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Hard-coupling PF motors experiment
nerdsforprez posted a topic in LEGO Technic, Mindstorms, Model Team and Scale Modeling
Hello All - As Technic MOCs get bigger and badder, with improved controllers and battery sources powering them, I have noticed that hard-coupling motors together in most supercars and large construction-themed Technic MOCs is becoming more and more common. Now, Philo has shown that there really is no difference in RPM and power between hard-coupling motors or coupling them through an adder http://www.philohome.com/motors/motorcomp.htm Despite this, the assumption that Lego motors run at similar RPMs is still important to consider when hard-coupling them because if one is slower than the other it can degrade performance over time, and perhaps even permanently damage a motor (another good source of information is Sariel’s book, 1st version, The Unofficial Lego Technic Builder’s Guide, Chapter 18). During a recent build of mine I was challenged to really think about how consistent motors are when I accidentally noticed how inaccurate the RPMs were when running two M motors off a single PF battery pack. I noticed that even with the naked eye I could detect difference in their RPMs. This led me to ask "How much difference is there really?" and to wonder if more damage than previously thought is caused by coupling PF motors. This led to the little experiment below. (Note: caution when setting the volume; set it fairly low and adjust from there. It changes greatly during the film as I slowed down, then ran it at regular speed. Usually I run a song or something over the audio but not here, I felt the audio an important part of the actual content of the experiment). Little information of the experiment. 4 M motors, and 4 XL motors were compared against each other for speed. 10 seconds each trial. Trials compared each motor, one against the other. Example: 1 versus 2, 1 versus 3, 1 vs. 4, 2 vs. 3, etc. (you get the picture). RPMs for the same motor, but different trials slightly differed because although I tried to run each trial for exactly 10 seconds, when I examined the film with digital software I noticed that there were slight deviations in the trial time (10.4, 9.6 seconds, etc.). These deviations are so small, but when you are running motors at several hundred RPMs then even fraction of a second can result in several additional turns. These little deviations did not matter because trials were independent. Winners in each bracket (M motors, XL motors) beat all other motors in independent trials. As can be seen in the video, the differences were not huge. M motors and XL motors were 100% similar in their average inconsistency (5.5 percent). To summarize, I think this is little variability and likely really does not lead to much damage or decrease in overall output (same conclusion made by Sariel and Philo; referenced above). Two things to consider however, and I think are new contributions not made by others, is that although there really is only 5.5% difference (average) between motors, when they operate at like 405 RPMs (M motor, Philo for reference) then we are still talking about quite a bit of difference, at least in revolutions per minute between differing motors (~23). When you put it that way, the difference seems a little more significant. Lastly, if one is going to hard-couple two motors together, he/she may want to examine the speed of both independently first. Although average difference is not that bad, at the tail ends of the distribution large differences can exist. The largest difference in my little examination in the M and XL motors was around 9%. For M motors that is around 40 RPMs different and for XL motors that is around 20…. Which really sound bad, especially when considering over the long haul! So, check motors for reliability first before hard-coupling them, you don’t want to get combinations such as 1 vs. 2 in my experiment, where unreliability between motors is fairly high!