# RPM of a motor from frequency

I have a very old motor which I can't find any information about. I need to calculate its RPM for some work. I've attached a cog to it, held a piece of paper up to it and recorded the pitch. It was around 300Hz.

Can I use this in any way to calculate its RPM and if not, are there any other tests I could possible carry out?

I'm using it with a gearbox including 5 30:8 gear ratios. The final RPM of the last gear is about 1.5RPM. By this, it should be about 750RPM but this doesn't seem right. The starting cog has 8 teeth.

• Doubtful. Is it an induction motor? Maybe a picture could help.
– TQQQ
Commented Apr 18, 2023 at 18:06
• I forgot to mention. It is a motor that was used in a vacuum. Does this help? I have also added more information about the project which may be useful. Commented Apr 18, 2023 at 18:09
• (30/8)^5 ~= 741.6 so if your output shaft is 1.5 rpm, your input must be 1112 rpm. -- why not state the number of teeth on your gear? Commented Apr 18, 2023 at 18:30
• @ChristophRackwitz Thanks - I'll add it now, but it's 8. Commented Apr 18, 2023 at 18:32
• well 300 hz / 8 teeth * 60 s/min is 2250 rpm. after those five reductions, that would have to be 3 rpm. to check: 1.5 rpm means one revolution in 40 seconds. how did you measure those 300 hz? Commented Apr 18, 2023 at 18:33

Of course you can! 300Hz is also 18000 cycles/minute. Now all you need to to is to divide by the number of teeth on your cog and you have the RPM.

Let's say there are 100 teeth on the cog. So that gives us:

300 Hz * 60 sec/minute = 18000 cycles/min
18000/100 teeth = 180 RPM

Now all you need to do is count the number of teeth on your cog!

• So there are 8 teeth on the cog (it's very small) meaning this would be 2250 RPM. However in my gearbox there are 5 30:8 gear ratios ending at 1.5RPM (10 seconds to go 45 degrees). This suggests the starting RPM is 750. I'm not sure which is correct. Commented Apr 18, 2023 at 18:30
• @ChrisMartin Well, 2250 is exactly three times 750. Is it possible that the fundamental frequency is actually 100 Hz and you're measuring the third harmonic? Is there a significant amount of sound at 500 Hz or 700 Hz? Commented Apr 18, 2023 at 19:59
• "18000/100 teeth = 1800 RPM" may contain an arithmetic error Commented Apr 18, 2023 at 20:57
• Yes, you're right. I corrected it. Commented Apr 18, 2023 at 21:44