Poor tuning of the sensorless vector control loops. Step-by-Step Troubleshooting Workflow
Disconnect drive terminals and measure resistance at room temperature. Should be ~600Ω. Use a hairdryer to warm probe—resistance should increase smoothly.
In hoisting or vertical axis applications, a mechanical holding brake is often used. If the brake fails to release (due to a blown rectifier or sticky mechanism), the drive attempts to move against a locked brake, resulting in an immediate 7902 fault.
| Root Cause | Solution | |------------|----------| | PLC in STOP | Set PLC to RUN or reconfigure drive to ignore missing telegram ( p2040 = 0 – not recommended for safety) | | p2040 too short | Increase p2040 to at least 2–3× bus cycle time | | Loose connector | Reseat or replace bus connector | | Wrong telegram type | Match p0922 with PLC configuration (e.g., standard telegram 1 for speed control) | | No termination | Activate termination on both ends of PROFIBUS segment | | EMI | Check cable routing, add ferrites, verify shield grounded at both ends | | Faulty option board | Replace communication board (e.g., CBE20 for PROFINET) | siemens drive fault code 7902
The drive lacks accurate motor data, preventing effective speed/torque calculation.
Ensure match the motor nameplate precisely.
In Siemens SINAMICS drive ecosystems (such as the G120, S120, and V90 lines), the inverter continuously monitors the relationship between the applied frequency, output current, and actual rotor speed. Poor tuning of the sensorless vector control loops
| Cause | Explanation | |-------|-------------| | | Motor is consistently operating above its rated torque or current (e.g., conveyor jam, heavy cutting load). | | Insufficient cooling | Blocked cooling fins, failed external fan, or dusty environment reducing heat dissipation. | | High ambient temperature | Motor located near ovens, furnaces, or in non-ventilated enclosures. | | Low speed operation with self-cooling motor | Motors relying on shaft-mounted fans lose cooling at low RPMs but may still draw high current. | | Pulse frequency too high | Higher switching losses in motor iron core generate extra heat; parameter p1800 may be set above recommended value. | | Incorrect motor parameters | Wrong rated current or thermal time constant (p0305, p0611) leads to miscalculated temperature model. |
Once resolved, implement these preventive measures:
): Give high-inertia loads more time to accelerate without oversaturating the current loop. Verify Current Limit ( p0640p 0640 Use a hairdryer to warm probe—resistance should increase
Attempting to accelerate a heavy, high-inertia load too quickly via an aggressively short ramp-up time ( p1120p 1120
A critical note for troubleshooting: . If you are in V/f control ( P1300 = 0 ), the drive’s stall detection is largely inactive, and you will not see this code.