Troubleshooting Simovert Masterdrive F025 F026 F027 UCE Trip

Overview of F025, F026, and F027 on SIMOVERT MASTERDRIVES 6SE70

Faults F025, F026, and F027 are the most reported inverter-side trips on the legacy Siemens SIMOVERT MASTERDRIVES family (6SE70 series, also referenced as 6SE702x in compact and compact-plus variants). All three faults belong to the same diagnostic class: UCE monitoring (collector-emitter desaturation detection on the IGBT stack). Each code is phase-specific, so the three codes can be treated as one diagnostic family with three phase tags.

Per the official Diagnostics and Troubleshooting manual for the MASTERDRIVES platform, the cause of the detected fault must be removed before the drive can be reliably reset. F025 and F028 can be reset remotely (a benefit of the on-board CUVC status word), but F026 and F027 typically require a power-cycle reset on the CUVC/PMU operator panel once the root cause is cleared. The fact that the fault auto-clears at the customer site is a strong indicator of an intermittent condition, not a hard failure — but the drive will eventually latch permanently if the root cause is not addressed.

Key components involved in the UCE detection path:

• IGBT modules (3-phase inverter bridge)
• IGD (IGBT Gate Driver) card — drives the gates and returns the desaturation status
• IVI (Inverter Interface) card — fiber-optic gateway between the low-voltage CUVC control and the high-voltage IGD stack
• FOC (fiber-optic cables) — command and status paths to each IGD
• CTs (current transformers) — phase current feedback (compact type, ~23.9 Ω / 53.5 Ω windings)
• LBA (Line Base Adapter) — present on AFE/regen units, can degrade the DC link if corroded
• CUVC (Control Unit Vector Control) — evaluates UCE status and current feedback
• Cooling fan + starting capacitor — loss of airflow can thermally stress the IGBT stack and trip UCE

Fault Code Definitions and Phase Mapping

State-word bit (reference): the CUVC status word (r001) bit 13 maps to the UCE group; the diagnostic word r947 reveals the exact sub-code for the active latch. Always read r947 and the alarm buffer (r947 indexed, plus the operator panel's History menu) before any reset attempt.

Root Cause Hierarchy and Probability Matrix

Field experience on the 6SE70 platform shows the same subset of components behind the majority of F025/F026/F027 reports. Use the matrix below to triage in the field.

Pre-Diagnostic Safety and Preparation

The 6SE70 DC bus stores lethal energy for several minutes after mains removal. Follow this order without exception:

1. Lock out / tag out (LOTO) the upstream breaker or contactor.
2. Wait at least 5 minutes after the last ON command before opening the cabinet (longer if the unit is a compact-plus with high DC-link capacitance).
3. Verify zero energy with a Cat IV 1000 V tester on the DC-link test points (+DC, -DC). The drive should read < 5 V DC.
4. Verify the CUVC PMU is dark and the 24 V control supply is off.
5. Confirm the motor is mechanically isolated (coupling removed or shaft locked) if any rotation will be commanded during the test.

Stage 1 — Motor, Cable, and Output Reactor Insulation Test

Rule out the load first. A short between a phase and ground will pull the IGBT out of saturation, which the IGD will read as a UCE trip.

1. Disconnect the motor leads from the drive output terminals (U2, V2, W2).
2. With a 1 kV insulation tester, measure phase-to-ground and phase-to-phase for 60 seconds.
3. Pass criteria: > 100 MΩ per phase at 40 °C, with the trend stable. Anything < 5 MΩ indicates insulation breakdown.
4. Repeat the test on the cable and any sine-wave output filter or output reactor. A 37 kW unit such as the 6SE7027-2TD61 commonly ships with an output reactor for long-cable applications — these reactors can develop inter-winding shorts after years of thermal cycling.
5. If insulation is good, reconnect motor leads and move to Stage 2.

Quick fault code heuristic: if F025, F026, and F027 all appear within 50 ms of each other, the cause is almost always upstream of the IGBTs (motor/cable) or the IVI/FOC path — not the IGBTs themselves.

Stage 2 — Current Transformer (CT) Resistance Test

Bad CTs cause the CUVC to interpret normal current as an overcurrent, which can in turn command the IGD into hard switching and trip UCE. The compact-type CTs used in 6SE70 compact and compact-plus have two distinct secondary windings. From the field reference values: 23.9 Ω on one winding and 53.5 Ω on the other (the difference reflects the two turns ratios used for the control and regulation loops).

1. With the drive de-energized, isolate the CT leads from the IVI board.
2. Measure each secondary winding with a 4-wire ohmmeter.
3. A reading outside the acceptance band, or any open circuit, mandates CT replacement before further testing — a marginal CT will report back to the CUVC as a noisy current loop and can drive the IGD into UCE trips during high-dI/dt transitions.
4. Inspect the CT mounting for cracks, loose clamps, and signs of thermal discoloration.

Stage 3 — IGBT Diode and On-State Test

This is the single most common hardware repair on 6SE70 units. Always test the anti-parallel diodes before assuming an IGBT module is healthy.

