Resolving A031 Alarm on Siemens 6RA70 DC Drive Near Base Speed

Overview of A031 Alarm on the Siemens 6RA70

Alarm A031 on the SIMOREG 6RA70 DC master is the "speed controller at limit" warning. It is generated by the n-controller (speed controller) when its output, which represents the torque demand and ultimately drives the armature current setpoint, reaches a positive or negative limit for a configurable time. Unlike F-class faults, A031 is a status warning and does not latch the drive off, but it indicates that the controller can no longer command the torque that the mechanical load is demanding at the current operating point.

On a 6RA70 the speed controller output is fed into the current and torque limit chain. If the controller output saturates because the limit chain is clamped, the actual motor speed will deviate from the setpoint, the field loop will start to weaken even though speed is below base speed, and the armature current will rise. This is the symptom set reported in the field case: A031 near base speed, P590 (speed setpoint) and P591 (speed actual) match nominally, yet the speed controller output remains pinned against a limit and the field collapses. The current setpoint displayed at K0131 sits well below the configured torque limit P181, which is the diagnostic fingerprint of a current-side saturation rather than a torque-side saturation.

Reference: SIMOREG DC-Master 6RA70 Operating Instructions (Siemens Support entry 109757751), function block diagram of the n-controller and the four-input limit chain.

Reported System Configuration

The reported installation is a rolling mill DC drive with the following observed parameter set. The combination of values is a textbook recipe for repeated A031 events and is used throughout this article as the working example.

With P171 = 70% the drive is permanently clamped to 0.7 pu of device rated current. A rolling mill bite can transiently demand 1.5 to 2.5 pu, so every pass will drive the controller into saturation. The 3% / 3 s A031 trigger is also aggressive and will fire on normal biting transients.

A031 Trigger Logic and Parameter Roles

The A031 alarm is the logical OR of two sub-conditions inside the n-controller block:

1. The speed controller output saturates the positive or negative torque/current limit for longer than P390 (seconds).
2. The speed deviation between setpoint and actual exceeds P388 (percent) for longer than P390.

The limit against which the speed controller output is clamped is the minimum of the following four values, all expressed in percent of rated device current:

• P171 (positive) and P172 (negative) — system current limit.
• Speed-dependent current limit P104-P108 — active only when P109 = 1.
• Variable limit from connector K1, selected via P603.
• Variable limit from connector K9, selected via P604.

The torque limit is a second stage clamped by P181/P182 and the variable K2 source (P510/P511). The displayed current setpoint K0131 is the smoothed output after the entire limit chain. Status word R40 reports which branch of the limit chain is active: bit 11 is "speed controller at positive current limit," bit 10 is the negative equivalent, bit 9 is torque limit, and bit 2 is system current limit. In a healthy 6RA70 application bits 9, 10, and 11 should pulse briefly during a transient and return to zero; if any bit latches high for more than a few hundred milliseconds, the controller is permanently in saturation.

Reference: SIMOREG 6RA70 Parameter List (Siemens Support 23102135) for parameter definitions, units, BICO sources, and signal flow diagrams.

Root Cause Categories

When A031 appears near base speed on a 6RA70 in a rolling mill, the following five root causes cover the vast majority of field cases. Each category has a distinct trace signature.

2.1 Current Limit Saturation Due to Under-rated P171 / P172

This is the most common cause. P171 and P172 act as a hard ceiling on armature current. A rolling mill bite transient can demand 1.5 to 2.5 pu of rated motor current for several hundred milliseconds; if P171 is set to 70%, the controller can never command more than 0.7 pu, the speed will collapse, and the controller output stays at the limit until the load event passes. The trace data in the field case shows current setpoint at 80% with K0131 at 65%, well below P181/P182 — proof that the saturation is on the current side, not the torque side. The fix is to raise P171/P172 to a value that the motor and gearbox can mechanically absorb, typically 100% to 120% for a mill motor.

2.2 Field Weakening Below Base Speed Due to Supply Sag

The 6RA70 field loop begins to weaken as soon as counter-EMF approaches supply voltage minus the firing-angle headroom required for current control. If the AC supply sags to 90% of nominal at the same time the load demands higher current, the available armature voltage headroom shrinks and the field loop pulls field current down to keep the firing angle inside its operating window. The net effect is reduced back-EMF, more armature current for the same torque, and a faster trip into current limit. The trace files show the field collapsing as setpoint rises — a textbook signature of supply sag-induced field collapse. The mitigation is upstream, on the AC supply side, not in the drive parameters.

