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    Default G0720R Inverter died

    My less-than-one-year-old G0720R benchtop mill died. In the middle of drilling some small holes, I pushed the Start button and got only a small click. Some diagnostic work determined that some of the power transistors on the inverter board had shorted out. I have no clue as to why they died when the machine was under no load or even stopped.

    Does anyone have experience with inverter problems on this machine?

    Grizzly have agreed to send me a new inverter board, but would not tell me when I might get it--including the possibility that it might have to come from China.

    Has anyone swapped out the supplied inverter for another type? Is this sort of failure common?

    Any ideas about what I can do to fix and avoid this problem in the future will be appreciated.

    Dr. Wiggly

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    Default Re: G0720R Inverter died

    Mine died as well. It was a G0720 and it had been in sporadic use for two years when the board died. My mill was already out of warranty so I had to shell out $ 400+ for a replacement. I did try to find another source because I could not believe that a board that had only seen less than 20 hours of use would fail. The motor does exist as a stand alone with its built in position sensor but the inverter board was not available in single quantities. My new board has been working since its installation but I have been wary of using the E Stop switch to stop the motor given that the inverter board failed when I pressed the same to stop the motor. Good luck with your search. If you do find another possibility please post your findings.



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    Default Re: G0720R Inverter died

    I looked for a different inverter that would work with the mill since I also felt that it was wrong for the electronics to fail so quickly. I think that alternate inverters are available, but any of them will likely cost as much or more. More importantly, the Grizzly inverter module has several other functions built into it, so just an inverter wouldn't be a useable replacement.



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    Default Re: G0720R Inverter died

    Dr. What other functions are involved in the inverter? If the wiring is anywhere similar to a G0704, they are just using a copy of a KBMM125. The tach is a separate circuit.

    A lazy man does it twice.


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    Default Re: G0720R Inverter died

    Fastest1,

    You're right that not much other stuff is on the inverter board but the control panel seems to be fed mostly from there. The inverter board connects to the Start, Stop, Tach, and Milling/Tapping controls. Probably most of the work just would be tracing the existing circuits and making a separate circuit diagram for use with a different inverter.
    Another option would be to have on hand the parts to repair the Grizzly board when the power IGBTs go pop. I didn't try that when mine died because I was still under warranty. Analyzing the inverter with an ohmmeter, everything seemed okay except for two of the IGBTs and the fuse.
    Someone could do us all a big favor by tracing the circuits and writing a "how-to" for using a different inverter or repairing the Grizzly one.
    I hadn't seen the KBMM125 before. Looking at the documents on the web site, I don't see how it can be used with a motor that has Hall effect sensors as the Grizzly motor has. Did I miss something?

    Last edited by DrWiggly; 11-25-2014 at 12:44 PM. Reason: add comment


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    Default Re: G0720R Inverter died

    The G0720R has a brushless DC motor, so the KBMM drivers will not work. You have few choices, other than getting a replacement from Grizzly, as a BLDC driver needs to be matched to the motor.

    Sadly, there have been many machines over the last 5+ years that used BLDC motors, and BLDC "upgrade" kits sold for many machines. I can't think of a single one that hasn't been pretty much a total train wreck in terms of performance and especially reliability. A BLDC spindle motor nearly killed Novakon, and did kill Mikini. If it were me, I'd ditch the BLDC motor, and get a good 3-phase AC motor and VFD. In the end, it would probably cost less, work better, and last a LOT longer.

    Regards,
    Ray L.



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    Default Re: G0720R Inverter died

    I was assuming the motor and drive were similar to the G0704's. I went the route Ray suggests with my 12 x 36 when its motor went South. I have quite a rpm range without changing gears.

    A lazy man does it twice.


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    Default Re: G0720R Inverter died

    My problem with the G0720R motor controller continues. The first failure occurred just before the warranty expired and Grizzly provided a replacement. Since then the replacement controller has failed the same way: I have repaired it three times and it has failed every time. After I repair it, it runs for maybe a hundred presses of the "Emergency Stop" button and then fails again. Every time it fails is under the same condition. It runs just fine if I never had to press the "Emergency Stop" button, but since it is the only way to conveniently stop the tool, I grit my teeth and press. Maybe I could turn the speed control all the way down and then turn it off, but before long, I would wear out the speed control.

