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Fanuc Al-12 troubles

52pickup

Plastic
Joined
Jun 18, 2019
Machine:

1990 Cincinnati Sabre 750 with Fanuc motors and drives
A06B-6059-H212#H570 Spindle Drive
A06B-0728-B104#3800 Spindle Motor


Back Story:


A few weeks back while dry running a program, spindle was at 8k (which is wide open on this machine), when suddenly I hear the RPM start to ramp down. I hit feed stop and spindle stop... spindle doesn't stop so I hit the E-stop.. still doesn't stop. The spindle ended up just free spinning until it finally slowed to a stop about what seemed like 5 minutes later. Get everything booted back up, and everything is as normal, until I try to call a tool. When the spindle goes to spin up and orient for the tool change the spindle turned VERY slowly, then the spindle drive errors out with an AL-12 code.

Tested the motor with a megohmmeter and got 200 megs on two legs, and 300 on the third, testing with the motor disconnected from the cables, leg to ground. Tests the same through the cables to the motor, and cables show no continuity when disconnected on both ends. Spindle turns free and smooth.

Next I tested the transistors. One transistor block tested out of spec, so I decided to replace both of them while I was in there. Go to fire back up, now spindle doesn't spin at all, same AL-12 fault. At that point I decided to just replace the drive, and got a used drive off eBay. Replace drive, no change, spindle doesn't move, still AL-12 fault.

I'm stumped at this point. I don't want to continue throwing parts at this thing. What am I missing? Are there tests I can do that I've overlooked?
 
Is the ebay drive a known good unit? With the motor leads disconnected try turning on the spindle. If the drive is bad you will probably still get the AL12. If you get a different alarm, the drive is probably not the problem.

Test the motor by wiring it to line power. While running (if it does without tripping a breaker) check the current on each leg with an clamp type ammeter. If they are all equal and lower than the name plate current, then the motor is OK.

One or the other of these tests should narrow down the problem area.
 
Drive was allegedly removed from a functioning machine, but it's ebay... so who knows the real story.

The original drive did throw the AL-12 with the motor disconnected. I haven't tested the replacement yet, but I will do that this afternoon, and try jumping the spindle motor. Should I just hook the R S T cables to the U V W cables to jump the motor?
 
......Should I just hook the R S T cables to the U V W cables to jump the motor?

That would be one way to do it. DO NOT leave the U V W wires connected to the drive if you do it this way. That will kill the drive. I just used a 3 phase "extension cord" and made my connections at the motor itself. That removes uncertainty about the machine wiring from the test.
 
I had this alarm on a customer's machine back in April where the operator stalled the spindle with a large dull drill. It took out the drive. I got a replacement from TIE (Fanuc World) the next day and all was good. If the motor megged good and the drive faults out with the motor unhooked I would think it's a bad drive.

I would send the one you bought on ebay back to the seller.
 
So I ran those tests. Motor runs when jumped as I described earlier. Legs are pulling 28.6(u) 25.0(v) and 26.7(w) amps. Name plate says 28a. I will say... it doesn't *sound* like its spinning full speed. This thing has always sang at 8k rpm, this sounded more like 4-5k to me but I don't really know how I'd check the actual speed.

Also, with the motor disconnected, this eBay drive also throws a Al-12 code.
 
The amps are not as closely balanced as I would expect. That might be explained also by the fact that you got some variation in the megger test. It won't be running full speed since it is just at 60Hz.

The AL12 on the drive with nothing attached is bad.
 








 
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