Author Topic: RPM measuring methods, tools, contraptions, ideas (Read 8697 times)

mixerman · « **on:** August 24, 2009, 02:43:46 PM »

I thought it might be fun and informative to see how mill builders from everywhere are

figuring the speed of there axial shafts. This is to show when different voltages are made,

when a device is supplied to take the place of wind power on the work bench or tree

stump.

I used two ele drills one was a Craftsman 19.2V dc, it was rated for 600 RPM's and

another older 110V ac 1/2 drill that was rated for 320 RPM's. I also have a small bench

press its slowest speed is 620 RPM's and goe's up from their. I have grid ele, but not

everyone does.

Flux · « **Reply #1 on:** August 24, 2009, 10:57:23 AM »

Drills are probably not very accurate for speed. The battery ones may be reasonable if not loaded to much. The induction motor driven drill presses may be quite good as long as the thing is not overloaded or the belt slips.

The 110v commutator type thing really doesn't have a speed at all. It will be what the manufacturers regard as a safe no load speed for devices that may be used with it.

Being series motors the speed will fall on load right down to standstill if you load it enough.

Lathes may be quite good but really the best way is to actually measure the speed with tachos, adapted cycle speedometers or by measuring alternator frequency.

Flux

zeusmorg · « **Reply #2 on:** August 24, 2009, 11:08:20 AM »

a good way to measure RPM's is use a bike spedo that has an rpm function. A cheap one is available ..http://www.dealextreme.com/details.dx/sku.2682 I'm using one on my recumbent, and it does fine.

spinningmagnets · « **Reply #3 on:** August 24, 2009, 01:33:18 PM »

I have read about using a photosensor. The part you want the sensor to look at is painted a flat black, and a small piece of mirror tape is affixed in one spot. An LED shines on that area, and every time the reflector passes by, the light is reflected onto the photosensor.

I havent used one, but users sounded happy with the results. I read that on sites that show how to convert a car turbocharger into a propane jet engine. Some used flat black and shiney gloss white.

imsmooth · « **Reply #4 on:** August 24, 2009, 03:36:15 PM »

reed switch with rpm computer

http://www.mindchallenger.com/wind/wgpage5a.html

TheCasualTraveler · « **Reply #5 on:** August 24, 2009, 06:48:33 PM »

Call me old school but I like my method.

There is a program called strobo.exe that can make a stobe disk to any rpm you input and at a frequency of either 50 or 60 hrz. It's located near the bottom of this page - http://pagesperso-orange.fr/jlf/erepro2.htm

Used primarily for setting the speed on phonographs. You just print one out, slap it on the back of the magnet rotor and view it under fluorescent light to see the RPM. Only cost is for a sheet of paper and the ink to print it.

I use a DC motor with a PWM speed controller to dial in the rpm I want and check it with the strobe disc. For those of us unfortunate souls without a Lathe. Suits me.

mixerman · « **Reply #6 on:** August 25, 2009, 07:08:34 AM »

I'm lost, what im I looking for? The words on the disk being upright maybe.

I used 33 rpms just to try out. Im missing something.

mixerman · « **Reply #7 on:** August 25, 2009, 07:12:36 AM »

I ordered one last night, your not going to beat that price shipped. About how many rpm's can the trike turn going down hill?

TomW · « **Reply #8 on:** August 25, 2009, 08:01:54 AM »

mixerman;

The hash marks will appear to stand still when at the Rpm indicated [or a multiple, i think] on the printout. it will appear to rotate slowly when near the stated RPM but not exactly on.

Tom

TheCasualTraveler · « **Reply #9 on:** August 25, 2009, 04:45:30 PM »

yes, what Tom said.

First you need to select the frequency, for U.S. it's 60 hrz. I believe Europe is 50 hrz.

Then pick a speed you want to measure output at and choose that RPM to print out a strobe disc.

Then you need to view it spinning under fluorescent light.

The light is pulsing at 60 hrz and when the rotor, in your case the box fan is spinning at 33 rpm the black hash marks will appear to stand still instead of looking like a blur.

The fan is probably going much faster than 33 rpm so you see a blur. If you could use a speed control to gradually slow the fan down, this would show you when you reach 33 rpm. (or as Tom said, a multiple of it)

This is not something you can put on anything to check the rpm, rather if you have a target rpm this will tell you when you are rotating at it.

It's very simple but I suppose I just made a hash of explaining it.

mixerman · « **Reply #10 on:** August 25, 2009, 10:28:43 PM »

I made a few more this time for the ele varable speed drill, I choose 500 rpm's, the drill is capable of 600 rpms. I tryed 600 and could not find it, not enough rpms to see the still image. At 500 I can sneek up on it, very hard to get exact with just a trigger. I suppose the image I see just prior to the target 500 rpm is multiple images moving clockwise and the counter clockwise images are just after 500 rmps. Kinda neat. What would be neater would be a lot of rpm ranges on 1 disk. Thanks!!

