motor stall current vs Victor ratings

[dawilliams]

We're rookies to FIRST, but experienced with BattlebotsIQ. We've seen the 05 and 06 power diagrams and have a question or two. If the motors draw 80+ amps at stall, the main breaker is only 120 amps, and the Victor 884's only do 40 amp continuous, how do FIRST teams allow for stall conditions? Four motors at stall would exceed the main breaker, and any Victor 884.

In Battlebots we would use higher rated Victor 885's, in parallel if needed, because we had motors with theoretical (if the batteries could keep up with the load) stall currents of near 300 A @ 24v.

1. Are FIRST robots allowed more than one battery?
2. Can we have more than one main breaker in parallel off the battery (if only one is allowed)?
3. Are the circuit breakers provided in the KOP auto resetting? Main breaker auto resetting?

Any other ideas along these lines?
Nervous excitment.
Thanks,
Gooooooooooo Teams!!!

[Adam Y.]

Are the circuit breakers provided in the KOP auto resetting? Main breaker auto resetting?
Yes though you definately don't want to rely on this fact. You actually want to gear the motors down so that they are not drawing that much current. Sufficient gearing means that the motors will not be drawing as much current as needed. As you pointed out the stall current far exceeds the rating on the Victors.
Are FIRST robots allowed more than one battery?
No.

[Richard Wallace]

We power the whole electrical system through a single 120A breaker (which can really do at least 160A for a minute or so), then each motor circuit is further protected by its own 20A, 30A, or 40A snap-action auto resetting breaker. The 40A variety can actually handle about 70A for a few seconds.

FIRST will publish an electrical system diagram for 2007 (and component datasheets) sometime shortly after Jan. 6, which will make all this a little clearer.

EDIT: Of course, FIRST tweaks the rules a little every year. We have no idea what changes we may see this time around. However, it is typical of our Game Design Committee to make rules that encourage robots built to perform a scoring function that usually requires handling or at least controlling a game piece. The high-power traction systems typical of BBIQ are generally not required for success in the FRC.

[Tristan Lall]

1. Are FIRST robots allowed more than one battery?Yes, but not in the way you're thinking. Only one 12 V battery (of the Exide ES18-12 or EX18-12 types) and one 7.2 V battery (any NiCd pack) may be used as described in the rules.
2. Can we have more than one main breaker in parallel off the battery (if only one is allowed)?This one should be a no, per <R54>, but there's a little bit of a catch here. If you read <R54> as meaning "use these devices in this general arrangement," (there's a diagram) "but don't worry about making the connections in exactly this way", then you've probably satisfied the rule as it was intended to be followed. But the loophole is, of course, that a 120 A circuit breaker (even a 2nd one) is a legal additional electrical part*. If you're reading the rule to mean that the connections don't have to be made in exactly this manner, then you have not restricted the nature of those connections in any meaningful way. You could therefore claim, that having two parallel 120 A breakers (in the usual place in the circuit) satisfies the diagram (because a subset of the electrical system is exactly as required, and the diagram does not preclude the possiblity of different or additional connections).

Now, to anyone who's been in FIRST a while, that's obviously not the expected conclusion—but how can we interpret the rule to make this configuration impossible? It seems to me that we would have to read the rule as meaning "use these devices in this exact arrangement, including making the specified connections". This, means that you must not add anything extraneous to, or remove anything from the specified portions of the circuit. All this is fine, until you run into the fact that the Maxi fuse panel and both ATC panels are in the circuit as drawn, and connected in a particular fashion. That implies that all must be present and connected exactly this way, even if not used. The Maxi block is heavy, and teams might understandably want to be rid of it, to make weight (sacrificing maximum power output in the process); similarly, it's likely that one of the ATC panels is redundant. The rule certainly wasn't interpreted and enforced according to this reasoning last year.

We could look for a middle ground (where the extra main breaker is illegal, but the rest of the diagram is less strictly defined), but no such thing was proposed during the season—indeed, it seems to have been overlooked by all.

