I’ve been designing brushless permanent magnet motors since the 1980s. In most applications, service life is cited as advantage vs. a similarly rated brush-type permanent magnet dc motor. Service is often traded-off vs. higher initial cost to show that cost of product ownership over a long-enough period will be lower when brushless motors are used.
CIM motors that have been used for hundreds of hours and then disassembled generally have their insides coated with brush dust. The dust coating causes armature coils to run hotter; think of it as an unwelcome thermal blanket. Things that run hot fail sooner; as a rule-of-thumb, motor insulation service life is reduced 50% by raising its temperature 10 Celsius degrees. And of course if the brushes wear until they are too short, their electrical contact with the commutator is degraded, causing them to drop more voltage and run hotter. So there are two negative effects of brush wear that a CIM must contend with. Neos and Falcons don’t have those problem, so their life is limited mainly by bearing wear and/or loss of lubrication (grease life).
The negative factor on service life for brushless motors is electronics. The only failure mode I have seen on brushless motors in FRC is fatigue of sensor leads, generally at the crimp contacts of their connectors, and generally caused by stress from disconnecting.
TL/DR; brushless motors should last much longer than brush motors, unless electronic components that are integral to the motor fail.
My understanding of brushless from the R/C world mirrors that, and while I’m sure neither VEX nor REV specced garbage on purpose I wonder just how good their stuff is at resisting wear. We’ve now got a fleet of 1-season-old NEOs, and I’d love to see someone do some tests like @Tom_Line describes on them to see how they compare to spec.
(And whoever’s been joyriding the inevitable Falcon 500 test chassis, feel free to join in too.)
Thank you for responding. In my head thinking through BLDC motors I couldn’t think of many failure modes, but since I have 0 experience in that field I didn’t want to just make a blanket statement.
I worked in the brushed motor industry for a little while, but long enough to know that there are quite a few failure points on a sealed motor like the CIM. Im surprised there hasn’t been a push to have a company develop an unsealed, replaceable brush motor that has a duel ended shaft and an endbell that is receptive to an encoder package. It would be at a higher price than a CIM, but it would have some features that would make brushed motors a bit more competitive. Happy to see the Venom motor come out as an option at the very least.
The only thing that kills BLDC motors is heat. You’ll burn out your motor controller before you burn out your motor. I’ve had RC car motors get so hot that the leads desolder themselves. Still runs just as new (which is confusing because i didn’t think the magnets would work after)
Are there certain tests or metrics an FRC team could do with standard FRC hardware (e.g. no readily available dyno) to approximate brushless motor health? If bearing and grease life are the main mechanical failure modes, would free current be a reasonable approximation?
For us, motors generally are not reused the following year, for one simple reason - we don’t take apart the robots that frequently. Some robots get trashed quicker than others, yes. But generally speaking, we always have the past two years of robots functional, with some robots hanging on well past that. Our 2010 robot lasted through 2016. Our 2012 robot only got trashed a couple of weeks ago. We currently have 2017, 2018, and 2019 functional, and a fall project was to do some rework on 2017 so it’ll keep running for several more years - it’s a great demo robot.
Generally speaking, after 3+ years of use, we’re going to consider a motor (and motor controllers) to be EOL for competition use. So, we end up going with new motors every year. It’s a cost that’s built into our budget and fundraising goals.
Free speed test at 2 voltage levels in both directions give you some good info especially if you do a baseline at the beginning of the motor life and record it as the season goes on. We do a health check after every match and monitor motor speed and current draw on all motors and it detects a bunch of things in system and was implemented to find bad brushes in the 775s. This same method can be used with a free spinning motor. Record enough info on new motors then compare that when the motors get old.
We are looking forward to not having to worry about brush failure anymore!
That makes sense. Coming from a fundraising impaired team. We are just used to scavenging.
The good thing about all these motors having the same bolt pattern is that: depending on desired functionality of a competition retired robot. You could in theory just swap in old cims and controllers and modify the code a bit. If you needed the motors from the retired robot.
Yes, I think free current is a good metric for the Falcon. I would like to hear from CTRE about that. High free dc current can indicate either an electronics issue, high friction, or weak magnets.
For many years, many teams (mine included) have been using free current as a health metric for brush motors.
Neos can also be checked for Back-EMF, using a multimeter that can read RMS voltage and electrical frequency. A decreased ratio V/f will indicate whether magnets have weakened.
I had a high-school friend whose father gave him a cannonball to play with, because he had destroyed all the other toys he had ever received. My friend would have loved competitive robotics, and very likely would have made a good FRC driver. But he was born too soon.
About 10min of run time so the motors outlast the tyres - just. Actually we used VEX Pro compliant wheels with the spokes cut out wrapped around 268kV BLDC out runners with a 3dp rim.
