Resolving SINAMICS G120 CU250S PN F31850 Resolver Encoder Fault
1. Problem Overview
When commissioning a Siemens SINAMICS G120 drive equipped with a CU250S PN control unit and a resolver-based motor feedback system, fault F31850 may appear immediately after activating an encoder configuration such as Encoder 1 Resolver 1001. In the field case that motivates this article, the operator reported:- Drive family: SINAMICS G120
- Control Unit: CU250S PN
- Commissioning tool: STARTER V5.3
- Control Unit firmware: V4.7.6
- Motor feedback: resolver on encoder interface 1
- Encoder configuration selected: Encoder 1, Resolver, code 1001
- Resulting fault: F31850
Field observation: The fault is typically reported as F31850, but the underlying chain often starts with F31150 carrying a meaningful fault value. Both faults must be cleared and their root causes addressed, otherwise F31850 will reappear after the next ON command.
2. Affected System Identification
Before applying any fix, verify the exact hardware and firmware in use. The troubleshooting path is firmware-sensitive, especially around how F31150 fault values are encoded.| Item | Required identification |
|---|---|
| Drive family | SINAMICS G120 (firmware V4.7 SP6 family) |
| Control Unit | CU250S PN (6SL3246-0BA22-1FA0 or successor 6SL3246-0BA22-1PA0) |
| Power Module | PM240-2 / PM250 / PM260 (matched to motor rating) |
| Encoder interface | Sub-D socket X9 (onboard) and/or SMC10/SME20/SME25 module |
| Encoder type | Resolver, 2-pole, 7 Vrms excitation typical (e.g. 1XP8001, 1PH8 resolver tail) |
| Commissioning tool | STARTER V5.3 or SINAMICS StartDrive V15 / V16 (TIA Portal) |
| Firmware | CU250S PN V4.7.6 (or V4.7 SPx in the same family) |
| Motor technology | Induction (asynchronous) or permanent-magnet (PMSM) |
3. Fault F31850 — Encoder A/D Converter Communication Fault
F31850 belongs to the encoder / sensor module fault group in the SINAMICS fault system. In the G120 fault list, F31850 indicates a fault inside the encoder evaluation channel, specifically an internal A/D converter communication problem on the Sensor Module. The fault value that accompanies F31850 is the principal diagnostic indicator: each bit pattern reports which sub-function inside the encoder ASIC has failed.3.1 Typical F31850 fault values and meanings
| Fault value (hex) | Meaning | Likely cause |
|---|---|---|
| 0x0001 | Resolver excitation error | Open/short on resolver primary, broken lead |
| 0x0002 | Resolver SIN/COS amplitude error | Resolver damaged, cable too long, shield broken |
| 0x0004 | Resolver SIN/COS out of tolerance | Resolver defective, mis-wiring (sin ↔ cos swap) |
| 0x0010 | Resolver tracking error | Mechanical, dynamic overspeed, encoder mis-config |
| 0x0020 | Resolver absolute position error | Resolver not aligned, multiple pole pairs mis-set |
| 0x0040 | Resolver wire break detected | Open circuit in resolver cable |
| 0x12001 / 0x10001 | A/D converter communication error | Sensor Module ↔ ASIC communication corrupted; encoder card suspect |
| 0x80000 | Combined A/D converter / SPI fault | Encoder card hardware defect or severe EMC issue |
The exact bit assignments depend on the encoder module firmware version. Always cross-check the active fault value against the Siemens SINAMICS G120 List Manual for the specific firmware in use. Do not interpret a fault value without the manual in front of you.
3.2 Why F31850 is often misdiagnosed
The common mistake is to clear F31850 and immediately attempt another run. The fault will reappear because the A/D converter fault is generated by internal data integrity checks between the Sensor Module and its ASIC. The trigger can be:- A hardware defect on the Sensor Module (SMC10, SME20, SME25) or the onboard resolver interface of the CU250S PN
- A cable or connector problem (cold joint, broken shield drain, wrong pin crimp)
- An EMC disturbance that corrupts resolver signals at high dI/dt switching
- A wrong resolver interface parameterization (e.g. wrong transformation ratio or excitation voltage)
4. Fault F31150 — Encoder Interface Fault with Value 0x80000
In the field case, F31850 was preceded by F31150 with fault value0x80000 (or decimal 524288; the value is displayed by STARTER in hexadecimal). The expert in the case indicated that this value reflects a commutation-with-zero-flag condition. The F31150 fault value is the most useful single piece of information when triaging this class of problems, because it points at the function block in the encoder evaluation chain that is misbehaving.
