Resolving Siemens 6RA70 A031 Alarm and Negative Torque Direction

1. Problem Definition

On a SIMOREG DC MASTER 6RA70 drive controlling an intermediate DC motor used for paper support in a printing machine, two coupled symptoms appear in EMF (back-EMF) control mode with an analog speed/torque reference:

• The drive enters alarm state A031 (speed controller monitoring) during operation, especially after extended dwell periods in which the displayed torque symbol shows "–" (no active quadrant).
• The diagnostic value r021 (actual torque) intermittently reads approximately -100 %, indicating the internal torque setpoint is being clamped or driven into the negative (motoring II / regenerating I) quadrant even when the process physically requires motoring torque only.

The operator initially set P165 = 1 to block negative torque. After that change, the drive stayed in the "–" quadrant much longer than before, current collapsed to near zero, and A031 reappeared more frequently. The problem is therefore not just an alarm-clearing exercise; it is a sign that the controller setpoint chain is no longer matched to the mechanical load characteristic of the paper-support section.

2. SIMOREG 6RA70 Hardware and Firmware Context

The 6RA70 series is a fully digital three-phase, fully controlled B6 bridge DC converter rated for armature outputs up to 1200 A (chassis) and field outputs up to 30 A in the standard product. Firmware releases from V1.x through V3.x are in service; parameter numbering and alarm codes are stable across these releases, but a small number of monitoring parameters (notably in the P38x speed-controller block) were refined in V2.1 and later.

The reference parameter list for this troubleshooting case includes:

• P165 – Torque direction enable (limits torque setpoint to one polarity)
• P388 – Speed deviation threshold for n-controller monitoring
• P390 – Monitoring time before A031 is raised
• P150 – Torque setpoint smoothing time constant
• P151 – Speed setpoint filter time constant
• P155 / P156 – Positive / negative torque limit 1
• P157 / P158 – Positive / negative torque limit 2
• P160 – Torque limit source selection
• P082 – Analog input 2 function assignment (typically EMF feedback or auxiliary reference)
• r021 – Actual torque display (signed %, +100 % = P100/P101 scaling)
• r026 – Actual speed setpoint at controller input
• r027 – Actual speed (encoder or EMF-derived)
• r039 – Armature voltage actual value

Alarm A031 is described in Section 10 of the 6RA70 Operating Instructions; refer to the official Siemens diagnostic FAQ for the canonical alarm list and remedy chain: FAQ for 6RA70 DC MASTER and SIMOREG CM – Alarm Messages (PDF).

3. Drive and Motor Specifications

The intermediate paper-support section uses a separately excited DC motor. The nameplate values quoted in the service report are:

At 400 V armature and 56 A, the converter apparent armature rating is approximately 22.4 kVA DC, comfortably inside a 30 A-class 6RA70 (typical selection: 6RA7018-6DS22 or 6RA7025-6DS22 depending on overload). The 3.6 A field current fits inside the standard 5 A field module on the same unit. For the actual converter selection use:

kVA_DC = V_A × I_A / 1000 = 400 × 56 / 1000 = 22.4 kVA

Confirm that the chosen 6RA70 has P100 (armature current) and P101 (armature voltage) entered at the commissioning stage to match these nameplate values; otherwise the internal torque scaling (used to compute r021) will saturate before the motor reaches rated torque.

4. EMF Control Mode and Quadrant Behaviour

EMF (back-EMF) regulation closes the speed loop on the induced voltage computed from armature voltage minus IR drop, without a tachogenerator. It is a four-quadrant-capable mode by default: positive reference drives motoring (I-quadrant), negative reference drives motoring (III-quadrant), and the firing-angle logic automatically regenerates when the load overhauls the reference.

