The circuit shown in Figure 1 uses the voltage source to control the speed or power of a motor in a robotics or mechatronics application. The power supply is a dual-voltage source as shown in the figure.
By selecting the voltage, it is possible to control the speed or torque of the motor. For instance, we can use a 12 V motor and power it from a 9 + 3 V power supply. With the switch in one position (A), 12 V is sourced to the motor, and in position (B), 9 V is sourced. If we use multiple cells and a switch with one pole and many positions, we can control the applied voltage in Steps of 1.5 V.
Remember that small DC motos acepts short voltage burst to start or increase the power by moments. A 9 V DC motor, for example, can be powered with 12 V to start or to increase the power by some seconds.