Re: [CAD_CAM_EDM_DRO] Advice, BEFORE I BLOW UP my Sherline?

You can't effectively solve this problem without knowing the source of the
noise - because it needs to be snubbed as close to the source as possible.
If you really want to solve the problem properly, you'll need to find and
identify the bothersome transients by means of a fast capture scope that
lets you see the transitions at both the switches and the commutator. While
it's possible that the motor brushes are the main cause, it's just as likely
that DV/DT at the switching devices in the speed controller are launching
monstrously tall transients with harmonic ringing that digital devices like
stepper controllers don't tolerate.
The analysis is not one you'll accomplish in a few minutes on a cocktail
napkin, because as previously mentioned, the capacitance your snubber will
introduce can often cause new problems or the conditions will change as the
brushes wear, etc. A correct solution involves measuring the width of the
transients, their typical frequency and then identifying the actual spikes
from the harmonics - then calculating snubber values for the particular
transients you want to snub, followed by empirical testing. This can take a
week or two in a lab with all the best tools.

If you want a 'fix' without having to do all the analysis, for a 115V hybrid
or SCR phase-controller make a snubber out of a .1uF 250V poly cap with a 10
ohm 1/2 to 1W resistor in series. If it's a transistor controller, opt for a
.01 uF cap or smaller. Place this across the load wires and start it up.
depending upon the length of the load wires, you may be able to tell what
the source of the troublesome transients is by adding the snubber close to
the load and alternately near the controller while observing. This works
particularly well with universal motors.

At 240V & above, you'll have another problem - transients being generated
by the controller's switching devices and carried back into the building
wiring. If your linear stepper p/s is not isolated from the same AC line
you'll need a completely different snubber there. The process used to solve
for cap value is similar, but it can take the 1,000 pc assortment of caps by
the time you find what works with whatever line capacitance and inductance
is present. The solution can be site-dependent also.

I wouldn't bother with trying MOV's since they will have a very limited life
in this kind of application, and the failure mode is very ugly.

Hope this helps.

----- Original Message -----
From: "Abby Katt" <cncgroupnospam@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Friday, October 07, 2005 5:03 PM
Subject: [CAD_CAM_EDM_DRO] Advice, BEFORE I BLOW UP my Sherline?


> I've opened the Sherline motor, and have run wires from the two brushes
> to the outside of the motor (through the top), I am now going to connect
> a cap between the brushes, and each brush through a cap to ground (the
> motor casing). The caps are (big chunky) 400VAC rated 0.68uf.
> I'm about to step out for a bit.. and test it when I return, so....
>
> Can anyone advice me if this setup is going to work? Should I put a
> series resistor inline with any of the caps just in case they decide to
> short out?
> Or is this setup okay. The plan is to reduce some of the noise which is
> causing so much pain (steppers doing their own spindle-dance etc)
> Thanks!
> Ab
>
>
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