Resolving F011 F012 Overcurrent and I-Too-Low on Siemens 6SE7026

Problem Statement and Engineering Context

A Siemens SIMOVERT MASTERDRIVES unit, MLFB (machine-readable product designation, German Maschinenlesbare Fabrikatebezeichnung) 6SE7026-0TD21, used on a chute telescoping application, trips on fault F011 the moment a run command is issued. With the motor leads physically disconnected for a no-load bench test, the same drive trips on F012. An attempt to bypass the protection by editing parameter P060 from 7 to 5 was rejected by the PMU (Parameterization and Monitoring Unit, the front-panel LCD and keypad), which displayed two horizontal lines ("==") and refused to commit the value. The discussion below lays out the engineering meaning of each fault, why P060 in particular refused to change, and a field-proven sequence to bring the drive back online without compromising protection.

The reported sequence of events — F011 with motor connected, F012 with motor disconnected, parameter change refused — is a classic fingerprint for one of three underlying problems:

1. Closed-loop vector control (with encoder feedback) on a vector card, but the encoder is faulty, mis-aligned, or the wrong direction. The controller commands excessive current trying to track a non-existent rotor position and the output stage trips on F011. With the motor unplugged, the same controller reads zero current, fails the I-min check, and trips F012.
2. An open phase or high-resistance connection in the motor cable, the motor winding, or the current-transformer (CT) feedback path. The drive sees an imbalance that drives one phase into the overcurrent limit, and it also sees the average current fall below the I-min threshold.
3. A genuine overload or short circuit, with the encoder contribution as a secondary effect. A winding failure to ground, a wet terminal box, or a cable insulation breakdown produces both F011 (high phase current) and F012 (current imbalance tripping the I-min window) depending on exactly which phase has faulted.

The remainder of this article walks through identification, fault-by-fault diagnosis, and a step-by-step procedure that resolves all three cases without masking a real fault.

Drive Identification and MLFB Breakdown

The Siemens 6SE70 family (later marketed as SIMOVERT MASTERDRIVES, with the modern successor line being SINAMICS G150/G180/S120) is identified by a 12-character MLFB. The first eight characters 6SE7026 decode as follows:

CU2 vs CUVC Control Card: First Verification Step

The original SIMOVERT MASTERDRIVES were supplied with a CU2 control card (sometimes written "CU II" or "Compuverter 2"). When vector-control firmware became standard, the slot was repopulated with the CUVC card. The two cards use different firmware, different parameter sets, and different fault dictionaries. Confirming which one is fitted is the single most important verification step before any further troubleshooting.

To confirm which card is fitted:

1. Power down the drive, lock and tag the disconnect, wait for the DC-link capacitors to discharge. The PMU continues to flicker for several minutes after power-off; the standard five-minute rule applies.
2. Open the front cover and read the part number on the large PCB occupying the upper slot of the control bay. The label will read 6SE7090-0XX84-... for CUVC, or one of the CU2 part numbers for the older card.
3. Apply power, press the PMU "P" key to enter parameter mode, and read r005 (firmware version) or r006 (control card type). CUVC firmware reports "VC" or a version string in the 4.x range; CU2 firmware reports earlier version identifiers.
4. As a cross-check, navigate to P060. If the parameter responds with a current-related function (e.g., "current limit" or a percentage of rated current), the unit is almost certainly a CUVC. If the parameter is a setpoint source or a small-integer selector, it may be a CU2 or a CUVC operating with a non-default parameter set; cross-reference against the firmware manual.

All references below assume the CUVC control card, which is the dominant population of the 6SE7026 in service today. If the drive is confirmed CU2, the fault codes carry the same names but the parameter numbers in the recovery procedure will differ. Cross-reference the Siemens Industry Online Support portal for the CU2 firmware manual.

F011 Overcurrent - Definition and Trigger Conditions

F011 in the 6SE70 family is the overcurrent fault. The inverter's gate-driver stage measures the actual phase current on every PWM cycle and trips when the instantaneous current exceeds the configured trip threshold. The trip is hardware-validated — it is enforced by the gate-driver ASIC itself, not by the CUVC firmware — so F011 cannot be "talked away" by parameter changes once the trip has fired.

