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A traction motor is an electric motor used for propulsion of a vehicle, such as , electric vehicle or , or electric multiple unit trains.
Traction motors are used in electrically powered railway vehicles (electric multiple units) and other including electric , , , , and , as well as vehicles with electrical transmission systems (diesel–electric locomotives, electric ), and battery electric vehicles.
The first experimental electric traction motor of 1875 was rapidly developed internationally for city use. In the 19th century traction motor passenger car companies began to compete with the dominant citywide Horsecar transportation system.
A variant of the DC system is the AC series motor, also known as the universal motor, which is essentially the same device but operates on alternating current. Since both the armature and field current reverse at the same time, the behavior of the motor is similar to that when energized with direct current. To achieve better operating conditions, AC railways are often supplied with current at a lower frequency than the commercial supply used for general lighting and power; special traction current power stations are used, or used to convert 50 or 60 Hz commercial power to the 25 Hz or Hz frequency used for AC traction motors. Because it permits the simple use of , the AC system allows efficient distribution of power down the length of a rail line, and also permits speed control with switchgear on the vehicle.
AC induction motors and synchronous motors are simple and low maintenance, but up until the advent of power semiconductors, were awkward to apply for traction motors because of their fixed speed characteristic. An AC induction motor generates useful amounts of power only over a narrow speed range determined by its construction and the frequency of the AC power supply. The advent of power semiconductors has made it possible to fit a variable frequency drive on a locomotive; this allows a wide range of speeds, AC power transmission, and the use of rugged induction motors that do not have wearing parts like brushes and commutators.Andreas Steimel Electric Traction - Motive Power and Energy Supply: Basics and Practical Experience Oldenbourg Industrieverlag, 2008 ; Chapter 6 "Induction Traction Motors and Their Control"
Usually, the traction motor is three-point suspended between the 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 weight, increasing unwanted forces on the track. In the case of the famous Pennsylvania Railroad GG1, two frame-mounted motors drove each axle through a quill drive. The "Bi-Polar" electric locomotives built by General Electric for the Milwaukee Road had direct drive motors. The rotating shaft of the motor was also the axle for the wheels. In the case of French TGV , a motor mounted to the power car'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 car's frame, rather than to the bogie, better dynamics are obtained, allowing better high-speed operation.
As traction motors use a reduction gear setup to transfer torque from the motor armature to the driven axle, the actual load placed on the motor varies with the gear ratio. Otherwise "identical" traction motors can have significantly different load rating. A traction motor geared for freight use with a low gear ratio will safely produce higher torque at the wheels for a longer period at the same current level because the lower gears give the motor more mechanical advantage.
In diesel-electric and gas turbine-electric locomotives, the horsepower rating of the traction motors is usually around 81% that of the prime mover. 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: 0.9 × 0.9 = 0.81
Individual traction motor ratings usually range up .
Another important factor when traction motors are designed or specified is operational speed. The motor armature has a maximum safe rotating speed at or below which the windings will stay safely in place.
Above this maximum speed centrifugal force on the armature will cause the windings to be thrown outward. In severe cases, this can lead to "birdnesting" as the windings contact the motor housing and eventually break loose from the armature entirely and uncoil.
Bird-nesting (the centrifugal ejection of the armature's windings) due to overspeed can occur either in operating traction motors of powered locomotives or in traction motors of dead-in-consist locomotives being transported within a train traveling too fast. Another cause is replacement of worn or damaged traction motors with units incorrectly geared for the application.
Damage from overloading and overheating can also cause bird-nesting below rated speeds when the armature assembly and winding supports and retainers have been damaged by the previous abuse.
Typical cooling systems on U.S. diesel-electric locomotives consist of an electrically powered fan blowing air into a passage integrated into the locomotive frame. Rubber cooling ducts connect the passage to the individual traction motors and cooling air travels down and across the armatures before being exhausted to the atmosphere.
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