Traction motor typically refers to those motors that are used to power the driving wheels of a railroad locomotive, electrical multi-unit train (such as a subway or light rail vehicle train), or a tram.
Traditionally, these are large DC series motors usually running on approximately 600 volts. Nowadays, the availability of high-powered semiconductors (such as thyristors and the IGBT transistor) has now made practical the use of much simpler, higher-reliability AC induction motors (see asynchonous traction motor ). Synchronous ac motors are also occasionally used (as in the TGV).
Before large scale dieselization in the mid-20th century, a single large motor was often used to drive multiple driving wheels through connecting rods that were very similar to those used on steam locomotives. It is now standard practice to provide one traction motor driving each axle through a gear drive. Usually, the traction motor is simply suspended between the truck (bogie) frame and the driven axle; this is referred to as a "nose-suspended traction motor". The problem with such an arrangement is that a portion of the motor's weight is unsprung, increasing forces on the track. Occasionally, other mounting arrangements are made. In the case of the GG1, two truck-mounted motors drove each axle through a quill drive. In the case of the TGV power units, each axle is driven by a motor mounted to the power unit's frame; a "tripod" drive allows a small amount of flexibility in the drive train allowing the trucks (bogies) to pivot. By mounting the relatively-heavy traction motor directly to the power unit rather than to the truck (bogie), better dynamics are obtained allowing much-improved high-speed operation.
Because of the high power levels involved, traction motors are almost always cooled using forced air.