Zetopan
Member
lolachampcar said:I somehow came away from the Elon fuse discussion thinking Tesla was employing active "pyro" fuses which have circuitry on board to monitor instantaneous current (within the sample time frame of the circuit) and "blow" the fuse link in a fault condition.
Firstly, my original posting had some errors in it that I need to correct. While the power dissipated in a passive fuse is I^2*R the actual critical value is I^2*T.
I used I^2*R in some places where I should have been using I^2*T. That could just add to the confusion, sorry about that.
The resistance obviously has a tolerance, but that can be pretty well controlled in manufacturing by using what is known as a Kelvin connection to accurately monitor the resistance as a QC check. What has the greatest source of error is the I^2*T, a measure of how long it takes the fuse to blow.
As a simple example, take two fuses that have identical resistances but slightly different geometries. One fuse is slightly thinner and shorter, with the other obviously being slightly thicker and longer. The thicker longer fuse will be able to dissipate slightly more power before self destructing since it has more mass and a larger cross sectional surface area to conduct heat to the non-fusing regions as well as slightly more surface area to the air. These small differences can cause relatively large changes in the time that the fuse takes to do its intended job.
To get a tighter tolerance on the I^2*T Tesla employes active circuitry to monitor the voltage drop across the relatively accurate resistance of the fuse. When the desired I^2*T has occurred the pyro is activated to blow the fuse, which could have otherwise taken too long or too short of time to blow due to the thermal characteristics of the material and geometry variations. You get the T component in the measurement by integrating, also known a filtering. Computing I^2 is not a difficult task these days since the circuitry measures a current proxy (the voltage) and squares it. The result is then filtered and then thresholded to establish the I^2*T limit that is desired. By tightening the I^2*T tolerance in this way the actual current threshold can be increased much closer to the desired limit without suffering from premature fusing.
[QUOTEDo you have a reference for the fuses still being set to 1300 amps?[/QUOTE]
These are not 1,300 amp fuses, they are much lower than that when subjected to a continuous current. The 1,300 amps is the fusing current when subjected to higher currents for relatively shorter time intervals. Musk has publically stated that the original (I^2*T) fusing was set to 1,300 amps because of the tolerance problems and by using pyro with active circuitry the limit could be raised to 1,500 amps (without risk of premature destruction).