[Antennas] Tesla antenna, DC mains, etc.

[Adam Farson]

Hi Norm,

There is a very good engineering reason why DC is favoured for traction - in
such applications as railways, subways, streetcars, trolleybuses and
high-speed elevators.

Some railway systems are powered from low-frequency AC (16.67Hz in Europe,
25Hz in the north-eastern US, as I recall). The lower frequency allows for
somewhat better control of the AC slip-ring induction motors used in such
systems.

A compound-wound DC motor is much easier to control, and has a much higher
starting (standstill) torque, than an AC induction motor.  In the modern
"industrial-frequency" traction system used on main-line rail routes in
Europe, transformers and rectifiers aboard the locomotive convert the 25kV
50Hz AC from the catenary to 1.5kV DC for the bogey motors. Another
advantage of the industrial-frequency system is that it simplifies trackside
plant. Simple transformers suffice to feed the catenary from the grid, as
opposed to frequency-converter stations for the low-frequency system or
rectifier stations for the 3kV DC system used in some parts of the world.

The third-rail system used on suburban railways and subways uses 500 to 600V
DC. The London Underground system uses two feed rails, to allow use of the
running rails for signalling.

HVDC (high-voltage DC) power transmission is also becoming popular. An HVDC
submarine-cable system, with rectifier/inverters at both ends, has been in
service across the English Channel for many years.

I lived in Johannesburg, South Africa, from the end of WW2 until the late
sixties. One district had 250V DC mains (derived from the 500V traction
supply) as late as 1967. They may have switched to the 220V 50Hz standard in
later years. Even in AC areas, the municipal electric utility supplied
250/500V DC to several older high-rise buildings to power elevators and
escalators. DC electricity meters were a science unto themselves.

A radio-electrical store in the DC district had a big rotary inverter in the
back room. One could hear this machine start up every time a salesman
demonstrated a piece of AC-powered merchandise such as a radio.

Cheers for now, 73,
Adam VA7OJ/AB4OJ