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 Chief Delphi Breaker Modeling
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#16
11-22-2017, 01:30 PM
 Ryan-Greenblatt A foolish programmer FRC #0900 (Zebracorns) Team Role: Programmer Join Date: Oct 2015 Rookie Year: 2015 Location: NC Posts: 57
Re: Breaker Modeling

Quote:
 Originally Posted by Ryan_Todd If you have any inclination to integrate a proper thermal analysis into your model, I've already done most of the legwork. ... 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
By applying the model OP created, I think I have a pretty good framework for breaker simulation that just requires information available on the breaker data sheet, knowing the electrical resistance of the breaker (which is theoretically not too hard to figure out), and knowing the internal temperature required to trip the breaker. Using the method OP outlined in the paper, we can calculate the effective thermal conductance (watts/degree C) to be equal to: (rated amperage)^2 * (R) / (internal temp required to trip - ambient temperature of testing on data sheet). This calculation assumes that if the rated load is passed through the breaker indefinitely it will asymptotically approach tripping. See the paper for more detail. For my calculations, I assumed the ambient temp of testing was 25 C, which is supported by the temperature derating curve. Then, we must calculate the specific heat for the breaker (degree C/joule). The calculations are discussed in the paper, but specific heat = Log(1 - ( internal temp required to trip - ambient temperature of testing on data sheet)/(I^2*R/thermal conductance)) / (thermal conductance*time) where I is the current of a point on the current vs time curve on a data sheet and time is the corresponding time. This calculation may be done for several points and then the specific heats can be averaged. Additionally, you can choose points on the lower bound, upper bound, or middle of the curve depending on what you want you estimate of internal temperature to represent, a lower bound, an upper bound, or an average value. Then, (as discussed in the paper) the differential equation DT/dt = specific heat * (I^2*R – thermal conductance*(T – ambient temp)) may be numerical integrate with variable I (current) to estimate the internal temp, T. I used this approach to recreate the current vs time curve and temperature derating curves for the 120 Amp breaker to test the models accuracy. In this case, I was trying to estimate the lower bound of how long it would take to trip. I used c++ and once I clean up my code I will post it somewhere. The current vs time curve has current in units of multiples of rated current (120 Amps).

The derating curve has temp on the x axis and effective rated current on the y axis.
Both curves match closely, though the derating curve looks quadratic rather than linear, I am not sure why that curve has the wrong shape (the values very close toward the middle).
#17
11-30-2017, 03:31 PM
 Al Skierkiewicz Chief Robot Inspector AKA: Big Al WFFA 2005 FRC #0111 (WildStang) Team Role: Engineer Join Date: Jun 2001 Rookie Year: 1996 Location: Wheeling, IL Posts: 11,113
Re: Breaker Modeling

Guys,
You are all correct in talking about internal temperature but the majority of temperature rise internally is due to the resistance of the contact. The series resistance of the other parts is a fraction of this resistance. Additionally that resistance rises with operation cycles and any pitting that might occur during breaker life. One of the contacts is welded to a rather large bi metal strip.
Here is a picture of the inside of the breaker for reference. More heat is generated by loose connections to the terminals of the breaker on many robots.
https://www.chiefdelphi.com/forums/a...6&d=1460405454
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#18
12-08-2017, 10:00 AM
 Ryan_Todd ye of little faith FRC #0862 (Lightning Robotics) Team Role: Mentor Join Date: Apr 2006 Rookie Year: 2005 Location: Plymouth, MI Posts: 122
Re: Breaker Modeling

Quote:
 Originally Posted by Al Skierkiewicz ...the majority of temperature rise internally is due to the resistance of the contact. [...] Additionally that resistance rises with operation cycles and any pitting that might occur during breaker life...
Good point!
Makes me wonder whether the widths of the time-to-trip and temperature-derating curves might come from more than just manufacturing tolerance... For improved model accuracy, we might need a whole new derating curve based on number of operation cycles.

Quote:
 Originally Posted by Al Skierkiewicz ...More heat is generated by loose connections to the terminals of the breaker on many robots.
And here's another useful insight!

