Electric rail traction - Converters and motors

The political decision to increase train speeds and the performance of freight trains led SNCF to move towards the use of three-phase – synchronous and asynchronous – motors from the 1980s onwards.

These motors are characterized by a higher power density than DC motors. This is necessary, as the mass of the traction unit is a critical factor in limiting train speed, due to problems of track degradation.

Since the end of the eighties, this type of motorization has become widespread on all new rolling stock: commuter and regional railcars, TGV railcars and locomotives.

Wound-rotor synchronous motors are not industrially attractive for self-propelled railcars, as the unit power of these motors is only a few hundred kilowatts. Below 1 MW, rotor sizing does not vary proportionally with the motor's design power.

Three-phase motors are controlled by inverters supplied from :

• a DC power source for synchronous and asynchronous motors;

• a DC voltage source only for the asynchronous motor.

SNCF did not opt for the use of a synchronous motor fed from a DC voltage source, as the inverter structure is considerably more complex and costly than the one chosen.

This choice of motorization has only become interesting thanks to the recent and progressive evolution (over the last two decades) of power semiconductors. Optimizing the mass and volume of on-board equipment is an essential criterion, and minimizing the number of power semiconductors is often at the limit of current technological know-how.

Powering the traction unit from the catenary involves adapting the supply voltage and/or current of the three-phase inverters using input converter structures of varying complexity. Before designing the structure and control of input converters, it is important to define their electromagnetic compatibility with respect to the railway and public environment.

In view of these various technical aspects, we will approach this study of static converters and traction motors in the following order:

• electromagnetic compatibility in railway applications ;

• power semiconductors ;

• DC and single-phase catenary input converters ;

• synchronous and asynchronous motors.