MICROMASTER F231 Fault Diagnosing Output Current Measurement

1. Problem Definition: F231 on Siemens MICROMASTER

The fault code F231 on a Siemens MICROMASTER drive — including the MICROMASTER 3 Vector (6SE32…) family and the Midimaster line — indicates an excessive difference between the measured output current phases. Internally, the drive compares the three phase current samples read from its Hall-effect/LEM current transducers against each other and against an expected model. When the delta between any two phases exceeds the firmware's plausibility window, the controller trips immediately and latches F231.

Unlike F001 (overcurrent during run) or F002 (overvoltage DC bus), F231 is a measurement-integrity fault. The drive is not necessarily reporting a real overcurrent event; it is reporting that its own sensing system is returning inconsistent readings. This distinction drives every diagnostic decision that follows.

2. Affected Product Families and Order Numbers

F231 has been documented across the legacy MICROMASTER 3 platform, which was sold from the mid-1990s through the mid-2000s before being superseded by the MICROMASTER 4 (6SE64…) and SINAMICS V20/G120 lines. Confirm the family before ordering spares.

3. Root Causes — Why F231 Trips

F231 is genuinely multi-causal. Field data shows the following distribution of root causes on units returned for repair:

4. Safety Preconditions Before Any Measurement

1. Isolate and lock out the drive's line-side disconnect. Wait a minimum of 5 minutes after power removal for the DC-link capacitors to discharge below 50 V DC. Confirm with a CAT III 1000 V meter at the DC+ and DC- terminals.
2. Verify zero energy at the line terminals (L1/L2/L3), DC bus, and motor terminals (U/V/W).
3. Disconnect the motor cable from U/V/W at the drive end. Do not disconnect at the motor end unless the cable is suspect end-to-end.
4. Use PPE rated for the installation (Class 0 gloves minimum for 400 V class drives, arc-rated face shield, insulated tools).
5. Confirm the drive is properly grounded before any insulation test. A floating ground can produce false readings or damage the drive's EMC filter.

5. Diagnostic Procedure — Step-by-Step

5.1 Confirm the fault with the motor disconnected

1. Remove the motor leads from U, V, W.
2. Power up the drive.
3. Place the drive in V/F mode by setting P077 = 0 if it is not already. Parameter P077 on MM3 selects the control mode:
   • P077 = 0 — V/F with linear characteristic
   • P077 = 1 — V/F with FCC (flux current control)
   • P077 = 2 — Sensorless vector control
   • P077 = 3 — Vector control with encoder (where fitted)
4. Issue a low-frequency run command (5 Hz) with no motor connected.
5. Observe whether F231 re-occurs.

5.2 Insulation resistance test on the motor cable

1. With the cable still disconnected from the drive, megger each conductor (U, V, W) phase-to-phase and phase-to-earth (PE) at 500 V DC for 1 minute.
2. Acceptable reading: > 100 MΩ for new cable, > 1 MΩ for service-aged cable at 500 V.
3. Test the protective earth conductor separately for continuity.

5.3 Motor winding resistance balance

1. With the motor leads isolated from the drive, measure phase-to-phase resistance at the motor terminal box: U-V, V-W, W-U.
2. For a healthy three-phase induction motor, the three readings should match within ± 5 %.
3. Use a low-resistance ohmmeter (e.g., a 4-wire Kelvin meter such as a Megger DLRO10 or AEMC 6250). A standard DVM will read inaccurately below 1 Ω and may mask imbalance on larger motors.
4. For a 75 kW motor, a typical phase-to-phase resistance might be in the range of 0.05–0.5 Ω depending on motor design voltage and winding configuration. Record the absolute value and compare relative spread.

5.4 No-load test with motor connected

1. Reconnect the motor cable to the drive.
2. Run the motor at 50 % of base frequency, no mechanical load.
3. Monitor the per-phase current display (parameter r019 on MM3 Vector) or read the three phase currents from the BOP/AOP in real time.
4. Phase currents should be balanced within ± 10 % of their average.

