Hey Bill, hey all,
you can usually find how to connect the VFD in the VFD manual. At least on my VFD manual, this is the case. There you can find that a magnetic contactor on the input prevents restarting the spindle in case the power was lost, which is a safety measure, but there is also a VFD setting that already prevents this.
As of updated IEC 60204-1, an emergency power cutoff is only recommended if it is absolutely necessary. One such case would be a fire, when you would cut all the power in case of fire with a big paddle switch at the door, so that the firefighters can be sure there is no electricity present anymore. In this case, you would cut everything with the paddle switch, not only a VFD, but both the CNC controller and the VFD and everything.
In all other cases, it is an emergency stop of all motors and acutators that is wanted instead, with Safe Torque Off (STO) as final state. A VFD needs to remain powered on to reach this state.
I know of no useful setup where a CNC controller would cut the power to a VFD. A CNC controller should be able to send the VFD a Safe Torque Off (STO) which is both possible via ModBus or via VFD input terminal like the Masso CNC controller uses.
According to the updated IEC 60204-1, a VFD’s spindle STOP command is considered as a valid Safe Torque Off (STO) that fulfils the stop category 0 requirement.
A category 0 or 1 stop safety circuit has to be wired in a way that after the Emergency Stop has been triggered, the setup cannot be restarted again without having pressed a dedicated RESET button. Therefore for an emergency circuit, you use a (ISO 13849) Safety relay such as a Omron G9SE. See here for a circuit diagram that describes how to wire it.
That’s why on VFD control cabinets, you have usually in the front door:
- A manual power switch (can be a magnetic contactor that will prevent to be on when power comes back after a power outage) for powering up the entire setup. Don’t cut power to the VFD when the VFD is not in STOP mode!
- A red/yellow mushroom-shaped Emergency Stop switch that is wired to the safety relay circuit, that will lead to a Safe Torque Off (STO) of all motors and actuators of the entire CNC machine
- A reset pushbutton without which it is not possible to restart the motors and actuators of the system after Emergency Stop was triggered.
You can see these three components in the back of the door:
If you use the emergency stop mode of the Onefinity/Buildbotics firmware (pin 23 on 25-pin I/O port), which will make the stepper motors and the spindle powerless (Safe Torque off), you can let it be triggered by the safety relay circuit, and wire your big red/yellow button to the safety relay as recommended.
As for the big red/yellow button in the Onefinity Controller case, that just cuts to power to the Onefinity Controller internal power supply, I see no case where it could be useful. I would never use it, as it lets the spindle and/or the router running (!)
Usually you power on the VFD and the CNC controller on at the same time with a manual power switch, and switch everything off after you have put everything in STOP mode (and, in case of the Onefinity/Buildbotics, have shut down the Raspi inside).
When selecting the power switch, be sure to find out what the VFDs input current requirements are. On cheap chinese VFDs, usually the real input current is willingly omitted on both manual and nameplate because they want to sell 110 V VFDs that there exist no domestic supply strong enough in the U.S. for See here for details.
On my VFD, the current requirement at the input is 24 A on 230 V, so on my VFD control cabinet I bought a 32 A switch and a 32 A plug/socket and a 2+PE 4.0 mm² (11/3 AWG) cable
