Troubleshooting 6RA80 SINAMICS DCM Current Hunting and F60068

1. SINAMICS DCM (6RA80) Drive Architecture Overview

The SINAMICS DCM controller, marketed in earlier product generations as the SIMOREG DC-MASTER 6RA80, is a fully digital three-phase thyristor converter used to feed the armature and field circuits of separately excited DC motors. In a 500 kW hot-strip rolling-mill application the unit typically operates as a four-quadrant armature bridge with a separately controlled field channel. The hardware consists of an electronic power section (thyristor stacks with RC snubbers, current transformers, fuses), a Control Unit (CUD) that hosts the closed-loop firmware, and a Basic Operator Panel (BOP20) or AOP30 for parameterization. The CUD card carries the firmware version, the motor data set, the BICO interconnections, and the closed-loop coefficients; replacing it without restoring the parameter set is the single most common cause of post-commissioning faults on this platform.

The complete drive must always be referenced against the current edition of the Siemens operating instructions. Always cross-check the procedure below against the official SINAMICS DCM DC Converter Operating Instructions (109478240) and the SINAMICS DCM Control Module Manual (109763558). Catalog data is published in the SINAMICS DCM catalog D23.1.

2. Symptom Matrix - Current Hunting After Load Impact

Three distinct but related symptoms are reported on 500 kW rolling-mill stands equipped with the 6RA80 / SINAMICS DCM controller:

3. Root Cause Analysis - Speed / Current Loop Interaction

On the 6RA80 / SINAMICS DCM the inner control cascade is fixed by firmware:

1. Field current controller (FI) - innermost loop, output = field firing angle
2. Armature current controller (I) - mid loop, output = armature firing angle
3. EMF / speed controller (n) - outer loop, output = current setpoint
4. Superimposed position / web tension loop (optional)

When a hot bar engages the rolls the speed loop instantly demands a large additional armature current setpoint. If the speed-controller proportional gain P50155 (speed controller P-gain) is too high, or its integral time P50156 (speed controller reset time) is too small, the current setpoint overshoots. The armature current controller must then chase a moving target. If the armature P-gain P50225 (current controller P-gain) is also too high, the inner loop becomes underdamped and the system rings at the closed-loop natural frequency. The visual signature is the 10 A → 320 A swing reported in the field log, with armature voltage oscillating in anti-phase because the thyristor bridge is commuting too aggressively.

The tail-of-bar transient (60–70 RPM overshoot) is the mirror image of the engagement transient: the load suddenly disappears, the integral term of the speed controller continues to integrate error, and the integral action must be drained through the current loop before speed recovers. Two minutes of healthy running followed by instability only after a real load event is the diagnostic fingerprint of speed-loop tuning that was done on a no-load spinning motor.

Field-side issues compound the problem. With field weakening in use, the field controller must hold flux constant while armature current varies; a sluggish field current controller widens the mechanical time constant seen by the speed loop and makes the entire system non-linear at exactly the operating point where the bar engages.

4. Armature Current Controller Tuning (P50225, P50226)

The armature current controller parameters that the engineer adjusted are listed below with their canonical meaning. The numeric indices match the SINAMICS DCM parameter list described in the operating instructions manual.

For a 500 kW rolling-mill motor with a typical armature time constant of 30–80 ms, the rule-of-thumb starting point is Kp at 0.3–0.6 (normalized to the Siemens internal scaling) and Tn at 8–15 ms. If a stable value cannot be obtained by manual adjustment, run the automatic optimization routine:

1. Confirm that motor data P50078 (armature inductance La), P50081 (armature resistance Ra), P50083 (field resistance Rf), P50100 (armature rated current IA,nom), P50101 (armature rated voltage UA,nom), and P50102 (field rated current IF,nom) have been entered from the motor nameplate with the correct unit prefixes.
2. Set P50051 (optimization run selection) to "Armature current controller" and trigger via P50052 = 1 (start optimization). The drive will inject a controlled current step and measure the closed-loop step response.
3. When the run completes, read back P50225, P50226, and the new P50228 reference-model damping factor. Save the parameter set with P50180 or to the optional memory card.

