Replacing 1FT6 Absolute Encoder on SIMODRIVE 611U Drive

Overview

The Siemens 1FT6086-1AF71-3EH1 is a three-phase synchronous servo motor from the 1FT6 family, factory-paired with an absolute encoder (EnDat interface, indicated by the "E" segment in the part number). When the encoder is damaged in service and must be replaced, the SIMODRIVE 611U drive cannot self-learn the new commutation offset the way a SINAMICS S120 with automatic commutation tuning can. The replacement encoder must therefore be installed at a known angular relationship to the rotor's permanent-magnet reference axis; otherwise the drive commutates the wrong phases the moment torque is enabled, the motor "runs away," and the drive trips on overcurrent.

Siemens factory service is the recommended path for absolute-encoder replacement on 1FT6 and 1FK7 motors because the procedure depends on mechanical tolerances that are not guaranteed outside a service workshop. The procedure documented below is the field-engineering workaround that has been used successfully on multiple 1FT6 and 1FK7 motors and aligns with the maintenance section of the 1FT6 installation/configuration manual (1FT6_IA.pdf, distributed historically through Siemens automation.siemens.com support) and with the SIMODRIVE 611U commissioning documentation for SIMOCOM U.

Encoder Identification and Motor Family

The 1FT6086-1AF71-3EH1 part number decodes as follows:

For 1FT6 motors the pole pair count (and therefore the mechanical-to-electrical angle conversion) is fixed by the frame. Verify the pole pair count from the motor data sheet or rating plate before any mechanical correction step. The relationship is:

θ_electrical (deg) = θ_mechanical (deg) × p

where p is the number of pole pairs. Typical 1FT608x motors are built as 8-pole machines (p = 4).

Safety Preconditions

Before opening the motor or energizing the drive, observe the following:

• Disconnect the 611U from the line filter / line supply and wait for the DC-link to discharge (the DC-link voltage indicator on the 611U front panel must read below 50 V DC, typically 5 minutes after power-off).
• Release the holding brake electrically before any test run; mechanical rotation against a seated brake will damage the brake and likely the encoder bearing.
• Mount the motor outside the machine on a rigid bench so that uncontrolled motion cannot injure personnel or damage equipment.
• Identify and lock-out any mechanical load that could be back-driven by the rotor during EMF testing.
• Confirm the motor nameplate rated voltage and current match the 611U power section being used (8 A, 15 A, 25 A, 50 A modules are typical).

Tools and Materials

Mechanical Marking Procedure

The 1FT6 motor's encoder mounting flange and shaft end face have alignment features that must be referenced before the new encoder is installed. Some 1FT6 motors are delivered from the factory with three small black or colored dots: one on the encoder shaft face, one on the encoder housing flange, and one on the motor shaft end. These dots are co-linear when the encoder is in the factory-commissioned position. Note that the dots are not present on every 1FT6 unit; treat them as a hint, not a guarantee.

1. With the motor de-energized and isolated, scribe a continuous line across the encoder housing flange, the encoder shaft flat (or key), and the motor shaft end face. The line must cross all three components at the same angular position.
2. Mark the encoder housing relative to the motor front flange with a second line, 90° offset from the first, so you can re-establish any rotational error after one full mechanical revolution.
3. Photograph the marked position before loosening any fastener. The reference mark is the only record of the factory commutation offset if the original encoder cannot be returned to its exact seat.

Encoder Mechanical Installation

1. Inspect the motor shaft end face and the encoder bore. Remove any burrs, debris, or residual thread-locking compound. The mating surfaces must be clean and dry.
2. Slide the new encoder onto the motor shaft, rotating it as required so the encoder shaft flat (or key) is aligned with the scribed reference line. There must be no axial play once the clamp screw is tightened.
3. Tighten the central encoder shaft clamp screw to the torque specified on the motor data sheet. For 1FT608x, this is typically 8 Nm. Do not exceed; the encoder shaft is hardened but the threaded interface is not.
4. Align the encoder housing flange to the scribed line and tighten the two housing screws evenly. Verify that the encoder housing cannot be rotated by hand against the motor flange.
5. Re-check the alignment lines. Any visible mismatch after tightening indicates that the encoder slipped during clamp-up; loosen, re-seat, and re-tighten.

