Fanuc VFD acting up on CNC machine. Chasing down culprit.

[fastline12]

This is a full Fanuc VFD/motor unit for a spindle. The spindle is high mass and accelerates/decelerates constantly. On slower RPM accelerations, I can hear pulses of acceleration from the spindle rather than smooth. Once the motor is running around 2000rpm, it runs smooth through RPM changes.

While running, I tested the incoming voltage to the VFD to detect voltage drop. It fluctuates a bit but mostly stable. Nominal voltage should be 220V and my voltages are running 234, 231, 236. Those numbers will drop about 5-7V during acceleration.

However, I decided to test the frequency on the input side of the VFD and seeing frequency all over the map. spikes to 70hz and lows down to 20hz.

The motor is rated for 28A cont at 200V (Jap). I am running a double motor rotary phase converter that I did balance very well with caps. You can hear and see them shake during this hard accel issue. The sizing for the converter system is 20HP. This is all fed with a 25KVA utility transformer that is shared so I feel certain the transformer is being over driven but that usually causes voltage drop, not frequency issues?

During these slow accelerations, you can almost hear those IGBT gates going to 100% duty trying to get the spindle up. I am not sure if the slower speed of the motor and inherent very low VFD output frequency is amplifying the issue?


Any thoughts, ideas, or system checks we can do would help. I am not sure that my frequency checks are reliable as the harmonics on the line could do that. Might be time to drag the scope out.

 

[waross]

From the little information given it looks as if the motor is around 10 HP to 12 HP.
You may have been better advised to use an oversized VFD and run it on single phase.
A 1760 motor would be tracking the frequency changes.
At 20 Hz the motor would run at 560 RPM and at 70 Hz the motor would run at about 2060 RPM.
You can probably forget about frequency changes on the input. Whatever you are measuring it is not the line frequency nor the rotary convertor basic frequency.
Did this system ever work properly or is it a new system?
How about full specs for both the motor and the VFD.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Line voltage and line frequency is going to be completely irrelevant to the output frequency stability of the drive as Bill said. Don't waste your time on that.

I'll reiterate. Is this something new happening to a system that has been working fine, or is this newly installed? Makes a big difference in what to look for.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[fastline12]

I apologize for any missing info. This is a complete machine so simply swapping the VFD to something else is not an option. This machine is working and running right now. I am simply trying to determine why the spindle seems to accelerate in pulses and not smooth at lower rpms and if this should be ignored as a function of the very low frequency output of the VFD or if we should look into this further.

The drive and motor are somewhat proprietary by Fanuc. They don't publish certain things but the motor is a model 18P.

The utility transformer is a 25 kva and shared for other loads and is being loaded beyond 100A. I am not sure if that transformer could be the source of this issue?

 

[jraef]

Again, HAS it been working fine and just STARTED this behavior, or has it been doing this from day one?

If it has always been doing this, it may be as simple as someone not having performed the "auto tune" procedure for the vector control algorithm in the drive. For vector control to work properly, the drive must have an accurate mathematical motor model in its memory. It then monitors the motor performance and modifies the output to correct any errors. It creates this motor model by either direct entry of the data, which is difficult to attain, or you have the drive "tune" itself to the actual motor either statically or dynamically. If it has no model, it "guesses" and if the guess is wrong (which is almost always going to happen) the output is unstable as the drive "hunts" for performance, which sounds like what you are describing.

If it was fine for a while and started exhibiting this, and it turns out someone replaced the motor or drive but didn't redo the tuning procedure, there's your answer. If it was fine and started doing this but nothing has changed, it starts to point to a failing component in the drive.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[fastline12]

Jraef, This is a factory matched system and is 20yrs old. The fanuc drives do not have auto tune systems as they are designed to match an exact motor and tuned in with the parameters settings to match their motor. I know the drive and motor are original. I also believe if the machine was put on a better 3ph utility service, it would not exhibit this behavior so I do believe either the 3ph converter system or utility is the cause and just trying to determine how I might source the issue by testing? Could the tiny utility transfer be the issue? Is it possible that I am indeed seeing low voltage dips when the drive demands power but I am unable to see such fast responses with my FLuke meter? Need to scope it?

 

[itsmoked]

fast:

Typical VFDs would complain bitterly if their DC bus voltage dropped precipitously but I don't believe 20y old Fanuc units do. This leads me to agree with your assessment that trying to squeeze all that HP thru a phase converter is causing the issue.

While phase converters work ok when faced with a proper 3phase motor load they work less than well when confronted by nothing more than a 3ph rectifier feeding a capacitor bank. In that case small differences in voltage can perhaps result in one phase of the rectifier bank possibly not doing its fair share.

Further, trying to pull that much current thru your 1ph building and out thru the rotary phase converter could be causing your DC bus voltage to modulate with current load peaks on the DC bus.

I would expect to see horrid dips in the DC bus voltage during your shuddering events. It would be fun to see on a scope. On the other-hand it would also be exceptionally dangerous to attempt to see it with normal scopes. I paid an extra $3K so I could execute such stunts with my scope. Unless you have an isolated scope or an isolated probe don't get involved.

Instead, I would beg/barrow/steal a 3ph 10,15, or 20HP motor and add it between the rotary phase converter and the mill. It would represent a rotary stored energy source close to the mill which Xs out the rotary converter limits and to some extent, the building wiring by storing energy close-up to the mill.

If you have a smaller motor than a 20HP, wire it in and see if there is an improvement. You should be able to extrapolate what a 20 would do for you if, say, a 10hp helps.

