{"id":92,"date":"2007-01-13T06:35:15","date_gmt":"2007-01-13T06:35:15","guid":{"rendered":"http:\/\/www.danmarx.org\/blog\/?p=92"},"modified":"2007-01-20T05:46:58","modified_gmt":"2007-01-20T05:46:58","slug":"method-for-measuring-rpms-of-an-unknown-device","status":"publish","type":"post","link":"https:\/\/www.danmarx.org\/blog\/method-for-measuring-rpms-of-an-unknown-device\/","title":{"rendered":"Homemade Tachometer"},"content":{"rendered":"

I thought of another way to more accurately measure the true RPMs of the drill I was using to measure the kV of the stock motor on the Super Cub. Instead of measuring another motor whose kV I assumed to know, I ran the drill into a 10k potentiometer and measured how many times it rotated with a frequency counter. I tore the back off of a 10k pot so it would spin indefinitely, then put 5VDC on one leg while running ground and the other leg to my DMM. A 1k ohm resistor across the the DMM to creates a voltage divider network that isn’t dependent on the high impedance input of the DMM for proper operation. The rotating pot creates a sawtooth waveform at the rate of 1 Hz per revolution which the DMM (frequency counter) can easily read. Take that number and multiply by 60 and you get RPM of the whatever device was used to spin the pot, in this case, my cordless drill. (Since the pot spins with a certain amount of friction, the drill has to have plenty of torque to overcome the friction and not affect the measurement. This method propably wouldn’t work for directly measuring the free-running RPMs of just a motor because the friction of the rotating pot would adversly affect the measurements. If the motor were geared high enoough though, it could still work).<\/p>\n

At this point we’ve removed just about all the variables and we’re left with only the accuracy of the DMM, which according to the data sheet is about +\/- 1Hz. The reading I got toggled between 20 and 21 Hz, so I took the average and figured 20.5Hz. Take that number x 60 and you get 1230. So the final measured RPMs of my drill came out to be 1230 RPMs. So the kV of the stock motor on the Super Cub turns out to be 1230 \/ (1.420 \/ 3) = 2599 RPMs\/Volt.<\/strong> Since the DMM is only accurate to 1Hz, we still have about a 5% error in that number, but it’s about as close as I can measure with the test equipment I have at home. An o-scope on the other side of the potentiometer would give us a much more accurate reading, but I’ll save that test for another day.<\/p>\n

So when determining a motor upgrade for the Super Cub, anything that provides about 2600 RPMs\/V should do the trick, granted the motor has the power as well. Since the cub is geared, there is a 3:1 reduction on that number bringing the prop RPM to 866. Of course this is unloaded, but the absolute highest free running RPMs at the shaft is around 866 RPMs. So if you’re looking to change the Cub from an in-runner to an out-runner and connecting a prop directly to say a brushless motor, anything that has a kV rating of at least 866 should perform as well or better than the stock motor, again granted that it’s a big enough motor and can provide the torque and power necessary. Makes sense that the Park 450 motor is a popular upgrade, since it has a kV of 890, and can probably provide a lot more power, especially at 11.1V from a 3S lipo. Putting a motor with a kV at or over 1000 in a direct-drive app would just cause the SC to absolutely scream. Or a kV of 3000 or more would be great for keeping the stock gear box with the 3:1 gear ratio. My Graupner Speed 480BB has a kV 3890 which is significantly more than the stock motor and a lot more the the popular brushless setup. Oh no, could be bad news, because nothing great doesn’t come at the cost of something else. I suspect my runtimes will be 6-8 minutes tops and there may be smoke and fire, maybe. Tomorrow we get the motor into the gearbox and finish out the tests. If it draws too much current, I simply won’t run it. But I am anxious to see how much more thrust I can generate with the new motor, should be fun if nothing else.<\/p>\n

rotationalrpmmeter.gif<\/p>\n","protected":false},"excerpt":{"rendered":"

I thought of another way to more accurately measure the true RPMs of the drill I was using to measure the kV of the stock motor on the Super Cub. Instead of measuring another motor whose kV I assumed to … Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"enabled":false},"version":2}},"categories":[11],"tags":[],"class_list":["post-92","post","type-post","status-publish","format-standard","hentry","category-rc-planes"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/www.danmarx.org\/blog\/wp-json\/wp\/v2\/posts\/92","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.danmarx.org\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.danmarx.org\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.danmarx.org\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.danmarx.org\/blog\/wp-json\/wp\/v2\/comments?post=92"}],"version-history":[{"count":0,"href":"https:\/\/www.danmarx.org\/blog\/wp-json\/wp\/v2\/posts\/92\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.danmarx.org\/blog\/wp-json\/wp\/v2\/media?parent=92"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.danmarx.org\/blog\/wp-json\/wp\/v2\/categories?post=92"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.danmarx.org\/blog\/wp-json\/wp\/v2\/tags?post=92"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}