# What's the RPM



## nctoxic (Oct 10, 2011)

Is there an easy way for a not-too-electrically-inclined guy to determine the rpms of a motor that is missing the data plate?

I have an old motor without the usual data plate.  I've determined it draws appx 5.2 amps when running.  That's close to 3/4 hp, but I can't determine the revolutions per minute.  

Thanks for any helpful hints. 

Tim,,,

Sorry for the double post.  I couldn't figure out how to delete the second one.


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## Kennyd (Oct 10, 2011)

I bought one of these laser tachs a few years ago, it's very handy for a variety of things and the price is right: http://www.amazon.com/Professional-Laser-Photo-Tachometer-Measurements/dp/B000MMW0PY


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## Hawkeye (Oct 10, 2011)

A laser tach is worth having, even if it is only to satisfy a tool craving. Generally, single-phase motors come in two speeds - 17425 and 3450 RPM. Fasten a pulley or something to the shaft and run it up. Compare that to a known motor. You'll be able to guess whether it's the same speed, twice as fast, or half as fast.

In use, the motor will actually turn a bit slower. But this should help.


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## Ed. (Oct 11, 2011)

Hi NCTOX, I also bought one of those cheap digital tachometers similar to the link a few posts above, they seem to work well.

Regards

Ed.


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## hq308 (Oct 11, 2011)

Hawkeye said:


> A laser tach is worth having, even if it is only to satisfy a tool craving. Generally, single-phase motors come in two speeds - 17425 and 3450 RPM. Fasten a pulley or something to the shaft and run it up. Compare that to a known motor. You'll be able to guess whether it's the same speed, twice as fast, or half as fast.
> 
> In use, the motor will actually turn a bit slower. But this should help.



That's a very fast motor, lol. I'm guessing you meant 1745?


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## Tony Wells (Oct 11, 2011)

1725, half of the 3450 would be my thinking on that typo.


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## Hawkeye (Oct 11, 2011)

No, HQ, it was supposed to be 1725, like Tony said. Darn laptop keyboard and thick fingers. No space between keys.


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## hq308 (Oct 11, 2011)

Hawkeye said:


> No, HQ, it was supposed to be 1725, like Tony said. Darn laptop keyboard and thick fingers. No space between keys.



Thanks Hawkeye I figured that out after seeing Tony's post, I knew there was an extra digit in there somewhere.


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## Tony Wells (Oct 11, 2011)

That's always my excuse, and I'm sticking with it. Big fingers; small keys.


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## Tony Wells (Oct 11, 2011)

OK, here's a challenge: Find a neon lamp test light. Calculate the circumference of the shaft. Put white marks (whiteout will do) spaced such that they are an even multiple of 60 Hz at either 1725 or 3450. Hold the lit neon lamp near the spinning shaft. If you have calculated it right and marked it correctly, you'll get a stop-action view of the marks. If they don't appear to be stationary, you haven't got the shaft marked for the correct speed.

Anyone remember seeing the turntables with the strobe speed indicators? Works like a timing light on an automobile engine. Another near obsolete tool.

On small shafts, a disc can be afixed to it for marking.


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## nctoxic (Oct 12, 2011)

Tony Wells said:


> Calculate the circumference of the shaft. Put white marks (whiteout will do) spaced such that they are an even multiple of 60 Hz at either 1725 or 3450.



Tony,
Okay, I like this kind of challenge.  Give me a hint about the above quote.  My old brain is spinning and smoking, but I'm lost on the spacing of the marks.  I understand the methodology, just ain't sure about the spacing.


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## Tony Wells (Oct 12, 2011)

OK, imagine the circumference of the shaft flattened out. Now at 1725 RPM, how many feet per second (or inches, or MM or CM, as you wish) does it travel? Calculate how far apart the marks need to be to represent 1/60th of a second. Now convert it (mentally) back to a circumference, and you know how far apart to mark the shaft. Of course, there are limits to consider, a 1/4" shaft may not be "long" enough to mark, as the circumference may be shorter than the distance traveled in 1/60th of a second. I haven't done this kind of tach work in ages, but it does work. The 60Hz  (or 50 in places) is generally very accurate. It's what keeps out plug-in clocks running


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## Highpower (Oct 12, 2011)

Tony Wells said:


> OK, here's a challenge: Find a neon lamp test light. Calculate the circumference of the shaft. Put white marks (whiteout will do) spaced such that they are an even multiple of 60 Hz at either 1725 or 3450.



Ok, that had me going for a minute Tony. When I first read that, I was thinking how in the world am I going to get that many marks around my neon test lamp bulb? 

