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.