Traction motor
Traction motor refers to an electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...

 providing the primary rotational torque of a machine, usually for conversion into linear motion
Linear motion
Linear motion is motion along a straight line, and can therefore be described mathematically using only one spatial dimension. The linear motion can be of two types: uniform linear motion, with constant velocity or zero acceleration; non uniform linear motion, with variable velocity or non-zero...

Traction (engineering)
Traction refers to the maximum frictional force that can be produced between surfaces without slipping.The units of traction are those of force, or if expressed as a coefficient of traction a ratio.-Traction:...


Traction motors are used in electrically powered rail vehicles such as electric multiple unit
Electric multiple unit
An electric multiple unit or EMU is a multiple unit train consisting of self-propelled carriages, using electricity as the motive power. An EMU requires no separate locomotive, as electric traction motors are incorporated within one or a number of the carriages...

s and electric locomotive
Electric locomotive
An electric locomotive is a locomotive powered by electricity from overhead lines, a third rail or an on-board energy storage device...

s, other electric vehicles such as electric milk floats, elevator
An elevator is a type of vertical transport equipment that efficiently moves people or goods between floors of a building, vessel or other structures...

s, conveyors, and trolleybus
A trolleybus is an electric bus that draws its electricity from overhead wires using spring-loaded trolley poles. Two wires and poles are required to complete the electrical circuit...

es, as well as vehicles with electrical transmission systems such as diesel-electric, electric hybrid vehicles and battery electric vehicles. Additionally, electric motors in other products (such as the main motor in a washing machine) are described as traction motors.


Traditionally, these were series-wound brushed DC 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
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

 induction motor
Induction motor
An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are robust and have no brushes...

s known as asynchronous traction motors. Synchronous AC motor
Synchronous motor
A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets at the same rate as the power supply frequency and resulting rotating magnetic field which drives it....

s are also occasionally used, as in the French TGV
The TGV is France's high-speed rail service, currently operated by SNCF Voyages, the long-distance rail branch of SNCF, the French national rail operator....


Before the mid-20th century, a single large motor was often used to drive multiple driving wheel
Driving wheel
On a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotive's pistons...

s through connecting rods that were very similar to those used on steam locomotive
Steam locomotive
A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...

s. Examples are the Pennsylvania Railroad DD1
The Pennsylvania Railroad's class DD1 were semi-permanently coupled pairs of third rail direct current electric locomotives built for the railroad's initial New York-area electrification. They operated between Manhattan Transfer and Pennsylvania Station in New York City, and from there to the...

, FF1
The Pennsylvania Railroad's class FF1 was an American electric locomotive, a single prototype numbered #3931 and nicknamed "Big Liz" by its crews. It was built in 1917 for the task of hauling freight trains across the Allegheny Mountains, which the PRR planned to electrify; this was never...

 and L5
The Pennsylvania Railroad's class L5 were the railroad's second generation of production electric locomotives after the DD1, and the last to use a jackshaft and side rods to drive the wheels. The L5 was a single-unit locomotive instead of the twin-unit DD1. Thirteen were built...

 and the various Swiss Crocodiles
Crocodile (locomotive)
Crocodile electric locomotives are so called because they have long "noses" at each end, reminiscent of the snout of a crocodile . These contain the motors and drive axles, and are connected by an articulated center section. The center section usually contains the crew compartments, pantographs...

. It is now standard practice to provide one traction motor driving each axle through a gear drive.

Usually, the traction motor is three-point suspended between the bogie
A bogie is a wheeled wagon or trolley. In mechanics terms, a bogie is a chassis or framework carrying wheels, attached to a vehicle. It can be fixed in place, as on a cargo truck, mounted on a swivel, as on a railway carriage/car or locomotive, or sprung as in the suspension of a caterpillar...

 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. In the case of the famous Pennsylvania Railroad GG1
The PRR GG1 is a class of electric locomotives that was built for the Pennsylvania Railroad for use in the northeastern United States. A total of 140 GG1s were constructed by its designer General Electric and the Pennsylvania's Altoona Works from 1934 to 1943....

, two bogie-mounted motors drove each axle through a quill drive
Quill drive
A quill drive is a mechanism that allows a drive shaft to shift its position relative to its driving shaft. It consists of a hollow driving shaft with a driven shaft inside it...

. The "Bi-Polar
Milwaukee Road class EP-2
The Milwaukee Road's class EP-2 comprised five electric locomotives built by General Electric in 1919. They were often known as Bipolars, which referred to the bipolar electric motors they used. Among the most distinctive and powerful electric locomotives of their time, they epitomized the...

" electric locomotives built by General Electric
General Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...