Common root causes of F011, ordered by frequency in the field:

1. Output short circuit. Phase-to-phase or phase-to-ground short in the motor cable, the motor terminal box, or the motor winding. Inspect the cable run for crushed insulation, water ingress, and animal damage. Megger the motor windings to ground; a healthy 6SE7026-class motor reads >100 MΩ per phase to ground with the cable disconnected.
2. Encoder fault in closed-loop vector mode. Discussed in detail below. The vector controller commands excessive current trying to align a non-existent rotor flux axis. This is the most common F011 in vector-controlled applications on CUVC drives.
3. Excessive load / stalled motor. The chute telescoping mechanism in the original report may be jammed. Manually verify free movement of the screw, belt, or chain that the motor drives. A 4:1 or higher reduction gearbox can stall a healthy motor if the downstream mechanism is jammed, and the CUVC will pull rated current trying to track the speed setpoint.
4. Ramp time too short. A very fast acceleration setpoint drives the current limit. Check P462 / P463 (ramp-up / ramp-down times) and the speed setpoint ramp on the upstream controller.
5. CT (current transformer) failure. On older 6SE70 units, the CTs in the DC link can fail with age. A failing CT reports phantom current and trips the gate driver. This is rare but worth noting on a drive with 15+ years of service.
6. Power module (IGBT) failure. A shorted IGBT inside the power stack draws uncontrolled current the moment the gate is fired. The trip is immediate on the first run command. Diagnosed by resistance checks across the DC+ and the three output phases with the cables disconnected and the drive powered off.

F012 I Too Low - Definition and Trigger Conditions

F012 in the 6SE70 family is the I-too-low fault. The output current has fallen below the configured I-min threshold for longer than the configured debounce time. The I-min monitor is implemented in firmware (unlike the F011 hardware trip), so it can be tuned or disabled, but disabling it is not a repair — it is a temporary measure only.

Why F012 appeared on the bench test with the motor disconnected:

• Vector control without a motor. The CUVC in closed-loop vector mode expects the connected motor to draw magnetising current. With the cables off, the measured current is zero, the I-min monitor times out, and F012 fires. This is a feature, not a fault — the monitor is protecting the drive from running with a missing load.
• Open phase in the motor cable. A loose terminal, a broken conductor, or a contactor with a welded-closed main and an open aux can present as "current too low" in the phase that has lost its return path.
• Current-transformer (CT) open circuit. Less common, but a failed CT primary or secondary can read zero current to the controller.
• Field weakening or speed setpoint out of range. In some applications, an over-ambitious field-weakening setpoint or an encoder-fitted motor running above base speed will draw very little current; the I-min monitor can trip if the threshold is set too high for the operating point.

Vector Control Mode and F011/F012 Sensitivity

The chute telescoping drive is most likely configured for vector control (closed-loop with encoder, or sensorless vector). Vector control is preferred for any hoist, telescope, or synchronised motion because the speed and torque accuracy of a V/Hz drive is insufficient for the application. But vector control makes the drive sensitive to faults that a V/Hz drive tolerates:

To check the current control mode in the CUVC:

1. Read P100 (control mode). Typical values: 0 = V/Hz, 1 = sensorless vector, 2 = closed-loop vector (with encoder), 3 = torque control (sensorless), 4 = torque control (with encoder). Confirm against the firmware manual for the exact mapping in the installed version.
2. Read P151 (encoder pulses per revolution). A value of 0 means "no encoder configured" and rules out closed-loop vector.
3. Read r040 (actual rotor position, in degrees electrical). If this value is 0 or stuck when the motor turns, the encoder feedback is broken.

Encoder Failure as a Hidden Cause of F011

Field experience with the 6SE70 CUVC and its successors (SINAMICS S120) shows that a failing encoder is the single most common cause of an unexplained F011 in a vector-controlled application. The failure mode is well-documented and counterintuitive on first inspection: an encoder fault produces an overcurrent fault, not a "speed feedback loss" fault, because the controller's reaction to bad feedback is to command more current trying to drive the rotor to the commanded angle.

The cascade looks like this:

1. The encoder signals (channels A, B, Z for an incremental encoder; CW/CCW for a single-channel) become noisy, intermittently shorted, or disconnected.
2. The CUVC's encoder interface reads a rotor position that is not the real rotor position.
3. The vector controller, believing the rotor is at angle θ_cmd, commands stator current on the wrong axes.
4. Instead of producing torque, the current produces a braking or accelerating force in the wrong direction. The controller sees the rotor not moving to the commanded position and increases the current command to track harder.
5. The current command reaches the overcurrent threshold. F011 fires.