I should note that depending on the circumstances, it may well be ruled as unsafe per se, and therefore in violation of <S01> and/or <R40> (which pertain to unsafe design, operation or additional part usage, as judged by referees and inspectors, respectively), but without direction in that regard, various officials will have different feelings on the matter, depending on their perspectives and knowledge. (And indeed, if the parallel arrangement is unsafe, consider that the very same reasoning might lead a single 120 A breaker to also be considered unsafe—I can think of some arguments which might lead down this road....)

I don't recommend trying to exploit this potential loophole, as it would probably be quashed very quickly upon being submitted to the Q&A.
3. Are the circuit breakers provided in the KOP auto resetting? Main breaker auto resetting?The individual 20, 30 and 40 A breakers are self-resetting; the 120 A main breaker is not.

All of the preceding applies to the 2006 game, and may not be true in 2007.

*It is legal to have on your robot (and not just as a decoration), because it satisfies the additional electronics rules and is not specifically prohibited anywhere. Its precise function (beyond its mere presence) is the issue.

[eugenebrooks]

R54 refers to the battery and the 120 amp breaker, directly,
in the singular. That, in combination with the diagrams and the
fact that the breaker clearly protects the 6 gauge wire, battery
connector, and battery from overcurrent, makes the intent of
R54 clear. Seeing this as a loophole is quite a stretch. I don't
think that you would get this one past any technical inspector.

Eugene



But the loophole is, of course, that a 120 A circuit breaker (even a 2nd one) is a legal additional electrical part*. If you're reading the rule to mean that the connections don't have to be made in exactly this manner, then you have not restricted the nature of those connections in any meaningful way. You could therefore claim, that having two parallel 120 A breakers (in the usual place in the circuit) satisfies the diagram (because a subset of the electrical system is exactly as required, and the diagram does not preclude the possiblity of different or additional connections).

[Richard Wallace]

I don't think that you would get this one past any technical inspector.Well, maybe someone could get it past one ;) -- Tristan is an experienced FRC robot inspector.

But I agree with Eugene about the intent of the rule.

[dcbrown]

The EX18/ES18 battery specification lists a maximum rating of 230A for 5 seconds. Various estimates of a 120A load based upon the the battery specification chart and different derating values yields between 20-40 seconds of rated battery life. Real life experience may differ, but the these are the rated values from the specification.

So on the practical side, pushing the battery beyond 120A for all but short brief periods will mean the 'bot will die before the round is over. Running two 120A main breakers would seem to indicate the design only wants the robot to run a few seconds before completely draining the battery AND it would allow the battery to be run outside of its specifications.

Running the battery anywhere up around the maximum rated value of 200A+ for any time at all is flirting will damaging the battery at least or causing some safety issue like melting or worse. Also, the number of discharge/charge cycles the battery can go through is highly dependent upon the rate and depth of discharge. Draining capacity at high loads can severely shorten the lifetime of the battery. Some data on AGM/GEL cells indicates as few as 10s of cycles under these extreme circumstances.

So even if you wanted to run multiple 120A breakers, there is no practical reason to do so -- the battery would at best drain very quickly and not last through the competition. All such a design would be doing is causing a potential risk to the robot as well as others.

Of course there have been rumors of changing the battery for this upcoming season, so there may be a whole different set of issues in two weeks to discover.

On the practical side, the motors when geared down seldom fully stall but instead start spinning the wheels even with grippy treads.

Bud

[dawilliams]

Thank you all for your replies. This is certainly a credit to the FIRST gracious professionalism creed.

Still one curiousity comparing my BBIQ experience with FIRST robots. Most teams at BBIQ used 24v systems for drive, motors from 1-3 hp PER SIDE (or 2-6 hp total in the differential drive), and batteries with 5-6 AH ratings. The playing surface was sandtextured paint on steel, which ended up fairly slick by the third day of competition. If two bots pushed against each other, one would move or the wheels would spin. This lack of traction compared to rubber on carpet as with FIRST would protect BBIQ bots from burning up Victors or motors. But I still saw several burned motors or Victors (pink/grey smoke) each year.

The FIRST motors are approx 1/2 hp for the largest ones. The battery has higher AH rating letting it keep up with whatever the motor draws. The playing surface with rubber tires is less forgiving (no slipping to protect the Victors and motors). To my thinking, this means robots pushing against each other won't be able to move each other. Thus more likely to stall the motor. Even geared down, if you push against the side of the opponent, you won't be able to move it, and that is still a stalled motor.

