Traction inverters & converters
A traction inverter is the power electronics unit, fitted to electric locomotives and multiple units across the European rail network, that converts DC power from the catenary or intermediate DC link into the variable-frequency AC supply required to drive electric traction motors.
Electric traction systems operate at fixed catenary voltages — 750 V or 1,500 V DC on urban and regional networks, 3 kV DC on parts of southern and central Europe, 15 kV AC at 16.7 Hz in the German-speaking countries and Scandinavia, and 25 kV AC at 50 Hz across much of the European high-speed and intercontinental network.
Regardless of the supply system, the traction motor requires a controlled variable-frequency, variable-voltage AC supply to manage torque and speed. The inverter provides this conversion.
On AC-fed systems a traction transformer and four-quadrant converter first step down and rectify the catenary voltage to an intermediate DC link, typically 1,500–3,600 V. The inverter then converts this DC link voltage to three-phase AC for the motors. On DC-fed systems the catenary voltage feeds the DC link directly.
Semiconductor technology
Insulated-gate bipolar transistors (IGBTs) at voltage ratings of 1,700 V, 3,300 V, or 6,500 V — selected according to catenary voltage — have been the standard switching device in railway traction converters since the 1990s.
Silicon carbide (SiC) metal-oxide-semiconductor field-effect transistors (MOSFETs) are displacing IGBTs in new designs. SiC devices switch faster, generate lower losses at a given power level, and enable higher switching frequencies, which reduce harmonic distortion and allow smaller passive components.
A Europe’s Rail Joint Undertaking demonstrator project testing SiC-based converters on a dual-system tramway found twice the power density of an equivalent silicon-based system, alongside a 25–30% reduction in losses.
The shift to SiC is incremental: hybrid modules combining a silicon IGBT with a SiC freewheeling diode were commercially available before full-SiC assemblies. Full-SiC modules from suppliers including Infineon, Mitsubishi, and Wolfspeed are now entering series production for rail.
Regenerative braking
The inverter also manages regenerative braking. When a train decelerates, the traction motors operate as generators, returning energy to the DC link. This energy can be fed back into the catenary for use by adjacent accelerating trains — the most efficient outcome — stored in on-board or wayside energy storage, or dissipated as heat in roof-mounted resistor banks when no absorbing load is available.
Modern electric trains recover 15–35% of total traction energy through regeneration depending on service pattern and the availability of receptive loads on the same electrical section.
Regulatory framework
Power converters installed on rolling stock are subject to IEC 61287-1 (adopted in Europe as EN 61287-1), which defines terminology, service conditions, and test methods. The broader electronics standard EN 50155 covers environmental and electromagnetic compatibility requirements for all electronic equipment on rail vehicles. Electromagnetic compatibility for rolling stock apparatus is governed by EN 50121-3-2.