Troubleshooting Siemens Simovert 611 Infeed DC Bus Folding

Problem Overview

A Siemens Simovert 611 (6SN11 series) infeed module installed on a Sinumerik 840C-controlled machine exhibits DC-bus voltage folding: the regulated 600 V DC bus ramps up toward 600 V, droops back to roughly 580 V, the internal precharge/contactor relay drops out, the yellow "DC bus OK" LED extinguishes briefly, and the relay re-energises to repeat the cycle. Two successive replacement infeed modules reproduced the identical folding behaviour, while each module behaved correctly when bench-tested with a minimum set of hardwired links and no load.

This article documents the bench-versus-machine divergence, the role of terminals 9, 48, 63, 64, 112, NS1, and NS2, the significance of the 400 V ±10% three-phase supply tolerance, and the procedure used to isolate the load (LT modules) from the infeed as the suspected root cause.

Siemens Simovert 611 Infeed Module Background

The Simovert 611 family uses an I/R (Infeed/Regenerative) module - referenced in some Siemens literature as the UE module (Untersetzungs-Einspeisung) - that converts a three-phase 400 V AC supply into a regulated 600 V DC bus. This bus feeds the LT (Leistungsteil) inverter modules that drive the spindle and axis motors. The same bus returns regenerative energy from the motors back to the line during braking.

The DC bus is normally a stiff 600 V regulated by the IGBT chopper and the line filter. The yellow "DC bus OK" LED on the front of the infeed module is the primary visual indication of bus regulation. When the bus droops below the undervoltage threshold, the LED extinguishes and the precharge/line contactor is de-energised, disconnecting the line reactor from the bus.

Affected Hardware and Configuration

The reported fault occurs on a Sinumerik 840C-controlled machine. The relevant hardware stack includes:

• One 611 infeed module (UE / I/R) - the device exhibiting folding DC bus output.
• One or more 611 LT modules (inverter stages) mounted in the same DC-bus rack.
• The Sinumerik 840C NCU providing enable signals (terminals 63/64) and contactor control (terminals NS1/NS2).
• An external infeed line filter (line-side reactor + EMC filter).
• Hardwired control wiring between the NCU and the infeed module X111 connector.

Observed Symptoms

The fault exhibits the following reproducible behaviour:

1. Module powers up and DC bus begins to ramp.
2. Yellow "DC bus OK" LED illuminates as the bus approaches 600 V.
3. DC bus voltage droops to approximately 580 V and falls.
4. Yellow LED extinguishes.
5. Internal precharge relay drops out; line contactor opens.
6. DC bus decays toward zero.
7. Precharge cycle repeats.

The cycle period is typically 1 to 3 seconds. Removing the DC-bus links to the LT rack and the ribbon-cable signal interface did not stop the folding, indicating the infeed module itself - not the LT load - was oscillating in the installed configuration.

Root Cause Analysis - Why the DC Bus Folds

The 611 infeed module regulates the DC bus by chopping the rectified line voltage through an IGBT. Regulation requires continuous gating pulses. When pulse enable (terminals 63 and 64) is removed, gating stops; the bus is supported only by the line-side rectifier and bulk capacitors. Under that condition the bus voltage tracks the rectified peak of the line minus the drop across the precharge resistor, and any sudden load transient (including the bus capacitance discharge) will collapse it below the undervoltage threshold.

The reported behaviour - bench test holds 600 V after terminals 63/64 are tied to 9, but the same module folds in the machine - is consistent with the machine's NCU not driving terminals 63/64 high during the no-load bench-style test scenario. When the NCU is removed from the equation (or its enable relay is open), the I/R module interprets the missing enable as a commanded shutdown and pulses the contactor off.

