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Motor Wire Length

JasonK

Participation Award Recipient
#21
Agreed but the size of the pulse does, as NCT's tests showed, hence the energy that has to be absorbed by the capacitor and the heat generated.
I have a BSEE, which is why I am curious to the data behind how they came up with 'longer power leads = broken ESC' as it doesn't make sense to me to have the ESC dependent on the battery acting as additional power filtering for the standard switching behavior. I wouldn't consider NCTs tests to show that the capacitor is going to 'burn out' from the additional length that he tested, unless was only rated for the rest voltage of the input, but that would be a _design_ issue to have a cap that isn't rated higher then the expected voltages.

without seeing waveform details of of the test (IE was the spikes just related to the voltage drop because of current over the transmission lines, back EMF off the motor, electronics switching effect, or even something else), I am not going to jump to conclusions that the longer leads are such a big deal (at least if the power wires are apprprately sized for the expected current load - we run more current in some of our planes then the breaker would brake for your house wiring and that wire is typically thicker then much of the stuff in our plans).
 

JasonK

Participation Award Recipient
#22
for NCT's test, there was 0.15ohms added from the additional wire length and an additional .2V power change was measured.
i = v/r
i = .2v/.15ohm
i = 1.333333333A

that would only take a ~1.3A change in the current drawn between on/off to make that much voltage difference from the shorter wires.

if your over rating your ESC like you should, I wouldn't be to worried about it. if your pushing 20A through a 20A ESC... while your just asking to blow out the ESC at that point.
 

bwarz

Master member
#23
yea OK lets think logically here too for those who have experienced failures. If you have shorter wires on the motor and using this in an airframe, likely in a twin config, where is the ESC? Shoved in the engine nacelle behind the motor which by its nature gets hot while it is running. Reference my previous post about what kills caps. Now if you shortened the power wires and lengthened the motor wires, the ESC would need to move. Away from the heat. Likely to a more roomy space. And if you were thinking about it, adding some method for airflow across the space. Assuming that you were using the exact same model of ESC, at that point, which was the solution to the failures? The wire length -or- the physical relocation with likely better thermal characteristics?
 

Mr NCT

Elite member
#24
If I get a chance I'll set it back up and see if I can get a pic of the wave form. I had to run the 0 ref. way off the bottom of the screen to be able to expand the display enough to see the difference in the amplitude of the spike. Imagine a straight up line and a concave rapid down slope. So any picture will be misleading - kind of like charts newspapers show that only show a small segment but not in reference to a 0 point.
 

bwarz

Master member
#25
If I get a chance I'll set it back up and see if I can get a pic of the wave form. I had to run the 0 ref. way off the bottom of the screen to be able to expand the display enough to see the difference in the amplitude of the spike. Imagine a straight up line and a concave rapid down slope. So any picture will be misleading - kind of like charts newspapers show that only show a small segment but not in reference to a 0 point.
I hate that. Its called creative marketing...
 

JasonK

Participation Award Recipient
#26
If I get a chance I'll set it back up and see if I can get a pic of the wave form. I had to run the 0 ref. way off the bottom of the screen to be able to expand the display enough to see the difference in the amplitude of the spike. Imagine a straight up line and a concave rapid down slope. So any picture will be misleading - kind of like charts newspapers show that only show a small segment but not in reference to a 0 point.
don't need to do it for me, my comment was more related to the intense "this is how you must do it" comments, that weren't backed up with data/explanations. But if you want to setup the test stuff, I would be more then happy to look at the data with you (or if your going to be at Flite Fest and wanted to bring the gear, we could find each other there and look at it).