Sinumerik 810D 1FT6 Motor Restore After PLC Archive Recovery

Problem Summary: C Axis Alarms After PLC Archive Restore

An ultrasonic machining centre controlled by a Sinumerik 810D developed a persistent C axis alarm set immediately after the maintenance team restored a PLC archive. The C axis drive motor had been replaced in 2007 with a Siemens 1FT6034-1AK71-3AG1 permanent-magnet synchronous servo motor and the Wintec field engineers had entered the new motor parameters into the controller at that time. After the recent PLC restore, the alarm list shows drive-side faults that point back at motor identification, current limits, encoder alignment, and torque monitoring.

This technical reference walks through the engineering analysis of why a Sinumerik 810D can lose drive-relevant parameters during what looks like a "PLC restore", how to recognise the failure mode from the alarm pattern, and the exact recovery workflow to bring the C axis back to production quality. The same procedure applies to any 1FT6 motor retrofitted into a 611D-controlled 810D machine, including the 1FT6044, 1FT6052, 1FT6062, and 1FT6081 families.

Affected Hardware and Software

The components involved in the incident, as found on the machine and read from the original alarm list:

The motor data plate lists two operating points, both of which must exist in the controller's drive machine data:

• Standstill: M0 = 2.0 Nm / I0 = 2.6 A / nmax = 9700 rpm
• Rated (S1): Mn = 1.4 Nm / In = 2.1 A / nn = 6000 rpm

Root Cause: How a "PLC Restore" Affects Drive Data

On the Sinumerik 810D, persistent storage is partitioned into three logical areas that survive mains cycles independently:

1. NC data (NCMMA / NCK): axis machine data, channel MD, tool tables, work offsets, R parameters, compensation tables, drive configuration block.
2. PLC data (PLCMMA): ladder / FBD program, PLC machine data, interface DBs to the NC.
3. Drive data (611D FEPROM): motor code, current controller parameters, speed controller, torque limits, encoder configuration, pole pair number, motor EMF constant.

What a user calls a "PLC restore" can mean three different operations depending on which backup file was selected from the HMI or Series Start-up Tool:

Because the C axis motor swap happened in 2007, an archive written before 2007 will not contain the 1FT6034-1AK71 parameters. The drive will revert to the originally commissioned motor (typically 1FT5 with a different pole pair count) or to default 0 (unconfigured). This produces the alarm pattern the user is observing.

Siemens 1FT6034-1AK71-3AG1 Motor Data Plate Decoded

For Sinumerik commissioning, the motor is selected by its Siemens motor code. The motor code encapsulates the data plate values plus internal motor constants (Kt, Ke, resistance, inductance, inertia, pole pair number) so that the drive controller can stabilise the current loop on first enable. The 1FT6 catalogue number decodes as follows:

Data plate parameters and their role in the 611D drive machine data set:

Approximate motor constants derived from the plate (used only if a motor code is unavailable):

• Torque constant Kt = M0 / I0 = 2.0 / 2.6 ≈ 0.77 Nm/A
• EMF constant Ke (line-to-line) = UIN / (nmax / 1000) = 294 / 9.7 ≈ 30.3 V per 1000 rpm
• Phase resistance Rph ≈ R_U-V × 0.5 (delta to wye) ≈ 1.5 Ω typical for frame 63
• Rotor inertia Jr ≈ 3.0 × 10⁻⁴ kg·m² (1FT6 63-frame catalogue)

Sinumerik 810D and SIMODRIVE 611D Architecture

The 810D integrates a CCU (Compact Control Unit) that contains the NCK, the PLC runtime, and a digital drive bus interface. The 611D power modules connect to the CCU via a parallel drive bus running over ribbon cable. The C axis motor is driven by a 611D 1-axis or 2-axis power module, with the encoder feedback coming through the 611D ER (encoder) module mounted on the same chassis.

