Why back emf is used for a dc motor? highlight its significance

Back emf is produced as a result of the relative motion between the magnetic field and the coil.
Going by Lenz's law the back emf is produced in such a way as to oppose the motion producing it, therefore the back emf acts as an opposing force on the motor motion.
It acts like a natural brake.
The direction opposes the terminal p.d, therefore we usually subtract back emf from it.
The faster the speed, the higher back emf.
Significances of back emf include 1.
It leads to increase in resistance of the motor leading to lower power consumption during rotation.
2.
It limits the speed of the motor

The induced emf developed when the rotating conductors of the armature between the poles of magnet, in a DC motor, cut the magnetic flux, opposes the current flowing through the conductor, when the armature rotates, is called back emf.
Its value depends upon the speed of rotation of the armature conductors.
In starting, the value of back emf is zero.

To know the significance of back emf lets first talk about the Lenz's law so according to Lenz's law

• The direction of current induced in a conductor by a changing magnetic field due to Faraday's law of induction will be such that it will create a magnetic field that opposes the change which produced it.
• Now In the case of DC motor, the supply Voltage is the cause of generated magnetic field or emf hence the generated emf will try to oppose the voltage.
• Therefore supply voltage V has to forcefully induce the emf in the armature conductors. (Why forcefully? .. because back emf is opposing the supply voltage in dc motor.)
• Hence the total electrical work done to oppose the back emf and to induced current in the armature conductor is converted into mechanical energy which is developed by the armature conductor.

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