On Friday 11 August 2017 02:29:36 Gene Heskett wrote:
> On Thursday 10 August 2017 17:10:17 Les Newell wrote:
> > Hi Gene,
> > > I've had a heck of a time with noise when using a 7i90 interface,
> > > noise so bad it destroyed several 7i90's by punching thru the 3.3
> > > volt circuitry.
> > That's some serious noise. Do you know where it is coming from? A
> > near field probe is great for finding noise sources. Wrap a dozen
> > turn of wire around an insulating former, say around 2" diameter
> > then wrap some tape around it to hold it in place. Plug the loop
> > into your oscilloscope, crank up the gain and wave it around any
> > suspicious areas. It is usually pretty easy to home in on the noise.
> > With smaller probes you can even trace the noise to a particular
> > area of a circuit board.
> My probe was a ferrite cored standup choke of a few microhenries, and
> it found lots of noise about the pair of switchers driving the
> steppers, more noise than the stepper drivers were making. Analog
> supplies for the steppers helped a lot, but moving the pi and
> interfacing out of the power electronics box, combined with the
> 7i42TA's and its all happier than a clam. I put a 20amp corcom brick
> wall in the line power to the pi, and another in the 256 volt feed to
> the vfd. Probably overkill but it did help, and the vfd doesn't seem
> to care. Its in the line feed, not the motor load.
> > Noise can come from unexpected places. For instance my inverter
> > has a built in switch mode power supply to power it's logic, as do
> > most inverters. On mine that power supply stuffs more noise back
> > down the supply than the inverter's main drive section!
> > Les
Re-reading this, Les, I have to ask how old that VFD is?
Anything over 4 or 5 years, I would expect that the capacitors would be
suffering from poor ESR, which isn't a test feature available on a
utility DVM with a microfarads test function.
A gismo called a Capacitor Wizard, around $200 from a guy in Kansas City
does the definitive such test, measuring the equivalelent series
resistance, aka ESR, and any capacitor that reads over 2 ohms ought to
This unwanted resistance causes heat in what should be a zero resistance,
no power lost part.
If you open the vfd up, sometimes the eye can spot the major failures as
those will have bulged tops, or even split on the score lines on the top
of the cap and letting out some of the anti-freeze, or evidence of
heating in the discoloration of the plastic film shrinkwrapped label, or
evidence of bottom leakage, usually whitish stuff on the pcb they are
mounted on. Any of those 3 clues should put it in the waste barrel.
And your vfd will be happy, where it may fail for good if not so
That cheap, phony vfd I bought off ebay, and am using on the sheldon
lathe, came with a printed note that the big capacitors had a runtime
life of 5 to 10 thousand hours and should be replaced on about that
schedule, precisely because of this. As a way to prolong that time, I
have a couple i/o pins dedicated to controlling a pair of 40 amp SSR's,
that are .hal file netted so all motor power is off until the motion
enable button, 2nd from the left at the top of the axis display, is in
the enabled state. So a joint error that stops motion, also kills all
So, rather than shutting down the computers for the night, pure hell on
hard drives, I just touch that button with the mouse and disable all
that motor stuff.
If I ever find a round tuit, I'll move that 4000 lumen 4' led light stick
mounted across the top of the lathe, onto that same circuit, so its
nominally 30 watts isn't on 24/7 with the computer stuffs. Hindsight,
20-5 you know. ;-) Those are nice lights too. Worth the money.
I hope this helps.
Cheers Les, Gene Heskett
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Genes Web page <http://geneslinuxbox.net:6309/gene>
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