Signal paths on the C axis:

1. CCU → drive bus (X141 ribbon) → 611D power module (X411) - control words, torque setpoint, speed setpoint
2. 1FT6034 motor → motor power cable → 611D power module U/V/W terminal - phase currents
3. 1FT6034 encoder → encoder cable → 611D ER module (X311 or X312) - 1 Vpp sin/cos + commutation
4. ER module → power module - position, speed, commutation angle
5. Motor PTC / brake → 611D power module terminal strip - thermal protection

The PLC restore only reloads the user program. The drive machine data, axis MD, and 611D FEPROM persist independently. If a full archive with the "with drives" flag was reloaded, the FEPROM is overwritten and the 611D boots with the older configuration.

SIMODRIVE 611D Module Identification and LED Diagnostics

Before opening the cabinet, the LED state on each 611D module pinpoints the failure area:

Document the LED state of the C axis power module and the ER module before continuing. A red LED on the ER module almost always points at the encoder cable, not the motor data, and saves hours of misguided FEPROM work.

Drive Bus Ribbon Cable and Connector Reference

The CCU 810D and the 611D chassis are linked by a 34-pin shielded ribbon cable. The standard assignment on the 810D is:

Mechanical handling rules for these cables:

• Fold radius minimum 50 mm - any tighter fold breaks the inner conductors
• Lock both ejector levers evenly - asymmetric insertion bends pin rows
• Never plug/unplug with the drive bus powered - the line drivers latch up
• Replace rather than re-solder any cable that shows kinking, oxidised contact, or pushed-out pin

Pre-Diagnostic Safety and Setup

Working inside a live Sinumerik 810D cabinet requires a documented lockout/tagout (LOTO) sequence. Before opening the cabinet or connecting a meter:

1. Open the main disconnect and lock it with a personal padlock.
2. Wait 5 minutes for the 611D bleeder resistor to discharge the DC bus. The bleeder is internal to the LT module.
3. Verify zero voltage at the DC bus test points (labelled +DC / -DC on the LT module) using a CAT III 600 V DMM. Measure phase-to-phase and phase-to-PE.
4. Isolate the 24 V PLC supply if you are disconnecting PLC field devices.
5. Fit an ESD wrist strap on the cabinet and ground it before touching any ribbon cable or module connector.
6. Photograph the connector layout before any disconnection - ribbon cable pin orientation mistakes are the most common after a PLC restore.

Diagnostic Procedure: Step-by-Step

Use the following sequence to localise the failure. Each step is independent of the next so you can stop as soon as the alarm is resolved.

Step 1 - Read and Classify the Alarm List

From the HMI, navigate to Alarm > Alarm Log and capture the current and historical alarms with timestamps. Classify each alarm by its number range:

Step 2 - Verify CCU ↔ 611D Communication

On the 810D HMI open Diagnostics > Drive > Drive Status. Each configured axis must show:

• Drive ready (RDY LED green on 611D module)
• Encoder OK (ER module LED green)
• No communication fault on the drive bus
• Drive bus cycle time < 1 ms typical

If the drive status screen reports "Nock" (no link) for the C axis, the ribbon cable is the prime suspect. Re-seat before going further.

Step 3 - Check the Ribbon Cable and Connectors

1. Power down completely and discharge the bus (see Pre-Diagnostic Safety).
2. Remove the ribbon cable between CCU 810D X141 and the 611D power module X411.
3. Inspect each conductor under a magnifier for kinks, oxidation, or pushed-out pins. The 34-pin ribbon is sensitive to torsion.
4. Re-seat firmly and lock both ejector levers evenly.
5. Power up and re-check the drive status screen.

Step 4 - Measure the 1FT6034 Motor Winding Resistance

With the motor disconnected from the 611D power section, measure the U-V, V-W, and W-U resistance at the motor terminal box or cable end (after isolating the shield):

Allow 5% tolerance between phases. A delta of more than 10% between phases points to a developing turn-to-turn fault even if the absolute resistance looks normal. For temperature compensation use R_corrected = R_measured × (235 + T_ref) / (235 + T_measured) with T in °C (copper constant).

Step 5 - Check the Encoder Cable

1. Disconnect the encoder cable at the 611D ER module (X311/X312) and at the motor encoder connector.
2. Inspect pins for bent contacts or pushed-out sockets. The 12-pin or 17-pin connector on 1FT6 motors uses spring-cage contacts that can retract over time.
3. Verify shield continuity from motor housing to ER module case (low ohms).
4. Reconnect and torque the connector locking ring to the manufacturer spec (typically 0.6 Nm).

