[moldmker]

Hello...Did a search for this with no specific results:

Machine: 1981 Kitamura VMC
Control: Fanuc 6MB

Problem: I was in a very light cut when machine stops with a Servo 401 alarm.
Check the cabinet and replace 30A fuse and indicator fuse for the Z axis. Checked the other 2 and OK.
Power up, soon as the contactor pulls in, the 3ph. converter sounds loaded and the Z axis blows another 30A fuse.
Checked the Z axis brushes, worn down to service limit, some indication of heat, and lots of dust.
Blew everything out, visually inspected commutator, scraped some of the areas that looked bridged with a dull scriber. (not sure if they were actually bridged or just dusty).
Power up again, soon as the contactor pulls in, the 3ph. converter sounds loaded and I power down quickly.

Any suggestions?

Thank you,
Bob

[Al_The_Man]

Disconnect the motor armature, if the load is still there it is the drive shorted.
Al.

[moldmker]

Disconnected the armature wires from term. strip.

Power on

Brief load when contactor pulls in, then alarms out to Servo 401.

Does not blow the fuses.

Power off/on and same result.

Servo drive has lit the red TGLS LED. (I will do a search on this)

Is anything serviceable on the drive?

Thanks,
Bob

[Dan Fritz]

Check again to be sure that you don't have another blown fuse. Use an ohm meter this time. Sometimes, a 30A cartridge fuse will blow, but the corresponding indicator fuse will not pop up. When you repair the indicator fuses with new fuse wire, it's possible to get a little blob of solder on arm of the little spring-loaded flag. When you put the plastic cover back on, the arm can't pop up, so the fuse can blow but you'd never know it. If the little flag can't pop up to shut down the drive, you can run on single phase, which will overload the remaining phase and blow one of the OTHER fuses.

Usually, these fuses blow in pairs. You replaced one of them, but it's possible that you have another blown one that you're not aware of. The cartridge fuse and the indicator fuses always blow together, but only the indicator fuse can shut down the drive.

Also, have you tried to manually turn the ballscrew? Does it turn easilly? You could have a lube problem or a counterweight chain that's broken, putting too much of a load on the servo. Do you have an electric brake in the motor? Some Fanuc motors have a brake with a 110v signal to release it. If the brake starts to drag, or if the 110v power to release the brake is lost, then the motor shaft will be impossible to turn by hand. To test this, you may have to do this:

1) Block the Z axis from drifting down with some 2x4s or 4x4s
2) Remove all 3 cartridge fuses and all 3 indicator fuses
3) Power up the control. The brake should release, but the motor can't move.
4) Turn the ballscrew (Z up) by hand to see if all is free (brake, lube, counterweight, etc.)


You could have a blown SCR, but that would blow the fuse instantly whether or not the armature is connected. Also, make sure that you're getting all 3 phases to the terminals "A", "1" and "2" on the back of the drive. Use an AC voltmeter and check between A and 1, between A and 2, and between 1 and 2. You should see AC voltage in all three tests. The phases don't have to be exactly ballanced, but that would reveal a lost phase. These drives need 3 phases to run properly.

[Al_The_Man]

I have also found that the 30amp fuse can be blown and the indicator fuse is still OK so the alarm does not occur.
The drive is running on two phases.
I found that this was due to the High wattage resister in series with the Indicator fuse was open, I think they are around 1ohm.
I have come across this a couple of times.
Al.

[Dan Fritz]

Thanks for the tip. I never thought that the resistor might be open.

Dan

_______________

"Anything worth doing well is worth doing twice."

[moldmker]

Checked all fuses with ohm meter...OK

Checked between A...1 = 101 VAC ; 1...2 = 101 VAC; A...2 = 96 VAC

Removed all fuses on Z-axis.

Reconnected armature wires.

Control powers up OK , all contactors pull in.

Machine alarms out with Servo 401

Z axis screw turns freely.

Z axis PRDY LED is lit.

Next step?

Thanks,
Bob

[moldmker]

Talked with Fanuc today about problem.

Had me check motor leads for short to ground...checks OK.

They're leaning towards the drive (p/n A06B-6045-H005/H006) being bad, can rebuild for just less than $2k.

In my research, I see that there are 2 Fuji Transistor Modules p/n A50L-5000-0017 (50 amp) that control the motor current.

These appear to be scarce, but I have access to another shop's obsolete parts stash.

Am I on the right track?
Can these be tested?

Thanks,
Bob

[Al_The_Man]

They can usually be checked with ohmeter for short circuit, I am a bit leery changing these as it can also be the firing board which caused the problem in the first place, OR the firing board gets damaged when the IGBT or SCR blows.
Al.

[cncdiag]

If you need your drive tested/repaired give us a call.
CNC Diagnostic 813-983-1870

[moldmker]

I will be replacing the Z-axis drive with one from another machine.

Other than verifying jumper settings, are there any other areas I should check.

Thanks,
moldmker

[moldmker]

NOTHING is ever easy.

I got one 30 amp Z drive and one 15 amp X or Y drive off ebay.

The drives are all the right p/n's but the piggybacked board on the Z axis drive has a power supply on it and the X or Y drive does not.

This is exactly opposite of what I need.

Million dollar question: Can I just switch the board from one drive to the other?

Thanks,
Bob

[Dan Fritz]

Bob, You can switch the drive boards, provided that you set the jumpers on the board for the correct motor and tach voltage The jumpers determine the tach signal (volts per 1000 rpm), and the type of motor, etc. The pots (variable resistors) also have some "basic" settings for each type of motor, so if you see one of these pots that are wildly different between the boards, set them to what the other board is before the swap. Just copy the jumper and pot settings from the old board before powering up the drive.

The power supply is "shared" between drives, and the output of the one (usually the X) servo board is jumpered to the others with a ribbon cable. If you have two boards with a power supply, that shouldn't be a problem. Just don't connect the ribbon. The power supply gets 18vac and a CT (center tap) from the drive chassis, and outputs +24v, +15v, and -15v dc for the servo board. There should be check pins for each voltage output if you want to verify them.

Remember, the jumpers are the "rough" adjustment for the motor type and tach voltage, and the pots are the "fine adjustments". Don't mess with pots RV11 a, b, and c unless you have an oscilloscope and know how to adjust them. Those are for compensating for the 3 phases to get the same current from each phase. If you DO have a scope, put it on CH11 (the current feedback) using CH3 or CH4 as a ground reference to see the pulses of current going to the motor from each phase. Each "bump" on the scope should be about the same hight (amplitude) if the phases are well ballanced. A nicely ballanced drive results in a smoother running motor.

[moldmker]

Big Thank You,

Swapped boards, verified jumpers and voltages.
Up and running again.

Bob