Re: Reducing PMDC motor noise ?

Eric R Snow wrote:

On Thu, 11 Oct 2007 21:05:49 -0400, Jamie
<jamie_ka1lpa_not_valid_after_ka1lpa_@xxxxxxxxxxx> wrote:


Eric R Snow wrote:


I have a CNC mill in the shop that radiates profusely in the AM band.
The servo motors are brushed PMDC motors. I am wondering if I can use
capacitors across the motor leads at the motor to help attenuate the
noise. The motors are powered by modern servo amps and the power
supply output is 100 volts at 15 amps max. The CNC control is a
retrofit done by me and purchased from Ajax. Ajax has terrible
customer service and has not answered my questions about reducing
electrical noise. The motors are all properly grounded and the motor
cables are enclosed in braided copper conduit. If caps can be used at
the motors what kind at what capacity should I use? Better yet, how
can I calculate what capacitors to use?
Thank You,
Eric R Snow

That is normally not an issue in shops since you should be working and
not listening to the radio:) unless you are disturbing some one out side your premises? That would be understandable of course.

The brushes could be generating the noise, there are a few ways of
removing it but, I really don't think that is the mojor problem.

One of the problems could be the drive it self on the service generating the noise due to it's nature in operation. The most
common ways I can think of off the top of my head is an isolation
xformer or a reactor in the L1,L2,L3 lines to the drive.

The motor it self should be symmetrically sealed with it's metal case and metal seal tight leads from the drive cabinet, to the pecker head.

The armature leads should be in it's own metal race way or flex
seal. Twisting them also helps to prevent noise from coming back through
the leads since this, effective R.F.

Good luck..

Gretings Jamie,
I've been told by the boss that I'm not working hard enough but since
I am the boss I just ignore the old coot. The drives, AKA servo amps,
are fed DC from rectified and filtered 3 phase, the filtering being
one large cap. This power supply scheme is the one provided by the
maker of the drives. Would an oscilloscope be useful to track down the
source? Could one lead be used as an antenna that could be waved
around the various machine components to help spot noise emanations?
Thanks,
Eric

Using a scope with direct connections may not yield much because you
most likely will see this noise all over the inside of the cabinet.

Make a small loop of wire about 1 inch radius (2 inch across) with
a good amount of turns. Use some very thin wall wire like teflon, enamel
(bell wire) ect. Connect to scope across the leads. Set the scope
in the millivolt range and use the loop as a sniffer.


If you don't have an isolation xformer or reactors, the R.F. generated
from the drive firing circuits mainly on the output will still radiate
over the wires to the main.
The lead wires from the supply to the drive inside will generate a nice radiation of noise. From this point i'm assuming you may have a
PWM type drive that is using the motor's inductance as part of the
equation.
This type of system can be very noisy.

what you want to do is concentrate on using common mode chokes to
cancel out self generate noise and maybe an xformer.
a 3 phase reactor is a good starting point and maybe through in a bucket filter. In your case, you almost have one. Placing a burden (load) resisters on the PS helps in this matter.

To be honest, I would mount a 1:1 xformer on the outside of the enclosure where the drive is housed (metal I hope). Have the secondary only leads going in side the cabinet.
At the entry point inside, I would have a 3 phase reactor to help
things along.
Also, using rope constructed conductors reduces the skin effects
greatly that carries this signal, along with swing chokes on the output of the drive..

P.S.
Try putting loops in the wire as an experiment to see if it reduces
the problem.
I find putting a single loop at L1, L2, L3 serves as a service check
point for current measurements and, it also acts like a choke.




--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

.

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