5. Speed Controller Tuning (P50155, P50156) and Field Weakening

The speed controller is parameterized as follows:

For a stand with a high-inertia motor (typical GD² for a 500 kW mill motor is 80–200 kgm²), set the initial speed-loop gain by the symmetrical optimum criterion:

Tn_n = 4 · Tn_I · (1 + Ta/Tn_n_ref)        (approx.)
Kp_n = Tn_n / (2 · Kp_I · Ta · Tsum)

where Tn_I is the armature current controller reset time (P50226), Kp_I is the armature current controller P-gain (P50225), Ta is the armature time constant (La/Ra), and Tsum is the small dead-time sum of the current loop. In practice the engineering shortcut is to start with P50155 at 30–50% of the value suggested by automatic optimization and increase it in 10% steps while observing the bar-engagement transient on the trace.

If the drive operates in field weakening above base speed, the field controller must be re-tuned because the gain of the EMF loop is proportional to speed. Verify the field-weakening configuration:

1. Set P50102 to the motor rated field current (not field voltage).
2. Confirm that P50104 (field weakening start speed) and P50105 (field weakening end speed / EMF limit) match the motor nameplate and the operating profile of the stand.
3. Re-run field-current optimization (P50051 = 2) before re-running the speed optimization. A mis-tuned field loop shifts the gain seen by the speed controller and is the most common reason automatic optimization yields a sluggish speed loop on a mill drive.

6. CUD Replacement and F60068 Fault Clearing

Fault F60068 "Parameterization error in power unit" is raised by the firmware during the first power-on self-test after a CUD replacement when the parameters describing the connected power unit do not match the unit the CUD is now driving. The parameter set in a new CUD is the factory default, which is the smallest 6RA80 / SINAMICS DCM rating (often the 125 A unit referenced in the field log), so the firmware will clamp all current limits and field limits to those of the smaller box. This explains the reported symptoms:

• Power-unit identification reads 125 A / 10 A field (parameters r50072 / r50073) because the CUD still holds the default code.
• Field current clamps to 10 A and the drive cannot reach rated field flux, hence field voltage climbs as the firing angle is driven deeper into the unsaturated region of the field supply.
• Armature current scaling in the trace appears wrong because the current measurement is interpreted against the wrong rated value.

Clearing procedure:

1. Identify the actual power unit data from the rating plate on the converter (rated DC current, rated DC voltage, rated field current, three- or four-quadrant code).
2. Enter the values into P50071 (power unit rated DC current), P50072 (power unit rated DC voltage), and P50073 (power unit rated field current). On 4-quadrant units also confirm P50074 (bridge configuration code) is set to "4Q".
3. Download the previously saved parameter file from the memory card using P50180 (load from card) or via STARTER / SIMOTION SCOUT. Verify that r50072 and r50073 now read the correct unit rating after the load.
4. Acknowledge the fault with the standard sequence (binector-connector reset on P50170 or the BOP20 "Acknowledge" key) and run a power-on check in idle to confirm that F60068 has cleared and does not reappear.

7. Field Voltage Stabilization and F60043 Fault

Fault F60043 "Field current below minimum" is raised when the measured field current falls below the configured monitoring threshold during operation. The reported behaviour - field voltage climbing from 220 V DC toward 320 V DC while armature current is limited to 42% of rated - is the diagnostic fingerprint of a field-current setpoint/actual mismatch caused by incomplete field optimization after a CUD replacement.

