Siemens G120 Fault F07900 Motor Blocked

Getting the F07900 "Motor Blocked" fault on your Siemens SINAMICS G120? Learn how to check mechanical jams, adjust torque limits, and configure parameters p2175 and p2177.

 

If your Siemens SINAMICS G120 drive trips and displays Fault F07900, it means the drive has detected that the motor is "blocked." In plain English, the drive is trying to push the motor with significant torque, but it isn't seeing the motor speed reach the expected level.

This is a protective feature. If the drive continued to pump high current into a stationary motor, it could lead to burnt motor windings or mechanical damage to the drive's power unit. Here is a human-made guide to help you diagnose the root cause and get your machine moving again.

What Triggers the F07900 Fault?

The drive identifies a blocked motor when two conditions are met simultaneously for a specific period of time:

• The motor speed is below the threshold set in p2175.
• The drive is operating at its torque limit for longer than the time specified in p2177.

Step-by-Step Troubleshooting Guide

1. The Physical Check: Is it actually stuck?

Before diving into the software, rule out a mechanical failure. A seized bearing, a jammed conveyor, or a stuck pump impeller will trigger F07900 every time.

• Action: Safely disconnect the power (follow LOTO procedures) and try to rotate the motor shaft by hand or with a tool.
• Result: If the shaft won't budge, you have a mechanical jam, not a drive problem. If it spins freely, move to the next step.

2. Verify Torque Limits (r1538, r1539)

Sometimes the motor isn't physically blocked, but the drive's "strength" is limited so low that it can't overcome the normal friction of the machine.

• Action: Check the effective torque limits in parameters r1538 (Upper torque limit) and r1539 (Lower torque limit).
• Tip: If the power unit is too hot, the drive might automatically reduce the current limit (thermal derating), which in turn lowers the torque, potentially triggering a "blocked" message incorrectly.

3. Adjust Blocked Detection Parameters (p2175, p2177)

In some applications (like high-friction mixers), the motor might naturally move slowly while pulling high torque. If the detection settings are too sensitive, you will get "nuisance" trips.

• p2175 (Motor blocked speed threshold): This defines the speed below which the drive considers the motor "stopped." If your process requires very low speeds, you may need to decrease this value.
• p2177 (Motor blocked delay time): This is the "wait time" before the drive trips. If your machine takes a few seconds to "break loose" during start-up, try increasing this timer.

4. Check U/f Control Settings (p0640, p1120)

If you are running the drive in U/f (Scalar) control mode, the drive is less intelligent about torque. Check these specific settings:

• p0640 (Motor overload factor): Ensure the current limit is high enough for the starting load.
• p1120 (Acceleration time): If the accel time is too short, the drive hits the current limit immediately. Extending the ramp-up time can prevent the F07900 trip.

5. Verify "Flying Restart" Logic (p1110, p1111)

If you are using the Flying Restart function (starting a motor that is already spinning), the drive needs to know which direction it's allowed to turn.

• Check parameters p1110 (BI: Inhibit negative direction) and p1111 (BI: Inhibit positive direction). If the drive tries to catch a motor spinning in a direction that is software-inhibited, it may register as a "blocked" condition.

Summary Checklist

Checkpoint	Action
Mechanical	Manually rotate shaft to check for jams/seized bearings.
Monitoring	Check r1538/r1539 for active torque restrictions.
Sensitivity	Increase p2177 time or decrease p2175 speed.
U/f Mode	Increase acceleration time (p1120).

Final Note: If the motor is physically free and your parameters are set correctly, but the fault persists, inspect the motor cable for a loose connection. A missing phase can cause the motor to lose significant torque, leading to a "blocked" state even under light loads.