In most dc motors, the brushes supply the current to the coils as the rotor rotates one at a time, after another. The ‘width’ of the brushes should be well planned. In general, ‘charging’ the motor, its rotation does not vary much, but, will be requested more power of the power supply. To change the angular velocity, you must change the applied voltage.
https://www.mrosupply.com/v-belts/v-belts/a-section/1239708_a23_gates-rubber/The direction of rotation of the rotor will depend on the asymmetries of the motor and the direction of the electric current and inverting the direction of the current the motor will begin to turn backwards. This is how a toy train ‘walks backwards’: with the direction of the current in its rotor, inverted. The rotor of a DC motor rotates at an angular velocity proportional to the voltage applied to its coils. These coils have small electrical resistance and consequently would be traveled by intense electric currents if the rotor remained at rest. However, once in motion, changes in the magnetic flux over the coils generate a counter-electromotive force that removes energy from that current and decreases the electrical stresses on such coils. The resulting torque will be canceled when this force equals the applied electric voltage; the angular velocity becomes constant.