Breaker Trips

Technical Electrical
1

This year we had a good drive system with 4 andy mark sims. We used a medium-low single gear drive train but obviously had issues about getting pushed by other robots. If next years match requires pushing we will likely go to at least a 6 sim robot with two gears. Question is each of the andy mark sim motors have a stall amp draw of like 133 amps. We stalled out our 4 sims on more than one occasion without tripping any breaker (that I know of). How is this possible? My best guess is that the 40 amp circuits trip first (but we don’t realize it) so the motor never draws its full stall current.

Also how do teams who run 6 sims keep from tripping their main breaker? The main breaker does trip at 240 amps right? If they stalled all 6 of their motors at the same time then… 40 amp breakers times 6 motors = 240 amps. That’s not counting any amps from other motors they might have on their robot or the crio/network bridge.

Also, is it legal to put, say 8 drive motors on 30 amp breakers? If it is you would trip your motors quicker when getting close to stall range but it would provide more torque at higher rpm thus getting a higher top speed.

This is of special interest to us since we might try a swerve drive. If we put 1 drive motor on each pod and 1 steer motor for each pod and all those motors have to be on 40 amp breakers we have no room for other motors we may need. It would be nice if we could put the steer motors on 30 amp breakers or possibly put 2 drive motors per pod (this may be crazy because it would be 8 drive motors and 4 steer motors). Any idea what would happen in this situation?

Thanks for everyone’s input and I hope everyone had a great time at finals. (Curie rocks!!!)

2

Breakers don’t trip the moment they reach their rated currents. The main breaker is rated for 120 amps, but it will not trip when passing 240 amps for up to 40 seconds. It will take about 5 seconds to trip when passing 500 amps.

3

The main breaker is a thermal breaker, so it requires >120A for a good amount of time before it will trip. Same with the 40A breakers. They also need to have > 40A for more than a few seconds in order to trip and reset. So, although there is more than 120A when all of the motors are stalled, this doesn’t usually happen for a long enough period of time for the breakers to get hot enough to trip. I believe specific motors have specific breakers that you must use according to the rules, and I believe cims must use 40A breakers.

4

The 2014 rules have no such requirements. The 2015 rules may or may not differ.

5

Both the PDB breakers and the main breaker are thermal, meaning that they heat up as you put more current through them, and when the reach a certain temperature they trip. The main breaker is rated for 120 amps (not 240), but it will not trip immediately if you exceed that value. For example, at 240 amps, it will take anywhere from 10 to 40 seconds for it to trip. See the datasheet for more information. The PDB board breakers are the same way, but the trip significantly faster. Here is their datasheet.

As for having eight drive motors, you could do that but they could not be all CIMs because the rules limit us to using only six (this year). But you could use a CIM/minCIM pair (we are planning on doing this in the offseason).

6

Yes, you are correct, the only requirements this year were for specific gauge wire for use with different applications.

7

Breakers are chosen for the wire size they are used with. 40 amp for minimum #12 wire. If you only want #18 wire then you can only use a 20 amp breaker. CIM motors can be run on 5 amp breakers if you choose. The breakers are protecting the wire. If the load shorts or the wire shorts at the load end, the breaker will trip before the wire fails.

8

Thanks for all the responses. This helps clear up some info about how we could run our motors next year. Obviously no one knows what next year holds but are there any indications of changing to current motor styles or number of motors allowed? Also I noticed the new PDB has some feedback next year. Does anyone know if you could manually limit current through it? Lastly, while this is a bit off topic, we are thinking of having our robot and test robot at some events and charge people to test drive them. This is obviously to raise money. What are some things we can do to keep from burning up our motors and motor controllers? We can limit throttle in the code which should help the motors but is that better or worse on our motor controllers? We use Victor 888’s right now.

9

Not that I’m aware of or would anticipate. Over the past several years, FIRST has built broad supplier agreements with several motor manufacturers that I don’t see them breaking. That said, the 2015 Robot Rules will be revised more than in years past, due to the new control system.

Also I noticed the new PDB has some feedback next year. Does anyone know if you could manually limit current through it?

The PDB only supports current monitoring, but you could use that monitoring to detect overdraws in code and shut off/turn down the offending speed controller.

Lastly, while this is a bit off topic, we are thinking of having our robot and test robot at some events and charge people to test drive them. This is obviously to raise money. What are some things we can do to keep from burning up our motors and motor controllers? We can limit throttle in the code which should help the motors but is that better or worse on our motor controllers? We use Victor 888’s right now.

Talons and Victors are both extremely hard to fry (unless they’re wired backwards). Have you had any specific problems with your Victors?

10

With the new control system next year, one of the added features is the power distribution board has built in current monitoring. You can monitor the current that each power leg is pulling. Armed with this information you could modify the output to the motor controller, to prevent main circuit from tripping.

