Need Help!- Problem with 6M spindle drive

[BigDamo]

I've been having a perodical problem with the Spindle on my Fanic 6M milling machine. It is a DC spindle. From time to time it will blow either 1 or 2 of the 3 fuses on the spindle drive. It also sometimes when going to start up the spindle, almost starts, then errors and stops. On the spindle drive the second led from the top comes on "over current". I seem to be able to start the spindle at a low rpm and it goes ok, but when there is a sudden change in speed, or I press the spindle stop button, it will error. It seems that sometimes if I just let it sit there and rotate at a low speed for awhile, maybe 1/2 an hour, and then come back, it seems to have solved the problem and it all runs ok again. Other days it just won't start, always almost starting and then an error. I have been talking to a Fanuc engineer on the phone and he says that it could be a faulty SCR. I removed the wire from each of the large terminals on each SCR and checked the resistance to the smaller terminal and they all seem to be around 15 ohms, some around 13 and some 18. The technicial also said I need to check between the large terminal and the base with a megohm meter, which I did at 250V DC but am not sure I am doing it correctly as I get no resistace value at all. I have been reading here a little about what Dan has mentioned, hopefully he will read this and respond. Or anyone else that may be able to help. The Fanuc technician also said that the AOS02, AOS03 and AOS04 IC's may be faulty. They are very expensive to replace so at the moment I want to make sure it's not something else first. Any ideas? I just went out and bought this megohm meter hoping I would find a problem but as yet havent. I read Dan's explination of how the system works with the smaller SCRs controlling the field current, maybe there is something wrong with them? Any help is greatly appreciated.

BigDamo

[Dan Fritz]

I would guess that your SCRs are OK. When an SCR blows, it usually shorts clear through, which will blow one or more fuses as soon as you turn the spindle on.

The first thing I would check is the motor. When these DC motors are mounted vertically (as they are in a mill or machining center), you can get lots of carbon dust from the brushes in places where it shouldn't be. First, remove each of the big armature brushes and clean out the dust as best you can. Make sure that the brushes are not too badly worn. A Fanuc Maintenance manual should give the minimum brush length for each size motor.

Also, I've seen situations where the little bronze guide that the brush slides in gets damaged from arcing on the side closest to the armature. Be sure that each brush slides easily back & forth in the guide. If the brush sticks at all, use a tiny file to smooth out the guide, taking care to not nick the commutator on the motor shaft.

If carbon dust is the culprit, you may someday have to disassemble the motor and give it a good cleaning. Usually there's no need to have the motor rewound, just clean it thoroughly and put in some new brushes.

The tachs sometimes get dirty too, although I don't think this is causing your OVC alarms. Open the small cover on the back end of the motor and look for the tiny tach brushes. I like to just hold a pencil eraser against the little tach commutator while turning the motor by hand. This "cleans up" the tach feedback signal and lets the motor run at a more consistant speed. If the tach is dirty, this can cause the motor to "surge", which increases the current load on the drive.

If this doesn't help, please post the part number on the spindle drive board, and let me know what the motor size is. Maybe I can suggest a pot adjustment to acc/dec that will lessen the current load a bit on the drive.

[BigDamo]

Hello Dan, thanks for the reply. This morning before I checked the SCRs and then went out and bought a megohm meter, I did actually give the motor a good clean. It's a horizontal machine. I pulled out each brush and gave it a good blown with some compressed air. I checked the length of the brushes and can still see the line marked on them to indicate when they become too short. I removed the tacho cover and gave it a bit of a blow there too. One thing I should tell you is once while cleaning the motor I removed the small commutator and checked the resistance across each adjacent segment. I did notice that there was one pair of segments that were an open circuit. I did this ages ago, before I started having these spindle problems, and so thought it wasn't related to this problem. What do you think about checking the SCRs with a megohm meter? Am I doing it correctly, measuring between the large centre terminal and the base block? Is 250V the right setting to test them? I didn't get any result resistance at all. The spindle board is A20B 0008 0374 / 02 08D and the motor is a Gettys DC model 6. Thanks,
Damo

[salmve]

BigDamo, have you regognize any difference at motor voice when the fault is on?
I think i have a same problem. When i start the cold machine and turn spindle on, it starts normally but motor voice is like 3phase AC- motor whithout one phase. (Motor and spindle unit is same as you have.) When i stop the spindle, i get overload alarm. Only if speed is very low it stops without alarm. Warm machine works ok, or if i warm up the spindle speed unit with hairdryer or hotair gun. Area of warm up is below heathsink. Area is very brief.

