Author Topic: Stroboscopic photography & motors  (Read 249 times)

Online klugesmith

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Stroboscopic photography & motors
« on: September 27, 2020, 07:08:06 PM »
It's a bit frustrating that the science photography category at HVF is partitioned only by camera type,
with no "other" that might get, for example, work with microsecond flashlamps.

For little scientific videos I sometimes turn to compact digital camera, mostly idle since starter smartphone arrived.
Still pictures from smartphone often show artifacts of progressive "shutter" when subject is, say, a neon light.
Cheap smartphone's picture and video modes have even fewer control knobs than the point and shoot cam (Coolpix 550).

I wanted to measure the speed of an induction motor from a scrapped vacuum pump. 
Nominally 1/4 HP, 3450 RPM, but probably more like 3580 RPM with no load. 
It's neat that slip rate of 20 RPM (1/3 Hz) can induce enough rotor current to overcome bearing and windage drag.  Will motor turn even faster with cooling fan ports blocked, as do vacuum cleaners when hose is blocked?

Old-fashioned fluorescent lighting didn't give enough stroboscope contrast to be useful.
My power-line-synchronous LED stroboscope is buried in clutter.
Got useful strobe result with a xenon-arc lamp, powered by a NST.  Circuit dynamics apparently result in a brief, bright spark once each half cycle. There's substantial but tolerable jitter in time, arc path, and intensity.

Second picture is snipped from one frame of video with CP550.  Better production values, and numerical results, to follow.

Here's an old video made with the LED stroboscope:
https://www.flickr.com/photos/r_k_f/3688448113/
« Last Edit: September 27, 2020, 07:16:50 PM by klugesmith »

Online klugesmith

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Re: Stroboscopic photography & motors
« Reply #1 on: September 28, 2020, 08:16:46 AM »
First attempt to rectify the NST output using a HV diode, driving the high pressure Xe arc lamp, led to failure of the diode.
One from a lot of about 20, I think 20 kV 30 mA, bought long ago.
Tonight I had much better results with a microwave oven diode, even though it's not technically rated for NST end-to-end peak voltage.  The more pointy lamp electrode was taken to be the cathode.  Eye protection was in place.  With or without diode, lamp didn't light until NST primary voltage was above 50% of nominal.  For this picture the NST was at about 70%, and the apparent phase of light pulse jittered by up to 10% of cycle period.

Slip rate of the motor was about 10 RPM, and decreased to about 9 RPM when I blocked fan air inlet with my hand.

I tried several other lamps with the diode in series:  a curly CFL tube, a curly xenon photoflash lamp, and a mercury-arc lamp with a 9-pin minature base!

In those cases the stroboscopic view showed a blurry, very low contrast bright area on one side of motor coupler.  Not well suited to measuring apparent rotation rate.   Bet we could do better with variac, low voltage transformer, and simple white LED (how about a white and a blue in antiparallel configuration?)  The missing stroboscope energizes LED string, via a transistor, with short pulse from a 555 that's phase locked to AC power cycle.
« Last Edit: September 28, 2020, 08:27:45 AM by klugesmith »

Offline Zipdox

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Re: Stroboscopic photography & motors
« Reply #2 on: September 28, 2020, 09:09:45 AM »
Oh god that blue glow looks awfully familiar. Is it UVC? Better watch out.


Offline plasma

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Re: Stroboscopic photography & motors
« Reply #3 on: September 28, 2020, 09:15:09 AM »
I learnt you shouldn't have a fluoresce light over a lathe, because it might look stanary.
Just some info.

Offline TMaxElectronics

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Re: Stroboscopic photography & motors
« Reply #4 on: September 28, 2020, 10:50:31 AM »
It might be interesting to try using a very bright LED for this. The driver I use for them (LT3795) has a PWM input with a minimum on-time of three switching cycles (so 6us at 500KHz Fsw).
Might even be able to capture some larger things with it, as the LED could probably to 500W peak when pulsed slowly enough.

If I find the time I'll make up a board with a BNC input for control from a signal generator and try it out :)

Online klugesmith

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Re: Stroboscopic photography & motors
« Reply #5 on: September 28, 2020, 05:14:51 PM »
An unexpected new project is to measure the narrow pulses of current or light in high pressure Xe lamp on NST.

Good point about the peculiar OSRAM bulb maybe being an intentional radiator of ultraviolet light.  Label says Spektrallampe.

By the way, regarding eye safety around UVC-emitting things like mercury vapor rectifiers and germicidal lamps.
It makes no difference whether you avoid looking at the lamp.
The geometry that matters is whether the lamp can see your eyes.
254 nm doesn't even penetrate the cornea, much less get focused to a spot on the retina.
There are cases where you can get a good look at the glow, without receiving a hazardous exposure.  Protective equipment like real glass glasses or goggles can't hurt.

« Last Edit: September 28, 2020, 05:17:56 PM by klugesmith »

Offline Zipdox

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Re: Stroboscopic photography & motors
« Reply #6 on: September 28, 2020, 06:41:56 PM »
UVC poses the same risks as welding. Arc eye, sunburn, skin cancer. You're better off avoiding it at all costs. If you insist on using it, wear long sleeved clothes and a face shield.

Online klugesmith

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Re: Stroboscopic photography & motors
« Reply #7 on: September 28, 2020, 07:36:45 PM »
Appreciate your words of caution there.  My first & only sunburn from arc welding, where shirt was not buttoned up enough below the mask, was in about 1975.  Found stick welding to work better with a motor-generator than with a transformer box.

Didn't intend to suggest a cavalier disregard of UVC hazards.  As with many toxicological risks, it's best to just stay away if you don't have time to learn the numbers and measure exposure with instruments.  UVC is not found in natural sunlight, on Earth, but has been very well studied regarding human health.  It's not as sneaky as x-rays made on purpose, or radioisotopes.

Zipdox, have you noticed the recent debate about "deep UVC" excimer lamps?
Their developers claim that 222 nm kills germs while being much less injurious to people, compared to 254 nm, which has represented UVC for most of a century.  There are studies, sponsored by makers of traditional 254 nm equipment, that warn "not so fast!".
« Last Edit: September 28, 2020, 08:05:56 PM by klugesmith »

Offline Zipdox

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Re: Stroboscopic photography & motors
« Reply #8 on: September 28, 2020, 08:49:09 PM »
Hmm I did some Googling and couldn't easily find any research suggesting that far UVC is dangerous. However I do have my doubts about it being completely harmless. I speculate it's a situation comparable to the dangers of alpha radiation. While your skin protects you form it, exposed living tissue isn't exactly safe. I'd still worry about far UVC damaging my cornea since it doesn't have a dead layer and relies completely on diffusion and tear drops for the delivery of oxygen and nutrients.

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Re: Stroboscopic photography & motors
« Reply #8 on: September 28, 2020, 08:49:09 PM »

 


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