Comparison Between DC and AC Traction System

DC Traction System :

In the dc traction system, dc series motors are mostly used. Sometimes, in tramways and trolley cars dc compound motors are also used. For tramways and trolleybuses, the operating voltage is 600V. Whereas in suburban trains, the operating voltage is between 1500V-3000V. Unattended sub-stations are spaced at a distance of 30-40 km.

Comparison Between DC and AC Traction System

The spacing of sub-stations depends upon operating voltage and traffic density of the route. In order to reduce the cost and losses of transmission, sub-stations are fed with ac at 33 to 110 kV. Each sub-station has a transformer and rotary converter to step down the voltage and convert it to dc at the desired level.

Advantages of DC Traction System :

  • DC series motor is more suitable for traction when compared to ac motor.
  • Maintenance cost is low.
  • The weight of the dc motor per kW output is low.
  • Better speed control.
  • Efficient regenerative braking.
  • A single conductor is enough as rail acts as the return conductor.

Disadvantages of DC Traction System :

  • The overall cost is high because of the additional cost of substation equipment such as converting equipment and boosters.
  • Due to low operating voltage, a conductor of a large cross-section is needed.
  • A more number of sub-stations spaced at a shorter distance is needed.

Applications of DC Traction System :

  • Sub-urban services.
  • Road transport where the distance of the run is small with frequent stops.

AC Traction System :

In this system, single-phase ac series motors are used. The operating voltages are in the range of 300-400V at a frequency of 25 or 16 2/3 Hz. High voltage in the range of 15kV to 25kV at 50 Hz is either directly fed to the locomotive or to the substation. High voltages will be supplied to the locomotive if the operating range is less than 30 km. If the operating range is more than 30 km, the high voltage will be given to the substation.

The substations or locomotives have transformers and frequency changing equipment to obtain the desired operating voltage and frequency. Mostly, the motors will operate at a voltage level of 300 to 400V. Sub-stations can be spaced at a distance of 50 to 80 km.

Advantages of AC Traction System :

  • Rapid acceleration and retardation have less effect on the system.
  • Less cost.
  • Low-frequency operation improves commutation, efficiency, and power factor of ac series motor.
  • Low-frequency operation reduces the line reactance and hence voltage drop.

Applications of AC Traction System :

  • Mainline service.
  • Sub-urban railways.

Comparison Between DC and AC Traction System :

FactorDC TractionAC Traction
CostDC series motors are cheap.AC series motors are somewhat expensive.
Cost of ConductorSince high voltage levels cannot be obtained in a dc system cost of the conductor is more.The cost of the conductor is less in an ac traction system due to high voltage levels.
Substation CostThe cost of a dc substation is more due to additional converting equipment.The cost of substation for ac power supply is less.
EfficiencyDC traction system is more efficient.AC traction system is less efficient.
MaintenanceA dc locomotive requires less maintenance.An ac locomotive requires more maintenance.
AccelerationIn dc traction, dc series motors are capable of giving high acceleration.The acceleration of ac motor is less compared dc series motor.
Speed ControlThe number of speeds obtained by the dc series motor is limited.In ac series motors wide range of speed control is obtained.
InterferenceDC power systems produce less interference with communication lines.A considerable amount of interference is produced with communication lines in the case of an ac system.
Regenerative BrakingIn a dc series motor, regenerative braking is more efficient.In an ac series motor, regenerative braking is less efficient.
SubstationFor a given track length, the number of substations required is more as the voltage above the prescribed level cannot be obtained.For a given track length, the number of substations required is less since high voltage levels can be obtained by using transformers.
ServiceDC traction system is suitable for suburban service.AC traction system is mainly employed for mainline service.
Energy ConsumptionEnergy consumption is less in a dc traction system.Energy consumption is more in ac traction system.

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