Additional root-cause candidates that must be ruled out:

• Undervoltage line condition: 412-416 V line-to-line observed in the field, with 425 V on the workshop bench. 400 V ±10% allows 360-440 V, so all measured values are within spec. Line undervoltage is therefore not the cause, but borderline undervoltage can still cause nuisance trips if combined with weak regulation.
• Faulty LT module dragging the bus: The reported 5-700 kΩ DC resistance and similar capacitance across multiple LT modules is normal for an unpowered 611 LT module. Internal IGBT shorts would typically present as a near-short (well under 100 kΩ) or as thermal runaway under DC bus load. To prove an LT module is the cause, disconnect all P600/M600 links from the rack and re-test the I/R in isolation.
• Internal I/R damage: Pre-charge resistor failure, IGBT module degradation, or DC-link capacitor ESR rise. Bench test with no load should still hold the bus if these faults are present, so passing bench tests tend to exonerate the I/R.
• Line filter capacitor fault: A shorted X-capacitor in the infeed filter can pull the bus down. The reported attempt to remove the filter and re-test is the correct diagnostic step.

Diagnostic Procedure - Bench Versus Machine

The bench test in this case used the following minimum link set, which mirrors the bench-test recipe documented in Siemens service documentation:

Bench Test Minimum Link Set - 611 I/R Module Standalone
========================================================
  Terminal 9   <--- linked to --->   Terminal 48     (control 24 V loop)
  Terminal 9   <--- linked to --->   Terminal 112    (DC-ON / precharge command)
  Terminal 9   <--- linked to --->   Terminal 63     (inverter enable, channel 1)
  Terminal 9   <--- linked to --->   Terminal 64     (inverter enable, channel 2)
  NS1          <--- linked to --->   NS2             (contactor feedback loop)
  1U1          <--- linked to --->   2U1             (phase U bypass link)
  1V1          <--- linked to --->   2U1             (phase V bypass link)
  1W1          <--- linked to --->   2U1             (phase W bypass link)

Apply three-phase 400 V AC to 1U1 / 1V1 / 1W1.
Expected result: P600-M600 settles at 600 V DC and holds steady.
Yellow "DC bus OK" LED remains lit continuously.

Critical Terminal Functions Explained

The Sinumerik 840C drives terminals 63, 64, and 112 from internal enable relays that close once the NC has booted, the drives have been commissioned (NCK ready), and no drive fault is active. If any of those relays fails to close, the I/R receives no enable and will not regulate. On a freshly powered machine that has not yet completed NCK startup, the enable is intentionally absent.

Supply Voltage Analysis

The bench supply measured 425 V line-to-line; the machine supply measured 412, 412, and 416 V line-to-line. The 611 I/R specification is 400 V ±10%, i.e. 360 V to 440 V line-to-line. All measured values fall comfortably within tolerance, with the machine supply roughly 4% below nominal and the bench supply 6% above nominal.

A 4% undervoltage condition is not by itself sufficient to fold the bus, but it does reduce the rectified peak (sqrt(2) × V_LL) from a nominal 565 V to ~583 V, which is right at the lower edge of where the chopper can regulate to 600 V. When combined with a weak DC-link capacitor bank or with chopper enable that is intermittent, the bus can dip below the 580 V undervoltage cut-off threshold.

LT Module and Load Fault Isolation

To isolate a faulty LT module (servo inverter stage) as the root cause, perform the following:

1. Power down the machine completely. Wait 10 minutes for the DC bus to bleed through the discharge resistors. Verify zero volts on P600 to M600 with a CAT III meter.
2. Remove the DC-bus bars (P600/M600) between the I/R module and the LT rack. Insulate the bar ends with heat-shrink or terminal boots.
3. Disconnect the ribbon-cable interface from the LT modules (X311/X321 etc.).
4. Reapply three-phase power. With only the I/R module and no load, the bus should rise to 600 V and hold.
5. If the bus still folds with no LT load, the fault is upstream of the LT modules (wiring, NCU enable, or the I/R itself).
6. If the bus holds, reconnect LT modules one at a time. After each reconnection, power down safely and re-energise. The LT module that causes folding when reconnected is the faulty unit.

The reported 5-700 kΩ across the LT modules' DC connections is consistent with healthy modules whose DC-link capacitors have discharged through the bleeder resistors. The fact that all measured modules show similar values argues against a single shorted LT module as the root cause.

