On 8/6/24 16:46, Viesturs Lācis wrote:
Hello!
Recently I have posted some questions about the retrofit that I am
working on and now I have encountered an issue that I cannot figure
out.
The thing is that I am swapping original Yaskawa servo drives with
Mesa 8i20. One of the things for that was 3phase rectifier bridge (it
takes 230 VAC input and provides 340V output) and 4700 uF capacitor
for smoothing the ripple.
It took me some time and some patience from PCW to fix the config and
in the end I got one 8i20 to move one joint in the machine. So my next
step was adding 2 more 8i20 drives for remaining 2 joints. It ended up
with a blast in one 8i20. So I replaced it as well as I had to replace
rectifier bridge. But now my issue is that protection fuse is tripped
as soon as I enable machine power.
1) only rectifier bridge connected - all good
2) rectifier bridge + capacitor - fuse tripped immediately
Note that I have not yet connected DC bus to 8i20 drives.
I checked capacitor with multimeter, it was showing 180V DC (and it
did not decrease in that time that I was holding multemeters pins
there). I am not sure if that is some residual charge from previous
(which I doubt) or if that was some charge that was acquired before
fuse was tripped, but this seems to me like a good capacitor. Is that
correct?
I do not understand why was everything fine in initial testing - I did
turn machine on and off lots of times and capacitor was discharged
(and recharged!!!) numerous times. I have swapped that fuse for
identical unit from a machine that has yet to go through the retrofit
process. And it is the same.
So my question to electronics gurus - could capacitor be damaged or
was it just a beginners luck that everything worked and do I need to
introduce some inductor between rectifier bridge and capacitor to
limit the startup current that charges capacitor?
Your thinking is correct Viesturs. That 4700 if it needs recharged to
around 340, might be enough inrush to clear the fuse again. What I have
done is make use of controlling the machine power, all of it, with the
F2 key of linuxcnc, except I use 2 gpio pins to control 2 SSR's with
some time of 2-3 seconds from first on thru a 50 ohm 100 watt power
resistor to slow the capacitor charge, then the 2nd SSR shorts the
resistor after the cap is fully charged. Also fed to a pyvcp led to let
me know its live when it is. I was tripping a 30 amp breaker turning on
my go704 originally, now a 20 amp just barely stays up when the spindle
gets locked. Considering the normal draw is about 3 amps, the NEC frowns
on the 30 amp breaker in that scenario. Since your cap is much smaller
than mine, a 1 second delay to firing the 2nd SSR is probably more than
enough to save the fuse or breaker. I do all that sort of things in the
.hal file. I do the machine power with SSR's in all my stuff. Not all
has the delays though.
Viesturs
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Cheers, Gene Heskett, CET.
--
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