Problem Overview
A SINAMICS G120 drive system that has run for ten years on its original program begins to exhibit uneven motor current and torque under load, accompanied by rising stator temperature. With the original encoder-based vector control (P1300 = 21), the system passes no-load commissioning but destabilizes the moment mechanical load is applied. Component swap-out (control unit, encoder, encoder cable, PROFIBUS cable) and motor optimization (P1910 stationary, P1960 rotating) all return nominal results at no-load. The fault recurs only under load, isolating it to a control-loop interaction that grows with torque demand.
The field-proven workaround is to change parameter P1300 from value 21 (vector control with encoder) to value 1 (V/f with linear characteristic and FCC). After the change, the current and torque oscillograms stabilize, the winding temperature returns to its long-term baseline, and the machine can stay in production. The change is a workaround, not a cure; the underlying root cause must still be located before the next scheduled outage.
System Identification
The affected drive is a SINAMICS G120 assembled from a Power Module PM240 and a Control Unit CU240S DP F. The control terminals on the CU240S accept a maximum tightening torque of 0.25 Nm (2.2 lbf.in) and a nominal cable cross section of 1.5 mm², per the official CU240S compact operating instructions. The PROFIBUS DP variant of the CU240S is documented in the CU240S operator panel interface manual, which includes the parameter list and the safety parameter section.
| Component | MLFB | Function |
|---|---|---|
| Power Module PM240 | 6SL3224-0BE38-8UA0 | 3AC 380–480 V line-side inverter with integrated line filter |
| Control Unit CU240S DP F | 6SL3244-0BA21-1PA0 | Closed-loop control, PROFIBUS DP, Safety Integrated (STO/SS1) |
| Encoder (HTL or TTL) | Per P0400 | Speed/position feedback for vector control |
| Commissioning tool | STARTER / TIA Portal with SINAMICS Startdrive | Parameterization and trace recording |
Symptom Matrix
| Symptom | No-load | Loaded (P1300 = 21) | Loaded (P1300 = 1) | Diagnostic value |
|---|---|---|---|---|
| Motor current (r0027) ripple | Negligible | Severe, periodic | Smooth, follows load | Vector-loop instability indicator |
| Torque (r0031) oscillation | None | ±10–30 % swings | No oscillation | Speed controller mismatch |
| Winding temperature | Within class F baseline | Rises steadily, approaches class H | Returns to baseline | I²R + harmonic heating from ripple |
| Mechanical speed (r0063) | Stable setpoint | Visible hunt around setpoint | Slight droop, stable | Encoder feedback quality |
| Drive alarm buffer | Empty | Empty or transient F07900 | Empty | No hard fault — sub-failure mode |
The empty alarm buffer is itself diagnostic: the drive is not reporting a hard fault, so the cause is sub-failure degradation rather than a tripped protection. The most likely candidates are encoder feedback quality, DC bus capacitor aging, motor insulation, or power cable integrity.
Diagnostic Sequence
Follow the sequence below to localize the failure before applying the P1300 workaround. Each step is a gate: do not proceed to the next step until the current step passes.
- Capture a baseline trace in STARTER (or TIA Portal with the SINAMICS Startdrive plugin) on r0027 (current), r0031 (torque), r0061 (speed setpoint), and r0063 (speed actual). Set a 4 s time base, 2 s pre-trigger, 1 ms sampling on current. Save the trace to a project folder.
- Verify the encoder with the Control Unit test function: p0400 (encoder type selection), r0403 (encoder status), p0437 (encoder SSI baud rate if applicable). Encoder pulse count and direction must match the configuration in P0400 / P0408. Swap the encoder cable if any single bit error is logged in r0491.
- Replace the control unit only after verifying the encoder is clean. The CU240S DP F supports parameter-set cloning; back up the parameter set with p0971 = 1 before swap-out.
- Measure the motor insulation with a Megger at 500 V DC and 1000 V DC. A reading below 100 MΩ per phase, or an imbalance greater than 5× between phases, indicates a motor fault. Note that a Megger test does not catch turn-to-turn insulation breakdown; a HiPot / surge test is required for that.
