VFD failure modes...?

[RG Vieira]

Looking at used/surplus VFDs... because I'm a farmer that can't afford new :-). Provided everything is sized & hooked up correctly, what part of a VFD is most likely to eventually fail? Would it be good to oversize the VFD (vs HP of motor) so it isn't working as hard? By how much?
Looking to possibly have "likely to fail" spare parts on-hand.
Appreciate your thoughts!

 

[Billtodd]

Not sure what sizes you been looking at but many smaller ones now use a rectifier/IBGT module that , while available, is almost impossible to swap without board damage (BTDT) .

Of the last two units that I could fix, one required a board swap (happened to have a dead unit with working controller board ) and the other , surprise , surprise, was a cheap and cheerful Chinese unit that still used discrete and readily available, parts .

 

[RG Vieira]

Appreciate your reply. Sounds like the IBGTs, or support circuit around them, can get pushed pretty hard.
I'm looking for several sizes.
A 10-15 HP unit to drive a 7.5 HP air compressor & small Bridgeport (3/4 HP).
One for a 40 HP irrigation booster pump.
One for a 75 HP irrigation booster pump.

BTW, the 40 HP VFD will need feedback from a remote(wireless...2,500 ft range) pressure gauge & adjust pump speed accordingly...Need to figure out how to do that also as existing solutions I've found integrate into an expensive (& too complicated) Modbus TCP/IP gateway...

 

[Clive603]

The inevitable failure mode is the capacitors dying. If nothing else breaks first the capacitors will simply run out of life.

Start - stop cycles, heavy load changes, short term overloads and even frequent speed changes are the things that nibble away at lifetime. Moderate oversizing means larger capacitors that don't need to work so hard and will cope better with start - stops and short term overloads but if the device has always been used decently within its parameters it won't make much difference.

I guess best life is for something that sits in a dry, not too warm, not too cold environment running steadily at close to utility frequency putting small speed changes on a motor of about 2/3 rds its nameplate rating. Hardest duty is probably frequent stop starts on compressor duty close the motor power limits so the boost function has to be selected. Extra de-merits for being bolted directly to the compressor living the common outside semi sealed shed / hutch thingie subject to all outside temperature and humidity changes

Realistically best life for a second user means finding something that hasn't been abused or worked too hard. Then keeping it dry and cool. Lucking into something that has sat on the shelf for years means following the instructions on how to re-form the capacitors if you expect it to last. VFDs really need to be run up every month or three to stay healthy.

Clive

 

[DDoug]

One for a 40 HP irrigation booster pump.
One for a 75 HP irrigation booster pump.

BTW, the 40 HP VFD will need feedback from a remote(wireless...2,500 ft range) pressure gauge & adjust pump speed accordingly...Need to figure out how to do that also as existing solutions I've found integrate into an expensive (& too complicated) Modbus TCP/IP gateway...

You want to buy used junk for this application ?

 

[dalmatiangirl61]

Looking to possibly have "likely to fail" spare parts on-hand.
Appreciate your thoughts!

What is your skill level of diagnosing and repairing electronics? These are not water pumps or engines. I've parted out a few of them, but you would really need to know what you are doing to diagnose the individual components to be able to repair one.

 

[RG Vieira]

Electrical engineer...average technician so not huge confidence there, but...

 

[Joe Gwinn]

Electrical engineer...average technician so not huge confidence there, but...

To work on a VFD, no part of which is near enough to ground (power neutral or safety) to prevent explosions if one tries to attach the scope ground clip anywhere, you will need an isolation transformer, and/or isolated differential probes capable of full peak rectified voltage plus some headroom. A circuit diagram wouldn't hurt either.

 

[RG Vieira]

Maybe that was rhetorical, but looking for good, used at 50 to 75% off. Farmers tend to be pretty resourceful that way... especially when very little of what retail consumers pay makes it way back to the farm...but I digress.

 

[RG Vieira]

Understood. Thank you!

 

[RG Vieira]

Thank you, Clive.
Lot of great info here!

 

[crickets]

Sounds like you may have a few applications for just phase conversion, and maybe one or two for actual speed controls ? But also looking at the numbers you've provided, it doesn't sound like you'll be running that on a standard residential power. I mean 75HP alone is 56kW, which exceeds the capacity for a standard residential service. So what's the deal ? Are you on a commercial (3 phase?) service ? Off grid (that would be some massive inverter... or a diesel gen) ?

 

[JST]

EE also, designer of VFDs at the last employer.

1) Most VFDs are N.O.T. designed for component level repair.

2) dead IGBTs almost always means dead driver chips. Count on replacing them just because.... don't even test unless you replace them... I've repaired enough VFDs in development for various causes to see that. The shorted IGBT usually melts a spot on the die, which makes a gate connection, shorting V+ to gate (and driver).

3) Sometimes the failure "reaches back" through the driver to damage the control uP, or burns holes in the board, in which case you have a "problem".

You can check them in operation, so long as you have a 'scope that has isolated inputs. HP has made portables that do, and Fluke Scopemeters do also, most isolated up to 600VAC.

It's a very good policy to check gate waveforms, to make sure that there is good drive, with a sharp rise to the "charging plateau", and then a rise after that to well above the "plateau". Slow gate drive will cause another failure. You want 100 MHz to see that.

Overall, it is actually pretty reasonable to replace entire PWBs, or drives, and not parts. You can have more confidence in a unit that has not already failed and been repaired, especially since newer SMT parts are a real nuisance to replace. Many of them have the terminals underneath, inaccessible. You need a hot air soldering wand for those, and reheating boards increases failure rate.

If you can get a deal on a bunch of drives of one model, it may be worth it, if they all do not have the same problem. You can often mix and match PWBs to get working units. (watch out for potentially incompatible revision levels).

You can, but do not have to, match VFD output current to motor current (VFD a bit higher). If the discrepancy is too high, however, then the VFD may not have sufficiently fine control of current etc at the motor current level, so that you cannot protect the motor.

 

[RG Vieira]

Crickets--
The 75 & 40 HP motors are on ag service 3-phase.
The other is on residential, 2-phase 220V...but driving 3-phase 7.5HP & 3-phase 3/4HP motors (so yes, phase conversion).

 

[RG Vieira]

JST-- Thanks so much! Really good info. Your past experience is exactly what I was hoping to learn from, here.
Board/module level "repair" is fine by me. Appreciate the isolated scope probe & waveform insights!

 

[Steve in SoCal]

I have actually bought and used both 40 & 75HP inverters off ebay. These were purchased years ago but, they were both good and continue to function at a light usage.

The prices today are much higher, I paid 400.00 for the 40 and 1000.00 for the 75, the 75 was NOS the 40 vintage mid 1990's ABB the 75 is early 2000's. ABB

Steve

 

[RG Vieira]

thermite-- Thanks very much for your insight!

 

[RG Vieira]

Steve in SoCal-- Appreciate your experience, here.