1500 RPM Machine

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1500 RPM Machine

Postby holtzman » Mon Apr 26, 2010 7:20 am

I am experimenting now with 1500 rpm rotation. This is in order to use 2 spiral 60 lines scanning. And I want to use the same size disc, 45 cm, as in my existing 60 line tv.

Here are some results, they may be useful for designing a Nipkow disc device.

Theory
    The wattage needed to rotate a disc is proportional to its speed in the power of three (cube). Example: we increase speed x2, hence the power increase should be x2, but because also the aerodynamic resistance forse (and torque required for rotation) increases proportionally to the square of speed, we have the 2x2x2 increase required.
    The torque needed to rotate a disc is proportional to its diameter in the power of five (!!!) :shock: Why? Example: we increase diameter x2. The speed of the disc surface relative to air increases x2, and aerodinamic forses increases x2 in the power of 2 (x4). But the area on which these forses are acting increases x2 in the power of 2, as well. And this is not all! The torque increases additionally x2 because the points to which aerodynamic forses are applied are twice as far from the disc center. This all gives 2x2x2x2x2 (x32) increase in torque (and wattage) required.


Sorry for maybe unclear explanation, but I tried this out and it's very close to theory...
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Postby Marcus » Mon Apr 26, 2010 11:37 am

Image

My brain hurts
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Postby Steve Anderson » Tue Apr 27, 2010 12:17 am

Run it in a vacuum, as Baird did with some of his larger discs. Drag? Wot drag?

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Postby Klaas Robers » Tue Apr 27, 2010 4:30 am

In how far would it help to place the disc in an enclosure that is connected to the vacuum cleaner and sucked to low pressure? Does a lower pressure, not a vacuum, help enough?
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Postby holtzman » Tue Apr 27, 2010 5:36 am

:lol: The enclosure itself should have an impact on "aerobraking". The air would rotate inside, and less friction will be with the disc. And the disc won't work as "ventilator" throwing aside new and new portions of air. So probably we'll gain some energy using an enclosure without vacuum cleaner... But there should be also aerodynamic friction between air and enclosure.
By the way, the noise made by 45 cm disc rotating at 1500 rpm is comparable to that of working vacuum cleaner!

Some more theory: speed of the 45 cm disc outer rim at 1500 rpm is 127 km/h, which is quite impressive not to say dangerous :twisted:
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Postby DrZarkov » Tue Apr 27, 2010 4:58 pm

If you put the disc into an enclosed box and connect a vacuum cleaner, the spinning disc with will work as a pneumatic siren. Together with the noise of the vacuum cleaner it will become a very "intereresting" television experience.
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some progress towards 1500 rpm

Postby holtzman » Sat May 15, 2010 9:18 pm

Just wanted to share what I've been trying by now. First of all, a sewing machine motor is not that bad. I have a 60W one, connected via cheap triac dimmer. I was surprized to see that it is quiet enough and regulatable enough. This is for 1500 rpm/45 cm disc, for regular speed of 750 rpm it feels "heavy" a bit and too much power!
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rotating lightbox

Postby holtzman » Sat May 15, 2010 10:12 pm

Here is my concept of rotating lightbox, not finished or tested yet.

I left the idea of electronical commutation as well as stepper motor drive discussed in previous posts.

Instead, flexible shaft from a small power tool is used to transmit the movement. An improvised worm gear is for 1:16 reduction. Led array should be inside the stripes drum, it's not assembled yet.
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It works!

Postby holtzman » Thu Jun 03, 2010 1:38 am

Finally, after some modification - I moved the reductor "upstears" close to the lightbox - it works. Two spirals, one frame. The quality of my clip is not good, the real image is better. No artefacts are seen due to the new lightbox design, the image is just X2 larger. The only drawback is that it is darker, I added 3 leds but even 15 leds total is not enough.
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Postby Klaas Robers » Thu Jun 03, 2010 3:48 am

I fear that you need 4x as many LEDs to get the same brightness as a normal single spiral 60 lines disc. The surface is 4x as large, thus....

But the quality is very good. And that for 60 lines....
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Postby holtzman » Thu Jun 03, 2010 7:12 pm

Thank you, Klaas!
Yes i am aware of the fact there should be 48 leds at least. But there are many obstacles to put all of them there, beginning by that I have no idea where to purchase the "good" type leds with wide angle/high output.

Another issue - what will happen to gain and gamma after increasing the load on your led driver? I already have more than 300 mA on full white, so I reduced the source resistor to just 3 Ohm and gamma seems OK. The Mosfet is not even getting warm though, it's power limit is still far away.
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Postby DrZarkov » Thu Jun 03, 2010 7:59 pm

The result is far more better than I've expected by the description! Congratulations! :-)

BTW: The clip, is it from " Les Faucheurs de marguerites"?
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Postby dominicbeesley » Thu Jun 03, 2010 9:40 pm

That looks truly brilliant - I thought it might work but not that well!

If you want high power LEDs have you tried http://il.farnell.com/

As you've got limited space have a look at Luxeon emitters they should do what you want...though you'll need to mount them on a metal heatsink with a heat conducting epoxy resin...If not luxeon stars are easier to work with you can mount them with heat conducting tape pads.

Well done - almost worth a visit to Israel to see, any plans of bringing it over to the convention!

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Postby holtzman » Thu Jun 03, 2010 10:48 pm

Thanks everybody, I really appreciate your feedback! Dom, this link is really great, hope to be able to dig it once time allows.
The clip is from Hell's Angels film by Howard Hughes. Sorry for silent clip my camera has no audio.
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Postby AncientBrit » Thu Jun 03, 2010 10:54 pm

Well done Holtzman!

Excellent bit of construction.
Any chance that you might submit an article to the magazine?
The commutated light box is an unusual concept.

Well done again,

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