SINAMICS S120 Fault F07801 — Motor Overcurrent

Overview

The F07801 fault code on a Siemens SINAMICS S120 drive system indicates a Motor Overcurrent condition. This safety-critical trip occurs when the drive's control loop detects that the output current supplied to the motor has exceeded the maximum limit defined in parameters or the absolute physical capacity of the Motor Module's power unit. When this threshold is breached, the Control Unit immediately shuts down the drive (typically using an OFF2 coast-to-stop reaction) to prevent catastrophic thermal destruction of the IGBT power switches, motor stator windings, and associated cabling.

Symptoms

When the SINAMICS S120 encounters fault F07801, you will observe several immediate physical and system symptoms:

• LED Indicators: The Ready (RDY) LED on the Control Unit (e.g., CU320-2) and the corresponding Motor Module will turn solid or flashing red.
• HMI displaying Fault F07801: The machine interface or operator panel will display the fault code F07801 along with the descriptive text "Motor overcurrent."
• Immediate Motor Coasting: The motor will instantly lose torque and coast to a complete stop, regardless of any active decel ramp profiles.
• Audible Clues: Prior to tripping, you may hear physical straining, loud humming, buzzing, or metallic clicking from the motor, especially during rapid speed transitions.
• Trip Timing Clues: The fault may occur immediately upon giving a run command (indicating a dead short or ground fault) or only during rapid acceleration/deceleration phases (pointing to inertia mismatch or parameter tuning issues).

Possible Causes

Identifying the root cause of an overcurrent fault requires analyzing both the physical environment and the internal control parameters of the SINAMICS S120. Common culprits include:

• Mechanical Binding or Jamming: The driven load is physically locked, or there is an enormous frictional resistance (failed bearings, jammed gearboxes, stuck conveyers) forcing the motor to draw peak current to meet the speed setpoint.
• Motor Insulation Breakdown: A phase-to-phase short circuit or a phase-to-ground fault has developed inside the motor windings.
• Cabling Integrity Faults: The motor power cable (U, V, W) has been pinched, chafed, or degraded within a cable carrier, allowing conductors to make contact with each other or the machine ground.
• Incorrect Parameter Configuration: The basic motor nameplate parameters—such as rated current (p0305), rated power (p0307), or maximum current limit (p0640)—are mismatching the physical hardware installed.
• Aggressive Ramp Rates: Acceleration (p1120) or deceleration (p1121) times are set too low for the mechanical inertia of the load, causing massive transient dI/dt spikes.
• Defective Drive Hardware: The current sensing transformers (CTs) within the S120 Motor Module have drifted, failed, or the gate driver board/IGBT modules have suffered internal dielectric breakdown.
• Unstable Control Loop Tuning: High gains in the speed controller (p1460, p1462) or current controller (p1715) are causing unstable current oscillations that overrun safety limits.

Step-by-Step Troubleshooting

Follow these sequential diagnostics steps to isolate and rectify the cause of the F07801 fault:

Step 1: Secure the System and Perform Visual Checks

Before touching any electrical terminations, execute a formal Lockout/Tagout (LOTO) procedure on the main switch cabinets. Check the high DC-link voltage indicators on the S120 line and motor modules and verify that the voltage has discharged to a safe level (< 50 VDC) before proceeding. Verify the physical state of the motor. Try to turn the motor shaft manually (if decoupled from high-risk loads) to check for free, smooth mechanical rotation.

Step 2: Perform Insulation and Continuity Testing

Disconnect the motor power cable from the drive's output terminals (U2, V2, W2) on the Motor Module:

1. Use a high-quality Megohmmeter (Megger) to test the insulation resistance of the motor and cable system. Set the test voltage based on local standards (typically 500V or 1000V for a 400V/480V class motor).
2. Test each phase to ground (U-PE, V-PE, W-PE). The reading should be very high (typically > 100 Megohms). A low reading indicates a ground fault in the cable or motor.
3. Test phase-to-phase continuity and resistance balance (U-V, V-W, W-U) using a low-resistance digital micro-ohmmeter. All three phases must be highly symmetrical; unbalanced resistance indicates a shorted winding turn.

Step 3: Analyze System Parameters via STARTER or Siemens Startdrive

Connect your PC to the Control Unit using STARTER, Startdrive, or the web server. Evaluate the parameterized values against the actual physical equipment labels:

• Go to the parameter view for the affected drive object.
• Verify that the motor type (p0300) matches the physically installed motor.
• Check p0305 (Rated motor current) and ensure it matches the nameplate exactly.
• Review p0640 [Current limit]. Ensure this value is set to a reasonable level (typically 1.5 to 2.0 times the rated motor current, up to the maximum limit of the Motor Module).
• Look up the fault memory log (r0945, r0949). The fault value in parameter r0949 often contains diagnostic details on which phase or internal circuit detected the overcurrent condition.