The 4 largest motors draw 133, 96 or 63 amps at stall. That exceeds the 40 amp rating of the Victor 884. The Victor 884 doesn't list a higher current for any short time (as the Victor 885 does).

So do you suggest a single Victor 884 for one of these large motors, or two in parallel for each motor, each with an independent 40A circuit breaker?

Thanks again!

[Joel J]

What I usually see the robot designers do is to select a traction material for their wheels, then calculate the maximum torque they can get with those wheels, given the robots weight, as a first step. So lets say there is a 6 inch tire, a 150lb robot, and the coefficient of friction between the wheel and the floor is 1.3 (IFI roughtop tread). The gearbox designers would calculate the max torque at the wheel to be: 150*1.3*3(tire radius) = 585 in-lbs. Convert that to the oz-in units that's given on the CIM motor chart, and you get 9360 oz-in's at the wheel before it starts slipping. I've typically seen FIRST drivetrains use the 4 identical CIM motors, two per side, meaning that the load would be evenly shared across these motors. So each motor needs to provide 9360/4 oz-in's of torque, or 2340 oz-in. The designer would then determine the amount of torque they would get when pulling 40 amps from the CIM motor, as opposed to the 133 amps at stall (let's say its 105.43 oz-in's, as opposed to the stall torque of 343.4 oz-in). Divide the torque needed from the individual motor by the max load you want it to see, and you get the reduction needed between the motor and the wheel. In this case 2340/105.43 gives a reduction of 22.1948 minimum. When you reduce the free speed of the CIM motor (5310 RPM) 22:1, you get a final RPM of 239.245, which translates to a ground speed of 6.2634 fps. I didn't factor in losses (4.5253 fps, with losses, on most one speeds.. 4.1fps on a typical 2 speed). At this reduction, the idea is that the wheels would start spinning before the motors begin to pull more than 40 amps, preventing stall. I guess the idea is similar to having a low traction surface to play on. If the final speed you get after doing these calculations is too low, then you just lower the "grippiness" of the wheels you are using. If you want to be a strong pusher, you can only lower the grippiness so much before you start losing traction advantage. You'll start getting into the whole tradeoff between speed/torque/current pull that I just can't bother with; I'll stick to programming.

This was a bit rushed. If you want a detailed description of this idea, you can search around the forums for topics on drivetrain design, or gearbox design, or something..

[dcbrown]

At River Rage, drivers of our team bot which used two CIM motors, 6 wheel drive with 2" wide IFI roughtop tires side pushed all the robots it came up against to prevent shooters from scoring. Motors never stalled, breakers never popped. The robot used the KOPS transmissions and weren't geared down very far from there - that is top speed was still acceptable.

Bud

[eugenebrooks]

Unless the rules change this coming season, relative to last year, I suggest a single 40 amp breaker, feeding a single Victor 884, connected to a single CIM motor.

Yes, the Victor 884 is rated at 40 amps continuous, and the breaker is rated at 40 amps, but both of these parts will sustain higher currents for short time periods.

Eugene

The 4 largest motors draw 133, 96 or 63 amps at stall. That exceeds the 40 amp rating of the Victor 884. The Victor 884 doesn't list a higher current for any short time (as the Victor 885 does).

So do you suggest a single Victor 884 for one of these large motors, or two in parallel for each motor, each with an independent 40A circuit breaker?

Thanks again!

[JBotAlan]

Thank you all for your replies. This is certainly a credit to the FIRST gracious professionalism creed.
Well, the official hint has been released, so traffic here is picking up.
To my thinking, this means robots pushing against each other won't be able to move each other. Thus more likely to stall the motor. Even geared down, if you push against the side of the opponent, you won't be able to move it, and that is still a stalled motor.
Two things:
1) the drivers are trained not to keep pushing if we're not moving--that would release magic smoke for sure if the wheels weren't slipping which brings me to:
2) our wheels start to slip when pushing against something relatively static at somewhere around half power, so our motors don't stall unless the driver stalls them on purpose (holds them at quarter power for a long time). We still have a strong pushing force; ask anyone who has played against us.
So do you suggest a single Victor 884 for one of these large motors, or two in parallel for each motor, each with an independent 40A circuit breaker?
At first, I thought that having two victors on one motor was against the rules, but I re-read last year's rules and the only slightly relevant rule is <R86> in section 5:
<R86> No more than one motor may be connected to each Speed Controller.
That doesn't prohibit two Victors on one motor. However, I can see this being frowned upon during inspection. One victor is all that's needed to white smoke a motor; two seem like they would cause problems for sure. I just don't see the need for that much power. I don't think we've ever tripped a breaker. Just plan for the wheels to start slipping just as the motors start to stall.