4.1 Decoding F31150
F31150 belongs to the sensor module 1 fault group. Its fault value is a bit-mask identifying the precise sub-fault:| F31150 value (hex) | Bit | Meaning |
|---|---|---|
| 0x0001 | 0 | Encoder power supply fault |
| 0x0002 | 1 | Encoder overcurrent / short |
| 0x0004 | 2 | Resolver amplitude error |
| 0x0008 | 3 | Resolver SIN/COS quadrature fault |
| 0x0010 | 4 | Resolver absolute position lost |
| 0x0020 | 5 | Zero-mark position error |
| 0x0040 | 6 | Resolver wire break |
| 0x80000 | 19 | Commutation with zero-mark active / encoder flag combination |
5. Why a G120 Cannot Run a PMSM in Closed-Loop Speed Control
This is the single most important root-cause check in the case. The SINAMICS G120 firmware maps control modes as follows:| Motor type | V/f open loop | Sensorless closed loop | Encoder closed loop (G120) | Encoder closed loop (S120) |
|---|---|---|---|---|
| Induction (asynchronous) | Supported | Supported | Supported | Supported |
| Reluctance (synchronous reluctance) | Supported | Supported (limited) | Supported | Supported |
| Permanent magnet (PMSM) | Supported (limited) | Limited (small motors only) | Not supported on G120 | Supported |
Action: Before swapping cards or recabling, confirm the motor technology. Read the motor nameplate: if it lists a permanent-magnet rotor, an EMF constant, or a back-EMF line-to-line value at 1000 rpm, you almost certainly have a PMSM. Move the application to a SINAMICS S120 or SINAMICS V90 (for simpler servo applications), or change to an induction motor with a resolver.
6. Step-by-Step Diagnostic Procedure
Execute the steps in order. Do not skip ahead. The sequence is designed to separate firmware/configuration issues from hardware issues.- Capture both fault codes and values. In STARTER, navigate to Drive → Diagnostics → Fault/alarm buffer. Record F31150 value, F31850 value, drive firmware, and motor type used in the configuration.
- Verify motor technology. If PMSM is being used, the configuration must change before further diagnostics (see Section 5).
- Select sensorless control temporarily. Configure the drive as induction motor, sensorless vector control (SLVC), p1300 = 20, no encoder. Run the motor and confirm it accelerates cleanly. This proves the power section is healthy.
- Check the encoder parameter set. Open Configuration → Encoder in STARTER. Confirm p400, p401, p403, p404 (encoder interface selection) match the physical Sensor Module. For CU250S PN onboard resolver interface, p400 = 1 (encoder 1), p404 = 1001 (resolver type).
- Inspect the encoder wiring. With the drive de-energized and locked out, check the resolver cable pin-out against the wiring diagram in the CU250S PN manual. Pay special attention to REF+, REF− (excitation), SIN+, SIN−, COS+, COS−.
- Measure resolver signals at the drive end. With the drive de-energized, measure the resistance of the resolver windings from the drive end. Typical values: 20–120 Ω on the primary (REF), 50–200 Ω on each of SIN and COS secondaries. Open or shorted windings indicate a damaged resolver or cable.
- Test the resolver on a known-good drive. Connect the same resolver to a second CU250S PN or a working S120. If the fault does not reproduce, the resolver and cable are exonerated.
- Test the Sensor Module on a known-good resolver. If the resolver passes Step 7, mount a known-good resolver on the suspect Sensor Module. If F31850 reappears, the Sensor Module / CU250S PN encoder interface is defective.
- Replace the suspect card. Order a replacement SMC10, SME20, or SME25 (depending on the resolver type) or replace the CU250S PN control unit if the onboard resolver interface is integrated and damaged.
- Recommission. After the card swap, repeat the full STARTER offline/online configuration with motor identification (p1910 / p1960) and encoder identification (p1990).
