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Traction motor refers to an electric motor providing the primary rotational torque of a machine, usually for conversion into linear motion (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, electric motors in other products (such as the main motor in a washing machine) are described as traction motors.

Contents

Transportation applications

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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. See for example the Pennsylvania Railroad DD1, the PRR L5 and the various Swiss Crocodiles. 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.

Road vehicles

Traditionally road vehicles (cars, buses and trucks) have used diesel and petrol engines with a mechanical or hydraulic transmission system. In the latter part of the 20th century, vehicles with electrical transmission systems (powered from both combustion engines and/or batteries or fuel cells) began to be developed - one advantage of using electric motors is that specific types can regenerate energy (ie act as a dynamo) - providing braking as well as increasing overall efficiency.

Rating

In diesel-electric and gas turbine-electric locomotives the horsepower rating of the traction motors is usually around 81% that of the prime mover[1][2]. 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% × 90% = 81%.

Cooling

Because of the high power levels involved, traction motors are almost always cooled using forced air.

Manufacturers

See also

External links


Simple English

See also: Electric vehicle and Electric motor

Traction motor refers to a type of electric motor. A traction motor is used to make rotation torque on a machine. It is usually changed into a straight line motion.

Traction motors are used in electrically powered rail vehicles such as electric multiple units and electric locomotives. They are also used in electric vehicles, such as electric milk floats, elevators, and conveyors. Vehicles with electrical transmission systems such as Diesel-electric locomotives, electric hybrid vehicles and battery powered electric vehicles.

Contents

Transportation applications

Railroad

Railroad first used DC motors. These motors usually ran on about 600 volts. High-powered semiconductors were developed to control the switching of AC motors. They have made AC induction motors a better choice.[1] An induction motor does not require contacts inside the motor. These AC motors are simpler, and more reliable than the old DC motors. AC induction motors known as asynchronous traction motors.

Before the mid-20th century, a single large motor was often used to drive multiple wheels through connecting rods. This was the same way that steam locomotives would turn their drive wheels. Now, the normal practice is to use one traction motor to drive each axle through a gear drive.

Usually, the traction motor is mounted between the wheel frame and the driven axle. This is called a "nose-suspended traction motor". The problem with this mounting is that some of the motor's weight is on the axle. This causes the track and frame to wear out faster.[1] 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.

The DC motor is made in two parts; the rotating armature and the fixed field windings. The field windings, also called the stator, surrounds the armature. The field windings are made of tightly wound coils of wire inside the motor case. The armature, also called the rotor, is another set of coils of wire wound round the central shaft. The armature is connected to the field windings through brushes. The brushes are spring loaded contacts pressing against the commutator. The commutator sends the electricity in a circular pattern to armature windings. A series-wound motor has the armature and the field windings connected in series. A series-wound DC motor has a low electrical resistance. When voltage is applied to the motor, it makes a strong magnetic field inside the motor.[1] This produces a high amount of torque, so it is good for starting a train. If more current than needed is sent to the motor, there would be too much torque and the wheels would spin. If too much current is sent to the motor, it could damage the motor. Resistors are used to limit the current when the motor starts.

As the DC motor starts to turn, the magnetic fields inside start to join together. They create an internal voltage. This electromagnetic force (EMF) works against the voltage sent to the motor. The EMF controls the current flow in the motor. As the motor speeds up, the EMF falls. Less current flows into the motor, and it makes less torque. The motor will stop increasing its speed when the torque matches (is the same as) the drag on the train. To accelerate the train, more voltage must be sent to the motor. One or more resistors are removed to increase the voltage. This will increase the current. The torque will increase, and so will the speed of the train. When no resistors are left in the circuit, full line voltage is applied directly to the motor.

On an electric train, the train driver originally had to control the speed by changing the resistance manually. By 1914, automatic acceleration was being used. This was achieved by an accelerating relay in the motor circuit. This was often called a notching relay. The relay would watch the fall of current and control the resistance. All the driver had to do was select low, medium or full speed. These speeds are called shunt, series and parallel from the way the motors were wired.

Road vehicles

See also: Hybrid electric vehicle and Electric vehicle

Traditionally road vehicles (cars, buses and trucks) have used diesel or gasoline engines with a transmission. In the latter part of the 20th century, vehicles with electrical transmission systems began to be developed. These vehicles have a source of electricity from batteries or fuel cells. They may also be powered by an internal combustion engines.

A benefit from using electric motors is that some types can generate energy. They act as a dynamo during braking. This helps improve the efficiency of the vehicle.

Cooling

Because of the high power levels used by traction motors, they create a lot of heat. They usually require colling, often with forced air.

References

  1. 1.0 1.1 1.2 "Electric Traction Drives". railway-technical.com. Railway Technical Web Pages. http://www.railway-technical.com/drives.shtml#How. Retrieved 2010-09-17. 

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