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Aug 21, 2019, 09:21 PM
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ESC Capacitor ripple current?

Hi I have an old HV85 installed in an old TRex 600, running new 6S 5Ah batteries with the stock 600XL motor.

I have had to lengthen the wire from the ESC to the battery (about 6 inches) and after reading on the 'net I installed an additional capacitor (from memory about 1500uF 35V low ESR type) that I had on hand under the presumption anything is better than nothing until I can order in something more appropriate.

For interest sake I took some readings with my scope across a simple dumb 1mOhm shunt resistor connected between the battery and the ESC and got the following pictures.

The first image shows the heli start-up and settling into it's governed headspeed. The cursors show +28.8mv and +9.6mv indicating the battery current is always positive and varying (roughly) between 30A and 10A when the headspeed is stabilised, but always staying positive.

The 2nd image is another recording with a more appropriate time base to check the switching (about 12.5Khz) and it appears the duty cycle is (very roughly) 50 %. The third image shows the current is still varying roughly +30A and +10A it was just an unfortunate choice of vertical scale.

So my real questions are:

In an ideal world, shouldn't my battery current quickly drop to zero if not go a little negative during the "off" period?

As my battery current clearly isn't dropping to zero, is it safe to assume that the battery is still charging up the filter capacitors (including the extra one I added) during that "off" time to replenish what was taken out of them during the "on" time? and if so, would the the capacitor ripple current be the average, or RMS value of this "off" current, which would very roughly suggest my capacitor ripple current is around 20A?

I'm sort of stuck with the batteries and lead length, so I'm guessing more capacitors installed near the ESC (and or raising the headspeed a bit) would be a good thing?

Last edited by bjr_93tz; Aug 21, 2019 at 09:32 PM.
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Sep 06, 2019, 07:27 AM
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davidmc36's Avatar
If you look at what they recommend as a max voltage ripple, and download the logs from ESC memory, I bet you will find it to be no where near any concern point with 6 inches added. I did more than 12 both positive and negative and saw 1/5 of one percent ripple pushing 80 amp ESC to about 90% load.

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