Auction site proximity sensor tachometer

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Dsmroots

Joined
Nov 20, 2014
Messages
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Hey guys, so I am in the beginning stages of building a 2x72 grinder. I've been a member for a few years now but I have never posted my own thread, so moderators, I apologize in advance if this is in the wrong place.

I have a treadmill motor that is brand new. It puts out 2.8 hp max and 2.6 hp continuous. On the lable it doesn't list the rpm range. I cant determine the drive wheel size until I know what the rpm range is, so I ordered a cheap digital prox sensor tachometer off of the big auction site. I have hooked it up and ran the motor but for some reason it is only picking up about half of the motors range and I even question that readout.

Does anyone have any experience with a legitimate tachometer or may be able to help line out this current setup? I am restraining from listing the exact product that I bought because I know some forums are picky when it comes to name dropping or product posting.
 
Is your tac a visual one. If so put a white dot on the shaft do it has a reference point for rotations. But I will just put this out there sence I’m here. Don’t use a treadmill motor for a grinder. Third motors are DC and have magnets inside and are not sealed. Steel grit will get sucked inside and stick to the magnet. Find you a nice 2-3hp TEFC single phase motor.
 
The sensor is not optical or visual in nature. It uses a NPN proximity sensor that open/closes a circuit from the magnet running around the shaft or in this case the flywheel. There is a circuit board that accepts the signal and has a digital readout.

The treadmill motor and power supply I picked up for $70 total which is crazy cheap for variable speed and a brand new motor. My plan with the motor was to build a HEPA filtered shroud to keep dust out of the motors internals.

Plus, I don't have a dedicated 220 outlet for a 1.5hp+ 3 ph vfd setup. To complicate things even more on that, I tend to move into a new house every 2 years so being able to run variable speed off of a 15 amp 120v ac plug is super handy.
 
Perhaps make up a magnet tool to fit an electric drill or something of known RPM to prove its operation properly.
Fwiw, We put one of those cheap ’tachs on a woodlathe. It worked fine.

Fyi, some treadmill motors are TENV housings, will work fine in dirty environment, but be sure they obtain decent ventilation. We powered that woodlathe with one. Its mounting is fairly heavy. Between that mount heatsinking and air turbulance from the beltwheel seems to keep enough cooling on the motor.
 
Double checking the accuracy isn't a bad idea since I could use my drill press for the test. I am beginning to wonder how the resolution is of the actual sensor itself is. I suppose it is possible that the rpm is too high for the sensor to open and close quick enough. I don't beleive my multimeter has a hz readout but that is worth a look I suppose. At 3000 rpm it should be reading about 50 hz. I have a feeling it turns faster than that.

Thanks for all of the advice. I'll keep tinkering around with it to see if I can figure it out.
 
I have a cheap laser one that uses a piece of reflective tape on whatever you're trying to measure. It works very well.
 
I bought a hand held tach. Turns out it spins at 8200 rpm. Still don't know if the tach setup that I did have is junk or salvageable but for now I don't care.

So I guess I was planning on targeting around 5-6000 sfpm which means a 2.3-2.8" drive wheel. Or, I could speed limit the motor to say 6k rpm and go with a bigger wheel. I definitely need to do a little searching to see what is available.
 
The guys already told you that the motor/ VFD aren't right for a grinder ... and they are right.

I'll add that the HP ratings given on those motors are also not real. True 2.8/2.6HP won't run on any ordinary 110VAC circuit.
 
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