- Run stationary identification (p1910) with the motor cold and decoupled. The drive measures stator resistance (p0350), leakage inductance (p0356), and main inductance (p0360). Compare the results to the nameplate and to the as-commissioned values stored in the project's last backup.
- Run rotating identification (p1960) with the motor uncoupled from the load when possible. This step refines the torque controller and the flux model. If rotating identification cannot be run unloaded, restrict the speed range with p1965 and accept reduced accuracy.
- Capture a second trace identical to the one in step 1. If the ripple persists, the issue is not the encoder or the control unit — it is the motor, the cable, or the power module.
- Swap the motor with a known-good identical unit if available. Run the same trace. If the ripple follows the motor, the fault is in the original motor. If the ripple follows the drive, the fault is in the power module or the cable.
- Inspect the power cable with a TDR (time-domain reflectometer) or LCR meter. The PWM output of an inverter drive can transmit VHF-band noise that degrades encoder signals if the shield is compromised.
- Test the power module with the diagnostic procedure in the PM240 service manual: measure DC bus ripple with a floating oscilloscope at the DC link test points and compare to the maximum allowed value (typically < 5 % of Vdc). Aged electrolytic capacitors show increased 100/120 Hz ripple and elevated ESR.
Root Cause Analysis: Why P1300 = 21 Fails After 10 Years
P1300 = 21 selects vector control with encoder feedback. The drive runs field-oriented control (FOC), where the stator current is decomposed into a flux-producing component (id) and a torque-producing component (iq). The current controller updates iq every current-controller cycle (typically 125 µs at 8 kHz pulse frequency, set in p1800), and the speed controller updates the torque setpoint every speed-controller cycle (typically 1 ms). The speed controller proportional gain (p1460) and integral time (p1462) are tuned for a stable, stiff response under normal conditions.
This control structure is highly sensitive to three categories of error:
- Encoder feedback errors: A single dropped pulse or an intermittent connection causes the speed controller to misread speed, demand a compensating torque, and then over-correct. The result is a torque loop oscillation that grows with load. The drive will not always raise F07900 (Encoder signal lost) on a single missed pulse; the disturbance can sit below the trip threshold yet be visible in the trace.
- DC bus voltage ripple: Aged aluminum electrolytic capacitors in the PM240 can no longer hold the DC bus flat over a mains cycle. The increased 100/120 Hz ripple modulates the available voltage, which the FOC loop reads as a torque disturbance and tries to reject. With healthy capacitors the disturbance is below the controller's rejection bandwidth; with aged capacitors it exceeds it and the loop oscillates.
- Motor insulation weakness: PWM output has a dv/dt of several kV/µs. After ten years, partial discharge activity inside the windings can degrade interturn insulation. A motor with weakened insulation presents a non-linear impedance to the inverter, generating harmonics the FOC loop does not model.
With P1300 = 1, the drive runs a V/f law with Flux Current Control (FCC). The stator voltage is set by the frequency according to a linear V/f profile; FCC adds a dynamic current-stiffness term that boosts low-frequency torque. There is no flux model, no torque loop, and no encoder feedback used for control. The drive is more tolerant of feedback noise, DC bus ripple, and motor anomalies precisely because it does not try to actively reject them. The cost is a slight droop under step load and reduced low-speed torque.
P1300 Control Mode Comparison
| P1300 | Control mode | Encoder used | Dynamic response | Parameter sensitivity | Low-speed torque |
|---|---|---|---|---|---|
| 0 | V/f, linear | No | Low | Low | Limited (no boost) |
| 1 | V/f, linear + FCC | No | Low–Medium | Low | Improved (FCC) |
| 2 | V/f, parabolic | No | Low | Low | Limited |
| 20 | Sensorless vector (speed) | No | High | Medium | Good |
| 21 | Vector with encoder (speed) | Yes | Very high | High | Excellent |
| 22 | Sensorless vector (torque) | No | High (torque) | Medium–High | Good |
| 23 | Vector with encoder (torque) | Yes | Very high | High | Excellent |
The complete parameter list is in the SINAMICS G120/G120C List Manual on the Siemens support site. Engineers must always cross-check the supported P1300 values against the firmware version installed on their CU240S; firmware revisions have historically added and removed values, and a value valid on FW V3.2 may not be valid on FW V4.7.
