Strobe to Measure RPM

Discussion on Stirling or "hot air" engines (all types)
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DonnieB
Posts: 12
Joined: Wed Nov 15, 2006 3:41 pm
Location: Perry, Florida

Strobe to Measure RPM

Post by DonnieB » Wed Dec 06, 2006 9:06 pm

I needed a way to measure the rpm on my Stirling and could not find anything on the internet, so I made my own. If there is an easier way, I am open to suggestions. In the meantime, here is my schematic and a short video. http://www.gtcom.net/~dbarnes/rpm_strobe.htm

DonnieB

SScandizzo
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Joined: Tue May 16, 2006 5:06 pm
Location: California

Post by SScandizzo » Fri Dec 08, 2006 5:18 pm

Actually, Donnie, there is a VERY easy way to measure RPM.

http://www.sherline.com/rpmgage.pdf

Basically, its a paper wheel with black and white divisions on it that takes advantage of the fact that fluorescent lights flicker at 60Hz. The page is pretty self-explanatory, and I can tell you from my own experimenting that it is reasonable accurate.

-Stefan

valhalla418
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Joined: Mon Dec 11, 2006 8:59 am

Post by valhalla418 » Mon Dec 11, 2006 9:22 am

I looked at the black and white rpm disc and think its a great way to measure rpm. My problem is that I live in the UK so my A/C is 50Hz, not 60Hz like in the US. So if I used this disc it would give me an incorrect reading.

I want to create a 50Hz, but I've looked at the 60Hz disc and is seems to me that it's incorrect. For example if you look at the 2400rpm ring (innermost) it has 3 white intervals that complete the circle. So each flash of the fluorescent would count a third of a revolution. So 60 flashes would give 20 revolutions (60 x one third). This is 20 revs per second. So 20revs-per-sec x 60secs = 1200rpm, not 2400rpm as the disc states.

Is the disc wrong? or have I worked this out incorrectly?

If I am correct then I believe the disc rpm values should be 1200, 900, 600, 450, 400, 360, 300, 240, 200, 150 (from innermost to outermost), and not the values printed on the disc.

Ben

SScandizzo
Posts: 66
Joined: Tue May 16, 2006 5:06 pm
Location: California

Post by SScandizzo » Mon Dec 11, 2006 11:35 am

Good catch, Ben!

I thought this at first as well. Turns out that at 60Hz a fluorescent light flashes 120 times. I'm not sure of the electrical explanation, just that that's the way it actually works. Regardless, I built an electronic revolution counter using a tiny magnet as the trigger and a hall switch as the sensor, and I can confirm that the disk/light technique is accurate. Had I known that from the onset, I probably wouldn't have built the electronic one!

-Stefan

valhalla418
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Joined: Mon Dec 11, 2006 8:59 am

Post by valhalla418 » Mon Dec 11, 2006 2:08 pm

ok... that explains it. So if I want to make a UK version then I guess I'll need to work with 100Hz.


Thanks!

valhalla418
Posts: 3
Joined: Mon Dec 11, 2006 8:59 am

Post by valhalla418 » Mon Dec 11, 2006 5:18 pm

I've created a 50Hz timing disc to use with a fluorescent light in the UK:

http://www.globecc.co.uk/50HzRPMDisc.gif

Print it out and view rotations under fluorescent lighting.

DonnieB
Posts: 12
Joined: Wed Nov 15, 2006 3:41 pm
Location: Perry, Florida

Post by DonnieB » Mon Dec 11, 2006 8:46 pm

Thanks Stefan, for the info. I'll give it a try.
The reason for the 120 flashes instead of the 60 is that ac current is a sine wave that swings from a positive voltage to a negative. So, in one cycle, the voltage rises from zero to 120 and back to zero then rises, in the negative direction, to -120 volts. So you have 2 voltage peaks in one cycle. The flourescent doesn't care if it is positive or negative... at least, that is the way I understand it. By the way, I have started on my second stirling. Hope it works. Thanks again.
Donnie :grin:

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