Electrical Setup on the SIMODRIVE 611U

1. Mount the motor on the demo bench. Mechanically clamp the flange so the rotor can rotate freely but the assembly cannot translate.
2. Connect the motor power cables (U2, V2, W2) to the 611U power section according to the wiring diagram on the inside of the 611U terminal cover. Phase sequence must match the motor label; the 611U assumes a right-hand rotating field with U2-V2-W2 in that order.
3. Connect the encoder cable (EnDat) to the 611U encoder port (X411 or X412 depending on the module variant). Use the Siemens pre-assembled encoder cable, not a generic cable; EnDat requires 8-wire twisted pair with characteristic impedance 100 Ω.
4. Apply 24 V to the holding brake (terminals on the motor junction box, polarity as marked). Verify the brake releases audibly and the rotor turns freely by hand.
5. Connect the PC running SIMOCOM U to the 611U service port (X471, RS-232, 9 600 Bd default, 8-N-1).

Commissioning with SIMOCOM U

1. Power up the 611U. The drive runs a self-test and identifies the connected motor and encoder. Confirm in the SIMOCOM U online tree that the encoder type is reported as "EnDat absolute" and that the encoder position changes when the shaft is rotated by hand.
2. Open the commissioning wizard and accept the default motor parameters for the 1FT6086. Verify rated current, rated speed, and pole pair count against the motor data sheet.
3. Set the speed setpoint to a small value (for example, 50 rpm) and the acceleration ramp to a slow value (for example, 2 s) so that any commutation error produces a soft fault rather than a violent motion.
4. Enable the drive (RFG enable, pulse enable) and observe the motor. Three outcomes are possible:

Oscilloscope-Based Fine Alignment

For the first attempt, the mechanical marking procedure usually lands the encoder within one electrical pole of the correct position, which is enough for the motor to spin. For marginal cases (motor runs but exhibits cogging, torque ripple, or position drift under load), use the drive's DAC test outputs to align the encoder precisely.

1. In SIMOCOM U, navigate to Diagnostics > Test Sockets (or the equivalent in your firmware). Configure DAC1 (test socket 1) to source the signal "electrical rotor position." On 611U this is a 0 to 10 V analog representation of the absolute commutation angle, scaled so that 0 V = 0° electrical and 10 V = 360° electrical (verify the scaling in the SIMOCOM U help for your firmware).
2. Connect oscilloscope channel 1 to DAC1 output (test socket on the 611U front panel, observe ground).
3. Build a Y-network of three 1 MΩ resistors connected to motor phases U, V, W. The junction of the three resistors is the virtual neutral. This network loads the motor only with a few hundred nanoamps and does not disturb the encoder signals.
4. With the drive disabled but the DC-link charged, rotate the motor shaft slowly by hand or with a battery-operated screwdriver. The phases produce back-EMF that is accessible at the motor junction box.
5. Connect oscilloscope channel 2 to motor phase U (between the U terminal and the virtual neutral). Observe the sinusoidal back-EMF waveform.
6. Trigger the scope on the rising zero-crossing of phase U (this is the convention for the U-phase rotor flux axis on Siemens 1FT6 motors).
7. Read the DAC1 voltage at the same instant. The DAC1 voltage must correspond to electrical rotor position 0° (i.e., 0 V on the linear mapping). If it reads near 5 V, the encoder is offset by 180° electrical. If it reads near 2.5 V, the offset is 90° electrical.
8. To correct, mechanically rotate the encoder housing by the required offset. The mechanical rotation required is:

Δθ_mechanical = Δθ_electrical / p

For example, to correct a 90° electrical offset on an 8-pole (p = 4) 1FT6 motor, rotate the encoder 22.5° mechanically.

