Replacing SIMODRIVE 611A 2-Axis Module with Single-Axis Units

Problem Overview

A Siemens SIMODRIVE 611A modular drive system on a Sinumerik-controlled machine (saw in this case) develops a hard fault on one axis of a dual-axis power module, Siemens part number 6SN1123-1AB00-0CA0. Field troubleshooting isolates the failure to the axis controller / power-section path rather than the 1FT5-series synchronous servo motor. Because the machine is scheduled for end-of-life replacement in the near term, the maintenance strategy is to retire the failed 2-axis module and substitute two surplus single-axis modules (6SN1123-1AA00-0CA1, 50 A each) rather than send the original unit out for repair. The existing control card is 6SN1118-0AE11-0AA1 and the Sinumerik side module is 6FX1121-4BA03.

The migration is technically viable because the SIMODRIVE 611A backplane, control-card interface, and 1FT5 motor wiring are common to both the dual-axis and the single-axis power sections. However, three items must be reworked before the axes will run cleanly: (1) the module's DIP switch block must be re-addressed so the NCU recognizes two separate axis slots; (2) the per-axis pulse-enable and controller-enable wiring, normally bussed on the dual-axis module, must be split out to the two single-axis modules; and (3) the Sinumerik axis configuration in 6FX1121-4BA03 must be recommissioned so that drive parameters, encoder scaling, and current limits match the new 50 A hardware instead of the original 25 A per axis. This reference documents each of those steps, the supporting diagnostics, and the verification procedure that follows.

Affected Components and Part Numbers

Fault 1040 Root-Cause Analysis

The reported alarm on the 611A status display is fault 1040, observed at the moment the operator attempts to enable the affected axis. Three diagnostic observations from the field are decisive:

1. Fault present with motor disconnected. The 1FT5 power leads were lifted at the motor terminal box. On the next power-up, fault 1040 still raised. With the power stage idle (no stator current path), a motor-winding failure or cable short cannot be the cause.
2. Fault follows the controller, not the motor. When the motor cables were physically swapped between the two axes on the dual-axis module, fault 1040 stayed anchored to the originally-faulted axis slot — proving the failure is upstream of the motor terminals (current sensor, control board, or IGBT power section).
3. Fault present at enable, not at run-up. The drive reports the fault before motion is commanded, which is consistent with a current-loop monitoring or power-section self-test failure rather than a position / following-error trip.

On SIMODRIVE 611A, the diagnostic number 1040 belongs to the current-monitoring alarm group. With the failure pattern above (alarm with no load, axis-locked, controller-side), the most likely root causes are:

• Current sensor (Hall-effect transducer) failed on one axis channel. The 611A measures phase current through a closed-loop transducer; an open or shorted transducer forces the regulator into saturation, and the firmware raises 1040 on enable.
• Shorted IGBT or driver stage on one axis half-bridge. A short in the power section presents a non-zero current offset to the regulator at enable; firmware detects the imbalance and raises 1040.
• Control-card axis-1 channel failure on the 6SN1118-0AE11-0AA1. Less common, but if the same fault persists on the original module after a known-good dual-axis swap, the control card itself is suspect.

Because the dual-axis module is already pulled from the machine and the operator has two 1-axis LT modules in inventory from decommissioned equipment, the practical path is to discard the failed module and migrate to the two single-axis units rather than isolate the failure down to component level.

Migration Feasibility: Dual-Axis to 2× Single-Axis

The 611A backplane is slot-based: each LT slot carries DC bus, 24 V logic, and the analog setpoint / feedback bus from the control card. Both the 2-axis LT module (6SN1123-1AB00-0CA0) and the 1-axis LT module (6SN1123-1AA00-0CA1) use the same physical slot connector, so mechanically the swap is direct. The differences that matter are:

Because the per-axis current rating increases from 25 A to 50 A, the regulator can deliver more current to the 1FT5 motor than the original drive configuration specified. The 1FT5 motor itself is the limit on allowable current — the drive parameter that must be reset is the motor's continuous current limit (typically motor nameplate IN) and peak current (IMAX). Setting these conservatively at nameplate values keeps the motor within its thermal envelope; setting them at 50 A would exceed the motor's thermal limit and trip a different alarm later.

Control Card Compatibility (6SN1118-0AE11-0AA1)

The control card 6SN1118-0AE11-0AA1 is a 2-axis regulator card designed for the 611A family. It provides two independent current / speed / position loops, two encoder inputs, and two pulse-enable outputs — one per axis. The card does not care whether the LT (power) section for each axis is one half of a dual-axis module or a stand-alone single-axis module; the interface to the power stage is the same set of analog references and digital enable lines per axis.

What does change in the migration:

• Pulse-enable line for axis 1 must be wired to terminal 663 / 9 on the first single-axis LT module (instead of being internally bused to the dual-axis module's axis-1 half-bridge).
• Pulse-enable line for axis 2 must be wired to terminal 663 / 9 on the second single-axis LT module.
• Controller-enable (terminal 65 / 9, axis 1; 65 / 63, axis 2) must be split between the two new modules.
• Motor temperature sensor input (terminal 75 / 76 area on the original dual-axis wiring) is duplicated per single-axis module — each module must receive the motor's PTC / KTY input separately.

Reference the 611A Function Manual for the exact per-axis terminal mapping on the single-axis LT variant; the terminal numbering on the 1-axis LT module is condensed compared with the 2-axis module because there is only one set of power and feedback terminals to wire.

