Quick Guide for Fanuc System Troubleshooting: Is it the Cable, the Amplifier, or the Servo Motor?

Checking a Fanuc system to find out why it is faulting.

Finding fault with a Fanuc servo system

When your CNC machine outfitted with Fanuc system stops working, the person running it or maintenance personal must quickly figure out the root-cause and then formulate a plan to get the machine running again.  Seems simple, but rarely is it simple or easy.

Being a Fanuc repair facility, a common happening is that we will get a Fanuc motor or amplifier in for service, then go through our process to evaluate and we find out: the unit runs fineOr, the unit does need repair and we return it repaired and the customer calls and says their machine still doesn’t run.  Either way, we have a frustrated customer that still has a problem with their machine.

The first thing, most operators do, is check the fault code that comes up on their amplifier.  This is the best place to start.  Whether you need to pull the drive, cable, or servo motor and get it repaired, here are the most common codes that we see and how they can tell you which component is at fault:

Fault Code #8 and #9 (the second channel)

Finding Fanuc Faults

Fault code #8 on a Fanuc amplifier display is very common.

Overcurrent: To trouble shoot a #8 fault code, we see this fault most, make sure machine is powered down, then disconnect the motor cable from the drive.

Is it the Drive/Amplifier?  Power the drive back up, if the #8 stays on, the problem is within the drive and it will need to be repaired.  If you power the drive back up and the #8 goes away, then you may have eliminated the drive as the cause so the issue is either the servo motor or the feedback cable.  But, if the servo motor and cable test well (see below) – then the drive may be faulty due to weak or aged components and will need service.

Is it the Motor or the Cable?

The cable from a Fanuc motor to the drive.

A bad cable can be the reason a machine won’t run.

To check the power cable and servo motor, check resistance between phases of the cable with a multimeter.   Depending on the length of the cable and any connections in between (common of spindle motor junctions) you should get balanced readings of 3 ohms or less.  If your readings are higher, you will want to disconnect the cable from the motor and megger the phases of the cable in between each other and also from phase to ground.  In this procedure, you would like to see your readings as close to infinite as possible.  We typically don’t like to see readings below 200 Meg ohms but have seen motors that read less than 20 Megs that had no issues.  However, this is a sign that the insulation is degrading.

If your cable checks out good, it’s time to move on to the motor.  Again, using a multimeter, check resistance between phases and in most cases you will want to see readings in the fraction of an Ohm 0.3-0.5 is fairly common.  But don’t be alarmed if you don’t get readings in the 5 Ohm range, some motors are wound differently and you get that from time to time.  As long as they are balanced you are usually safe.  If the winding resistance checks out good, go ahead and use the megger and Meg out the motor phases to ground.  You will be looking for readings that are similar to that of the cable you checked.  Again, you will want to be as close to infinite as possible as possible but anything 200 Meg ohms or more will suffice.

Finally, these are only quick, easy ways to check your motor and cable.  Most facilities do not have access to a winding analyzer (surge tester) that can thoroughly test the windings and insulation; these quick tests will most likely unearth a fatal error.  Good luck!

Accurately finding what is at fault can save a lot of time.

Jason shoots video to show customer when his motor faults out and why

Fault Code #1

Internal fan stoppage:  Whenever the speed of a fan has changed, you can check to see if something got wedged in the fan assembly, or a build-up of debris can cause a change in fan speed.  Any change in fan speed will cause a #1 error and shut down.

Fault Code #3

Servo Amplifier – Low DC Link blown:  Depending on model if drive has circuit breaker, verify y it is on.  Then check the DC voltage.  If it is good, the drive needs to be repaired.  If bad, the fuse may be blown.

Spindle Amplifier – Fuse on DC link blown

Other Fanuc Spindle Amplifier common fault codes:

AL-12:  Overcurrent or blown IGBT

AL-31:  Motor lock or v-signal loss

AL-34:  Parameters setting error, check to make sure the parameters are set right.

For more information on your Fanuc system, or for repair services on your Fanuc servo motor, drive, power supply or monitor, please contact Repair Zone.

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