SINAMICS S120 Line Module on Two Phases: DC Bus & Faults

Problem Summary

Connecting a SINAMICS S120 Basic Line Module (BLM) or Smart Line Module (SLM) with only two of the three required line conductors (L1, L2, L3) instead of a balanced three-phase 400 V AC supply produces three independent failure modes simultaneously:

1. The uncontrolled 6-pulse diode bridge (BLM) or the IGBT bridge (SLM) only commutates over two phases, behaving as a single-phase full-wave rectifier. The DC link collapses from the nominal 540 V to approximately 360 V.
2. The phase-failure monitor in the SINAMICS Control Unit detects the missing third phase and issues a phase-loss fault within the first line cycle.
3. The precharge path of the SLM and of any Line Connection Module cannot close its charging contactor because the DC link never reaches the closing threshold (default 80 % of nominal). The downstream Motor Module is therefore left discharged and any drive enable is rejected.

For a small test bench running a low-power servomotor, none of these failure modes can be cleared by parameter manipulation. The trip is a hardware-level detection on the Voltage Sensing Module (VSM10) or the integrated line voltage monitor, and it exists to protect the line filter, the Active Interface Module, and the connected Motor Modules from inrush currents and unstable DC link operation. The full SINAMICS S120 Line infeed manual documents the line-side hardware required.

Three-Phase Rectifier Theory and the 1.35 Multiplier

A six-pulse diode bridge (the heart of every SINAMICS Basic Line Module) rectifies a balanced three-phase AC line by sequentially switching six diodes in pairs. Each pair of diodes conducts for 60 electrical degrees, producing a six-step DC waveform. The arithmetic mean of this waveform defines the no-load DC link voltage:

V_dc = (3·√2 / π) · V_L-L ≈ 1.35 · V_L-L

For a 400 V three-phase line this gives 540 V, the value printed on the rating plate of any 400 V class SINAMICS BLM (for example 6SL3130-1TE22-0AA0, 10 kW, 400 V).

The 1.35 factor is the integration of six sinusoidal half-segments over one line period. If a single phase is removed, only four of the six diodes can ever be forward-biased. The bridge degenerates to a single-phase full-wave rectifier driven by the line-to-line voltage of the two remaining phases:

V_dc,2φ = (2·√2 / π) · V_L-L ≈ 0.90 · V_L-L

For 400 V line-to-line this yields 360 V. The missing 180 V is the reason every drive enable on the test bench fails: the SINAMICS firmware will not release the inverter gates when V_dc < 0.85 · V_dc,nom.

Worked Calculation: 6SL3130-6AE21-0AA0 on 400 V 3AC, L3 Open

What Happens Inside the Basic Line Module

The BLM is an uncontrolled diode bridge followed by a line filter capacitor bank. Relevant part numbers in the S120 booksize format include:

• 6SL3130-1TE21-6AA0 (5 kW, 400 V, booksize)
• 6SL3130-1TE22-0AA0 (10 kW, 400 V)
• 6SL3130-1TE23-5AA0 (20 kW, 400 V)
• 6SL3130-1TE24-0AA0 (40 kW, 400 V)

The internal signal flow is:

L1, L2, L3 → 6-pulse diode bridge → DC link capacitor → DC bus bars to the Motor Modules

When L3 is left open, the diodes in the L3 half-leg of the bridge never conduct. The remaining four diodes form a full-wave bridge across L1 and L2. Output ripple increases from 4.3 % (six-pulse) to 48 % (single-phase full-wave), and the DC average drops to 0.9 · V_L-L.

Critically, the BLM does not contain an integrated precharge circuit. The DC link capacitor of the downstream Motor Modules is charged through the BLM's own diodes at the instant of contactor closure. With only two phases, the inrush current is lower, but the residual DC link is below the undervoltage threshold, so the Motor Module will not complete its own precharge handshake. It reports F07802 "Precharge monitoring time expired" (firmware V4.x) or A06005 "Line supply fault" (firmware V5.x) depending on the version of the SINAMICS List Manual installed.

What Happens Inside the Smart Line Module

The SLM is built around an IGBT bridge that supports energy recovery into the line (regenerative operation). Booksize part numbers:

• 6SL3130-6AE21-0AA0 (5 kW, 400 V)
• 6SL3130-6AE22-0AA0 (10 kW, 400 V)
• 6SL3130-6AE23-5AA0 (20 kW, 400 V)
• 6SL3130-6AE25-0AA0 (50 kW, 400 V)

The SLM has a precharge path with two anti-serial thyristors and a precharge resistor that closes for approximately 1.5 s after power-on. The precharge path monitors the DC link voltage through the integrated VSM10 (Voltage Sensing Module) and latches the main contactor when V_dc exceeds 80 % of V_dc,nom. With two phases present:

• The precharge circuit still operates, but the DC link stabilizes at ≈ 360 V.
• The 80 % closing threshold is never met (80 % of 540 V = 432 V).
• The SLM reports F07802 "DC link precharge aborted" and the main contactor remains open.
• The optional Active Interface Module (AIM, 6SL3100-0BE2x-2AB0) does not close its line filter contactor and reports F30002 "DC link voltage precharge" on the CU320-2.

If the precharge threshold is bypassed by an external 24 V interlock, the IGBTs can be gated, but the DC link ripple trips the Line Module firmware on F30004 "Line filter overtemperature" or F30005 "Line supply phase missing" within 200 ms.

Active Line Module Comparison

The Active Line Module (ALM), part number 6SL3130-7TE2x-xxxx for the booksize range, is the only member of the S120 Line Module family that is theoretically tolerant of brief line disturbances. It uses an IGBT bridge with closed-loop sinusoidal current shaping and can hold the DC link at 600 V (default) or 720 V (alternative) even when the line voltage sags. However, the ALM is not designed to operate with a missing phase. The integrated VSM10 measures all three line-to-line voltages; a missing phase is reported as F06310 "Line voltage asymmetry" on firmware V5.x within 100 ms. The ALM will not be a workaround.

