Is my VFD a doorstop?  - Page 1
Forum section: electronics / repair

author: Eire
author_rank: Newbie
timestamp: October 18, 2018, 07:39:23 pm
content: Hi all,I dun blown up my VFDBackground;I recently bought an ancient 2HP 3PH Milling machine, and bought a top shelf industrial quality 1.5kW VFD from Ali Express which has since exploded.It was powering the mill fine until one day it tripped the 40A breaker on the garage circuit as the motor was slowing down (me having pressed the stop button on the vfd). Subsequent attempts to power it up immediately tripped the breaker. A few days later I've measured the motor windings, they all have the same resistance and show open circuit to ground. After some googling to check if it was safe, I disconnected the motor from the VFD and powered it up off mains again. It lit up, display came on and all but about 5 seconds later there was a substantial flash, bang, magic smoke, and after replacement underwear I took the cover off to find a blown cap.Am I right in thinking this cap is to smooth the rectified mains input before feeding it into the inverter circuit?Given that there was no load connected, what are the odds that everything else is okay?Following this, should I try replacing the cap or just scrap the unit and buy another?I've got a multimeter but no scope, so testing is limited I guessPhoto here: I can't compress it on my phone it seems:https://drive.google.com/file/d/1z2sZUYG1vY6B0eV_awPqTLK3kbSsuIof/view?usp=drivesdkThanks in advance

author: taydin
author_rank: Frequent Contributor
timestamp: October 18, 2018, 07:52:35 pm
content: That cap seems to be the filtering cap that comes after the full bridge rectifier. Most likely the bridge diodes before it are shot, which will result in that cap receiving reverse polarity voltage 60 times a second.

author: janoc
author_rank: Super Contributor
timestamp: October 18, 2018, 07:53:36 pm
content: Quote from: Eire on October 18, 2018, 07:39:23 pmJust ... lolQuote from: Eire on October 18, 2018, 07:39:23 pmThe odds that the unit is OK and it is just the capacitor are pretty much zero. The first case when it tripped a breaker and then didn't power back up are indicative that something has cooked inside.The cap only blew up as a consequence of the previous problem and you trying to power the device up - it is very rare for a capacitor that has been fine before to explode "unprovoked".BTW, your google drive is not public, so I can't look at the photo.EDIT, now it worked OK.If your VFD was less than $100 or so, just trash it and buy a proper one from a reputable vendor, not some cheap Chinese special from AliExpress. These devices have to handle a lot of power and high voltages - you don't want a cheap piece of junk to burn your shop or house down.

author: ignator
author_rank: Regular Contributor
timestamp: October 18, 2018, 08:20:25 pm
content: Eire; I'll second what Janoc quoted as a big belly laugh "top shelf industrial quality 1.5kW VFD from Ali Express".I had the same experience with a 3Kw unit from eBay a few years back. Main capacitor explosion. They sent me a replacement which sits on the shelf. They don't use good enough components and there is no derating.I've purchased many name brand VFD from eBay. Just make sure they are 200 volt class, unless you have access to 440volt mains. They come in 200V and 400 volt class, and will trip out on undervolt for home use, if you get the 400v intended operation unit.

author: Eire
author_rank: Newbie
timestamp: October 18, 2018, 09:46:48 pm
content: Thanks all, glad we've gotten a laugh from itI'll keep an eye out on eBay for used branded stuff; there's obviously a wealth of the same shyte as the first one available for cheap but I'm assuming they would probably die a similar death. I've had some decent 'dumb' tools from aliexpress like collets / drills / cutters etc which have been fine so I suppose it's just a matter of learning what's useable and what to avoidThanks again

author: Mr.B
author_rank: Unknown
timestamp: October 18, 2018, 10:06:53 pm
content: I came in here wondering how big a Vacuum Fluorescent Display would have to be to qualify as a doorstop…Oops, wrong VFD.

