The fault code "7900" on Siemens drives indicates a "Motor Locked" condition. This fault occurs when the drive detects that the motor is unable to rotate freely or is mechanically jammed. Here are steps to address this issue:

1. Check Mechanical Obstructions: Inspect the mechanical components of the motor and driven equipment for any obstructions or binding that could prevent rotation. This could include debris, foreign objects, or misaligned components. Clear any obstructions and correct any alignment issues to ensure free rotation.

2. Verify Belt Tension: If the motor is connected to driven equipment via belts or pulleys, check the tension of the belts. Over-tensioned or misaligned belts can cause excessive friction and lead to a locked motor condition. Adjust the belt tension as necessary to ensure proper alignment and tension.

3. Inspect Bearing Health: Check the bearings of the motor and driven equipment for signs of wear or damage. Worn or damaged bearings can cause increased friction and prevent smooth rotation of the motor shaft. Replace any worn or damaged bearings as needed.

4. Check Load Conditions: Ensure that the load on the motor is within the rated capacity of the motor and drive. Excessive loads can cause the motor to become overloaded or locked. Reduce the load if necessary and adjust parameters such as acceleration/deceleration ramps to prevent overloading.

5. Verify Motor Parameters: Ensure that the motor parameters are correctly configured in the drive's settings. Incorrect motor parameters or mismatches between the drive and motor specifications can lead to locked motor conditions. Check parameters such as rated current, voltage, and frequency.

6. Inspect Motor Connections: Check the motor connections for loose or damaged wires, terminals, or connectors. Poor connections can cause increased resistance and prevent the motor from rotating freely. Ensure that all connections are tight and secure.

7. Review Drive Settings: Check the drive's control parameters and settings related to motor protection and fault monitoring. Adjust the settings as necessary to prevent the drive from entering a locked motor condition.

8. Inspect Cooling System: Ensure that the motor cooling system is functioning properly. Overheating can cause thermal expansion of motor components, leading to increased friction and a locked motor condition. Check for blocked ventilation openings, dirty cooling fins, or malfunctioning cooling fans.

9. Consult Technical Documentation: Refer to the drive's user manual, technical documentation, or contact Siemens technical support for specific guidance on troubleshooting and resolving motor locked faults.

By following these steps and addressing the potential causes, you can troubleshoot and resolve the motor locked fault in your Siemens drive.