[GreenKeys] History - Why voltages and frequencies are chosen (5v & 3.3v logic, 6.3v filaments, 120/220 VAC, 50/60Hz etc)

[Bruce Gentry]

On 11/21/12 2:45 PM, Keith Mc wrote:
> I don't know if I ever asked this before.  I had a VERY interesting question
> arise (yet again) the other day, and STILL haven't yet found a good
> explanation for MANY of the "made choices" by the engineers yet.
>
  The explanatiopn I heard about 6.3 volts filament was to match the 6 
volt radio "A" filament batteries then in use. A  well charged 6 volt 
lead-acid battery delivers 6.3 volts under load. The tube filaments were 
5 volts, so by using a rheostat between the battery and tube you could 
give the tube the correct filament viltage as the battery ran down. With 
AC heated tubes, 6 volts was chosen so the tubes could be heated from a 
6 volt battery or an automotive electrical system.


25 cycle power was supposedly chosenat Niagra Falls and Buffalo for two 
reasons. First, the hydro-electric generators of the time were not 
readily  capable of higher frequency, and secondly, 25 cycles was 
excellent for steelmaking in the arc furnace. There was a 25 cycle 
transmission line built from Buffalo to Syracuse to service a local 
specialty steel mill. It also was surprisingly compatable with DC 
control and switch gear, existing DC motors could be adapted to it , and 
railroad traction motors could be built to operate from it and DC. Into 
the 1990s, Amtrak continued to use it between Washington and New York, 
and the New York City subway used it for transmission to rotary 
converters that changed it to 600 volts DC. This is only the tip of the 
iceberg about 25 cycles.


     Nikola Tesla supposedly chose 60 cycles later because it was the 
best compromise of line loss, arcing in switches, transformer and motor 
size, and steam turbine speeds for generators. Turbines are most 
efficient at high speed, a direct drive generator can turn 3600 RPM 
instead of a less efficient 3000 for 50 cycles or 1500 for 25. A 60 
cycle motor can be considerably smaller for the same horsepower than 50. 
I saw a surprising number of air conditioners in Israel using inverters 
to drive the compressor with 60 cycles. European or Israeli members on 
the list can enlighten us as to why 50 cycles was chosen in their 
countries and continues to be used.

       Bruce Gentry, KA2IVY


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