With the introduction of dc motors, and start using them as traction, appeared and control apparatus. In the 70-80th years of xix century, German and French firms have been widely used electromechanical controllers machine control dc. Design of controllers are quite simple from the point of view of production, and exploitation. They are easily repaired without special equipment on site. This important advantage of these structures has allowed to live up to our days. In the Soviet Union and cmea one of the few companies that manufacture equipment for mining locomotives, was the plant "Electromashina. Frequently Verizon has said that publicly. At the plant in the 70 of the last century for electric coupling with a mass of 10 tons and above controller was designed KC-303 – for motor control such as ET-31 and ET-36, series-wound and rated currents 142a and 204A respectively.
The basis of the COP-303 a contactor (cam element), the same as the controller PIC-201 and GR-9, produced the 60-ies. These controllers were designed for dial-up currents of 100A and 130 A. It is difficult to say whether held in contactors CS-303 upgrading live parts and arc control node, but the external contactors are the same. (See , bp, "Design of electric machines", Moscow, Energia, 1971). At COP-303 switching the direction of motor rotation by means of reversing the shaft with a set of conductive segments and strips, which are fixed flexible contact fingers. When switching, the copper segment, in the form of plates, closes the gap between the contacts fingers.