The brushed DC motor is an internally rectified motor designed for use in DC power supplies. The brushed motor is the first commercially important electrical drive mechanical energy application. The DC power distribution system has been used in commercial and industrial buildings for more than 100 years. The working voltage or magnetic field strength can change the speed of a brushed DC motor. According to the connection of the magnetic field and the power supply, the speed and torque characteristics of the brushed motor can be changed to provide a stable speed or speed that is inversely proportional to the mechanical load. Brush motors continue to be used in electric propulsion, cranes, paper machines and steel rolling mills. Because the brushes are worn out and need to be replaced, brushless DC motors that use power electronics have replaced brushed motors in many applications.
When the current passes through the coil wound on the soft iron core, one side of the positive electrode receives an upward force, while the other side receives a downward force. According to Fleming's left-hand rule, force produces a rotating effect on the coil, causing it to rotate. In order to make the motor rotate in a constant direction, the "DC" commutator reverses the current every half cycle (in a bipolar motor) so that the motor continues to rotate in the same direction.
One problem with the above motor is that when the plane of the coil is parallel to the magnetic field, ie. When the rotor poles and the stator poles are at 90 degrees-the torque is zero. In the picture above, this happens when the core of the coil is horizontal-when it is about to reach the position. The motor cannot be started in this position. However, once it starts, it will continue to rotate in this position through momentum.