[PRCo] Re: voltage and frequency

[robert simpson]

Thanks Boris and Bob;
   
  A rectifier and battery system would make an excellent system.  It could also provide emergency lighting for a short period of time should there be an electrical interruption - and there would be no flickering/twinkling of lights.
   
  A previous responder mentioned they use variable transformers on European trains to control/regulate speed.  How did they control speed on a Direct Current system?  I remember watching the operators on PRCo pressing the "accelerator" (sorry, I don't know the appropriate lingo) to the floor when starting and the wheels didn't spin.  Was there some kind of regulator which limited the amount of power going to the motors?  A simple rheostat would seem to be very inefficient.  
   
  Did the Direct Current cars use resistive braking on long downhill grades?  
   
  I have recollections of a rather large, flat metallic object hanging between the wheels suspended only about an inch above the rails.  Was this part of the braking system?
   
  Bob Simpson
  from Krazy California
  
Boris Cefer <westinghouse at iol.cz> wrote:
  Yes, I understand and know this, but with lights fed from a battery instead 
directly from the AC system there is not such a problem.

Boris

----- Original Message ----- 
From: "robert netzlof" 
To: 

Sent: Saturday, September 01, 2007 5:11 AM
Subject: [PRCo] Re: voltage and frequency


> --- Boris Cefer wrote:
>
>> Why would the light flicker?
>
> With AC, for a brief moment (repeatedly), the current through the
> lamp filament is zero. So, for 1/4 cycle, the current, and hence the
> filament temperature, increases until the peak current is reached;
> then for a 1/4 cycle the current and the temperature decrease. Repeat
> during the next half cycle and the next and the next...
>
> Thus, the filament temperature increases and decreases on a cycle at
> twice the line frequency; twice, since the filament doesn't care
> which way the current flows. (Alternative explanation: power is
> proportional to the square of the current and the frequency of sine
> squared is twice the frequency of the sine).
>
> Whether the variation in filament temperature is great enough to be
> noticable depends in part on how much the filament temperature
> changes during a half cycle. The higher the frequency, the less time
> there is for the filament to cool before it gets another "jolt" of
> current so. Since there's less time to cool, the temperature doesn't
> decrease as much as it would using lower frequency current.
>
> The shape and thickness of the filament also play a part. A long,
> slender filament would lose heat faster than a short, thick filament.
> Low voltage lamps have shorter, thicker filaments than higher voltage
> lamps and so one would expect the variation in intensity to be
> smaller for lamps running on a low voltage.
>
> Besides all that, how easily can the human eye and brain notice small
> changes in light intensity? The higher the frequency the harder it is
> to notice the changes. Perhaps some people notice the effect more
> than others.
>
> Maybe flicker isn't a good word for the effect, but I can't think of
> another. Maybe twinkling?
>
> Bob Netzlof a/k/a Sweet Old Bob