JBot

[eugenebrooks]

Refer to "FIRST Guidelines, Tips and Good Practices"
and the "Robot Power Distribution Diagram." These
documents are updated each season. R80 refers to
the robot power distribution diagram.

I see our robot is traveling in circles...

Eugene

At first, I thought that having two victors on one motor was against the rules, but I re-read last year's rules and the only slightly relevant rule is <R86> in section 5:

That doesn't prohibit two Victors on one motor. However, I can see this being frowned upon during inspection. One victor is all that's needed to white smoke a motor; two seem like they would cause problems for sure. I just don't see the need for that much power. I don't think we've ever tripped a breaker. Just plan for the wheels to start slipping just as the motors start to stall.

JBot

[Alex698]

Now i have to ask:
How long before we melt the anderson power-pole connectors? them seem to be the weakest factor. Just a thought.

[Gdeaver]

Keep in mind that while a robot is in a pushing match they are not scoring points. Scoring point wins matches. Yes, defense comes into play, but a well coordinated offense can neutralize a defense with picks and such. First play is different than battle bots and the rules more limiting.

[RogerR]

Keep in mind that while a robot is in a pushing match they are not scoring points. Scoring point wins matches. Yes, defense comes into play, but a well coordinated offense can neutralize a defense with picks and such. First play is different than battle bots and the rules more limiting.

this isn't always the case; in 2000, 2001, 2002, and 2003 you got points based on where you or you're goals where (granted, in '01 you were fighting gravity, instead of opposing 'bots). in 2004, when most 'bots hung from the upper platform, being able to hold one's position while attaching to the bar was often dependent on how your drive train measured up to your opponents. likewise, the triplets' beefy drivetrain, coupled with effective programming, helped them to hold their positions while under attack from opposing defenders, which allowed them to unload massive quantities of balls into the high goal.

while the specifics depend on the game and startegy, an effective and powerful drive train can be a potent offensive weapon.

[Alan Anderson]

So do you suggest a single Victor 884 for one of these large motors, or two in parallel for each motor, each with an independent 40A circuit breaker?
Don't put Victors "in parallel". It won't work the way you think it will, and it could end up destroying one or both of them.

A Victor speed controller pulses the outputs between full power and off, changing the duty cycle and polarity based on the PWM input. Even with the same input, two Victors will not necessarily switch their outputs in synch with each other, and might be fighting each other.

[JBotAlan]

Now i have to ask:
How long before we melt the anderson power-pole connectors? them seem to be the weakest factor. Just a thought.

I have not once seen an Anderson pair melt. Not once. And that's 3 OCCRA seasons and 2 FIRST seasons. These are on a much smaller scale power-wise than you seem to think. I think you'd have to do something extremely scary to melt the Anderson pair (think of making your robot chassis hot enough to cook eggs...)

JBot

PS. what are those connectors rated for anyhow?

[lukevanoort]

PS. what are those connectors rated for anyhow?
50 amps

[Richard Wallace]

50 ampsThe 50A rating of these connectors includes a fairly significant safety factor. To qualify the initial design and the tooling, or to validate changes such as a new material or supplier, the connector manufacturers probably test at some service factor (maybe 1.25?) for a relatively long duration (maybe 2000 hours?) and in an elevated ambient temperature envirionment to ensure that 50A is a conservative rating.

As used in FRC (very short duration loading to 160A, typical 2 minute loading within marked rating) these connectors won't get warm enough to worry about, unless their terminals are incorrectly crimped. That won't be an issue if you use the factory crimped units that come in the KoP.

