Did you know you can measure RPM using just a mobile phone or a tablet?
The App I'm using for this is called Spectrum Analyze
and it's a free App though I paid a dollar to unlock some very useful features
Let's measure the RPM of my jointer.
So the fundamental frequency of this jointer appears to be 240 Hertz
and that repeats every 240 Hertz if we look here.
That is at 480
so a twice that so basically is what our residence is to that main frequency
and thats the rate at which the knives go past the table.
So let's go to the calculator
and we have 240 Hertz and I know there's three knives on the head
so it turns 80 times per second time 60
4,800 RPM on the cutter head. Next the table saw.
So we have a very strong fundamental peak at about 3020 hertz
so 3020 divided by 40 teeth on the blade
is 75.5 turns per second
times 60 seconds in a minute is 4530 RPM.
Now it's not quite that straightforward you kind of have to know
what you're doing and know what approximate RPM you're looking for
cause if I zoom way out on this
you can see all kinds of peaks here these are multiples of the tooth frequency
because they don't just excite the frequency of the teeth
but also essentially harmonics of it.
And if we zoom in way on the left
at the lowest frequencies we see some weaker peaks there
for instance this one here that's at 120 hertz
and that's essentially just line hum from the motor.
This sort of line home is quite common.
There is also 120 hertz coming from the transformer in this battery charger.
Transformer hum is always double the frequency
so the transformer gets 60 hertz and then the next field will vary
60 times per second but as the magnetic field
maximizes in one direction the force is maximum
then as it crosses zero the force is nothing
but then as it goes in the other direction the force is
again the same amount so for every cycle that goes both up and down
the magnetic field peaks zero twice so you get twice as many peaks
in the magnetic field causing a hum of twice the frequency.
Next less measure this dust collector.
So we have some interesting peaks
we have one at 354 hertz
and there's one back here
at very low frequencies this one is at 59 hertz
so it works out that 354 is 59 times six
and that tells us this is probably the motor frequency,
the rate at which the motors turning around this is for the vanes
so that the impeller on this blower has six vanes
causing six times the frequency
so we take the 59 turns per second times 60
is 3,540 RPM but if we look at the tug on the motor
it says 3,450 RPM and I know the no load speed of these motors
is 3,600 RPM it's rated for 3,450
and it's actually running at 3,540.
So the rate at speed is 95.8 percent idle speed
and the actual speed is 98.3 percent idle speed
so it's rated to run 4.16 percent below idle speed
but is actually running at 1.66 percent below idle speed
so basically the speed drop from idle is only 40 percent
of what it's rated for which is to say this motor
currently is only 40 percent loaded.
So we have a very pronounced peak at around 1,872 Hertz
divided by six teeth per inch on my blade
comes to 312 inches per second
divided by 12 inches in a foot is 26 feet per second
time 60 seconds in a minute
is 1,560 feet per minute what's the speed of my Dremel tool?
So looking at the spectrum here we have two strong peaks
that are close to each other I'm not sure which one of those
actually represents the RPM one is about 547
the other one is at 671.
So I'll put a small piece of tape on the chuck
to get more sound from that and we'll try again.
So before we had those two peaks
now we just added this peak I'm guessing that this tape
slowed down the drum a little bit so I'll shorten it a bit
to put less load on it and let's try again.
So with less load we get about this frequency
Oops.
And that works out to five or six turns per second
times 60 is 30,000 RPM.
Now curiously enough we have another peak in here as well
and it turns out those two peaks are 120 hertz apart
so that is some inter-modulation
between the actual rotation of this thing and the line hum.
So let's take the tape off
and try this again and I think those two peaks
that we saw earlier are just 120 hertz apart.
So we have one peak at 539
and this other peak is at 659
that's exactly 120 hertz apart.
So going back now I know that this was the actual idle RPM
at 543 turns per second is 32,580 RPM.
What's the speed of this fan?
A very clear peak here
Which is at 70 Hertz
70 hertz divided by three blades on the fan is 23.3 turns per second
times 60 seconds is 1,400 RPM.
Question is, does the filter in this fan slow it down?
Here's our 70 Hertz again, let's take the filter out
And sure enough it speeds up.
What's the speed of my router on the pantorouter?
So if we look on here
the first real strong peak is
268 hertz times 60 seconds in a minute
is 16,080 RPM
which happens to be very close to
what this rotor is actually set to.
Now looking at this some more there's some other peaks on here as well
for instance we have this peak here
which is 120 hertz that's the line frequency.
We also have this peak
which is 120 hertz higher
than the speed of the router so that's 120 hertz inter-mod
I don't know what this one is for
but if we zoom way out
way, way out
we can see a very strong peak here
that one is at about 4,800 hertz
that happens to be 18 times
the actual frequency that we're getting off the router
so that tells me that is probably the commutator noise
and that the motor has 18 points on the commutator.
So as you can see it's not always entirely straightforward.
It helps if you have experience using spectrum analyzers.
The App I'm using on here is called Spectrum Analyze for Android.
It's a free App, the upgrade for a dollar allows me to zoom in better
well worth it the other thing you want to do
is you want to set it so it's got 32,000 bins on the FFT.
I tried to find a equivalent App for IOS, For iPad
but most of the Apps on iPad tend to cost a fair bit of money
I bought a few of them but none of them turned out to be as useful as this one.
There are a few that cost on the order of 20 to 30 dollars that I didn't try
because if you spent that much money you could just spend that money
buying a cheap RPM meter from China.