Negative thermal resistance in SimScape

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Hi,
I'm trying to build a lumped parameter thermal model of a permanent magnet synchronous machine in SimScape. By doing so, I need to implement some negative thermal resistances. (For reasons, see below). Unfortunately, SimScape does not allow thermal resistances to be negative. So does anybody of you has an idea how to handle this (or had this problem himself)?
The problem background is the following: If we, for example, consider radial heat flow through a hollow cylinder with internal heat generation, it is generally acknowledged in related research that one should not apply the heat flow directly between the two nodes each representing half of the wall thickness of a hollow cylinder but instead include a negative thermal resistance connected to the aforementioned resistances in "T-configuration".
A short and comprehensive overview about the necessity of negative resistances is e.g. given in "Thermal modelling of a high speed permanent magnet synchronous machine, PhD, Grobler A.J, May 2011" (Chapter 3). The relevant chapter is freely accessable via: http://dspace.nwu.ac.za/bitstream/handle/10394/6528/Grobler_AJ_Chapter3.pdf?sequence=4&isAllowed=y
Thanks in advance for your help!

Accepted Answer

Jonas
Jonas on 17 Feb 2020
When you check the source code of the Thermal Resistance block, it is indeed hard-coded that the thermal resistance is larger or equal than zero.
component resistance < foundation.thermal.branch
% Thermal Resistance
% This block represents a generic resistance in a thermal network. The heat
% flow rate is proportional to the temperature difference and inversely
% proportional to the resistance.
% Copyright 2016 The MathWorks, Inc.
parameters
resistance = {1, 'K/W'}; % Thermal resistance
end
equations
assert(resistance >= 0)
if isfinite(resistance)
T == resistance * Q;
else
Q == 0;
end
end
end
Unfortunately, you cannot simply change that line of code in the standard Simscape library block.
You may want to investigate creating a custom Simscape block/library.
I have not done this myself but I believe this is your only possibility of solving the issue.

More Answers (1)

Kumar Bhivesh Sharma
Kumar Bhivesh Sharma on 19 Mar 2022
How to create negative resistance in thermal circuits !! i am facing the same problem

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R2019b

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