Comparison between AC and DC Traction System (AC Traction vs DC Traction)

DC Traction System

A traction system in which DC series motors are used for running the electric trains is known as DC traction system. The DC power to be fed to DC series motor is obtained from substations which are located at a distance of 3 to 5 km for urban and suburban heavy traffic and 15 to 30 km for main line service.

AC Traction System

A traction system in which AC motors (1-phase or 3-phase induction motors) are used for running the electric trains is known as AC traction system.

In AC traction system, a high voltage is fed to the overhead line from substations and is tapped by the locomotive which has a step-down transformer installed in it which converts this high voltage to low voltage of the order of 300 to 400 volts. This reduced voltage is fed to the AC motor.

Comparison between AC Traction and DC Traction Systems

The following table compares and contrasts the various features of AC and DC traction systems −

Point of ComparisonDC Traction SystemAC Traction System
EfficiencyDC traction system is more efficient.AC traction system is less efficient.
Cost of motorDC series motors are cheaper.AC motors are expensive.
MaintenanceDC traction system requires less maintenance.AC traction system needs more maintenance.
Speed controlThe speed control of DC series motor is limited.With the AC motors, wide range of speed control is possible.
Cost of systemThe overall cost of the DC traction system is less.The overall cost of the AC traction is more.
Overhead distributionIn DC traction system, overhead distribution of electrical power is possible.In the AC traction system, overhead distribution is used.
AccelerationDC traction system is capable of giving high acceleration.AC traction system gives less acceleration.
TorqueDC series motor produces less torque.The starting and running torque developed by AC motors is more.
Regenerative brakingDC series motors have more efficient regenerative braking.In case of AC motors, the regenerative braking is less efficient.
Interference with communication linesDC traction system causes less interference with the communication lines.AC traction system produces more interference with communication lines.
Number of substationsDC traction system requires more number of substations for a given track distance.AC traction system requires less number of substations for a given track distance.
SuitabilityDC traction system is suitable for urban line railway service.AC traction system is suitable for main line railway service.
Energy consumptionEnergy consumption in DC traction system is less.For same output, energy consumed by AC traction system is more.
Weight of locomotiveWeight of DC locomotive is less.Weight of AC locomotive is more.
Capital cost of substationFor DC traction system, the capital cost of substation is more due to converting equipment.Capital cost of AC substation is less.
Cost of conductorIn DC traction system, the cost of overhead conductor is more as the operating voltages are low.AC traction system uses higher voltages so the cost of conductor is reduced.

Updated on: 27-Apr-2022

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