[PRCo] Re: voltage and frequency

[John Swindler]

If still valid, this might be a useful link:  Were there not previous 
discussions concerning control systems??

  ---------

Raw archives are still at the location I posted before; However, there's
now "web archive", updated hourly. No search feature yet.

http://lists.dementia.org/mlist/pittsburgh-railways/

Choose the year and month you wish to look at, and you get a date-sorted
list. You can instead choose "threaded" and get that view.






>From: robert simpson <bobs at pacbell.net>
>Reply-To: pittsburgh-railways at dementia.org
>To: pittsburgh-railways at dementia.org
>Subject: [PRCo] Re: voltage and frequency
>Date: Sat, 1 Sep 2007 07:54:39 -0700 (PDT)
>
>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
>
>
>
>
>
>

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