VFD Loud bang.

[JST]

I like your idea of pull out the VFD and have a look. I'd have probably already done that.

No telling what you may find............. I've seen many things that I would not have believed except they were right there when I looked.

What you find may point out the solution.

BTW, your original post said the sound seemed to come from the motor, not the VFD. So the motor may be worth a look.

 

[johansen]

The max voltage you will get during regen for a 480v drive is as high as 1000v, and yes, you may kill the drive if this continues. The current isnt just fed by the motor, its the vfd during slowdown. Once it switches from regen to dc injection, the peak dc voltages will fall from the dc bus brake voltage (800vdc approx) to 600 volts (415vac *1.41).

The motor is savable. Wash and scrub out wherever is arcing and pour a coat of polyurethane or regular clear 5 min epoxy in the winding.

How far apart the wires are doesnt matter with regard to the thin layer of carbon and copper metal oxides deposited everywhere.

 

[SomeoneSomewhere]

Any short loud enough to be considered 'loud' over a spinning lathe will leave blast marks and pitted copper, as well as destroy the drive. Guarantee this isn't happening.

A 240V VFD will have a DC bus running about 330VDC normally, lifting up to ~400VDC under braking conditions, and the drive likely uses components rated for 450VDC continuous, that will handle 600VDC for a few milliseconds happily. Double these figures for a 480V drive.

Braking creates a slowly (in electrical terms) rising DC bus voltage. The motor doesn't really see anything different electrically during braking vs acceleration at a given speed, just the sign of the slip changes.

I agree that you:

• aren't using the lathe in the way it was designed - the original three-phase native setup will not have had electric braking, though it might have had electrically released mechanical brakes, or just a straight mechanical brake.
• are probably finding that something in the gearbox is not meant to see that sudden a torque change.

 

[JST]

We have not been told what voltage the machine runs on. I would think that if it was 480, that would have been mentioned, but..............

The mention that the noise seems to come from the motor is of interest and tends to support a mechanical cause, but does not prove it.

 

[johansen]

It is not definitely not mechanical. It is also fairly random. I'm running 3 phase 415 volts so I am up there. By the way, it is roughly 1000v per mm for free air. There is also a smell when it happens. Not sure if it is ozone or a burned component but the VFD and motor still work. Maybe there is a MOV in the VFD that is dying. May pull the VFD out and have a look.

415v means we should assume a 480v rated vfd and so the dc brake voltage will be in the 550vac*1.4 territory. Unless they make auto sensing vfds that can distinguish between 415 and 480v nominal or its a euro specific default programming.

 

[SomeoneSomewhere]

Even configured for 415VAC, almost all drives I've seen aim for a peak DC link voltage of ~800VDC, or about 565V AC equivalent.

No 415V industrial equipment should have clearance less than ~3mm, and creepage is normally about double that. The terminals in the motor should be spaced by at least that amount, and the windings themselves are insulated - you need to look at the punch-through voltages for that, not the clearance/creepage.

https://blog.se.com/energy-management-energy-efficiency/energy-regulations/2018/06/06/clearances-and-creepage-distances-in-lv-electrical-switchboards/

The motor electrically simply doesn't see any higher stress under braking than under acceleration. The VFD isn't tripping, so it's not a DC link overvoltage. You've almost certainly got a mechanical issue.