1. Confirm DC-link discharge per the safety procedure.
2. With the motor leads disconnected, set a digital multimeter to diode mode.
3. Probe each phase leg (U2 → +DC, U2 → -DC, and so on for V and W). Record the forward voltage of every diode.
4. Compare the three phases. All six diodes on a healthy module read within 50 mV of each other. A spread > 100 mV, or any open/short reading, identifies the failed module.
5. Replace the affected IGBT module as a complete phase leg; never mix old and new IGBTs in the same phase.

Torque check: re-torque the DC-link busbars and the IGBT main terminals to the frame-size-specific value (typically 8–12 Nm for compact, 15–22 Nm for compact-plus). A loose busbar will read intermittently as UCE because the IGBT collector does not sit at the expected potential during the desaturation check.

Stage 4 — IGD Card, IVI Card, and Fiber-Optic Path

When IGBTs test good, the next layer is the IGD → IVI → CUVC signaling path. Field data shows fiber-optic problems account for a large share of intermittent trips, especially on units that have been in cabinets exposed to vibration.

1. Visual inspection of FOCs. Pull each fiber at the IVI and IGD connectors and inspect the ferrule under a magnifier. A scratched ferrule or oil-contaminated tip will attenuate the optical signal and cause a false desaturation flag.
2. Re-seat each fiber. Many intermittent trips clear with a simple re-seat because the optical coupling was marginal.
3. Check the fiber bend radius. Minimum 25 mm; any kinks < 10 mm radius will fracture the fiber internally and the fault is invisible to the eye.
4. Test the IGD card. If a known-good spare IGD is available, swap it and re-test. If the fault clears, the original IGD is the cause. Pay attention to the gate resistor on the IGD — these are often the first component to drift on the card.
5. Test the IVI card. The IVI is the fiber-optic gateway. A bad IVI typically trips all three codes simultaneously within one CUVC scan. Replace the IVI as a unit; the on-board EEPROM holds the calibration data and must be transferred from the original card to the replacement.

Stage 5 — LBA Adapter, CUVC, and Auxiliary Cooling

These are the secondary causes but should not be ignored, especially on AFE/regen variants and on long-running installations.

• LBA corrosion. On Active Front End (regen) units, the Line Base Adapter board sits in the same airflow as the inverter. Corrosion on the LBA connector changes the DC-link voltage feedback, which causes the CUVC to command the inverter into over-modulation. The result can present as a UCE trip on the inverter side. Inspect, clean with isopropyl, and reseat.
• CUVC firmware. Read the firmware version from the operator panel (PMU shows it on power-up). Confirm it matches the latest release for your frame size. Phantom F025/F026/F027 events have been documented in older firmware revisions.
• Cooling fan + starting capacitor. A failing fan motor is a common indirect cause. With insufficient airflow the IGBTs run hotter and the desaturation threshold trips earlier. The starting capacitor on 6SE70 cabinet fans is a small (typically 2–4 µF) motor-run capacitor; a < € 2 component, but it can suppress the entire drive. Verify the fan spins freely on power-up and that the capacitor measures within ± 10 % of its marked value.

Reset, Restart, and Anti-Nuisance Delays

If the root cause is the operator pressing the start button faster than the drive's discharge and pre-charge cycles allow, the solution is operational rather than hardware. The MASTERDRIVES pre-charge circuit takes a finite time to recharge the DC link after a stop; re-enabling the inverter before pre-charge is complete causes an inrush that the IGD reads as a UCE event.

1. Add a restart interlock delay of 90–120 seconds in the upstream PLC or in the CUVC's parameter P464 (re-start on/off delay). 2 minutes is a common field-proven value.
2. Where the application uses an external interlock that opens and closes on a process event (e.g. a safety door), debounce the interlock in the PLC and gate the OFF-to-ON command through the same delay.
3. Confirm the operator's HMI displays the CUVC status word bit 13 (UET status) so the operator does not push reset while the drive is still resolving a UCE condition.

This single change has resolved nuisance F025/F026/F027 events on multiple process lines where the root cause was operator behaviour, not hardware.

Verification Procedure After Repair

Once a component is replaced, validate the drive in a structured loop before handing it back to production.

1. Re-apply control power only. Read the CUVC status word r001 and confirm bit 13 is clear and r947 (last fault) reads 0.
2. Apply mains. Wait the full pre-charge time (the PMU will indicate "Ready").
3. Run a no-load spin at low frequency (2–5 Hz) for 2 minutes. Monitor r019, r020, r021 (output currents U/V/W) and r022 (motor torque). All three currents should track within ± 5 % of each other.
4. Step the drive through 25 %, 50 %, 75 %, and 100 % speed reference. Hold each step for 60 seconds and watch the alarm buffer.
5. Run a full load cycle (the original customer load profile if available). Watch the IGBT heatsink temperature via r039. Confirm the fan ramps up with load.
6. If any F025/F026/F027 returns, re-open the diagnostic loop at Stage 4 — the IGD, IVI, or fiber path is the most common missed cause.

Field Commissioning Checklist

Diagnostic Flow Summary

Troubleshooting Matrix at a Glance

FAQ