Key Parameter Reference
| Parameter | Address | Function | Read/Write |
|---|---|---|---|
| P1300 | Control mode | Selects V/f or vector control | R/W |
| P1310 | Voltage boost, continuous | Compensates stator IR drop at low speed | R/W |
| P1311 | Voltage boost, acceleration | Boosts torque during acceleration | R/W |
| P1312 | Voltage boost, starting | Boosts torque at start | R/W |
| P1335 | Slip compensation | Active in V/f mode only | R/W |
| P0400 | Encoder type selection | 0 = no encoder, 1 = HTL bipolar, etc. | R/W |
| P0408 | Encoder pulse count | Increments per mechanical revolution | R/W |
| P1460 | Speed controller P gain | Active in vector mode (P1300 = 21) | R/W |
| P1462 | Speed controller integral time | Active in vector mode (P1300 = 21) | R/W |
| P1715 | Current controller P gain | Active in vector mode | R/W |
| P1717 | Current controller integral time | Active in vector mode | R/W |
| P1800 | Pulse frequency | Default 4 kHz; max 8 kHz for FSE PM240 | R/W |
| P1910 | Stationary motor identification | Measures p0350, p0356, p0360 | R/W (start) |
| P1960 | Rotating motor identification | Refines speed/torque controller | R/W (start) |
| P0304 / P0305 / P0307 / P0310 / P0311 / P0314 | Motor nameplate data | Rated V, I, P, f, n, cos φ | R/W |
| P9601 | SI enable | Safety Integrated enable | R/W (password) |
| P9650 / P9651 | STO debounce / tolerance | Safety reaction time | R/W (password) |
| P9761 / P9762 | SI password | Write-protect Safety parameters | R/W |
| P0971 | Save parameter set | 1 = save to non-volatile memory | R/W |
| r0027 | Current actual (smoothed) | Armature current RMS | R |
| r0031 | Torque actual | Nm | R |
| r0035 / r0037 | Motor / PM temperature | From PTC/KTY or thermal model | R |
| r0061 / r0063 | Speed setpoint / actual | RPM | R |
Implementing the P1300 = 1 Workaround
Changing P1300 is a control-mode change, not a quick commissioning toggle. Follow the sequence below to apply it without losing safe-state behavior.
- Lock out the Safety Integrated configuration before any control-mode change. The CU240S DP F holds the Safety configuration in a password-protected area (p9761 / p9762). Note the F-parameters in use, especially p9601 (SI enable) and p9650 / p9651 (STO debounce), before changing the control mode.
- Set the drive to "Ready for switching on but not running" (PROFIdrive control word 1, bit 0 = 0; bit 1 = 0). Drive state must be Ready, not Operation, when you write to P1300.
-
Write the new control mode:
P1300 = 1. Confirm the parameter is written and accepted (the drive copies the value to the volatile image and the change is active at the next ON command). - Re-enter the motor data if the drive prompts for a new identification. With P1300 = 1 the motor model is simpler and the controller does not need the magnetizing inductance, but it still needs p0304 (rated current), p0305 (rated power), p0307 (rated voltage), p0310 (rated frequency), p0311 (rated speed) for slip compensation (P1335).
- Set the V/f boost (P1310, P1311, P1312) for the low-speed torque. Factory defaults of P1310 = 50 %, P1311 = 0 %, P1312 = 0 % are a starting point. Adjust by 10 % steps and re-trace the current at 2 Hz to verify the magnetizing current stays below rated current.