To diagnose an encoder-induced F011:

1. Read r040 (rotor position feedback) and r041 (actual speed from encoder) at the PMU or via DriveMonitor / STARTER commissioning software.
2. Manually turn the motor shaft by hand (with the drive inhibited) and observe r040. The value must change smoothly by 360 electrical degrees per mechanical revolution (or 360/pole-pairs for a multi-pole motor). A stuck or noisy value confirms encoder failure.
3. Inspect the encoder cable for shielding integrity, ground connections, and routing away from VFD power cables. Older 6SE70 encoder interfaces have limited isolation; cable problems are the most common root cause.
4. Check the encoder supply voltage at the encoder terminals (typically 5 V or 24 V depending on the encoder). A drop below spec under load indicates cable resistance too high or encoder draw too high.
5. Swap in a known-good encoder on a temporary basis. If the fault clears, the encoder is the root cause.

With the motor leads physically disconnected, the same controller continues to expect current, reads zero, and trips F012. The two faults together — F011 with motor, F012 without motor — are therefore very strong evidence that the drive is in closed-loop vector mode and the encoder is bad, the motor is mismatched, or the encoder parameter group has not been commissioned.

Parameter 60 (P060) and the "==" Display Indication

The discussion records an attempt to change P060 from 7 to 5. On the CUVC, the function of P060 depends on the parameter set in use: in some factory default sets it is a current-limit value; in others it is a setpoint or control-source selector with a small-integer range (0-15). The change attempted — from 7 to 5 — is more consistent with the latter, since current limits are typically percentages and not single-digit integers. The exact function is in the CUVC parameter list for the firmware version installed; cross-reference the parameter list shipped with the unit or downloaded from the Siemens Industry Online Support portal.

The important diagnostic point is not what P060 was being changed to, but that the change was rejected with a display of two horizontal lines. In the SIMOVERT MASTERDRIVES PMU, the two-line display "==" (or "----" on some firmware versions) means one of the following:

For the rejection observed on the PMU during the bench test, the most likely cause is that the drive was in a fault state (F011 or F012 still active) when the parameter change was attempted. The CUVC will not accept parameter changes while a fault is latched. The correct sequence is:

1. Acknowledge the fault by pressing the "OFF" / reset key on the PMU, or by toggling the OFF1 / OFF3 input on the terminal strip.
2. Confirm the drive is in state o7.0 or o7.2 (ready to run, waiting for ON command) on the PMU status display.
3. Press P to enter parameter mode, navigate to P060, change to the new value, press P to commit.
4. If the parameter still shows ==, confirm that P053 (parameter access level) is set to a value that permits the change (typically 6 for expert).
5. If the parameter still shows == with expert access enabled, the new value is outside the firmware-defined limits and the change cannot be made.

Output Stage, Cable, and Motor Diagnostic Tests

With the drive locked and tagged, perform the following tests in this order. Each test is non-destructive; the goal is to localise the fault before applying power.

1. Cable and motor insulation

1. Disconnect the motor leads from the drive terminals (U, V, W).
2. Use a 500 V or 1000 V megohmmeter (Megger) to measure insulation resistance from each phase to ground and phase-to-phase. A healthy motor with clean, dry insulation reads >100 MΩ. A reading below 1 MΩ indicates insulation failure; a reading between 1 MΩ and 100 MΩ indicates moisture or contamination.
3. Measure the same way on the motor cable with the motor end disconnected.
4. If insulation is marginal, dry the motor and cable before re-energising. A wet motor on a vector drive will produce exactly the F011/F012 pattern observed.

2. Output stage health (IGBT modules)

1. With the drive powered off and locked out, measure the resistance from DC+ to U, V, W using a digital multimeter on the diode range. A healthy IGBT module reads a diode drop (~0.3-0.7 V) in one direction and open in the other.
2. Measure from DC- to U, V, W in the same way.
3. A shorted module reads near 0 Ω in both directions. An open module reads open in both directions. Either condition requires IGBT module replacement.
4. Repeat from U to V, V to W, and W to U on the output side. Phase-to-phase shorts inside the IGBT stack present the same way.

3. Motor winding balance

1. With the motor cable disconnected from the drive, measure the resistance from U to V, V to W, and W to U at the motor terminal box.
2. The three readings should match within 5% for a healthy three-phase induction motor. A spread of more than 5% indicates turn-to-turn failure or loose connections.

