Alarm 5 Troubleshooting for FANUC CNC Servo Amplifier

Servo Amplifier Alarm 5
CNC Alarm SV0435
Applies to Alpha i Servo


Alarm 5 is displayed when the DC link voltage is too low. This presentation will demonstrate the troubleshooting procedures involved.

An alarm 5 on a servo amplifier indicates that the DC link voltage is below minimum. The DC link provides the voltage used by the motors to turn. Nominal voltage of the DC link is 300 volts for standard Alpha i amplifiers and 600 volts for high voltage units.

SV435 is your onscreen sign that you are experiencing this problem.

The DC link voltage is directly proportional to the incoming AC power to the power supply module. In a little bit I'll show you some math on how you can know exactly what your DC link voltage would be. i know right....math.

The servo amplifiers are in the electrical cabinet and opening the cabinet with power applied potentially exposes you to danger.

In order to accurately troubleshoot the exact cause of this alarm, you will need to have power applied to the equipment. Don't attempt to perform anything discussed in this presentation if you are not both fully qualified to do so and have the equipment owner's permission to do so.

Once you are done troubleshooting and before performing any maintenance, follow all plant safety and lockout procedures. These procedures should include releasing all energies related to the maintenance procedure performed.

An alarm 5 requires you to look at the big picture. The DC link is created in the power supply module and shared among all the amplifiers that are connected to it. If one amplifier says the DC link is low, all of its friends should be saying the same thing. The spindle amplifier's alarm for dc link undervoltage is 51 and the power supply module's is 4. So with everybody watching the voltage on the DC link, if you only have a single unit claiming it is low, take a really hard look at that one unit.

As promised, math! To calculate the DC link voltage, take the value of the incoming AC and divide it by .707

The AC used for the DC link enters the power supply at the bottom. You determine the incoming AC value by checking it phase to phase. We have no interest in phase to ground for this.

The DC link voltage depends on whether your system is using 200 volt or 400 volt amplifiers. Check the label at the top of the power supply module if you're unsure.

For 200 volt drives the AC specification is

200 to 240 volts AC. No less than 170 volts, no more than 264 volts. This includes during highly kinetic energy events like accelerating and decelerating motors and lightning strikes in the area.

phases need to be within 2% of each other, so we're talking very balanced power

Based on the incoming power, your DC link voltage could be anywhere from 240 to 373 volts. Since we're talking about DC link undervoltage, we're most concerned with a DC link under 240 volts.

For 400 volt drive the AC spec is

380 to 480 volts AC. No less than 342, no more than 528. The same stipulations as the 200 volts units apply.

The DC link should be no less than 484 volts or alarms will occur.

The DC link is located along the top of the drive units. Flipping up the doors will reveal two sets of very substantial bus bars. The top is positive, bottom is negative.

In between the bus bars are LEDs that indicate whether the bus is charged up or not. In this example the bus is charged.

And in this one, it's not.

To check the DC link voltage, lift the doors at the top of the units. Using the screws as measurement points, check for DC voltage between the two bars.

In summary, if you're getting this alarm and it's only happening on one unit

Make sure the DC link bus bars are securely attached to the amplifier

Reseat the gray logic board. If you have other servo amplifiers with the same part number, the logic board is the same you can swap it. Bear in mind you run the risk of losing home position when you unplug feedback connectors (they're the ones with JF in the name)

If you have more than one saying the DC link is low then you need to look at where the DC link is coming from - check the incoming power. If it's low, get it fixed, if it's fine, your PSM is probably bad.