Siemens SINAMICS S120 Fault F07900 — Motor Overtemperature Fix

Siemens SINAMICS S120 Fault F07900 — Motor Overtemperature (Actual)

The Siemens SINAMICS S120 is a modular, multi-axis servo drive system used in high-performance CNC machine tools (Siemens SINUMERIK 840D sl/840D, 828D), servo presses, printing machinery, winding equipment, and industrial robots. It consists of a Line Module (ALM, BLM, or SLM) and one or more Motor Modules (single or double-axis) connected via DC bus and DRIVE-CLiQ communication.

Fault F07900 means the S120 Motor Module detected that the motor’s actual temperature (measured via a KTY84, PTC, or NTC temperature sensor integrated into the motor winding) has exceeded the trip threshold. The drive shuts down the affected axis to prevent motor insulation failure. F07901 is the companion fault — same alarm but triggered by the motor’s thermal model (software temperature estimate) rather than a physical sensor.

Jump to Fix

Related S120 Faults (Referenced in This Article)

Common Causes

• Sustained overload (F07900/F07901) — The most common cause is the motor running above its rated torque for too long. On CNC machine tools, this happens when axes are pushed beyond the motor’s continuous current rating in heavy-cutting cycles. The motor’s thermal time constant (typically 20–30 minutes) means the alarm may not appear until well into a long cycle.
• Missing or degraded motor cooling — Siemens 1FK7, 1FT7, and 1PH motors intended for constant-torque variable-speed operation (e.g., main spindle, rotary indexer) require forced cooling. If the cooling fan or liquid cooling circuit fails, the motor heats rapidly. A blockage in the coolant passages produces a gradual rise in A07910 warning before the F07900 fault.
• Incorrect thermal model parameters (F07901) — If the motor has been replaced with a different model and the parameters were not updated, the S120’s internal thermal model will be incorrect. The model uses rated current (p0307), rated speed (p0311), and motor mass (p0344) to estimate winding temperature. An incorrect p0307 (motor rated current) is the most common source of premature F07901.
• Temperature sensor failure — A failed KTY84 sensor in the motor winding will output a resistance that the S120 interprets as high temperature. A KTY84 at 20°C reads approximately 580 Ω; at 100°C it reads approximately 1050 Ω. An open sensor reads >>10 kΩ, causing immediate F07900.
• Motor sizing error — If the motor/amplifier was sized without adequate margin for the actual application duty cycle, the motor will run hot in normal operation. This is a commissioning error, not a maintenance fault.

Step-by-Step Fix {#step-by-step-fix}

1. Check the motor temperature at the time of fault. In STARTER or SINAMICS Startdrive commissioning software, navigate to Drive Object → Diagnostics → Fault buffer or use the SINUMERIK HMI alarm list to read the fault value. The fault value for F07900 encodes the measured temperature — a value like 125°C confirms the motor was genuinely hot. A temperature reading of 300°C+ indicates a sensor fault (open KTY84 or broken sensor cable).

2. Let the motor cool and inspect the cooling system. Allow at least 45 minutes cooling before restarting. During this time:

   • For air-cooled motors: verify the integral fan (if present) is running. Check for debris blocking the fan intake.
   • For forced-air cooled motors (separate fan): verify the fan motor is powered and running. On Siemens 1PH8 spindle motors, the forced cooling fan is a separate 3-phase motor on the motor’s non-drive end.
   • For liquid-cooled motors: check coolant flow rate and coolant temperature. Inspect for kinked coolant hoses.

3. Measure the KTY84 / temperature sensor resistance. At the motor terminal box (or DRIVE-CLiQ connector on newer Siemens motors), disconnect the temperature sensor leads (typically a 2-wire KTY84 connected to the motor’s encoder/temperature cable). Measure resistance with a digital multimeter:

   • At room temperature (20°C): KTY84 should read 580 ± 50 Ω
   • At 100°C: approximately 1050 Ω
   • Open circuit (>50 kΩ): sensor failed — replace motor or temperature sensor insert
   • Short circuit (<100 Ω): wiring fault or failed sensor — check cable, then sensor

   PTC sensors: should read <1 kΩ when motor is cold; will spike to >100 kΩ when tripped.

4. Verify thermal model parameters (for F07901). In STARTER/Startdrive, navigate to the Drive Object and check:

   • p0307 — Rated motor power (kW). Must match nameplate exactly.
   • p0311 — Rated motor speed (RPM). Must match nameplate.
   • p0344 — Motor weight (kg). Used to calculate thermal mass; incorrect value causes wrong model temperature.
   • p0612 — Overtemperature monitoring mode. Check if thermal model and/or temperature sensor are both active.
   • p0625 — Ambient temperature for model. Default 20°C; if installed in a hot environment, increase this.

5. Check the drive’s actual load current. Use STARTER or SINUMERIK HMI diagnostics to monitor the Motor Module’s actual output current during the machining cycle. Compare to the motor’s rated current (p0307 / nameplate):

   • If current is consistently above 100% of rated: the motor is undersized or the machining parameters are too aggressive. Reduce feed rate, depth of cut, or consult machine tool application engineering for proper sizing.
   • If current is normal but F07900 still occurs: the temperature sensor or cooling system is the likely fault.

6. For persistent F07900 with all checks passing — The KTY84 sensor inside the motor winding may be intermittently drifting high. This can only be resolved by replacing the sensor (requires motor disassembly by a motor rewind shop) or by replacing the motor if within warranty.

7. Check the DRIVE-CLiQ encoder/sensor cable. On Siemens motors with DRIVE-CLiQ (1FK7-CT, 1FT7-CT), the temperature sensor data is transmitted digitally over the DRIVE-CLiQ cable along with encoder data. A damaged DRIVE-CLiQ cable or failing connector can cause incorrect temperature readings. Inspect the cable and connector at both the motor and the Motor Module.

Parts Often Needed {#parts-often-needed}

When to Call a Professional

Siemens SINAMICS S120 systems in machine tool applications are typically commissioned with SINUMERIK CNC parameters tightly integrated with the servo drive parameters. Changing motor parameters (p0307, p0311) requires updating the machine data in the NC as well, or the axis may behave incorrectly (wrong max speed, wrong torque limit). Always back up machine data (NC, PLC, drive) using SINUMERIK HMI → Service → Data Backup before changing any parameters. Siemens has a 24-hour technical support hotline for machine tool drives (1-800-241-4453 in North America); for on-site service, Siemens Industry Service has certified machine tool technicians.

A persistent F07900 that returns within minutes of restart typically indicates the motor needs replacement — continued operation risks catastrophic winding failure and a much longer downtime event.

Pro tip: SINAMICS S120 stores a fault history buffer accessible via STARTER or Startdrive under Diagnostics → Fault/alarm buffer. Each entry includes the fault number, time stamp, and the parameter value that caused the fault (e.g., the temperature reading that caused F07900). For intermittent thermal faults, export the fault history and plot the temperature readings over time to identify whether there’s a gradual thermal rise (cooling system issue) or sudden spikes (sensor fault). This data is invaluable for convincing a machine tool manufacturer’s application engineer that the motor needs replacement.

See Also

• Siemens SINAMICS G120 Fault F00001
• Siemens SINAMICS G120C Fault Codes
• Siemens SINUMERIK 840D Alarm 380500
• Siemens Micromaster 440 Fault F001
• Yaskawa Sigma-7 SGD7S Servo Drive Alarm Codes

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