An electric motor used in a locomotive or motor car to drive one or more moving wheel shafts. There are many types of double tractors, such as dc traction motor, ac asynchronous traction motor and ac synchronous traction motor. Dc traction motor, especially dc series excitation motor has better speed regulation performance and working characteristics, which can meet the needs of locomotive traction characteristics and has been widely used.

The operating principle of the traction motor is the same as that of the general dc motor, but it has special working conditions: the space size is limited by the gauge and the diameter of the driving wheel; In the locomotive running through the rail gap and turnout to withstand considerable impact vibration; When the large and small gears do not mesh well, the armature will produce strong torsional vibration. It is easy to be invaded by rain, snow and ash sand in harsh environment. So traction motor has many requirements in the design and structure, such as to make full use of the body's internal space structure is compact, to adopt high insulation materials and magnetic materials, spare parts to have higher mechanical strength and stiffness, the motor needs to have good ventilation cooling conditions and dustproof moistureproof ability, to take special measures to cope with the difficult conditions of "reverse" to reduce the carbon brush of the spark and so on.