Traction inverters and converters
A traction inverter converts DC power from the catenary or intermediate DC link into the variable-frequency AC supply that drives the traction motors of electric locomotives and multiple units.
No European catenary voltage matches what a traction motor needs. The inverter closes that gap — and its semiconductor technology is undergoing the most significant shift since the 1990s.
Within the wider traction and propulsion chain, the inverter is the stage that turns a fixed supply into controlled motion.
Function in the traction chain
Electric traction systems operate at fixed catenary voltages — 750 V or 1,500 V DC on urban and regional networks, 3 kV DC in 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 long-distance network.
Regardless of the supply, the traction motor requires a variable-frequency, variable-voltage AC feed to control torque and speed. The inverter provides that 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 the DC link voltage to three-phase AC for the motors; on DC-fed systems, the catenary feeds the link directly.
Semiconductor technology
Insulated-gate bipolar transistors (IGBTs) at 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 them 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 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 is incremental: hybrid modules pairing a silicon IGBT with a SiC freewheeling diode reached the market 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 and return energy to the DC link.
That energy can be fed back into the catenary for adjacent accelerating trains — the most efficient outcome — stored in onboard 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.
EN 50155 covers environmental and electromagnetic compatibility requirements for all electronic equipment on rail vehicles, while electromagnetic compatibility for rolling stock apparatus is governed by EN 50121-3-2.