The torque symbol shown on the PMU is one of four states:

• I – positive torque, positive speed (motoring forward)
• II – positive torque, negative speed (regenerating reverse)
• III – negative torque, negative speed (motoring reverse)
• IV – negative torque, positive speed (regenerating forward)
• – – no active torque command (setpoint near zero, drive in coast/idle)

In a tension-controlled nip roll, only quadrants I and IV (forward pull with light hold-back) are physically meaningful when the paper is moving in the design direction. Quadrant II would imply the section is trying to push paper backwards, and quadrant III implies the section is dragging paper. Both produce slack-then-snap web behaviour, exactly the symptom reported before P165 was changed.

5. A031 Alarm Mechanism

A031 fires when the absolute difference between the speed setpoint input to the n-controller and the actual speed (EMF-derived or tachogenerator, depending on configuration) exceeds P388 for longer than P390 milliseconds. The two parameters are factory-defaulted to conservative values for unloaded bench testing; they almost always need site-specific adjustment.

Important: enlarging P388 or P390 only hides the symptom. A031 is the controller telling you the setpoint chain cannot be physically realized. The correct fix is to make the setpoint chain realizable, then bring P388/P390 back to their tight production limits.

6. Root Cause Analysis

Three independent factors combine to produce the observed behaviour:

1. Master tension loop swings through zero. A PID master outputting ±10 V for tension trim will pass through 0 V during steady-state correction. In EMF control that maps to 0 RPM setpoint and 0 torque. The drive sits in the "–" symbol and armature current collapses.
2. P165 = 1 blocks the negative half of the response. Once negative torque is forbidden, the controller cannot regenerate to hold back the section when the line momentarily tries to pull the section faster than the section's own torque would allow. The drive output becomes one-sided (positive only) but the load is two-sided; the integral term of the speed controller accumulates large wind-up, and on the next correction it slams into the positive torque limit.
3. Wind-up drives r021 to -100 % transiently. When the integrator is finally allowed to discharge, the controller walks the torque setpoint all the way through the negative rail before the limit clamps it. r021 reads -100 % during that discharge, even though the final applied value is +0 %.
4. Setpoint vs. realized EMF diverges. With the integrator wound up and the reference swinging around zero, the EMF-derived actual speed lags the setpoint by more than P388 for longer than P390, triggering A031.

The combination of factors 2 and 3 is why P165 = 1 made A031 worse, not better. It treated the symptom (negative torque) rather than the cause (improper tension-loop scaling and integrator behaviour).

7. Diagnostic Procedure

Execute the following steps in order. Record values to a commissioning sheet before changing any parameter.

1. Capture the alarm history. Use the PMU to scroll r047 (last fault code) and r048 (alarm word). Confirm that A031 is the only active warning; if other alarms (A034 overcurrent, A035 overvoltage, A041 field loss) appear simultaneously, fix those first.
2. Record the analog chain. With the machine at standstill, command a small positive reference and read r026 (speed setpoint), r027 (actual speed), r039 (armature voltage), and r021 (torque). They should move together within P388 limits.
3. Inject a step reference. Use the PMU parameter P085 or the analog test source to apply +5 %, then -5 %, then +5 % setpoint steps while watching the response on DriveMonitor or the PMU. The torque polarity should follow the polarity of the reference; if r021 shows -100 % on a positive step, the P155/P156 torque limit signs are inverted.
4. Verify the EMF feedback sign. Disconnect the analog reference. Run the motor briefly with a small positive torque limit (P155 = 5 %, P156 = 5 %) using P160 in test mode. If the motor runs the wrong way, swap P082 (analog input 2) sign or invert the EMF sign bit (P706 depending on firmware).
5. Capture the tension loop output range. On the printing machine, run a known web speed and read the tension PID output to the 6RA70 with a DMM. Most masters are scaled to ±10 V = ±10 % trim around the line speed. If the swing is larger than ±10 %, the tension controller is over-tuned and is itself driving the swings.
6. Read P155/P156/P157/P158. Default values often leave the negative torque limit at -100 %. This is what allows r021 to read -100 %.