11

Also note that while the motors are not turning (or struggling to turn), their effective resistance approaches ~0.1ohms (at perfect 12v stall draws 133 amps V=IR). All of the wiring adds some resistance, but basically it means there is significant resistance inside (citation needed) of the battery compared to outside between the poles. This, according to Ampere’s law, means that regardless of the voltage difference in the battery’s cells, the voltage difference across the terminals is (at least instantaneously) much less than 12v. This all means that the CIMs indeed stall at much less current than you think because batteries are not perfect emf suppliers.

On another point, The same exact robot, under the exact same (light duty!) conditions, with a 4cim drive vs 6cim drive, will pull (ok, slight inefficiencies, sue me) the same exact net current! This is because regardless of the number of motors (with the same robot weight), it take the same amount of energy to maintain a speed, so the total current draw is the same, but the current/motor is less.

That being said, nobody simply adds another motor to their 4CIM setup, they change the gearing for higher speed, etc. This forces the CIMs to work roughly as hard each as each motor in a 4cim drive, with 3/2 the available torque being used to push harder/accelerate faster, etc. This does put considerable strain on the 120 amp main breaker, but not necessarily the 40 amp ones, compared to a 4 CIM variant.

With a reasonably geared 3CIM drive, and perhaps some clever “ramp up” coding, you will have no trouble with the 120amp breaker. On our robot we used a 3CIM ballshifter from VexPro, geared to final speeds of 5.5 and 15.5fps. We were very happy with it, and although the top speed may not have been as high as it could have been, it sure reached that speed quickly, and I would be worried about the decreased acceleration time in a faster drive. Our low gear was also burly enough to deliver the torque we needed, and usable speeds, while keeping the main breaker cool. We had no trouble all season, despite playing an unfortunate amount of defense

12

Brian,
Actually adding a third motor isn’t as simple as all that. The motors are not efficient so adding the third motor adds another loss into the equation. Even if the motors are running at peak efficiency, they do not approach 100%. If I am reading the graphs correctly, it is between 60 and 70% efficient at peak.
The internal resistance of the battery is 0.011 ohms nominal at full charge. So that means four motors in stall should drop close to 6 volts while 6 motors could drop almost 9 volts. The reality here is that the DSC power supplies drop out preventing any PWM output at around 5.5 volts. The variable is the motor wiring losses and the length of #6 wiring feeding the PD. The longer the run on any of these lines affects the voltage available for the controller and then the motor. Under real world conditions with the average amount of wiring, both Rich Wallace and I have calculated a typical stall current of about 116 amps.
I would think that the CIM motors would be sticking around for a while. Overall they are a decent motor for our use and everyone knows how to mount and use them. (Remember please that these motors are known for having weak bearings and should not be designed to be used with side loads. Your mileage may vary.)

13

Our victor 888’s have done very well, no problems as of yet but we try not to abuse them and this would be likely if we allowed random people to test drive our robot, thus the reason I asked.

I guess I will post the main reason I am asking this question so that we can get some feedback about the idea. (I was going to keep it under wraps because I think this is a unique idea, but you guys are awesome helping us with this so maybe it can be of use to you).

We are looking at using a hybrid swerve/hi-traction drive. We would have two swerve pods that would be driving the robot at all times. The two high traction drives would lift up or drop down depending on what the driver needs. With this system the driver could drive the robot like a swerve drive to get the extra mobility, but when pushing power or high agility are needed the driver could drop down the high traction wheels and get the extra boost. This system would have a weight and power advantage over a traditional swerve drive because of two less steer motors and two less pods but would still give us the mobility of a swerve when needed.

We would use two large sim motors per swerve pod and two mini sim motors per high traction system for a total of 8 drive motors. (4 full time, 8 when high traction system is down). What we are worried about is what will happen when we try to push someone with all 8 motors. With the new PDB it sounds like we will have options for limiting our output but then is there any real point to having the extra motors? We also plan on trying to develop a gearing system so we have at least 2 gears which will help.

Anyone have any thoughts on this design? Has anyone tried it before? Will the 8 motors be too much? Should we drop it to 2 motors per pod and 1 motor per hi-traction system.

Thanks again for all the feedback!!!

14

B,
I think it has been tried before but I can’t remember the specifics. Obviously there is a weight tradeoff. We (WildStang) have used various methods with swerve more accurately called “crab” in my business, as the motion it emulates. If you have a game that needs that particular motion and does not require mechanisms for game piece manipulation, it might turn out to be a game winning drive train. I impress on all users of crab drives that it requires about three times the practice time to be learned. Drivers who do it effectively know from the practice exactly how to react to the field. With two styles of drive, I would think some time would be lost to switching driving tactics. Something akin to dual languages.