 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 power car
Power car
A power car is a railroad vehicle that is closely related to the locomotive. What differentiates the two is their construction or their use. A true locomotive can be physically separated from its train and does nothing but provide propulsion . A power car, on the other hand, is frequently an...

s, 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."Under the Hood of a TGV"

The DC motor was the mainstay of electric traction drives on both electric and diesel-electric locomotives, street-cars/trams and diesel electric drilling rigs for many years. It consists of two parts, a rotating armature and fixed field windings surrounding the rotating armature mounted around a shaft. The fixed field windings consist of tightly wound coils of wire fitted inside the motor case. The armature is another set of coils wound round a central shaft and is connected to the field windings through "brushes" which are spring-loaded contacts pressing against an extension of the armature called the commutator. The commutator collects all the terminations of the armature coils and distributes them in a circular pattern to allow the correct sequence of current flow. When the armature and the field windings are connected in series, the whole motor is referred to as "series-wound". A series-wound DC motor has a low resistance field and armature circuit. Because of this, when voltage is applied to it, the current is high. (Ohms Law: current = voltage/resistance). The advantage of high current is that the magnetic fields inside the motor are strong, producing high torque (turning force), so it is ideal for starting a train. The disadvantage is that the current flowing into the motor has to be limited, otherwise the supply could be overloaded and/or the motor and its cabling could be damaged. At best, the torque would exceed the adhesion and the driving wheels would slip. Traditionally, resistors were used to limit the initial current.

As the DC motor starts to turn, the interaction of the magnetic fields inside causes it to generate a voltage internally. This "back-EMF" (electromagnetic force) opposes the applied voltage and the current that flows is governed by the difference between the two. As the motor speeds up, the internally generated voltage rises, the resultant EMF falls, less current passes through the motor and the torque drops. The motor naturally stops accelerating when the drag of the train matches the torque produced by the motors. To continue accelerating the train, series resistors are switched out step by step, each step increasing the effective voltage and thus the current and torque for a little bit longer until the motor catches up. This can be heard and felt in older DC trains as a series of clunks under the floor, each accompanied by a jerk of acceleration as the torque suddenly increases in response to the new surge of current. When no resistors are left in the circuit, full line voltage is applied directly to the motor. The train's speed remains constant at the point where the torque of the motor, governed by the effective voltage, equals the drag - sometimes referred to as balancing speed. If the train starts to climb an incline, the speed reduces because drag is greater than torque and the reduction in speed causes the back-EMF to fall and thus the effective voltage to rise - until the current through the motor produces enough torque to match the new drag.

If the train starts to descend a grade, the speed increases because the (reduced) drag is less than the torque. With increased speed, the internally-generated back-EMF voltage rises, reducing the torque until the torque again balances the drag. On a sufficiently steep grade, the internally generated back-EMF voltage may rise higher than the full line voltage. On such steep grades, the motor acts as a regenerative brake
Regenerative brake
A regenerative brake is an energy recovery mechanism which slows a vehicle or object down by converting its kinetic energy into another form, which can be either used immediately or stored until needed...

 -- the motor generates electric power, returns that power to the electric lines, and acts as a brake to prevent run-away acceleration down the grade.
(On such a steep grade, adding resistors -- or disconnecting the motor from the line entirely -- would make the train go faster).

On an electric train, the train driver originally had to control the cutting out of resistance manually, but by 1914, automatic acceleration was being used. This was achieved by an accelerating relay (often called a "notching relay") in the motor circuit which monitored the fall of current as each step of resistance was cut out. All the driver had to do was select low, medium or full speed (called "shunt", "series" and "parallel" from the way the motors were connected in the resistance circuit) and the automatic equipment would do the rest.

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 internal combustion engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

s and/or batteries or fuel cells) began to be developed—one advantage of using electric motors is that specific types can regenerate energy (i.e. act as a regenerative brake
Regenerative brake
A regenerative brake is an energy recovery mechanism which slows a vehicle or object down by converting its kinetic energy into another form, which can be either used immediately or stored until needed...

)—providing braking as well as increasing overall efficiency.


Electric locomotive
Electric locomotive
An electric locomotive is a locomotive powered by electricity from overhead lines, a third rail or an on-board energy storage device...

s usually have a continuous and a one-hour rating. The one-hour rating is typically about ten percent higher than the continuous rating, and limited by the temperature rise in the motor.

In diesel-electric and gas turbine-electric locomotive
Gas turbine-electric locomotive
A gas turbine - electric locomotive, or GTEL, is a locomotive that uses a gas turbine to drive an electric generator or alternator. The electric current thus produced is used to power traction motors. This type of locomotive was first experimented with during the Second World War, but reached its...

s, the horsepower
Horsepower is the name of several units of measurement of power. The most common definitions equal between 735.5 and 750 watts.Horsepower was originally defined to compare the output of steam engines with the power of draft horses in continuous operation. The unit was widely adopted to measure the...

 rating of the traction motors is usually around 81% that of the prime mover
Prime mover (locomotive)
In engineering, a prime mover is an engine that converts fuel to useful work. In locomotives, the prime mover is thus the source of power for its propulsion. The term is generally used when discussing any locomotive powered by an internal combustion engine... This assumes that the electrical generator
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...

 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%.

Individual traction motor ratings usually range up 1,600 kW (2,144 hp)


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

See also

  • Air cooling
  • Electric vehicle battery
    Electric vehicle battery
    An electric vehicle battery or traction battery is a rechargeable battery used for propulsion of battery electric vehicles...

  • Rare earths
  • Torque and speed of a DC motor
    Torque and speed of a DC motor
    The torque and torque density of an electric motor is not necessarily dependent on its speed. It is rather a function of flux and armature current.-Effects:Increase in flux decreases the speed but increases the torque...

  • Virtual transmission
  • Induction motor
    Induction motor
    An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are robust and have no brushes...

     & Three-phase AC railway electrification

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.