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Hazard mitigation related to a VFD

Hazard mitigation related to a VFD

Hazard mitigation related to a VFD

Last night I got called to a slicer machine with about 20 knife blades.

The complaint was it kept stopping randomly with the random running from a minute to about 6 months.

Luckily it had dropped during the day down to staying off.

It was thought that the control cabinet was too hot as it was pretty roasty hot so filtered air was fanned thru it. That seemed to work for many months before it was back. They'd taken to opening the cabinet up, which required the disconnect to be opened, and then they'd push the RESET button on the unpowered VFD, close it back up and it would run for a while before checking-out again.

Turned out one of the wires from the operator panel speed control pot where it entered the VFD control terminal block had broken every (probably) non-annealed strand of its crappy wire except ONE. That one was the center strand in cage-clamp bunch of broken wires. This allowed temperature shifts, vibration, machine bumps, VFD Reset button presses, hell, probably moth farts, to connect and disconnect the speed signal.

Anyway here's my question:
This disconnect would feed the VFD a 0.0 speed command and it would decelerate down to stopped looking stopped! Of course workers might do something thinking the system was "stopped" but a bump would suddenly feed it a 57.7Hz signal and it would take off!

Got any ideas how to prevent this behavior? Something that could see the speed signal and would actually put the VFD into stop or ??.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Hazard mitigation related to a VFD

I haven't worked with drives in a while, so these may be infeasible, but I thought I'd throw them out there anyway. How about:
  • putting a ferrule on the wire?
  • installing wiring with a service loop or other type of strain relief? That wire looked under strain.
  • using a signal with a live/elevated zero like a 4-20 mA/1-5 V/2-10 V and triggering something that allows the drive to hold last speed when the signal falls to zero?
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RE: Hazard mitigation related to a VFD

I like the 4-20 ma suggestion xnuke.
If you are fortunate, many industrial 4-24 ma controllers will detect a 0 ma signal as a fault or a circuit failure.

"Why not the best?"
Jimmy Carter

RE: Hazard mitigation related to a VFD

Set a minimum speed is the VFD? Otherwise, look at 4-20mA because the VFD can likely be set to trip on it going below 4mA.

RE: Hazard mitigation related to a VFD

Great stuff! This is a 0-10V pot so the 4-20mA angle would require some changes. The 'Set Minimum Speed' is perfect! In this case it would've dropped to the minimum and not stopped so the failure mode would not get mistaken for 'OFF' mode. It would've also screamed what the problem was seeing it drop from 57Hz down to 10Hz then running back up. That would've gotten all the things out of the troubleshooting matrix that could cause a "STOP".


Keith Cress
kcress - http://www.flaminsystems.com

RE: Hazard mitigation related to a VFD

In a lot of VFDs, you can program the behavior on loss of speed reference, so it can "Hold Last", Alarm, Fault etc.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: Hazard mitigation related to a VFD

Yes, but you can't detect if 0V is due to a loss of reference or simply that the reference has been set to 0V.

RE: Hazard mitigation related to a VFD

LionelHutz just stated a very good reason to use an analog signal with built in possibility of an open vs a zero speed signal. 4-20mA is one possibility. Some drives have select-able analog input, and detect out of range low and high.

Personally if I was putting my fingers into this device, I would want the power supply locked out (padlock and red tag), and then dog the rotating parts to make sure I did not have the possibility of a rotating inertia problem. Relying on the operating controls for a safety stop is not wise, as the operating controls usually are not designed for that. Purpose designed safety stops are better, but still deserve a careful look first.

RE: Hazard mitigation related to a VFD

Most VFD speed references are your typical control panel pot. Of course pots are notoriously crappy and down-the-road end up having open spots worn in their windings/traces. Setting a minimum allowed speed will save the day on those. A clapped-out pot, instead of the motion going to zero on an open, will instead slow to something but not stop.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Hazard mitigation related to a VFD

Sounds like a training issue. Workers should never "do something" without performing a proper LOTO.

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