What is a servo press?

A servo press uses servo motors for precise force and position control during forming operations.

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Use our free tonnage calculator at servopress.club/tonnage-calculator.html

Siemens S120 F07801/F07802 fault - anyone dealt with this?

kevin_dies

Siemens S120 drive on our 400T is showing F07801 and occasionally F07802. The press was running fine until we had a power blip last Tuesday. Since then these faults pop up randomly.

F07801 is "motor overcurrent" and F07802 is "motor locked." But the motor runs fine when it's not faulting — smooth, no noise, normal temperature. It's like the drive thinks there's a problem that doesn't exist.

We've checked motor insulation (>100MΩ), cable connections, and power supply. Everything looks clean. Siemens support says to check the power module but that's a $6K part and I'm not convinced that's it.

Anyone seen phantom F07801/F07802 after a power event?

mchen_servo

Re: F07801. One thing I'd add from experience: on S120 with PM motors, always check the motor temperature sensor wiring before assuming the motor is actually overheating. I've seen corroded connectors on the motor feedback cable cause false F07801 alarms on three separate Bruderer presses. Cleaned the connector, alarm never came back.

anna_plc

**Related guides:** If you resolved the F07801 alarm, you may also want to check [Servo Press Annual Maintenance Schedule](https://servopress.club/d/83) to prevent recurrence, and [Yaskawa Sigma-7 A.74 Overload Alarm](https://servopress.club/d/98) if you see overload faults after the motor repair.

toolndye_dave

For F07801 on S120, check the motor cable phase resistance with a multimeter. Disconnect at the drive end and measure between U-V, V-W, and U-W. All three should be within 5% of each other. If one pair is significantly different, you have a cable or motor winding issue. This takes 2 minutes and eliminates the most common root cause.

carlos_mx

One more thing on F07801/F07802 that bit us last year — check your S120 firmware version. Siemens changed the overcurrent detection algorithm in V4.7 SP2 and it became more sensitive to current spikes during reversal. We had a press that ran fine for 2 years, then started throwing F07801 every 200-300 cycles after a firmware update during scheduled maintenance. Nobody connected the two events for weeks.

The fix was adjusting p1520 (current controller adaptation) from the default. Siemens tech note 109763694 covers it but good luck finding it on their support site. If you're on V4.7+, set p1520=1 and see if the fault frequency drops.

Also — F07802 specifically on press applications: if it only triggers during the return stroke at high speed, check your DC bus voltage with a scope during that phase. The motor is generating back-EMF during fast return and if your regen resistor is undersized or the chopper threshold (p1254) is set too low, the bus voltage spikes and triggers the fault before the chopper can react. We bumped p1254 from 760V to 780V and added a second regen resistor in parallel. Problem solved.

carlos_mx

Late to this thread but wanted to add — we had the exact same F07801 after a power event. Turned out the power blip corrupted one of the drive parameters. Specifically the motor data set (p0300-p0311). The drive was trying to run with slightly wrong motor constants and tripping on "overcurrent" that was really just a calculation error.

Fix was simple: re-run the motor identification routine (p1900=1, then p1900=2 for rotating measurement if you can uncouple the load). Took 10 minutes and the fault never came back. Worth trying before you start replacing hardware.