- Disable the encoder in the configuration (P0400 = 0, encoder not connected, or set to monitoring only). The drive will not fail if the encoder remains connected, but encoder-related alarms (F07900, F07901) may appear if the controller references a sensor it is not using.
- Re-arm Safety Integrated and run the safety acceptance test. The safety functions (STO, SS1) are independent of the control mode and must be re-verified after any control-mode change.
- Test the first run unloaded at 5, 10, 25, 50, and 100 % of rated speed. Verify current and torque are smooth.
- Test under load at the same speed points. Verify current is stable and torque follows the load profile.
- Run for 24 hours in production. Monitor r0035 (motor temperature) and r0037 (power module temperature). Document the values for the maintenance record.
Verification Procedure
The change must be verified in three modes: electrical stability, mechanical stability, and thermal stability.
- Electrical: Capture a STARTER trace on r0027 (current), r0031 (torque), r0061 (speed setpoint), and r0063 (speed actual) at 100 % speed and 100 % load. The current trace should be smooth with the 2× line-frequency envelope; ripple should be below 5 % of the RMS value. Compare to the trace taken before the P1300 change.
- Mechanical: Accelerometer on the motor housing and on the driven equipment. Compare vibration spectra to the as-built baseline. A 2× line-frequency sideband or a 4× line-frequency sideband suggests mechanical imbalance, not electrical ripple.
- Thermal: Read r0035 (motor temperature from the PTC/KTY model) and the physical winding RTD. The winding temperature should stabilize within 10 K of its ten-year baseline within four hours of full-load operation. If the temperature continues to rise, additional harmonic heating may be present even if the oscillogram looks clean.
Long-Term Root Cause: Power Module and Motor Tests
Operating the machine on P1300 = 1 is acceptable as a temporary measure, but the underlying root cause must be located. The two highest-probability root causes, in order, are the power module and the motor.
Power Module Capacitor Aging
The PM240 uses aluminum electrolytic capacitors in the DC link. After ten years of continuous service, especially in a warm cabinet, the capacitors lose capacitance and the ESR rises. The visible symptom is increased 100/120 Hz ripple on the DC bus. Vector control reads this ripple as a torque disturbance; V/f control is more tolerant because it does not model the torque disturbance. The diagnostic procedure:
- Disconnect the drive from the line and wait 5 minutes for the DC bus to discharge to a safe level (verify with a meter at the DC+ and DC- test points).
- Reconnect the line, enable the drive (no run command), and measure the DC bus voltage with a floating oscilloscope at the PM240 test points. Use a differential probe rated for the DC bus voltage (≈ 650 V DC for a 400 V line).
- Compute the peak-to-peak ripple and compare to the rated DC bus voltage. The expected ratio for a healthy PM240 is below 2 %; aged capacitors show 4–8 %.
- Run the drive at 50 % speed no-load for 30 minutes, then repeat the ripple measurement. The ripple should not change significantly; if it does, the capacitors are temperature-sensitive and failing.
- Read the operating hours counter and the load profile. Capacitor life is dominated by ripple current and temperature; a 10 K rise in operating temperature halves the life.
- If the ripple is excessive, replace the DC link capacitors (a service procedure documented in the PM240 service manual) or replace the PM240 outright.
Motor Insulation Degradation
The Megger test is a phase-to-ground test at low DC voltage. It catches gross insulation failure but not interturn insulation breakdown. PWM inverters stress the motor with a dv/dt of several kV/µs; the interturn voltage can exceed 1200 V on a 400 V system. After ten years, partial discharge activity inside the windings can erode the interturn insulation. The Megger will pass, but the motor will draw non-sinusoidal current and present a non-linear impedance to the inverter. The diagnostic procedure:
- Schedule a HiPot (high potential) test with a motor service provider. The test applies a controlled high-voltage AC waveform to each phase and measures the leakage current. Any phase-to-phase leakage above the manufacturer's limit indicates a winding fault.
- Request a surge test. A surge tester applies a fast-rising pulse (typically 0.1–5 µs rise) to one phase at a time and looks for a matched response. Mismatched response between phases indicates a turn-to-turn short.
