Siemens 611U A607 A608 Alarms on Arrow 2: Diagnostic Procedure

Overview

Siemens SIMODRIVE 611U digital servo drives are deployed on Cincinnati Arrow 2 series vertical machining centers (Arrow 2 750, 1000, and 1250C variants). Two of the most persistent and field-misdiagnosed alarms on this platform are A0607 (speed-controller output at limit during ramp-up) and A0608 (speed controller at torque or current limit for an inadmissibly long time). On the operator panel these typically surface as “X-Axis Pulse Enable,” “Y-Axis Pulse Enable,” or a generic “Axis Pulse Enable” message, while the 611U drive’s seven-segment display continues to flash the underlying alarm code. This article documents the diagnostic procedure used to isolate the fault to a mechanical, cabling, or drive-module root cause without unnecessary motor or drive replacement.

System Architecture

The Arrow 2 machine uses a SINUMERIK 840D (or 840C on earlier builds) numerical control commanding a 6SN11xx-series 611U drive rack. The rack is laid out as follows:

• Controller cards (6SN1118-xxx) — one two-axis controller slots into the upper position of each Power Module stack. The controller card holds the speed and position loops, the I/F interface to the NC, and the diagnostic seven-segment display.
• FEPROM memory module — a small plug-in cartridge carrying the axis-specific parameter set (Firmware versions typically 03.x and 04.x on Arrow 2 builds). The module MUST remain matched to its original Power Module and motor.
• Power Module (6SN1123-1Ax0x or 6SN1124-1Ax0x series) — contains the IGBT inverter bridge, gate drivers, current sensors (LEM-type transducers), and integrated bleeder.
• Line / power supply module (6SN113x series) — establishes the DC bus for all Power Modules in the rack and provides the auxiliary 24 V supply logic.

Spindle and feed-axis Power Modules in the Arrow 2 are never interchangeable even if they share a current rating sticker. The spindle module is engineered for high-inertia constant-torque loads; the feed-axis module is optimized for high-bandwidth follow-through. The X, Y, and Z Power Modules on the same machine are interchangeable with caution, provided the part number stamped on the front panel is identical.

A0607 and A0608 Alarm Definition

Both alarms indicate the digital speed controller inside the 611U has saturated at the torque or current limit. They are control-loop saturation alarms, not over-current trips, and are therefore correlated to load demand — not motor HP or current rating per se.

The parameter that bounds the integrator is p1140 (torque limit ramp-up time) and p1141 (torque limit ramp-down time) on the 611U parameter tree. Default settings are reasonable; persistent tripping means the load is demanding torque the motor cannot deliver without saturating the loop.

Root Cause Categories

The published Siemens warning (and decades of Cincinnati Arrow 2 field service) identify the following categories:

1. Mechanical — binding bearings, failed coupling insert, damaged ballscrew nut, contaminated linear guides.
2. Motor — shorted winding, insulation breakdown, open phase, degraded feedback device (resolver or encoder).
3. Power cable — chafe damage, broken conductor at stress relief, water ingress at the motor terminal box, mis-wired phases on a recent replacement.
4. Encoder / feedback cable — shield break, intermittent conductor, ground loop on the shield.
5. IR compensation module — resistor module (1FK6 / 1FT6 motors) drifted or open.
6. Drive module — memory module corruption, current-sensor failure, IGBT gate-driver fault, controller-card interface fault.
7. Tuning — Kp/Tn mismatched to load inertia, especially after a mechanical modification.
8. Cooling — cabinet fans failed or filters clogged; the 611U IGBT module will fold back torque at elevated heatsink temperature, driving the controller into limit.

Safety and Preconditions

Before opening the cabinet or disconnecting motor leads, observe these controls:

• Main disconnect OPEN and locked; verify with a known-good voltage tester on the line side of the disconnect.
• Wait five minutes for the DC bus capacitors to discharge (Siemens specification for the 611U > 4 min ≤ 800 V DC).
• Pin the axis to prevent free-fall on Z (Arrow 2 builds have a gravity column; do not rely on counterbalance alone).
• Set the control to “Commissioning” or “Service” mode if any motion is required for tuning.
• Confirm all stored energy (hydraulic counterbalance, pneumatic supply) is vented.

Diagnostic Procedure — Step by Step

Step 1: Capture the Alarm Buffer

1. Power on the machine; do NOT acknowledge alarms yet.
2. On the 611U drive front panel, press the P key repeatedly to step through the alarm history.
3. Record each alarm code, axis identifier (A=X, B=Y, C=Z on most Arrow 2 wiring), and the timestamp from the NC alarm line.
4. From the HMI (SINUMERIK Operate or earlier MMC103), navigate to Diagnostics > Drive > Alarm Buffer and back up parameters r947.0…r947.7 (time-stamped fault buffer on the 611U).

Step 2: Mechanical Free-Motion Test

1. Open the way-cover and inspect the ballscrew nut area for lubricant migration or chip contamination.
2. Mark the coupling flange with a felt-tip pen so any movement during hand-turn is visible.
3. With the drive de-energized at the controller card (pulse enable removed), rotate the ballscrew by hand through full travel. The expected feel is uniform drag with no detents, heavy spots, or grating.
4. Listen for the characteristic “hum” or buzz that operators describe as “dither.” This is the drive applying micro-corrections; if the hum is loud enough to hear from the operator panel, the speed loop is already operating near its limit — a leading indicator of A608 risk.
5. Inspect the coupling insert (typically R+W EK/series or KTR Rotex on Arrow 2). Polyurethane inserts appear black and matte; failure appears as cracked, glossy deformation; Hytrel inserts become milky-white under stress. Replace on suspicion.
6. Check axial play at the ballscrew preload assembly with a dial indicator; preload drift > 0.0002 in. after 20 years of service is normal, but > 0.001 in. requires bearing service.

