The non-obvious, but not very helpful, answer is that “it depends”. We don’t know how you have set up your drivetrain, or how your driver behaves, or any of those other things that will greatly affect how much current the Victors see.
No problem with these ones, they’ve been thru some tough competitions, and have Lexan mounted one inch above the tops of the fans. 4 small CIM motors, 6wd, grippy wheels, 2 speed andymark transmissions.
As Jim pointed out, there is no hard and fast rule as to which thermal design is too little cooling. No Victor is running at 40 amps for an entire match. You could model an average match but it would only hold true for a particular game and a particular robot design. Here is what we do know, the FET(IRL3103) is speced to run in the temperature range -55 to +175 C. Each FET can handle 64 amps at 12 mohm ON resistance at 25 C and the package is designed for about 50 watts dissipation. There are two pairs of three FETs in parallel in series for each direction so that translates to 8 mohm total series resistance (12/3 +12/3). If a speed controller is feeding a Chalupa in stall then the current is about 130 amps and the entire controller is dissipating 130^2 x .008=135 watts. So in the worst case, the fans must get rid of 130 watts or the controller will start to heat. As the heat in each FET rises, the operation of the FET changes. The ON resistance rises and therefore the max allowable current falls. At 125 C the max current rating is about half that at 25 C. Now as to failure, the junction temperature (that temp right at the silicon junction inside the case) can rise rapidly without the case temperature being affected. If that is the case, then no amount of cooling fan will protect the FET from abuse.
The better question might be “Have you experienced any Victor failures attributable to heat or blocked fans?” I have not seen one Victor that I could absolutely attribute a death to heat rise. The majority of failures have come from foreign objects (metal shavings and dust) or flat out abuse (rapid forward and reverse motor command or lengthy operation in stall such as repeated turning with tank drive and sticky treads).
There is a thought among many engineers in FIRST that, if you are not pushing the envelope, you are not being competitive…
Over the years, we have mounted Victors in almost every orientation conceivable, packed them very very close and enclosed them with polycarbonate covers which violated the 2" rule.
Two years ago, we even had victors with intermittent cooling fans because the students crimped ring lugs on the fan wires without properly stripping the wire’s insulation. The intermittent fans were not discovered until we had competed in two regionals and the Atlanta championship (we fixed it post season).
Our team has never lost a victor due to overheating.
Space Victors no closer than 1/2" apart, side to side, and 2" apart, front to back (this is more due to 10AWG wire access than to cooling), and leave 2" above the fan before locating your cover.
Do put a protective cover over the Victors. Flotsam (bits of aluminum from drilling, et cetera) from people working on the robot is the #1 cause of Victor failure that I have seen. The trade off between restricting air flow and protecting the electronics from debris is, to me, a no-brainer.
I think it would be interesting to see the data that IFI has collected during the manufacturing/testing phases of the Victor Speed Controllers. I’m sure they have quantitatively analyzed them and their thermal characteristics to determine their efficiencies, as this would be a very appropriate concern. Many people use their speed controllers, not only besides in the FRC (Robot Wars and all that) and in a lot of other robotics applications. At the university I go to, its not uncommon to see victor’s in many of the robots that the college of engineering develops, and most people would have never heard of FRC.
The best way we found to mount electronics while keeping weight down and not having them overheat is to mount thirty thousandth lexan pulled tight and screwed to the frame then just lining the victors up in a row leaving about 1/2 inch between them. I would not use an electronics box we tried that and it was such a hassle to get around inside it and fix any thing also it weighs more and you need to put some ventilation in it as well which is more weight so for us it is not the way to go.
In many of my non FIRST related robotics projects I regularly remove the fan from the victor and operate normally without it. I do this to reduce overall hight, weight and most importantly noise. Some projects require up to 12 of the things, and those fans make a racket. My non-first stuff usually has to run for very long durations under continuous draw (albeit less then a typical FIRST 'bot). I have never, since they were debuted, found a Victor that failed due to overheating or experienced any odd behavior as a result.
Let’s put it this way- If you manage to overheat a Victor, chances are the motor overheated and failed a long time ago.
I do however keep my victors well spaced and unobstructed, within reason. I do this not for any consideration of cooling but to keep wire runs clearly defined and easy to follow. I’ve seen some electronics boards, both in FIRST and outside of it, that are very compact but seem just about impossible to work on or troubleshoot. I usually favor ease of repair and understanding over tight packaging. I do agree with what Mike said though, if you aren’t pushing the envelope, your not being competitive. It’s a balancing act.
We recently made some off season mini-robots and we ran all the victors without fans on them. They worked fine, the victors never shut down or went up in smoke because of heat. I know that the victors are rated to handle much more voltage. (i believe up to 48 volts??) So at 12 volts its really not hard on the victor at all. It should have no problem running at 12 volts with not fan on.
In all seriousness though, I/we really don’t know, we have never had problems with them over heating… but then again we had them very well ventilated and did not run them longer than 5 min. at a time. (last year’s robot consisted of a plywood box with a two wheel drive train… you get the picture)
As far as I know, the Victor 883’s and 884’s are rated between 6 and 15 volts. In practice they don’t seem to run well at all below 9 volts, and I’ve run one 883 up to 24 by accident once. It functioned fine for about 20 minutes before I noticed the error. I think, but am not certain, that the older 883’s had a higher voltage rating but lower current rating. I still have a bucket of the things that are all over 5 years old and still working fine despite lots of varied use over their lives.
IF does, or has, made some variations of the Victor that operate at higher voltages, currents and/or with out reverse. These are aimed at the combat robot crowd for running big spinning weapons and very large drive motors.
I wouldn’t recommend running the Victors with out the fans in FIRST, and I think it’s actually illegal to do so. The added cooling probably does increase the Victors efficiency more then the small draw the fan reduces it. If I had the option I might consider removing the fans on Victors running low current motors, like the globe or seat motors. These motors just aren’t capable of overheating a Victor, and I’d rather save the weight.
Only the 885 is rated for 48 volts and this model is not available for use on FIRST robots. Although the controller may run at 24 volts, the internal voltage regulator may get too hot for continuous duty at this voltage. The 884 and it’s predeccessor the 883, are rated for 12 volts continuous duty.
I agree with the advice above, and you’d be well served to heed it.
Thinking a bit outside the box, this sounds to me like a perfect pre-season experiment: What, exactly, does it take to overhat a victor? And, does it go into shutdown or thermal liquidity (i.e., melt) when it passes the safe point?
Start with some research with the IFI folks, I am sure they’ll take your phone call and shower you with data. Next, take a Victor, and associated items to simulate various current loads, along with an accurate temperature probe (like a thermocouple). Measure the temperature at steady-state under variuos loads and both with and without the fan, and also in different kinds of restricted areas (a small box, for example).
Then, the important part: Publish your findings in a white paper for the rest of us to see. In this way, not only do you really learn the answer to your question, even the part about “it depends”, but so do all of us…
40A is about 50% above the specified normal load for a CIM motor, per its data sheet. CIM motors can operate at 40A for quite a while before they will overheat – much longer than a 2 minute FRC match. Stall current for a CIM is much higher, around 130A.