Resolving Simoreg DC Master A031 Speed Controller Fault

1. Problem Identification: Simoreg DC Master A031 Alarm

The Siemens Simoreg DC Master drive (6RA70 / 6RA80 series and successor SINAMICS DCM) raises alarm code A031 ("Speed controller monitoring") when the closed-loop speed regulator cannot hold the actual motor speed within a configurable deviation band of the active speed setpoint for a configurable minimum time. In a properly commissioned drive this is a diagnostic alarm: the drive is signalling that the inner current/torque loop is saturated, the speed feedback path has a problem (sensor, wiring, mechanical coupling), or both.

By default A031 is non-latching: the drive logs the event, sets the alarm bit, and continues running. If the speed error persists beyond the configured escalation window, A031 may transition to fault F031 depending on the configuration word settings (P390 family in 6RA70 / r390 family in 6RA80). Always treat A031 as a symptom and search for the underlying cause rather than disabling the monitor.

1.1 Drive Platforms and Firmware

• 6RA70 series (SIMOREG DC Master, 15 A to 1200 A, 400 V / 575 V / 690 V / 830 V) — parameter set is index-based (Pxxx.y).
• 6RA80 series (SIMOREG DC Master, second generation, up to 2800 A) — parameter set is BICO-based (r / p).
• SINAMICS DCM (6RA80 successor, firmware 1.4 and later) — same alarm code A031 carried forward.
• Parameter dumps from DriveMonitor save with .dnl extension; trace exports use .trc or .csv.

Siemens documentation for the Simoreg DC Master 6RA70 / 6RA80 is available through the Siemens Industry Online Support portal. Search for "6RA70" or "SIMOREG DC Master" to retrieve the operating manual, parameter list, and commissioning manual.

2. A031 Alarm Definition and Trigger Logic

The Simoreg firmware continuously executes the speed monitor block. Each scan (cycle time ~ 1-5 ms depending on the signal source) the block computes the absolute speed error and compares it to the threshold:

|n_setpoint - n_actual| / n_rated > threshold (P388) AND duration > t_delay (P389)

When both conditions are true the alarm is raised. Either condition alone is insufficient. A momentary speed dip during a heavy load transient will not raise A031 if the deviation settles within the configured delay.

2.1 Default Trigger Conditions

2.2 P390 Sub-Index Structure

P390 in the 6RA70 firmware is multi-indexed (P390.1 through P390.4, and in some firmware revisions P390.0 for the high word). The exact meaning of each sub-index is firmware-version-specific. If your .dnl file shows P390 sub-index values such as 1, 2, 3, 4, 3, 0, do not edit them blindly — each bit has a specific function. Open the parameter help (F1 in DriveMonitor) to confirm the meaning for the exact firmware version installed in the drive. Typical functions of the bits are:

3. Root Cause Categories

A031 is a symptom alarm. Three root cause families account for the majority of field incidents. Always classify the cause before changing parameters.

3.1 Speed Feedback Path Problems (most common, ~60 % of cases)

• Tachogenerator commutator wear, glaze, pitting, or uneven surface.
• Carbon brush end-of-life (length < 50 % of new).
• Tacho spring pressure weak or sticking.
• Tacho ripple > 10 % peak-to-peak of nominal DC output.
• Loose or oxidised terminal screws at the drive end or tacho end.
• Encoder cable shield broken or routed near power cables.
• Encoder signal level out of HTL / TTL specification.
• Encoder pulses-per-revolution (PPR) setting wrong for the application.

3.2 Mechanical Coupling Problems (~20 %)

• Flexible coupling elastomer cracked or missing segments.
• Angular or parallel misalignment between motor and feedback shaft.
• Loose mounting feet on tacho or encoder.
• Key or keyway worn, causing micro-slip under load.
• Radial play on the feedback shaft bearing.

3.3 Current / Torque Limit Saturation (~15 %)

• Load demand exceeds configured torque limit (P152).
• Field-weakening region operation (constant-power, reduced torque).
• Mechanical jam (bearing failure, foreign body, product overfill).
• Acceleration ramp shorter than the mechanical system can follow.