This would not work well if you have long periods of accel and decel back to back to back because there will not be any opportunity to "store energy".

If this has all been fine for years and only now you're seeing this new anomaly then it is likely the the DC caps loosing their mojo. If your process is doing fast decels against fast accels then you could increase the DC capacitance as you should be storing the decel energy on the bus instead of blowing it off on the braking resistor.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Has this unit been running on the phase converter for 20 years? as anything been changed or moved recently?
The spindle motor is rated for 200 Volts. That may be to allow the unit to function on commercial three phase 120/208 Volts systems, with a voltage drop.
As long as your voltage is above bout 210 Volts the unit should work. You could check the mains voltage ahead of the phase convertor. If low voltage is causing motor acceleration issues you should be able to see the voltage dip with a good multi-meter.
Have you checked the 20 year old caps on the DC bus?
What is the total motor HP of the phase converter motors?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

20 years old, I'm betting the caps are failing. 7-10 years is an average lifespan for DC bus caps on VFDs. As the caps start to fail, the DC bus ripple increases and that can result in the transistor firing becoming erratic.

A simple tell-tale sign is to open up the drives and look at the caps themselves for signs of swelling. In general it's not worth repairing compared to replacing.



"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[itsmoked]

In general it's not worth repairing compared to replacing.

Trust me.. with respect to 20 year old Fanucs it is worth replacing the caps verses drive replacement. Those obscure controllers can be out-of-availability and the new ones may not talk the same or have the same parameters. So after all the work and cost of replacement the machining center refuses to ever run again over some single parameter it's unhappy about.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[fastline12]

I am figuring the VFD/motor are demanding about 20HP for accel. My RPC is 20HP in which I bring one 10hp up with start caps, then bring another 10HP online. The system is balanced only when both are running. I realize this may just be too small for the VFD.

However, I was certainly already thinking about a supplemental cap bank tied in parallel with the factory caps to give extra capacitance and stability to soften inrush currents? I do wonder how the VFD will respond to this extra capacitance in charge up? IIRC, most of these have something in place for slower charge up? If that is not the case, I would worry that the extra current needed to charge the extra caps could overload the VFD during power up?

 

[waross]

Read the previous posts and check the DC bus caps!

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Adding caps to an RPC helps with real inductive loads such as motors running across the line, but likely will be worse for the VFD. With the VFD, the DC bus caps are simply pulling current from the line at the peaks of each sine wave (hence the term "non-linear"). They pull that current at the available level at the terminals, think of it as the Available Fault Current level. So because the manufactured phase of the RPC is created by the limited current capacity of the idler motor winding but is supported by caps in the RPC, and caps discharge instantly, the peak current draw on the two phases fed by that can become more "peaky" to the diode bridge and cause excessive ripple on the DC bus of the drive. That added bus ripple then increases the thermal stress on the VFD bus caps. You are just setting yourself up for even worse stress by adding even MORE caps on the RPC side.

And if you already have a problem with your 20 year old caps, you are going to accelerate the total failure and possibly cause collateral damage.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[fastline12]

Jraef, I was referring to adding caps to the VFD on the DC buss to there is more available storage for inrush. I agree that probably taking a look at the existing caps should come first but I suspect either the smaller RPC or utility is causing this.

 

[jraef]

OK. But adding more caps to the DC bus creates other problems too, such as the pre-charge circuit being inadequate to handle the capacitor charging current on initial energization. Nothing is as simple as we want it to be sometimes.

Bonitron makes a unit that you can use to beef up the capacity of a drive when fed from single phase, it basically does what you are thinking of, but it's a stand-alone cap bank and rectifier with its own pre-charge circuit, then you connect it to the VFD's DC bus.

http://www.bonitron.com/m3712.html

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[waross]

Have you been having problems for 20 years?
If the problems have developed slowly over the last year or so, you can probably restore performance by replacing the original caps with new, good caps of the same rating.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[fastline12]

I am uncertain how long the problem as existed. We typically run the machine a bit faster and I think stayed out of this problem range. The machine or VFD have not had any errors and there are no other signs there is a problem other than to hear the spindle sort of "stair step" the speed up to about 1000rpm.

I do know the motor is 4 pole and is 1:1 with the spindle. The motor indicates 500/6000rpm and the spindle curve indicates constant HP from 500-6000rpm. I am not sure if operating below 500rpm would normally be a tough task for the drive as the would be at 10-15hz.

Itsmoked - I assume you are referring to looking for AC ripple on the DC buss? Caps go bad and ripple causes erratic IGBT operation? I would sure think the Fanuc drive is constantly monitoring for ripple?

Jraef - you hit on the point I was getting at on the extra DC buss caps. I had concerns that the extra precharge current could be a problem. Could this be as simple as installing a manual switch or timer so all the caps could not charge at the same time? Just for discussion, I was considering matching the capacitance to what is currently installed and bring that bank in say 10 seconds after the factory bank charges? I would think the precharge system would be able to handle another identical bank? Or maybe I am thinking wrong.


The reason I ask is we will be moving a couple machines to an area where inrush at the utility is limited. What is odd to me is they limit the HP of motors blanketly but do not consider VFD starts, just across-the-line. I might be looking to soften the inrush seen at the utility side.

For consideration is a 10HP limit on motor starts. That is approx 300A inrush for 240V/1P, but you can get utility service for as many amps as you want! The way I look at it, as long as I don't have an inrush that exceeds 300A, would should be good. Then there is the VFD factor which already cushions inrush by having built in over current protection so they technically can only do what is on the plate?