7. Hardware Verification Procedures
7.1 Cable integrity
Use a megohmmeter to test insulation between each resolver lead and ground. The reading should exceed 100 MΩ at 500 V. Lower values indicate moisture ingress or insulation damage. Inspect the connector backshells: the resolver cable shield must be terminated 360° at both ends. A pigtail shield termination is a common cause of EMC-driven F31850 in cabinet installations.7.2 Resolver winding direction
The forum expert in the case specifically called out resolver winding direction. Resolvers are wound such that SIN leads COS by 90° electrical at positive rotation. If the SIN and COS leads are reversed:- The drive will report inconsistent position values
- F31150 with a position-related fault value can appear
- F31850 can appear if the A/D converter detects impossible quadrature
7.3 Sensor Module swap
If the CU250S PN is used with an external Sensor Module (SMC10 on the option module slot), swapping the Sensor Module is a ten-minute diagnostic step. Order a spare SMC10 (6SL3055-0AA00-5AA3) or SME20 (6SL3055-0AA00-5BA3) for the next on-site visit. With the spare installed, repeat commissioning. If F31850 disappears, the original Sensor Module is defective.7.4 Onboard resolver interface check
If the resolver is connected directly to the CU250S PN Sub-D connector X9 (no external Sensor Module), the encoder interface is on the control unit itself. There is no field-replaceable sub-assembly; the entire control unit must be replaced. Replacements are hot-swappable if the drive is offline and the Control Unit is keyed correctly.8. Commissioning Procedure for Induction Motors with Resolver
Once the hardware has been cleared, perform the commissioning sequence below for an induction motor with resolver on a G120 CU250S PN. This is the same sequence that the field expert recommended as a "step by step" approach.- Restore factory settings: p0010 = 30, p0970 = 1. Wait for the drive to complete the reset.
- Enter commissioning mode: p0010 = 1.
- Enter motor nameplate data into p0300–p0340 (motor code, voltage, current, power, power factor, frequency, speed).
- Configure motor control: p1300 = 21 (sensorless vector control with speed controller) for first-run, then change to p1300 = 22 (vector control with encoder) once the resolver is verified.
- Configure the resolver encoder: p400 = 1, p401 = 1 (encoder 1 active), p404 = 1001 (resolver type), p404.1 = pole pairs of resolver (typically 1).
- Run motor data identification: p1910 = 1, then give the ON command. The drive performs a static and rotating identification (the latter requires the motor be free to rotate without load).
- Run speed controller optimization: p1960 = 1, with the motor coupled to the load. The drive runs a series of speed steps and tunes Kp / Tn.
- Run encoder identification: p1990 = 1, with the motor at standstill. The drive measures the resolver offset and stores it in p0431.
- Save the parameters: p0971 = 1.
- Test operation: jog the drive in both directions, verify p0418 increments correctly, then run at low speed under no load and confirm the speed actual matches the setpoint.
9. Key Parameter Reference
| Parameter | Description | Typical value |
|---|---|---|
| p0010 | Drive commissioning parameter filter | 0 (operation), 1 (commissioning), 30 (factory reset) |
| p0300 | Motor type selection | 1 (induction), 2 (PMSM, S120 only) |
| p0400[0] | Encoder type selection, encoder 1 | 1001 (resolver) |
| p0404[0] | Encoder configuration, encoder 1 | Bit-coded; check manual |
| p0410[0] | Encoder inversion, encoder 1 | 0 or 0x80000000 (invert direction) |
| p0431[0] | Commutation angle offset | Set automatically by p1990 |
| p1300 | Open-loop/closed-loop control mode | 20 (V/f), 21 (SLVC), 22 (vector with encoder) |
| p1910 | Motor data identification routine | 1 (automatic) |
| p1960 | Speed controller optimization | 1 (automatic) |
| p1990 | Encoder adjustment / pole position identification | 1 (automatic) |
10. Troubleshooting Matrix
| Symptom | Most likely cause | Confirm with | Fix |
|---|---|---|---|
| F31850 only, no F31150 | Sensor Module A/D converter defect | Swap test with known-good module | Replace Sensor Module or CU250S PN |
| F31150 0x80000 then F31850 | PMSM in closed loop on G120 | Check motor nameplate (PMSM?) | Change to induction motor or move to S120 |
| F31850 with F31150 0x0004 (amplitude) | Resolver signal amplitude | Measure resolver signals on terminal | Check cable length, shield, excitation |
| F31850 with F31150 0x0040 (wire break) | Open circuit | Resistance check at drive end | Replace cable, check connector pins |
| F31850 disappears in sensorless | Encoder chain (resolver + module) | Re-enable encoder, watch for fault | Follow Section 6 procedure |
| F31850 only with long cable | Signal attenuation | Check cable length vs. spec (max 100 m typical) | Shorten cable or use repeater |
| F31850 intermittent, EMC nearby | Shielding or routing | Inspect cable path, separation from VFD output | Re-route with 200 mm separation, re-terminate shield |
11. Verification
After the corrective action, perform the following checks before releasing the drive to production:- Clear the fault buffer and confirm both F31150 and F31850 are no longer pending.
