Siemens has developed Sitrac, a new system for controlling the drive motors of rail vehicles. Sitrac requires fewer sensors, which reduces down time of the drive units and ultimately increases availability of trains. As reported in the research magazine Pictures of the Future, the system developed by Siemens rail technology experts in Erlangen is equipped with very robust control technology. The system recognizes the motor’s rotational speed, without requiring a rotational speed sensor, and uses the current converter to adjust the torque, and therefore also the rail vehicle’s traction.

Prior to Sitrac, control systems used direct measurement of the motor’s rotational speed to control the torque. These systems are equipped with a wheel speed sensor on the motor shaft. Such sensors use a magnet wheel and a magnetic sensor to measure the rotations — and thus the motor’s speed — as electrical pulses. The wheel speed sensors are built directly onto the motor, where they are subjected to strong mechanical and thermal stresses that lead to wear and tear. And because technical defects are almost impossible to predict, parts of the rail vehicle’s drive unit are also often out of action.

With the help of a computer model, the Sitrac system calculates the rotational speed on the basis of the measured currents and voltage supply. It then uses these values to determine the torque required at any given moment. To adjust the torque, the system controls the inverter that supplies power to the rail vehicle’s electric motor. In addition to ensuring less down time — which means fewer delays — the system provides passengers with a more comfortable ride. This is because Sitrac generates less vibration than conventional systems. Its control technology is also more resistant to mechanical and electrical faults.

Sitrac is already in use in a number of cities, including Shanghai, Boston and Paris. It can be adapted to almost any kind of train, from subways to long-distance transport systems. And regardless of whether trains run on direct current or alternating current, Siemens offers special software that adapts the complicated control of the drive units to the power grid in question.