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

[Paul Wills]

Actually it's 16 2/3 Hz.  It's exactly 1/3 of 50 Hz.  It allows a simple 1 
to 3 ratio of poles on each end of the MG set.

Amtrak still runs 25Hz from the east end of Sunnyside Yard (New York City) 
and Washington, DC as well as the Harrisburg line.  SEPTA (The Philadelphia 
commuter authority) also runs 25 Hz but mostly because they are the "tail" 
being wagged by the Amtrak "dog."

They're committed to it for the long term as most of the rotary frequency 
changers have already been replaced with solid state units.  (There are also 
two, I believe, hydro-electric generators on the Susquehanna river at Safe 
Harbor dam.)

PDW

----- Original Message ----- 
From: "Keith Mc" <acti at provide.net>
To: <Greenkeys at mailman.qth.net>
Sent: Wednesday, November 21, 2012 4:37 PM
Subject: Re: [GreenKeys] History - Why voltages and frequencies are chosen 
(5v & 3.3v logic, 6.3v filaments, 120/220 VAC, 50/60Hz etc)


> Now that is VERY interesting.
>
> Ah, but that now begs the question, "why 16.7Hz"?   Why not 16.5, or 17?
>
> Did that somehow zero in on EXACTLY the speed they needed for an app?
> Did it somehow satisfy some nice integer number or physical need in
> a design (eg the maximum number of turns in a winding of the
> required wire gauge, that they could reasonably "stuff" into an
> armature, etc)?
>
> OR, was the choice totally arbitrary, "within a general range"
> of acceptable numbers, and it just "felt right" once encountered?
>
> I'm trying to get a sense of what drives SPECIFIC number choices
> for both the V&F that we see every day, yet never understand.
> Especially when it comes to "the VERY odd numbers" you often
> see chosen (like your "16.7Hz"... ;-).
>
> At first glance many seem "totally arbitrary" numbers. But often they
> have colorful stories behind them, or have VERY good (but often
> not well understood) reason for existence, that suddenly makes
> great sense once you hear it!   (Like this one...)
>
> Some of those uncovered stories are amazing. Others are amusing.
> ...and in hindsight some are "just plain dumb, but now we're stuck
> with the choice because it would cost too {darn} much to FIX"... {grin}
>
> Which is it, for the Swiss' "16.7Hz" decision?
>
> - Keith Mc.
> .
> douglas-w-jones wrote:
>> Railroads used lower frequencies because they wanted low RPM
>> induction motors for their locomotives.  Lower frequency makes
>> the motors run slower.  The Pennsylvania Railroad's choice of
>> 25 Hz was on the high side.  The Swiss used 16.7 Hz.
>>
>> The low frequency also cuts down on eddy current losses and
>> allows high inductance motor windings in "universal" (commutated)
>> motors to still carry enough current to work well.
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