Thread: VFD safety

I've been having an interesting off-line discussion with a member about the inherent safety of VFDs.
The key question appears to be, at what point while operating/working a VFD powered machine do things become critical enough to warrant the VFD being completely switched off.

The assumption being that while an operator is doing something on/to a machine the VFD electronics is reliable enough not to suddenly go feral and switch on a motor.
The specific issue being, is the switching electronics (including any remote logic on-off switch) on a VFD as reliable as a manual switch?


For example, how many of you would completely switch a VFD off while changing a lathe or mill chuck?
What about changing/movng a workpiece or headstock tool /fitting?
What about measurements of workpieces using inside callipers etc
Changing belts, gears etc?
How about other VFD powered machinery and their operations/servicing?


How about something that places you in significant danger like working under a VFD powered hoist?
My take on something like this would be I wouldn’t work under it unless there was a physical barrier of some sort involved anyway.

I was going to make this a poll but lets see how the discussion pans out.

That's an interesting question Bob, I am yet to use a VFD but have one sitting on my desk for my belt grinder project.

I think as with all things machinery it's a judgement call on does it have the ability to hurt / maim you if something goes wrong. I think most us of look for a fail safe / 2nd level isolation if it has that ability, so something more then just a switch re-engaging.

On my lathe for example, virtually anytime I need to do anything with the chuck I hit the big stop button on the front, for the lathe to start unexpectedly it would need to have the stop button re-engage AND the start button would then need to be re-energized AND the switch on the carriage flicked (or I suppose the jog button on the lathe would also move it). This is a minimum 2 things to fails before it would kick back in.

On a drill press tho I'm less likely to turn it off at the wall when changing drills as it's not likely to cause much carnage if it spins up unexpectedly.
Unless they have changed design's or gotten lazy on car hoists the motor is only used to raise / lower it and when its at height a mechanical stop is engaged and the hoist lowered slightly until the mechanical stop engages. Once again this is the 2nd level of safety.

So to answer you question, would I switch off the VFD / put in a second level isolation (if the VFD takes time to start you could have a inline switch between the motor & VFD that physically disconnects the motor from the VFD) on something like a lathe that was driven by a VFD, hell yes
Would I / will I turn off the VFD when changing belts on the belt grinder that will be VFD driven, probably not.

lathe : stop button needs to be reengaged to start
tom senior mill ( still building remote control box) will have reengaged stop button to start
clarkson T&C grinder just press start
john

Interesting thoughts there Bob.
I admit to only ever switching my VFDs off when I finished with a machine. All have a locking OFF switch and a start switch. But they are controlling the VFDs, not the mains power.

Hi Joe,

An interesting question.

If you had any electrical work to do to a machine, then you would turn off and isolate the VFD. Not quite the question you are asking.

To back up a bit, what would be required for the VFD to fail that turns on the output? Either the logic/control circuit fails, turning on the output or the semiconductors in the output stage fail and DC is supplied to the machine. The later fault should/probably/possibly trip some sort of protection device putting an end to that problem, it's also unlikely that most ac motors would do much when dc is applied. Chance of logic/control circuits failing in such a mannner to turn on the output? likely pretty slim. Much more likely that someone will drop/lean/spill something on the VFD controls or remote control that triggers the VFD. So to prevent that probably good practice to secure the control panel/remote where they cant be operated and or operate either an emergency stop or mechanical switch between the VFD and the motor. This is the eqivalent of taking the car keys out of the ignition.

Good question, I had not thought about this before

Cheers

The Beryl Bloke

Originally Posted by Theberylbloke

Hi Joe,

An interesting question.

If you had any electrical work to do to a machine, then you would turn off and isolate the VFD. Not quite the question you are asking.

I don't believe I asked a qurestion. Did you mean "Hi Bob"?

Originally Posted by Theberylbloke

This is the eqivalent of taking the car keys out of the ignition.

While going down a steep hill?
I think it's more like putting the car in neutral and tucking your feet under the seat.....

Yes, I in fact did mean "hi Bob", sorry for the mistake.

So does anyone have any real hands on experience with VFD failures? I have seen one where the internal power electronics developed a short circuit and that blew the protective fuses. Not a fault that would cause the VFD to turn on the output circuits.

I am aware that the requirements for working on 11kV distribution systems has changed over the years. Once you had to have a visual open point (amongst other precautions). Now, not so much due to the increase in fully enclosed gas insulated switchgear. The other precautions remain of course. I am just attempting to draw a parallel with an area I know about. i.e. with the inverter output off it is probably good practise to have other precautions in place to prevent the most likely cause of the inverter turning back on. That cause is most likely human error, unless someone can show a report on the VFD failures. If you turn off the supply to the inverter that is also a precaution and could be argued that it is eliminating the hazard completely as is opening a switch between the VFD and the spindle/machine.

Cheers

The Beryl Bloke

Several of us have - including me - have seen failing or failed VFDs. All of the faults affected the power stages. The result of which is always a LOSS of output (a failsafe mode).
Anyone heard of or had a control module/board failure?

Don't forget the possible interaction with a brake.
My lathe has a spring-applied, power released brake on the motor.

An internal VFD fault would be trying to start against the brake.

But, the more likely fault is a failed control circuit that could release the brake and start the VFD, as described by others.

John

This is my sketch for my Sheraton Cub 600 Lathe (in the project section)

EDIT; //metalworkforums.com/f303/t198...aton-cub-600-a

If you have a look at the Emergency Stop Switch in the bottom left, you will see that I have all the switches being pulled low (ground) when the emergency switch is pressed. There is no floating inputs.

So to get the lathe to run, Power on at power point, release emergence switch, then Start/Jog will make it move.

instruction book mentions not disconnecting the mains when the motor is spinning/VFD running it as if you do there is a significant back EMF that needs to be dissipated. if this is done you may melt something, fiy a board or possibly liven up you whole machine.

I am considering fitting a braking resistor onto my VFD as well to help with this and to stop the chuck quicker. My lathe does not have a brake.

if i'm working on the chuck, I hit the emergence switch. That is it.

Ryan

Safe Operating Procedure's or SOP's as they are called demand that that safety/emergency stop switch be activated whenever one's limb, finger or whatever is in near proximity to rotating items be they a chuck or drill bit etc etc.

I have begun to use the emergency stop on the mill when ever I need to change endmill/tool holder etc etc.
Cant on the lathe as it doesnt have one...yet!!

and knowing that idiots are in abundance on many work sites, having had first hand experience of what some idiot did by putting me in danger, I now always now lock out the main switch and I might only be 2metres away from it.