Step-by-Step Resolution Procedure

1. Confirm the bench test result with full link set. Tie terminals 9-48, 9-112, 9-63, 9-64, NS1-NS2, and 1U1-2U1, 1V1-2V1, 1W1-2W1 on the I/R module. Apply three-phase power. The bus must hold 600 V DC continuously. If it folds on the bench with all links present, the I/R module is internally damaged.
2. If bench test holds, the I/R is good. Re-install the I/R in the machine with all control wiring disconnected from X111 except the three-phase supply. Repeat the link set as on the bench. The bus should hold. If it does not, the rack, bus bar, or filter assembly is interacting with the I/R - move to step 4.
3. Verify NCU enable outputs. With the machine fully wired (no bench links), power the Sinumerik 840C. Allow NCK startup to complete (5 to 10 seconds after the HMI shows "Ready"). Measure terminal 9 to 63 and terminal 9 to 64. Both should read +24 V DC. Measure terminal 9 to 112. It should read +24 V DC. If any of these read 0 V with the NCU ready, the NCU enable relay or the wiring is the fault.
4. Inspect the NCU-to-I/R cable. Pin-by-pin continuity check between the 840C terminal strip and the I/R X111 connector. Look for broken conductors, cold solder joints at the Phoenix connectors, and corroded terminals. Pay particular attention to pins 63, 64, and 112 which carry the enable chain.
5. Verify NS1-NS2 loop closure. NS1 and NS2 form a feedback loop that proves the external line contactor has actually closed. If the line contactor auxiliary is wired normally-open (NO) instead of normally-closed (NC) to this loop, the I/R sees "contactor open" and refuses to enter run state.
6. Re-test with load. With all control wiring restored, command the NCU to enable the drives. The bus must hold 600 V continuously. Drive a slow jog on each axis in turn. Any axis that triggers bus folding under load is suspect - investigate that LT module for IGBT degradation.
7. If folding persists under no load with full NCU enable, the I/R has internal damage that the bench test did not expose - typically ESR-elevated DC-link capacitors or an intermittent gate-driver fault. Replace the I/R module with a known-good Siemens-tested unit.

Verification and Commissioning

After resolution, perform the following commissioning verification:

• Measure P600-M600 with a true-RMS voltmeter. Confirm 600 V DC ±2% (588-612 V) over a 10-minute idle period.
• Verify yellow "DC bus OK" LED is solid, not flashing.
• Monitor bus voltage on the Sinumerik 840C HMI service screen (Diagnostics > Drives > DC Bus). Confirm steady 600 V reading.
• Execute a spindle speed sweep 0 to maximum RPM in both directions. Bus must remain within ±5% of nominal.
• Execute rapid traverse on each axis. Regenerative energy must return through the I/R without triggering overvoltage (the regenerative path is bidirectional).
• Run a complete part program. Monitor the bus for the full cycle - no folds, no LED dropouts.

Fault Code Matrix and LED Indicators

For 7-segment fault codes on the I/R module, common entries include "F-01" (line undervoltage), "F-02" (DC bus undervoltage), "F-05" (chopper overcurrent), and "F-08" (precharge timeout). Always cross-reference the specific fault code against the Sinumerik 840C diagnostics manual for the exact remedy.

Preventive Recommendations

• Document the wiring between the NCU and the I/R module on every commissioning job. The link-set bench test should be recorded as part of the FAT (factory acceptance test).
• Inspect line-side fuses and the contactor auxiliary contacts annually. A pitted NO contact on the line contactor can mimic an open NS1-NS2 loop.
• Log DC-bus voltage to the HMI's trend display. A 600 V reading that decays below 595 V under steady-state load is an early warning of capacitor degradation.
• Replace electrolytic DC-link capacitors on the I/R module after 10 years of service, regardless of bench-test results.
• Keep spare I/R modules powered up on a maintenance bench with a 400 V supply and the minimum link set - this keeps their internal capacitors formed and provides a known-good reference for field comparison.