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'Visual Persistence'
1997\02\11@234655 by Martin McCormick

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       We once had a major power failure on the Oklahoma State University
campus and the power plant fired up a stand-by generator to get the computer
center and a few other vital spots up and running again.  The trouble was
that the speed governor wasn't too accurate and our mains frequency swung
from 53 HZ to around 65 HZ before they got it settled down around 60 HZ.

       Several people commented upon the flickering or strobing fluorescent
lights so there is a visual difference between 50 and 60 HZ.  It makes
me wonder whether fluorescent tubes made for the 50 HZ market have a longer
persistence phosphor?

       Another couple of observations:  Our UPS's never came back on line
but displayed error indications since the frequency was too far away from 60
HZ for the ferro-resonant transformers to work. It was also kind of strange
to hear the large air handlers in the building gradually speeding up
and slowing down.  I thought I was loosing my mind at first:-)

       At least we did not burn anything up that day.

Martin McCormick

1997\02\12@002931 by John Payson

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>         We once had a major power failure on the Oklahoma State University
> campus and the power plant fired up a stand-by generator to get the computer
> center and a few other vital spots up and running again.  The trouble was
> that the speed governor wasn't too accurate and our mains frequency swung
> from 53 HZ to around 65 HZ before they got it settled down around 60 HZ.
>
>         Several people commented upon the flickering or strobing fluorescent
> lights so there is a visual difference between 50 and 60 HZ.  It makes
> me wonder whether fluorescent tubes made for the 50 HZ market have a longer
> persistence phosphor?

A normal fluorescent light fixture which is operating properly will flicker
at 120Hz (if running off two-phase power) or 360Hz (if running off three-
phase).  This flicker is not going to be noticeable to anyone; even if it
were only at 100Hz, it would still be well above the threshhold of percep-
tion.

On the other hand, a typical fluorescent fixture is a tuned system which
relies upon the input frequency to be within its capture range.  If the
input falls outside the capture range the system will go into and out of
lock.  Depending upon the system characteristics, this may result in the
light flickering at a submultiple of the line rate or in the light strob-
ing chaotically.  A light which, e.g., misses three cycles out of four
but occasionally catches a few in a row will be very noticeable, and such
behavior is not uncommon for an out-of-tune ballast.  Anyone who has been
around flurescent lights has probably seen a light whose ballast has gone
out of whack which exchibits precisely these symptoms.

By the way, if you see a fluorescent light fixture acting up because of
wierd current going into it or because of a bad bulb, shut it off.  When
operating out-of-lock, many ballasts may generate excessive heat and may
be permaently damaged if not shut down.

>         Another couple of observations:  Our UPS's never came back on line
> but displayed error indications since the frequency was too far away from 60
> HZ for the ferro-resonant transformers to work. It was also kind of strange
> to hear the large air handlers in the building gradually speeding up
> and slowing down.  I thought I was loosing my mind at first:-)

It may be just as well if the UPS' didn't come back on line, since some types
of equipment may have problems if the line frequency is not within spec.

>         At least we did not burn anything up that day.

All the light ballasts survived?

1997\02\12@174300 by Karoly Hoss

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Martin McCormick wrote:
>
> our mains frequency swung
> from 53 HZ to around 65 HZ before they got it settled down around 60 HZ.
>         Several people commented upon the flickering or strobing fluorescent
> lights so there is a visual difference between 50 and 60 HZ.

the visual fx can be on the edge if you move .
try to shake your finger in front of the screen
the phosphor can decrease the effect but it's still there .
in scientific american I read an article that stated visual flicker
reception
can be more sensitive if you hear audio freQs close to the flicker freq.

it's possible but I don't remember the biophisical basics .

bye
charley

1997\02\15@112129 by Dwayne Reid

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Just a minor correction, John,

I have not seen a flourescent ballast which operates from 3 phase.  The
large office towers that I have pulled wire in all use high voltage lighting
circuits (277 Vac or 347 Vac).  This is the phase to neutral voltage for 480
Vac or 600 Vac 3 phase respectively.  The ballasts are all connected between
a phase conductor and neutral, taking care to split the total number of
ballasts equally between the 3 phases.  Your figures of 120 Hz (60 Hz
supply) and 100 Hz (50 Hz supply) are correct, 360 Hz is not correct.

>A normal fluorescent light fixture which is operating properly will flicker
>at 120Hz (if running off two-phase power) or 360Hz (if running off three-
>phase).  This flicker is not going to be noticeable to anyone; even if it
>were only at 100Hz, it would still be well above the threshhold of percep-
>tion.

Most of the time, I find that flourescent lighting flicker is not too
objectionable.  The exception is when flourescent lighting is used in an
office with a computer monitor operating in interlaced mode - the flicker
resulting from beat frequencies can render the monitor useless.  I'm in that
exact situation: my OLD 20 inch Mitsibushi monitor operates in 1024 x 768
resolution only if the graphics card is configured as an IBM 8514 monitor
(60 Hz interlaced).  I get around the problem by using a halogen floor lamp
reflecting off the ceiling.

Dwayne
Dwayne Reid   <spam_OUTdwaynerTakeThisOuTspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, Alberta, CANADA
(403) 489-3199 voice     (403) 487-6397 fax

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