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#1




Breaker Modeling
With the popularity of 775 pros and generally high current robots modeling the 120 Amp breaker* seems like a good idea. By using the 120 amp breaker data sheet, I made a function (if you want exact specifications just ask) to map any current greater than 120 amps to the lower bound of how long it will take the breaker to pop**. Then, to figure out how close a breaker is to popping, I can just sum dt/function(current) where function is as described in the sentence before***. So, if the breaker would take 12 seconds to pop at 240 amps and 4 seconds to pop at 360 amps, then if it was at 240 amps for 2 seconds and 360 amps for 1 second it would be 43% "popped" (2/12+1/4). However, this model doesn't account at all for the situation where current is below the rated maximum. Does anyone have any data or ideas on what occurs in contexts such as when the breaker is at 360 amps for 3 seconds then at 40 amps for 2 seconds then at 360 amps for another 2 seconds? Any data or better ideas on how to generally model breakers would also be great.
*The model also should work for any of the PDP legal breakers as they also work (from my understanding) by having a piece of metal deform due to heating up from high current. Their data sheets are also easily available. **This assumes an additive property which hopefully isn't too unrealistic. ***That function can also pretty easily take into account temperature derating , that calculation assumes around 2535 degrees C 
#2




Re: Breaker Modeling
I'd be interested to hear how this works out, specifically how accurate to real life it is. I think someone from 1678 posted on here a while ago that the graph on the spec sheet isn't very accurate and they saw a lot of variation in their testing they had a hard time accounting for in the graph. You might want to reach out to them and see if they want to share the results of whatever testing they did.

#3




Re: Breaker Modeling
Any breaker analysis ignoring its temperature is probably going to fail. Notice on the data sheet there is another graph describing the temp derating curve.
Over the course of a match the breaker is constantly heating up. So if the breaker is cold a the beginning of a match you might get reliably get over 6 seconds at 360amps, and by the end of the match less than 4 seconds. You need to model the temp changes over time. Stick an accurate temperature sensor on the breaker and gather data on how it warms over time at various loads. 
#4




Re: Breaker Modeling
You should probably also monitor the temperature as it cools (with no current, but nothing actively cooling it) as well. Maybe not so critical for use within a match, but it would be essential for predicting longer terms.

#5




Re: Breaker Modeling
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The large industrial breakers I used at previous jobs behaved the same way and were influenced by the same factors. We did some basic calculations and always did extensive testing to verify correct operation. 
#6




Re: Breaker Modeling
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I am curious what those basic calculations were and what testing was done. Also, did you end up modeling the breakers in real time? 
#7




Re: Breaker Modeling
To be sure it is not the ambient temperature that is important, it is the temp of the bimetal strip inside the breaker that matters. In my opinion, the wire does a great job at removing heat, drawing it away from the interior of the breaker. One side of the metal is welded to the contact on the outside of the breaker. The interior space will be near ambient at the start of the match.
The variation in the specification curve is more a production variance than anything else. Please remember that FRC multiple motor drives may not have a 100% duty cycle at max current. If the robot is actually moving, CIM motor current should be considerably less than stall current. While worse case stall current on a CIM motor is 131 amps, wiring and speed controllers enter significant loss on FRC robots. On average, I would place stall currents in the 100110 amp range for FRC robots. Many teams use a "turbo" mode in their software that runs the robot at somewhat less than full throttle for the majority of the match and then only use "full throttle" when needed for speed. While less than full throttle, max current will not occur in all motors at the same point in time. The PWM output of the motor controllers is not in sync and the motor commutators are neither running at the same speed or commutator position. 
#8




Re: Breaker Modeling
Yep, I was planning on using the PDP's measured current as the input to the model for the 120 amp breaker. So it will account for all current flow, including from items other than motors.

#9




Re: Breaker Modeling
If you have any inclination to integrate a proper thermal analysis into your model, I've already done most of the legwork.
Original post here, discussing the effect of precooling a main breaker with canned compressed air before the start of a match: https://www.chiefdelphi.com/forums/s...4&postcount=44(WARNING, this original post made an incorrect assumption of the main breaker's series resistance, and so the final numbers are off by a factor of 6.) I unfortunately can't edit the original post anymore, so I updated the calculations and published the result as a white paper: https://www.chiefdelphi.com/media/papers/3404 Last edited by Ryan_Todd : 11212017 at 12:14 PM. Reason: word choice 
#10