Step 6 - Verify the 611D Power Module IGBTs

1. With the motor disconnected, measure diode forward drop between DC+ and U, V, W and between DC- and U, V, W using a DMM on diode mode.
2. All six readings should be 0.3-0.7 V in one direction and OL in the other.
3. Any shorted reading (near 0 V in both directions) means the IGBT module must be replaced before the drive is re-enabled.

Step 7 - Read the Actual Drive Machine Data

On the HMI, navigate to Start-up > Drive > Drive MD. Inspect the motor-related fields. The exact MD numbers depend on the drive firmware version (e.g. SW 4.x vs 5.x for 611D); the parameter set is the same:

• Motor type → should be PM synchronous (1)
• Motor code → should match 1FT6034-1AK71
• Rated current → 2.1 A
• Standstill current → 2.6 A
• Rated speed → 6000 rpm
• Max speed → 9700 rpm
• Pole pair number → 3

If any field shows the default (0) or an old 1FT5 value, the 611D FEPROM was overwritten with archive data. Refer to the official Sinumerik 810D Commissioning Manual for the exact MD layout of your firmware version.

Drive Alarm Code Reference

The most common drive alarms seen on a 1FT6034 with corrupt or default motor data:

Restoring Motor Parameters on the 810D

If the drive MD shows the wrong motor code, you must re-enter the parameters. Use Method 1 in preference; fall back to Method 2 only if the motor code is not in the 810D firmware library.

Method 1 - Enter the Siemens Motor Code

1. Open Start-up > Drive > Motor Selection on the HMI.
2. From the motor list, choose 1FT6 family, frame size 63, length 4, winding 1AK71.
3. Confirm encoder option 1C 2048 S/RB25.
4. Press Calculate to load the auto-derived motor constants (Kt, Ke, R, L, J).
5. Press Save and follow the prompt to save to FEPROM. A FEPROM save is mandatory - a RAM-only save is lost on the next power cycle.

Method 2 - Manual Parameter Entry

For older 810D firmware versions where the 1FT6 motor code library is incomplete, enter the values from the data plate directly. The MD numbers depend on drive firmware; the values do not:

Motor Constant Calculations

Two derived quantities must be calculated and entered alongside the data plate values:

Torque constant Kt (Nm/A):

Kt = M0 / I0 = 2.0 / 2.6 ≈ 0.77 Nm/A

The 611D uses Kt to convert the measured current Iq into a torque estimate for monitoring and the i²t thermal model. A 10% error is acceptable; a 30% error is not.

EMF constant Ke (V per 1000 rpm, line-to-line RMS):

Ke = UIN / (nmax / 1000) = 294 / 9.7 ≈ 30.3 V / 1000 rpm

The 611D uses Ke to compute the field-weakening current Id above base speed and the overspeed protection threshold. An overestimated Ke causes the drive to enter field-weakening too early and reduces available torque; an underestimated Ke lets the motor EMF exceed the DC link voltage and trigger an overvoltage fault.

Phase resistance Rph:

Rph = R_line-to-line × 0.5 (if delta), or R_line-to-line × 0.5 (if wye); here the 1FT6034 is wye-wound, so Rph = R_L-L / 2

The 611D measures the actual Rph during the current controller commissioning step and writes the measured value back to the drive MD, overriding the manual entry. Always run the commissioning step after a manual entry.

Commissioning the C Axis

Once the motor data is restored and saved to FEPROM:

1. Power down and back up to re-initialise the 611D with the new FEPROM contents.
2. On the HMI open Start-up > Axis > Commissioning for the C axis.
3. Run Current controller measurement first. The drive pulses the motor with low current (10-20% of I0) and measures the actual current loop response. The measured resistance and inductance overwrite the manual entries.
4. Run Speed controller optimisation. The drive performs a defined acceleration ramp and tunes Kp/Tn automatically. Listen for oscillation or growling noise - if present, abort and re-check encoder direction (swap A/A- or B/B-).
5. Run Position controller optimisation (KV factor). Accept the default until first part proves out.
6. Set the torque limit to M0 (2.0 Nm) for normal operation. Set the rapid traverse speed limit to nn (6000 rpm) for S1 work; nmax (9700 rpm) is only permitted with field-weakening validated.

