ELECTRIC TRACTION - PART - 05 - SPEED TIME CURVE FOR TRAIN MOVEMENT & CATEGORIES OF RAILWAY SERVICES

DIFFERENCE BETWEEN ACCELERATION AND RETARDATION

ACCELERATION refers to the rate in change of velocity of a moving body. If a body is moving at a constant velocity, there is no change and hence it has no acceleration.

RETARDATION is the application of a force that produces negative acceleration. Synonyms would be braking, deceleration, damping, etc. Gravitational force operates downward (in a negative direction) so, in most frames of reference, gravity is a retarding force.

SPEED – TIME CURVE

The curve drawn between speed (km per hour) and the time (in minutes or seconds) is called ‘speed time curve’.

Speed-Time curve is a graph showing the variation of speed as function of time.

The speed time curve consists of

1. NOTCHING OR STARTING PERIOD [OA]

During this period, the traction motor accelerates from rest.

2. ACCELERATING PERIOD [AB]

During this period, according to the torque-speed characteristic of the motor, the torque gradually decreases and the speed increases, but the train still continues to accelerate.

3. FREE RUNNING PERIOD [BC]

During this period, the train runs at constant speed.

4. COASTING PERIOD [CD]

(Running with power switched off and therefore, there is retardation due to frictional and windage forces)

During this period, the power is cut off and the train moves due its kinetic energy.

The train slows down due to retardation production.

5. BRAKING PERIOD [DE]

During this period, brakes are applied and the train is brought is stop.

INFORMATION OBTAINED FROM SPEED – TIME CURVE

1. The curve can give speed of the train at any instant.

2. The slope of the curve, at the instant, gives acceleration or 
    retardation (a decrease in rate of change).

3. If the slope is positive it is an acceleration, if negative, it is 
    retardation and if zero, the speed is constant at that instant.

4. The area under the curve give the total distance covered.