Source for know RPM reading

I'm super friggin tired so if I'm missing something please be kind.

But you should be able to just point the meter at a florescent light.

With the meter set for 1 pulse per rev it should read exactly 7200RPM.

2 pulse per rev, 3600 rpm.

(Drives be nuts b/c I can't check my rcheli's headspeed w/o going outside or shining an old incandescent flashlight onto the blade grips)

Sent from my SM-N975U using Tapatalk
 
+1 on "any synchronized motor"

The AC motor on your drill press, lathe, mill, etc, running with no/low loading will spin at the RPM listed on the attached tag.

In a 60 Hz power region this is most often 1800 or 3600 RPM

Alternatively, I have measured the chuck RPM on my gear-driven lathe. The posted chuck RPM in a given gear is just the AC motor speed reduced by the gear ratio selected. For even more sanity you can measure the chuck RPM as well as the motor RPM.

You could do the same with a belt driven drill press if it has a tag with chuck RPM of each belt location.
 
Ha! That's kind of what sent me down the hole.
I figured the same thing. My understanding is the same that a generic AC motor for a machine - like me old SB 9a would to fine as a reference.

The motor plate claims RPM of 1725 - which I recall is very common. My RPM meter says it spins at 1793 RPM with no load (belt off, checking the output pulley).

No idea what's going on with that.
 
Just checked my low speed grinder - also should be 1725 and it's 1782 - so at least the grinder and the lathe motor are close to the same, as they should be.

Seems like my rpm meter is off by about 4%?
 
I'll give my turntable a shot later just for yuks. It's 50yo belt drive but is audiophile quality, but turntables can also be off a bit. I vaguely remember getting into the load/slip rpm stuff last time I went down this hole.

This diversion is an interesting exercise in metrology - having to link your measurements to a trusted source of known quality.

The spinning disk suggestion is actually quite clever and relies only on the 60hz strobing effect of fluorescent lights for a time base.

I came up with another crazy idea that would work but I'm not going to try it out. My lathe has accurate machined gears to produce feeds in distance / time. A cell phone has the most accurate clock you can easily get your hands on. So if I set my lathe feed pretty slow and timed how long it took the carriage to travel two inches, measured with a dial indicator, I could then use the time and gear ratio (which might be a little tricky to figure out) to determine the RPMs of one of the larger driving gears. This method does not rely on an assumption about the motor speed.

Sometimes I think too much.
 
dbb-the-bruce said:
Just checked my low speed grinder - also should be 1725 and it's 1782 - so at least the grinder and the lathe motor are close to the same, as they should be.

Seems like my rpm meter is off by about 4%?
Click to expand...

No, that sounds about dead on. In THEORY, a 1725 motor is actually an 1800 rpm motor. In PRACTICE there is some "slip". The 1725 RPM is rated at some practical, continuous load for the given motor.

When you ADD load to a motor, you get some "slip", where the motor does not "quite" keep up with the frequency of the AC power. The slight "slip" you're seeing is not "reading too high", it's a motor with very low load (friction and air resistance to the rotor assembly... It's just below theoretical, and well above a practial speed in use. This WILL vary some small amount from motor to motor, for a multitude of reasons. Taking some liberties, throwing away a PHD's worth of electrical theory, and greatly simplifying... That "slip" is what allows a motor to increase or decrease it's current draw to where the input power it accepts matches the output power demanded. IE, why the basement lights dim when the table saw starts up for example...

If you're shooting inside of four percent (and not knowing the EXACT motors, it "appears" that you are WELL within four percent of where those oughtta hit in a no load situation), then you've pretty much reached the end of where most entry to middle ground type photo tach's are going to be. Hang on to that one, treat it good....
 
What meter do you have?

And now that I'm fully awake/caught up, yeah, all you need to do is "measure" an old school fluorescent light to confirm optical tach accuracy.

It's actually quite common to see the instruction in manufacturer's sanity check/FAQ's/troubleshooting procedures: https://monarchserver.com/Files/pdf/Verify Tach Ops.pdf

Number you want is 60Hz * 60sec * 2 = 7200 ÷ meter's expected pulse/rev

Jake is spot on re the slip & to stay with simple: no load = no slip = theoretical speed based on motor & power properties, and it's pretty easy to calculate.

And 1800rpm is exactly what's expected out of an idealized 4 pole AC induction motor on 60Hz mains. i.e., no electric/magnetic losses & no mechanical losses.

B/c that doesn't exist, you're left with an expected RPM somewhere between 1725 - 1800.

Trying to find my motors text book but that thing was so expensive I think I may have sold it back to the bookstore after graduation.
 
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dbb-the-bruce said:
Just checked my low speed grinder - also should be 1725 and it's 1782 - so at least the grinder and the lathe motor are close to the same, as they should be.

Seems like my rpm meter is off by about 4%?
Click to expand...

It's probably the motor is 4% off it's data plate.

Stu
 
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