SINAMICS G120 PM240-2 Ready-But-Not-Running: Diagnostic Guide

A SINAMICS G120 PM240-2 (6SL3210-1PE32-5UL0, 132 kW) with a CU240E-2 control unit on firmware V4.7 returns a steady green RDY LED after a field-level PCB repair, no faults in the buffer, a 563 V DC bus, and a healthy 24 V rail, yet the drive refuses to start the motor. The output stage is almost never the cause after a clean repair - the issue is one of four systemic blockers: a missing start command, a held STO input, a wrong macro or wiring mismatch, or a leftover motor-identification state. This article walks the diagnostic tree in field order and closes with the supply-topology pitfall the original report itself flags: a 132 kW PM240-2 is a three-phase device and cannot be run from a two-phase 400 V test tap.

1. Equipment Identification and Baseline

Confirm the exact catalog numbers before touching anything. The combination in this case maps to a specific frame size, line filter class, and CU variant that determine which terminals, macros, and parameters apply. Cross-check the MLFB against the SINAMICS G120 PM240-2 operating instructions before trusting any terminal assignment.

The MLFB 6SL3210-1PE32-5UL0 decodes per the SINAMICS G120 catalog as: 6SL3210 = PM240-2 family; 1PE = 400 V 3-phase, 132 kW; 3 = no integrated line filter; 2 = standard variant; 5 = FSE frame; UL = push-through air-cooled; 0 = no options. The 'CU240E-2 V4.7' label in the original ticket does not specify the full MLFB of the CU - the variants (CU240E-2, CU240E-2 F, CU240E-2 S) differ in their Safety Integrated terminal layout. The diagnostic logic below applies to the standard CU240E-2; F and S variants add two-channel STO terminals and PROFIsafe handling that must be considered separately.

2. The 'Ready but Not Running' State Machine

On the CU240E-2, the green RDY LED is the consolidated state of the inverter logic. The most common states encountered in the field after a no-fault, no-run condition are:

The original ticket explicitly states 'RDY LED not blinking' and 'no fault present', which lands the case in the steady green row. The drive is logically ready, the power stack has been released by the CU, and the inverter is waiting for a permissive - it is not a hardware damage symptom in the output stage. The remainder of the article is built around that single observation.

3. Supply Topology Pitfall - Two-Phase 400 V on a Three-Phase Drive

The original report cites a two-phase 400 V test feed. This is the single largest red flag in the case, and it must be addressed before any other troubleshooting is meaningful.

Every PM240-2 frame FSD through FSE (75 kW and above) is a true three-phase rectifier front end. The 6SL3210-1PE32-5UL0 is rated 3 AC 380-480 V +/-10%, three-phase. It is not a single-phase drive, and Siemens does not publish a derating to run it on two phases with the third leg floating. The consequences of attempting this are:

1. Asymmetric line current. The third phase carries only the small magnetizing current of the line reactor; the other two phases carry the full load. Expect immediate F30002 (DC bus overcurrent) or F30001 (line-side overcurrent) on heavier loading.
2. DC bus droop and 100/120 Hz ripple. A two-phase 400 V feed through a six-pulse rectifier produces a DC bus of roughly 1.35 x V_LL = 540 V nominal, but with peak-to-peak ripple around 60-90 V. The reported 563 V is consistent with three-phase 400 V at 50 Hz, not two-phase. Either the meter is reading line-to-line between two phases of an otherwise healthy three-phase system, or the drive is in a partially powered state that should not be trusted.
3. Brake chopper latch. FSE PM240-2 drives have an internal brake chopper. Asymmetric DC bus ripple will trigger F30003 (DC bus overvoltage) on the chopper side whenever the load regenerates - a normal event on a compressor during shutoff backspin.
4. Safety Integrated reliability. The CU240E-2 derives its STO permissive from the DC bus sensing chain. With an out-of-spec DC bus, the STO logic may refuse to release even if the EP and STO inputs are wired correctly, or it may release intermittently under load.
5. Capacitor stress on the original failure. Two blown PCB caps in a row are a strong signature of a supply event (surge, phase imbalance, missing phase). Operating the repair on the same two-phase feed risks repeating the failure on the new caps within hours.