- Run an offline partial discharge test if available. PD activity correlates with winding age and is the most sensitive indicator of insulation degradation.
Cable Integrity
The encoder cable and the motor cable are both stress points. The PWM output of the inverter is a VHF-band source; a compromised shield or a failed insulation strand can couple noise back into the encoder feedback. The diagnostic procedure:
- Measure the shield-to-ground resistance on both cables. A reading above 1 Ω indicates a broken shield.
- Time-domain reflectometry on the motor cable. A reflection more than 5 % of the launch pulse at a distance consistent with the cable length indicates a discontinuity.
- Insulation resistance at 1000 V DC phase-to-ground and phase-to-phase. A reading below 100 MΩ per phase indicates a fault.
Diagnostic Trace Procedure
To capture the current and torque ripple that confirms the fix, use the SINAMICS trace function in STARTER or TIA Portal. The procedure is identical for both tools; the parameter addresses are the same.
- Open the drive in STARTER. Right-click the drive in the project tree, then Commissioning → Trace.
- Configure two traces:
- Trace 1: r0027 (current actual), r0031 (torque actual), 4 s window, 1 ms time base.
- Trace 2: r0061 (speed setpoint), r0063 (speed actual), 4 s window, 1 ms time base.
- Set the trigger to Torque actual > 50 % of rated with a 1 s pre-trigger. The drive starts recording the moment load torque exceeds half the rated torque.
- Run the machine through the production cycle. The trace records the current and torque in steady state and during transient events.
- Export the trace as a CSV. Calculate the RMS of r0027 over the 4 s window and the peak-to-peak ripple. The ripple is the peak-to-peak divided by the RMS, expressed as a percentage. A ripple below 5 % is acceptable; above 10 % indicates a fault.
Grounding and EMC Considerations
The PWM output of the PM240 has a rise time of around 100 ns and a peak voltage of twice the DC bus voltage (line-to-line, 6 kV/µs). At this rise time, the cable behaves as a transmission line, and any impedance discontinuity (loose terminal, broken shield strand, corroded gland) creates a reflection that raises the peak voltage at the motor terminals. The standard mitigation is:
- Use a symmetric, shielded VFD cable (e.g., Siemens 6FX5008 or equivalent) with a continuous shield bonded to ground at both ends with 360° glands.
- Keep motor cable length below the EMC limit specified in the PM240 manual (typically 25 m for 8 kHz pulse frequency with the integrated filter, longer with output filters).
- Separate the encoder cable from the motor cable by at least 200 mm; cross at 90° if they must intersect.
- Verify the cabinet PE bus is bonded to the building ground with a low-impedance connection (resistance below 0.1 Ω to the main grounding electrode).
Firmware Considerations
Firmware version on the CU240S affects which P1300 values are available and how the FOC loop is tuned. The supported firmware streams for the CU240S DP F are V3.2, V4.4, V4.5, and V4.7. The following are field-proven notes:
- V3.2: Stable for 10+ years; default P1300 = 21. Known to be sensitive to DC bus ripple from aged capacitors.
- V4.4 / V4.5: Improved FOC loop, better rejection of DC bus ripple. P1300 = 21 with these firmware streams is more tolerant of aged capacitors than V3.2.
- V4.7: Latest at the time of writing; supports the same P1300 values as V4.4 / V4.5 plus a few additional diagnostic parameters.
Check the firmware version on the CU240S against the latest service pack on the Siemens support site. Firmware updates have historically fixed vector-control stability issues and improved the rejection of DC bus ripple. Plan a firmware upgrade during the next planned outage if the unit is below V4.5.
Preventive Action Plan
Once the machine is stable on P1300 = 1, schedule a root-cause investigation during the next planned outage.
- Power module health: Measure DC bus ripple, capacitor ESR, and heatsink temperature. Replace the PM240 if the ripple exceeds 5 % of the DC bus voltage or if the heatsink runs more than 10 K above its as-built baseline.
