pic: burnt up TETRIX motor

[colin340]

loading photo ...

[wilsonmw04]

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.

[Cyberphil]

Very interesting!

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!

[colin340]

Originally Posted by wilsonmw04

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.

yes we are planing on shuting it off but a slip of the hands and boom motor gone

[blackiceskier]

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.

[Bill_B]

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.

[colin340]

Originally Posted by Bill_B

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.

yes the resistor is very burnt up and cracked right in half
the shunt capacitor looks fine

[Tristan Lall]

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....

[vincentkee]

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.

[Bill_B]

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.

[ayeckley]

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.

[Bill_B]

Originally Posted by ayeckley

Has anyone experimented with the use of a 10A fuse in place of the 20A "default" fuse? Haven't seen it discussed elsewhere.

...

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.

[colin340]

Originally Posted by ayeckley

Has anyone experimented with the use of a 10A fuse in place of the 20A "default" fuse? Haven't seen it discussed elsewhere.

my electrical people said a fuse would take to long to blow as the resistor or inductor was gone in under 2 sec

[Al Skierkiewicz]

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.

[colin340]

Originally Posted by Al Skierkiewicz

The "resistor" is an inductor.

Al why do u think it is a inductor? just to quiet the brush noise?

also any idea on value of the inductor?