Realigning Siemens 1FT6105 Incremental Encoder After Bearing

Problem Definition: Post-Service Hunting on 1FT6 Motors

After a bearing replacement on a Siemens 1FT6 series synchronous servo motor (in this case a 1FT6105-1AC71-1AG1 integrated into a SINUMERIK 840D powerline CNC), the rotor and the mounted incremental encoder can lose their original angular relationship. When the drive is re-enabled, the SIMODRIVE 611 (or third-party drive, here a Pro-motion S18) reads an incorrect rotor position from the encoder and commands torque at the wrong electrical phase angle.

Typical field symptoms:

• Motor accelerates to high RPM immediately after the drive enable, even with zero commanded setpoint (open-loop runaway)
• High-frequency oscillation ("hunting") at standstill with audible chatter
• Excessive current even with no motion demand (visible on the drive's Smooth Current Actual Value screen)
• SINUMERIK alarms: 25050 Contour monitoring, 25060 Speed setpoint limitation, 300608 Axis %1 drive fault, or 300504 measuring-circuit error depending on the firmware build
• Follow-up error (025201) on the NCK axis

The root cause is the loss of the mechanical relationship between the encoder's commutation reference (the C/D tracks of an incremental sin/cos encoder) and the rotor magnet ring. The encoder cannot independently determine the absolute rotor position on power-up; it relies on the C/D reference once per mechanical revolution being aligned to the back-EMF zero crossings of the motor windings.

Why Encoder-to-Rotor Alignment Matters on a Synchronous Servo

An incremental rotary encoder produces three functionally distinct signal groups:

• A and B (incremental tracks): 2048 sine periods per revolution (S/R) on ERN 1381/1387, quadrature-decoded by the drive to derive incremental position
• R (index/reference mark): one pulse per revolution used only for homing
• C and D (commutation reference tracks): one sine period per revolution, used after power-on to establish the absolute rotor position within one electrical revolution

On a permanent-magnet synchronous motor, the back-EMF of phase U crosses zero at a specific mechanical angle that is fixed by the magnet ring geometry. The drive must apply current to phase U at this zero-crossing angle; applying current at the wrong angle produces negative d-axis current (demagnetising) or pure q-axis current (torque) at the wrong phase, causing rotation or oscillation.

The C and D tracks therefore must be mounted on the encoder shaft so that their zero crossings coincide with the U-phase (and V, W) back-EMF zero crossings within the manufacturer's specified electrical tolerance:

When a TAMAGAWA incremental encoder (as observed in this field report) replaces the original Siemens encoder on a 1FT6105, the alignment principle is identical: the encoder's commutation reference (typically track Z or the C/D pair on TAMAGAWA TS / OH series) must be phased to the motor's U-phase back-EMF zero crossing.

1FT6105 Motor and Encoder Identification

The Siemens 1FT6105 is a frame-size 100, length-5 synchronous servo motor in the 1FT6 family. Typical nameplate data:

• Standstill torque: ~21 Nm (frame 100/Length 5, natural cooling)
• Rated speed: 2000 - 3000 RPM depending on winding variant
• Rated current: ~17 A (variant dependent)
• Pole count: 8-pole (4 pole pairs) for frame-size 100 1FT610x variants
• Protection: IP65
• Insulation class: F

With 4 pole pairs, the mechanical angle between successive U-phase zero crossings of the back-EMF is 360°/4 = 90° mechanical, and the angular spacing between U, V, W is 30° mechanical (120° electrical).

The encoder designation 1AG1 in the Siemens 1FT6 order code identifies the resolver/encoder family and connection system. The service report on this motor specifies a TAMAGAWA incremental encoder (commonly TAMAGAWA TS2640N141 or OHE-25-1024 family), which provides incremental A/B quadrature plus a single commutation Z pulse or C/D tracks.

