Direct current (DC) motors operating from DC convert electrical energy into mechanical energy as rotary torque, just as AC motors convert electrical energy into torque.

The fundamental physical principle on which every electric motor operates is that a conductor carrying a current within a magnetic field experiences a force. If a conductor carrying DC current is placed at right angles to a magnetic field formed by either an electromagnet or a permanent magnet, it will experience a force perpendicular to the field and to itself.

This force is proportional to magnetic flux density, the current in the wire, and the length of the wire conductor.

By winding the wire into loops or coils, its effective length can be increased, thus increasing the interaction between the wire and the magnetic field. DC current enters one end of the coil and exits from the other end. The resultant forces acting on a single wound wire coil generate a rotary torque.

The rotating member of a DC or universal motor is usually called the armature, to distinguish it from the rotating member of an induction motor, usually called the rotor. Torque produced on the coil of the armature is proportional to the sine of the angle between the magnetic field and the coil.

Thus, as the coil becomes perpendicular to the magnetic field, its rotation ceases and the armature and shaft will stop unless a means is found to keep it moving in the same direction (clockwise or counterclockwise). The device most often used to perform this switching action is the mechanical commutator, a ring assembly formed from insulated conductive segments.

DC motor armatures have multiple coils wound on cores made of stacked steel laminations. The ends of each coil are brought out and connected to opposing commutator segments.

DC enters the commutator segment from a carbon brush, flows through the coil, and exits from the opposite segment to another brush. Commutation keeps the armature rotating so that current flows through the each coil in succession continuously until the power is shut off.

Because motor torque is proportional to the number of coils and commutator segments, as their number increases the torque will increase and armature rotation will become smoother.

The three most common DC motors are shunt-wound motors, series-wound motors, and compound-wound motors. Because each of these motor designs has advantages and disadvantages, the selection of the most appropriate motor for an application depends on the motor characteristics best able accomplish the desired task.

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