Traction Rolling Stock (TRS) covers the electrical systems on board electric locomotives and EMU/MEMU rakes — how they draw power from the OHE via the pantograph and convert it to traction effort at the wheels.
Onboard Power Flow
- Pantograph collects 25 kV AC from the OHE contact wire.
- Circuit breaker and main transformer step the voltage down to a level usable by traction converters.
- Modern locomotives use converter-inverter (IGBT-based) systems to produce variable-voltage, variable-frequency AC for three-phase traction motors.
- Three-phase traction motors drive the wheels through a gearbox, converting electrical energy to mechanical tractive effort.
AC vs. Older DC Traction Motors
- Older locomotives used DC series traction motors, fed via tap-changer transformers and rectifiers.
- Modern three-phase AC traction motors (induction motors, converter-fed) are largely maintenance-free compared to DC motors, which need brush/commutator upkeep.
- Regenerative braking — modern AC locomotives can feed braking energy back into the OHE, unlike older DC systems that dissipated it as heat.
Exam Focus
Know why modern locos use three-phase AC traction motors over DC (lower maintenance, higher power density, regenerative braking capability) — this comparison is a recurring theme.
Real Exam Facts — Key Locomotive Classes
- WAP5 — a high-speed passenger electric locomotive rated for a maximum speed of 160 kmph, among the fastest conventional locos in service.
- WAG9 — a heavy-haul freight electric locomotive with a maximum axle load of 20.5 tons, matching the standard axle load limits on most Indian Railways routes.