    Grizzly says they have never heard of this problem, but it seems like Poseidon reports the same failure in his reply. Nice to have some company in misery Poseidon;-)

    Lacking any solution, so far Grizzly tells me that they don't do refunds nor trade-ins, Given that lack of support, I'm dead in the water or doomed to work two or three days and then spend a day and money to repair it yet another time. If any others of you readers know of the same problem, please post it here or elsewhere on the web so Grizzly cannot be unaware of the problem.

    Any ideas for salvaging my $3300 investment?

    In the meantime, if any of you have a dead motor controller lying around, it may have failed in any number of ways, but I will guess that this failure is one of the very common ways it fails. It shorts out two of the six power transistors when the "Emergency Stop" switch is pressed. If you contact me directly, I will provide instructions on how to diagnose this failure and how to repair it. You will need an ohmmeter and soldering skills.

    I hope none of you need to accept my offer.

    Update: I jumped the gun a bit on saying that Grizzly offered no help. I just got a call to tell me that they were sending me a new controller and motor. They think maybe the problem occurs from an interaction of the parts. I will be trying the new parts to see if this fix does it.


    Last edited by DrWiggly; 02-13-2015 at 04:01 PM. Reason: Update


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    Default Re: G0720R Inverter died

    Hi Doc,

    Sorry to hear of your inverter woes. Thankfully, someone else is carrying the Sieg SX4 here in the US(Milling Machines) and at $ 300.00 less than Grizzly. Support from Grizzly is almost non-existent. I even tried contacting Sieg but they told me that I had to go through the distributor which, at that time, was only MTW in Canada and Grizzly in the US. My replacement has been working since its installation but I have always been using the POT to turn the motor down. Do you have a source for the transistors that sit below the main board?

    BTW, is your unit already CNC'd? I ask because the E-stop becomes even more important with the conversion but, given the reliability problem with the inverter board, I have been keeping the keyboard handy for its Escape(Esc) key to stop the automated motion of the table.



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    Default Re: G0720R Inverter died

    I do not have CNC connected.

    See the update to my recent post. Grizzly has not forsaken me! In my discussions with Tech Support, we speculated that since the problem does not occur on most of the machines they sell, and since replacing the controller doesn't fix the problem, some other part of the machine has to make my unit different from most others. The "Emergency Stop" switch operates by simply turning off the AC power to the control unit, so the motor is the only other part that seems suspect.

    The DC motor has Hall magnetic sensors in it to detect the rotor position and switch the power phases as needed. If the sensors were behind or ahead of the rotor a little or noisy or some other way not exactly right, that might do it. I'll post what happens with this experiment.

    The power transistors used in the motor controller are mounted directly on the heat sink and their leads are soldered on the circuit board. They are Fairchild FGL60N100BNTD insulated gate bipolar transistors (IGBTs) and cost about $6.42 from mouser.com or many other electronic parts suppliers. It is important to remove the old solder to clear the holes so that you can slide in the leads of the replacements. I also replaced the insulating film between the transistor and the heatsink just for good measure.



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    Default Re: G0720R Inverter died

    I am happy to report that Grizzly has continued to work with me to fix my G0720R mill problem. They sent me a new motor and motor controller after my most recent plea for help. I don't know whether the motor has been updated since they built my original motor, but the controller board has clearly been updated in hardware and software. Their willingness to supply parts after the warranty period had expired apparently is due to deciding that I had an unresolved warranty problem.
    The new parts fixed my problem. I have operated the mill for several hours and hundreds of operation of the "Emergency Stop (ES)" button with no problem. I suspect that the modifications to the controller are responsible for fixing my problem, but of course, I cannot rule out that they may have modified the motor also.
    For any of you who have a similar problem you could buy a new updated controller, but since it's so expensive you may not want to gamble on it. As discussed in previous posts, a work-around for lethal stops is to turn the speed control down before hitting the ES button. That takes time and will eventually wear out the speed control. A better way would be to add a switch to redirect the speed control output to a fixed zero-speed setting. Then you could just flip the switch to stop the tool. You might also hit the ES button for safety if you are going to be changing tools or otherwise risking your fingers. I was prepared to install this custom fix if the new controller didn't fix my problem, but I didn't have to try it. If anyone wants to try this work-around, let me know and I'll explain how I planned to do it.
    Thanks to Grizzly for sticking with me.