I never thought of it that way, but a loose terminal connection would generate enough heat to dramatically reduce the current required to trip one of these breakers.
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#19
12-08-2017, 10:06 AM
 marshall Online Stripe & Drape Sales FRC #0900 (The Zebracorns) Team Role: Mentor Join Date: Jan 2012 Rookie Year: 2003 Location: North Carolina Posts: 3,007
Re: Breaker Modeling

Of note, the main breaker uses 1/4-28 nuts and studs. As far as I'm aware, nothing is preventing teams from using nylock 1/4-28 nuts with the main breaker.
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Last edited by marshall : 12-08-2017 at 11:02 AM.
#20
12-08-2017, 10:33 AM
 GeeTwo Somebody Else AKA: Gus Michel II no team Team Role: Mentor Join Date: Jan 2014 Rookie Year: 2013 Location: Slidell, LA Posts: 5,960
Re: Breaker Modeling

Quote:
 Originally Posted by marshall Of note, the main breaker using 1/4-28 nuts and studs. As far as I'm aware, nothing is preventing teams from using nylock 1/4-28 nuts with the main breaker.
2017's R71 F allows it. (Emphasis mine)

Quote:
 Originally Posted by 2017 Game Manual, R71 The Driver Station software, roboRIO, Power Distribution Panel, Pneumatics Control Modules, Voltage Regulator Modules, RSL, 120A breaker, motor controllers, relay modules, Wireless Bridge, and batteries shall not be tampered with, modified, or adjusted in any way (tampering includes drilling, cutting, machining, rewiring, disassembling, etc.), with the following exceptions: . .F. Fasteners (including adhesives) may be used to attach the device to the OPERATOR CONSOLE or ROBOT or to secure cables to the device.. .
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#21
12-08-2017, 10:42 AM
 philso Mentor no team Join Date: Jan 2011 Rookie Year: 2009 Location: Houston, Tx Posts: 1,703
Re: Breaker Modeling

Quote:
 Originally Posted by Ryan_Todd I never thought of it that way, but a loose terminal connection would generate enough heat to dramatically reduce the current required to trip one of these breakers.
The resistance of the loose connection at the breaker (or at the battery terminals) will cause larger voltage drops than one would expect at any particular current, possibly causing unexpected brownouts.
#22
12-13-2017, 08:50 AM
 Al Skierkiewicz Chief Robot Inspector AKA: Big Al WFFA 2005 FRC #0111 (WildStang) Team Role: Engineer Join Date: Jun 2001 Rookie Year: 1996 Location: Wheeling, IL Posts: 11,113
Re: Breaker Modeling

The point I was hoping to make is that loose connections heat the outside terminals significantly and that heat is conducted directly to the parts inside. I can't tell you how many robots I see each year with loose connections. Teams are shocked when I walk up and wiggle the #6 leads and often I will give a small tug and pull the wires right out of the terminals.

I will again make a statement about terminals I see way too often. The terminals teams buy at Home Depot (or other big box stores) that use a screw to hold the wire are designed for solid wire. Type ADR2-B2-5 is one of these terminals. The screw will push the strands of the wire up the sides of the terminal reducing your #6 to a #12 or smaller equivalent wire size. You want a terminal where the screw tightens a clamp on the wire. When you use a clamp type terminal be sure to strip the wire sufficiently to allow at least 1/8" of wire to extend beyond the clamp. This will prevent the wire from being pushed out of the terminal over time.
__________________
Good Luck All. Learn something new, everyday!
Al
WB9UVJ
www.wildstang.org
________________________
Knowledge is power. Power UP!
#23
12-23-2017, 04:04 PM
 jaustinpage Coach/Mentor AKA: Austin FRC #6377 (Howdy Bots) Team Role: Engineer Join Date: Mar 2011 Rookie Year: 2007 Location: Austin, Tx Posts: 6
Not sure if it helps, but here is one way to get the closed form solution for the trip time in Python (without heat modeling, just using the spec sheet). Not sure if that helps with the analysis: https://github.com/jaustinpage/frc_r...ain_breaker.py

I should probably update this to be heat based.
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