Required Tools and Setup

Pre-Alignment Safety Checks

1. Lock and tag-out the SINUMERIK 840D main power at the cabinet disconnect. Open the cabinet disconnect (Q1), confirm DC bus discharge indicator is at zero volts on the SIMODRIVE 611 LT module, and verify absence of voltage with a CAT IV multimeter on phases L1/L2/L3 and the DC-link test points (P600, N600).
2. Mechanically decouple the 1FT6105 from the spindle or ballscrew it drives. The motor shaft must rotate freely without connected load. If the motor is back-mounted through a gearbox, decouple the gearbox input coupling to prevent back-driving the load.
3. Verify encoder cable continuity from the motor junction box to the SIMODRIVE 611 regulator card (6SN1118-xxx) or Pro-motion S18 feedback port. Use the encoder test parameter on the drive's Service screen (for SIMODRIVE 611: p1700 = 5 commissioning mode; for SINUMERIK 840D: MD38000 $MA_SAFE_ENC_SSI_FORMAT is not applicable for incremental; instead verify via the drive commissioning software SIMODRIVE COM).
4. Confirm the encoder is recognised by the drive before any mechanical work. Incremental encoders on 1FT6 motors must register a valid signal without alarms. If the drive reports 300504 or similar at this stage, fix the cable first.
5. Ensure the work area is ESD-safe. Encoder head electronics are ESD-sensitive.

Star-Resistor Wye Test Method (Field-Proven Procedure)

The technique described in the field service report and in the Siemens SIMODRIVE 611 service manual uses the motor windings themselves as a generator. With the encoder still connected to the drive and the LT power module disconnected from the bus, the motor is rotated externally by a portable drill. Each phase winding produces back-EMF (BEMF) proportional to speed. A three-resistor star provides a neutral point; the scope measures BEMF phase-to-neutral and compares it to the encoder's C/D reference signal.

Wiring the star resistor

1. Solder the three 1000 Ω resistors together at one end, forming a star (Y) point. This is your measurement neutral.
2. Solder an insulated test lead to each of the three free resistor ends. Label them U, V, W.
3. Connect the U, V, W leads to the corresponding pins on the motor power connector (the connector that normally plugs into the SIMODRIVE 611 LT module output, or the Pro-motion S18 output). On Siemens power connectors (size 1.5 / M40), U, V, W are the three large power pins; on TAMAGAWA-equipped retrofits, follow the encoder/motor wiring diagram supplied with the motor.
4. The motor connector must remain disconnected from the drive's power output, so the drive is electrically isolated from the motor windings while the encoder cable is still live.

Wiring the oscilloscope

1. Channel A probe tip → U-phase motor power pin. Ground clip → star-point neutral.
2. Channel B probe tip → encoder reference signal. For an ERN 1387 this is the C or D track; for a TAMAGAWA incremental encoder this is typically the Z (index) track or the C/D pair. The probe ground must share the encoder cable shield ground, ideally at the drive end of the cable. Confirm the correct reference signal pin against the encoder datasheet.
3. Set both channels to AC coupling, 200 mV/div initially, timebase 5 ms/div, sweep triggered on Channel A. As the motor rotates, BEMF amplitude rises with speed; increase voltage scale as needed.

Step-by-Step Encoder-to-Rotor Alignment

1. Loosen the encoder mounting screws. On 1FT6 motors, the encoder is mounted via three M4 cap screws on the rear bell housing. Back them off so the encoder body can be rotated by hand against the friction of the encoder centering ring.
2. Spin the motor shaft with the portable drill at approximately 50-100 RPM. The encoder cable is live, the LT module is disconnected, so the drive is reading valid A/B/C/D signals without driving the motor.
3. Identify the U-phase BEMF waveform on Channel A. At constant drill speed this is a clean sine wave (or near-sinusoidal trapezoidal) at a frequency of (RPM × pole_pairs) / 60. For a 1FT6105 with 4 pole pairs at 60 RPM, expect ~4 Hz on the BEMF.
4. Identify the reference pulse on Channel B. The C/D pair or Z reference will appear as a sine pulse (C/D) or short index pulse (Z), one period per mechanical revolution of the motor shaft.
5. Rotate the encoder body in small increments while observing both traces. The C/D (or Z) zero crossing should track relative to the U-phase BEMF zero crossing as you rotate the encoder.
6. Adjust until the zero crossings coincide within the specified electrical tolerance. On Siemens 1FT6 with ERN 1387 the documented tolerance is ±2° mechanical (±8° electrical for 4 pole pairs). On TAMAGAWA-equipped motors the tolerance is typically ±3-5° mechanical; consult the encoder datasheet.
7. Tighten the encoder mounting screws to the specified torque (typically 4-5 Nm on 1FT6) while holding the encoder at the alignment point. Re-check the alignment after tightening; the friction of the centering ring can pull the encoder slightly.

