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What is the best strategy to roughly estimate the RPM of a simple DC motor as seen in this video?.

I want to estimate the max possible rotation of the coil that has $n$ number of turns and has moment of inertia $I$ for a uniform field $B$. The one side of the axle has been scraped so what current only flows along one half of the rotation.
enter image description here

There are factors like friction that slos down motion and back emf that reduces current. How do I put those into one equation for estimation?. Is there any better way?

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Friction is terribly difficult to estimate. If you could, you could calculate the speed by measuring the power input and deducting the static losses. What tools do you have available? If you have an oscilloscope, display the current input as a function of time. There will probably be a variation in the resistance over the cycle that will repeat and you can measure the periodicity. A frequency counter might work as well. If you have a strobe light, put a dot on one point and find the frequency that makes the dot stand still.

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  • $\begingroup$ is there no way to theoretically estimate the RPM of the given set up? now, I have only Digital Multimeter available. $\endgroup$
    – Mula Ko Saag
    Commented Sep 14, 2014 at 3:15
  • $\begingroup$ What is the purpose of the estimate? That determines how good an estimate you need. If you can't see it spin, it must be at least high tens of RPM. Unless it is carefully designed it doesn't spin a few thousand RPM, probably much less. Is that range good enough for your purposes? $\endgroup$
    – Ross Millikan
    Commented Sep 14, 2014 at 3:28
  • $\begingroup$ Yes, to be honest, I only need in what range is THE motor spinning on video youtube.com/watch?v=elFUJNodXps and how much max it can go in type of set up in 9V battery, ceramic 3.5 in motor with 40 turns. $\endgroup$
    – Mula Ko Saag
    Commented Sep 14, 2014 at 3:32
  • $\begingroup$ I only watched the first bit. The fact that you can see it changing means it isn't turning very fast. 100 RPM would be a good guess. You could try to find out what the frame rate of the camera was, then download the video and look at it frame by frame to do better. In theory, it could be spinning very fast and the camera frame rate could be almost the same so the images get slowed down (as the difference of frequencies) but that is unlikely. $\endgroup$
    – Ross Millikan
    Commented Sep 14, 2014 at 3:38

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