Thanks for everyone's responses thus far.
I think the best solution would be a brake motor or some kind of mechanical brake, as was suggested, but we just don't have the room. I don't have a problem with using timed locks on the guard doors, but I have no doubt that they would eventually be removed at management's request, after continual griping by the operators. (Unfortunately it seems that keeping everyone happy is sometimes more important than safety.)
There doesn't seem to be a consensus on whether having a mechanical contactor between the motor and drive is a good idea. I have read some drive manuals that warn against it, although my Delta book does not. One post mentioned that dropping the enable line on their brand of VFDs with a safety relay is an acceptable e-stop method per the documentation. My Delta book doesn't say anything about that but I left a message with our Delta rep to find out.
I added some braking resistors to the drives this morning and found that the motors could ramp down in as little as 1-2 seconds (even with the heavier brushes), which would be acceptable. So if I could safely remove the contactors from the system, I think my problem would be solved. We are using a Banner PicoGuard safety controller to handle the E-stop buttons and optical guard door switches. We could easily tie the safety relay in to the enable lines on the drives and... Poof! problem solved... Or is it?
To answer a few questions:
- Yes they're only fractional motors but still very dangerous. If we leave the system the way it is, it's only a matter of time before somebody would lose a finger. This I am sure of.
- The operators do need to get inside the machine, maybe up to several times an hour for setups.
- You can't rely on training. It's a nice idea, but from a safety standpoint, not worth the risk. My last machine project had a number of razor-sharp cutters inside. I went to great lengths to guard the machine and make it safe. One operator still had a hospital visit because he removed a guard (with the machine off) and accidentally slammed his hand into one of the non-moving cutters. So we had to make a special cover that the operators are supposed to place over the cutters whenever they're working inside the machine. The last time I was over there, the cover was hanging on the wall with about 1/2" of dust on it.