The fault code "7801" on Siemens drives indicates a motor overcurrent condition. This fault occurs when the current flowing through the motor exceeds the maximum allowable limit. Here are steps to address this issue:

1. Check Motor Load: Verify that the load on the motor is within the rated capacity of the motor and the drive. An overcurrent condition can occur if the motor is overloaded or if there are sudden changes in the load. Reduce the load if necessary and adjust parameters such as acceleration/deceleration ramps to prevent sudden spikes in current.

2. Inspect Mechanical Issues: Check for any mechanical issues with the driven load or motor, such as binding, misalignment, or excessive friction. Mechanical issues can increase the current drawn by the motor. Correct any mechanical issues and ensure that the motor operates smoothly.

3. 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 overcurrent issues. Check parameters such as rated current, voltage, and frequency.

4. Inspect Motor Connections: Check the motor connections for loose or damaged wires, terminals, or connectors. Poor connections can cause increased resistance and higher currents. Ensure that all connections are tight and secure.

5. Review Drive Settings: Check the drive's control parameters and settings related to current limits, acceleration/deceleration profiles, and motor protection. Ensure that the settings are appropriate for the application and adjust them as necessary to prevent overcurrent conditions.

6. Check for Ground Faults: Inspect the motor and drive system for ground faults. A ground fault can cause abnormal currents to flow through the motor. Use a multimeter or insulation tester to check for ground faults and address any issues found.

7. Inspect Cooling System: Ensure that the motor cooling system is functioning properly. Overheating can increase the resistance in the motor windings and lead to overcurrent conditions. Check for blocked ventilation openings, dirty cooling fins, or malfunctioning cooling fans.

8. Review Environmental Conditions: Consider environmental factors such as ambient temperature and humidity. High temperatures or humidity levels can affect the motor's performance and increase the risk of overcurrent conditions. Take appropriate measures to control environmental conditions if necessary.

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 overcurrent faults.

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