Fuji VFD shutting down-special installation
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    Default Fuji VFD shutting down-special installation

    About 4 years ago I bought a Fuji FRN025G11 VFD on EBay new in the box. I wanted a quiet phase converter so I purchased a DV/DT filter out of a Liebert UPS. I added a DC choke and additional capacitors in the DC circuit and it worked great until now. I have run Hurco and Centroid machines as well as manual lathes. Everything was great. I purchased a Kitamura with a 15M control and hooked it up. More times than not when the spindle shuts down during a tool change or just manual operation the VFD shuts down. It throws a OV3 alarm, meaning high DC voltage during running. I tried running the lathe to absorb some of the counter EMF if that was the problem.. I can add braking circuit fo a couple thousand dollars, but I do not think that is th real problem. Looking for ideas. Put your thinking caps on and thanks for any input.

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    What is the overvoltage set to?

    What is your line voltage during operation when it shuts down?

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    I went down this morning and started things up. The single phase line into the VFD is 243, a little over Max per manual. The line out of VFD is 231 balanced. The DC buss voltage is 326. There is not a parameter to adjust the line in max or the DC buss max. They are preset at factory. Those numbers were without spindle load. Started spindle at 500RPM, DC voltage is 327, AC in is 243 and AC out is 230 balanced. I stopped the spindle and it did not trip. Like I said, it doesn’t drop out every time. My shop is fed with 480V single phase, believe it or not. I have a transformer on each end of the feeder so I could change the taps to get the line down a little. Right now I am running a short program with 6 tool changes to see if it quits.
    Bob

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    A quick update. I just went down to the shop and everything was down. I had the Fluke recording the DC buss and it tripped out at 408V. The manual says the trip is set to 400, so that is the problem. I think I am going to change the transformer taps and get the line voltage down a bit. It might just get the buss voltage down enough to stop the tripping.
    Bob

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    Another update: Changed the raps on both transformers to lowest tap. Line voltage at shop is now 226v. The DC buss voltage is 301v. Tried my program and on third tool change it all went to hell.... DC buss tripped at 408.6 volts. I am guessing the machine is kicking it back, but I do not understand how I get CEMF from the spindle drive in the machine.

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    When/if the drive slows the spindle, the energy has to go somewhere, and that somewhere is back in the buss, raising the voltage. It is not certain that you have that problem, but it sure looks like it, since the tool change would trigger a slow down first.

    What is your decel rate set to?

    If you can, without messing up the program, make the deceleration rate less.

    Otherwise, with 480 input, it is likely that there are some "industrial loads" elsewhere in the system you are connected to. Disturbances on that system due to load-shedding may cause short periods of high voltage.

    But the most likely at the moment seems to be the decel rate and energy dissipation. It may be added to a high mains voltage.

    The other alternative is to add a "braking resistor", which will dump the excess energy (bus voltage returned current) to an external resistor. It shoud not cost thousands, unless your unit has no provision to add one. Many fairly low cost units do.

    the downside is that a braking resistor can dump returned spindle energy fine, but is not the tool to use for a mains voltage surge. It will not work for that, and may easily be burned up if the surge is too long.

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    There are no industrial loads. I built my shop on an RV pad that was 180’ from my house. The conduit to supply power to the RV was only 3/4”. I stepped up the voltage at the house to 480 volts ran #4 copper in the conduit and stepped it back down at the shop giving me 200 amps at the shop. Fuji makes a braking resistor kit for the drive. Because it is a 25hp drive it has to be installed outside of the enclosure for heat. That kit less the resistor is about $1400. It monitors the buss voltage and clamps it thru the resistor in a self contained package. I might try to figure out how to build one, but I just hope that will cure my problem. I wonder if I can install a braking resistor in the Fanuc control at the spindle drive???? I wonder if that will suppress the back EMF?

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    You cannot suppress the back EMF. It is a fact of physics, energy that has to go somewhere.

    What you CAN do is:

    1) Reduce the decel rate, so that there is more time for the control power supply to consume the excess energy without letting it go over voltage.

    2) Add a separate system to take the energy. $1400 for an external unit is pretty high, I assume there is no internal switch for the purpose, as in many 1HP to 10 HP drives.

    3) In some drives, you have a range over which you can adjust the trip point. Yours apparently is not one of those.

    4) Change from controlled decel to coast-down, if a slow enough controlled decel is not available. You may have an option of DC braking for at least part of the decel, to make it less time to stop.

    Seems as if you need to look at #1 and #4, as the others appear to be unavailable.

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    Hi JST, I really appreciate you working with me on this. This morning I took my Fluke 125 Scope meter and recorded the dc buss and it pretty much showed what my other meter was telling me. I put the scope on the AC out of the VFD and when I shut the spindle down there was a fast transient of off the scale current with a drop to zero volts. Basically killing the VFD with the pulse. Another item I discovered was this only happens above 1000 RPM on the spindle. I found a dynamic brake module on Automation Direct for $200. I am going to call them Monday and see if they have specs as to how fast it reacts. The last question I have is, do you know if I can slow the decel in the parameters of the Fanuc spindle drive? The spindle does stop like an emergency stop every time it stops. It goes to a dead stop in a split second. Have a great weekend.