3.4 Tuning and Configuration (~5 %)

• Speed controller P gain (P150) too high → oscillation → A031.
• Wrong tacho gain (P084 / P085) → magnitude error.
• Wrong direction wiring (analogue input polarity inverted).
• Encoder direction bit set incorrectly.

4. Pre-Diagnostic Preparation

4.1 Required Tools

• DriveMonitor software (6RA70) or Starter / Startdrive (6RA80 / SINAMICS DCM).
• PC with USB-to-serial converter (RS232) or USB cable for 6RA80.
• Digital multimeter with mV / mA / Ω ranges and diode test.
• Oscilloscope, ideally ≥ 20 MHz bandwidth, with AC coupling.
• Hand tachometer or optical strobe for speed verification.
• Fine-grit polishing paper (P800-P1000) for commutator cleaning.
• Allen keys, feeler gauge, dial indicator for coupling alignment.
• Insulation tester (megger) for cable / winding check (with armature disconnected).

4.2 Safety Precautions

4.3 DriveMonitor Connection

1. Connect the PC to the drive:
   • 6RA70: serial cable from PC to connector SS52 / X300 on the CUD1 board.
   • 6RA80: USB cable to X300 service port, or PROFIBUS / PROFINET to the CUD.
2. Launch DriveMonitor. Configure the COM port: Settings → Serial Port → select COMx, baud 9600 (default), parity even, 8 data bits, 1 stop bit (or 19200 for newer firmwares).
3. Online → Read All Parameters to upload the live parameter set from the drive.
4. Open the .dnl parameter dump (Parameter List → File → Open).
5. Set up a four-channel trace on: n_setpoint, n_actual, I_armature, and P152 (torque limit). Trigger on A031 if available.

5. Tachogenerator Feedback Diagnostics

A DC tachogenerator produces an output voltage proportional to speed (commonly 60 V / 1000 rpm for a 6-pole industrial tacho, or 100 V / 1000 rpm for some European standards). Three diagnostic tests are recommended for any A031 that originates from a tacho feedback path.

5.1 Visual Inspection of the Commutator

1. Isolate the drive and lock out the AC supply.
2. Remove the tachogenerator cover and lift the brush holders.
3. Inspect the commutator surface:
   • Light copper colour with a thin brown patina: healthy.
   • Heavy black carbon track, grooving, pitting, or uneven wear: brush material contamination, weak spring force, or both. Clean the commutator with fine-grit polishing paper (P800-P1000) by rotating the tacho shaft manually; do not use emery cloth (silicon carbide embeds in the copper).
4. Measure brush length: brushes shorter than 50 % of original length are at end of life.
5. Check spring pressure: weak or sticky springs cause intermittent contact and ripple.

5.2 Static Resistance Test

1. Disconnect tacho leads at the drive terminals.
2. Set the multimeter to the Ω range, connect to the two tacho output leads.
3. Rotate the tacho shaft by hand through a full revolution.
4. Read the resistance at multiple angles (every 30-45°).

5.3 Dynamic Ripple Test (Oscilloscope)

The dynamic test is the most informative. Run the drive at a stable, known speed (e.g. 50 % of rated) and capture the tacho output at the drive terminals — not at the tacho itself, so the cable losses are included.

1. Connect scope CH1 to tacho+ terminal, CH2 to tacho− terminal, both referenced to PE.
2. Set scope to AC coupling, 100 mV/div, 20 ms/div.
3. Run the drive at 50 % speed, hold for 10 s, capture a clean trace.
4. Measure peak-to-peak ripple voltage (V_pp) and DC component (V_dc).
5. Compute ripple ratio: η = Vpp / Vdc × 100 %

5.4 Tacho Voltage Scaling Check

The Simoreg scaling parameter P084 (and P085 for direction) maps the tacho voltage to the rated speed. If the tacho has been replaced with a different voltage constant, the scaling must be re-entered or the actual speed reading will be wrong, and A031 will fire at high speed even though the mechanical speed is correct.

After any tacho replacement, verify the actual mechanical speed with a hand tachometer and compare to the drive's n_actual reading. A 5 % mismatch is acceptable; more than 10 % means P084 must be re-entered.

