Motor control system makes trains more reliable

9 August 2006 Power Electronics / Power Management

Developed by Siemens rail technology experts in Erlangen, Germany, for controlling the drive motors of rail vehicles, the Sitrac system requires fewer sensors, a design factor that reduces downtime of the drive units and ultimately increases availability of trains. Equipped with very robust control technology, Sitrac recognises the motor's rotational speed without requiring a rotational speed sensor, and uses a 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 torque. These systems are equipped with a wheel speed sensor on the motor shaft that uses a magnet wheel and magnetic sensor to measure the rotation - and thus the motor's speed - as electrical pulses. These speed sensors are built directly onto the motor, where they are subjected to strong mechanical and thermal stresses that lead to wear and tear.

With the Sitrac system, a computer model helps calculate the rotational speed on the basis of the measured currents and voltage supply, and 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.

Besides ensuring less downtime, the system provides passengers with a more comfortable ride, as 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 across the globe. 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 or alternating current. Siemens offers special software that adapts the complicated control of the drive units to the power grid used.

For more information contact Siemens Southern Africa, +27 (0)11 652 2000.

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