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    Default Re: G0720R Inverter died

    Turning down the rpm prior to hitting the Estop wears something out? I doubt it, or it is such a little amount it is insignificant.

    Also would seem to defeat the purpose of an EStop to require any other moves.

    A lazy man does it twice.


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    Default Re: G0720R Inverter died

    Good to see some help from Grizzly on this model. My three year old 0720 died a while ago and it's been most disappointing trying to get good technical advice from Grizzly. Being in Canada, with the shipping, broker fees and the declining dollar, my new controller cost me $740 to my door. If that would have fixed the problem, I would have accepted it. Unfortunately, after installing the new board, still no action. Next step was to source a new potentiometer, no stock at Grizz till end of March. Got a new one out of the UK the next day from the order date, WOW, FED EX. Installed that part and still no go. Another go-round with the tech on the phone, meter in hand, results, no output from the board, input good. Total frustration on my part by now, got the ok to send the board back for a refund. To make a useable mill out of this paperweight I ordered a 1 1/2 hp 3600 rpm 3 ph motor along with a suitable vfd, and extension cord for the removable control panel. Did the required mods to the cast belt cover to mount the motor and adjust the belt, fab up an adapter bushing and step key for the drive pulley and I'm in business. Overdriving the motor to 80 hz gives a higher spindle speed. Low speed still seems to be adequate for tapping, instant reverse on the motor works great at low speeds. Still have to find out if the controller is compatible with the tapping switches on the quill handles, as it's just a grounding signal to the original control. The mill is useable now, drilled a 1inch hole in some 1/2 in steel without problem, ran a few cuts with a 1/2 in end mill. A spindle read out will be the next order of business, the vfd does give motor speed, a chart would also do the trick.



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    Default Re: G0720R Inverter died

    I did read some notes about G0720R Heavy Duty Benchtop mill from Grizzly on this side and have made the very same experience. At max RPM the chip guard popped open (the little magnet does not work too well) and the mill stopped, 230 V/30A fuse on controller board broken, replace that fuse, when switch system back on ICs on controller board exploded. It appears that any of the safety features (e-stop, limit switched, chip guard) when used can destroy the controller board. There are several notes on this website with similar experiences. This is a flawed and dangerous design in my opinion. By the way I have the second machine, first was replaced as warrantee. Similar issue - controller board, capacitor exploded. Now the warrantee is expired. This machine costs $3000 and weight 600 lb. Grizzly should recall that product. LatheCity - contact me if you like. I am just a customer of that mill running a small metal shop.



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    Default Re: G0720R Inverter died

    Sorry for coming late to the party. I am actually,as of today, a lucky (?) owner of a G0720R and, guess what, hitting the E-switch with the spindle at full throttle was one of my first experiments after I removed that thing from the crate. Didn't burn. Yet!

    After hitting the E-switch the spindle stops much faster than quickly reducing RPM with the potentiometer. A fast reduction in RPM results in a very sudden increase in the capacitor(s) charge and, consequently, voltage. If this increase is fast enough, it will induce (pun intended) both bottom and top IGBTs to conduct simultaneously. This shoot-through current will very readily destroy the IGBTs. If this is what is happening, there are basically two solutions, one easier than the other.

    The first solution is to increase the amount of capacitance following the rectification stage. With the original capacitors kept in place, add in parallel, probably on a second board, a capacitor bank. This way, although the increase in charge remains the same in a situation of sudden spindle stop, the increase in voltage will be considerably less, resulting in a lower probability of causing the shoot-through scenario described above.

