Hi,

I am interested in designing a bus bar and want to use Q3D to extract the inductance. I have the option of getting the inductance at an AC frequency, or the DC inductance. I am a little confused because I thought that there is no inductance at DC? I guess I thought inductance is treated as a short at DC and capacitors are treated as an open. Can someone help explain?

 

Q3D?

 

Ansys Q3D Extractor. It allows you to import a 3D file such as STEP or PCB files to to extract parasitics.

 

A simulator question? A simulated parasitic?

 

Yes, it is a simulator. I am not asking anything about Ansys Q3D specifically though. I am asking if inductance is real at DC and if so, how is that since I've always been taught that inductance is treated as a short at DC.

 

There is something called reactance. The unit of reactance is ohm. Thus, reactance is what an inductor exhibits, much like resistance, in that it opposes the flow of current.

reactance is calculated as X = 2 x pi x f x L ohm
where pi = 1.414
f = frequency in Hz
L = inductance in henries

Hence when f = 0, reactance X = 0

In other words, a bus bar has zero reactance when there is no AC (i.e. at DC). A bus bar can still have inductance. Inductance is not dependent on frequency, DC or AC.

 

Although, if anyone has any tips for simulation in Q3D for my inductance I'd greatly appreciate that too.

 

O ok that makes sense. I was definitely thinking about reactance.

 

Hi Mr Chips,
I have spotted an error in your post #6. You have wiriten the square root of 2 down (1.414) instead of the value of pi (3.142) I'm sure this is just a slip up as these are numbers we all remember.

Les.

 

Hi Mr Chips,
I have spotted an error in your post #6. You have wiriten the square root of 2 down (1.414) instead of the value of pi (3.142) I'm sure this is just a slip up as these are numbers we all remember.

Les.

I was afflicted with the "Friday night curse" having imbibed one can of cider. Thank goodness that at least one AAC member is alert.

 

Of course a busbar has inductance, all conductors have some inductance, but it has no effect until current flows. For a busbar carrying DC the effect will not be seen except at the start of current flow and the change in current flow, where V=(inductance) x(rate of current change). In an AC circuit the voltage tends to counter the flow at that instant, which leads to the effect of reactance, which looks like resistance with a phase shift. There are formulas for calculating the inductance of a long straight conductor, with long being a quite relative term. It would be educational to compare the calculated inductance from the formula with the simulation results, if you do both please let us know.
Also, the inductance is relatively independent of the frequency, while the reactance is directly proportional to the frequency. Thus you must specify the frequency of interest.