4. Check 1 - Is the Setpoint Actually Reaching the Drive?

On CU240E-2 firmware V4.7, the default control word source depends on p0015 (macro) and the active command source. In macro 0 (factory), the source is the BOP-2, terminals, or fieldbus depending on p0700 / p1500. The relevant control bits in the standard telegram are wired as follows:

Procedure on the BOP-2:

1. Press ESC to back out of any menu, then OK to enter the parameter menu.
2. Navigate to p0840, set to 1, press OK to confirm. The drive should accelerate to the setpoint in p1000 / p1058 / p1060.
3. If the drive runs under forced p0840 = 1 from BOP-2, the issue is in the upstream command source - the terminal wiring, the PLC handshake, or the macro selected.

If the BOP-2 also fails to start the drive, the problem is on the permissive side, not the command side - move to Section 5.

5. Check 2 - STO (Safe Torque Off) Is Released

STO is the most common cause of 'ready but no run' on a CU240E-2 that returns no fault code. STO is a hard-wired, non-software interlock; the inverter will not gate-fire the IGBTs until both STO channels are released, and the firmware raises no fault if it is held by design. The default terminal layout for STO on the standard CU240E-2 is single-channel via DI (configurable, often DI4), while the F and S variants add the dedicated two-channel STO terminals:

Verification procedure:

1. Measure X1.13 and X1.15 against X1.12 with a calibrated DMM. Both should be 18-30 V DC. If either is 0 V, STO is held.
2. From the BOP-2, navigate to r0002 (drive status) and r0046 (missing enables). Bit 14 in r0046 indicates STO active.
3. Read r9772 (SI diagnostic, CU side) and r9872 (drive-side). Both should read 0xFFFF when STO is released. Any other value indicates a channel mismatch.
4. Inspect the wiring for broken jumpers between X1.13/X1.15 and the 24 V source. Field failures of these jumpers are extremely common after a PCB repair because the rework may have disturbed nearby terminals or pulled on a harness.
5. On the basic CU240E-2, check p9660 (source of STO via DI) and p9661 (active level). A common commissioning error is leaving p9660 assigned to a DI that is not wired.

If STO is not the cause, check the basic enable chain in r0046 bit-by-bit. The parameter is a bitmask; each missing enable is a flag:

• Bit 00: OFF1 missing (p0840)
• Bit 01: OFF2 missing (p0844)
• Bit 02: OFF3 missing (p0848)
• Bit 03: Enable operation missing (p0852)
• Bit 04: Ramp generator inhibit (p1140)
• Bit 05: Ramp generator stop (p1141)
• Bit 06: Setpoint inhibit (p1142)
• Bit 14: STO active
• Bit 15: SS1 active (F/S variants only)

6. Check 3 - Macro and Terminal Wiring

Parameter p0015 selects the macro. Each macro reconfigures the digital inputs, analog inputs, and command sources to a vendor-recommended wiring pattern. If the macro does not match the physical wiring, the start command never lands on the right input. Common macros for CU240E-2:

After a PCB repair, it is very common for an electrician to reconnect the field wiring to the terminals they remember, but a macro change earlier in the project life - or a default reset that put p0015 = 0 - leaves the wiring mapped to non-existent inputs. The drive is then 'ready' but every command lands on a parameter that has no source assigned.

Procedure:

1. Read p0015 on the BOP-2. Note the value.
2. Cross-check with the wiring diagram from the project. Verify each terminal against the macro: in macro 3, DI0 must be 24 V to start; in macro 5, DI0 is start and AI0 must have a live 0-10 V or 4-20 mA signal.
3. If a macro is active that does not match the wiring, set p0015 = 0, factory-reset the drive with p0970 = 1, then either re-run the commissioning wizard on the IOP-2/BOP-2 or set the macro to the correct value and re-load the parameters.
4. If no macro is selected (p0015 = 0), the I/O is not assigned. Either run commissioning from the BOP-2 wizard or set p0700 / p1000 / p1500 manually to map the sources you want.