9. Loosen the encoder housing screws, rotate the housing by the calculated amount, re-tighten, and repeat the measurement. Repeat until DAC1 reads 0 V at the U-phase rising zero-crossing within ±0.1 V (which corresponds to ±3.6° electrical).

Mechanical Correction Cycle

If the first commissioning attempt fails with a "run-away" or hard fault, the encoder is grossly misaligned. Use the iterative correction procedure:

1. Disable the drive and isolate power. Wait for DC-link discharge.
2. Loosen the encoder housing screws and remove the encoder from the motor shaft.
3. Re-seat the encoder, rotating it relative to the motor shaft by a fixed mechanical step. A common starting step is 45° mechanical, which corresponds to 180° electrical on an 8-pole motor and effectively swaps the commutation phase (useful when the only symptom is reversed rotation). For fine-grained correction use 22.5° mechanical = 90° electrical on an 8-pole motor.
4. Re-tighten the shaft clamp screw to specified torque.
5. Reconnect and repeat the commissioning attempt from Commissioning with SIMOCOM U.
6. If the motor still runs away, repeat steps 1 to 5 with an additional rotation in the same direction. If the motor now runs but in the wrong direction, the previous step overshot; back off by half the rotation.
7. Typical success is achieved within 1 to 3 attempts when the original mechanical mark was preserved.

Troubleshooting Matrix

Verification and Acceptance

1. Run the motor at no load across the full speed range (from 50 rpm to rated speed) with a slow ramp. Monitor the 611U's actual-speed feedback; it must track the setpoint with less than 5 rpm error at steady state.
2. Apply a small load (a brake or a known frictional load) and run at 25%, 50%, and 75% of rated speed for 5 minutes each. Monitor drive current; it should not exceed 110% of the motor's no-load current plus the calculated load current.
3. In SIMOCOM U, open the trace recorder and capture the torque-producing current Iq and the rotor electrical position over a full mechanical revolution. The Iq waveform should be smooth; any periodic disturbance indicates residual alignment error.
4. Re-tighten all mechanical fasteners to specified torque. Re-mark the final encoder position with a paint pen so any future service has a reference.
5. Document the final encoder mechanical offset (in degrees, relative to the original mark), the firmware version of the 611U, and the SIMOCOM U parameter set. Store with the motor's maintenance record.

Field-Proven Notes and Caveats

Several practical points are worth recording for engineers who attempt this procedure:

• The mechanical marks on 1FT6 motors are not always present. Treat their absence as normal, not as a sign of a non-standard motor.
• Always test the motor on a bench, never inside the machine, during the alignment phase. A misaligned encoder will produce uncontrolled motion the moment torque is enabled.
• Do not attempt this procedure on motors with brake units without first electrically releasing the brake. Back-driving a seated brake will destroy the brake lining and often the encoder bearing.
• The 1FT6 absolute encoder carries the motor's commutation offset in the drive, not in the encoder itself. The drive must be told which motor it is connected to via SIMOCOM U; otherwise it uses a default motor that does not match the actual 1FT6086 parameters.
• If the encoder cable is the suspect component, replace the cable rather than the encoder. EnDat signal integrity is sensitive to cable capacitance and shield termination.
• For motors that have been in service for many years, the encoder bearing may be the actual failure mode. Replace the encoder, not just the code disc; Siemens does not sell the code disc separately.
• Use the Siemens service partner portal (https://www.automation.siemens.com/partner) to locate the nearest authorized service center if the in-field procedure does not converge within 8 attempts.

For a broader reference on encoder installation practices (shaft alignment, coupling selection, grounding), the IFM installation guidelines provide a useful general-purpose overview that complements the Siemens-specific procedure:

• IFM Encoders: Installation Guidelines

FAQ