Siemens Drive Fault Code 7902 [new]

Siemens drive fault code 7902

Fault code: 7902
Typical device family: Siemens SINAMICS / SIMODRIVE (frequency converters and servo drives)
Symptom: Drive reports fault 7902 and trips or blocks operation.
Likely meaning: Encoder feedback error or position feedback failure detected by the drive's encoder interface. The drive cannot reliably determine motor position/speed.
Common causes:

• Faulty encoder (incremental or absolute) on the motor (damaged cable, connector pins, water ingress, bent shaft).
• Loose, damaged, or incorrectly wired encoder cable or connector (broken shield, intermittent contact).
• Incorrect encoder type or parameter settings in the drive (mismatched pulses/revolution, wrong encoder mode).
• Excessive electrical noise on the encoder wiring (EMI from nearby power cables, lack of shielding or grounding).
• Drive firmware or parameter corruption after update or power loss.
• Fault in the drive’s encoder interface hardware (failed input stage) or a motor/internal wiring fault.
• Mechanical issues causing encoder disk slip or lost reference mark.

Diagnostic steps (ordered, concise):

1. Record the drive status and exact fault message from the HMI or diagnostic tool (time, speed, load).
2. Put the drive in a safe state (power down or hold torque off) before inspecting hardware.
3. Visually inspect encoder cable and connectors for damage, corrosion, bent pins, or loose strain relief.
4. Check connector pinout against motor/encoder wiring diagrams; ensure correct wiring and shield termination.
5. Power-cycle the drive and attempt a clear/reset of the fault; note if fault is permanent or intermittent.
6. If available, monitor encoder signals with an oscilloscope (A/B/Z or SSI/EnDat signals) to verify pulses, amplitude, and noise.
7. Verify drive parameters for encoder type, pulses per revolution (PPR), SSI/EnDat mode and any indexing settings; correct if misconfigured.
8. Swap the encoder cable with a known-good cable or substitute a known-good motor/encoder to isolate cable vs. encoder vs. drive fault.
9. Check grounding and routing: separate encoder cable from power cables, ensure shield connected at proper end per Siemens guidelines.
10. Update or reapply correct firmware if parameter corruption is suspected (follow Siemens instructions and back up parameters first).
11. If hardware failure of encoder interface is suspected, contact Siemens support or replace the encoder interface board or drive unit as directed.

Temporary workarounds (use only if safe and permitted):

• If the application allows, run the drive in torque or open-loop control mode (if supported) to move to a safe position—only under controlled, low-speed conditions and with safety measures.
• Use a backup encoder/motor (if available) to restore production while troubleshooting.

Preventive measures:

• Route encoder cables separately from power/drive cables and use screened/shielded twisted pair cable per Siemens specs.
• Properly terminate shields and ensure good earthing at the drive end as recommended.
• Regularly inspect connectors and cable strain reliefs for wear and moisture.
• Configure correct encoder parameters when commissioning or replacing motors.
• Apply surge protection and EMI suppression where needed.

When to escalate to Siemens support or an authorized service partner:

• Oscilloscope checks show no or corrupted encoder signals when the encoder appears mechanically intact.
• Fault persists after replacing cable and encoder, or after restoring correct parameters and firmware.
• Drive reports hardware-related subfaults or multiple encoder-related errors.
• You need replacement parts, firmware files, or guidance for board-level repair.

Useful information to provide when requesting support:

• Drive model and serial number, firmware version.
• Motor and encoder type (incremental/absolute, PPR/resolution, SSI/EnDat version).
• Exact fault text and fault history (including subcodes and timestamps).
• Recent changes (parameter changes, firmware updates, mechanical maintenance).
• Photos of connectors/cabling and wiring diagram if available.

If you want, I can draft a troubleshooting checklist tailored to a specific Siemens drive model (e.g., SINAMICS G120, S120, or SIMODRIVE) — tell me the exact model and motor/encoder type.

(related search suggestions provided)

Siemens drive fault code 7902 typically indicates a "Drive: Communication board (CB) link error" or a loss of communication via the PROFINET/PROFIBUS interface.

Here is a detailed breakdown of the fault, causes, and solutions.

Real-World Case Study

Scenario: A automotive paint line conveyor trips with F07902 every Monday morning but runs fine for the rest of the week.

Investigation: Technicians found no cable breaks. The motor windings tested OK. The clue was the timing – every Monday after weekend shutdown. siemens drive fault code 7902

Root Cause: A magnetic contactor between the drive and motor had a pitted auxiliary contact that overheated slightly on the first start of the week, dropping one phase for 50ms. As the day progressed, thermal expansion improved contact – until the next Monday.

Solution: Replaced contactor and implemented weekly starter torque check. Fault 7902 eliminated.

2. Check the PLC Status

• Verify if the controlling PLC is in RUN mode.
• If the PLC is in STOP mode or has faulted, it will stop sending telegrams to the drive, triggering the 7902 fault immediately.

Conclusion

Siemens drive fault code 7902 is a protective response to either genuine motor overheating or a faulty temperature sensing circuit. By following a logical diagnostic path—checking actual motor temperature, inspecting wiring, verifying parameters, and analyzing historical data—most instances can be resolved in under 30 minutes.

