VFD spindle control safety

[m_c]

Also you can factor in that there is little to no torque on start up of the spindle with these type of spindles .

This.
Unless you have a spindle with lots of torque (I.e. milling machine/lathe spindle that's been geared down from a multiple KW motor), then holding the spindle with your hand will be enough to stop it from spinning up.

Plus most VFD manufacturers advise against regularly power cycling VFDs.


If you were worried about damage, I'd put the risk of crush injuries on any reasonably sized/powered CNC, far higher than a spindle starting up when you've got your hands near it.

[Neale]

If you were worried about damage, I'd put the risk of crush injuries on any reasonably sized/powered CNC, far higher than a spindle starting up when you've got your hands near it.

Probably right. Thinking about it, I've stalled my 2.2KW spindle by trying to push it too fast/deep through steel - with a 4mm cutter. That was at around 5K RPM - no torque worth talking about. A spanner on a spindle would easily hold it.

[routercnc]

I’m in the minority here and I start and stop my spindle manually through large push buttons on the control box main panel. These are linked back into the VFD inside the control cabinet through the built in terminals under the front cover.
Same goes for the speed control which is a potentiometer on the control box main panel linked back to the VFD.

Some jumpers in the VFD have to be set to use external controls.

This means the PC is not in charge of the spindle at all except for an e-stop where it will stop and inhibit the spindle running, again using terminals on the VFD.

I’ve never had the spindle come on when changing tools, and have never forgotten to start the spindle before hitting cycle start. In the same way that I have never tried to drill a hole on a drill press without starting the motor! It’s a preference thing and I just like to use it that way, plus it feels a bit safer.

[Doddy]

I've had my G&T for the night so am feeling somewhat ambivalent. The OP questions safe operation - and he's correct to do so. I hold a view which I think is shared by some that, gauging an acceptable level of personal risk against convenience that I/we are prepared to cut some corners. But, please understand that doesn't make it safe. I wouldn't pretend for a second, or promote that nipping up a cutter in a spindle with your hands/body/spanner anywhere near is "safe" in the event of either hardware or software failure.

Right!, where's that blasted cat?

[Kitwn]

At present I'm following Doddy's principle of taking what I see as an acceptable risk when tool changing but I'm aware my machine has a built in risk which is that the relay that starts the spindle will pulse briefly during power up/down of the controller. I use a cheap Chinese breakout board and LinuxCNC. The relay on the board operates a relay on the wall that activates the spindle and the dust extractor.

Not a problem when it's all switched on but if I wait long enough there'll be a power cut just as I have a firm grip on a tool to pull it out of the (still quite tight) collet and there'll be enough stored charge in the VFD capacitors to modify my finger prints.

It's all a balance of probabilities but reading this thread does remind me I should add some kind of interlock. Perhaps a nice big switch in series with the VFD control relay which includes a back contact for a warning light. I have all the necessary bits in the shed so it's not exactly beyond what is 'reasonably practicable' as the legislation says.

I would never criticise routercnc's KISS approach but know myself well enough to predict the odd oversight, especially towards the end of a tiring day and am therefore in favour of the more automatic control for my own machine.

[ericks]

Been using my new diy machine for a while but must admit i have not completed the safety design...too busy with other stuff:)

I should know better!!

[AndyUK]

You are correct - as far as providing a completely safe solution you would require to disconnect the supply. The manual does indicate an electromagnetic contactor (relay) but notes that this should not be used as a general on/off control.

I ordered a relay the other day intending to use it for exactly that function, then I read this in the manual. I'm sure I'll find another use for that relay, so the £5 I've spent on it is immaterial, but whats the reasoning for this exactly? Is it the noise that the VFD is spewing out, or is it more of a surge current thing?

[Doddy]

I ordered a relay the other day intending to use it for exactly that function, then I read this in the manual. I'm sure I'll find another use for that relay, so the £5 I've spent on it is immaterial, but whats the reasoning for this exactly? Is it the noise that the VFD is spewing out, or is it more of a surge current thing?

I wouldn't like to speculate, however, wording from the manual:-

"No contactor should be installed between the power supply and the inverter to be used for starting or stopping of the inverter. Otherwise it will affect the service life of the inverter."

My guess (and it is a guess) is that these have been engineered by an accountant and power-up surges, and inductive EMF stress the power components.

[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.