5.5 Coupling Check Between Motor and Tacho

• Hand-rotate the assembly and listen for clicks (slipping key).
• Inspect flexible coupling elastomer for cracks or missing segments.
• Measure the gap between the two shafts with a dial indicator; parallel and angular misalignment should be < 0.1 mm.
• Check radial play on the tacho shaft bearing — > 0.05 mm play is a problem.

6. Encoder and Pulse Generator Feedback Diagnostics

When the speed feedback is an incremental encoder (P083 = 2 or P083 in encoder mode in 6RA80), check the following.

6.1 Signal Levels (HTL vs TTL)

6.2 Wiring and Shielding

• Use twisted-pair cable for A / A/ and B / B/ pairs.
• Shield must be bonded to PE at the drive end only (not at the encoder end) to avoid ground loops.
• Maximum cable length: 50 m for HTL at 100 kHz, 25 m for TTL.
• Route the encoder cable at least 200 mm away from VFD or DC armature cables. If crossing is unavoidable, cross at 90°.
• Inspect the connector at the encoder end for bent pins or contamination.

6.3 Direction and PPR Test

1. Run the motor in the positive direction at low speed (10 % rated).
2. Verify the encoder count increments (DriveMonitor trace on n_encoder_raw or equivalent).
3. If n_actual shows negative when n_set is positive, swap A and A/ (or B and B/), or invert the count direction bit in the encoder configuration parameter (P083 or p083 depending on platform).

6.4 Encoder PPR Verification

The Simoreg scales the encoder pulse count to the motor's rated speed via a PPR (pulses per revolution) parameter. If the encoder has been replaced with a different PPR, the speed reading will be off by the ratio.

Calculate the expected pulse frequency: f_Hz = (n_rpm × PPR) / 60. Measure with a frequency counter on the A channel at rated speed. A 5 % deviation is acceptable; more indicates wrong PPR setting.

7. Back-EMF (EMF) Feedback Operation

In cost-sensitive applications the Simoreg can be run in EMF (armature voltage) feedback mode. The actual speed is derived from the back-EMF, calculated as V_emf = V_armature - I_armature × R_armature. A031 in EMF mode typically indicates one of the following:

• Armature resistance value wrong in the parameter set (P110 series on 6RA70) — re-measure with a Kelvin bridge or calculate from V / I.
• IR compensation mis-tuned (P111 / P112 on 6RA70).
• Field current fluctuating (field supply problem, loose field terminal, weak field stack).

Check field current stability (P260 or trace variable) before assuming a tachogenerator problem in EMF mode. If the field current sags under load, the calculated EMF will sag, the actual speed reading will sag, and A031 will fire even though the mechanical speed is correct.

8. Current and Torque Limit Verification

A drive sitting in current limit cannot accelerate the motor to the demanded speed, so the speed error grows and A031 fires after the configured delay. This is the second most common root cause in production environments.

8.1 Identifying the Limit Condition

1. Run the drive from a low speed to rated speed while monitoring the trace.
2. Watch I_armature (P170 / r170) and the torque setpoint (P160 / r160).
3. If I_armature equals the configured current limit (P171 / P172) and the speed error is large, the drive is in torque limit.
4. Compare the actual torque to the configured limit (P152 / r152). If they match, the limit is the cause.

8.2 Current and Torque Limit Parameters

8.3 Common Load Issues

• Mechanical jam (bearing failure, foreign object in the process).
• Over-loaded product (process transient, batch overfill).
• Field-weakening region: at high speeds the field is weakened, so available torque is reduced.
• High-inertia load with insufficient acceleration ramp time (setpoint ramp too short).

9. Parameter Adjustment: P388 and the P390 Family

P388, P389, and P390 control the A031 monitoring function. They should NOT be changed to mask a fault, but in some commissioning situations a calibrated adjustment is appropriate.

9.1 When Adjustment Is Appropriate

• The drive is operating in a known-good condition but A031 fires during fast acceleration ramps. Widen the deviation band (e.g. 5 - 7 %) or extend the time delay (e.g. 5 - 6 s).
• The application has high cyclic load transients that mimic a tacho error and have been confirmed to be safe for the driven load.
• After a motor / gearbox swap where mechanical dynamics have changed.

9.2 When Adjustment Is NOT Appropriate

• A031 fires at steady-state speed — there is a real problem; do not mask it.
• A031 fires within the first 3 s of start-up with the default values — the load is too high or the speed controller is unstable.
• Tacho ripple is high, or coupling is loose — fix the cause, not the alarm.

