Material handling: Monorail system with non-contact power transfer demonstrated in Germany

01/25/2007


discrete control; motors, drives & motion control

SEW Eurodrive's electric monorail system inductively couples electric power from a set of three conductors carrying high-audio-frequency alternating current through a soft-magnetic core into a pickup coil. The middle conductor carries current in twice the quantity and opposite sense as the upper and lower conductors. This current arrangement forms a pair of poles (as seen in this cross sectional view), which couple maximally to poles in the magnetic core. Source: Control Engineering

Nicola Tesla's dream was to distribute electric power without wires. At least, that was one of his dreams! SEW Eurodrive demonstrated a practical monorail system that features non-contact power transfer at the SPS/IPC/Drives Exhibition in Nuremburg, Germany. The system, called Movipro, rides on a monorail track with embedded power-carrying conductors that inductively couple power to pickup coils in the vehicle. Servomotors on the vehicle drive it along the track.

Despite Tesla's success developing (with George Westinghouse) wired infrastructure for distributing electricity in the form of alternating current, he was never able to develop a wireless electricity distribution system. There are a number of reasons wireless power distribution as Tesla imagined it couldn't be made practical. The most significant problem being that Tesla's broadcast system was inefficient, sending power out in all directions with only a fraction of it being picked up by users.

There's a very good reason to want non-contact electric power transfer in a material transport system. Conventional material transports incorporate a vehicle and guide rails. That's been the pattern since before railroads. Moving the vehicle requires carrying a power source with the vehicle, or supplying it through the guide tracks system.

Railroad trains, of course, carry an energy reserve, usually combustible fuel (mostly diesel fuel), and a means of extracting that chemical energy and using it for vehicle propulsion (diesel-electric drive). The problems of this system are limited energy storage capacity and the relative inefficiency of engines for converting chemical to mechanical energy.

Cable cars and aerial trams make the prime mover (usually an electric motor) part of the stationary plant, where it can have a continuous source of energy (such as the electric power grid). While such systems don't have to make frequent stops to pick up additional fuel, their mechanical drives have even lower efficiency and poor reliability due to their physically extended (to the far limits of vehicle range) drive systems.

For many applications, the best system combines electric power generated externally with a prime mover (electric motor) carried on board the vehicle. The fly in this ointment is the need for a metal-to-metal sliding or rolling contact to carry power from the stationary generating station to the moving motors mounted on the vehicle. Maintaining electrical contact in the face of an uncontrolled external environment is difficult.

SEW Eurodrive has solved the contact problem by transferring power from the track to the vehicle electromagnetically. Large cables built into the track serve as one side of a transformer, while pickup coils wound around iron cores serve as the other.

The main difference between Tesla's less-than-successful wireless power distribution system and SEW's system is that, while the former uses far-field phenomena to span long distances. SEW's system uses near-field technology to couple power from track to vehicle over fractions of an inch. As the "Non-contact power distribution system" figure shows, the monorail track has three insulated conductors. Current flows through the top and bottom conductors in one direction, while double the current flows in the opposite direction through the center conductor. The magnetic field these currents set up strong peaks between the conductors. The core's poles sit in those spaces, where they sample the strongest magnetic field.

In contrast to the very strong near-field magnetic intensity, the far field—well away from the conductors—is very weak. This minimizes radio wave power radiation.

The current in the conductors alternates at a high audio frequency (tens of kilohertz). The frequency is high enough to couple well inductively, but low enough that even a large installation fails to act as an antenna.

SEW Eurodrive spokespeople said that there are a small number of installations testing this technology successfully. It has been introduced as a standard product in Europe and the company plans to introduce it to the U.S. market early in 2007.

Click here to read an earlier Control Engineering mention of such a system in a U.S. application.

For more information, visit SEW Eurodrive's Website at www.sew-eurodrive.com

C.G. Masi , Senior Editor





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