In order to understand the significance of back EMF, let us see what happens if there is no back EMF.

Let’s make up an example of a fictitious DC motor that has permanent magnets (i.e. fixed strength field).

Let’s say a VOLTAGE of 12 V is applied to the motor. The armature winding has a resistance of 0.2 Ohms (neglecting other resistances in the path e.g. brush contacts).

If there is no back EMF, the armature willdraw a current of 60 A and produce a heating of 720 Watts in just the (I^2)R loss. Outrageous ! Right ?

However, as the armature’s current interacts with the field winding’s magnetic field, the armature experiences the torque, and it starts moving. The movement induces a voltage in the armature winding that opposes the external voltage. This opposing voltage is what we call the back EMF. The back EMF is PROPORTIONAL to field, and it is also proportional to the angular speed of the armature.

Now, the external voltage drops across the armature due to the sum of the back EMF and the armature I-R drop.

Vs - Vb = Ia * Ra

Where

Vs is supply voltage,

Vb is back EMF,

Ra is resistance of armature winding, and

Ia is the armature current

In practice, the resistance of the armature is SMALL enough that the IR drop (at reasonable currents) is a very small fraction of the external voltage.

Thus, Vs = Vb (approximately)

That is, in practical terms, the back EMF almost balances out the external voltage.

At the start, the armature is not moving, and the back EMF is ZERO. The armature current is very high. The torque is the product of the field and the armature current. The initial torque is high. As the armature moves, the back EMF appears, and grows higher. The armature current drops gradually. The torque drops gradually, and the speed approaches the stead STATE speed.

Thus, the back EMF plays two roles

It limits the currentIt plays a self-regulating role on speed and helps run the motor at a close to constant speed (a small percentage of variation) from no LOAD to full load.

Due to back EMF, you can take an ideal voltage source, and a near-ZERO resistance armature winding, and the motor would still limit itself to a finite speed, current, and torque. That would not be possible without back EMF.

Hope this helps.