Pictures of the Future    Spring 2006

The Competitive Drive

Rail systems have to compete with airplanes and cars. That’s why only the most economical, reliable, fastest and comfortable trains will attract passengers. With its versatile, high-performance drives, Siemens is ensuring a competitive future for rail travel.

A man is listening to classical music on headphones while his wife reads in a leather armchair and their son watches a video on a flat-screen display. This family isn’t sitting in their living room. Outside, the landscape is rushing by at more than 300 km/h. The scene is from 2007, and the trio are passengers on the Velaro E train from Madrid to Barcelona (High-Speed Rail).

"With the Velaro concept, which is based on Deutsche Bahn’s ICE 3, we have a platform for high-speed trains that can be used worldwide," says Friedrich Moninger, Innovation Strategist at Siemens Transportation Systems (TS) in Erlangen. There is lots of room in the air-conditioned train, because all the technology is installed "below the floor," that is, under the passenger compartment. There are no heavy traction units, as in the ICE 1 and ICE 2, and there are no more electrical switchgear cabinets in the carriages. All of which makes the train 20 % more spacious. It also offers technical advantages: Half of all the axles now have their own drive. This gives the train powerful acceleration, thanks to optimal wheel-rail contact. "And it also makes it possible for the train to climb grades of up to 4 %," explains Moninger. "That’s twice the performance of conventional passenger trains." What’s more, the train’s maintenance-friendly construction, with easily replaceable modules and parts with long service life, make the Velaro platform very economical.

No Gearbox Needed TS is also developing innovations for local trains. One example is a project with the Automation and Drives Group on "a very energy-efficient drive system," as Dr. Lars Löwenstein of TS Advanced Development reports. Today’s rail vehicles usually have three-phase asynchronous motors, which work most effectively at high rotational speeds. Transfer of force to the slower-running wheels therefore requires a gearbox, which increases energy consumption, as a result of frictional losses. A gearbox also generates noise, needs maintenance and must be replaced occasionally.

"The new drive doesn’t have these disadvantages," says Löwenstein. "It has no gearbox and forms a single unit with the wheelset axles." Its rotor’s magnetic field is generated by strong permanent magnets made of rare earth materials, which have recently become economical to use. And this kind of motor also delivers high torque, even at very low engine speeds.

"We’re conducting the first tests on a subway vehicle," Löwenstein reveals. Here, engineers have built the world’s first completely-integrated, encapsulated and water-cooled direct drive without gearbox on the vehicle’s bogie. The encapsulation protects against dirt and moisture and reduces noise and interfering fields. Except for two roller bearings, the motor has no parts that can wear out.

Permanently-excited motors have the disadvantage that losses occur as soon as the motor is moved, even with a switched-off drive. But passive cooling does a very good job of discharging the heat that’s produced. "Permanent excitation has the advantage of being fail-safe, which means that we can use the motor as a safe, second electric service brake," says Löwenstein. No second mechanical service brake is required.

Direct drives can also ensure optimal construction of bogies, which are mounted under the train’s main frame. This summer, Siemens will present the Syntegra chassis concept, which fully integrates drive, bogie and braking technology. "With Syntegra, we’ll save lots of weight and open up new possibilities in rail car structures," promises Löwenstein. This means rail cars can be built longer and wider, giving passengers more room. And wear and energy consumption will be reduced.

Greater comfort —especially due to quieter motors —will be provided by a new type of controller. "We have developed a compact module for the control and regulation of the drives and current converters," reports Moninger. The Sitrac (Siemens Traction Control) system calculates the voltage for the electric motors with a precise computer model instead of requiring complicated measurements. It thus controls the inverter module, which supplies the drives with current and quickly regulates torque and acceleration. Without Sitrac, precise, dynamic measurement of the motor rotation speed would be necessary. Today, this is done by wheel speed sensors consisting of a wheel on the motor shaft and a magnetic sensor. These are subjected to high temperatures and mechanical loads, which limit their reliability. Sitrac doesn’t require any sensors. "That’s a big advantage," says Moninger. "Dispensing with the wheel speed sensors and their wiring cuts costs and boosts reliability." Sitrac can be adjusted to any rail car. It only needs to "know" all the relevant parameters. Sitrac was used for the first time in early 2003, on the Renfe Tren Civia in Spain —and today it’s installed on several subway systems.

Storing Braking Energy. Power supplies for trains are also becoming more reliable. "In the future, passengers won’t even notice a power outage if the contact wire or the contact to it is interrupted briefly," explains Dr. Andreas Fuchs, Development Manager for drives at TS. Siemens is developing an "integrated auxiliary converter" that gets power for all auxiliary devices, such as interior lighting, not from a transformer connected to the overhead line, but from the drive’s power supply. This allows the motors to also serve as generators, which means all electrical auxiliary systems can function even if the overhead line fails.

Researchers also are studying gas-powered locomotives, which emit fewer pollutants. And diesel-electric drives can be more efficient too. A large share of motion energy is lost to resistance while braking. Some of it could be stored by flywheels or supercaps (high-performance capacitors) and released during acceleration. With such capacitors, the diesel can even be shut off entirely in stations allowing trains to pull away on stored energy, without emissions. In several cities, Siemens has already installed Sitras SES, a similar system for temporary storage of braking energy (see Pictures of the Future, Spring 2004, Saving Energy).
                                                                                                                 Bernhard Gerl