Back-emf or Counter-emf is a voltage developed in an electrical winding by Faraday’s Law that opposes the source voltage, thus limiting the current in the winding. For example,
- Often the current regulator used invariable-speed drives is unable to track the commanded current because of insufficient voltage difference between the motor back EMF and the supply.
- A permanent magnet brushless DC motor with concentrated stator phase windings. The concentrated windings create a square wave flux distribution across the air gap and a trapezoidal shaped back-EMF.
- A representation of a machine in which the stator voltage equations are modeled as a voltage source in series with a reactance (and typically a resistance). The voltage source represents the back emf present on the stator windings due to the coupling between the stator and rotor circuits.
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When a current passes through a coil wound on an iron core, it generates magnetic flux in the iron core, especially if the iron core passes through the centre of the winding and back up to the other end around the outside of the coil, without any air gaps.
This magnetic flux is now in the centre of a coil of wire and it then generates a voltage across it. It does not matter if the coil is the same one that caused the magnetic flux or another coil.
So, if it is the same coil, it will generate a voltage that opposes the original voltage. This is back EMF.
If it is a different coil, a new voltage will be generated across it. This is a transformer secondary.
Back EMF happens in some motors, too.
A motor is rotating because of a voltage across it and there is a magnetic field.
Now, you have a rotating set of coils in a magnetic field. This will cause these windings to generate a voltage which will oppose the voltage which caused the motor to rotate.
This, too, is back EMF.
In both cases, back EMF is very important as it limits the current which will flow in the transformer primary or the motor windings when the device is lightly loaded.