Expected waveform (Fig 110.1 reference)

The reference image referenced in the SIMODRIVE 611 service manual (historically hosted at the Siemens DOCON WEB portal under item 13245, catalog sinumerik_simodrive_04_2010_e) shows the U-phase BEMF as a clean sine wave with the C/D zero crossing superimposed at the BEMF zero crossing. Visual reference at Siemens DOCON WEB — Sinumerik Simodrive 04/2010, item 13245.

Verification and Re-Commissioning on SINUMERIK 840D

1. Reconnect the motor power connector to the SIMODRIVE 611 LT module output (or Pro-motion S18 U/V/W output).
2. Close the cabinet and remove the lock-out. Restore SINUMERIK 840D to commissioning mode.
3. Run the drive's automatic commissioning routine. On SIMODRIVE 611 with 840D powerline: NCK MD30130 $MA_CTRLOUT_TYPE = 1; on the drive: enter p1700 = 5 to enable the commutation alignment test. The drive commands a small controlled motion to verify the new commutation angle is within tolerance.
4. Observe the axis Service screen: smooth current actual value should drop to near zero within 200 ms after enable. Any oscillation > 0.5 A peak-to-peak indicates remaining misalignment.
5. Run a controlled axis move at low velocity (e.g., 1000 mm/min) over 10 mm of travel. Monitor following error in the Service axis display. A correctly aligned axis shows < 0.5 mm following error with default feedforward.
6. Re-verify the encoder parameter block on the drive: p1011 (encoder type, should match the nameplate), p1013 (encoder resolution, 2048 for ERN 1387 / TAMAGAWA 2048-line), p1021 (commutation angle offset, recorded before the bearing work).

Drive-Specific Notes: SIMODRIVE 611 vs Pro-motion S18

The same physical alignment procedure applies to both drives because the alignment is between the motor and the encoder, not the encoder and the drive. Differences appear at the verification stage:

Troubleshooting Matrix

Preventive Practices and Field Caveats

• Mark the encoder position before removal. Before loosening the encoder screws, scribe a line across the encoder flange and the motor bell housing with a fine marker or scriber. This gives a coarse re-assembly reference even before electrical alignment.
• Record the commutation offset parameter (p1017 on SIMODRIVE 611) before any encoder or bearing service. Even if the mechanical alignment is restored exactly, having the original drive parameter lets you confirm the encoder was not disturbed.
• Use new encoder centering rings if the original ring shows wear. The polymer ring controls friction during alignment and can cause drift after tightening if worn.
• Do not rely on the encoder's R (index) pulse for commutation. R is for homing, not commutation. The C/D tracks (or TAMAGAWA Z pulse) are the commutation reference.
• After re-assembly, always run the drive's automatic commutation calibration before putting the axis into production. On SIMODRIVE 611 / 840D this is the p1700 test mode; on the Pro-motion S18 this is a drive-side command in the configuration software.
• Encoder-to-rotor angle changes with temperature. For high-precision spindles (rotating-tool indexing), allow a 15-minute warm-up before final verification.

Refer to the Siemens 1FT6 configuration manual and SIMODRIVE 611 service documentation for your firmware version. Current Siemens documentation is available on the Siemens Industry Online Support portal at support.industry.siemens.com. The historical DOCON WEB reference sinumerik_simodrive_04_2010_e, item 13245 covers the SIMODRIVE 611 alignment waveform. Incremental encoder fundamentals are summarised at Wikipedia: Incremental encoder.