Problem- 1982 fanuc 6m control (Enshu) 401 servo error

[Rob Lacy]

Hello everyone , we have used Enshu vertical machining center running 20 hrs per day .
first we got the 401 servo error , solved by opening spindle drive door and putting a fan on it but after 2 months , we are getting the 401 servo Alarm
again ..

please send advise

[cncdiag]

Do you get an alarm led on the servo drive?

[WGoyer]

Rob,
In addition to looking for alarms on the servo drive (Velocity Control Unit), you might want to note if the alarm comes on power up, or just during normal operation. Sometimes the VCU can drift and cause other alarms. Sometimes, the axis can have a bind and cooling the VCU just cools the thermal overload device.

Are any of the motors hot?

Warren

[Dan Fritz]

Check to be sure that all axes are getting some lube. A clogged lube line can let an axis run dry, overloading the servo. The motor will get hotter than normal too.

The 401 alarm doesn't indicate what happened to shut the servo down, but something happend on one of the servo boards that caused a servo to shut off. Later model servo boards had alarm LEDs that might tell you something: OVC = Overcurrent, TGLS = Tach Generator Lost Signal, etc.

Are all servos running smoothly when they run? You might be missing a phase on one of them, causing the servo to have little power and overload easily. Check the 3-phase fuses on back of each servo. Use an ohm-meter (don't just look at the little flags on the "indicator" fuses). I've seen situations where an indicator fuse would blow, but instead of the little flag popping up to indicate a blown fuse, the flag sticks inside the fuse and you continue to run with a missing phase. The smaller Fanuc motors have three 15A indicator fuses. The larger motors have three "Cartridge" fuses with three 1.3A indicator fuses in parallel. When a 30A cartridge fuse blows, the indicator fuse blows along with it, so you have to replace them both. NEVER replace the cartridge fuse with anything other than a fast-blow "rectifier" fuse. Slow-blow fuses won't protect the SCRs because they blow too slowly.

[Rob Lacy]

Thank you Fritz , we check a few things today and it looks like overcurrent is occurring at different times of the day .. have you seen this before ? and would running thru a transformer help this ?
250 volts down to 242 volts

[Dan Fritz]

You might need to adjust the taps on the servo transformer, especially if the machine has been moved to a different shop lately with a different line voltage. A line voltage from 190v to 550v should be possible with the standard Fanuc 3-phase multi-tap transformer. This is a big 3-phase transformer, usually located at the bottom of the servo cabinet. There is a diagram in the 6M mainenance manual for adjusting this transformer. There is a set of jumpers for each phase, and each phase must be jumpered the same.

Also, there is a smaller single-phase transformer inside the CNC that can adjust the CNCs power supply input voltage. That transformer has a single set of taps from 190 to 550vac. The output on the secondaries should be 100vac and 200vac for that one.

If the taps on the transformer are close (+/-10%) to your actual line voltage, then you may have a problem with LOAD on the servo. Is it always the same axis that alarms out? If so, check that motor for heating, and check the ballscrew with the power off to see if it turns easily. Lube is a big problem on older machines. Also, have you checked the motor brushes lately? They could be getting worn to their limits, or you might have a lot of carbon dust & muck inside the motor.

Is the Z axis your problem? If so, look carefully at any counterballance that might be used on the Z axis. Sometimes a counterballance weight is lifted by a chain & pulley, which can break and put too much load on the motor. A broken counterballance can overload the motor even if it's not moving. Another possibility on the Z axis is a bad brake in the motor. Z motors on VMCs usually have a brake that is released when the servo is enabled. The brake s meant to keep the Z axis from drifting down when you turn off the power. If the brake isn't releasing, then the Z motor is always trying to overpower the brake, which can cause an overcurrent. If you have a motor with a brake, pull the cartridge and indicator fuses out of the Z servo and turn the control on. The Z axis should drift down (possibly causing an "excess" errro alarm). If the Z does not drift down, try to turn the ballscrew by hand (it should turn easily up to the point of an excess error alarm). If it does not turn easily, the brake is not releasing like it should. Most Fanuc motors use 110vac to release the brake, so you could just connect it to an extension cord and plug it into the wall to test it. X and Y motors won't have brakes on VMCs.

[cncdiag]

Swap the top boards on the servo drives and see if fault follows.
If so you have a bad top card.
Make sure jumper are set correctly for the axis.

[ckirchen]

I second checking under the caps on the motors.

I have an '83 Roku which was giving 401's fairly regularly, probably a couple of times a week. Cycling the power usually got rid of the alarm, but there we several agonizing sessions where no amount of OFF-ON would get the alarm to clear - I say agonizing because they only happened when I really needed the machine to work.

I did the trick with the fans, checked and rechecked fuses, and wire shaking in general. On the advice of another post, I popped one of the caps and carbon dust came pouring out. I cleaned it out (wear a respirator) and the 401's are almost a thing of the past. The still appear every now and then, but cycling the power always clears it (knock on wood). Could be because I didn't clean the Z motor, it's inside the casting and I don't have enough room above it in my little garage shop to pull it out. I'm OK with that; it's a 25 year old machine which is finally making me a little bit of money. Maybe the remaining 401's are the machine's way of reminding me not to rely on it too much.

Chris Kirchen