I have a 30 feet line with DC supply 12 V. A motor on the end draws about 0.5 Amps initially (100 ms). It's logic shuts down because of the line's voltage drop caused by inrush current.
A capacitor of 1000 uF prevents the shutdown.
However, if I calculate the size of the capacitor that prevents a drop of more than 1 V, a much larger (x50) value comes out (0.5 A * 0.1 s / 1 V = 0.05 Farad).
I figure this is because the supply isn't disconnected but continues to supply power, so the capacitor does not have to provide the current alone.
How can I improve my model so the calculation is closer to the real thing?
Best Answer
You need to treat the line as an in series inductor.
This may be the limit of lumped element mode analysis(treating the line as a simple inductor is an example of this), but it will at minimum give you a good starting point.
The line has two values, an inductance and a capacitance, if you place the supply entering an inductor, then a capacitor from the line in parallel with your capacitor it should work quite well.
There are many tools for calculating your inductance, I quickly turned up this one.
Since you are putting a large capacitance at the end it will probably be so large in comparison to the capacitance of the line that the line capacitance is negligible.