[Al Skierkiewicz]

Boy,
You take a few days off for the holidays and look what happens. Here are a few answers for you...
The battery can supply more than 400 amps for short periods of time when fully charged, limited by it's internal impedance, life cycle and state of charge.
The 120 amp circuit breaker is capable of more than that for periods of less than 10 seconds but must be derated for increased temperature. (the breaker is a temperature only controlled device as are all of the self resetting breakers.)
Yes the rules allow more than one 120 amp breaker but only one can and must be used for all power to the robot. That makes the use of a second breaker downstream redundant and provides a second source of failure and added series resistance.
As pointed out before, only one Victor and one breaker per motor. Using two on the same motor is asking for trouble and will not pass inspection. You may feed two Victors/motor combinations the same PMW signal if you choose by using a "Y" PWM cable. This rarely works as intended but is a quick and easy solution to some problems.
Self resetting breakers and Victors are also capable of significant overcurrents for short periods of time without trippping or damage. Don't depend on the overcurrent handling to make up for deficient designs.
The 50 amp Anderson Power Pole connectors can handle the extreme currents because their rating is based on continuous current at 50 amps. The temperature rise in a two minute match is usually enough to prevent any damage. I have seen melting with these connectors but it usually was a case of damaged contacts caused by the use of alligator clips on the charger to connect the charger to a battery. I always suggest that teams change out the clips for a mating Anderson connector on the charger(s) for best results.
There are two limiting factors in electrical system overloads that prevent excessive damage in First Robots. 1) The resistance encountered in normal wiring and electrical design places a significant resistance in series with the battery which will limit maximum current. 2) The Robot Controller will mute all PWM outputs during any period of time when the main power supply falls below about 8 volts. Maximum loads on the battery will drop the terminal voltage below this point, shutting off motor control and thereby reducing current. Should the backup battery also be discharged or fail, then a delay of several seconds will occur while the RC reboots and reacquires communication.
Hope all this helps, good luck this season.

[dawilliams]

I have used Victors with Y-cable to synch the PWM input to each Victor and never had a problem. This was with much higher current than Victor 884. IFI lists max currents for Victor 885 for 1 and 2 seconds. Since none is listed for Victor 884, where are some of you getting this over-rating info for the Victor 884? There are other threads that discuss burning Victors in matches (not just reversing polarity) which does indicate some teams are drawing too much current for some time. And defensive bots do push for quite some time.

I like the analysis done by Joel J (team 229) but I suspect the problem may lie in the desire/need for speed higher than 6.2 fps (that's less than 4 mph ie medium fast walk). Then a similar analysis would show the wheels don't slipand higher current would be expected. I also think the line:

"When you reduce the free speed of the CIM motor (5310 RPM) 22:1, you get a final RPM of 239.245, which translates to a ground speed of 6.2634 fps"

should use the motor speed at the load equivalent to 40A current, not the free speed.

[Joel J]

I have used Victors with Y-cable to synch the PWM input to each Victor and never had a problem. This was with much higher current than Victor 884. IFI lists max currents for Victor 885 for 1 and 2 seconds. Since none is listed for Victor 884, where are some of you getting this over-rating info for the Victor 884? There are other threads that discuss burning Victors in matches (not just reversing polarity) which does indicate some teams are drawing too much current for some time. And defensive bots do push for quite some time.

I like the analysis done by Joel J (team 229) but I suspect the problem may lie in the desire/need for speed higher than 6.2 fps (that's less than 4 mph ie medium fast walk). Then a similar analysis would show the wheels don't slipand higher current would be expected. I also think the line:

"When you reduce the free speed of the CIM motor (5310 RPM) 22:1, you get a final RPM of 239.245, which translates to a ground speed of 6.2634 fps"

should use the motor speed at the load equivalent to 40A current, not the free speed.
www.andymark.biz

Look at that company's (created by FIRST veterans from team 45) website for info on shifting gearboxes that give you the option of having a speedy gear and a pushing gear. You can buy one of their shifters, or you can design your own (this is not a simple thing to do, if you've never done it before). You shift into the pushing gear (low gear) to play defense, or to hold your position, and then move into the higher gear to fly around the field. The drivers would have to remember to shift to low to do any pushing, as they would probably pull too much current in high gear.

should use the motor speed at the load equivalent to 40A current, not the free speed.
Well.. under an amount of load that would cause the robot to pull 40A, it would certainly be moving near the rated load speed for the proportional amount of torque (multiplied by the efficiency of the gearbox); however, under normal driving conditions (that is, when not pushing), the motors are probably pulling much less than 40A.. they are pulling something closer to the free speed current draw divided by the gearbox efficiency. So, in my example above, with an AndyMark transmission, the robot would normally drive around at 4.1 fps in low gear, but when it started to push, it would slow down to a slower speed. I bet you can visualize this happening if you've ever watched a robot get into a pushing match.

