The transmission and transfer of alternating current electrical power from the generating source to the end user requires particular consideration of many variables when the source is stationary and the user is in motion, as in the case of an electrical railway. One of the principal factors to be considered is that of money outlay, either as a first cost or for the rehabilitation and updating of existing trackage and railway cars or vehicles. With the usual number of vehicles per mile of track, the cost of electrification of the track is many times the cost of the electrical equipment on the vehicle, and it thus behooves the designer to point efforts to a vehicle system in which the cost of track electrification is minimized. It is recognized that highest possible voltage, single phase power with reasonably high power factor, results in lowest track costs.
Insofar as voltage is a factor, it is desirable for the wayside power distribution system to furnish alternating current power at 25,000 or even 50,000 volts in the case of station stop distances of the order of 50 miles, for example. With voltages of this level, it is no longer practical to furnish polyphase power through the pickup, and it becomes essential to utilize a third rail or overhead catenary system and pantograph pickup with current return in the grounded reaction rail to supply single phase power. Given this situation of power supply from the track electrification it is seen that the equipment on board the vehicle must be adapted to convert the high voltage single phase alternating current into power at the wheels and, at the same time, present a reasonable power factor window for the source to look into. It is apparent, of course, that such on board equipment ought not exact any undue penalty of weight and envelope.
The voltage aforesaid is too high for the traction motor or motors, so a transformer is needed aboard the vehicle. Furthermore, if the traction motors are of the induction type (either rotary or linear), polyphase power is required at least at starting. On the other hand, either rotary or linear induction motors present a poor power factor.
To meet this problem there was devised the Kando System used on the Hungarian State Railways for a period extending from 1932 to sometime during the 1950's. This system was featured by a free-running, rotating synchronous machine having a direct current excited rotor and a stator provided with single phase power input and polyphase power output windings. This converting machine provided step-down transformer action, phase conversion, and power factor correction in a single entity, whose weight was not of paramount importance since it was used on locomotives. For example, a typical Kando System, including the necessary auxiliary accessory equipment such as oil and water coolers and pumps, water tank, air circulating fans, and the like, occupied an envelope volume of about 4,500 cubic feet and weighed over 13 tons.
Modern high speed transportation methods, utilizing either rotary or linear types of polyphase induction motors, are not adapted to such unwieldy, cumbersome and massive converting machinery exemplified by the Kando System. Hence, later technology has tended to look in other directions for solutions to the problem of providing either tracked-wheel or tracked-air-cushion transportation vehicles or magnetic levitation vehicles adapted to speeds upwardly to 300 or 400 miles per hour.