TheCasualTraveler · « **Reply #11 on:** August 26, 2009, 06:29:25 AM »

They make them with multiple rpm's on one disc but they are for phonographs so the rpms are like 33, 45,78.

I find these most useful for simply checking output at your desired cut-in when you are determining how many turns you want in your coils.

Flux · « **Reply #12 on:** August 27, 2009, 01:59:19 PM »

I wouldn't recommend a strobe disc for checking rough speeds. They are fine for setting a precise speed on a decently controlled device ( gramophone turntable for instance).

You will get stationary images with dozens of harmonics and you will be hard pushed to decide which is correct and unless the speed control is precise you may not even manage to hold the image at all.

For decent results you need a light source much better than a fluorescent light. Even at the primary frequency the pattern is not well defined with the afterglow of a fluorescent lamp and you will not see it unless you can hold the speed very exactly.

We used them for holding the speed of large machines under test driven by a dc motor. We never used them alone but they were convenient to keep the speed constant once set. The actual speed was always checked against a tachometer to be sure the strobe pattern was not on a harmonic. We used special gas discharge trigger tubes and even when set with those tubes it was not always possible to see the stationary image in fluorescent lighting and it was never clear enough to control the thing at a distance.

Flux

TheCasualTraveler · « **Reply #13 on:** August 27, 2009, 02:24:24 PM »

I agree, it takes patience to dial my test rig in to an exact rpm which is why I use a DC motor with speed control and a steady power source. I had no problem seeing the stationary images though, and for the RPM range of 150 to 200 ( common cut-in area) I was confident when I had the correct rotational speed and not a harmonic.

That said, I defer to the experts and only offer this as my humble contribution to the topic of, "RPM measuring methods, tools, contraptions, ideas"

mixerman · « **Reply #14 on:** August 27, 2009, 10:20:35 PM »

Bonehead had a idea in this post

http://www.fieldlines.com/story/2005/10/26/72644/745

where you hack a calculater and add a switch and magnet, said it could be done for about 4 bucks. Some other ideas in that post also.

zeusmorg · « **Reply #15 on:** August 28, 2009, 12:22:21 AM »

You know, I've not looked at the actual RPM's going downhill. I can sustain it at 30 MPH however.

Usually on a severe downhill I'm not too interested in reaching over and switching it, I'm more concerned with keeping it stable!

Flux · « **Reply #16 on:** August 28, 2009, 01:47:35 AM »

With your decent speed control it is fine. With his crude drill as a drive and no experience I don't think he will make it.

Fluorescent lights are ok for your application where you are close to the strobe disc and you can see what probably looks like a stationary image on a grey background. With a decent strobe lamp the image looks just like the actual disc and is clear enough to see from several yards away.

If you choose the correct number of bars for your particular speed then the first harmonic is far enough away for you to get the correct speed as long as you have a rough idea to start with.

For a newby using a drill and not being familiar with the fairly poor image from a fluorescent light I think the chances of hitting the wrong speed are very high if he can get the speed near enough to get an image at all.

Flux

ghurd · « **Reply #17 on:** August 28, 2009, 05:24:38 AM »

I tried that, and tried the same idea with a pedometer.

They both must have some kind of switch de-bouncer circuit.

The max RPM the calculator would register was about 250 RPM. The pedometer max was a lot lower.

I use bike speedometers.

The cheap one in the link above works fine. Feels a bit fragile with the tiny wire. A stronger magnet helps by increasing the distance the sensor can register.

G-

willib · « **Reply #18 on:** September 07, 2009, 12:04:47 AM »

I use a multimeter with frequency measuring capabilities and use the equation...

rpm = Hz times 60 divided by half the number of poles

This assumes that your machine is a 3/4 ratio of coils to poles.

say you get a reading of 51 HZ.

and your machine has 12 coils and 16 poles

rpm equals 51 times 60 divided by 8

rpm = 382 .

mixerman · « **Reply #19 on:** September 09, 2009, 02:16:23 PM »

My spedo came yesterday, been trying to figure out the different modes, this link may come in handy if you loose your instructions. Also shows one hooked up it a turbine.

http://www.reuk.co.uk/Use-a-Cycle-Computer-to-Measure-Turbine-RPM.htm

mixerman · « **Reply #20 on:** September 09, 2009, 02:21:58 PM »

I have a Craftsman meter that someone gave me , there is a selection on the meter for "FREQ % duty" , Is this the type meter your speaking of? How would sombody hook this up for a test?

Mixerman

willib · « **Reply #21 on:** September 09, 2009, 08:20:48 PM »

Yes thats the type of meter.

to test with this type of meter i just select the freq function and put the leads across the circuit under test.

what you will get for a reading on a motor conversion is anyones guess though.

i strictly use it for axial flux machines and other stuff with clean sinusoidal waves.

bill.

ghurd · « **Reply #22 on:** September 09, 2009, 09:22:36 PM »

It works on conversions.

G-

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Author Topic: RPM measuring methods, tools, contraptions, ideas (Read 8697 times)