Tractive effort is the force required to haul a load. Tractive effort when multiplied with speed at which the train is required to run gives the Horse Power. Importance of Tractive effort lies with its capability to start a train on gradient and run it up to a speed of around 30 kmph whereas that of Horse Power is its ability to achieve and run at the maximum permissible speed. The most important source to provide Traction Mechanics – Train, grade, curve and Acceleration Resistance power in the locomotive is Traction Motor. Tractive Motor is a class of Electric Motor which has the characteristics to match the load requirement of traction such as high starting tractive effort and self corrective action with variable load.
Tractive Effort vs Speed Characteristics
Traction Motor used for traction purpose is DC Series Motor, Separately Excited DC Series Motor, Three Phase induction Motor, Synchronous Motor and Permanent Magnet Synchronous Motor. Whatever may be source or principle of working of traction motor, all provides Tractive effort characteristics to encounter train resistance and provide acceleration.
AB This is constant tractive effort line generally up to a speed of 30-50kmph. Point B is the maximum power point. OA is decided based on the continuous rating of Traction Motor and adhesion limits. This curve is lifted upward by overloading traction motor utilising thermal capacity of the traction motor.
BC This is a constant power line in which tractive effort drops with increasing speed in inverse proportion. For passenger locos, need for tractive effort is less as compared to speed therefore tractive effort characteristics at lower end is at reduced power.
CD Maximum limit of speed is imposed on the mechanical design of the traction motor and mechanical transmission.
TR This is train resistance curve. Point of intersection determines the speed at which tractive effort equals train resistance. Acceleration reserve exists up to the point of intersection. The curve TR moves up and down while running on up and down gradient.
EF This line gives the limit of adhesive weight and Tractive effort has to be within this limit.
Thermal Overload of Traction Motor
It is possible to overload a motor for a short period because the temperature rise is slow as compared to time period required for acceleration. This time period is in the range of 2, 10 and 30 minute overloading capability of 150, 120 and 110% during which the temperature rise is within the class of insulation for which the traction motor is designed. This helps in taking advantage during start of the motor when it has to exert high tractive effort where current drawn is more than the continuous rating and thereafter for some more time when it provides higher acceleration effort.
Gearing on Electrical Locomotive and Electrical Multiple Units
Driving force of Traction motor is transmitted to wheel through reduction gearing arrangement to meet the requirement of high tractive effort and low speed as compared to high speed and low tractive effort of traction motor. Transmission may be either through only reduction gear called partly suspended motor and via flexible coupling and reduction gear called fully suspended motor.
Let number of teeth & rpm of traction motor pinion and wheel gear are t & Ntm and T & Nw (note t is always smaller than T) respectively, wheel diameter is Dw in meter and Ntm in rpm then we get
; T/t is also called gear ration Gr
Tractive effort available at wheel is
Where ɳ is efficiency, τ= torque produced by TM, Dw is wheel diameter and Gris called gear ratio
The function of gearing is similar to transformer in electrical circuit where voltage and current represents torque and speed respectively.
A high gear ratio locomotive produces more tractive effort at start as compared to low gear ratio locomotive and decides the suitability of locomotive for freight and passenger application with same rating of traction motor.
Comparison of Gear ratio for different class of locomotive and Electrical Multiple unit