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VFD low voltage alarm... Can I increase voltage safely?

M

mmurray70

Stainless
Joined
Jan 11, 2003
Hi I have an older Mitsubishi Z300 spindle drive in a Fadal 4020 and im getting an occasional undervoltage alarm while the spindle is ramping up or taking a heavier cut. Only seems to happen when the grid levels are on the lower side (I see about 233-244 single phase in my area).

What should the max DC bus voltage be? Right now im seeing a range of 318-328 with machine idle, depending on grid. I think transformer is tapped for 250v (I was recommenced to set voltage on lower side). Do I have room to safely tap it at 240?

Also, this has been running fine for years and problem seems to be more common lately. Could the capacitors be loosing some of their capacity? What would replacements typically cost for a 7.5kw drive? Anybody have ballpark idea?
 
Looks like I have this problem under control for now. I noticed the light inside the machine would flick for a split second every time the coolant pump started (larger pump then stock). And this was happening while the spindle was still ramping up to speed. So combine this with my poor power supply (phase coverter) this was just enough to throw the alarm.

I added in a delay to wait for the spindle to reach full speed before coolant comes on and now everything seems fine. Just thought I would post this up in case it might help others sometime.

No idea why this only started causing problems lately? Maybe the drive is getting weaker? Or maybe the coolant pump could be pulling even more current on start for some reason? I think I was very close to getting the alarm all along. In any case I bet it will run fine now. Fingers crossed anyway.
 
As capacitors dry out they lose their capacitance. This results in more drop on the DC bus on each cycle.
 
DC bus voltage is 1.414 x applied RMS AC voltage.

Optimally, a 240VAC fed drive would have a DC bus at 340VDC and Dynamic Braking Resistors typically kick on at 360V or higher if measured in absolute scale or at some margin above expected voltage when a line voltage compare circuit is used. Generally a High Bus alarm triggers at 360VDC or above.

Sounds like you solved your voltage sag problem with a delay of start on the coolant pump . . . if you are concerned about low bus faults during heavy cuts - if you have a transformer with taps in front of the machine - you could change taps to increase voltage until you are at 340VDC on the DC bus.
 
By memory, most drives trip at 410-420V bus voltage and start dumping into resistors around 380-390.

You've got room to move upwards but if your problem is failing capacitors, this will only delay the inevitable and the higher voltage will probably further shorten their life.
 
motion guru said:
DC bus voltage is 1.414 x applied RMS AC voltage.
Click to expand...

Kinda - the 1.414 is the peak so that assumes the caps are keeping the voltage at peak levels. Rectified 3 phase DC is 1.35 without caps so running the answer is somewhere in between.......... 1.35 x line voltage is the absolute minimum.
 
Thanks for the help guys. Everything still running fine so far. If problem comes back I will adjust transformer higher or look at replacing caps.

I asked Mitsubishi about replacement caps and they recommended replacing the drive, so guess i would be on my own to source them. What voltage ratings would I need here? Any rule of thumb in terms of capacitance vs size of drive? If I knew ratings maybe I could order some ahead of time before taking it apart.
 
mmurray70 said:
Thanks for the help guys. Everything still running fine so far. If problem comes back I will adjust transformer higher or look at replacing caps.

I asked Mitsubishi about replacement caps and they recommended replacing the drive, so guess i would be on my own to source them. What voltage ratings would I need here? Any rule of thumb in terms of capacitance vs size of drive? If I knew ratings maybe I could order some ahead of time before taking it apart.
Click to expand...

Just open the case and look at the original capacitors. The capacity and voltage rating should be printed on the cases.
 
mmurray70 said:
Hi
What should the max DC bus voltage be? Right now im seeing a range of 318-328 with machine idle, depending on grid. I think transformer is tapped for 250v (I was recommenced to set voltage on lower side). Do I have room to safely tap it at 240?
Click to expand...

AM I thinking sideways here, or should you have the transformer tapped for the voltage you have IOW, 240. The transformers that I am familiar with have a number of taps corresponding to input voltages
 
gustafson said:
AM I thinking sideways here, or should you have the transformer tapped for the voltage you have IOW, 240. The transformers that I am familiar with have a number of taps corresponding to input voltages
Click to expand...

I've heard several people recommend keeping the voltage on the lower side with Fadals to prolong spindle drive life. To get lower voltage at the drive you tap the transformer for a higher input voltage. Also, my drive is Japanese and everything about it in the manuals says 200v, not 240, even though its getting closer to 240. Maybe thats why they suggest this? Im not too sure.

I think if I were to adjust transformer for more voltage it would certainly help with this current problem. But who knows, the whole thing might blow up that much sooner? I feel slightly more comfortable sneaking by on the low side for now based on recommendation of others and the fact that the manuals say 200v
 
It is difficult to not buy Chinese made, but price frequently equals quality, and manufacturers frequently to not overpay, and do not overspec. So if a cap is in a place that sees higher voltages, it is not usually expensive to up the voltage number
 
Hey Guys looks you guys were right. This problem has gradually gotten worse. I pushed it off by delaying start of coolant pump and finally have bumped voltage to drive up 10 volts but you can see this problem is slowly getting worse and worse over time. So most likely bad capacitors as you guys said. Plan on pulling this apart on Monday to see what caps are in there.

Where should I be looking to buy replacements and what brands should I look for and any to avoid? Thanks
 
Pulled it apart today, the drive has 2 capacitors, Nichicon LNR 1800MF and 400v. The LNR series doesnt go to 400v anymore so ordered the LNX series which is similar but longer life. Moved up to 450v rating to get same size case and connections. Hopefully it all works out.

Blocked
 
Seems you have already selected parts, but.........

For this application, the key spec is "ripple current". If you can determine the rating on the original type, and match it with a replacement that also fits, and has the same capacitance and voltage rating, you should be good.

Parts with lower ripple current ratings will have shorter lifetimes. Sometimes "very" short, if the rating of the replacement is dramatically lower than the original.
 
JST said:
Seems you have already selected parts, but.........

For this application, the key spec is "ripple current". If you can determine the rating on the original type, and match it with a replacement that also fits, and has the same capacitance and voltage rating, you should be good.

Parts with lower ripple current ratings will have shorter lifetimes. Sometimes "very" short, if the rating of the replacement is dramatically lower than the original.
Click to expand...

Yeah this is my only machine so had to get new ones ordered asap. The new LNR and LNX Both are rated for "high ripple" but unfortunately I dont know exact rating of the original. New one is 7.6a low frequency and 10.6a high frequency. They are mounted in parallel so would handle double that. I think the max output of drive is less then 30a (its a 7.5kw drive). Hopefully these will do the trick. You think ill be ok?

I considered getting 400v 2200MF as these would have also been direct fit, but they were out of stock.

Untitled.jpg
 
Those sound reasonable, and, as a W.A.G., should do fine. You don't get a lot better numbers per part than that, in general.
 
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