Author 
Topic 

maba1971
Germany
3 Posts 
Posted  Apr 08 2019 : 16:16:39

I try to simulate a current shunt with fourterminal kelvin connection.
I specified two external ports: ... .external nout nin current_path .external nsense2 nsense1 voltage_sense
But I can't understand the impedance matrix in Zc.mat: Row 2: nsense2 to nsense1, port name: voltage_sense Row 1: nout to nin, port name: current_path Impedance matrix for frequency = 10 2 x 2 0.0894106 +9.63507e07j 0.0187796 +2.67151e07j 0.0187796 +2.67391e07j 0.118482 +5.27858e07j Impedance matrix for frequency = 100000 2 x 2 0.0896302 +0.00770253j 0.0188284 +0.00188867j 0.0188287 +0.00189023j 0.118695 +0.00389998j
I fact I have a current path nout <> nin and need the resistance (voltage drop) between nsense2 nsense1. Can anybody help to interpret the impedance matrix? Thanks in advance!
Mario
mb 

Enrico
399 Posts 
Posted  Apr 08 2019 : 19:09:10

Hi Mario,
the resistance is the real part of the impedance matrix. At your frequencies vs. your structure dimensions, it seems it does not change with the frequency. Now your port 'voltage_sense' is between nsense2 and nsense1, so I would say that you are interested in the resistance of this part, i.e. 0.119 ohm. However I am a bit puzzled by the fact that you use two frequencies, as your statement is 'need the resistance (voltage drop)'. In AC the voltage drop is determined by the overall impedance, not only by the resistance; but as you talk about current shunt with 4 terminal kelvin connection, I expect you are interested in DC current.
If the resistance result makes sense for you, ok; otherwise, please provide a small drawing of the structure as you defined and simulated it.
Ciao, Enrico



maba1971
Germany
3 Posts 
Posted  Apr 08 2019 : 21:04:00

Hey Enrico, thanks for your fast answer and sorry for the inaccuracy of my question!
I analyze FoilShunts for the use at higher frequencies and compare the "DC" impedance (at 10 Hz) with the impedance at 100 kHz. In addition to the simulation I also do some measurements and in my measure setup it is more easy to use 10 Hz than DC, the impedance should be almost the same as at DC.
I am interesed mainly in the realpart of the impedance due to skineffect, current distribution and other effects. Of course the voltage drop rises due to the inductance too, but this can be eliminated in my setup  the problem is the increase of the real part.
I did not find a possibility to upload the postscript file generated by fasthenry and zbuf, so I did a small asciidrawing of the foil structure:
.units mm

 thick=0.075 
 sigma=758 
    
       
       
     
  nsense1 nsense2  
 
nin nout
The material of the complete foil structure is NiCr. Nearby the foil is the base plate made of aluminum with eddy currents.
I think the real part of the impedance from nin to nout is: 89 mohms. What I need is the real part of the impedance which is responsible for the sense voltage between nsense1 and nsense2, so maybe: 89 mohms  2*18 mohms??
Mario 
Edited by  maba1971 on Apr 08 2019 21:12:39 


Enrico
399 Posts 
Posted  Apr 09 2019 : 16:41:22

Not really. If I read correctly your drawing, this is a single conductive structure with a common part and the 4 contacts (as per Kelvin, but including the 'measured' part in the simulation).
Now from a black block perspective you have a twoport network (as you defined two ports) so your matrix structure is:
V1 = Z11 Z12 I1
V2 Z21 Z22 I2
(where the bars indicate matrices and vectors. Note also that in your case V2 is Vsense and V1 is Vcurrent_path).
But internally your network is this:
I1 > R1R2 < I2

^ R ^
V1    V2
R1R2
where R1 and R2 are the small segment resistances, and R is the central part resistance (I'm using symmetry considerations here, and using only the real part)
Now from simple circuit theory you have:
Z11 = 2*R1+R Z12 = Z21 = R Z22 = 2*R2+R
So this gives you
R = 0.0188 ohm R2 = (Z22R)/2 = (Z22Z12)/2 = (0.1190.019)/2 = 0.05 ohm R1 = (Z11R)/2 = (Z11Z12)/2 = (0.08960.019)/2 = 0.035 ohm
If nsense1 and nsense2 contacts are narrow, it is reasonable that their resistance is higher.
Does this make sense for you?
BTW why do you not use FastModel for visualization or, even better, FreeCAD E.M. workbench for modelling the FastHenry 3D structures? They make your life easier. BTW If you want to attach pictures, please send me via email (see contacts page). As users are not verified on the Forum, because of spam and virus uploading images and/or posting links is limited. We may move to user email verification in the future to avoid this issue.
Best Regards, Enrico



maba1971
Germany
3 Posts 
Posted  Apr 09 2019 : 18:21:09

quote: Originally posted by Enrico ... Does this make sense for you?
Yes, your interpretation of my small drawing was correct. I understood the meaning of the impedance matrix completely wrong  thanks for your explanation! I expected 50 mohms for the central part resistance R. And less than 2*35 mohms for the current path pins R1. I think I have to check and correct errors in the structure of the NiCr part!
quote: BTW why do you not use FastModel for visualization
because I love my Linux so much ;)
quote: or, even better, FreeCAD E.M. workbench for modelling the FastHenry 3D structures? They make your life easier."
I think I will try this, if the structure in the next project is maybe more complex ...
Mario 


Enrico
399 Posts 
Posted  Apr 09 2019 : 20:34:37

quote: because I love my Linux so much ;)
..then definitely you need to use FreeCAD ;)




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