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We noticed you attached photosto your comment.
Use a shaft rotaiton sensor of course.
I smell homework...
We can't help without the gear ratios, pulley diameters and possibly shaft diameter and rope thickness as well.
Motor RPM X gear box ratio?
Not sure what you need to measure.
As Jack says buy a tacho, their not expensive. At a push you could build one plenty of curcuits on line.
Know what gets me, most AC motors are 1750 RPM. I think 750 might be the gear output.
Quite likely by the looks - iDspite the picture this could be homework as well.
He can do his own homework.
I think there is some simple math connecting mains power frequency, and the speed of induction motors. Specifically the synchronous speed
is the mains power frequency, in cycles per minute, divided by the number of poles, and the number of poles is always even.
E.g. for 60 Hz power, which is 60 cycles/second = 3600 cycles/minute
So for 2 poles, SS = 3600/2 = 1800/min
for 4 poles, SS = 3600/4 = 900/min
for 6 poles, SS = 3600/6 = 600/min
The number they print on the plate is a little bit slower than synchronous speed, by about 2 or 3 percent, and I think that is the speed they expect the motor to turn at, under a load. E.g. 1750 = 0.972*1800, but the rotating field produced by the stator is turning at 1800 rpm.
There is, 10 or 12 pole can drop the RPMs down to 800 and 700 at 60 cycle. You can even get a chart with all the math done.
Sounds like it may be quite slow.
Paint a dot and count the number of times it goes around in six seconds multiply that by ten. And you have your RPMs.
Use a timing light, start by painting a white dot on the shaft, shine the timing light on the dot. The timing light flashes matches the RPMs of the shaft when the dot remains still, and you have your RPMs.
RPM counter you put the counter up to the end of the shaft and it tells you the RPMs.
Put a rope on a pulley, count the feet of rope that passes through the pulley in a minute. Pie times diameter of pulley gives you the feet in 1 revolution of the pulley. Divide the feet that passes in a minute by the feet in 1 evolution and you have the RPMs.
Build an RPM counter, all you need is a frequency counter light and a light sensor.
So what are you capable of doing and maybe we can be more helpful.
I am guessing you are asking how to measure the speed of a moving shaft.
If you can paint, or attach a sticker to, a section of the shaft, then light reflected from this section of the shaft will vary in intensity as the shaft turns.
Then you have this light signal, basically one pulse of brighter (or dimmer) light for each turn of the shaft. The light signal can be converted to an electrical signal, by way of a light sensor. Then a microprocessor can count the pulses, in a given time interval, and output a number for the shaft speed, in turns per second, or turns per minute (rpm), or whatever.
As a sort of packaged tool, this thing is called "optical tachometer".
Here is a manual for one. I found it via Google(r) search for "manual for optical tachometer",
And of course you can search for this thing on eBay too. I actually have one of these gizmos, a cheap one, that looks a lot like this one,
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