For a compressor on the basic CU240E-2, the typical macro is p0015 = 5 (pump/fan with analog setpoint), with p1300 = 2 (sensorless vector) for high starting torque, or p1300 = 6 (quadratic V/f) for energy-optimized partial-load operation. With p1300 = 0 (linear V/f) the drive will run, but the energy consumption on a compressor is 8-15% higher than the quadratic mode.

7. Check 4 - Motor Identification State (p1900 / p1910 / p1960)

The second thread in the field report pointed at p1900, p1910, and p1960. These are the motor-identification routines that the firmware runs during a fresh commissioning:

If any of these are non-zero and the conditions for the identification run are not met - missing enable, STO held, motor decoupled, terminal start not active, or a wrong macro - the firmware enters the 'Identification phase' and the RDY LED will flash 2 Hz green. The drive is not running the motor; it is waiting for the start command that triggers the ID sequence, or it has started the ID and stalled partway.

The source confirms the user has already set p1900 = 0, p1910 = 0, p1960 = 0. That removes the ID lock. If after that the drive still does not run with RDY steady green, the problem is elsewhere - go back to Sections 4-6. Note that simply zeroing these parameters is not a substitute for running the ID; it leaves the motor model un-tuned and the compressor will run with default Siemens induction-motor parameters. Plan a proper ID on a real 3-phase supply as a follow-up, with the motor decoupled from the load to avoid damage to the compressor coupling and to get a clean inertia measurement.

The p1910 routine in particular requires the motor to spin freely. If the compressor is coupled and you set p1910 = 1, the firmware will attempt to ramp the loaded compressor, which can stall, overspeed (if the compressor is unloaded but the firmware expected a light load), or trip F07901 (motor stalled). Always decouple before running rotating ID, then re-couple and re-check p1496 (acceleration pre-control) and p1470 / p1472 (speed controller P-gain / integral time).

8. Check 5 - Local Operation from BOP-2

The fourth suggestion in the source thread is the single most useful field test: drive the motor from the BOP-2 in local HAND mode. This bypasses the entire command source chain and isolates the issue to the power stack and the motor.

1. Press OK on the BOP-2 to enter the menu.
2. Select HAND (green hand icon) or toggle AUTO/HAND. In HAND, the BOP-2 owns the start command and setpoint.
3. Press the green I key to issue ON, the red O key to stop. The motor should run at the setpoint in p1000 / p1058 / p1060 (default 50% of p1082 maximum speed in HAND mode).
4. Observe the RDY LED. If it is steady green and the motor runs in HAND but not AUTO, the issue is in the command source chain (Section 4) or the macro wiring (Section 6).
5. Press the AUTO key to return remote control to the configured source.

If the BOP-2 also fails to start the motor and the RDY LED stays steady green with no fault, the only remaining blockers are STO (Section 5) or a 24 V permissive on a digital input that has been reconfigured to act as an enable. Inspect r0046 again with the BOP-2 in HAND. The HAND mode is fully independent of the AUTO command source; if it does not run in HAND, the fault is on the permissive side, not the source side.

9. LED and Status Word Diagnostics with No PC

When a PC with Startdrive, STARTER, or the TIA Portal is not available, the CU240E-2 and BOP-2 expose enough state through the menu tree to do full diagnosis. Walk this sequence in order:

The combination of r0002 = 'Ready' + r0046 = 0 + r0945 = 0 + r9772 = 0xFFFF is the unambiguous 'ready, all permissives present, awaiting start' state. If you are in this state and the drive still does not start, you have a command-source problem (Section 4) or a wiring-vs-macro mismatch (Section 6), not a hardware fault. The hardware tree should not be replaced.

For a deeper view, read r0052 (status word 1 bit-by-bit) and r0053 (status word 2). r0052 bit 0 = Ready, bit 1 = Ready to start, bit 2 = Operation enabled, bit 3 = Fault present, bit 4 = Coast stop active (OFF2), bit 5 = Quick stop active (OFF3), bit 6 = Switch-on inhibit active. If bit 6 is high after a power-up, the drive is in 'switch-on inhibit' state and requires a 0 to 1 edge on OFF1 (p0840) to clear - it is a one-shot lockout that survives parameter changes and is a frequent cause of the original symptom on a freshly powered drive.