Re: Breaker Modeling
The nominal main circuit breaker trip current is an i^(constant) x t relationship.
This FRC 120 amp breakers trip appears to be around i^(.4) x time based on the spec sheet. The circuit breaker is in a molded (insulated) case which will reduce the effective heat transfer rate. At room temperature (77° F) the trip current is about 113% of rated. The resistive heating will be related to a function of integrated V*I with time. At some point the heat transfer rates will tend to stabilize the internal temperature. I’d suspect this would be warm enough to bring the internal temp to close to 100° F or more, and reduce the trip current to the rated 120 Amps or a bit less. You could measure the breaker metal terminal temperatures to get a better idea of internal temperature vs time. 
#11




Re: Breaker Modeling
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Definitely better than the case. 
#12




Re: Breaker Modeling
Let's put some physics to this.
Breakers trip because of heat. So we should do an energy balance and consider this an open system with no work being done. Heat energy enters the system because the breaker / wire assembly has a small resistance. Heat energy leaves the system via conduction or convection. Q_breaker = Q_resistance  Q_cooling All power dissipated in a resistor is converted to heat energy. This is expressed as P = I^2 * R. This gives you a result in Watts. Current can be measured with acceptable accuracy through the PDP. Modelling the heat loss via conduction or convection gets a bit more complicated but can be assumed to be linear over time based on the governing equations. To test the validity of this method let's generate a curve that hopefully matches reasonably well with the data sheet using these equations. Using arbitrary numbers let's say we believe the breaker trips after applying 150 amps for 500 seconds (based roughly on the data sheet, in reality you'll want to determine this experimentally). We'll assign a resistance value of 0.001 Ohms (it's arbitrary and doesn't really impact the calculations). We'll also arbitrarily say that the breaker has 2 W of cooling via conduction. Q_resistance = [(150 amps)^2 * 0.001 Ohms * 500 seconds] = 11250 Joules Q_cooling =  (2 J/s * 500 seconds) = 1000 Joules So we can say that the breaker will trip when it reaches 10250 Joules of heat (this corresponds to a temperature via a specific heat value) Note that I didn't look up specific heat values or resistances for anything for the sake of quick math so my Joules are probably way off from realistic numbers. Now to solve for time to breaker trip at a different current we can do some algebra. Q_breakerpop = I^2*R*t  P_cooling*t t = Q_breakerpop / (I^2*R  P_cooling) This reasonably creates a similar arc as the data sheet generates, but likely needs a lot of tuning via experimental values. Since I is obviously not constant in a match a model could be generated tracking heat generated using the PDP data. 
#13




Re: Breaker Modeling
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Thus, one would have to characterize the breaker behaviour for each particular robot to get an accurate estimate of the trip point. This characterization work makes sense if one is building a bunch of very similar robots. Typically, each team will have a different environment for their breakers so it will be difficult to use the results for one team on a different robot. It seems that the OP may be trying to use the breakers as a current limiting device. Breakers are meant only to serve as protection devices and are never designed to have a precise trip level. If your objective is to prevent the breakers from tripping in high current applications, the proper way would be to set the current limit of all of the motor controllers in each application (drivetrain, winch, shooter) to the same level, ensuring that level is below the minimum trip current of the breakers, taking into account any derating factors. The current limit level of the motor controllers should be more consistent, from one unit to another. It should also be largely insensitive to the ambient temperature and the history. Last edited by philso : 11212017 at 02:19 PM. 
#14




Re: Breaker Modeling
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All of that said, under the RoboRIO, we were far more likely to experience brown outs, and adjusting everything to avoid/minimize brownouts will make it very unlikely to ever trip the main breaker. We ran test runs, adjusted motor ramping, current limits, etc until under abusive driving conditions with a fresh battery, brownouts were very rare and when they did occur they were of the very short variety. This combined with a fairly smart autodownshift, and a traction limited low gear; we did not have any issues with the main breaker all season. 
#15




Re: Breaker Modeling
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It will be better for teams to implement current limit through the motor controller than to have the breakers open, even the snap action ones in the PDP. Pushing the envelope, as you are suggesting, might give you 510% more peak current, for periods of 1 second or less, but you will have a greater risk of opening a breaker. It also makes you vulnerable to variations in factors such as the ambient temperature and parttopart variations (breakers are not tightly calibrated). Will that slightly higher peak motor current really make a measurable difference in your performance? Is the risk of opening a breaker acceptable to your strategy? Can you devise a way to get better overall performance by not pushing the motor power envelope so hard so you can have reliable power? Why are you allowing abusive driving conditions? The sustained high current draw conditions will wear out your batteries faster. 
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