[Al Skierkiewicz]

DA,
Maximum rated current for a Victor was published a few years ago but can be derived from the current specifications for the MOSFETs used. Keep in mind that each FET is in parallel with two others for current sharing but that there is derating due to lack of heatsink. The FETs have a very low "on" resistance and so very little power is dissipated within the device which keeps internal heating to a minimum. However, there are other factors...usually at maximum current, a) the speed controller is no longer using a PWM output, it is at full "on", b) the motor is at or near stall, c) the motor may be back driven by a pushing robot, d) bad electrical connections at the Victor will raise it's temperature due to heating of the contact area, e) the fans become inefficient due to the lower battery voltage. Keep in mind that about 90% of all speed controller failures are due to foreign and conductive material inside the controller. The remaining failures are a result of incorrect wiring, over heating, repeated high current due to stalled motors or rapid direction changes or defective mechanical systems.
As pointed out, many teams design for at least 6 ft/sec but speeds from 8-12 ft/sec are acceptable. Higher speed designs will result in mechanical systems that will easily stall and draw excessive currents when starting or accelerating.

[dcbrown]

"The battery can supply more than 400 amps for short periods of time when fully charged, limited by it's internal impedance, life cycle and state of charge."

Since the specification for the battery from the manufacturer states "Max. Discharge Current 230A (5 SEC.)" - I'm not sure I'd be willing to assume the consequences of any discharge rate that exceeds the manufacturers claim as maximum. Just the safety risks alone give me pause.



"The 120 amp circuit breaker is capable of more than that for periods of less than 10 seconds but must be derated for increased temperature. (the breaker is a temperature only controlled device as are all of the self resetting breakers.)"

The bussmann specification for the 120A breaker indicates that at room temperature, the breaker is nominally good with a 200% load for anything from 10 to 40 seconds before tripping. At 125% load, it may run for upwards of 500s or more before tripping or as little as up to 50 seconds depending on individual part. Either limit is within specification. Derating at 100F can be anywhere from -5% to +25% over rated current. Of course there is no graph given for the number of times it has been tripped vs derating. Now that would be interesting! The purpose is as a failsafe device, just design steady state current draw to be under the 120A rating and it won't be a problem -- any short high draw transients won't matter.

Bud

[MikeDubreuil]

In general FIRST robots have a very conservative safety strategy. This is mandated by the Rule book which comes out at kickoff in January. Tristan talked about loopholes in rules and in general loopholes which involve safety are frowned on in the FIRST community. In fact in previous years the first rule in the safety section of the rule book is a catch all rule which allows FIRST to enforce safety rules they didn't specificly address in the rule book. I would reccomend wiring your robot following the exact set of rules given in the rule book paying specific attention to the electrical wiring diagram. Just because your wiring layout is theoreticly safer an inexperienced robot inspector might not pass you on this test. Robot inpection usually happens on the practice day (Thursday) of the regional. If your robot does not prass inspection it can not play during competition.

FIRST robots generaly rely on novel mechanical design for scoring game pieces rather than pure power to push around opponents. Sure powerful drive trains are encouraged. However, FIRST has been moving in a direction with the games where offensive robots are more encouraged. I don't expect that to change.

We favor beauty in mechanical design over electrical brawn :D

[Joe Ross]

I have not once seen an Anderson pair melt. Not once. And that's 3 OCCRA seasons and 2 FIRST seasons. These are on a much smaller scale power-wise than you seem to think. I think you'd have to do something extremely scary to melt the Anderson pair (think of making your robot chassis hot enough to cook eggs...)

JBot

PS. what are those connectors rated for anyhow?

