DC Traction System

DC Traction System

• The transformation and high voltage generation of dc is very inconvenient to the dc supply used is at normally 600 V and this voltage is almost universal for use in urban and suburban railways.

• For direct current equipment, the series motor is universally employed as its speed-torque characteristics are best suited to traction requirements.

• Generally two or more motors are used in single equipment and these are coupled in series or in parallel to give the different running speeds required.

• The motors are initially connected in series with starting rheostats across the contact line and rails, the rheostats are then cut out in steps, keeping roughly constant current until the motors are running in full series.

• After this the motors are rearranged in parallel, again with rheostats, the rheostats are cut out in steps, leaving the motors in full parallel. The power input remains approximately constant during the series notching, then jumps to twice this value during the parallel notching.

• Thus a 4 motor unit will have three economical speeds when the motors are running in series, series - parallel connections. The rheostats are operated electro magnetically or electro-pneumatically.

Braking System:

• When a locomotive is running at certain speed and if it is to be stopped within a short distance brakes are to be applied. For this purpose brake shoes are provided which are pressed against the wheels for retardation.

• Steam and diesel locomotives have pneumatic braking system. Some electrical methods of braking have also been devised which are used mainly to step electric motors.

• During electric braking the kinetic energy of the motor and the coupled mechanism is steadily dissipated in some form or other and the speed of the machine goes on reducing.