Resolving SIMOREG 6RA7085 r019 Armature Current Zero Reading

Problem Overview

The Siemens SIMOREG DC MASTER 6RA7085-6DV62-0-Z is a fully digital 6-pulse or 12-pulse (with option) armature converter rated at 420 V DC output with 600 A DC continuous armature current. The unit serves legacy DC motor drives in metals, paper, wire-drawing, hoist, and test-bench applications where modern AC retrofit is undesirable. One of the most frequently reported field faults on this model is the loss of the actual armature current actual value (parameter r019). When the parameter reads 0.0 or freezes at no-display, the drive itself often continues to regulate armature voltage and field correctly; only the feedback path to the operator panel, DriveMonitor PC software, or Profibus cyclic telegram is broken.

This article documents the complete diagnostic ladder for an r019 = 0 condition, including the terminal-voltage measurement at the CUD1 analog tap (X175-12/13), the current-transformer (CT) verification on the A2 armature-current-transducer card, the parameter set that gates the displayed value, and the hardware orientation problem (reversed current sensors) that has been the root cause in multiple documented field incidents on this 6RA70 sub-family.

Affected Products and Configuration

Symptom Set and Initial Triage

The reported incident combines the following observables:

• DriveMonitor (PC service tool) shows every actual value normally — line voltage, field current, armature voltage, speed feedback — except r019 (Act I val armat).
• r019 on the PMU (front panel) reads 0.0 regardless of motor load.
• Measurement at CUD1 analog terminals X175-12 (Armature current actual value) and X175-13 (reference/ground for that channel) with a bench DMM returns 0 V DC to a few millivolts even with the motor loaded.
• No drive alarm or fault is active. When the speed setpoint is raised on the test bench with a small motor, Alarm A031 appears, generally a current-monitoring or speed-dependent warning, consistent with under-utilization of the 600 A device.
• Output torque develops; armature voltage r018 responds correctly to setpoint ramps. Only the actual-current feedback path is dark.

Because the regulation loop continues to function and only the actual-value display path is broken, the fault is isolated to one of three subsystems: (a) the CT secondary circuit on the A2 card, (b) the analog front-end of the CUD1, or (c) the parameter scaling that determines which current value the firmware reports as r019.

Root Cause Family

For the 6RA7085-6DV62-0-Z the documented root causes for a hard-zero r019 trace to the following chain, ordered by probability from field experience on this series:

1. Current transducer (CT) wiring reversed or open on the A2 card. The CTs deliver a bipolar secondary current; if polarity is inverted the firmware reports near zero because the rectified mean cancels.
2. Load resistors on the A2 card open or drifted. Per Siemens manual chapter 6.2, the A2 board C98043-A7002 carries the burden resistors across CT pairs X3.1/X3.2 and X3.3/X3.4. Field-measured nominal is approximately 10 Ω; either open or shorted condition causes the analog tap at X175 to collapse.
3. CUD1 analog input failure. Damaged op-amp stage on the control board — rarer but valid on aged units.
4. Parameter scaling mis-match. p076.001 (or p067, depending on firmware version) — drive-rated armature current — and p100 — motor-rated armature current — set such that the displayed r019 resolution rounds 1.2 A to 0.0 on a 600 A device (≈ 0.2 % of full scale).
5. Defective ribbon cable between A2 and CUD1, or bent pins in the X3 plug.
6. Wrong Z-option firmware loaded at last service — r019 scaling differs between options.

The case history on this incident was unambiguously category (1): reversed current sensors. The pre-existing service technician installed the CTs backwards; once corrected, the display recovered without board replacement.