DIP Switch Configuration

The 611A LT module carries a small DIP switch block (S1, sometimes S2) on its front edge that the 611A firmware uses to determine the slot address and current-range scaling. For the migration, two switches are critical:

Wiring and Enable-Signal Considerations

The largest practical job in the migration is rewiring the enable and feedback signals. The dual-axis module combined these on a single connector; the two single-axis modules need them split out. The minimum wiring changes are:

1. Pulse enable (P-EN). Originally a single wire driven by the control card's axis-1 P-EN output, internally routed to the dual-axis module's axis-1 half-bridge. After migration, run a dedicated wire from the control-card axis-1 P-EN terminal to LT-module-A terminal 663. Repeat for axis-2 to LT-module-B terminal 663.
2. Controller enable (C-EN). Same pattern. Run C-EN axis 1 to terminal 65 / 9 on module A; C-EN axis 2 to terminal 65 / 9 on module B.
3. Motor temperature. The 1FT5 PTC / KTY input must land on each new module individually. If the original wiring had a single twisted pair entering the dual-axis module, that pair must now split — the motor PTC is single-sensor and must parallel into both modules' temperature inputs (per 611A practice) or, alternatively, only the module driving that motor reads the sensor.
4. Resolver / encoder cable. Each single-axis module has its own feedback connector. Route the 1FT5 feedback cable to the appropriate module; the second module's feedback input is unused unless the second axis's motor is wired into it.
5. Power leads U / V / W. Each single-axis module has one set of power output studs. Land axis 1's U/V/W on module A and axis 2's U/V/W on module B using the same gauge and length as the original dual-axis wiring to keep current-loop impedance balanced.
6. DC bus link. Both single-axis modules draw from the common 611A DC bus; the bus bars or wiring between the modules and the infeed must carry the higher of the two axis peak currents.

Enable-Signal Truth Table

Sinumerik Configuration (6FX1121-4BA03)

The Sinumerik side module must be told that the axis hardware has changed. In the Sinumerik 810D / 840D-era commissioning flow (compatible with the 6FX1121-4BA03 hardware), the relevant machine data blocks are:

The drive's current-loop gain is a function of the LT module's current-sensor bandwidth and the motor's leakage inductance. Because the new 50 A module has a higher current-measurement range and a slightly different transducer gain, the speed-loop gain and integral time constants will likely require a re-tuning pass. Plan for one iteration of automatic commissioning followed by a step-response check on each axis.

Recommissioning Procedure

1. Lock out / tag out the 611A infeed. Verify DC bus discharged to < 50 V before any module insertion.
2. Remove the failed dual-axis LT module 6SN1123-1AB00-0CA0. Inspect the backplane connector for bent pins or arc damage. Clean with isopropyl if needed.
3. Set the DIP switches on both replacement 6SN1123-1AA00-0CA1 modules per the table in the DIP Switch Configuration section above. Use the failed module as a reference for everything except current-range and slot-address.
4. Insert module A (axis 1) into the original slot for axis 1. Torque the mounting screws to the 611A specification (typically 2.5 N·m for the front-panel screws).
5. Insert module B (axis 2) into an adjacent slot if available, or into the same physical slot if the chassis layout allows. Many 611A chassis have a free slot adjacent to the original; verify before re-routing.
6. Re-wire the per-axis signals per the wiring table above. Use the original cable colors and ring-lug terminations where possible to keep the schematic traceability intact.
7. Re-apply power (main contactor on, DC bus pre-charge). Confirm no faults on the 611A status display at idle.
8. Enable each axis individually from the Sinumerik HMI. Listen for fan operation on each new module — both fans should spin up.
9. Check for fault 1040 with motors disconnected. This is the same isolation step the original fault demanded; if fault 1040 still raises on either axis, the new module is suspect and the issue is upstream (control card or wiring), not the LT.
10. Land the 1FT5 power leads. Reconnect motor 1 to module A and motor 2 to module B. Reconnect the feedback (resolver) cables.
11. Run an automatic commissioning pass on each axis via Sinumerik commissioning tools. Verify the motor ID, encoder direction, and current limit match the 1FT5 nameplate.
12. Execute a low-speed jog (e.g., 100 mm/min equivalent) on each axis. Verify direction, smoothness, and absence of fault 1040.
13. Run a step-response test at 50% rated speed. Adjust speed-controller proportional gain and integral time if overshoot is excessive or response is sluggish.
14. Run a full machine cycle (cut cycle on the saw) under no load, then under partial load, then under full load. Monitor the 611A status display throughout.

Verification Checklist

Troubleshooting Matrix

Field-Engineering Notes

Three points the original troubleshooting thread raised but did not fully resolve are worth committing to memory:

• Symptom fidelity at idle. The user reported "they were getting the same fault high current" with the motor unhooked. This is the single most useful diagnostic on the 611A: a 1040 with no motor rules out the motor, motor cable, and load-side fault, and confines the search to the control card, current sensor, or IGBT section. Always run that isolation step before pulling a module.
• Cable-swap evidence. The user also cross-swapped the motor leads between the two axes on the failed module and confirmed the fault stayed with the original axis slot. That confirms the failure is upstream of the motor terminals and rules out the power-stage wiring. Note that on the migration to two separate LT modules, this isolation test no longer applies directly — each module is its own physical entity.
• Spare-parts readiness. Two 1-axis LT modules from decommissioned equipment were already on the shelf. On SIMODRIVE 611A equipment approaching end of life, stocking one or two 1-axis LT modules as spares is cheaper than holding a repaired dual-axis module because the 1-axis variant is more flexible (any single-axis failure can be covered by cannibalizing a working module).

Lastly, the failed dual-axis module should be retained for bench-level failure analysis if budget allows. Even a faulty LT module can yield useful spares: the bus capacitors, fans, terminal blocks, and the unused axis channel (if it was not the failed one) are typically reusable. Label the module clearly as "DO NOT INSTALL — FAULTY 1040" before storing it on the spares shelf.

FAQ