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pic: burnt up TETRIX motor
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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.
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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! :D |
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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.
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Re: pic: burnt up TETRIX motor
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.
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the shunt capacitor looks fine |
Re: pic: burnt up TETRIX motor
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.... |
Re: pic: burnt up TETRIX motor
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. |
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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:
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Re: pic: burnt up TETRIX motor
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. |
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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. |
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also any idea on value of the inductor? |
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There is this
http://www.legoeducation.us/store/de...2&s=32&ID=2135 Use the promo code FRCT3340 (thanks Travis!) and it's $9. |
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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. http://parts.ftcrobots.com/store/det...&c=1&t=0&l= 0 |
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Does smoke = no longer competition legal motor?
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. |
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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... |
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That being said, it makes perfect sense that it is an inductor. The inductor and cap create an LC low pass filter, thus keeping the higher frequency noise from being radiated or conducted to other components or systems. |
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I will see if the motor I have open at work can CSI'd.
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Now that you see there is a problem, time to engineer a solution. Shouldn't be too difficult to design a way to turn off motor power when you strike the target.:) :)
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Al says that is an inductor and he is usually right. But it does look like a resistor to this mechanical engineer who remembers just about nothing from my EE courses 20 years ago. So I could very easily be mistaken.
Somebody asked about the value of part. If it is a resistor, you can tell by the color of the bands. I can't tell what the inner 2 bands are from the photo due to the crispiness. Maybe somebody who has cracked a motor open can post what the band colors are. Here is a link about how to decode the color bands to a value. http://www.electronicsteacher.com/tu...a-resistor.php I would expect there may be an equivalent code for inductors? |
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Thanks for the link unfortunately it is not currently part of the allowed list of Tetrix parts. Please standby... |
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http://www.tetrixrobotics.com/Buildi....aspx?moid=533. |
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OK,
I will check on this. I have a doc that is the allowed parts for inspectors. I cannot load the Tetrix page, it appears to be having an issue. My doc does not list the thermal protectors. |
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In Team Update 3, FIRST gives a 2nd link to Lego Education, FRC Tetrix parts, which is notably expanded over the Pitsco catalogue in the original manual.
www.legoeducation.us/FRC That link should take you to a 9 page catalogue of additional Tetrix acceptable parts.:) :) |
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TecheD,
There in lies some of the confusion. The lego link is not in the Team Update. It is a place that can be used to obtain parts. From TU#3 Please note that the Rule <R92> only allows Tetrix components. While Tetrix components are a subset of FTC components, it is essential to realize that not all FTC parts are Tetrix parts. As such, not all FTC parts are permitted on the MINIBOT. Please be sure to vet your components against the above list before constructing your MINIBOT. I am receiving a corrupt file error when trying to access the Tetrix page. Therefore, I cannot compare it to my file. |
Re: pic: burnt up TETRIX motor, excellent post
Thanks for the G2 on this motor and the implication that we must have a limit switch.
Hopefully someone, perhaps you yourself, will put this up in the Q&A so we can get a clarification. Have you seen the resister stripes of a new one ? Like what is the resistor rated at ? Maybe we could use a small cricuit braker in series (they go down to 3 Amps, although hard to source in that rated size) |
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The motor I have here at work failed in a way that it opened the case and exposed the interior of the inductor. It is a 3.9 microhenry RF choke in series with the motor. The shunt cap is .039 microfarad. The motor interior looks OK but the choke is open circuit. It would appear that the thermal protection is a good fix. Please stand by...
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Re: pic: burnt up TETRIX motor, excellent post
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Alex,
The trip characteristic for the snap action breakers would likely require something like the 1 amp type. A 3 amp breaker may never trip on 7 amps. |
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By now, everyone should have read Team Update #5 where the thermal protected wire for the Tetrix motors is now approved for your use. Enjoy.
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I hope they have a lot in stock!
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Go look up the MINI Blade fuse performance curves here to get an idea of how long a, say, 2A fuse can carry 10A
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how do i post my own post? i need help:confused: please help me
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I burnt a motor today using a 3:1 gear ratio (not counting gearbox). The smoke came from the gearbox, but both the gearbox and motor smelled. By my calculations, our robot's fastest speed would occur at a 6.48:1 gear ratio (not counting gearbox). I checked my calculations, but why does the tetrix motor smoke like this?
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Re: pic: burnt up TETRIX motor
Your motor is smoking because you are stalling it and drawing too much current. Don't expect it last long - and have someone else check your calculations.
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Could anyone using the Tetrix fuse protected motor power cable answer a couple questions:
1. Does it prevent burning up the Tetrix motors (eg. live up to its promise)? 2. Does the fuse act like a circuit breaker (eg. when it cools down it works again) or do you need to replace the fuse/power cable once it has prevented a motor stall ($9 vs. $30 for motor)? 3. If it is a "fuse" that is destroyed while preventing damage to the motor; can just the fuse (hopefully less that $9) be replaced on this cable without violating the FIRST rules? 4. Is this the cable people are referring to regarding thermal protection? Thanks. |
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We recieved our fuse protected power cable and it has so far worked very well. It does not use a fuse, it uses some sort of solid state circuit that inturrupts the power. The circuit then resets after a short time.
Very good investment. |
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Re: pic: burnt up TETRIX motor
Yamil,
It is an inductor. Search for my other post on this subject. I give the values and a method for repairing a motor with an open inductor. This is not a competition legal fix, but one that will give you motors to use for prototyping and practice. |
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And besides, now that it's been identified as an inductor and not a current-limiting resistor, there's a pretty good practical reason to retain it (over and above what the rules dictate). |
Re: pic: burnt up TETRIX motor
We had the same problem, a burned out motor. I assume that it is a blown inductor. How many amps will blow out the inductor? And is there an acceptable fuse that will prevent this from happening, preferably one that fits in the battery's fuse clip?
Thanks, David |
repairing burnt up TETRIX motor
In order to resore our blown Tetrix motor for "development" use (working on them is an "illegal" mod), I took it apart as follows:
- remove from gearcase - mark the orientation of the brush plate relative to the case -file away the areas where the case is "lipped over" to hold the brush plate. I used a small triangular file. - carefully tap the opposite end (the gear) on a hard surface to pop the brush plate up. - do not pull up on the brush plate! This will damage the brushes. Instead, slide it sideways off the commutator. Once I had it apart, I could see that the inductor was obviously smoked. I unsoldered it, and in its place, soldered a solid wire (don't worry, it will still work!) To reassemble: - create 2 loops using fine thread. Hook one loop around each of the brushes. - with the loops, carefully pull the brushes far enough apart to clear the commutator. - pinch the threads around the circumference in a way that will hold the brushes while you reinstall the brush plate on the shaft. - when it's in place, snip each thread loop and pull one end to remove them. - place the motor on its side and using a small hammer, gently "tab over" the housing adjacent to the notches you filed away to remove the brush plate. If you are as lucky as me, the motor will run like new! Granted, it doesn't have the "inductor", but it seems to work just fine for testing and development. It pains me to have to spend $30 each time a motor blows! |
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What current should the inductor be rated for?
I see its legal to repair it as per <R93> b) a burned out motor may be repaired by replacement of the burned-out inductor with an identical part |
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I believe that the very has been stated somewhere but no one can find a replacement part |
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Brian,
The highest current device I have found thus far for this size package is 900ma. Unfortunately, those are non-stock at Mouser. |
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Thanks Al, what size package is it? (length, width)? Ours is locked away in our trailer since we got home Saturday night from Palmetto. We have a week and a half to get ready for our next regional.
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