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some history: Why 16 2/3Hz was chosen

The earliest operational electric railways adopted DC power because the characteristic of series-connected DC motors correspond closely to the requirements of railway traction: the torque is highest at standstill, and decreases gradually as the speed increases. Because it was not feasible then to step up or step down the voltage of DC power, it was supplied to the motive power through an insulated third rail or overhead wire at the maximum rated voltage of one such motor (or a pair of motors connected in series). In order to avoid insulation problems, this voltage was typically 600-750V per motor, and so the rectifier / feeder stations had to be located every few km along the track in order to keep the drop in voltage between the nearest feeder point and the train relatively low.

While this arrangement was (and still is) satisfactory on urban and suburban lines, it is unsuitable for fast intercity routes. An obvious solution was to supply the train with high-voltage AC, which could then be stepped down efficiently by a transformer to a voltage suitable for the traction motors. But in that case, either the AC must then be converted to DC, eg by using a rectifier, or new traction motors suitable for AC power had to be developed. Both approaches were tried, but it was found that while the mercury-arc rectifiers then used at feeder stations were too fragile for mobile use, it was possible to design AC traction mo-tors with characteristics similar to DC ones, provided the frequency was low enough. (A third solution, the use of three-phase AC at 50Hz with induction motors, was also used for some time, notably in Italy, but this was ultimately abandoned because a complicated dual catenary had to be used and trains could operate at only a small number of different speeds.)

The first railway to have scheduled operation with low-frequency high voltage AC power was the branch line from Murnau to Oberammergau in Bavaria, electrified at 16Hz by Siemens in 1904. In 1912, the German Railways adopted this 15kV 16 2/3Hz system as standard for low-frequency high voltage electrification.

Shortly afterwards, several other countries adopted this standard too. This system has a series of advantages for railway operation which have ensured that it has been retained and expanded in those countries, despite the progress in developing the 50Hz system, which is now preferred for most new electrification. Indeed, with a few exceptions, all the first generation of locomotives designed to operate with 50Hz power supplies had DC motors supplied through rectifiers - which in many cases were their weak point. It is only recently that the development of high-power semiconductors has not only made reliable rectifiers available, but also enabled motive power to use three-phase induction motors, which are particularly suitable for this application because the windings on the rotor are robust and do not have to be connected electrically to any power supply. In this case, of course, the power does not have to be obtained from the primary source at 50Hz AC, nor even through a catenary or third rail, as it can also be generated by a diesel motor on board the motive power. But wherever the electrical power supplied to the traction motors need no longer be the same as that supplied through the catenary, substantial economies can be obtained by feeding the catenary with 25kV 50Hz obtained directly from the public distribution network, thus eliminating the cost of private low-frequency distribution and/or converter stations.
© 1999 Alan Reekie

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