Types of traction system by supply source

Electric traction is the propulsion of vehicles (trains, trams, trolley buses, electric buses) using electric motors, with the electrical energy taken from a supply line or produced or stored on the vehicle.

Types of electric traction based on supply source

A. Self-contained (non-electrified track) systems — the vehicle carries its own source.

1. Diesel-electric: a diesel engine drives a generator that feeds dc or ac traction motors.
   • Advantages: no overhead line, can run on any route, low line cost, high availability.
   • Disadvantages: low overall efficiency (about 25%), heavy locomotive, high running and maintenance cost, pollution, limited overload capacity.
2. Battery-electric: batteries feed dc motors.
   • Advantages: no pollution, simple, quiet; good for short distances, shunting, mines and e-rickshaws/e-buses.
   • Disadvantages: limited range, heavy batteries, long charging time, high battery replacement cost.

B. Electrified track (supply from distribution network) — power is taken from an overhead wire or third rail.

1. DC system: 600–750 V (tramways, metro), 1500–3000 V (suburban, main line), using dc series motors.
2. AC system: single-phase 25 kV, 50 Hz (most common), single-phase low frequency, or three-phase.
3. Composite system: ac in the line and rectifier or inverter on board.

Advantages of electric traction from line supply

• Clean, no smoke; ideal for cities and tunnels.
• High starting torque and acceleration, so higher schedule speed.
• Low maintenance and running cost; long life of locomotives.
• Regenerative braking returns energy to the line.
• Higher overall efficiency when fed from hydro power (important for Nepal).

Disadvantages

• Very high initial cost of overhead line, substations and electrification.
• Failure of supply stops all trains on the section.
• Trains can run only on electrified routes.
• Interference with telecommunication lines (ac systems).
• Additional cost of signalling changes.