Richard-
Offhand it sounds like ground loop problems. Make sure all the grounds only
connect at one point-the conduit to the motor may be the source of the problem
if it is grounded at both ends and there is a ground differential or a ground
fault.
Ground loops are a major source of noise problems, so adding grounds may not
help, but taking them away can. In your case it sounds like a star ground at
the drive is probably the way to go, with all the grounds lifted at the end
points. This might be difficult to do with motors that ground through the
frame, so what you can do is replace a part of the metallic conduit with
nonmetallic conduit-for a test you can probably just disconnect the conduit and
pull it back a bit, but code will probably not allow you to leave it that way.
If this conduit is where the ground loop is occuring it could have a dangerous
voltage on it due to a ground fault or just differential potential, so be very
careful when doing this. Ground loops can also damage equipment like the drive
if the current flows through the unit. They are tricky things and more than a
little equipment has been zapped by them.
If the signal from the encoder is a differential signal such as EIA-485 then
the twisted pair will have much greater noise immunity by itself than the
shield provides, particularly at lower frequencies, say under 1 MHz. Note that
the high noise immunity is due to a twisted pair driven with a differential
signal-if the signal is single ended it is no where near as high. But if it is
differential then lifting the ground may help.
Newer equipment often uses lower voltage chips, which can explain why a new
product is more sensitive to noise than an old one. A good design will
compensate for this, but that also adds cost.
My company makes some data optoisolators that might also help if lifting
grounds does not work.
Thanks,
Javid Butler
----- Original Message -----
From: richard harris
To: [email protected]
Sent: Thursday, February 01, 2007 8:30 PM
Subject: [Emc-users] System Noise
Hello,
This is a bit out of the standard EMC domain, more of an integration issue.
I have a Rutex 2200 drive, coupled to the Rutex motherboard. The encoder
wires are twisted pair, with shielding grounded at the drive; the power wires
are also shielded and run to the motor in a separate conduit. Somewhere I am
getting noise in this system. I have Rutex 9x drives on my other machine and
they run great, Rutex says that the new drives are more susceptible to noise.
How does that work your new product is inferior to the one it replaces?
I have tried shortening wires, adding grounds, checking grounds, even asked
the double E's at school.
Any ideas on how to reduce the noise, looking for any ideas at this point no
matter how crazy. If I have to stand on one foot while operating my mill so be
it.
Thanks
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