Ground fault error light on 1991 Partner 1D servo amps

[Jim Shaper]

I just got this machine and was told all it needed was a DC cap replaced to be put back into service. That has since been done, but I'm still getting a 450 Error code and the servo amps all trip their ground fault lights if I have the 3 conductor plug connected to the amps.

The very top right green relay light is triggered, as is the D6 one second from the bottom of that same string when those connections are made.

If I leave those connections hang, I still get an over travel in Z error when I reset the machine. Is this due to a bad limit switch? I've raised the head to the middle of it's travel (manually) and this error trips as the brake is released on the ball screw.

I don't have the schematic, so I'm pretty much running blind here.

[Jim Shaper]

So I pulled my head out of my butt and disconnected the DC supply wires from the servo amp's and the ground fault lights are off with the other two plugs still connected.

Like I said, the caps have been replaced, and have been verified to be good (as I was informed being brand new doesn't guarantee anything). What else could've failed when the cap went bad? The two shunt resistors measure lower than their rated 3000 ohms, but they're identical resistance, and one of them looks to have gotten hot (which is the one which was across the failed cap).

Is it possible that the bridge rectifier is shot, and that's what's causing the error light? What else in that end of the circuit could've been damaged to cause this issue?

[Jim Shaper]

The bridge is fine, voltage was high since the shop it came from had 208 service and I'm running it single phase off 240v, so that's been fixed.

I get a GF light on two drives if one power connection is on, but not on that drive. Might be the resistors on the caps, but haven't checked them all out. Might also be the motor wiring, but I haven't looked at that either.

[jpawelk]

Jim,
Typically a ground fault indicates a short in the 110VAC circuit or a short in a motor and/or cable. Typcially the resistor that is placed on the blue cap goes bad when you get this type of short. Replacing the resistor will not cause the GF alarms to go away!! You may want to try and disconnect one motor at a time and power up. See if you still get the alarms then. You may be able to determine what axis is giving you the trouble. Otherwise it is something in the 110AVC circuit. Start removing items that use the 110, typically items connected to terminals 1 and 2. After you get the ground fault alarms taken care of, replace the resistor.

[Jim Shaper]

Thanks JP.

So you're saying to take those bleed resistors off for the testing, then put them back on after I've determined the problem?

What about the one that goes from the neg side of the caps to the chassis?


I'd hate to lose a drive at this point because I shocked it not having something connected.

Also, the fault gets bounced around when one of the drives has the red and black wires connected (the plug on the right) but the others don't (I'm assuming this is just a connection to the DC bus, and not also the motor connections, but I could be wrong). So if one is connected and the other two aren't, the ones that aren't connected display the ground fault light but the one that is connected does not. If I connect two, but not one, only the one that's not connected shows a fault error. If all three are connected, all three ground fault lights are on.

I'm assuming the drives are getting their power from another section of the power supply than the DC bus, right? It seems that only the DC bus connection causes the issue.


The machine was functioning prior to one of the big caps failing. Is there another reset having replaced those I'm missing? I don't have any of the maintenance documentation for the machine.

I also haven't ruled out motor wiring issues, as I'm aware this is problematic on these machines of this vintage. What wire would you recommend for rewiring the motors and encoders? Gauge and insulation rating/shielding?

[Jim Shaper]

I pulled the wires off the terminals where they come off the transformers and found a ground connection across both 34 and 35. So there's our fault.

I'm going to head to the store and get some wire and start pulling.

Does the encoder wiring ever have problems? I pulled out my Y motor to have a look at the brushes and that encoder's insulation is pretty rock hard. The tie wraps disintegrate on contact. Should I replace all of that wiring, or just do the motors power wires? I figure if it's got issues after 19 years, it's just best to fix all of them at once. The rest of the machine is in pretty darn good shape.

[Jim Shaper]

I've gotten all the motors out and dumped a pile of carbon out of the X axis motor.

Something that concerns me, but I haven't verified with the other servos is how deep a groove the brake brushes seem to have carved into the rotor. There's plenty of brush material left, and there's no sign of copper in the dust, so I'm wondering if maybe that's just how the motor was made?

After cutting the DC leads off the X axis I wasn't able to measure a fault, so whatever was causing it is not there anymore.


I need to get some wire pulling lube, as I can only drag the new wires half way up the flexible conduit dry, but it's getting there.

[Jim Shaper]

I was wrong, the brushes I was thinking were the brake are actually the tach generator.

Verified a current leak to chassis via a motor repair shop, so I tore the whole motor apart and cleaned it out with compressed air. I'm going to run it back over to the shop and have him check it again to make sure I got it, but I'm not measuring any continuity to the chassis now.

I wasn't able to get the tach rotor off the shaft, so I couldn't scrub it out or turn the armature.

Then I was down at Milltronics today getting replacement brush caps and 5 of the 6 I bought are .003 oversized. I ended up having to grind one of them out after discovering they didn't fit by trying to install it and not getting it seated then having it crack apart when trying to back it out. I did get a nice tour of the joint and met a couple really nice people there. I'm sure Jill will send me some that work with my motors.

[Jim Shaper]

Got the motor back together and now I get a bed dance when resetting the drives on power up. Not homing, just hitting the reset e-stop message when the machine is initially started.

[Jim Shaper]

Got it all sorted out with the help of many friends and the good people at Milltronics!

For anyone finding this, the solution to the run-away travel was reversing the polarity of A and A not at the encoder once the phase of the motor and tach are configured for correct direction of travel when homing.

Now it's time to make some chips.