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Hard-coupling Lego PF Motors Experiment
About this creation
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 adders 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!




Comments

 I like it 
  July 30, 2017
Interesting stuff, and Gabor's observations are right on the money!
 I like it 
  July 28, 2017
A very interesting and informative experiment!
 I made it 
  July 28, 2017
Great comments Jerry. Admittedly, I think we more or less came to the same conclusion for most cases. I indicated that the results for NLS were actually pretty similar.... in most cases. But, in cases where there is near 10% difference, like in my M motor example (motor 1 and 4) when average RPM is 405 then that is ~ 40 turns per minute difference, or 2/3 turn per second difference. I am not sure this is "so small" of a difference in motors of high RPM. Heaven forbid there are RC Buggy motors with 10% difference, because that would equate to nearly 100 revolutions of difference per minute. Nearly two full revolutions of difference per second. I do think this is quite a bit. Therefore my recommendation to screen motors prior to direct-coupling, just in case there are large differences as seen in my experience. Also, don't forget to remember this little experiment has an extremely small sample. Much larger variation is likely to occur with larger, more realistic sampling. If I were to have a more adequate sample, say like 30-40 motors..... what kind of difference can I expect? 15? 20%? Possible, if I found nearly 10% difference with only 4 motors.
Quoting Jeremy McCreary Good demo of how much no-load speeds (NLS) can vary for a given LEGO motor type (not just PF). Matching motor NLS is definitely worthwhile in vehicles (like differential-drive tanks or twin-screw boats) with uncoupled motors on each drive wheel or prop. But I'm having a hard time understanding the risk of motor damage when NLS variations are so small. Electric motors almost always die a thermal death -- mainly from chronic overheating of the windings -- and prolonged high current draws (as in stalls) are almost always the root cause. Of all the things =likely= to cause NLS to vary, the permanent magnets are the prime suspects. But I can't see any of them making much of a difference in the currents drawn by coupled motors of the same kind -- even with substantial NLS mismatches.
 I like it 
  July 25, 2017
Good demo of how much no-load speeds (NLS) can vary for a given LEGO motor type (not just PF). Matching motor NLS is definitely worthwhile in vehicles (like differential-drive tanks or twin-screw boats) with uncoupled motors on each drive wheel or prop. But I'm having a hard time understanding the risk of motor damage when NLS variations are so small. Electric motors almost always die a thermal death -- mainly from chronic overheating of the windings -- and prolonged high current draws (as in stalls) are almost always the root cause. Of all the things =likely= to cause NLS to vary, the permanent magnets are the prime suspects. But I can't see any of them making much of a difference in the currents drawn by coupled motors of the same kind -- even with substantial NLS mismatches.
 I made it 
  July 23, 2017
Agreed! Hopefully, if anything, our younger generation will learn not to repeat historical mistakes by learning a somewhat archaic system.
Quoting Gabor Pauler Nirds, this is an excellent insight in the electric motor technology of middle ages. It is really sad - and probably due to monopolistic position of TLG against clone manufacturers - that in the age of high performance brushless motors and LiPo batteries, we have these stuff as official TLG parts. I know it is a first world problem, and there are much more hard problems in the real world, but Lego IS the tool which should teach our kids innovativeness and preemptiveness. Instead of teaching sluggish monopolistic sneaking... This is what leads next generation of engineers to next world diesel scandal, instead of really solving real world problems.
 I made it 
  July 23, 2017
No problems to a single application, but they can decrease the life of a motor through motor wear/resistance. If a motor has extreme resistance, it dies more quickly. Small resistances, true, don't has as negative an impact, but they still do affect motor longevity in the long haul.
Quoting jds 7777 I've noticed these differences before, but keep in mind, the differences in speed could be due to a variety of reasons. Internal friction, electrical resistance, slight differences in motor efficiency. But how much does this effect torque? That is what is really important. If 2 motors run at slightly different speeds, but have the same torque, then coupling them together shouldn't cause any problems at all.
 I like it 
  July 23, 2017
I've noticed these differences before, but keep in mind, the differences in speed could be due to a variety of reasons. Internal friction, electrical resistance, slight differences in motor efficiency. But how much does this effect torque? That is what is really important. If 2 motors run at slightly different speeds, but have the same torque, then coupling them together shouldn't cause any problems at all.
 I like it 
  July 23, 2017
Nirds, this is an excellent insight in the electric motor technology of middle ages. It is really sad - and probably due to monopolistic position of TLG against clone manufacturers - that in the age of high performance brushless motors and LiPo batteries, we have these stuff as official TLG parts. I know it is a first world problem, and there are much more hard problems in the real world, but Lego IS the tool which should teach our kids innovativeness and preemptiveness. Instead of teaching sluggish monopolistic sneaking... This is what leads next generation of engineers to next world diesel scandal, instead of really solving real world problems.
 
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