Back EMF in an Electric Motor

When an electric motor is running, current is used to turn the motor and provide movement.

This movement is produced because current flowing in the coil while the current is in a magnetic field leads to a force on the arms of the coil, the direction of which is given by Fleming's left hand rule. This does however lead to a back EMF, E b . in the coil, opposite in direction to the EMF applied to the motor to drive it. The resultant current flowing through the coil windings is the difference between the applied EMF and the back EMF, divided by the resistance of the coil.

The work done by the current in flowing against the back EMF becomes transferred to useful work done by the motor. The work done by the resultant terminal pd E-Eb in driving the current through the motor is wasted to overcome friction, the resistance of the wires, wasted as eddy currents in the magnets and because some of the magnetic field produced by the current leaves the region so does not aid in turning the motor. For maximum efficiency, the back EMF should be as close to the applied EMF as possible.

If for any reason, the motor is brought to rest while the current is still flowing, the back EMF becomes zero and and all the energy of the supply current is now turned into internal heat energy in the coil windings. The coil can heat up very quickly and possibly cause the motor to burn out.

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