Why Will Future Automobiles be Electric? – How to Design Road Inserts for Electric Cars
By John Talbot
(Under the Direction of the Lord Jesus, Prov 3:6; Col 3:17)
Electric cars powered by external sources require some form of electric connection to the source. In this article we describe the design of a suitable road insert for this purpose. The design requirements are:
It should be intrinsically safe
It should have adequate lightning protection
It should be able to carry large electric currents safely
It should be weatherproof
It should have adequate backup in the event of the main supply failing
The voltage should be chosen to suit conditions at each point along the route
It should be securely anchored to the road, so as not to present a traffic hazard
It should be flush with the road surface
To ensure 1, there should be a safe voltage in places, such as parking lots, where people walk. In places where there are no pedestrians, such as fenced freeways, higher voltages can be used to allow speeds up to, but not beyond, the speed limit. As we have mentioned in a previous article, the motor can produce full torque from as little as one volt provided the frequency is reduced in the same proportion as the voltage. The ideal voltage in a parking lot is 15. This will enable the auxiliaries to operate while the 12 volt battery (necessary for auxiliaries) is being charged. This is a safe voltage. No one has ever been electrocuted by a 15 volt supply. In our experiments we use DC voltages between zero and 282. This enables us to obtain three-phase voltages between zero and 200 after inversion. For light vehicles we use 200 volt, 400 hertz three-phase induction motors. For heavy vehicles we plan to use a three wire Hopkinson/Edison system. This uses 282 volts between one wire and the earth return and -282 volts between the second wire and the earth return. This enables us to get up to 400 volts three-phase. Three-phase motors of 400 volts 50/60Hz are available off the shelf. They are relatively heavy (250W/kg) but are rugged and reliable and can withstand a heavy overload for short periods.
We use an extruded metal channel as the earth lead. The “live” lead is located inside the channel and is insulated from the latter by a wood or paper insulator. The “live” lead is made up of a copper channel with a strip of cast iron inside. The contact which is attached to the vehicle is a thin sheet of beryllium copper. The lower edge of the latter runs on the cast iron strip. Beryllium copper is a good conductor of electricity, is resilient, and has a very long fatigue life. The cast iron contains graphite which serves as a lubricant, making a good bearing surface, resulting in little wear of either material
Paper is for many purposes the best insulator known. The highest quality coaxial copper cables are paper insulated, as well as undersea power cables. Paper has a very low permittivity, which enables it to damp transients. Prior to 1950 the gold mines on the Witwatersrand used electric motors of 6.6kV. The industry was expanding rapidly and the new mines were using 11kV motors. The insulation of the 11kV motors on surface of the mines was found to be breaking down. At first it was believed that lightning was responsible since it was only the motors on the surface (6000 feet above sea level) that were having the problem. That proved not to be the case. The problem was that corona discharge was breaking down the epoxy insulation. At that altitude the air density is only 1kg per cubic meter, compared with 1.226kg per cubic meter at sea level. Corona discharge is more likely to occur at low air densities. The solution may surprise you, as it surprised me at the time. Instead of epoxy resin, they substituted newsprint to insulate the stator windings. (Newsprint is the kind of paper that newspapers are printed on.) That was over half a century ago and some of those motors are still running today.
The metal extrusion is grounded along its full length. It serves to shield the conductor inside it and will heavily damp any lightning strike. The result is an insert that satisfies all eight conditions above.