10. When to Stop Troubleshooting and Send the Drive to Siemens

Two blown PCB capacitors on a 132 kW FSE PM240-2 are not a normal failure. The most likely root causes, in order of frequency, are:

1. DC bus overvoltage from regenerative load. Compressor backspin on shutdown, missing or undersized brake resistor, or weak supply absorbing no regen. The brake chopper IGBTs in FSE are vulnerable to repeated regen transients, and the failure cascades into the snubber caps on the control PCB.
2. Input supply event. Voltage surge, phase loss, lightning-induced transient, or DC injection from a weak transformer feeding a heavily loaded bus.
3. Cooling failure on the FSE heat sink. FSE frames use forced-air cooling with 24 V fans. A failed fan overheats the rectifier diodes and the DC link, then the gate-drive bias caps on the control PCB. Check the fan in the next service window.
4. Insulation breakdown in the compressor motor. Reflected voltage transients from long motor cables can back-propagate into the drive and destroy the gate-driver. A 132 kW drive typically feeds a 30-80 m cable; a dU/dt filter or output reactor is mandatory if the cable exceeds 50 m, per the PM240-2 manual.
5. Manufacturing defect on the PCB. A 132 kW drive is a long-tail product and some early production runs had sub-spec snubber caps. Siemens tracks these by MLFB suffix and serial range in the support knowledge base.

If the cause was an upstream event (1, 2, 3, or 4), repairing only the visible damage and powering up without addressing the root cause will blow the new caps again. The drive must go back to an authorized Siemens repair center for:

• Root-cause analysis with the original failure data downloaded from the SD card or memory module (p0964, fault buffer r0945-r0949)
• Replacement of the entire control board, not just the two visible caps
• Replacement of the gate-driver board (often co-damaged and not visible)
• Re-coating and HiPot testing of the PCB
• Re-commissioning with the original parameters from the backup file
• Re-validation of Safety Integrated STO function (mandatory after any safety-relevant component swap)

11. Post-Repair Commissioning Checklist for the Replacement Unit

When a refurbished or replacement drive arrives from Siemens, follow this sequence before energizing the motor:

1. Verify MLFB and serial. Confirm firmware version matches what was on the failed unit, or update to the latest Siemens-approved Hotfix for V4.7 (the current HF for V4.7 is in the support database - search by MLFB).
2. Verify input is true 3-phase 380-480 V +/-10% on L1, L2, L3 with a DMM. Measure line-to-line and line-to-neutral; they should be balanced within 2%. Verify phase rotation (L1-L2-L3 clockwise for default forward direction).
3. Verify the motor cable length, gauge, and shielding match the EMC installation guide. A 132 kW PM240-2 driving a 50 m motor cable without output reactors or dU/dt filters is a common overstress path and a probable contributor to the original cap failure.
4. Check the brake chopper / brake resistor sizing. A compressor application must have a brake resistor rated to absorb the full regenerative energy of the motor under backspin. Verify p0219 and p0218 (brake chopper threshold and duty) are correct for the resistor fitted.
5. Insert the SD card with the parameter backup, or use Startdrive to upload the project file. Verify p0015, p0700, p1000, p1500 match the project as-commissioned.
6. Run the commissioning wizard on the IOP-2 or BOP-2. This will set p1900, p1910, p1960 to the correct values and prompt for the motor data. Re-verify the motor nameplate data on p0304 (rated voltage), p0305 (rated current), p0307 (rated power), p0311 (rated speed), p0314 (motor pole pair number).
7. Run the motor identification with the motor decoupled from the load (compressor coupler disengaged). This is critical - identifying against a loaded compressor skews the inertia measurement and produces a poorly tuned speed controller. The compressor must be free to spin and the rotating ID p1910 must be allowed to run to completion.
8. Test STO by removing 24 V from X1.13 / X1.15 (F/S variants) or the STO-assigned DI (basic CU). The drive should immediately drop to safe state with the SAFE LED on. Restore 24 V and acknowledge. The STO function is mandatory to test on every unit, every restart, per EN 61800-5-2.
9. Test the emergency stop chain end-to-end from the safety relay output to the drive terminal. Verify the response time is within the safety function specification (typically < 20 ms for STO on SINAMICS G120).
10. Run the drive in HAND mode from the BOP-2, then in AUTO from the PLC / control source, verifying direction and setpoint scaling.
11. Capture the parameter set with p0971 = 1 to the SD card and store a copy off-drive. This is the live 'as commissioned' backup that you restore on the next PCB event.