The field voltage is not directly regulated by the operator; it is the consequence of the firing-angle output of the field current controller trying to push the requested current through a load whose impedance is higher than expected. The cascade is:

1. Firmware reports rated field current from P50073 (default 10 A) because the CUD has not been re-parameterized.
2. Field current setpoint saturates at 10 A.
3. Real field circuit requires ~30–40 A to establish rated flux, so the firing angle drives to its upper limit, raising DC field voltage toward the line-side peak.
4. EMF regulation cannot hold rated volts/hertz ratio, so the speed controller backs off the armature current setpoint to protect the field-weakening map. Armature current clamps at ~42%.
5. When the firing angle briefly snaps back the field current momentarily falls below the F60043 threshold and the fault latches.

Corrective sequence:

1. Re-enter the power-unit rating as in Section 6.
2. Verify motor field data: P50102 (rated field current), P50103 (field nominal voltage if used), P50083 (field resistance cold/hot).
3. Run field optimization (P50051 = 2). The drive will sweep the field firing angle, identify the saturation knee, and load the correct P50102 reference and the field controller gains P50330 / P50331.
4. After the optimization completes, check the steady-state field voltage. It should settle within ±10% of the motor nameplate value and remain stable under armature load transients.

8. Commissioning Procedure for a Rolling-Mill Stand

The procedure below merges the cold commissioning steps from the Siemens operating instructions with the field-proven sequence used on rolling-mill stands.

1. Mechanical and electrical safety check. Confirm the drive is isolated, the armature contactor is open, the field is off, and the motor is uncoupled or safely de-rated for no-load spin. Verify the line-side fuses, the line reactor (if fitted), and the field supply fuse.
2. Power-on with CUD parameterized. Restore parameters from memory card. Confirm r50072 / r50073 match the rating plate. Acknowledge all faults.
3. Motor data entry. Enter P50078, P50081, P50083, P50100, P50101, P50102, and the encoder / tacho data in P50090–P50093 from the motor plate. Confirm the pulse-number setting of the encoder against the tacho calibration.
4. Field current optimization. P50051 = 2, start via P50052. Wait for completion (typically 20–40 s). Save parameters.
5. Armature current controller optimization. P50051 = 3, start. The drive will lock the motor and inject a controlled armature step. Wait for completion.
6. Speed controller optimization (n_optim). P50051 = 4, start. Drive accelerates through the configured speed range and identifies the mechanical time constant and gain.
7. Field weakening optimization (if used). P50051 = 5, start. Drive ramps to the configured field-weakening speed and adjusts the EMF controller.
8. Save parameter set. P50180 to the memory card, and export a STARTER project to the engineering archive.
9. Loaded rolling test. Engage the first bar at reduced speed, capture a STARTER trace of r52161 (armature current), r52162 (armature voltage), r52163 (speed actual), and r52165 (field current). Look for ringing at the bar-engagement edge.
10. Fine-tune. If ringing is visible, reduce P50155 by 10% per iteration. If the speed loop is sluggish, increase P50155 by 10%. Re-save parameters after every change.

9. Inline Schematic - Closed-Loop Cascade

The diagram below shows the cascade that the engineer must keep in mind when adjusting any one parameter, because every gain you touch moves the bandwidth of every outer loop.

10. Verification and Field Test

After any tuning change, perform the following verification sequence before returning the stand to production:

1. Spin the motor at 50% of rated speed with no load and confirm stable current, voltage, and field readings on the trace.
2. Apply a controlled step disturbance to the current setpoint via STARTER and confirm the response settles within three open-loop time constants with overshoot below 8%.
3. Engage one warm bar at reduced line speed and record the engagement transient. The current peak should match the calculated peak (typically 1.6–1.8 × rated) and the oscillation envelope should decay within 200–400 ms.
4. Engage a hot bar at full line speed. Monitor r52161 for 60 seconds. The 10 A → 320 A swing reported in the field log must be replaced by a stable trace with ripple no greater than ±5% of the demanded average.
5. Confirm the F60068 / F60043 faults do not reappear in the diagnostic buffer r50100 / r50101 after a full shift of loaded operation.

11. Troubleshooting Matrix

12. Frequently Asked Questions