Is an axis shunt the same as an emergency stop circuit?

[Setlab]

I started dissecting my 90's model mill to find out what components I can use yet and came across something I hope isn't an issue.

My old CNC motherboard has a 5 pinout cable labeled 'axis shunt' leading to the main servo axis and spindle motor power supply. Could this be the emergency stop cutout of the old machine? There are a couple of weird things with this power supply. One it is powered on and off by a solid-state relay and two it has a second solid-state relay and 2 fairly large resistors (brown things in the last picture) mounted separately that I can't say for certain what the purpose is for, though I suspect when the axis shunt happens the second solid-state relay is activated and all power is immediately dumped into the resistors. It sounds plausible anyways, but it also sounds like something that will be dead weight and serve no purpose with the Acorn board.

Just for fun, here is a picture of the power supply in question, the axis shunt wires lead to the bottom right 5 wire connector. One really frustrating part of this power supply is nothing is really labeled, no input or output amps or voltages anyways. I multimeter tested it and it seems to produce about 170 volts of DC, who knows about the amps.

[Muzzer]

Yes, it looks like an active discharge circuit that dumps all the volts into those wirewound resistors. I suppose they might have argued that it's a failsafe approach to completely kill the power if the machine goes native on you. You'd want to kill the AC feed at the same time to avoid a fight. But as you say, it's not normal in Acorn systems.

You'd typically have a "charge" LED on a mains powered VFD or servo drive to warn you that there is a hazardous (>60Vdc) voltage inside the box but that's a different issue.

Assuming you are running this off 120Vac, you'd expect to see around 170Vdc, given that is appears to have a full wave rectified input circuit. As for the current rating, that's down to the bridge rectifier, not least its heatsinking. I suspect it could be good for 1-2 kW from where I sit. What sort of plug did the machine originally have? That might give you a clue.

[Setlab]

Now that you mention bridge rectifiers I think there are 2 of them heat pasted to the black aluminum plate. I'll see if I can't find some numbers off of them to lookup. The machine Originally came with a transformer that allowed either 110v or 220v to be used. I don't see any reason I need to run this machine on 220v so I'm not going to be useing that, though maybe I will depending on what kind of current strain all of the axis put on my 110v breaker.

Thanks for the help

[cncsnw]

That circuit could also be used for dumping excess charge during regenerative braking. In that case, the SSR that leads to the resistors would have to switch on in response to a voltage threshold.