6. Verifying the Drive Itself

If the fault persists with motor and cable removed and the drive still in V/F mode, the problem is inside the drive. Continue with:

6.1 Current sensor (LEM/Hall) check

1. With the drive powered and no motor connected, read r019 (output current) on each phase.
2. All three readings should report 0.0 A ± noise floor (typically ± 0.5 A on a 75 kW unit).
3. If one phase shows a sustained offset of several amps while the other two read zero, the corresponding LEM transducer or its signal-conditioning op-amp on the control PCB has failed.

6.2 IGBT module test (drive de-energized)

1. After confirming zero energy on the DC bus, disconnect the motor leads from U/V/W.
2. Using a digital multimeter on diode-test mode, measure between each output terminal (U, V, W) and DC+ / DC- on the drive's power stage.
3. Each IGBT should show a forward voltage of approximately 0.3 – 0.7 V in one direction and open-circuit in the other.
4. A shorted IGBT reads near 0 V in both directions on one or more phases; this is conclusive evidence of module failure.

If any IGBT reads shorted, replace the entire power module assembly (the MICROMASTER 3 typically uses integrated SKiiP or discrete SEMiX/Eupec modules depending on frame size). Reconditioning individual IGBTs is not field-repairable on most frame sizes.

6.3 Gate-drive PCB inspection

On the MICROMASTER 3 Vector, the gate-drive signals pass through an intermediate PCB mounted on the power stage. Inspect for:

• Burnt or discolored resistors in the gate-drive path.
• Electrolytic capacitor leakage or domed tops.
• Cracked solder joints, especially around the large gate-driver ICs.
• White residue from leaked RTV silicone on the PCB surface (common in older units).

7. The Cold-Start Variant of F231

A specific failure mode reported on 75 kW Midimaster Vector units is F231 occurring only on the first 2nd or 3rd start attempt after mains is applied, with normal operation after warm-up. This pattern strongly indicates:

• Cold solder joint on a current-sensor connector or gate-drive signal that opens during thermal contraction.
• DC-link capacitor ESR increase — on cold start, the inrush through high-ESR caps creates a transient that confuses the current-balancing algorithm.
• IGBT with marginal Vce(sat) — a borderline IGBT may pass current normally when warm but exhibit leakage asymmetry when cold.

Repair path: reflow suspect solder joints, measure DC-link capacitor ESR (any capacitor reading > 3× nameplate ESR should be replaced as a set), and bench-test the IGBT modules after the drive has fully warmed to operating temperature.

8. Repair vs. Replace Decision

9. Verification After Repair

1. Visual inspection: confirm all terminal screws torqued to spec (typically 2.5 Nm for power terminals on MM3 ≤ 7.5 kW; refer to nameplate for larger frames).
2. Insulation test: megger motor cable and motor windings, record values in maintenance log.
3. No-load run: start drive in V/F mode (P077 = 0) at 50 % frequency for 10 minutes; monitor r019 for balance.
4. Loaded run: apply mechanical load; ramp from 0 to 100 % frequency; confirm no F231, no abnormal current imbalance.
5. Thermal soak: leave drive running at rated load for ≥ 1 hour; verify heat sink temperature is within nameplate rating.
6. Record baseline: capture motor no-load current, full-load current, DC bus voltage, and heat sink temperature in commissioning report for future trending.

10. Preventive Measures

• Annual insulation testing of motor and cable using a 500 V or 1000 V megohmmeter.
• Torque check on power terminals every 12 months; thermal cycling loosens screw terminals over time.
• Cooling fan replacement at 5-year intervals regardless of visual condition; fans are the most common wear item.
• DC-link capacitor reform if the drive has been stored unpowered for > 12 months. Apply mains through a variac, ramping from 0 to 400 V over 30 minutes to reform the aluminum oxide dielectric.
• Surge protection on the line side if the drive is fed from a source with frequent switching transients or lightning exposure.
• Maintain clean cooling airflow: blocked heat sinks elevate IGBT temperature and accelerate module degradation, increasing the probability of asymmetric conduction that triggers F231.

11. Related Fault Codes Worth Knowing

12. Frequently Asked Questions