Current Draw vs. Stall current

I am currently working on a new drive train for our off-season robot chassis. I am using JVN’s Design Calculator, and it is saying that, at Max Continuous Load for our High Gear (15fps), the current draw per motor is 121.57 Amps. I looked at the Cim Motor specs, and it says that the stall current is 107 Amps.

Does this mean that if the robot stalls in High Gear, the robot will trip a fuse, or blow the Jaguar into my favorite blue smoke of death?

Adam Garcia
Team 4 Element

I am seeing the spec to be 133amp at stall

As for breakers

Based on that spec sheet at about 120amperes a 40 amp would trip in .5-1.1 (300% load)

Their graph is flawed I believe, if you count there are 11 graduations starting at any number x and going to 10x including x’s and 10x’s, conventional logarithmic scale should be 10, eg 1 2 3 4 5 6 7 8 9 10. Additionally the data seems to be a little different than what they say in text.

Maybe provide some more information so people more experienced than I can help figure out your issues.

What is your:
Wheel Diameter?
Gear Reduction?
Coef-of Friction (if the calc has it)?

of CIM’s in Drivetrain.

Wheel Diameter: 4 in
Gear Reduction: Still being determined, but prob going with 4-5 fps in low and 14-15 fps in high.
Coef-of-Friction: 1.4

of Cims in Drivetrain: 2 per gearbox for a total of 4 in total.

I would check your calculations as your result sounds pretty high. The net effect on the Jaguars would be that they would go into current protect mode at that high current. The stall spec on the CIM is 133 amps but generally the wiring teams use and the length of wire will result in lower currents at stall. As to the breakers, they may trip but effectively reset within milliseconds. As suggested the breakers are a temperature controlled device and so can stand loads in the 600% of rated current range for a couple of seconds. However, once at an elevated temperature, the trip point becomes much lower.
I always ask why teams want to go so fast. At 15 fps you will be covering end to end in a little over two seconds. Control becomes an issue and collisions will cause some serious damage. We recommend speeds of 10-12 fps for high gearing as a good balance of speed, maneuverability, control and the ability to avoid potentially fatal collisions. While many teams like to go fast, only a select few do it well.

I can think of one reason… one that has haunted me for quite some time… we were scrimmaging against the robowranglers where they were playing defense against us. We could not get past them at all! The only way I could think that would have worked would be to go faster doing a lateral maneuver, and then curve past them. (A short burst of speed here).

Depending on tire slippage, friction, motor loading, etc expect ~5:1 gear ratio for 4" wheels at 15FPS.
5,300 RPM x 80% / [15FPS x 60sec/min / (3.14 x 4"Dia/12)]
Check your calculations. Perhaps you are reading 30Amps per motor for a total of 120.

There are a few ways to defeat defense…

**Speed **(works great against most, but who is to say your defender isn’t just as fast?)

**Momentum **(a 150-lb loaded robot at 15fps is going to make whatever it is hitting move backwards at least a little…as long as the rules - and your robot’s construction - permit)

**Pushing **(works great against most, but just as with speed, you will eventually encounter a bot that can push you to a standstill)

**Agility **(turning on a dime and/or strafing let you outmaneuver some opponents, but fast, powerful bots can still be a problem)

**Deception **(in a hectic match, most defenders go do something else when they see you driving away from a goal. Take advantage!)

Teamwork/Blocking (every sport has some variation of a “pick and roll” or “lead blocker”. So does FRC!)

Teamwork/Blitzing (“send more men than the enemy can block”)

Against a great robot and great drive team, the last two options are often your best (and only) hope.

Good info here… Thanks… Jared :slight_smile:

I have done some further calculations, and I have a potential gearbox which goes 4.17 fps in low, and 12.51 fps in high. In high gear, the current draw per motor at Max Continuous Load is 119.41 Amps. Would this be sufficient to use without having to worry about tripping the breakers?

If not, what is the max current draw you would recommend for a robot in high gear?

Thanks for all your help!

That is the max current draw, i.e. you are pushing against an immovable object. As long as you’re not pushing you should be fine.

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Where did you get your 1.4 CoF #?


According to Chris is me,

"For calculation purposes in a drivetrain, I’ve been told 1.4 is a safe static CoF for roughtop on carpet in the direction of motion (basically, when gearing for traction limited pushing power / acceleration)

If you need more precision than a good estimate, I would experimentally determine this for yourself."

In retrospect, something like 1.2 or 1.3 is probably more accurate, but 1.4 does give you a nice and comfortable safety margin.

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As pointed out your calculation is based on pushing against an immovable object. Typically designing for something under 40 amps per motor while driving is ideal. Battery life factors in here as well. If you draw high amounts of current a great deal of the time you are driving, your battery might not last the match. Many teams do not take this into account. The result is usually intermittent driving near the end of the match when performance is needed most. Modify your design if your strategy requires you to drive while moving/lifting/scoring game objects. While a freshly charged battery is capable of producing 600 amps, it can only do that for seconds, not minutes. You may need to make a decision between lifting or driving at the end of the match. While some teams listen and check battery life, many do not. Our mechanical team has always managed to make fairly efficient designs. This allows us to be conservative on battery usage often running 10-15 minutes full out during practice without battery change. As a rule of thumb, a team should be able to run two competition matches on a single charge. You should change batteries between matches, this is just an indicator of the efficiency of your design. If you can only run one match, someday, a great opposing alliance will run your battery down before the end of the match.

The only reason I asked was because our team has done a ton of testing in regards to different wheels and tread. Based on those results, 1.4 is a bit high, but I’m not going to split hairs over it. I was just curious as to where he got the number from as its been a point of interest for me and my team for a couple years now.

Safety margins are nice to include, but you do have to use them carefully. If you are adding a margin of safety into every variable involved in a design, you’ll end up building something thats way overbuilt. Using one here is fine, but just make sure you are not also accounting for extra margin somewhere else and compounding your margin.


Thanks, I will keep that in mind.

Now ask your self how long will a 17 amp hour battery last while four motors draw 110 ish amps? How long untill your wire melts? I feel there is some thing wrong with your calculations.

The 121 Amps only occur when at max continuous load (ie. We are pushing against a wall, or an immovable robot). There is no feasible reason for pushing against an immovable object for more than a few seconds.

To get around ever having to go near this 121 Amps, our programmer is working on some code which will sense if the Amps rise too high while in high gear. If the robot senses high Amps, it will automatically shift to low gear in order to prevent possible breaking.

We just saw this a few days ago while looking at 399’s drive code. Ingenuity to the max!

To add to your discussion, a motor draws stall current whenever the motor is not moving. That is why there is a current spike when you first apply power to the motor. The current is proportional to the applied throttle value. If you are at full throttle and using fully charged battery and low resistance wiring, you can expect near the rated stall current when starting.