Prerequisites for Diagnosis

• Valid SIMOREG DriveMonitor commissioning software (compatible with the firmware on the unit; refer to Siemens ordering option for the matching version, see Siemens Industry Online Support).
• Current firmware documentation: SIMOREG DC Master Operating Instructions (Siemens order code 6RA7085 series), Chapter 6 — "Functions", and Chapter 11 — "Faults and Alarms" (A031 family).
• Calibrated digital multimeter (Fluke 87 V or equivalent), DC volts and ohms, 10 MΩ input.
• Clamp-on DC ammeter (or tong tester) for the field verification step.
• Oscilloscope (Fluke or Tektronix) with isolated channels to confirm unrectified CT secondary waveform if necessary.
• PMU (front panel) with the parameter navigation unlocked, or a Profibus/Profibus-equivalent connection to the HMI.
• Personal protective equipment: arc-rated gloves, face shield, insulated tools rated for the DC bus voltage.

Diagnostic Procedure

Step 1 — Confirm the symptom under load

Lock the line, apply auxiliary, and confirm that r019 holds 0.0 on the PMU regardless of setpoint step. Log the value of r018 (armature voltage actual) to ensure the power section is energised. If r018 is also zero, you have a power-section problem, not an r019 problem, and the remainder of this article does not apply.

Step 2 — Measure the analog tap at X175

Place the DMM in V DC across CUD1 terminals X175-12 (Armature current actual value) and X175-13 (reference):

The nominal scaling at X175 is ±10 V for ± armature rated current, i.e. 10 V ≡ 600 A on a 6RA7085-6DV62, so a healthy tap with the drive near rated load should show a few volts at least. At 1.2 A load the tap should still show approximately 20 mV — not zero.

Step 3 — Inspect the A2 armature-current card

Power down, discharge, and open the door to expose the A2 board, Siemens part number C98043-A7002. This board hosts the burden resistors and the rectifier/amplifier stage that converts the CT secondary currents into the ±10 V signal sent to CUD1 via the ribbon cable. Inspect:

1. The CT secondary pairs at X3.1/X3.2 and X3.3/X3.4. Confirm the conductors land in the correct polarity as silk-screened. Reversed polarity across both pairs is the textbook ground fault; reversed on a single pair produces a partial signal — the typical "small reading" symptom on dual-path designs.
2. Burden resistors on the PCB. Power-off resistance across the burden network measures approximately 10 Ω on the 6RA7085 class; any open circuit or burnt smell confirms board damage.
3. The ribbon cable that carries the rectified signal to CUD1. Pins can be bent; conductors can be chafed at the bend.
4. Connectors at X3 — ensure both locking tabs are down.

Step 4 — Cross-board swap test

If the source drive and a known-good sibling unit of identical rating are available, swap the CUD1 and the Communication Board between them and re-test. In the source incident this swap conclusively proved that the A2 card and CT wiring were at fault, and not the controller. This is the single most valuable bench test available in the field because it isolates the failing subsystem in a few minutes.

Step 5 — Verify parameter scaling

Even with healthy hardware, r019 will display 0.0 if the scaling parameters are wrong. Verify and record the following via DriveMonitor or PMU (P 0xxx access):

Do not enter the test-motor current as p100. P100 must always reflect the motor nameplate. With p100 = 1.2 A and p067 = 600 A, the firmware may generate diagnostic or alarm behaviour, and r019 resolution can be unacceptably coarse. The motor under test should be reasonably scaled to the drive for accurate parameterization; if not, use a load bank or a properly rated motor for commissioning.

Step 6 — Replace or refit the A2 / CT chain

If Steps 1–5 point to the A2 card or the CT wiring:

1. Document the existing wiring with photos.
2. Reverse the CT secondary leads at X3.1/X3.2 and X3.3/X3.4 one pair at a time, powering up between changes, and observing r019.
3. If reversal restores the reading on one or both pairs — that was the root cause. Confirm the value matches the clamp-meter measurement at the armature cable.
4. If reversal does not change anything, measure the burden resistors with the DMM; if any are open or burnt, replace the A2 card (C98043-A7002) using original Siemens spare-part procedure.
5. If the A2 card is healthy and CT polarity is correct, replace CUD1 as a final step. Note that the source incident did not require a CUD1 swap once the CT wiring was corrected.

