On 07/12/2018 02:12 PM, John Dammeyer wrote:
Is it standard practice to connect the DC Servo and DC Instrumentation Bus
to the machine frame ground which is connected to power line earth? Or is
it more normal practice to keep the DC isolated from the 'earth' ground.
The "DC Link" in a multi-axis servo system incurs large DC
offsets when the drives are powering the motors. This will
shift constantly as motors accelerate and decelerate. If
the drives have complete isolation between the power stage
and the control side, this should not be a problem. But,
you need to insure that the DC link is grounded at ONE POINT
only to some other system ground.
If the drives are NOT fully isolated, then ground loops are
guaranteed to occur, and everything else connected to ground
needs to accommodate this. If these are analog servo amps,
then using differential amps on all inputs (especially tach
and command) mostly solves it, as long as the DC link
variation doesn't exceed the common mode rejection of the
diff amps. These DC shifts can be surprisingly large, even
with beefy ground conductors.
And, of course, you CAN isolate the DC link, but have to
consider the possible dangers when something goes wrong. On
the other hand, having the DC link minus grounded to case,
and then somehow the DC link PLUS gets shorted to ground,
can lead to massive damage as XX hundred Volts rushes around
through the wall plug grounds and fries the CNC control,
computer, etc. I know because I have repaired some of my
(Pico Systems) gear when this has happened to customers.
Jon
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