lights and 3-phase motors

[mjmike]

I had a person once tell me someting interesting they used to have to do in the navy years ago. This is in regards to light fixtures in a mechanical space with 3-phase motors spinning some equipment on the output shaft such as a pully or pump. He noted to not put all the light fixtures in the space on the same phase as it had the possibility to pulse at the same rate as the motor shaft giving the illusion it wasen't spinning. In the navy, they had to break up the lights in the space onto all 3 phases.

I have been thinking about this, and could see the possibility of it happening with incadescent lights but not with flourescent. I was thinging of it in terms of a timing light. Anybody have any thoughts on this concept?

Thanks

 

[broadgage]

It was regarded as good practice to split the lighting over at least 2 and preferably all 3 phases in a factory or workshop with rotating machines.
The risk was greatest with flourescent or discharge lamps, since these flicker at twice line frequency. That could give the dangerous illusion that rotating machinery was at rest and safe to touch.

If only a single phase supply was available, special twin lamp flourescent fixtures were used, these had a capacitor in series with one lamp, and therefore gave a steady light in total from the two lamps.
(also had a good power factor)

Less of a problem these days due to the increasing use of electronic ballasts, these flicker much less, and at a very high frequency.
Also each ballast will be slightly different due to component tolerance, they wont all flicker at twice line frequency like magnetic ballasts.

Flicker is unlikely to be a problem in the USA with most incandescent lamps since the filament has enough thermal mass to remain lit continually on an AC supply.
Flicker can be a problem with very low current incandescents, since the thin filaments cool appreciably when no current is passing.

In the UK flicker from incandescents was slightly worse, due to the lower line frequency, and higher voltage. The higher voltage means a thiner filament for a given wattage.
Flicker is noticable with a 240 volt lamp of 25 watts or less, and can be detected with a 40 watt lamp.
It is insignificant with lamps of 60 watts and up, as would normally be used for factory lighting.
Low power filament lamps on machine tools are normally worked at 25 volts from a transformer, both to reduce flicker and enhance safety if the lamp or lampholder should be broken.

In the old days, some AC supplies were at very low frequencies such as 16.6, 20 or 25 cycles.
Such low frequencies are not ideal for lighting, and it was usual to either have a DC lighting supply, or to use gas lights.
If lighting had to be worked from a low frequency supply, 6 volt lamps from a transformer were sometimes used, the very thick filaments remain lit continually .

Note that lamp flicker could give the dangerous illusion that moving machinery was stationary, no matter how the machine was powered, did not have to be a three phase motor.
Single phase, or steam engine drive, or line shafting could appear stationery, or to be moving very slowly, under the right conditions.

 

[iwire]

That was very interesting, I leaned something new today.

Thanks!

 

[LarryFine]

I first learned about that years ago, when I went to what was a military school at the time. The movies Toy Soldiers and Major Payne were filmed at this school.

Added: For those interested, it's The Miller School of Albemarle, west of Charlottesville, Va. See here.

They had a metal- and wood-working shop with one large motor, and shafts with pulleys and leather belts, and linkages for shifting each machine on and off.

The teacher explained the same thing, about 1 phase lighting, stroboscopic effects, and the hazards of machines appearing stationary when they were moving.

 

[ptonsparky]

I remember it as having to do with the single lamp 8' slimlines. Two lamps did not have the problem because one was on while other off. Just how I remember being taught and I have always been careful to put in 2 lamp fixtures. May have to get out the old Electricians Handbook.

 

[BJ Conner]

I had a person once tell me someting interesting they used to have to do in the navy years ago. This is in regards to light fixtures in a mechanical space with 3-phase motors spinning some equipment on the output shaft such as a pully or pump. He noted to not put all the light fixtures in the space on the same phase as it had the possibility to pulse at the same rate as the motor shaft giving the illusion it wasen't spinning. In the navy, they had to break up the lights in the space onto all 3 phases.

I have been thinking about this, and could see the possibility of it happening with incadescent lights but not with flourescent. I was thinging of it in terms of a timing light. Anybody have any thoughts on this concept?

Thanks

the effect is called "Strobbing". We have streets around here that are illuminated with sodium vapor fixtures. You can ride along at night and see the wheels on the cars turning slower or faster.
About 5 years ago I saw a spec for some work on an airforce base in Alaska, Sodium vapor lights were specifically forbidden. Anything rotating at 60 cycles or multiples of it would appear to be still.

 

[mjmike]

very informative, thanks for all the responses. I learned something also.

 

[dicklaxt]

Another example of this is the timing lights used on automobiles and for us old guys remember the wheels of the stagecoach or buggy in the early movies and how they seemed to not be turning and somtimes have the illusion of rotating backwards?

dick