author: jmelson
author_rank: Super Contributor
timestamp: October 18, 2018, 10:40:54 pm
content: Quote from: Eire on October 18, 2018, 07:39:23 pmDo you have a braking resistor connected? You really need it on machine tools, as the inertia of the motor+spindle is substantial. Some cheap VFDs do not have provisions for this.When slowing down, the VFD extracts energy from the spinning mass, and the DC bus voltage surges. Usually the VFD will trip off with an "OV" error message to protect the transistors. But, the main DC link cap also takes a beating every time you stop.You can get replacement 6" stove top burners on eBay that are just about perfect as braking resistors. In fact, the Haas company uses exactly that on their machine tools. Just make sure the replacement VFD has provisions for external braking resistors.If it trips the breaker with the motor connected, it almost certainly means that TWO transistors on different half-bridges are shorted. Measure from the main cap + and minus to each of the output terminals (U, V and W). If you get a low resistance reading on any of these connections, there is a popped transistor.Also, you might have a warranty on the old unit.Jon

author: janoc
author_rank: Super Contributor
timestamp: October 19, 2018, 12:09:58 pm
content: Quote from: Eire on October 18, 2018, 09:46:48 pmAliExpress is good for buying cheap tools but you often get what you pay for. With electronics be careful, especially with anything meant for high power - there are a lot of products that are literal deathtraps there, waiting to electrocute you or to explode in your face. Under-rated components, poor quality components, often counterfeit components, ignorance of all common sense safety standards, if it has firmware, the firmware was written by some monkey in a jungle and not by an engineer, knock-offs of knock-offs with increasingly worse construction, etc.It is possible to find decent products but be very careful about doing your homework - look for reviews, ask around whether someone else is using that specific model and don't buy things nobody seems to have hands-on experience with. Also before plugging anything in I will take it apart and inspect it for any obvious problems - like the Atten hot air stations where some came miswired with the metal nozzle live instead of it being grounded.And, even better, if it concerns mains, just buy the device locally from a reputable vendor where it needs to comply with your local electrical standards, you get a warranty on it and you also get to sue them if it burns your house down. You will pay more but you likely won't have these problems.BTW, eBay is not any better - usually the same sellers are in both places and you will find also people who import stuff from AliBaba/AliExpress and then resell it for inflated prices on eBay.

author: Eire
author_rank: Newbie
timestamp: October 19, 2018, 07:01:57 pm
content: Thanks for the tips all; I've got some manual tools/consumables off Alibaba but lesson learned on this one!!Quote from: jmelson on October 18, 2018, 10:40:54 pmI had actually ordered an Allen Bradley VFD used off eBay, then read this & discovered it doesn't support braking resistors & the seller contacted me to say he checked the parts before boxing & the ad showed the wrong power rating, so we've cancelled the saleI'll keep an eye out for one with a braking circuit in.On that note; is it possible for the VFD to allow the motor to freewheel to a stop (I think this could be achieved by just opening the U,V,W IGBT's and holding them open?) without using that interia to charge the caps? The Alibaba VFD was definitely doing some braking as the mill seemed to slow significantly quicker than one would expect if it were freewheeling, and it definitely doesn't have a braking resistor so that's probably not done it any good.As a matter of interest, the mill in question is almost identical to that in the attached photo, so as you can imagine there's a significant amount of inertia in the motor/driveline/spindle when it's going full tilt. It's a pretty intimidating piece of kit to be standing beside!!

author: ignator
author_rank: Regular Contributor
timestamp: October 19, 2018, 09:41:17 pm
content: Quote from: Eire onTodayat 03:01:57 pmThat is a nice looking vertical mill. Heavy duty from a structural standpoint.The VFD comes with acceleration and deceleration times with default values. I think they are about 5 seconds. There is a program setting to enable freewheel operation vs controlled deceleration.You need to look at the manual to see all the programmable features.One feature I use is the LED display shows spindle RPM vs Hz. Again this is a setting, and you have to key in the reduction ratio of the motor to spindle in one of the settings. I leave the belts on my mill set to a middle setting, and as the motor is ~1800 RPM, I enable 120Hz max operation, giving a very wide range of speed control.