Traction motor

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See also: Electric vehicle and Electric motor

Traction motor refers to an electric motor providing the primary rotational torque of a machine, usually for conversion into linear motion ie traction.

Traction motors are used in electrically powered rail vehicles such as Electric multiple units and Electric locomotives, other electric vehicles such as electric milk floats,elevators and conveyors as well as vehicles with electrical transmission systems such as diesel-electric and electric hybrid vehicles. Additionally the electric motors in other products such as the main motor in washing machines are described as traction motors.

[edit] Transportation applications

[edit] Rail vehicles

Traditionally, these are DC series-wound motors, usually running on approximately 600 volts. The availability of high-powered semiconductors (such as thyristors and the IGBT) has now made practical the use of much simpler, higher-reliability AC induction motors known as asynchronous traction motors. Synchronous AC motors are also occasionally used, as in the French TGV.

Before 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. The "Bi-Polar" electric locomotives built by General Electric for the Milwaukee Road had gearless motors. The rotating shaft of the motor was also the axle for the wheels. In the case of the TGV power units, a motor mounted to the power unit’s frame drives each axle; 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.

[edit] Road vehicles

[edit] Rating

In diesel-electric and gas turbine-electric locomotives the horsepower rating of the traction motors is usually 81% that of the prime mover^{[citation needed]}. This assumes that the electrical generator converts 90% of the engine's output into electrical energy and the traction motors convert 90% of this electrical energy back into mechanical energy. Calculation: 90% x 90% = 81%.