Troubleshooting Fanuc CNC Power Supplies

Of the several variables that determine whether and how well a CNC machine tool works over time, the power supply may be the most important. The power supply for your Fanuc CNC mill or lathe has several elements that may fail without warning or begin to malfunction. You want to be aware of how this part of every machine tool can be vulnerable to failure, what can go wrong, and how a CNC repair tech can fix power issues. Good CNC machine maintenance practices can prevent some issues, but this post focuses on finding power supply issues. A quick review of the subsystems that run a machine tool will make it clearer how electrical issues could disable a machine or degrade its performance.

A Quick Review of Fanuc CNC Power Supplies
A Fanuc CNC tool will typically have several AC and DC power supplies for internal subsystems. There may be a power supply for the printed circuit board (PCB), the monitor, and the input/output card, the door interlock circuit, and a connection to the external power source. The machine will also have one or several fuses, which can wear out or blow.

Some performance issues with your Fanuc lathe or mill may be caused by the site’s power supply. If the facility gets a low-quality (variable and/or weak) supply of power, any electrical device is likely to have performance problems. Over time, relatively sensitive parts may be degraded by the peaks and valleys of the location’s power supply. However, most power issues in CNC machine tools are probably going to be found in one or more components of the tool.

Understanding Possible Failure Points
Your CNC mill, grinder, router, or lathe will have subsystems that take power from the workshop, which gets power from the local electrical grid. Once the current gets to the machine it will go through various power handling devices that step down the voltage, convert AC to DC or the reverse. That voltage will then go to solenoids and motors that run servos, hydraulics systems, spindle drives, monitors, and so on. 

That last paragraph covers many of the potential failure points for a Fanuc CNC machine tool’s power system. The details are a bit more involved, but at the same timeAssume the machine gets a good and steady voltage from the primary power source and the power cord is intact, you are left with a few sources of trouble inside the device. Many of your power issues are going to come from things like:

Current at the input or output side
Reversed polarity
Temperature issues
Age (normal wear)
Missing external components
The last item is the only that will be easy to identify and deal with but anyone with CNC machine maintenance experience will be able to find and fix other power problems.

Troubleshooting Power Problems
Ideally, the Fanuc machine itself will send a signal that indicates what kind of power issue has developed. Sometimes, the first sign of trouble will be when a machine tool component stops working or the machine shuts down or speed drops. Whatever the specific symptom, a CNC repair technician or engineer will need to do some analysis to track down the failure point and make a repair.

Never forget the possibility that something simple went wrong. We’ve all had devices turn off because someone unplugged. If the building has power, it could be the specific machine’s power cable or a blown fuse.

The fuses will take a little work to evaluate. The technician needs to find the relevant fuse, pull it and check that voltage goes in and comes out. If not, the associated power supply might be bad. Look for an LED indicator on the power supply. If there is one, you naturally should expect to see a light. If the light is absent or very weak, you may have found the problem. Check the power unit with a voltmeter.

Once those possibilities have been dismissed, a technician will have to evaluate some of the machine’s internal components. Aside from the internal power supply working, it needs to deliver the right amount of voltage, consistently. If voltage fluctuates dramatically or goes too low, the machine’s performance is sure to be compromised.

How much degradation or variation is acceptable? A 24v DC power supply that actually delivers anything below 20 volts will cause problems. On the other hand, a variation of two or three volts in a 380v power source is not a problem but a drop of 10 to 15 volts may indicate that part will need to be refurbished or repaired in the near future.

In some cases, the bad Fanuc CNC part will need to be expertly repaired or replaced. Many machine tool owners see the value in buying refurbished power supplies or calling on experienced technicians to diagnose and repair a defective power supply. Ideally, you will also have extra fuses on hand.