Is your Siemens G120 drive showing Fault F7900? Learn how to diagnose motor blocked issues, verify mechanical loads, and tune parameters.

This guide provides a technical breakdown and troubleshooting workflow for Fault F7900 (Motor Blocked) on the Siemens SINAMICS G120 drive. This is one of the most common protection trips, designed to prevent damage when the motor cannot reach its target speed despite pushing full torque.

Troubleshooting Siemens SINAMICS G120 Fault F7900: Motor Blocked Guide

Siemens G120 Troubleshooting: Fault F7900

Fault Code: F07900 (N, A)
Description: Motor Blocked / Speed Controller at Limit

In Siemens SINAMICS G120, Fault F7900 occurs when the drive detects that the motor speed is below the threshold, while the speed controller has reached its torque limit (saturation) for longer than the time. Essentially, the drive is trying to turn the motor, but it isn’t spinning as expected.

Common Root Causes

Mechanical Jam: The load (pump, fan, conveyor) is physically stuck or overloaded.
Torque Limits Too Low: The effective torque limits are set too low for the application’s required starting torque.
Wiring Issues: Phase leads (U, V, W) are swapped, or the star/delta connection is incorrect for the voltage.
Encoder Problems: If using an encoder, a mismatch in direction between the motor and encoder will cause the drive to think the motor is blocked.
Short Ramps: Acceleration time is too short for a high-inertia load.

Troubleshooting Steps

1. Mechanical Check: Try to rotate the motor shaft manually (with power off). Check for seized bearings, guide rail obstructions, or foreign objects in the machinery.
2. Verify Rotation Direction: Ensure the drive and encoder (if present) agree on direction. Swapped motor phases are a primary cause for new installations.
3. Check Parameters: If the motor is actually turning but tripping, Speed threshold might be too high, or Delay time might be too short. Adjust these to give the system more “patience.”
4. Review Torque Limits: If the drive is limiting current/torque too early, it won’t be able to overcome the initial load.
5. Perform Motor ID: Re-run the stationary and rotary measurement (Motor Identification) to ensure the drive has an accurate model of the motor’s electrical characteristics.

Key Parameters for F7900

Parameter Name	Standard Action
Motor blocked speed threshold	Decrease value if trip occurs at low speed.
Motor blocked delay time	Increase value to allow longer “stall” time.
Current limit	Increase if motor requires more starting current.
Acceleration time	Increase for high-inertia loads.