Solution: Corrected Wiring Confirmed by Bench Test

In the source incident the fix was simple but easy to miss: the previous service technician had installed the current sensors backwards. Once the engineer at the bench corrected the CT polarity on the A2 board at X3, the r019 reading immediately tracked the clamp-meter value of armature current across the full speed and load range. The CUD1 and Communication Board swaps that had been performed for diagnosis were returned to the unit. No firmware change was necessary; no parameter change was necessary once the wiring was correct.

Verification

1. With the line locked and the drive in "Operation Enable" (run enable active), apply a measured load via a brake or motor under controlled conditions.
2. Read r019 on the PMU. It should now match the clamp-meter reading at the armature bus within ±2 %.
3. Read r019 in DriveMonitor. The value must match the PMU exactly (PMU is the canonical source; DriveMonitor is a polling client).
4. If the HMI panel reads via Profibus, verify the relevant PZD (Process Data Word) mapping. For S7-style cyclic telegraph the actual-value word is typically PZD4 in PPO type 2/4 — confirm against the Profibus parameterization tool for the drive's GSD file.
5. Step-load and step-unload the drive; confirm that r019 tracks the load monotonically, with no dead zone at low currents. Sensitivity at low currents on a 600 A drive is approximately 60 mA per display unit, so a 1.2 A test motor will read 1.2 on r019, which will appear as a small but non-zero value.
6. Run the drive for at least 30 minutes under load, monitor for reappearance of A031 or any other alarm.

Related Fault and Alarm Codes

Field Tips and Edge Cases

• Reading non-zero at PMU but zero on HMI: If r019 is correct on the PMU but the Profibus HMI displays zero, the fault is not on the A2 path — it is the HMI mapping of the cyclical PZD. Verify the telegram selection in DriveMonitor → Communication → Profibus, and ensure the SIMATIC side references the correct word offset.
• Reading small but non-zero (e.g. 0.1 A): Often normal. The CT secondary at 1.2 A primary on a 600 A device is operating at the lower decade of measurement resolution. Use a load that draws at least 10 % of rated current to perform meaningful tests.
• Field replacement sequence: When replacing the A2 card in the field, always re-verify all CT polarity marks before powering up the unit. Many service technicians rely on the silk-screen arrows on the A2 board; on aged equipment these can fade.
• Dual-redundant CT chain: The 6RA7085 uses two CT pairs (one per bridge half of the 6-pulse rectifier). Each pair must be correctly oriented; reversing one only, while leaving the other correct, produces a half-amplitude r019 — easy to mistake for a calibration error.
• Firmware vs hardware fault indicator: When you see the F004 message you are looking at a hardware cause on every documented incident. The firmware does not throw F004 for a parameter scaling error.
• Z-option relevance: The "Z" suffix in the order code (e.g. 6RA7085-6DV62-0-Z) defines the language pack, optional features, and a portion of the licensed parameter set. A mismatch between two drives of the same MLFB but different Z-options may explain subtle display differences but will not zero out r019 on a healthy unit.

Preventive Measures

• Add a quarterly readout of r019 at a known load to your preventive-maintenance checklist and log it. A drift toward zero at the same load condition is an early indicator of CT or A2 degradation.
• Label the CT secondary pairs at X3 with phase marks (U1/U2, V1/V2) immediately after commissioning; do not rely on technician memory.
• Maintain a known-good spare A2 card (C98043-A7002) in stores for high-availability lines.
• Whenever a CUD1 is replaced, follow it with a verification of the X175-12/13 voltage at a known current. This takes two minutes and rules out the failure mode discussed here.
• Keep DriveMonitor project files version-controlled; r019 scaling is non-trivial and a parameter set from the wrong drive rating can look correct on every screen except the actual-value tab.

FAQ