3 phase variable frequency drive cutting out

[bill.shitner]

Hello all.

A new-to-me lathe has an Allen Bradley Powerflex 40 variable frequency drive to deliver power to the motor. It eats 1 phase 240v mains and produces 3 phase 240v to the motor, with frequency control to adjust speed. The unit appears to have a fault such that it suffers total power loss a few minutes (about 10) after turning on. The power loss occurs whether or not the motor is actually being driven (I discovered this after spending some time wading through the various settings, trying to see whether certain of them might be incorrect).

Advice on a sensible diagnostic procedure would be appreciated. I'm thinking that first port of call should be the mains power to the unit (faulty cable, fuse etc.). After that, I would tend to suspect some kind of overheat on the unit itself, especially if the power loss is reproducibly time-dependent after simply turning on (need to check this). I guess an overheat could be cooling fan failure, fluff and crap in the heat sink etc.

In the remote possibility that anyone has experience with one of these units and a fault of the specific nature described, input would be appreciated.

Cheers.

Bill.

[tabbypurr]

Sounds like you just need to get on & feel the possibly hot things once it's cut out, after disconnecting power & discharging large caps of course.

NT

[John Rumm]

If you then leave it alone for a bit, does it power up again next time?

It sounds typical of a thermal issue... could be fan related. I would probably check the power consumption of the VFD with no load connected and see if that is excessive as a starting point.

[The Natural Philosopher]

Yes. Sounds like thermal shutdown. I've had tonnes of experience of the teensy little examples of these used in model aircraft. Take some DC and run it through 6 HEXFET power transistors in 3 pairs to make a three phase H Bridge, sense the rotor angle from unpowered phases, do some clever magic and drive the phases up to the desired frequency. If it works AT ALL 99% of it is working OK.

I would if its new return it as faulty. I had a TV that did this. Its replacement does not

[Brian Gaff (Sofa 2)]

I did not see whether the originator had a manual or a circuit or description. I mean, is it merely an inverter driving a transformer to derive the three phases from, or something more complicated. The fact is, if its an invertors the low voltage part is probably where the overheating is happening, but I've only ever seen small scale items like this when I could see and tech has no doubt moved on a lot since then. Brian

[bill.shitner]

Thanks for the replies. It took me a while to get round to messing around with the thing in an attempt to fix it. To tie up the loose end and perhaps provide useful info for anyone else with a similar problem, here's what happened:

I checked resistance across the fuse (with agitation of the fuse, in case poor/intermittent connection was the problem). No fault.

I checked the 240v 1Ph mains in lead and picked up an apparent intermittent lack of continuity on the live wire (resistance varied up to infinity with certain wire-bending actions). This line of investigation was confounded when I discovered that one of the leads to my multimeter was dodgy and itself showed variable resistance on flexing. Regardless, I chopped off 6 inches from each end of the mains lead where there were some tight bends in the wire, just to be sure.

I should have tested operation of the device thoroughly at this stage, if I wanted conclusive proof of whether the power lead in was responsible for cutout, but being an impatient type, I moved on to looking at basic and advanced settings on the unit. Turns out that certain parameters were incorrect (frequency in set at 60Hz, motor name plate amps set incorrectly and at different values in basic and advanced settings). I corrected those values and ran the motor. This time, no cutout after a duration of operation longer than any achieved so far, but new error messages appeared, including a checksum error.

I performed a reset to factory values and then re-entered correct voltage/current/frequency, start/stop modes etc. Finally, all is well. I was reluctant to try a full reset at an earlier stage in proceedings because the device's settings are many and complex (160-odd page manual for this unit). However, the last resort seemed to do the job.

Cheers. Bill.

[Brian Gaff (Sofa)]

This of course is what you get when you don't know the history, and who has been meddling with it before you did. Brian