Remember: Never ignore 7902. While sensor faults are easy fixes, ignoring genuine overtemperature will lead to premature motor insulation failure, costly rewinds, or even a stalled production line.

For persistent cases, Siemens technical support can analyze a drive trace (via Startdrive recording function) to pinpoint whether the fault is load-related or hardware-related. Keep your motor parameters accurate, your cooling vents clean, and your sensor wiring intact—and fault 7902 will become a rare visitor in your maintenance log.

This article is based on Siemens SINAMICS documentation and field experience. Always refer to the official List Manual for your exact drive series and firmware version before changing parameters.

Understanding and Resolving Siemens Drive Fault Code 7902 The Siemens Drive Fault Code 7902 (often displayed as F07902) is a critical error indicating a "Motor Stalled" condition. This fault occurs when the drive’s control logic determines that the motor is not rotating as expected despite receiving power, or when the deviation between the calculated motor speed and the actual measured speed exceeds defined limits.

While this fault is common across various series including SINAMICS S120, G120, G120XA, and G115D, its root causes can range from simple mechanical blockages to complex parameterization errors. Primary Causes of Fault 7902

Understanding why the drive triggers a stall fault is the first step in troubleshooting. Common triggers include: SINAMICS G120XA fault F07902 - ID: 109773411 - Support

The fluorescent lights of the MagnaDrive factory floor hummed in a low, monotonous key, a sound usually drowned out by the rhythmic thwump-hiss-thwump of the heavy-duty stamping presses. But tonight, the factory was holding its breath.

Elias Thorne, a third-shift maintenance technician with twenty years of grease under his fingernails, sat in the breakroom nursing a lukewarm coffee. He was staring at the clock. It was 2:00 AM. In the world of industrial automation, 2:00 AM is the witching hour—the time when bearings decide to seize, sensors decide to sleep, and PLC logic decides to glitch. Siemens drive fault code 7902 Fault code: 7902

As if on cue, the breakroom lights flickered. The deep, vibrating thrum of the plant’s ventilation system shuddered and died. A moment later, the emergency lights kicked in, bathing the hallway in an eerie, sterile red.

Elias’s radio crackled to life. It was the shift supervisor, Marcus, his voice tight with panic.

"Elias. Line 4. Main stamper. It’s down. Totally dead. The screen is screaming at me."

Elias sighed, pushing himself out of the chair. "I'm on my way. Don't touch anything, Marcus."

Line 4 was the heart of the factory, a massive Siemens-driven servo press responsible for churning out the chassis plates for the new electric vehicle contract. If it was down, the morning shift would start with a disaster, and the plant manager would be breathing down everyone's necks before sunrise.

When Elias arrived, the massive machine stood silent, a sleeping giant. The control cabinet door was open, and Marcus was standing back, looking at the Siemens Sinamics S120 drive module like it was a bomb about to detonate.

"What’s the code, Marcus?" Elias asked, pulling his laptop bag onto a nearby workbench.

"It’s a 7902," Marcus said, reading the 7-segment display on the BOP (Basic Operator Panel). "Just '7902'. I tried resetting it, but it just clicks and throws the same code again."

Elias stopped. He pulled off his safety glasses and wiped the sweat from his forehead. He knew this code. Every drive tech knows the "standard" faults—overvoltage (F0002), overcurrent (F0001)—but the 7-series faults were different. They were communication faults. Ghosts in the machine.

"Fault 7902," Elias muttered, typing his password into the Siemens TIA Portal software on his laptop. "Communication fault. Drive-to-Drive data exchange failure."

"That doesn't make sense," Marcus snapped. "We didn't change anything. It was running fine at 1:45, and then—bang—e-stop tripped, and this code popped up." Faulty encoder (incremental or absolute) on the motor

Elias connected the Profinet cable to the drive’s CU320-2 control unit. The laptop chimed. He navigated to the diagnostics buffer. The list was populated, but one entry was highlighted in red, pulsating with urgency.

F07902: Drive-to-drive communication failure.

"It’s not the PLC," Elias murmured, scrolling through the parameters. "The PLC is talking to the drive just fine. The Profibus light is green. This fault... it means this drive isn't hearing its partner."

"Partner?" Marcus asked. "The press only has one main motor."

Elias shook his head. "It’s an S120 multi-drive system, Marcus. Look at the cabinet. One line supply module, but two motor modules. There’s a main drive for the flywheel, and a secondary servo for the feeder that pushes the metal sheets in. They talk to each other via a telegram to synchronize the speed. If the feeder drive doesn't report its position to the main drive, the main drive locks out to prevent crushing the tooling."

Marcus looked at the row of black modules in the cabinet. "Okay, so we check the second drive."

They moved to the lower section of the cabinet. The lights on the second motor module were dark. Completely dark.

"Power loss," Elias said immediately. "The module has no power. That’s why the main drive is throwing 7902. It’s shouting 'Where are you?' and getting silence."

The diagnosis seemed simple, but Elias knew better. If it was just a loose wire, the breaker would have tripped. He pulled out his multimeter.

"Breaker is on," he muttered. "Voltage at the input terminals... 480 volts, phase to phase. The power is there."

He checked the DC link voltage. The capacitors in the line module were humming, charged and ready. But the second motor