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    Quote Originally Posted by flylo31 View Post
    Hi JST, I really appreciate you working with me on this. This morning I took my Fluke 125 Scope meter and recorded the dc buss and it pretty much showed what my other meter was telling me. I put the scope on the AC out of the VFD and when I shut the spindle down there was a fast transient of off the scale current with a drop to zero volts. Basically killing the VFD with the pulse. Another item I discovered was this only happens above 1000 RPM on the spindle. I found a dynamic brake module on Automation Direct for $200. I am going to call them Monday and see if they have specs as to how fast it reacts. The last question I have is, do you know if I can slow the decel in the parameters of the Fanuc spindle drive? The spindle does stop like an emergency stop every time it stops. It goes to a dead stop in a split second. Have a great weekend.
    I do not know. Check the manual, if you have access to it. It may have a fast stop pre-set and depends on timing past the stop time to be sure it is stopped before attempting a tool change.

    With the brake module, be certain to set it so that it will not be activated by an actual power line surge. You cannot stop a line voltage change with it, only a surge of limited energy and duration that is from a transient condition in the unit.

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    Many drives have a parameter to pause breaking until the DC bus voltage is back within limits. This should stop the tripping (provided it can react fast enough - you may need to adjust the ramp rates and the curves for how fast it starts to decelerate), but will slow down the braking.

    More capacitance on the DC bus will help.

    It looks like there's no comms between the drive and the braking unit, so you should be able to use one of these and a suitable resistor capable of sinking 40-50A at 400VDC.

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    The VFD is not braking, it is being used as a 3 phase power source, so when the machines drives shut off the back EMF causes the over voltage spike. The primary means to dissipate the energy if the machine drive deceleration cannot be adjusted would be a braking resistor/module. It may be that decreasing the input voltage might give a small margin of headroom.

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    Quote Originally Posted by mksj View Post
    The VFD is not braking, it is being used as a 3 phase power source, so when the machines drives shut off the back EMF causes the over voltage spike. The primary means to dissipate the energy if the machine drive deceleration cannot be adjusted would be a braking resistor/module. It may be that decreasing the input voltage might give a small margin of headroom.
    Ok, maybe this needs a bit more clarity......... 'cause I am getting a little confused.

    The 3 phase source is separate from the machine, and is shutting off when the machine stops the spindle?

    Do the machines have their own internal variable speed drives, or are they single speed direct-driven?

    If they have drives, do these machines back-feed the AC line to dissipate their stopping energy?

    If they do, then that is a problem, and solving it may involve the other machines, or installing a different 3 phase source. VFDs cannot handle returned energy on their output, unless they control it themselves.

    A Phase Perfect can handle it "perfectly", and an RPC can handle it to a substantial degree.

    If they were direct driven, then stopping would be by disconnecting the motor, an action which would not have a back EMF issue.

    What power are these machines? That model number did not produce any hits.

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    I an sorry for being away but I got busy and had to let the machine set for a while. To try to clarify things, the VMC is a Kitamura 3X and it does have its own VFD with braking resistors for the spindle, which is working. That is my confusion, the VFD in the VMC should be suppressing the back EMF with its braking circuit. I am looking into a Phase Perfect and I am sure that it will solve the problems. I was hoping to make my existing phase converter work, but I guess that might not be possible. Just a little more clarification; the spindle on the Kitamura is a 20HP and the Fuji drive is a 25HP. I did install additional filtering and huge amount of capacitance on the Fuji’s DC line to prevent problems like I am having. But all for not.... It also was tripping during a tool change so no load on the spindle motor. I guess it’s time to save a little $$$$$$ for a Phase Perfect.

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    Reading this again, it sounds like you have one VFD (the Fuji) acting as a dumb phase converter, producing constant 3 phase from mains.

    You have a second VFD (the Kitamura) which runs off the three phase supply from the first drive, and does not trip.

    Are you sure the Kitamura uses a braking resistor, and is not a regenerative drive? If it's trying to return power to the grid, you'll see that.

    It could also be that the load transients from the Kitamura are too abrupt for the Fuji to handle. A DC link reactor or line filter may actually make this worse as it keeps pushing current into the drive once it stops drawing.

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    Interesting idea Someonesomewhere. It has a braking unit and two large resistors, so it is not regenerative. Interesting idea about my link reactor making it worse. I will try to get around to jumping it out today and see what happens. I thought the extra reactance and capacitance would be a good thing, maybe I am wrong.... I think your last point about the load transients is the root problem. Maybe it is Phase Perfect time because it should be able to absorb the transients.


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