On 7/7/25 10:22, Viesturs Lācis wrote:
pirmd., 2025. g. 7. jūl., plkst. 16:55 — lietotājs gene heskett
(<ghesk...@shentel.net>) rakstīja:
On 7/7/25 02:58, Viesturs Lācis wrote:
Hello!
I have a question if anyone from EU has a spare 8i20 drive (can be
used as long as it works correctly) that I can purchase. It seems that
I have to replace 3rd Yaskawa servodrive in my Biesse retrofit.
Eusurplus are out of stock, I did not find any other sellers that have
them and I really do not want to deal with customs bureaucracy.
Viesturs
Some Q's Viesturs:
Is Yaskawa gone?
and do you still have the failed units?
What is the power supply voltage?
Do you have an instrument that can measure both a capacitors capacity
and its ESR?
And make basic checks on power transistors, including big VFET's?
With that ESR instrument you can probably restore them by finding and
replacing the capacitors that have now aged out and failed, usually
effectively open.
Yes, I not only still have the failed units, I still NEED them to
provide proper encoder signal to LinuxCNC (I wrote about that last
year or so - A and B channels from encoder are normal quadrature, but
third channel has a pattern that, relative to A and B channels provide
information about rotor's position, basically doing the same as Hall
sensor signals).
The problem with previous drives were error messages. The problem with
this drive is that it is burning analog output channels on 7i48 board.
It has been few hours at max of total time the machine has been turned
on and enabled and I already have 3 analog outputs permanently stuck
at 13-14 V (last of which happened today; I need 2 channels for C axis
and for VFD, so only one spare analog channel left). I thought that it
was something random after the first, got suspicious after second and
now I am sure after the third. I have done multiple retrofits with
older Yaskawa drives, this is the first time I see something like
this. I have checked the wiring, I blame the servodrive.
My most sophisticated tool for measuring electrical signals is digital
oscilloscope. The other one is simple multimeter. Since I am not sure
that I understand the suggestion about measuring ESR (and what that
is) I should not get involved in attempts to repair.
So I still would appreciate any offer to purchase 8i20 from somebody
in EU to save my time on paperwork.
Viesturs
I can appreciate that. So some educational comments I've learned:
ESR stands for Equivalent Series Resistance, which dynamically s/b
under an Ohm, usually measured at 100KHZ and only 50 milivolts of signal
so as not to temporarily self heal and is caused by the deterioration of
the capacitors internal connection to the foil and paper normally used
as capacitance plates by the common electrolytic capacitors. The
ethylene glycol used as the electrolyte must be very very pure else this
"gastite" connection gradually degrades as the alu of the foil slowly
oxidizes. The capacitor may measure at or even above its nameplate
capacity in microfarads, but 2 ohms of ESR drastically reduces its
ability to absorb the higher frequency stuff it was used to absorb them
in the original design. The inability to absorb the higher frequency
noise may be the reason its eating the 7i48's outputs. Your scope should
be able to show those spikes, typically needing at least 100 MHZ of
bandwidth to show them with good accuracy, probe ground leads as short
as practical. My own best scope (I have 3, none below 100MHZ) is a $3500
Siglent that goes to 350 MHZ on all 4 channels.
Electrolytic caps should always be used at 90% of rated voltage or even
higher., a 16 volt cap used in a 3.3 volt circuit has a failure time
generally under a year. The under voltage allows the cap to "deform"
losing capacity in the process.
OTOH, an occasional overvoltage reforms them. I use that to advantage 2
places in my machine room, using two 65 volt caps in series as that was
what I had sack of, in a 124 volt dc circuit. But Jon's PWM-Servo is a 4
quadrant servo, capable of stopping a 1 hp motor in around 150
milliseconds. It does this by sucking the power back out of the motor,
which runs those 130 volts worth of caps up to a red one below 170 volts
as it reverses direction. Then it uses that overvoltage to re-accelerate
the motor back to whatever speed it was running but in the opposite
direction. That overvoltage reforms the caps by the leakage induced, but
the overvoltage is gone long before any life affecting heating has
occurred. The shop lights don't blink as there is no power drawn from
the line to reverse that motor during wide open operation, and its been
running like that since I rebuilt that mill while converting it to
linuxcnc, about 12 years ago.
VFD's can also be considered as 4 quadrant devices, programmed right I
can reverse the spindle motor in about the same 3 or 4 hundred
milliseconds at 400 rpms with a 40 lb 8" chuck mounted. All I hear is a
thump when it does.
This is also a quite convincing argument in favor of single point
grounds where all grounds go to a single ground bolt and NO place else.
Often called a "star" ground. Grounds all over the machine can cause
ground loops, and the noise that can inject into the system can easily
be card killers. Its also great insurance against nearby lightning
strikes.
Yeah, I'm an at times frustrated teacher. No insult intended.
Cheers, Gene Heskett, CET.
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author, 1940)
If we desire respect for the law, we must first make the law respectable.
- Louis D. Brandeis
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