When the DC voltage is applied to the armature of the DC motor,the motor draws enormously high current at the starting time and it decrease as the motor accelerates.Why the DC motor draws high armature current at the starting and how the current get decreased as the motor accelerates is because of the counter potential force developed across the armature winding. The counter voltage induced across the armature winding is called the back EMF. The diagram of the separately excited DC motor is as given below.
The DC motor has two main parts- the armature and the field winding. The field current flowing in the field coil generates the flux in the motor. The flux get linked to the armature conductor. when the motor armature( A rotating part) is stationary, the flux of constant magnitude linking to the armature conductor does not produce the voltage in the armature conductor.Therefore, the back EMF is zero when the motor starts. When the motor starts accelerating the back EMF starts developing across the armature winding. The induced EMF opposes the applied voltage according to the Lenz’s law.
The magnitude of the counter EMF or back EMF depends on the followings;
The field flux
The Speed of the motor
No. of conductors in the armature winding
The equation of the back EMF is as given below.
Eb= ΦNZ/60 *(P/A)
Where, Φ= Flux /Pole
N = Armature Speed
Z = Total number of armature conductor A = Number of parallel paths in the armature winding
The back EMF is proportional to the speed of the motor.
At start when N=0, Eb=0 and the motor draw very high armature current. Due to an interaction of the field flux and the armature current the torque is produced.
T= K*Φ Ia
The torque is exerted on the armature and motor starts accelerating. The back EMF starts to develop as the motor accelerates because the back EMF is proportional to the speed of the motor. The magnitude of the back EMF is always less than the applied DC voltage because IaRa drop in the armature.
Significance Of Back EMF
Back EMF regulates the armature current and, as a result the armature current automatically maintains the load requirement. Let us understand how the Back EMF regulates the armature current.
How Back EMF regulates the flow of armature current?
Case 1: When the applied armature voltage is increased
The back EMF limits the armature current and make the motor self regulating. If the applied voltage is increased the armature current increase momentarily and the speed of the motor increase. The increased speed of the motor increases the back EMF and the armature current gets reduced.
Case 2: When the speed of DC motor gets decreased
If the speed of the motor gets decreased because of the increased loading, the driving torque becomes less than the load torque and motor slow down and the back EMF gets decreased.The decreased back EMF will allow the more armature current to flow in the armature winding.The increased armature current will produce more torque which will produce the torque required by the load.When the motor attains the normal speed the back EMF will get reduced and thus the armature current get reduced. Thus the self regulation of the armature current is achieved.
Case 3: When the speed of DC motor gets increased
If the load on the motor is decreased, the driving torque becomes more than the load torque and, the motor speed increases and the increased speed cause increase in the back EMF. The increased back EMF opposes the applied DC voltage and thus the armature current gets decreased.The motor stops accelerating and the speed again bring back to the point that meets the exact load requirement.
Thus the back EMF regulates the armature current according to the load requirement.