Check the alarm log between each step. Any drive alarm must be cleared before the next step - the 611D will refuse to continue the optimisation with an active fault.

Verification and Acceptance Test

Use the following tests to prove the C axis is back to production quality before signing off:

1. Jog the C axis in both directions from the HMI. Listen and feel for smoothness. Any detent, cogging, or noise suggests encoder contamination or magnet damage.
2. Program a 360° rotation at rapid traverse (G0) and verify the actual position matches the commanded position in the axis-specific MD read-out. Tolerance: 1 increment.
3. Apply a 50% rated load test. Monitor motor current in the drive status screen. It should remain below 1.5 A continuous.
4. Run the part program with ultrasonic engaged. The C axis load is highest during the ultrasonic-on segment. The i²t utilisation should stay below 80%.
5. Capture the alarm log after one production shift. No 300xxx drive alarms should be present. A single 25010 at start-up is acceptable; persistent 300xxx alarms indicate the FEPROM save failed.

If the alarm persists after motor code restore and the ribbon, winding, and IGBT checks are all clean, the original alarm list attached by the user must be reviewed line by line. Specific codes (e.g. 300504 "Motor data set invalid") point at FEPROM corruption and require a 611D firmware reload, not just a parameter re-entry.

Alternative Causes and Field Pitfalls

Other failure modes that mimic motor data loss after a PLC restore:

• PLC interface DB mismatch: the PLC-to-NC handshake bits (DB3x.DBX6.0 drive enable, DB3x.DBX6.1 pulse enable, DB3x.DBX6.7 controller enable) can be reset by a PLC restore if the DB was hand-edited before archiving. The motor data is fine but the drive never gets enabled.
• Encoder cable shield broken at ER module: ground loops on the encoder shield produce intermittent 300700 "speed controller monitoring" alarms that look like motor data issues but are not.
• Brake engaged during commissioning: if the motor holding brake wiring is on the same cable as the PTC and the brake leads are crossed with the PTC, the brake stays engaged. The drive sees a heavily loaded axis and trips an i²t alarm.
• Drive bus cycle time doubled: a PLC restore that includes a modified PROFIBUS configuration can slow the drive bus to 2-4 ms, which trips the 611D speed controller monitoring. Check the bus diagnostics screen.
• Pole pair number wrong: an archive from a 1FT5 motor (p = 2) restored onto a 1FT6 motor (p = 3) produces immediate commutation alarms. Always verify p against the 1FT6 catalogue.

Migration Path: 611D to SINAMICS S120

Many 810D users consider migrating the 611D power section to a SINAMICS S120 chassis when the 611D becomes obsolete. The 1FT6 motor can be reused, but the motor code changes:

Use the Siemens Drive Technology Configurator to validate the S120 part numbers before quoting a migration. The 1FT6034-1AK71 can be re-driven by an S120 1-axis module in the 3A or 5A frame; the 1FT6 motor code is included in the S120 motor list under the same catalogue number.

Preventive Recommendations

To avoid a recurrence of the motor data loss after a PLC restore:

1. Always create a separate drives.arc backup in addition to the full nc_plc.arc. The drives archive contains the 611D FEPROM contents and can be restored without touching the NC or PLC.
2. Record the motor code, encoder code, and motor data plate values in the machine's electrical documentation. The 1FT6 factory test report belongs with the machine, not in the engineer's desk.
3. After every archive restore, compare the drive MD list (motor code, pole pair, current) against the documentation before commissioning. A 5-minute verification saves hours of fault chasing.
4. Add a 6-monthly preventive maintenance task: cycle the C axis through full travel with the drive status screen open. Any i²t or speed controller alarm at low load is an early indicator of motor or encoder degradation.
5. Document the 1FT6 1AK71 motor swap in the machine's history book with date (2007), vendor (Wintec), and motor serial (YFC 706953101001). This is the single fastest way to resolve the next PLC restore.

Frequently Asked Questions