View attachment 309


View attachment 308
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View attachment 308


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## nctoxic (Oct 12, 2011)

Tony Wells said:


> OK, imagine the circumference of the shaft flattened out. Now at 1725 RPM, how many feet per second (or inches, or MM or CM, as you wish) does it travel? Calculate how far apart the marks need to be to represent 1/60th of a second. Now convert it (mentally) back to a circumference, and you know how far apart to mark the shaft. Of course, there are limits to consider, a 1/4" shaft may not be "long" enough to mark, as the circumference may be shorter than the distance traveled in 1/60th of a second. I haven't done this kind of tach work in ages, but it does work. The 60Hz  (or 50 in places) is generally very accurate. It's what keeps out plug-in clocks running



Thanks Tony.  I just needed a kick start,  but it makes sense to me now.   I'll look for a strobe light (60Hz), however, it might be just as cost effective to get that laser tach from HF mentioned above.

Tim,,,


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## Kennyd (Oct 12, 2011)

nctox said:


> however, it might be just as cost effective to get that laser tach from HF mentioned above.
> 
> Tim,,,



I think so, plus you will find lots of uses for the tach once you have it.  I have used mine on my lathe. mill and drill press, as well as tuning up an old snowblower engine.


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## BRIAN (Oct 12, 2011)

i have a rev counter that i bought 50 years ago and it still works as good as new but you require a watch with a second hand to use it.

BRIAN.


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## Kennyd (Oct 29, 2011)

Kennyd said:


> I bought one of these laser tachs a few years ago, it's very handy for a variety of things and the price is right: http://www.amazon.com/Professional-Laser-Photo-Tachometer-Measurements/dp/B000MMW0PY



Update 10/29/11: The price is now $7.47 at this link: http://www.amazon.com/Digital-Photo...QY66/ref=sr_1_1?ie=UTF8&qid=1318825432&sr=8-1

That is to cheap not to own one...


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## endiathetos (Nov 5, 2011)

What a great post...   Just a comment on the little electronic RPM device.  I bought one for around $15 and it came with a set of reflective stickers.  Apply a small piece to a pulley, shaft or belt and the device gives you the RPM.  If you run out of the stickers, you can apply black electrical tape to the pulley except for a small spot and the thing will pick up the rotating bright spot.

Rex

I have little reflective stickers on most rotating surfaces.


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## martik777 (Dec 16, 2011)

Hmmmm, how about setting your camera to 1/30 sec for 1725 (flash OFF)?  1725/60= approx 30 revs/sec 1/30 sec should freeze it, 1/60 for 3450 rpm. I too have that laser tach for < 10 from ebay...works great


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## Inflight (Dec 19, 2011)

The rev counter tool, of which Brian provided the photo, is very quick and easy to use. I somehow find opportunities to utilize this device at least once a month, if not more often. Feels better in the hand than some of that HF stuff, too.



 Matt


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## churchjw (Dec 19, 2011)

Tony Wells said:


> OK, imagine the circumference of the shaft flattened out. Now at 1725 RPM, how many feet per second (or inches, or MM or CM, as you wish) does it travel? Calculate how far apart the marks need to be to represent 1/60th of a second. Now convert it (mentally) back to a circumference, and you know how far apart to mark the shaft. Of course, there are limits to consider, a 1/4" shaft may not be "long" enough to mark, as the circumference may be shorter than the distance traveled in 1/60th of a second. I haven't done this kind of tach work in ages, but it does work. The 60Hz  (or 50 in places) is generally very accurate. It's what keeps out plug-in clocks running




Here is a page that kind of goes with what Tony is saying.  I used this method years ago for a project and it worked well.

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

Jeff


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## brucer (Dec 21, 2011)

I've been thinking about making a tachometer for my step pulley bridgeport... I was thinking of milling a pocket in the upper bearing locknut and epoxy a magnet in the pocket, then mount a housing on the bearing sleeve locknut and mount a hall effect sensor in the housing, then just use one those cheap digital tachometer breadboard kits.. then mount the readout in my vfd remote.

 I dont really need it, but would be cool to have an actual tach readout on my vfd remote, instead of the normal frequency readouts that are available.

 might get around to it when I get with the cnc plasma finished.


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## Tony Wells (Dec 21, 2011)

Just so you guys don't think that idea was mine. Many of you may remember the days of the turntable,  the ones that played LP's. Some of them had a neon lamp, which flashes at 60 times/second near the edge of the platter, which in turn had marks properly spaced for each speed the tt was able to run, usully 33 1/3 and 45 RPM. There was an adjustment to "freeze' the timing marks, just like the old days of a timing light on a harmonic damper.


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