Step 3: Motor Insulation Test

1. Isolate motor leads at the 611U Power Module terminal block (typically X1, U2, V2, W2 for the 1FT / 1FK motor family).
2. Meg motor leads with a 500 V megohmmeter: lead-to-lead and each lead-to-ground. Acceptance: > 5 MΩ at 20 °C, derated linearly to > 1 MΩ at the motor’s rated class temperature.
3. Measure phase-to-phase resistance with a 4-wire milliohm meter. Phases should match within 2%; a deviation > 5% indicates an open or high-resistance joint in the motor or the cable.
4. If the insulation or phase resistance test fails, the motor set is the most likely culprit. Do not re-energize before repair.

Step 4: Power and Encoder Cable Test

1. Visually inspect the motor power cable along its entire run; pay attention to the cable carrier bend radius and any chafe against the coolant line.
2. With the cable disconnected at both ends, ring out each conductor end-to-end. Resistance should be < 0.5 Ω for runs < 15 m, < 1 Ω up to 30 m.
3. Inspect the encoder cable for shield continuity. Heidenhain and Siemens 611U feedback cables are shielded twisted pair; shield is bonded at the drive end only and isolated at the motor end. Verify with an ohmmeter.
4. With the encoder cable disconnected at the drive, measure each signal pair from the drive end; expected: open-circuit on the drive side and a defined impedance through the cable at the motor end. A shorted pair indicates cable damage.
5. If the cable is suspect, replace with a Siemens OEM or approved equivalent cable. Field-terminated cables require paired foil + braid shield and drain wire per Siemens installation guide.

Step 5: Drive Module Swap Procedure

1. Verify the controller card part number stamped on the front panel (e.g., 6SN1118-0DJ21-0AA1). Only identical part numbers may be swapped.
2. Verify the FEPROM is seated firmly and matches the original axis label.
3. Disconnect X1, X2, X411 at the controller card and remove the card. Place the FEPROM back on the matching memory module location; do not swap FEPROM modules across axes.
4. Install the spare (or known-good) controller card in the X-axis slot; reuse the X-axis FEPROM.
5. Power up and observe which axis now reports A608.

If the alarm follows the controller card, the controller is faulty (rare; current sensor or interface ASIC). If the alarm stays on the X axis, the Power Module or motor is suspect. Continue with Power Module swap.

Tuning Adjustments (Temporary Mitigation Only)

When all mechanical and electrical tests pass and the alarm still recurs only on aggressive moves, the loop gain may be marginally above the system’s tolerance. The 611U parameters exposed through the commissioning interface are:

For the X-axis on Arrow 2 1250C, the default Kp is approximately 0.8–1.2. A Kp reduction of 10–20% typically removes marginal A608 occurrences with no observable shift in contour performance. Adjustments should be made and signed off by a Siemens-certified field service technician with the SinuTrain or SimoCom commissioning tool. Loop tuning is a maintenance activity, not a permanent fix — a reoccurring alarm is a symptom of a load condition.

Cooling Verification

The 611U Power Module derates approximately 25% per 10 °C of heatsink temperature above 60 °C. On Arrow 2 builds the cabinet exhaust fans are mounted to the bottom outside of the cabinet and draw air across the power-module heatsinks. Verification steps:

1. Verify both fans are spinning in the correct direction (typically exhaust down or out).
2. Measure heatsink temperature at steady state with the machine in rapid-traverse cycle; should not exceed 70 °C.
3. Inspect and replace filter media at the service interval (90 days in dirty environments, 6 months in clean).
4. Verify the 611U internal fan on each Power Module is operating; a stalled internal fan will produce localized heating even when the cabinet fans run.

Verification After Repair

1. Reassemble coupling and re-energize the drive.
2. Run a G01 controlled move at 25% rapid (e.g., 1,000 mm/min on a standard Arrow 2 X axis) for 30 minutes. No alarms expected.
3. Run a full rapid traverse cycle (G00) covering full travel in both directions.
4. Run a contouring test (G02/G03) with the spindle loaded at 50% to simulate full-load conditions.
5. Verify positioning accuracy: laser interferometer or dial indicator at three points across travel. Typical Arrow 2 spec is ±0.0005 in. for X/Y, ±0.0003 in. for Z (note: a fresh backlash compensation may be required after coupling service).
6. Confirm a clean alarm buffer (r947) over the test cycle.

Pitfalls and Field Misconceptions

• Replacing the motor when the cable is the fault. Always measure insulation resistance and phase resistance first; a single chafed conductor in the power cable will produce the same alarm signature as a damaged motor.
• Swapping a non-identical controller card. Different firmware revisions trigger parameter-compatibility faults and may drive the drive into over-current on the first move.
• Using the spindle drive to substitute for an axis drive. Spindle and feed-axis modules use different current-loop tuning and, frequently, different current ratings. Mismatching produces immediate hardware damage.
• Disabling the scales via axis settings without selecting “distance-coded” reference marks. The SINUMERIK rejects the change because axis reference is fundamentally position-based; this configuration must be set during initial commissioning, not retrofitted for service convenience.
• Ignoring cabinet heat. Operators in conditioned machine shops rarely notice a stalled cabinet fan. The 611U module will silently fold back torque, producing A608 well before the heatsink reaches overtemperature shutoff.

Frequently Asked Questions