8. Parameter Reference – Corrective Values

The following table summarizes the recommended changes for a tension-only paper-support section in EMF control. All values assume commissioning against the 56 A / 400 V / 3.6 A motor in Section 3.

The key change is P156 = 0 % instead of P165 = 1. The P156 = 0 approach:

• Allows the n-controller integrator to function normally (no direction lockout).
• Clamps the applied torque setpoint at zero on the negative side, achieving the same physical result as P165 = 1.
• Prevents r021 from walking to -100 % during integrator discharge because the limit is applied at the output stage, not the input stage.

9. Step-by-Step Corrective Procedure

1. Place the drive in commissioning / parameter mode via the PMU (S1 switch on CUD board to position 1 if DriveMonitor is not available).
2. Navigate to P156 and enter 0.0 %. Confirm the parameter is accepted (PMU returns no error).
3. Navigate to P165 and enter 0 (restore two-quadrant torque direction).
4. Adjust P150 to 0.100 s and P151 to 0.050 s.
5. Adjust P388 to 12.0 % and P390 to 1.500 s.
6. Slowly reduce P401 (Kp) by 30 % from current value; increase P402 (Tn) by 50 %.
7. Save parameters (P052 = 1 to save RAM to EEPROM if not done automatically).
8. Run the paper at nominal line speed. Monitor r021 continuously on DriveMonitor; torque should remain in the 0 % to +25 % range with short excursions, never negative.
9. After 30 minutes of stable running, tighten P388 to 8 % and P390 to 0.500 s for production sensitivity.
10. Re-tighten the n-controller Kp/Tn if mechanical response feels sluggish, in 10 % steps, alternating between Kp and Tn.

10. Verification Tests

Run the following four tests after the parameter changes. The drive must pass all four before being declared production-ready.

If T4 still raises A031, the master tension PID is too aggressive. Adjust the master's proportional band upward or its integral time upward; do not loosen P388/P390 again.

11. Engineering Notes and Edge Cases

• Field weakening above 3000 RPM: This motor is rated at 3000 RPM base speed. If the section ever runs above that, ensure field-weakening entry (P103) and field current ramp (P257) are correct. Above base speed, armature voltage is clamped at P101 and torque falls off; the EMF control loop becomes more sensitive and A031 can reappear for an unrelated reason.
• Three-phase supply quality: A031 can also be raised by heavy line-side distortion. If multiple 6RA70 units share a transformer with non-linear loads, check the DC link ripple with a scope on r039; values above 8 % of P101 indicate the armature reaction loop is fighting line noise.
• Cooling and derating: At 56 A continuous with no overload, the 6RA70 runs well below its thermal limit. Confirm fan operation (if fitted) and that the cabinet ambient stays under 40 °C; otherwise the I²t foldback can momentarily reduce available current and produce apparent A031 events during long dwells.
• Encoder fallback: If A031 persists after the corrective procedure, the EMF-derived speed signal is too noisy for the application. Switch the drive to tachogenerator mode (P083 assignment, depending on firmware) or fit a pulse encoder on the motor and assign it via the encoder option board.
• Firmware version check: The P388/P390 behaviour was tightened in firmware V2.1 and again in V3.0. Before assuming a parameter mismatch, upgrade the CUD firmware to the latest service pack listed on the Siemens 6RA70 product page. Re-commission after every firmware change.

12. Summary of the Corrective Strategy

Replace P165 = 1 (which disables the n-controller) with P156 = 0 % (which clamps only the torque output). Restore two-quadrant torque direction. Add modest setpoint filtering via P150 and P151. Set P388 and P390 to site-realistic values for a tension section, and tune the speed controller Kp/Tn to a lower bandwidth than factory default. Verify with the four-test sequence above. This combination eliminates the root cause of A031, prevents r021 from reading -100 %, and keeps the paper-support drive in continuous positive torque as required by the process.

For canonical alarm definitions and a wider fault-diagnostics reference, consult the official Siemens document: FAQ for 6RA70 DC MASTER and SIMOREG CM – Alarm Messages.