I am apprehensive in postings it
But it has the ability to save many teams a great deal of money in prototyping.
Today this motor burned out after approximately 2 seconds of stalling at the top of the pole. We were all very disappointed in our $30 motor so my father set out to determine why it failed.( he could not believe a motor could fail that quickly)
It appears as if a current limiting resistor burnt up when the motor stalled.
He says is possible to replace the resistor and close the motor but THIS IS VERY ILLEGAL but for prototyping perfectly fine and does not have a effect on performance.
For our test we will be replacing this resistor with a piece of wire and externally mounting a new resistor for easy replacement.
I’m nervous people will see this as an opportunity to increase their motors performance but I warn you a simple check over motors resistance by a robot inspector and your team’s credibility will be gone.
I know budgets are tight so hope this is helpful
Thanks for the info. I would advise all teams not to continuously run these motors, at speed, when at the top of the pole. They will burn out. It might be wise to design a cutoff for your minibot when it hit the top. This bit of forethought will save many teams 30 - 60 bucks.
We were just messing around today and thought we burnt out one. But the weird thing was when we plugged it back into the battery without any load, it ran just fine, so much so we mounted it back on and used it for another 5-7 trials.
This is a very good tip though! This will make prototyping much less expensive!
Coming from a FTC member before disassembling a DC motor from the kit plug it back in to a power source with no external load. Sometimes the motors smoke and we think they are burnt out but they just smoke and are still usable in 3 years we have gone through 3 motors actually burning out, and that was only from a 1:27 gear ratio on last years shooter for Hot Shot.
The part that failed is the cylinder-ish one in the back, right? The blue blob is a shunt capacitor I think. It tries to reduce the brush noise on the power input line. If that shorts, you will blow a fuse every time. The smoke some others mention may also be from the part that failed here; just on the verge of failing.
Hypothetically, if one were to install a wire in parallel with the resistor, without making any modifications to the motor, <R47> would not stand in the way.* Since I don’t have the motor in front of me, I can’t say whether this would actually be possible: in all likelihood it would involve laparoscopic surgery through the vent holes (if any), and wrapping a shunt around the leads.
Perhaps that’s not what FIRST intended…but wrapping a wire around a lead is no more a modification than wrapping a chain around a sprocket. Of course, irrespective of what the rule says, it’s unlikely that FIRST would give its blessing to this technique if you inquired about it in the Q&A.
In any event, I wouldn’t recommend doing this. In fact, I only bring it up because it would be a plausible explanation to give an inspector checking the resistance at the motor terminals. (Not that such checks are common practice.)
*Sometimes you just need to think like a criminal…
Does anyone have problems with the tabs of the motors frequently breaking off?
We have our minibot climb up a steel pole that doesn’t have a top so it falls down. We don’t always catch it and then the tabs of the motors keep breaking, rendering them useless unless we solder on replacements.
Zip tie the leads to the motor body to give some strain relief? Solder is not mentioned in <R92> nor are electrical connectors. I hope this is an oversight. I’m not looking forward to trying to get a wire nut onto those tabs. :yikes: :eek:
Has anyone experimented with the use of a 10A fuse in place of the 20A “default” fuse? Haven’t seen it discussed elsewhere.
As long as the fuse survives the inrush current, it appears that it should hold during a “normal” climb using two motors near the max power operating point, yet still open reasonably quickly once a locked rotor condition occurs. Perhaps this would have prevented the OP’s issue altogether - or maybe not.
Our plan was to do this during build season, then revert to the 20A fuse for competition (and even then, replace that fuse every few matches). The worst-case scenario seems to be that there might be a lot of nuisance fuse openings, but it would be far less expensive to replace fuses than motors.
Sounds like a good idea in light of the OP’s situation. I’ve blown one 20a so far but I’m presuming that Colin’s failure was with the same fuse in place. Also interesting to note that “fuse” is not mentioned in the minibot rules section.
The “resistor” is an inductor. Opening the motor is a illegal modification and it pretty much screws up the brush assy if not done carefully.
<R93> Motors may not be modified with exceptions of those in Rule <R47>.
A 5 or 6 amp fuse in line with each motor might be a good protection. If installed properly with insulation, etc., I do not see any rule that would prevent it. The stall current on the motor is listed at 7 amps I believe.
Pitsco sells just such a modified motor power wire:
It’s hugely expensive for what it is, but it’s clearly 100% legal (since it’s a standard TETRIX part) and it’s ready to go for those not wanting to do soldering.
We failed both our motors this way with less than 1 second of stall, and it’s quite disappointing - but if the smoke is just the thermal breaker tripping and it happened to reset, then hey, we don’t have to spend $60.
I’m almost certain we will be direct driving our minibot now not because it’s fastest but just to avoid stall even though I think it will still happen.
From my experience, no, smoke does not not make the motor illegal. I can also say that smoke does not mean the motor is bad either. We have used every motor we have “smoked” again in competition. In face, I think we still have all the motors we started with 3 years ago. The only real problem we have is that we snapped two posts before they made the new adapters.
We have had both our motors burn up so far… I opened up one of ours yesterday and shorted the burnt resistor, planning on putting a new resistor on the outside. Does anyone know what resistance the resistor is supposed to be??
If these motors burn out this easily mini bots are going to get expensive very quickly…
