The DC motors are similar in their construction to the electric generators, when we observe in general 3/4 hp-56c-motors. From this finding the engines could be described as generators that work the other way around. When the current passes through the armature of a DC motor it generates a pair of forces by the magnetic reaction, so the armature rotates. As the speed of rotation will control the current flow in the armature, special devices must be used for DC motors starting.
The action of the commutator and the motor field coil connections are precisely the same as the generators use. The armature revolution induces a voltage in the coils and this voltage is in the opposite direction of the external voltage that is applied to the armature, this is known as induced voltage or counter-electromotive force. When the motor rotates faster, the induced voltage increases until it becomes almost equal to that applied. The current is then small, and the motor speed will be constant whenever the motor does not remain at low load and has to do other mechanical work than is required to move the armature.