12. Parameter Backup and Restore Strategy

SINAMICS G120 supports three backup paths, and using all three is the difference between a one-hour recovery and a one-day re-commission.

After any PCB repair or any firmware update, immediately do a triple backup: SD card, Startdrive upload, and paper printout of p0015, p0700, p1000, p1500, p1900, p1910, p1960, p0304-p0311 (motor nameplate), p1082 (max speed), and the user-defined parameter set (p0010 = 0 menu, p0003 = 3 user access). The paper printout survives the next cap blow when the electronics do not.

13. Common Fault Codes for 'No Run' on SINAMICS G120

These are the fault codes most likely to surface in a 'ready but no run' condition on a PM240-2 with CU240E-2. Refer to the SINAMICS G120 List Manual for the full list and the response value interpretation.

None of these are present in the current case (r0945 = 0, r0947 = 0, r2110 = 0, r9772 = 0xFFFF), which is why the RDY LED is steady green. The drive is not faulted; it is waiting.

14. Field Notes and Lessons Learned

• A steady green RDY LED with no fault is a command source problem, not a hardware problem, in approximately 95% of field cases. Start with r0046 and r9772 before opening the drive.
• PCB-level repair on a Siemens drive is almost never the right answer. The visible damage is the symptom; the PCB itself is the casualty. Replace the board at an authorized repair center.
• The 'two-phase 400 V' test feed in this case is a serious red flag. A 132 kW PM240-2 is a 3-phase device; do not connect L1/L2 only. If the only available test supply is single-phase, return the drive to a site with 3-phase, or use a 3-phase rotary or static phase converter sized to at least 250 A per phase for the FSE frame.
• The RDY LED state (steady vs 2 Hz flash) is the single most useful signal in the field. 2 Hz fast green = motor ID running. Slow 0.5 Hz green = BOP-2 local HAND mode active. Steady green = command-source problem. Steady red = fault. Flashing red = alarm. The flash rate is a 1-bit oscilloscope for the entire drive state.
• Always record p1900 / p1910 / p1960 before touching the drive. After a factory reset these revert to 0, and re-running the ID against the actual motor is the only way to recover correct performance. A 'ready' drive that has been p1910 = 0'd will start but will run on default motor parameters until the next maintenance window.
• The CU240E-2 has two STO channels; both must be released independently. A single-channel release is reported as a fault (F01611 or F30611), but a held channel with no fault and no release action is invisible except in r9772 / r9872. Always read these two parameters on a 'no run' complaint.
• Macro p0015 and command source p0700 are independent. p0700 selects command source (BOP-2, terminals, fieldbus, IOP). p0015 selects terminal assignment macro. Mixing them is the most common commissioning error after a parameter load.
• For a compressor with quadratic torque, the most common macro is p0015 = 3 (pump/fan) or p0015 = 5 (pump/fan with analog setpoint) with p1300 = 2 (sensorless vector) or p1300 = 6 (quadratic V/f). The drive will not energy-optimize a compressor on p1300 = 0; use p1300 = 2 or 6 for efficiency.
• Switch-on inhibit (r0052 bit 6) is a one-shot lockout that survives parameter changes. After any power-up on a freshly repaired drive, OFF1 must see a 0 to 1 edge before the inverter leaves the inhibit state. This is a frequent cause of 'no run after power-up' and is not a fault.
• A compressor that backspins on shutdown is regenerative. Without a brake resistor, the regen energy charges the DC bus above 770 V and the drive trips F30003. A 132 kW compressor on a 100 hp drive typically needs a 30-50 kW peak brake resistor, sized to the motor's rotational inertia at the worst-case backspin speed.

15. Diagnostic Matrix Summary

FAQ