SINAMICS G120 F07900: Motor blocked on start, what to check?
My SINAMICS G120 is throwing an F07900 fault. It happens right at the start of the process, and the motor barely turns before the trip. What should I check first?
The F07900 fault on a SINAMICS G120 indicates the motor has operated at its torque limit for longer than the configured delay time (p2177) while remaining below the speed threshold (p2175). This often points to a mechanical issue preventing motor rotation or an incorrectly set torque limit. It can also be triggered by an oscillating speed loop or thermal monitoring reducing the current limit.
Likely causes and checks
1Mechanical jam or unreleased brake
- Points to it
- The motor cannot rotate freely, causing it to hit the torque limit immediately at start-up.
- Check
- Verify that the motor can rotate freely and that any holding brake is releasing correctly.
2Torque limit too low for the load
- Points to it
- The drive's torque limits (r1538/r1539) are set lower than what is required to move the load, causing the motor to operate at the limit.
- Check
- Review the positive (r1538) and negative (r1539) torque limits to ensure they are appropriate for the application's heavy load conditions.
3Detection thresholds too tight
- Points to it
- The speed threshold (p2175) or delay time (p2177) for detecting a blocked motor are set too restrictively for normal, heavy starts.
- Check
- Examine parameters p2175 (speed threshold) and p2177 (delay time) to ensure they are suitable for the application's operational characteristics.
4Current limit reduced by Power Module thermal monitoring
- Points to it
- The power unit's thermal monitoring has reduced the current limit, causing the motor to operate at this reduced limit for an extended period.
- Check
- Check parameter p0290 (power unit overload response) and assess the cooling of the cabinet, especially if the fault occurs after long running in a hot environment or is preceded by A05000 alarms.
Next check: Given the motor barely turns before the trip, start by checking for any mechanical obstructions or if the holding brake is not releasing properly. This is the most common cause for immediate blocking at start.
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