A direct current machine (DC machine) is a rotating electric machine capable of converting direct current electrical energy into mechanical energy (as a DC motor) or mechanical energy into direct current electrical energy (as a DC generator). It is an electric machine that can convert direct current electrical energy and mechanical energy into each other. When it operates as a motor, it is a DC motor, converting electrical energy into mechanical energy; when it operates as a generator, it is a DC generator, converting mechanical energy into electrical energy.

The DC motor is equipped with a ring-shaped permanent magnet. As current passes through the coils on the rotor, it generates an ampere force. When the coils on the rotor are parallel to the magnetic field, the direction of the magnetic field they continue to rotate into changes. Consequently, the brushes at the end of the rotor alternate with the commutator, thereby reversing the direction of the current in the coils. The direction of the Lorentz force remains unchanged, allowing the motor to maintain a consistent rotational direction.

The working principle of a DC generator is to convert the alternating electromotive force induced in the armature winding into direct current electromotive force by the commutator's cooperation with the brush's reversing action, so that it is output as DC electromotive force from the brush end.

The direction of the induced electromotive force is determined according to the right-hand rule (the magnetic flux lines point towards the palm, the thumb points in the direction of the conductor's motion, and the direction of the other four fingers indicates the direction of the induced electromotive force in the conductor).

The direction of force on the conductor is determined by the left-hand rule. This pair of electromagnetic forces creates a torque acting on the armature, known as electromagnetic torque in a rotating motor. The direction of the torque is counterclockwise, attempting to rotate the armature in a counterclockwise direction. If this electromagnetic torque can overcome the resistive torque on the armature (such as that caused by friction and other load torques), the armature can rotate counterclockwise.