Resolving Siemens SIMOREG 6RA70 F030 Fault: Blocking Voltage Time Area

The F030 fault on a Siemens SIMOREG 6RA70 DC Master drive trips when the drive controller detects that the blocking voltage time area for a commutating thyristor pair has dropped below the minimum required for safe commutation. The drive logs fault value r949.01 = 1 to indicate the specific F030 sub-condition described as: "The blocking voltage time area for the commutating thyristor pair was too small."

This document consolidates field-proven diagnostic procedures, parameter inspections, and corrective actions for the F030 fault on rolling mill applications where 6RA70 drives are used for stand motors.

1. F030 Fault Definition and Sub-Code Mapping

The F030 alarm group in the SIMOREG 6RA70 covers commutation monitoring. The fault number is decoded via parameter r949 (fault value) in conjunction with r947 (fault number) and r948 (fault time). The sub-code stored in r949.01 is critical because F030 aggregates multiple commutation conditions.

2. Documenting the Fault: Required Parameters

Before clearing the fault, capture the following parameters from the drive monitor or via Siemens Industry Online Support SIMOVIS/DriveMonitor trace:

• r021 – Power section actual line voltage (rms line-to-line, in V)
• r022 – Armature voltage actual value (V DC)
• r025 – Armature current actual value (% of rated)
• r026 – Armature current setpoint (%)
• r038 – Firing angle α actual (degrees electrical)
• r040 – Line frequency actual (Hz)
• r078 – Field current actual (A DC)
• r080 – Field current setpoint (A DC)
• r650 – Control word 1 (binary)
• r651 – Control word 2 (binary)
• r652 – Status word 1 (binary)
• r653 – Status word 2 (binary)
• r949.01 – Fault value of last F030 trip

3. Root Cause Analysis of Blocking Voltage Time Area Fault

The blocking voltage time area is the integral of the reverse-bias voltage across a thyristor after it has been commutated off, multiplied by the time for which that reverse-bias persists. If this product falls below the thyristor's turn-off time (typically 100–400 µs for the devices used in 6RA70), the device may re-fire spontaneously and cause a shoot-through, which the drive prevents by tripping F030.

The mathematical condition for safe commutation can be approximated as:

∫ V_reverse(t) dt > t_q × V_threshold

where t_q is the thyristor turn-off time and V_reverse(t) is the voltage across the outgoing thyristor pair. The drive evaluates this integral continuously through the firing angle control loop.

3.1 Conditions That Reduce the Blocking Voltage Time Area

1. Line voltage depression – When the rolling mill stand load increases suddenly (such as at billet tail end exit), the line voltage can sag below the threshold that guarantees commutation margin.
2. Firing angle α too small (rectifier mode over-advancing) – If α drops below the minimum value programmed in P150 (minimum firing angle), the drive intentionally holds α back, but transient excursions below this point cause the blocking voltage time area to collapse.
3. Motor back-EMF approaching line peak – At speeds near base speed in the field-weakening region, the motor counter-EMF (E = k·Φ·ω) approaches the rectified line voltage peak. The difference available for commutation shrinks.
4. DC link inductance insufficient – A reactor value below the recommended minimum for the line short-circuit power (S_k) reduces the commutation overlap and the time available for thyristor recovery.
5. Line frequency excursion – A drop in line frequency (e.g., from 50 Hz to 49.5 Hz) increases the commutation period by the inverse ratio and shortens the available recovery time.
6. Defective thyristor or gate lead – A partially failed thyristor exhibits increased leakage or a longer effective turn-off time. Check firing pulse symmetry.

4. Field-Proven Diagnostic Procedure

Use this procedure when the fault has been confirmed as F030 with r949.01 = 1.

4.1 Step 1: Confirm the Fault Sub-Code

1. Connect DriveMonitor (RS232 on X300 / PROFIBUS via CBP2) to the 6RA70.
2. Read r949.01, r947 (fault number = 30), and r948 (fault time in operating hours).
3. Record the values along with the operating speed setpoint and armature current at the time of trip.

4.2 Step 2: Capture a Synchronous Trace

Configure a 4-channel trace to record the following at 5 ms sample interval for 10 seconds around the expected trip window:

• Channel 1: r038 – Firing angle α
• Channel 2: r025 – Armature current actual
• Channel 3: r078 – Field current actual
• Channel 4: r021 – Line voltage

Set the trigger to fault event so the trace freezes at the moment of F030. The trace will reveal whether the line voltage dipped, the firing angle bottomed out, or the field current collapsed prior to the trip.

