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Question 10: Explain the back emf in an electric motor.

ANSWER

Back emf in electric motor

An electric motor has the same design as that of a generator; the former (electric motor) converts electric energy into mechanical energy and the later (generator) converts mechanical energy into electric energy.

The current in the armature of the motor turns in the magnetic field and creating the necessary torque. However, the turning armature in the magnetic field has an emf induced in it. This induced emf opposes the emf of the running motor (applied voltage) and is called back emf in the motor.

Now let V is the potential difference across the terminals of the coil, I is current flowing in it and R is the resistance of the coil, then the applied voltage V must be equal to the sum of the voltage across the resistor R and the back emf.

It should be noted that the electrical work done against the back emf is obtained as the output mechanical energy of the motor. The output power is

This equation gives the work done against the back emf.

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