[FTC]: Max amps for DC motor fuse

[WRA]

We, like a lot of team I assume, have had a big problem toasting motors. A power distribution panel was recommended so we ordered and installed a RigRunner 4005. Know we are trying to find the optimal fuse size for our DC motor controllers. Each of thr 4 on the robot are I individually supplied by the RigRunner and 3 of the 4 power 2 v2 motors. I can find that the max stall amps for these is 4.55 amps. What I can't seem to figure out is what size fuse I should use when current is going to 2 motors at one time. Anyone have some advice? I'd really like to pop the fuse right before the motor breaking point if possible.

Food for thought on this. Currently S1C1 powers the left and right rear drive so they both may not stall at the same time. Would it be better to put left front and left rear on the same controller so that in most cases they would stall at the sametime?

Thanks in advance. Hope to see everyone at the North Super Regional next week

[maths222]

Food for thought on this. Currently S1C1 powers the left and right rear drive so they both may not stall at the same time. Would it be better to put left front and left rear on the same controller so that in most cases they would stall at the sametime?


I would advise against that, mostly because having them on different controllers allows for you to continue operating your drivetrain for defense even if a controller stops working/locks up/looses power.

[Sasha]

I do not think it is legal to insert a fuse between motor and controller. The list of allowed materials doesn't include fuses.

[dcribbs]

I do not think it is legal to insert a fuse between motor and controller. The list of allowed materials doesn't include fuses.

Q&A Forum: Robot Electronics and Power Question #45

http://ftcforum.usfirst.org/showthread.php?1921-Robot-Electronics-and-Power-Answer-Thread/page5

They are legal.

[Sasha]

Q&A Forum: Robot Electronics and Power Question #45

http://ftcforum.usfirst.org/showthread.php?1921-Robot-Electronics-and-Power-Answer-Thread/page5

They are legal.
Indeed they are - sorry for confusion.

Note though that the fuse will be between the battery and controller, not between the controller and the motor.

[WRA]

Thanks everyone. Yes, we have the RigRunner supporting the controllers not between the controller and the motor. Sorry for the confusion. We have been doing some stall testing and still have not come up with a good answer on the fuse size. A 5 amp fuse protects the motor by killing the current immediately in a stall. I grabbed some 10s today and we will test it tonight but I am not sure that I don't need to be somewhere in the middle.

[cgmv123]

A 5 amp fuse protects the motor by killing the current immediately in a stall.

Breakers/fuses protect wiring, not devices. A 5 amp fuse can and will pass current significantly in excess of 5 amps.

[Danny Diaz]

Breakers/fuses protect wiring, not devices. A 5 amp fuse can and will pass current significantly in excess of 5 amps.
Indeed. A "fast-blow" fuse will generally require twice the rated current for a period of 0.1 seconds in order to blow, a standard fuse generally requires twice the rated current for a full second before blowing, and a slow-blow fuse requires twice the rated current for a period of 10's of seconds to blow. The danger of putting a fast-blow fuse on a motor is that the motor may pull current near the stall current level for a short time while they're starting up, so a fuse innocently (and incorrectly) used to protect a motor from stall may blow as the motor is spinning up.

LEGO has a thermal fuse (http://www.pitsco.com/Parts/TETRIX_Fuse-Protected_Motor_Power_Cable) that they designed to help protect a motor during robot development. It's not fool proof either, many motors have met their maker due to poor design (the fuse generally can't pop fast enough to save a motor from a sudden and absolute stall), but just as many have been saved from near-stall situations allowing the fuse to blow before doing fatal damage to the motor. The benefit of the LEGO power cable is that it's a thermal fuse, so when it cools it will close the connection back and the motor will come back to life; the problem is that you don't want that on your competition robot because once the fuse pops it may take a minute or two to come back, and the fuse's protection ability seems to degrade each time it pops.

-Danny

[nuclearnerd]

I'm new at this. The Jags and Victors have the ability to use feedback don't they? Have any teams have gone to the effort of adding encoders to their 9015 / RS775 / Bag motors and then programming the controller or cRio to provide stall protection? It seems like that would be a useful off-season project. It might be nice to design an encoder output on the versa planetary gearbox too.