4. Encoder cable

1. Inspect the encoder cable for shielding integrity. The shield must be bonded to ground at the drive end, ideally through a 360-degree backshell, and must not be bonded at the motor end (to avoid ground loops).
2. Measure the encoder supply voltage at the encoder terminals under load. A drop of more than 0.5 V below the nominal supply indicates cable resistance too high or encoder current draw too high.
3. With the drive inhibited, oscilloscope the encoder A and B channels while slowly turning the motor shaft. Both channels must produce clean square waves at 50% duty cycle, 90 electrical degrees out of phase. Noise, missing edges, or asymmetric duty cycle indicate encoder or cable failure.

Current Sensor and I-Feedback Path Verification

If the motor, cable, and IGBT modules all pass the static tests, the F011/F012 pair most likely originates in the current feedback path. On the 6SE7026, the current feedback uses Hall-effect CTs mounted on the DC-link bus and on the output phases. The signal path is:

To verify the current feedback path:

1. With the drive inhibited but powered (DC link charged, gate drivers off), read the actual current monitors r019 (U-phase), r020 (V-phase), r021 (W-phase) at the PMU.
2. All three values must be near zero (within 1-2% of rated current). A reading significantly different from the other two indicates a failed CT or signal conditioning component on the I-Feedback card.
3. Run the drive in V/Hz mode (P100 = 0) with no motor connected, at low frequency (e.g., 5 Hz) and zero speed setpoint. The current monitors should all read a small magnetising current and remain stable. If one phase drifts or oscillates, the CT or feedback card has failed.
4. If the I-Feedback card is suspect, swap with a known-good card from a sister drive (6SE70 units of the same frame use the same I-Feedback board). Re-test.

Step-by-Step Diagnostic Procedure and Verification

1. Lock out and tag out. Open the upstream disconnect, lock it, and verify zero voltage at the drive input terminals with a known-good voltage tester. Wait 5 minutes for the DC-link capacitors to discharge. Confirm the PMU has gone dark.
2. Identify the control card. Open the front cover, read the part number, confirm CU2 vs CUVC. If CUVC, read the firmware version from r005 and download the matching parameter list from the Siemens Industry Online Support portal.
3. Read the active fault memory. Navigate to the fault buffer (r047 for the most recent fault, r048 for the prior). Record the fault, the actual current at the time of trip (if displayed), the actual speed, the IGBT temperature, and the DC-link voltage. This is the engineering evidence for what actually happened.
4. Inspect the encoder. Follow the encoder cable from the motor to the drive. Verify shield bonding, connector pin engagement, and cable routing. Manually turn the motor shaft and observe r040 on the PMU. The value must change smoothly.
5. Test the motor and cable. Megger the motor windings and cable to ground. Measure winding balance. Inspect the terminal box for moisture, animal damage, and loose connections.
6. Test the IGBT modules. Resistance checks on DC+/-, phase-to-phase, and phase-to-ground with the drive powered off.
7. Test the I-Feedback path. With the drive inhibited but powered, read the actual current monitors. All three must be near zero. If not, replace the I-Feedback card or the affected CT.
8. Make a no-load bench test in V/Hz mode. Temporarily set P100 = 0 (V/Hz open-loop), P151 = 0 (no encoder), P357 = 0 (disable I-min for the test only). With the motor cables connected, command 5 Hz. The drive should accelerate the motor to a slow speed. If it does, the motor, cable, and power stage are healthy.
9. Re-enable vector mode. Set P100 = 2 (closed-loop vector with encoder), P151 to the correct encoder PPR, and the motor nameplate data into the motor parameters. Run a 5 Hz command. If F011 returns, the encoder is the root cause and must be replaced or its cable repaired before the drive can be returned to vector control.
10. Restore production parameters. Once the drive runs cleanly in vector control, restore P357 to its application-appropriate value (typically 5-10% of rated current). Confirm the fault memory shows no new faults.
11. Functional test under load. Command the chute telescoping cycle at the production speed setpoint. Monitor the actual current on the PMU; it should track the load profile without approaching the I-max limit. Run at least ten cycles.
12. Documentation. Record the firmware version, the encoder PPR, the motor nameplate data, and the final parameter values for the next maintenance event. Save the fault buffer snapshot as a baseline.

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