VFD Line (input) Power Control

I am in the process of building a VFD electrical/control panel and I would like advice on how to apply line power… I can imagine two ways to do this:

      1)	Have a magnetic switch on the 220v line service that I manually turn on at the beginning of a session… OR
      2)	Have a magnetic contactor that is controlled by the Onefinity controller (via a control signal from the breakout board) that automatically powers up the VFD when the 1F controller is turned on… This way, I have one button (the 1F controller) that turns on everything.

Assuming that I can use the 3.3v (pin 6) as a “power on” signal to drive a Solid State Relay (SSR) to turn on the magnetic contactor.

I would prefer to use the 1F controller signal to power the VFD…. From a safety perspective, the hardware emergency stop button on the 1F would then be the last ditch “kill all power” option as it would also kill power to the SSR and open the magnetic contactor (cutting power to the VFD)… and the software e-stop button (on the screen and Breakout board pin 23) would stop the 1F and the spindle via the RS485 signal.

If you have connected your VFD this way (line power through a contactor controlled by the 1F controller), I would appreciate your insights. Thanks

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:

  1. 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!
  2. 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
  3. 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




Thanks as always for the response.

I clearly understand that one does not want to cut power to a VFD that is in run mode unless it is an absolute emergency (e.g. fire)… My thought is to use the hardware emergency power cutoff located on the onefinity controller box as THAT power cutoff for both the controller and the VFD (via a contactor), and install an emergency STOP button (via pin 23) on the top of my bench that provides a STO condition without removing power to the controller or VFD.

One reason for having the 1F controller power on the VFD is so that I simply have one switch that turns the system on/off… in this case, that would be the onefinity power button.

Thoughts? If this is a bad idea, let me know… thats why I am asking.


Hey Bill,

I think it’s a bad idea, for the reasons explained above.

The power button on the Onefinity Controller box does not power on the internal power supply. The internal power supply is always on as soon as you plug the Onefinity Controller power plug in. Therefore I would not use it as power switch.

To turn on a system, I would take a switch that turns all contacts on the input side on and off. The switch on the Onefinity Controller box does not do that.

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I appreciate you letting me bounce the idea off of you…

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Hey Bill,

are you asking this because you have the Controller with the Rev. 5 AVR Mainboard with the round power pushbutton (and not Rev. 4 with the Rocker switch version), which does not go on by itself when you power it on with a power strip or an external power switch? In this case, you could make that the controller is always on by shortening J18 on the Rev. 5. AVR mainboard (pcb). You could even retrofit the rocker switch by attaching it to J18.

But both versions do not power on the internal power supply, it is already on when the controller is plugged in.

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Yes, I have a controller with a round power pushbutton… given that this switch does not power on the internal power supply (as I had assumed it did), my idea of using the 3.3v pin on the breakout board as a “power on” signal does not work. Interesting idea to use J18… I’ll think about that… I believe that for the time being, I am going to build my VFD cabinet with a magnetic contactor with on/off pushbuttons and may try other ways of starting it when I get it up and running.