Phase-Loss Detection and the Associated Fault Codes

Phase-loss detection is performed in three independent places, which is why a single parameter work-around is never sufficient:

These fault codes are documented in the SINAMICS S120/S150 List Manual, which is the authoritative reference for firmware-revision dependent fault code numbers. The same physical condition can map to different code numbers between V4.5, V4.7, V4.8 SP1, and V5.2 SP1 firmware.

A partial parameter mapping (Firmware V5.2 SP1, HF17) for the line-side monitoring on a CU320-2 PN:

Setting p0210 = 230 V (single-phase line-to-line equivalent) and accepting the 360 V DC link is theoretically possible but disables p3451 and triggers F30005 from the VSM10 asymmetry detector.

Why the DC Link Drops to ~360 V on Two Phases

A numerical example using a SINAMICS 6SL3130-6AE21-0AA0 SLM (5 kW, 400 V) on a 400 V three-phase source with L3 disconnected:

• V_dc,3φ = 1.35 · 400 = 540 V (no load)
• V_dc,2φ = 0.90 · 400 = 360 V (no load)
• V_{dc,ripple,3φ} = 4.3 % peak-to-peak (23 Vpp at 300 Hz dominant)
• V_{dc,ripple,2φ} = 48 % peak-to-peak (173 Vpp at 100 Hz dominant)

Under even a few hundred watts of load on a 5 kW SLM, the DC link collapses further. The single-phase line-to-source impedance of a laboratory 400 V outlet is typically 100-300 mΩ, which gives 5-15 % additional droop at 10 A DC load. With the small servomotor test load (typically 50-200 W), the measured DC link will sit at 330-350 V.

This is below the SINAMICS minimum operating point:

• p0210 = 400 V → V_dc,nom = 540 V → p0280 = 459 V (undervoltage trip)
• p0282 = 432 V (precharge closing threshold)
• p0281 = 621 V (overvoltage trip, set with line filter reactor)

A DC link of 360 V fails both the undervoltage and precharge thresholds simultaneously.

Safe Test-Bench Alternatives

For a small servomotor test setup that only needs a few hundred watts and where a 400 V three-phase outlet is not available, the following configurations are supported by SINAMICS and preserve all safety monitoring:

1. 1-phase AC supply on a 200 V class SINAMICS. The 1AC 230 V BLM (for example 6SL3130-1TE13-0AA0 for 1.6 kW, 200 V) is designed for single-phase 200-240 V operation. The DC link settles at 0.9 · 230 = 207 V and all thresholds in p0210 / p0280 / p0281 / p0282 auto-scale to the 200 V class.
2. 1-phase supply on a 400 V class BLM with a step-up autotransformer. A 1 kVA Variac 0-270 V stepped up to 400 V. The DC link reaches 360 V, which is below the 400 V class thresholds, so the SINAMICS will still fault. This is therefore not a viable work-around.
3. 3-phase variable transformer (Three-Phase Variac). A 0-450 V three-phase Variac provides the missing third phase at any desired amplitude. This is the recommended test-bench solution when the 400 V mains is single-phase or unavailable.
4. DC link from a controlled laboratory DC supply. Set the supply to 540 V / 5 A, connect to the DC bus bars of the S120 with a fuse and precharge resistor. Suppress line-side monitoring by setting p0221 = 0. The SINAMICS will then run from an external DC bus, which is a standard commissioning scenario for SINAMICS cabinet modules (chassis format).
5. Substitute a 1AC 230 V drive such as SINAMICS V20, V90, or S210 for the small servomotor test. V90 part numbers 6SL3210-5FE2x-xxxx accept single-phase 230 V and drive 1FL6 servomotors from 50 W to 7 kW.
6. Add a braking chopper and resistor on the DC link if the test load is regenerative. The 6SL3100-1BE2x-2AB0 chopper modules work with both the BLM and SLM and limit the DC link to 750 V during regeneration.

Step-by-Step: Building a Safe Two-Phase Test Bench

If a three-phase Variac is not available and the test load is purely a small low-inertia servomotor, the most reliable workaround is to use a 200 V class SINAMICS instead of the 400 V class. Procedure:

1. Identify the motor. Check the motor's rated voltage on the rating plate. A 1FL6 0.05 kW servomotor is typically wound for 220 V delta, so it can run from a 200 V class inverter.
2. Select a 200 V class BLM. For the 1FL6 0.05 kW motor use 6SL3130-1TE13-0AA0 (1.6 kW, single-phase 230 V).
3. Set p0210 = 230 in the SINAMICS project. This is the default and matches the nameplate.
4. Connect the laboratory 230 V single-phase outlet to L1 and L2 of the BLM. Do not connect L3; the 200 V class BLM is designed for 1AC 200-240 V operation and the 6-pulse bridge tolerates the missing L3.
5. Power on. The VSM10 detects single-phase operation, sets p0221 = 0, and reports no fault. The precharge path closes within 1.5 s.
6. Verify the DC link with the Starter / Startdrive commissioning tool. Expected value 207-215 V. Below 195 V the undervoltage alarm triggers; above 230 V a 1AC 270 V supply is required.
7. Enable the drive and run the motor. Use p1120 / p1121 to set a 3 s ramp-up / ramp-down to limit inrush on the 1AC line.

Verification Checklist

Before any reduced-supply test is started, verify each of the following:

If any of these checks fails, the test must not be started. Reconnect the missing phase or replace the supply with a true three-phase source.

Frequently Asked Questions