Why does adding a mechanical load make a direct-current motor draw more current?

When a direct-current (DC) motor is connected to a constant-voltage source, it generates an internal voltage (often called back electromotive force) that is proportional to its rotational speed. This generated voltage opposes the source voltage, which serves to limit the amount of current flowing through the armature.1

When a mechanical load is applied to the motor, the rotor slows down. Because the generated voltage is proportional to speed, this decrease in speed causes the generated voltage to drop. With less opposing voltage present, the difference between the source voltage and the generated voltage increases, which results in a higher current draw. This increased current allows the motor to produce the additional torque required to match the applied mechanical load.1