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.
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.
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.
“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
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 
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.
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.
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 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 there was some discussion of the short duration current capacity you should expect from a Victor 884.
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!