2.3 Masked F035, F036, F037 Hiding Real Diagnostic Information

Faults F035 (drive blocked), F036 (no armature current can flow), and F037 (I²t monitoring) are protection-class events. When masked, the drive will not annunciate them, so the controller simply clamps at the current limit and the operator sees A031. Unmasking these faults turns them into diagnostic events that point to the real root cause — a mechanical jam, an open thyristor path, or thermal overload of the motor.

2.4 Variable Limit Sources Below Fixed Limits

P603 (K1) and P604 (K9) can pull the effective current ceiling below P171/P172 if the connector source value is lower. P603 = FIX 100 means a constant 100% on K1; P604 = K9 (block-specific) means the limit is set by a block output that may not be at 100%. If the K9 block-specific source is at 50% at the moment of alarm, the current ceiling is silently 50% of rated, and K0131 will display 65% (or whatever the lower of P171 and the variable limit allows) regardless of how high P181 is set. This is consistent with the K0131 = 65% observation when P171 = 70% — K9 is acting as the binding limit.

2.5 Mechanical Load Increase Without Process Tuning

If the rolling mill screwdown, reduction, material grade, or line speed has changed without a corresponding P171 increase, the new steady-state current demand can sit just below the current limit. The controller output will then sit at the limit during every pass, and A031 will appear on every billet. The drive parameters have not changed; the load has. The cure is a process review, not a parameter change.

Diagnostic Procedure and Trace Interpretation

Follow this sequence before changing any parameter. The 6RA70 trace buffer (StorTrace in DriveMonitor) gives the high-resolution evidence required to distinguish between the five root cause categories.

1. Open the DriveMonitor trace that captured the A031 event. Identify the timestamp of the alarm rise.
2. Inspect the trace for: armature current actual (K0111 or r029), current setpoint (K0131), firing angle alpha (K0103), speed setpoint (K0050), speed actual (K0051), field current (K0102), and limit word (R40).
3. Confirm whether R40 bit 11 is set — that bit is "speed controller at positive current limit" and is the direct internal flag for A031 on the positive side. Bit 10 is the negative equivalent.
4. Note the value of K0131 at the moment of alarm. If it is below P171, the binding limit is one of P603/P604 (variable limits) or the speed-dependent limit, not the system current limit. If it equals P171, the system current limit is binding and the load is too high for the configured ceiling.
5. Note the field current. If field current falls as speed rises through the A031 event, the field loop is being pulled into weakening below base speed and the cause is voltage headroom loss (supply sag or a mis-tuned field loop).
6. Note the firing angle K0103. If alpha exceeds 25° to 30° during a non-field-weakening pass, the bridge is voltage-starved.
7. Capture a one-second window before and after the A031 rise and export it for the parameter tuning step below. The exported CSV is the only durable evidence for the maintenance team.

Current Limit Chain Analysis

The 6RA70 applies a four-input minimum selector to derive the current ceiling. The four inputs are:

1. P171 / P172 — system, fixed in the parameter set.
2. P104-P108 — speed-dependent, active only when P109 = 1, scaled by n-actual / n-rated.
3. K1 connector — selected by P603.
4. K9 connector — selected by P604.

The minimum of these four is the effective current ceiling. The output then passes through the torque limits P181/P182 and the K2 connector selected by P510/P511. With P109 = 0 and P510 = P511 = FIX 200, the variable limits are the only source of flexibility. P603 = FIX 100 (constant 100%) and P604 = K9 (block-specific) means K9 is the only way the effective limit can be below 100%. To diagnose:

• Read the connector display for K9 from DriveMonitor and confirm its value at the time of A031.
• If K9 is, for example, FIX 70% in a block that the engineer did not realize was live, P604 = K9 will clamp the drive to 70% — exactly matching the trace.
• Temporarily change P604 to a fixed 100% or to 0 (no variable limit) and re-test. If A031 disappears, the cause is the K9 limit source, not the field or the load. If A031 persists, the cause is upstream.

The 6RA70 also applies speed-dependent current limiting through P104-P108. With a 500 RPM rated motor, a typical P104 = 100% at 0 RPM, P105 = 100% at 100 RPM, P106 = 100% at 200 RPM, P107 = 70% at 250 RPM, and P108 = 50% at 300 RPM is sometimes left active by mistake. Confirm P109 = 0 before assuming the speed-dependent limit is off.

