pic: burnt up TETRIX motor

[Al Skierkiewicz]

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...

[ayeckley]

Originally Posted by caffel

Maybe we could use a small cricuit braker in series (they go down to 3 Amps, although hard to source in that rated size)

The response times for circuit breakers are typically much longer than for equivalent-value fuses. If you go this route (and if it's determined to be FRC2011-legal), make sure to check that the response time of the device is within whatever time you decide is acceptable. Based on what I'm reading, it sounds like it would need to trip within 1 sec @ 7A (assuming one per motor) to prevent the "smoking" issue.

[Al Skierkiewicz]

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.

[Bill_B]

Originally Posted by Al Skierkiewicz

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...

Cursory search for a choke like that with a suitable current rating for this motor is perplexing. The ones I found are all too big to fit inside the motor. The common factor is a resistance so low as to be practically undetectable at .013 .024 ohms for instance. This would sorely test my FIRST Jiminy Cricket if I had two dead motors and less than a week to competition.

[Al Skierkiewicz]

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.

[IndySam]

I hope they have a lot in stock!

[DonRotolo]

Go look up the MINI Blade fuse performance curves here to get an idea of how long a, say, 2A fuse can carry 10A

[U5ER NAM3]

how do i post my own post? i need help please help me

[Nishant]

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?

[Tom Line]

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.

[WarehouseCrew]

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.

[MechJim]

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.

[elbuo]

Originally Posted by billbcc91

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?

Can someone provide the resistor value? I'm experiencing the same issue

[Al Skierkiewicz]

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.

[Tristan Lall]

Originally Posted by 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.

While my previous comment was true, and at least a plausible interpretation absent a direct ruling, I neglected to consider <R03>. So just in case anyone was considering implementing that: don't. <R03> prohibits custom circuits affecting motor outputs on the entire robot, not just the hostbot.

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).