Hi Guys.
thanks for your replies, welcome and instructive.
Rob, when I meggered (is that a term you use?) the motors were certainly disconnected from the drives.
I went thru much of the procedure you outlined, disconnected every circuit that goes to the machine and
bought each one online one at a time. The culprits are the steppers, not identical but all near enuf to 10mA
a piece.
Heres the odd bit, the leakage occurs with small variation only wether the axis is driving or not. Even when
stationary the drives still PWM current to the motors.
I've been thru the wiring again and again. All circuits from the BOB to the drives are opto types, I had the
limits and home switches disconnected, in fact removed, the spindle was dismounted and on the other side
of the room!
Vexta specify a 500v isolation result and max rated input voltage of 150V for the motors. The drives themselves
are 230V input. Their size tells me that they have inverter powered DC link. I have not pulled one to bits yet but am
thinking that the DC link may not be isolated from earth. Certainly most AC servo drives are not isolated (what
a surprise that was!!!). I was guessing that the leakage was in the drives but each drive frame is earthed back
to the common rail and I could not for the life of me see why leakage would flow in the mill frame earth.
Since I did that testing I have bought a Tectronrix current probe. One day soon I'll apply it to this problem,
I promise!
In absence of anyother explanation I concluded it was induced current, aka homopolar current in defiance of
Kirchoff. I thorougly agree it way too high but I'm not throwing these things out... they're superb in every
other respect.
I am very careful about power supplies fighting one another. I repair welding equipment or a living and have
seen numerous occasions where weld voltage, a very beefy power supply, blow up boards and their power
supplies should it inadvertently get connected.
Craig