4.3 Step 3: Inspect the Field Current Path

The original observation in this case showed field current dropping after the fault, which is consistent with the drive going into a fault state and removing excitation. Verify the field current path independently:

1. With the drive stopped and locked out, measure the field winding resistance (cold) and compare with nameplate data. The expected field resistance at 20 °C can be calculated from V_field / I_field_nominal; verify this matches the actual measurement at ambient temperature using the copper temperature coefficient of 0.00393 / °C.
2. Check field fuse continuity. The 6RA70 internal field supply uses semiconductor fuses; a partially blown fuse can pass nominal current but fail under transient demand.
3. Inspect the field thyristor bridge temperature and heatsink airflow.
4. Verify P082 (field current setpoint source) and P083 (field current reference) match the rolling schedule. A misconfigured field reference can drive the motor into deep field weakening at low speeds, where the armature firing angle has no margin.

4.4 Step 4: Verify Supply and Transformer Sizing

The minimum supply short-circuit power for a 6RA70 of rated current I_dN is given by:

S_k_min = U_d0 × I_dN / (k × cos(α_min) − cos(α_max))

where U_d0 is the ideal no-load DC voltage, k is a margin factor (typically 1.1–1.3), and the firing angle limits are α_min and α_max. If the actual system S_k is below this minimum, the F030 trip is a structural issue and requires either a transformer with higher impedance rating or a line reactor.

4.5 Step 5: Check Mechanical Coupling

Although the original report confirmed the mechanical system was in good order, F030 can be triggered by shaft load transients. Verify:

• Backlash in the gear box or pinion stand is within tolerance.
• The coupling is properly aligned (laser alignment check).
• The work roll surface is free of damage, which could cause sudden load steps.

5. Parameter Adjustments to Mitigate F030

The following parameter changes have been applied successfully in rolling mill retrofits. Adjust one parameter at a time and verify the result with a test run.

5.1 Limit the Minimum Firing Angle

Increase P150 (minimum firing angle limit) from the default 5° to 15–20° to provide additional commutation margin. This sacrifices a small amount of usable DC voltage headroom in exchange for guaranteed commutation time.

; Recommended change (DriveMonitor parameter editor):
P150 = 18  ; was typically 5–10
; Note: At α = 18°, the rectified output drops to:
; U_d = U_d0 × cos(18°) ≈ 0.951 × U_d0
; Net loss: 4.9% of no-load DC voltage

5.2 Configure the Field-Weakening Threshold

Restrict the field-weakening range so the motor does not enter deep field weakening at low line speeds:

• P082 = 1 (internal field reference)
• P103 – Field current lower limit: raise from 30% to 50% of nominal to keep the motor EMF away from line peak during stall-type conditions.

5.3 Adjust the Current Controller Response

Slow the armature current controller to reduce the firing angle step at load impact:

• P155 – Current controller P-gain: reduce by 20%.
• P156 – Current controller integral time: increase by 30%.

This change reduces the dα/dt slew at the moment of load impact, giving the commutation monitoring loop time to react without tripping F030.

5.4 Increase the DC Link Reactor

If the line is weak (S_k below minimum), add a line reactor to limit commutation overlap. The reactor inductance L_d is calculated as:

L_d (mH) = (U_d0 / (2 × π × f × I_dN)) × (u_k_new − u_k_existing)

where u_k is the per-unit short-circuit voltage of the supply seen by the converter. A reactor raising u_k by 4–6% is usually sufficient to eliminate sporadic F030 trips in weak-grid installations.

6. Verification Procedure

After applying any of the above corrective actions, perform the following verification sequence before returning the stand to production:

1. No-load run-up test: Ramp the drive from 0 to base speed over 60 s while monitoring r038, r025, r078, and r021. Confirm no F030 event is logged and no fault bit sets in r652.7 (status word 1, bit 7 = fault active).
2. Loaded test at typical rolling load: Pass a billet through the stand at the planned reduction schedule. Capture a synchronous trace. Confirm the firing angle never drops below the new P150 setting.
3. Worst-case load step test: Simulate a heavy bite (e.g., entering a slab) at the maximum planned speed. The 6RA70 firing angle should swing forward smoothly and recover to the new operating point within 200 ms without F030.
4. End-shift review: Read r947 and r949.01 at the end of the shift to confirm no F030 was recorded during production. Compare operating hours with r049 to verify the trace window.

7. Reading the Control and Status Words

The 6RA70 exposes the control word and status word in two locations:

• Bus-side (PROFIBUS DP / CBP2): PZD1 (control word) and PZD2 (status word) at the cyclic I/O address defined by the master configuration. Refer to the SIMOREG 6RA70 function diagram G180 for control word 1, G181 for control word 2, G182 for status word 1, and G183 for status word 2.
• Local display parameters: r650 (control word 1, 16-bit binary), r651 (control word 2), r652 (status word 1), r653 (status word 2). These are read-only and display in binary via DriveMonitor.

For decimal or hex display of the control/status words in DriveMonitor, set the parameter format to Hex in the toolbar; DriveMonitor will then show the value as a 4-digit hex number (e.g., 0x040F = bit 0, 1, 2, 3, and 10 set). The native format in the device firmware is bit-coded binary; a nibble view is not provided by the standard firmware.

Key status word bits for fault diagnosis:

8. Troubleshooting Matrix: F030 r949.01 = 1

9. Frequently Asked Questions