When a current carrying conductor is placed in a magnetic field, it experiences a force.
In the case of a DC motor, the magnetic field is produced by the field winding. And the armature winding is connected to external supply voltage hence it plays the role of current carrying conductor placed in the magnetic field.
Therefore a force is exerted on the armature placed in the magnetic field, it starts rotating. The direction of rotation of DC motor can be determined by Fleming’s left-hand rule
When a DC motor is switched on, direct current flows through the armature conductors and field windings. This flow of current produces armature field and pole field. Now, there are two magnetic fields in the air-gap between field shoes and armature. These two fields react with each other to rotate the armature.
The commutator reverses the direction of flow of armature current at regular intervals so that the armature field is always repelled by pole field. Since the armature is continuously repelled by the field poles, it keeps rotating the armature in the same direction.
Back Emf in DC Motor
When the armature winding of a DC motor rotates in the magnetic field produced by the field winding, it cuts the magnetic flux. Hence an EMF is induced in the armature winding according to the Faradays laws of electromagnetic induction.
But as per Lenz’s law, this induced EMF acts in opposite direction to the armature supply voltage.
Therefore, this EMF is known as the back EMF and it is denoted by Eb. This back EMF induced in a DC motor can be expressed mathematically as,
Eb = (PφNZ)/60A volts
Where P = number of poles
Φ = flux per pole in Wb
N = speed of motor in RPM
Z = number of armature conductors
A = number of parallel paths
Significance of Back EMF in DC Motor
When no load is applied to the motor, very small torque is required by it, which is necessary to overcome the friction and windage losses. Hence motor draws very small armature current.
Armature current, Ia = (V – Eb)/Ra
Therefore, the difference between applied voltage and back EMF is very small.
But when we start loading the motor, this decreases the speed of armature. This results in fall in back EMF. This reduced back EMF causes a larger current to flow through the armature winding and larger current produces increased driving torque.
Thus, the motor produces larger driving torque as it slows down. So motor will start running on the speed at which armature current is just sufficient to produce the required torque by the load.
On the other hand, when the load on the motor is reduced, the speed of armature increases due to excess of driving torque. This increases the back EMF which results in decreased armature current. In this way back EMF in a DC motor regulates the flow of armature current according to the load requirement automatically.