- Run the drive in sensorless mode for 10 minutes at 50% rated speed; confirm no faults and that the measured speed matches the setpoint within ±2%.
- Switch to closed-loop vector control (p1300 = 22). Run the drive from 0 to base speed in 10% steps. Use the STARTER trace function to record r0061 (actual speed) against the setpoint.
- Verify the direction of rotation: with a positive setpoint, the actual speed (r0021) must be positive and the encoder count (r0094) must increase.
- Reverse the direction: with a negative setpoint, the actual speed and encoder count must follow.
- Apply a step load of 50% rated torque; confirm the speed dip and recovery are within 5% / 200 ms, with Kp / Tn from p1960 optimization.
- Save the project to the CF card (if fitted) and to the PG/PC.
12. Safety Notes
Electrical hazard: The DC bus of a G120 PM240-2 retains lethal voltage for several minutes after the line is disconnected. Always wait for the DC-link voltage to drop below 50 V (pumps or fans of the unit audible) before touching the encoder terminals. Use a properly rated measurement device to confirm zero energy state.
Mechanical hazard: Encoder commissioning with rotating identification (p1960, p1990 with rotation) will spin the motor. The motor must be uncoupled from the load or the load must be confirmed safe to rotate. Always post a "Personnel clear of machine" notice before starting identification routines.
Functional safety: If the drive is part of a Safety Integrated (SI) function (STO, SS1, SLS, SDI, etc.), encoder faults can have safety implications. After any hardware change, re-validate the safety function with the appropriate acceptance test as required by the application standard (e.g. ISO 13849-1, IEC 62061, IEC 61800-5-2).
13. Frequently Asked Questions
What does F31850 mean on a SINAMICS G120 CU250S PN?
F31850 is a Sensor Module encoder fault indicating an internal A/D converter communication error. The accompanying fault value identifies the specific sub-function: 0x12001 / 0x10001 typically indicates an A/D converter communication failure, 0x0040 indicates a resolver wire break, and 0x0004 indicates a resolver amplitude error. Cross-check the active value against the SINAMICS G120 List Manual for firmware V4.7.
Why does F31150 with fault value 0x80000 appear before F31850?
F31150 0x80000 is the encoder interface flag indicating commutation with zero-mark active. In the field case, this combination appeared because the operator attempted to run a permanent-magnet motor in closed-loop speed control on a G120. This configuration is not supported; G120 closed-loop vector control with encoder requires an induction (asynchronous) or reluctance motor. For PMSM closed-loop control, migrate to a SINAMICS S120 system.
Can I use a G120 CU250S PN with a permanent-magnet motor and resolver?
Only in V/f open-loop or limited sensorless operation. Closed-loop speed or torque control with resolver feedback on a PMSM is a feature of the SINAMICS S120 family, not the G120. If the application requires PMSM closed-loop control, select an S120 CU320-2 / CU310-2 PN configuration with the appropriate SMC10 or SME20 Sensor Module.
How do I confirm the Sensor Module is defective?
Run a swap test. Mount the suspect Sensor Module on a known-good drive with a known-good resolver, or mount a known-good Sensor Module on the suspect drive. If F31850 follows the suspect module, the module is defective. In the field case, the resolver worked on a different drive, but the same drive still produced F31850, confirming the encoder card was at fault.
Do I need to re-run motor identification after a Sensor Module replacement?
Yes. After replacing the Sensor Module or the CU250S PN, re-run p1910 (motor data identification) and p1990 (encoder adjustment) at minimum. If the application uses speed controller auto-tuning, also re-run p1960. The commutation angle stored in p0431 is module-specific and must be re-learned to avoid uncontrolled motion at the first start command.
What STARTER or StartDrive version is required for firmware V4.7.6 of the CU250S PN?
STARTER V5.3 or later supports the V4.7 SP6 family. In TIA Portal, use SINAMICS StartDrive V15.1 or later with HSP for the matching firmware. Always install the HSP that matches the control unit firmware exactly; mismatched HSPs can produce inconsistent parameter dialogs and fault code interpretations.