Step 4: Run the Siemens Motor Identification (Motor ID Run)

If the motor, wiring, and parameters appear correct, but the overcurrent trips when trying to start up or run at specific speeds, the control loop may be misaligned:

1. Set parameter p1910 = 1 (Trigger stationary motor identification at next enable).
2. Provide a run command. The drive will pass a safe DC test signal through the motor to measure stator resistance, rotor resistance, and leakage inductances.
3. Update the calculated controller adaptations automatically via the software.

Step 5: Test the S120 Motor Module Hardware Independently

If the motor pass-through measurements are flawless, test the S120 Motor Module in an unloaded, V/f control state if possible:

1. Decouple the motor shaft from the load.
2. Switch the control mode parameter p1300 to 0 (V/f control with linear characteristic).
3. Set a long ramp-up time of 10-15 seconds.
4. Attempt to run the motor. If it runs smoothly without tripping, the issue is related to vector/servo control tuning dynamic limits or mechanical overload, not a hardware fault in the drive.
5. If the drive trips instantly even when disconnected from the motor cable, the internal current sensors or IGBT gate logic are damaged, necessitating a drive module replacement.

Recommended Actions

To prevent recurrence of the F07801 fault, implement the following operational safeguards:

• Mechanical Maintenance: Establish regular PM schedules to lubricate gearboxes, check drive belt tension, and monitor bearing temperatures to prevent sudden motor stalling.
• Ramp Time Optimization: If your process permits, increase the acceleration ramp-up time (p1120) and deceleration ramp-down time (p1121). This lowers the instantaneous dI/dt torque demands.
• Output Filters: If the motor cable length exceeds the maximum limits specified in the SINAMICS S120 manual (such as unshielded lengths over 100 meters or shielded over 50 meters), install a Siemens dv/dt or sine-wave filter at the drive's output terminals to suppress capacitive charging current spikes.
• Vector Loop Tuning: Optimize the speed controller gains to prevent torque overshoot during dynamic speed corrections.

Recommended Replacement Parts

If diagnostics confirm that the physical hardware has failed, refer to these critical replacement components:

• Active/Double/Single Motor Modules: Select the exact MLFB order code of your module (e.g., 6SL3120-1TE13-0AA3 for standard booksize modules). Both the current rating and chassis footprint must match.
• Control Unit standard memory cards: Siemens CompactFlash cards containing firmware licences can be swapped into replacement control units.
• Siemens Motion-Connect Cable Kits: High-flex, pre-assembled power cables (e.g., 6FX5002-5CG01-...) to ensure reliable shield termination and minimize capacitive leakage currents.
• Internal Power Stack Modules & Current Sensors: For block-format and large chassis units, consult the Siemens spare parts portal for the precise internal current transducer (CT) replacement part numbers.

Related Articles

• /knowledge/replacement/siemens-s120-motor-module-swap-guide
• /knowledge/compatibility/matching-siemens-motors-to-sinamics-drives
• /knowledge/guide/troubleshooting-ground-faults-in-servo-cables

FAQ

Q: Can I bypass the F07801 fault by masking it in the software?

No, F07801 is a critical hardware protection fault and cannot be bypassed or ignored. Attempting to suppress overcurrent monitoring would result in immediate catastrophic damage to the IGBT power electronics, posing severe fire and safety hazards.

Q: Why does the SINAMICS S120 only trip when the motor slows down?

This is typically caused by rapid deceleration. When a high-inertia load is forced to slow down faster than its natural decelerating curve, the motor acts as a generator. This dumps massive regenerative energy back through the Motor Module, causing a voltage and current surge that forces an F07801 or dynamic overvoltage trip. Solve this by increasing the decel ramp time or adding a dynamic braking module/brake resistor.

Q: What is the main difference between F07801 (Motor Overcurrent) and F30001 (Power Unit Overcurrent)?

While closely related, F07801 is a software/closed-loop limitation fault calculated or detected at the motor control level, often caused by configuration limits, overload, or tuning. F30001 is triggered directly by the physical hardware-bound overcurrent detection circuits inside the power unit (IGBT desaturation or physical CT trip), which typically indicates a direct short circuit or immediate hardware failure.

Q: Will updating the S120 firmware resolve regular F07801 occurrences?

Only if the issues are caused by software-based calculation anomalies in early firmware revisions. If you are using standard induction or PM synchronous motors, updating firmware rarely resolves F07801; instead, focus on checking physical motor mechanics, insulation values, and testing parameters.