I've seen one melt. http://www.chiefdelphi.com/pics/bin/1045466779dsc00631.jpg

But you're right, it's not common and probably a result of damage to the connector, although excessive current draw would be a contributing factor.

[Al Skierkiewicz]

Since the specification for the battery from the manufacturer states "Max. Discharge Current 230A (5 SEC.)" - I'm not sure I'd be willing to assume the consequences of any discharge rate that exceeds the manufacturers claim as maximum. Just the safety risks alone give me pause.

The bussmann specification for the 120A breaker indicates that at room temperature, the breaker is nominally good with a 200% load for anything from 10 to 40 seconds before tripping. At 125% load, it may run for upwards of 500s or more before tripping or as little as up to 50 seconds depending on individual part. Either limit is within specification. Derating at 100F can be anywhere from -5% to +25% over rated current. Of course there is no graph given for the number of times it has been tripped vs derating. Now that would be interesting! The purpose is as a failsafe device, just design steady state current draw to be under the 120A rating and it won't be a problem -- any short high draw transients won't matter.

Bud
The spec is for manufacturer recomendations to guarantee life cycle spec and stay within the terminal voltage spec, but the battery is capable of much more in the short term at reduced terminal voltage. A dead short will of course draw max current while at a terminal voltage of zero. Catastrophic failure of the battery is likely under these conditions.

The short term charachteristics of the breaker can withstand these loads. (as shown in your attached data sheet) I have not yet witnessed a main breaker trip during competition in a normally operating robot. Since a match is less than 200 seconds at operating current, the breaker will not trip even with four motors in stall for short periods of time.

I should mention that there are occasional manufacturing defects in the 120 amp breaker that will make it intermittant. All teams should check the breaker by tapping the red reset button when the breaker is turned to the "ON" position. A defective breaker will turn off (intermittantly) when the button is tapped. Replace any breaker that exhibits this defect.

[dawilliams]

DA,
Maximum rated current for a Victor was published a few years ago but can be derived from the current specifications for the MOSFETs used.... The remaining failures are a result of incorrect wiring, over heating, repeated high current due to stalled motors or rapid direction changes or defective mechanical systems. .

Can you direct us rookies to this published info? We're not electronics engineers and able to calc from the MOSFET ratings. Its too bad IFI doesn't publish these ratings on the Victor 884's like they do on the Victor 885's.

[Richard Wallace]

Can you direct us rookies to this published info? We're not electronics engineers and able to calc from the MOSFET ratings. Its too bad IFI doesn't publish these ratings on the Victor 884's like they do on the Victor 885's.Courtesy of FRC Team 358, here (http://team358.org/files/electrical/VictorFET_irl3103.pdf#search=%22irl3103%22) is a link to the IRL3103 power MOSFET used in the Victor 884 motor controller. The 884 is an H-bridge using a parallel connection of three of these FETs in each leg. In an earlier thread (http://www.chiefdelphi.com/forums/showthread.php?t=48284) there was some discussion of the short duration current capacity you should expect from a Victor 884.

[Al Skierkiewicz]

Thanks to Richard and Mark in the referenced thread above for the discussion. I would like to add that for much of the match the output of the controller is likely in a PWM mode and not at full throttle. With this in mind, the temperature rise in the FET (for the majority of users) will remain below 100 C for the two minute match. Adding to all the other factors including the resistance of the wiring, breakers, connectors etc. my guess is that the Victor is very conservatively rated at 40 amps in our application. I have faith that for a match duration 50 amps per FET are acceptable limits. The motor with the highest current draw is the Chalupa with a stall current of 129 amps. Considering the operation curves of the 40 amp breakers feeding the controllers, my suspicion is that the breakers will start to react before the damage occurs in 99% of the cases.
A very agressive driver in a full on pushing match with sticky wheels and no slip might on rare occassions exceed all of the limitations of the FETs and the breakers. There is still the other catastrophic failures to contend with i.e. broken transmissions, damage to drive components from a collision, etc. which will lock up the motors under normal conditions. Drivers need to be aware of these types of failures and not try to overcome them with brute force. Smoke and damage are the result of such abuse.
When using your robot for demonstrations, keep in mind that there are limitations for current and heat and robot maintenance and monitoring are required. Releasing smoke during a demo although momentarily exciting is not desirable!