Back EMF In DC Motor (English)

Back Electro Motive Force generated in a DC motor:

When the motor is connected to the power supply, the armature of the motor starts rotating in the magnetic field due to the force, then the armature or conductor cuts the magnetic field lines of the main field, which causes the EMF induced in the motor armature conductor, these emf direction is just Opposite to applied source voltage, it is called back EMF due to its opposite direction to the applied voltage used. The voltage supplied to the motor must be high enough to eliminate the effect of the back EMF.

With the following formula, we can show the opposing electrical carrying force generated in the motor.

Because

Vb=Eb+IaRa

Where at-

V= is the value of applied voltage used in volts

Eb= Back EMF in volt

Ra= Resistance of Armature in Ohm

IaRa= Voltage drop in armature

Ia=(V-Eb)/Ra

Or

Eb=V-IaRa Volt

This back EMF depends on the quantity of flux(Ø) and the speed (N) of the motor.

The time taken in a full cycle is 1/N minute Or  60/N sec

Hence 

Eb=KNØ volt

K= The value is a constant whose value 

K=ZP/A

Here

Z= Number of conductor in armature

P= Number of poles

A= Number of parallel paths in the armature

N=  Armature round in one minute (RPM)

Hence

Eb=(ØZNP)/60A Volt

The value of the back EMF also depends on the speed of the armature. If the speed of the armature of the motor is high then the back EMF will be more, but the armature current (Ia) will be less. If the speed of the motor is low, then the back EMF will also be low and more current will flow in the armature, which will cause more torque.
You Can Also Like  This Post -
Construction Of DC Machine (English)
DC Shunt Motor Rotation Direction Change Method (English)