Field Weakening, Armature Voltage, and Supply Sag

For a 6RA70 with a 600 V armature bridge, the maximum DC armature voltage is:

For a 575 V line and alpha_min = 5°, this gives roughly 783 V minus thyristor and commutation drop, so the bridge can comfortably supply 600 V DC at full conduction. However, the trace data shows the AC supply dipping to 90% of nominal during the rolling pass. At 0.9 * 575 = 517.5 V line, the maximum DC voltage drops to about 700 V. With a motor counter-EMF of, say, 540 V and a 100 V drop across the armature resistance and cables, the firing angle must advance to about 30° to maintain the required current. The 6RA70 field regulator responds to the rising alpha by reducing field current, which drops back-EMF and pushes the drive further into the current-limited regime.

Mitigation: install a power quality logger at the line side and capture the voltage during a representative pass. The 575 V supply at -10% is a separate A031 trigger in its own right, independent of the current limit setting. Address the upstream transformer tap, the capacitor bank, the line reactor, or the feeder cable sizing before any drive parameter change.

Reference: SIMOREG 6RA70 Fault and Alarm List (Siemens Support 26283645) for the field loop alarm thresholds and the counter-EMF model.

Masked Faults and Their Hidden Consequences

Masking an F-class fault does not stop the protection logic from acting; it only stops the fault from being latched and displayed. The drive will still take protective action, and the side effect is a generic A031 that hides the real cause. F035 (drive blocked, speed actual stuck below threshold with full current) and F036 (no armature current can flow, Ia below threshold with full command) in particular are diagnostic gold when investigating current-limit saturation: F035 directly indicates a mechanical jam, F036 directly indicates an open thyristor or a blown fuse, and F037 directly indicates that the motor thermal model is exhausted. The recommended setting for a rolling mill is to keep all three unmasked, route them to a trip or warning log via the BICO matrix, and let the maintenance team see them on every event.

Parameter Tuning to Resolve A031

Apply changes in the following order. Verify after each step with a no-load run, then a single pass, then a full rolling sequence.

1. Unmask F035, F036, and F037. Set the appropriate fault mask bits to 0 so the faults are reported. Recapture one A031 event with the faults visible. This step alone often reveals the actual cause.
2. Verify P604 (K9) connector source. Use DriveMonitor to read the value feeding P604 at the moment of A031. If the value is below P171, either raise the K9 source or change P604 to 0 (no variable limit) or to FIX 100 (a constant 100% connector).
3. Raise P171 / P172 to a process-appropriate value. For a rolling mill, 100% to 120% of rated motor current is common. The current limit should be set to the maximum value the motor and the gearbox can mechanically absorb, not to the motor nameplate. Verify with the motor thermal model (P383-P386) and the I²t limit (P387) that the new ceiling is thermally sustainable.
4. Correct the AC supply sag. If the line is dropping below nominal during the pass, address the upstream transformer tap, the capacitor bank, or the line reactor. The 6RA70 cannot compensate for a sagging line indefinitely.
5. Re-tune the speed controller. With a new current ceiling, the n-controller gain P225 and integral time P226 may need re-tuning. Use the DriveMonitor step-response trace to bring the closed-loop bandwidth back to the original design value, typically 0.3 to 1.0 pu of the mechanical natural frequency.
6. Re-evaluate P388 and P390. The 3% / 3 s trigger is aggressive. If the new current ceiling is high enough, the A031 trigger can stay where it is as a diagnostic flag. If process conditions warrant, raise P388 to 5% to 8% to suppress nuisance A031 during normal biting transients. P390 should stay at 3 s or longer to avoid spurious triggers from normal load steps.

Verification and Commissioning Checks

After the parameter changes, run a structured commissioning check. The following sequence has been proven on 6RA70 installations in rolling mills.

For each pass criterion, capture a 10-second DriveMonitor trace. The traces are the deliverable to the maintenance team. Store them with the parameter set and the date of the change so that future A031 events can be compared against the baseline.

Field-Proven Tuning Example

On a 500 RPM, 600 V DC mill motor with a gearbox rating of 200% torque for 30 seconds, a typical 6RA70 setup that has run for years without A031 uses:

• P171 = 110%, P172 = -110%
• P181 = 250%, P182 = -250%
• P603 = FIX 100, P604 = 0 (no variable limit)
• P510 = FIX 200, P511 = FIX 200
• P388 = 5%, P390 = 3 s
• P109 = 0 (speed-dependent limit OFF)
• F035, F036, F037 unmasked
• AC supply tapped to nominal, line reactor installed

With this configuration, the K9 block-specific source can no longer clamp the drive below 110%, the current ceiling matches the gearbox rating, and the A031 trigger threshold is wide enough to ignore normal biting transients. If the trace shows K0131 still pinned at the limit, the problem is mechanical, not electrical, and the mill stand, the screwdown, or the material grade should be reviewed.

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