- Motor insulation: Schedule a HiPot and surge test with a motor service provider. Replace the motor if the leakage current exceeds the limit or if the surge response is mismatched.
- Cable integrity: TDR the motor cable and the encoder cable. Replace any cable that shows a reflection above 5 % of the launch pulse.
- Encoder: Replace the encoder if r0403 reports any bit error. Note that an encoder can pass the no-load test and fail under load if the bearings are worn; a vibration test on the encoder itself is a useful supplement.
- Firmware: Check the firmware version on the CU240S against the latest service pack on the Siemens support site. Firmware updates have historically fixed vector-control stability issues and improved the rejection of DC bus ripple.
- Cabinet ventilation: Verify the cabinet ventilation is intact. A 10 K rise in ambient reduces electrolytic capacitor life by half (Arrhenius, 10 K rule). A cabinet that ran at 35 °C ambient for ten years has significantly aged capacitors; one that ran at 45 °C is at the end of its life.
Safety Acceptance Test After Control-Mode Change
After changing P1300, the Safety Integrated functions (STO, SS1) must be re-verified even though the safety logic is independent of the control mode. The acceptance test is documented in the CU240S safety manual and consists of:
- Force the STO input and verify the drive enters Safe Torque Off within the configured debounce time (p9650). The drive should indicate Safe Stop active on the BOP/AOP and on the safety status word (r9771).
- Release the STO input and verify the drive requires a positive edge on the OFF1 command to restart.
- Verify the safety diagnostics bits in r9771 match the wiring.
- Document the test in the safety logbook. The logbook is required for compliance with IEC 61508 and ISO 13849.
Do not skip this step; an undocumented Safety acceptance test is treated as a missing test by the auditor.
FAQ
What does P1300 = 21 versus P1300 = 1 actually change in the SINAMICS G120?
P1300 = 21 enables field-oriented vector control with encoder feedback. The drive runs a full flux model and a closed torque loop; encoder pulse quality, motor magnetizing data, and DC bus voltage quality all directly affect torque accuracy. P1300 = 1 enables V/f open-loop control with Flux Current Control. The drive sets stator voltage proportional to frequency, with FCC adding a dynamic current-stiffness term. Encoder pulses are not used for control, and the flux model is not active.
Why does vector control develop current and torque ripple after years of service?
The most common causes are aged DC link capacitors in the Power Module (increased 100/120 Hz ripple that the FOC loop reads as a torque disturbance), degraded interturn insulation in the motor (non-linear impedance that injects harmonics the FOC model does not predict), and intermittent encoder feedback (a single dropped pulse causes the speed controller to misread and over-correct). V/f control is more tolerant of all three because it does not try to actively reject them.
Can I keep running the machine on P1300 = 1 long-term?
It depends on the load profile. Constant-torque loads (extruders, conveyors) will see reduced torque at low speed and a slight droop on step loads. Quadratic loads (fans, pumps) are well matched to V/f control. High-inertia or high-transient loads (hoists, mixers) usually require the dynamic response of vector control, and the root cause must be fixed. As a rule, treat the P1300 change as a workaround, not a fix, and schedule the root-cause investigation for the next planned outage.
How do I capture a current and torque trace to verify the fix?
Use the SINAMICS trace function in STARTER or TIA Portal. Record r0027 (current), r0031 (torque), r0061 (speed setpoint), and r0063 (speed actual). Set a 4 s window with 1 ms time base and trigger on r0031 > 50 % of rated torque with 1 s pre-trigger. The peak-to-peak ripple of r0027 should be below 5 % of the RMS value when the machine is in steady state.
Is a Megger test enough to clear the motor?
No. A Megger measures phase-to-ground insulation at low DC voltage and catches gross insulation failure. It does not catch interturn insulation breakdown, which is the failure mode produced by long-term PWM stress. Schedule a HiPot test (high-voltage AC leakage current per phase) and a surge test (matched pulse response per phase) with a motor service provider. The motor service provider can perform these tests on site, avoiding motor removal.