    The second solution is only a solution if the board was badly designed. If the current path between gate and emitter is being shared with the main current path between collector and emitter for a considerable length, such as a couple of centimeters, for example, that again may induce both top and bottom IGBTs to conduct simultaneously. In this case, maybe a direct wire connection between IGBT driver and IGBT gate solve the problem. But for this to be minimally effective, the path for this return current would have to mirror the path the current takes between IGBT emitter and IGBT driver.

    Once I fully setup the thing, I'll hook up my oscilloscope and try to observe what happens. Now, it is time to remove all that goo and put the machine on top of the stand.



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    Default Re: G0720R Inverter died

    Congrats on purchasing a great mill.. It really is a nice machine even though it has a major fault. I suppose in retrospect one should always dial down the rpm before turning it off. Mine died a nasty death, still don't know if the motor itself is good or not, no way to test it. Tried a new board and potentiometer with no luck. The 1.5 hp 3ph motor and VFD I purchased from Automation works very good. There does seem to be a lack of low speed torque, although I did drill a 1" hole through a bit of half in plate. The torque boost in the vfd may solve that. Still haven't checked out to see if I can utilize the tapping switches on the quill through the vfd. For normal mill type operations, it does well. It will be interesting to follow your testing and results.



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    Default Re: G0720R Inverter died

    Congratulations on the new mill. I really hope to see another CNC conversion using this mill. It is unfortunate that DRO Pros out here in CA dropped the SX4 from their roster of available machines. They told me that their greatest obstacle in carying this mill was that they could not find a reliable carrier that could deliver the unit undamaged. Pictures of your conversion would be greatly appreciated. It becomes a resource for those following us down the rabbit hole.

    While unsolicited, I will advise you to stay with reputable vendors when you make your choice of which drive motors to use for your conversion. I ended up with three sets of stepper motors because I went cheap on the first two. First set was from ACT with their drivers and the second set was from Longs with their drivers. Both sets had resonance issues and were losing steps with almost any part I tried to cut using CNC. Perhaps if these motors are driven with Gecko drives I may have more luck with them.

    The third set that is currently moving the mill is from Leadshine, the same company Tormach uses for their mills. I opted for their easy servo system that utilizes encoder feedback stepper motors driven by their fully digital drives that tracks the actual motor revolutions as commanded by Mach3. A wee bit more expensive but all the frustrations built up from the use of the first two sets made the expense worthwhile.

    The ballscrews and ballnuts came from linearmotion2008, an eBay vendor. There are some on this forum who have had problems with this vendor but most opinions of this vendor are actually positive.

    Btw, my replacement motor control board is still working. I have since been using the POT to turn the motor off but there have been a few times when I had to press the E-stop because of mistakes I had made during part set up. I still have my original control board and I will be replacing the IGBTs in the hope that it would solve that boards problems. If that works it gives me a spare just in case this second board decides to throw in the towel.



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    Default Re: G0720R Inverter died

    Hi guys. I'm looking to purchase this machine as I'm not a fan of the gear drive set up on the g0755 or the PM-932. Now seeing this tread i am unsure which way to go. This will just be for manual operation and not sure on a CNC version as of yet. What are your thoughts?



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    Default Re: G0720R Inverter died

    Quote Originally Posted by Fastest1 View Post
    Turning down the rpm prior to hitting the Estop wears something out? I doubt it, or it is such a little amount it is insignificant.

    Also would seem to defeat the purpose of an EStop to require any other moves.
    Yes the problem with when they hit the Estop while the motor is at speed,this will send regen voltage back to the drive & can blow the IBGTs

    If someone was to make a camp to stop this, then the problem would go away, something like what is in the PDF would be needed to fix this

    Attached Thumbnails Attached Thumbnails G0720R Inverter died-regenclamp_920-0024-1-pdf  
    Mactec54


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    Default Re: G0720R Inverter died



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