Back EMF, or counter EMF, is an induced electromotive force that acts in the opposite direction to the applied voltage in a DC motor. When the armature of a DC motor rotates under the influence of the driving torque, the armature conductors move through the magnetic field, and an EMF is induced in them by the generator action. This induced EMF in the armature conductors acts in the opposite direction to the applied voltage and is known as the back EMF. The magnitude of the back EMF is given by the equation Eb = (NPφZ/60A), where N is the speed, P is the number of poles, φ is the useful flux per pole, Z is the total number of conductors in the armature, and A is the number of parallel paths through the armature between the brushes of opposite polarity.

The back EMF in a DC motor makes it a self-regulating machine, which means it makes the motor draw a sufficient amount of armature current to develop the torque required by the mechanical load. The back EMF opposes the supply voltage, and the electrical work required by the motor for causing the current against the back EMF is converted into mechanical energy. The significance of back EMF in a DC motor can be explained as follows:

• At no load, the DC motor requires small torque for controlling the friction and windage loss. The motor withdraws less current, and as the back EMF depends on the current, its value also decreases. The magnitude of the back EMF is nearly equal to the supply voltage.
• If the speed of the motor is high, then the value of back EMF is large, and hence the motor will draw less armature current and vice versa.

In summary, back EMF is an important concept in DC motors that helps regulate the flow of armature current and develop the torque required by the mechanical load.