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How to Solve Fanuc PSM Alarm 8?

MAR. 20, 2025

In a factory, once a machine tool stops, there is an immediate sense of anxiety in the air. Especially Fanuc system equipment, such as CNC lathes, machining centers, milling machines, we all know that their heart is the power supply module (PSM). It is responsible for providing stable DC power to the servo drive, motor, once the problem, the entire production line to stop.

We often receive calls from customers: “The machine suddenly reported PSM alarm 8, how to do?” Sometimes the customer's voice with helplessness, because a minute of downtime is money. What is this alarm all about? How to check? How to solve? Let's talk about it with the cases we usually encounter.

Alarm 8 What Does It Mean?

Simply put, Fanuc PSM alarm 8 is "DC link overvoltage”. In other words, the DC voltage inside the power module has spiked above a safe value. In order to protect itself, the system immediately stops and alarms.

If you don't pay attention to it, the consequences are serious: not only the power module may be burned, but also may involve the servo amplifier, spindle motor, the cost of repair is even greater. So once you see alarm 8, don't try to force reset and continue to run, but find out the problem first.

Why Do You Report Psm 8? We Have Several Common Situations

During the on-site troubleshooting, we summarized several high incidence reasons:

* The regeneration circuit is broken: when the motor decelerates, it generates energy that should have been consumed through the regeneration unit. If the regeneration unit is bad, this energy is piled up on the DC bus, and the alarm will be raised as soon as the voltage rushes high.

* Problems with the braking resistor: It is common that the resistor is burned black, disconnected, or the wiring is loose. The resistor does not work and the voltage is not suppressed.

* Grid voltage instability: some factories have large voltage fluctuations, especially at night when there are many equipments, high input voltage can also lead to over-voltage.

* PSM itself aging: such as IGBT damage, capacitor bulging, internal circuit problems.

* Poor wiring or grounding: This may seem trivial, but loose terminals and oxidized grounding can sometimes trigger voltage fluctuations.

We have encountered almost all of these situations, one by one on-site troubleshooting, nine times out of ten can find the cause.

How Do We Usually Troubleshoot Step by Step

If the customer wants to check first by himself, he can start from these places (pay attention to safety, disconnect the power first, and the capacitor must be discharged):

1. Look at the power supply

Measure the input AC voltage with a meter to see if it is out of Fanuc specifications. If the voltage is too high, it is recommended to add a voltage regulator.

2. Look at the braking resistor

Are there any burn marks? Is the resistance value normal with a multimeter? Is the wiring loose? This is the easiest thing to overlook.

3. Look at the return circuit

Check whether there is any abnormal heat, whether the fuse is broken, whether the transistor has been broken.

4. Look at the wiring and grounding

Re-tighten the cables and grounding, especially on older machines.

5. Suspect the PSM itself

If all the previous problems are OK, then it is likely that the power supply module internal failure. At this point it is recommended to send it for repair, do not dismantle it yourself, the voltage is too high to be safe.

Repair or Replace? Our Advice

Some customers may ask: "Can this be repaired? Should I just replace it with a new one?"

We usually judge it like this:

* If it's just a bad braking resistor, or a minor fault in the regeneration circuit, it's cost-effective to repair it.

* If the motherboard inside the PSM has been burned black, or repaired a few times or still report the same error, it is more economical to directly replace it.

In fact, many customers will choose refurbished PSM modules because the price is much lower than brand new ones, and after testing, they can be used right after installation, which is very cost-effective.

What We Can Do for Our Customers

At Songwei, we deal with similar Fanuc alarms every day. Whether it is the repair of PSM module or the replacement of new and old spare parts, we have stock and program. For example, for the common A06B-6077-H111 PSM, we have both brand new and tested used parts, which can be shipped according to customer's situation.

Meanwhile, our repair team will help customers to test out the old Fanuc parts by our Fanuc test bench and tell you whether it is cost-effective to fix it or more appropriate to replace it. Often times, with one phone call, we can help customers reduce downtime.

One Final Note

Fanuc PSM Alarm 8 is essentially a DC link overvoltage. The common reasons for this are the power grid, the resistance, the return loop or the module itself.

If you can't do it by yourself, the most reliable way is to find a professional team to deal with it.

At Songwei, we have been helping our customers to minimize downtime losses by providing the fastest solutions, whether it's for repair or replacement of spare parts. If you encounter PSM alarm 8, feel free to contact us.

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How to Solve Fanuc PSM Alarm 8?

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