9.3 Recommended Conservative Settings

Use these as a starting point only; verify with a full commissioning test.

P388.1 = 5.0   ; widen deviation to 5% (was 3%)
P388.2 = 4.0   ; extend delay to 4 s (was 3 s)
P390.1 = 1     ; keep alarm-only mode
P390.2 = 0     ; disable fault escalation
P390.3 = 0     ; default source routing
P390.4 = 0.5   ; small hysteresis

If A031 still fires after parameter adjustment with no mechanical or electrical fault, the speed controller tuning (P150 / P151 on 6RA70, p150 / p151 on 6RA80) is the next item to inspect.

10. Speed Controller Tuning Interaction

A mis-tuned speed controller can drive the actual speed into oscillation, which looks like a "speed error" to the monitoring block and triggers A031.

10.1 Signs of Tuning Problems

• Speed oscillates with a period of 0.1 - 1 s.
• Drive sounds rough or "hunts".
• Speed error oscillates around zero rather than settling.

10.2 Tuning Reference Values

Re-tune per the Siemens commissioning manual: increase P150 until the drive starts to oscillate, back off 30 %, then reduce P151 (i.e. increase the integral time constant) until steady-state error is acceptable. For the full procedure see the Simoreg 6RA70 / 6RA80 Commissioning Manual on the Siemens Industry Online Support portal.

11. Verification After Repair

11.1 Static Test

1. Power up the drive, clear all alarms, run to 0 setpoint.
2. Apply a small step in setpoint (5 % of rated).
3. Watch n_actual follow with a 200 - 500 ms first-order response, no overshoot, no oscillation.
4. Run to 50 % rated speed, hold for 60 s, confirm A031 does not appear.

11.2 Dynamic Test

1. Program a setpoint profile: 0 → 50 % → 100 % → 50 % → 0, with 5 s dwells.
2. Run the profile from DriveMonitor.
3. Verify the following in the captured trace:
   • No A031 events.
   • n_actual tracks n_setpoint within 3 % at all times.
   • I_armature does not stay at the current limit for more than 0.5 s.
   • No oscillation of n_actual around n_setpoint.

11.3 Long-Run Test

1. Run the drive on the actual production load for at least 1 hour.
2. Monitor the alarm log: no A031 events.
3. Verify the alarm history is clean: alarm count, last alarm timestamp, total run hours since last alarm.

12. Troubleshooting Matrix

13. Related Alarms and Cross-Reference

For related procedures, see the Simoreg 6RA70 / 6RA80 parameter list available from the Siemens 6RA70 product page on the Industry Online Support portal.

14. Field Notes and Caveats

• On 6RA70 firmware versions older than 2.5, P388 default is 5.0 % and P389 default is 5.0 s. Verify the default in the parameter help for the firmware loaded in your drive before interpreting the alarm.
• Some retrofit installations replace the original tacho with a digital encoder. If A031 appears only after the retrofit, suspect scaling (P083 / P084 / P085) and direction (A vs A/ swap) before suspecting the encoder itself.
• The Simoreg alarm log retains the last 8 (6RA70) or last 64 (6RA80 / DCM) alarm events with timestamp and frequency counter. Use it to distinguish "A031 fires once per shift" (transient) from "A031 fires every 30 s" (systemic).
• Drives operating above 1000 rpm with marginal tacho ripple can pass bench tests but fail in the field because of vibration-induced brush bounce. Verify ripple at the actual operating speed, not at a bench speed.
• Always check the drive's armature current at the moment A031 appears. If I_armature equals the current limit (P171 / P172) for the entire duration of the alarm, the load — not the feedback — is the cause. This is the single most useful 30-second check before tearing into the tachogenerator.
• If the tachogenerator's commutator has visible carbon deposition, clean it with fine-grit polishing paper (P800-P1000) by rotating the tacho shaft manually. Do not use emery cloth — silicon carbide particles embed in the copper and accelerate wear.
• Brushes shorter than 50 % of their original length, or with chipped / glazed contact faces, must be replaced as a set, not individually. Mixing new and old brushes causes uneven current distribution and accelerates the failure of the remaining old brushes.