[PRCo] Re: voltage and frequency

[Boris Cefer]

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" <wb3iqe at rocketmail.com>
To: <pittsburgh-railways at dementia.org>
Sent: Saturday, September 01, 2007 5:11 AM
Subject: [PRCo] Re: voltage and frequency


> --- Boris Cefer <westinghouse at iol.cz> 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