (generated) in a motor as it spins.

At a dead stop, a motor produces no voltage. If you apply a voltage, and the motor begins to spin, it will act as a generator that will produce a voltage that opposes the external voltage you apply to it. The 'back' part of 'back emf' reflects this opposition.

It is the reverse voltage generated when you power a motor. Motors & generators are almost the same in principle. The difference between applied voltage & back emf is what supplies power to the motor.

If you stop feeding power to a motor, it'll keep spinning and generate a reverse voltage. That is back emf, and directly proportional to motor speed.

Hence this is often used for motor speed control.

When a current flows through a conductor it generates a magnetic field around the conductor. with that being said in a solenoid the exact process take place, The magnetic fields around each turn on the coil link with the rest of the other fields on other turns to form complete loops around on the out side and the inner core of the coil. These line of flux will determine the polarity and strength of the solenoid. No matter how tight are the turns there will be flux lines that will always remain around each turn, these smaller flux lines will induce a current in the coil when there is an applied voltage(these currents that are induced are known as Eddy currents). But when these currents are induced they will be in a opposite direction with the applied current and since it is in a counter direction therefore it is known as the back EMF.

When a armature is moving in a permanent magnetic field than induced current in the coil and this current also produced magnetic field. Permanent and newly produced magnetic field make interaction between them. Due to this coil move and torque produced in the coil and anti.torque wants to stop the motion of coil this is the back emf in the dc generator.

A typical electromagnetic (EM) circuit, like most systems occupying our environment tend to resist change. For mechanical systems we have Newton's laws of motion describing their nature, and his First Law basically states that a system at rest or moving at constant velocity and not acted on by an outside force will remain at rest or at its constant velocity. I.e. it will resist change. Well an EM circuit is similar to this, somewhat, in that if you change the current in the EM circuit, this will cause a change in the magnetism the current generates. Back emf is the force generated that will oppose this change in current that you make, as you make it.

AC voltages change polarity every half cycle. In an inductor each half cycle generates a magnetic field According to the polarity of that cycle. At the moment that the voltage changes polarity the first half cycles magnetic field starts to collapse and the new half cycle starts to build the collapsing half cycle resists the building of the building half cycle and this is what I was taught is CEMF or some call it back emf