In a DC motor, when current is passed through the coil in a DC upon contact with the brush, it seems to be moving at a constant speed. But after half a rotation, a current is passed through the coil again.
Now, since more current is passed through it, shouldn't the DC motor accelerate? And thus it should keep accelerating infinitely?
But this does not happen. Why?
Besides static and dynamic friction between the rotor and stator or rotor and atmosphere, and besides load that acts against the power source and keeps acceleration of the rotor in check, there is also the effect of current decrease with increasing angular velocity, due to back emf. Back emf acts on current against the battery/voltage source powering the DC motor and decreases current. The stronger the back emf, the lower the current and the lower the torque accelerating the rotation.
The faster the coil rotates, the stronger the back-emf. This back-emf is actually motional emf due to motion of conductor in magnetic field, and at each element of the wire it is proportional to velocity of the wire. At some critical angular speed (which is practically unreachable, due to load and even friction), net motional emf in the whole winding would be so strong that it would completely counteract the emf of the DC power source; net emf would become zero, and since the winding has non-zero resistance, current would fall to zero (stop). In this hypothetical rotation with zero current, the accelerating force becomes zero and the rotor does not accelerate anymore.
In practice this state of things with zero current can't be reached, because there is always some resistance to rotation (due to load or at least due to friction), so the actual equilibrium rotating speed is lower than the critical speed described above. During normal operation the back EMF has always smaller magnitude than the driving EMF of the power source, and thus some current in the winding is present, and thus torque on the rotor remains non-zero.
The current flowing through the rotor must be controlled to keep a constant RPM. If there is no external load, there is a small current only to overcome the frictions in the bearings (of the motor itself, and also in the gear boxes assembled in the shaft, if it is the case).
Depending on the external loads, it necessary to increase the torque for the motor keeps in the same RPM. It is achieved by increasing the current of the rotor. The current oscillates around an average value depending on the variations of the external load. The objective of the motor control is exactly to keep the RPM as stable as possible, even for a variable load.
The control must balance the power of the motor, that is the product of the torque generated by the magnetic force by the angular velocity, and the required power from the external loads.