Then i purchased freezer spray and tried to find out the faulty component. I freezed component by component when spindle is on and get a few times the fault on. With hotair gun i can get the fault off. (As i said i can hear from motor sound when all is ok.) After few freezer/warmer cycle this phenomenon stops and the spindle has worked ok about 3 weeks now. Amazing.

Machine is Nakamura TMC-3, Fanuc 6TA

Vekku

[BigDamo]

Hi Salmve, that's very interesting. When you say you heat the area below the heatsink, do you mean on the circuit board? Which is where the little black square shape IC's are.
Damo

[Dan Fritz]

Are you sure that you have all 3 phases? Take an AC voltmeter and check between the bottom of fuse #1 and fuse #2. You should see something between 200 and 220 vac. Now, check between the bottom of fuse #2 and fuse #3 (you should see about the same voltage). Now check between fuse #1 and fuse #3 (same thing). If all three phases are there and about equal in voltage, you're OK on the supply side.

Also: be sure that all 3 fuses are good. Make the same check as above, only on the TOP of all 3 fuses. A blown fuse or a missing phase can make any 3-phase spindle drive run like crap.

[BigDamo]

Dan, the machine will alarm if any of those fuses are blown. I think it's interesting what Salmve has written as this area he is talking about is where the majority of the AOS03 and 04 IC's are. Where he has been heating with the heat gun. What's your thoughts on that?
Damo

[BigDamo]

Salmve, Just before the machine was cold so I started it up and tried starting the spindle straight away at 3000 rpm. Sure enough it did as per usual and tried to start, then errored. Next I got out the hair dryer and carefully warmed up all the black IC's at the lower section of the board, the AOS03's and the AOS04's, then went and tried to start at 3000 rpm again, no worries, no error! Well, although it is a temporary fix I think it proves where the trouble is. The sad part is the cost of getting those IC's. Before I was a bit hesitant as I wasn't 100% sure after replacing them, it was going to solve the problem. Now I am almost certain it is the problem. Great Job, I hope this helps other people with a similar problem. Dan, do you know what those IC's are for? Damo

[Dan Fritz]

From the looks of things, there is an OS03 and an OS04 for each phase, so there should be 3 of each. These ICs are proprietary "hybrid" ICs that Fanuc makes just for the servo and spindle amplifiers. It looks like these ICs are responsible for taking a pulse-width modulated input signal and using it to determine where on the sine wave each SCR should turn on and off. If these ICs malfunction, it could cause an SCR to fire at the wrong time relative to the AC sine wave.

These ICs all have analog circuits with op amps inside, and these circuits rely upon the +15v and -15v power supplies. The spindle board should have it's own power supply, with a couple of little voltage regulators fastened to a heat sink. There should be a +15v regulator and a -15v regulator. If one of these voltage regulators went bonkers, that could create the problem also. Look for a check-pin for +15v, -15v, and for +24v. Measure the voltage at these pins relative to a "0v" pin to see if they're OK. The +/-15 volts supply should be VERY close to this target voltage. Any variance would indicate a blown regulator or maybe another bad component in the power supply. A + or - 15v regulator would look like a power transistor - about 1/2 inch square by maybe 1/8 inch thick, with 3 pins coming out one side (soldered to the board) and a screw tab to attach it to a heat sink. These can be purchased a any electronics supply store for a couple of dollars. If these voltage are stable, then you can assume that you have a bad hybrid IC. Try heating them one at a time to see if you can isolate which one it is.

[salmve]

Hello
Pleasant to hear that with my tips you find something from your machine.

Sounds same problem. Just that area i warmed up.

Next time when you start the cold machine, try with very slow spindle speed. If it's as mine, you can hear when fault is on. Motor sound is not normal. I can increase the speed normal but decrease have to be done very carefully.

Freezer spray is very helpful. Maybe you can find bad componet with that. Or if you have good luck as i had you can work with machine without problem few time. I know this isn't final solution but now this is enough for me.

Thanks Dan again, your advice are very very informative as always.

Dan Fritz Wrote:
"From the looks of things, there is an OS03 and an OS04 for each phase, so there should be 3 of each. These ICs are proprietary "hybrid" ICs that Fanuc makes just for the servo and spindle amplifiers. It looks like these ICs are responsible for taking a pulse-width modulated input signal and using it to determine where on the sine wave each SCR should turn on and off. If these ICs malfunction, it could cause an SCR to fire at the wrong time relative to the AC sine wave."

Maybe this is the reason for unnormal motor voice. And now i understand little more of this unit

BigDamo good luck, hope this helps.

Vekku

[BigDamo]

Thanks Vekku, and thanks Dan. That's quite interesting to know about the OS IC's, and the voltage regulators. I know the ones you mean, screwed to the heat sink. I had checked all those voltages once before and they are all ok.
Damien