What causes back EMF in motors?

Back EMF in motors is caused by the change in magnetic field around the coils as they rotate.

In more detail, back electromotive force (EMF) is a voltage that is induced in electric motors when there is a change in the magnetic field surrounding the motor's coils. This phenomenon is a direct result of Faraday's law of electromagnetic induction, which states that a change in magnetic field within a closed loop of wire induces an electromotive force in the wire.

In the context of a motor, when it is powered and begins to rotate, the coils of wire within the motor are moving through the motor's magnetic field. This movement changes the magnetic field around the coils, which in turn induces a voltage in the coils. This induced voltage is the back EMF. It is called 'back' EMF because it acts in the opposite direction to the current that is driving the motor, effectively opposing the input voltage.

The magnitude of the back EMF is directly proportional to the speed of the motor. When the motor is first switched on, the speed is zero, so the back EMF is also zero. As the motor speeds up, the back EMF increases. When the motor reaches its operating speed, the back EMF is almost equal to the input voltage, with the difference being the voltage drop across the motor's resistance. This is why the current drawn by the motor decreases as it speeds up, despite the input voltage remaining constant.

In summary, back EMF in motors is a result of the change in magnetic field around the rotating coils, as described by Faraday's law of electromagnetic induction. It acts to oppose the input voltage and its magnitude is directly related to the speed of the motor. Understanding back EMF is crucial in the design and operation of electric motors, as it affects the motor's speed, efficiency, and power consumption.