VFD spindle control safety

[A_Camera]

I have bought a CNC machine which has a Huanyang vfd and a spindle pre-wired. At the moment, the vfd is powered up when the control box is switched on so there is no independent control over the power to the vfd. The spindle can be controlled by software so when a tool change is needed the spindle stops.

I would like to be 100% sure that when I am changing the cutter the spindle cannot start and in my inexpert opinion this can only be done by disconnecting the vfd from the power or the spindle from the vfd with a switch. Switching off the vfd (which I cannot do at present but is easily achievable) feels wrong but the manual is very clear that the vfd must never be powered up with the spindle disconnected. If I went down the spindle disconnection route I cannot imagine that I would never accidentally power the vfd without switching the spindle back on.

So, as far as I can work out, the only safe way to do a tool change is to turn off the vfd. Is that what people do?

The way I do it is simple. I have a 3 phase safety switch installed between the motor and the VFD. Simple and no electronics involved.



Every time I have to change tool I simply flip it to off and when ready I flip it to on. This procedure makes sure that regardless if my VFD, or control software, or whatever else would start the spindle due to an error, the spindle won't spin and my hands and fingers are safe. This may be regarded as overkill by some DIY machinists, but in my opinion the only safe way to work with these machines. It also gives me a sort of ultimate emergency stop possibility, but it should never be used as an on/off switch. However, I have to admit that that I did start the spindle a few times with it because I forgot to flip it to on after a tool change before starting milling and because the movement already started when I realized my mistake. Everything is still fine, but I know this is risky and should push the e-Stop and restart from there, instead of doing this.

Anyway, this is what I use for safety and it is 100% safe as long as I remember to flip it to off before starting the tool change. I do not turn off the VFD because it takes too long to start it ever time.

[Kitwn]

Connecting/dissconnecting a motor from a VFD or stepper controller when it is powered is generally considered a risky business due to the voltage spikes that can occur. Power transistors have been described as 'the fastest fuse on three legs' for good reason. If your switch has a back contact that can be used to prevent the VFD from trying to start when the motor is not connected that might be a good move.

[ericks]

Using a field isolator on drive driven motors are an Industry standard. They are lockable isolators.....obviously you would isolate after you have "stopped" the motor:)

[A_Camera]

Connecting/dissconnecting a motor from a VFD or stepper controller when it is powered is generally considered a risky business due to the voltage spikes that can occur. Power transistors have been described as 'the fastest fuse on three legs' for good reason. If your switch has a back contact that can be used to prevent the VFD from trying to start when the motor is not connected that might be a good move.

Perhaps I was not clear, it is NOT meant to be on/off switch, just safety switch to make sure the spindle will NEVER be started by accident or error while there is a tool change in progress. Of course, the spindle is not spinning when the switch is flipped to off, and the VFD is not feeding the motor power outputs when the switch is flipped to on. Starting and stopping the motor is of course done by the CNC software, in my case through Modbus commands.

Power transistors provide ZERO safety, quite the opposite, they give you a false sense of safety and may not operate when needed. A mechanical switch will always work because it is just what it is, simple and 100% independent from electronics and software/firmware.

[magicniner]

This is like a health and safety thread on a model engineering site! ;-)

Commercial CNC machines rely on the spindle motor drive, with a suitable signal from the controller, to prevent the spindle starting up whilst changing tools, that works, failure to understand why will lead you to think you need breakers and stuff between the motor & VFD when you don't.

If you have a decent VFD it has the facility for an E-Stop data input, this will suffice on equipment of a reasonable quality.

[Kitwn]

A-Camera,
I appreciate that a proper procedure for stopping and isolation protects the VFD. My concern would be the occasional operator error, possibly starting the VFD before closing the contactor which is why I suggested the back-contact interlock to prevent the VFD from operating with the contactor open. This ties in with my earlier comments about the 8 second delay etc. These spindles are a big investment for a hobbyist and anything you can reasonably do to prevent one of those potentially expensive "Oooops!" moments is worth considering.