determining RPM of motor

[jwaterdawg]

I have a motor of unknown RPM's. How can I determine the RPM's? I was thinking of winding up some string of a given length onto the spinning axle and timing it but there must be a better way. Suggestions most appreciated.

[sacherjj]

If you get a model airplane tach and mount a "blade" to it, you should be able to rear the RPM.

http://www2.towerhobbies.com/cgi-bin...&I=LXPT31&P=ML

If the motor runs at any decent speed, I think it would be hard to get an accurate string wind measurement. Although it should be in the ballpark, as long as you allow for the increased diameter of the reel when the string goes on it. If you can go reel to reel for a decent length of line, this could be pretty accurate.

[Russianwolf]

I have a motor of unknown RPM's. How can I determine the RPM's? I was thinking of winding up some string of a given length onto the spinning axle and timing it but there must be a better way. Suggestions most appreciated.

Not taking into account the fact that the string would overlap and cause a fluctuation in the circumfrance making it hard to measure. a 1000 rpm motor would eat through over 83 feet of string in less than a minute assuming a 1 inch circumfrance shaft.

You would need a mark on the shaft and be able to count how many times it rotated in a given time. which if it's a high rpm motor, will be pretty hard.

[LCHIEN]

I have a motor of unknown RPM's. How can I determine the RPM's? I was thinking of winding up some string of a given length onto the spinning axle and timing it but there must be a better way. Suggestions most appreciated.

The only real easy way to do a good job is to get a stroboscopic light you can adjust the flash rate on and turn it to 1700 or 3400 RPM and around to see when a mark on the sxle "stops" motion.

Used to be a strobe light was expensive but these days a bright LED and a oscillator chip may do the job if you have access to some simple electronics equipment.

Another way is even the cheap digital voltmeters nowadays have frequency counters, a bright light, a reflector and a photo pickup and said meter can do the job.

Again, some electronics know-how is required.

Otherwise, go to the store and buy a strobe or a photo tachometer.

[Jim Boyd]

The Redneck way would be to first tie a good heavy cord, like weedeater cord, around the shaft. Next start the motor and stick your tongue close enough to touch the spinning cord. Then count how many times you say OUCH!

[just4funsies]

Loring, the problem with the strobe light is that BOTH the 1800 or the 3600 RPM settings will "freeze" the mark on the motor shaft, since they are multiples of each other, regardless of whether the motor is running at 1800 RPM or 3600. If you set the flash rate at 1800, and the motor is running at 3600, there will be a slight visual effect caused by the passage of the mark once between flashes, but it would be difficult to see, especially without the ability to compare side-by-side with a motor running at the other speed.

One alternative would be if one could find a pocket-type motor tach. It's a small device that looks like a tire pressure gauge, with a pocket clip for carrying it around. It will either have a small rubber wheel on the end opposite the "meter" if it's designed to be placed alongside the spinning motor shaft, or a smaller rubber cup, if it's designed to be touched to the END of the motor shaft. These units are not too expensive (just a few bucks) but sometimes hard to find.

One other alternative, if there is a make and model or serial number stamped on the motor, is to take that info to the manufacturer, or even a local motor shop. They should be able to tell you the rated RPM's from that info alone.

[LCHIEN]

Loring, the problem with the strobe light is that BOTH the 1800 or the 3600 RPM settings will "freeze" the mark on the motor shaft, since they are multiples of each other, regardless of whether the motor is running at 1800 RPM or 3600. If you set the flash rate at 1800, and the motor is running at 3600, there will be a slight visual effect caused by the passage of the mark once between flashes, but it would be difficult to see, especially without the ability to compare side-by-side with a motor running at the other speed....

yes, that's why you run the strobe at 7200 RPM - then if you see 2 marks you know its twice the motor speed (the motor is running 3600). If you see 4 marks its four times the motor speed (the motor is running 1800).

[sneezer17301]

Find a motor that you know is 1725 (or 3600 rpm)take the pulley off turn it on. take your motor and sit it next to it turn it on. it should be real easy to tell if they are the same or different. The other option is to look inside and count the poles or groups of coils in the outer set of coils. Two big sets is a two pole 3600 rpm motor. 4 smaller sets is a 4 pole 1725 rpm motor. six sets is 1100 rpm and so on and so forth.

[JimD]

All my motors have nameplates which state the rpms. Did you look real well for a nameplate?

Jim

[jwaterdawg]

Even better. I found the manual. 10000 rpm and my tongue is still intact.

[just4funsies]

yes, that's why you run the strobe at 7200 RPM - then if you see 2 marks you know its twice the motor speed (the motor is running 3600). If you see 4 marks its four times the motor speed (the motor is running 1800).

You'll have to mark the end of the motor shaft (near one edge) to see it this way, because you can't see (or flash) all four sides of the motor shaft at once from the side view.

[vaking]

Tachometer is nice, strobe is even nicer but if I had to get results using only 19th century technology - I would convert the rotational speed into linear speed using rack & pinion setup. In other words - put small stick on a bench, lay the motor shaft accross the stick, turn the motor on and see how far the stick flies.

[bigsteel15]

Tachometer is nice, strobe is even nicer but if I had to get results using only 19th century technology - I would convert the rotational speed into linear speed using rack & pinion setup. In other words - put small stick on a bench, lay the motor shaft accross the stick, turn the motor on and see how far the stick flies.

ROTFLMAO at the visual.

[AlanJ]

Tachometer is nice, strobe is even nicer but if I had to get results using only 19th century technology - I would convert the rotational speed into linear speed using rack & pinion setup. In other words - put small stick on a bench, lay the motor shaft accross the stick, turn the motor on and see how far the stick flies.

OK. Explain the math to me. My stick is 12" long, 1" wide and 1/8 thick. It weighs 4oz. How far will it fly (off a frictionless level surface) if my motor is 10,000 RPM? Take into account normal gravity and assume the floor is 3 feet below the launch surface.

I expect this to be a competition between Loring and vaking (after all it was his idea)

[LCHIEN]

OK. Explain the math to me. My stick is 12" long, 1" wide and 1/8 thick. It weighs 4oz. How far will it fly (off a frictionless level surface) if my motor is 10,000 RPM? Take into account normal gravity and assume the floor is 3 feet below the launch surface.

I expect this to be a competition between Loring and vaking (after all it was his idea)

yeah, well it will start at V0=0 still be accelerating towards final speed Vf
and be launched long before reahing Vf unless you have a really long launch (12" is NOT enuf).