Resolving F7404 Fault on SINAMICS G120 with Long Motor Cables

Problem Overview: F7404 on a Pump-Fed SINAMICS G120

A SINAMICS G120 drive built around a CU240E-2N control unit and a PM240 45 kW power module is configured for a centrifugal pump. The motor runs cleanly on the ramp, but each time the stop command is issued the drive occasionally drops with F07404 – Drive: DC link voltage monitoring Vdc_max. No external braking resistor is fitted. After a 500 m motor cable and a Schaffner three-phase output wave filter were added, the same F7404 fault still appears intermittently on both ramp-down and free-run entry, and in some installations even appears during acceleration.

F7404 is not a drive hardware failure. It is the Vdc-max watchdog inside the control unit detecting that the DC-link bus has been charged above its programmed threshold. On a pump with no external load torque pulling the motor backwards and no resistor to dump the energy, the only credible charging path is the cable/filter network that the drive must swing every PWM edge.

F07404 Fault Code Definition and Default Response

The default p1284 threshold is derived from p0210 (device supply voltage) and typically sits ~1.15 × sqrt(2) × V_line. On a 400 V three-phase line the threshold is therefore close to 740–760 V DC. Each PWM edge charges the cable capacitance; the filter stage also stores reactive energy. If the resulting bus pump-up exceeds p1284 for longer than the configured debounce time, F7404 trips and OFF2 is forced, removing the IGBT gate signals and letting the motor coast.

Why a 500 m Cable and Filter Trips Vdc-max

Three physical effects compound on long motor cables driven by an unfiltered IGBT bridge:

1. Distributed cable capacitance. A typical 4-core PVC motor cable of 3×16 + 6 mm² shows roughly 0.3–0.5 µF/km of charging capacitance between conductors and screen. At 500 m that is on the order of 150–250 nF of capacitive load per phase that the drive must alternately charge and discharge every PWM cycle. The stored energy per cycle is small, but the reactive current at 4 kHz switching is not.
2. Reflected-wave / transmission-line overshoot. When the cable surge impedance (typically 30–90 Ω) does not match the motor impedance, the leading edge of the IGBT pulse reflects at the motor terminals. With unfiltered cables the terminal voltage can reach 2× the DC bus, and with a sine/output filter in the path the filter inductance and cable capacitance form an LC tank that rings on every edge, raising the effective bus load.
3. Regenerative pump-back during ramp-down. Even on a centrifugal pump (which is normally a square-law load that decelerates naturally with speed) the residual momentum of the impeller and water column can keep the motor above synchronous speed for a few hundred milliseconds after the stop command. The drive cannot regenerate into the line through the diode bridge; that energy lands on the DC bus. Without a brake chopper and resistor, the bus climbs until p1284 fires.

Diagnostic Procedure Before Changing Parameters

Before raising p1284 or extending the ramp, capture data with Starter, Startdrive, or a BOP-2 to confirm the root cause. Always log the bus voltage, output frequency, output current, and drive status word simultaneously.

1. Verify the line voltage at the drive input terminals, not at the panel busbar. A 415 V tap rather than 400 V pushes the rectified bus to ~590 V and reduces headroom before p1284.
2. Read the actual Vdc value in r0026 just before the fault. If r0026 sits within 5 V of p1284, the trip is the cable/filter; if r0026 is 30–60 V higher, the supply is high or the brake chopper (if present) is miswired.
3. Inspect the filter rating plate. Confirm the filter is rated for the carrier frequency in use and for the cable length. A Schaffner FN 3410 / FN 3440 family sine filter is rated for 50 m of unshielded cable, and longer runs need a re-rated unit or a different topology.
4. Look for F07801 / F07802 / F07900 in the fault buffer. These signal IGBT or motor-side issues that can look like Vdc-max when the DC bus is in fact collapsing from overcurrent.
5. Record the drive status word r0002 at the moment of the trip. Status codes 0 (commissioning), 1 (ready), 4 (run), and 7 (fault active) tell you whether the trip happens during the run, the ramp-down, or only on a second start attempt.

Solution Path 1: Stop Behaviour and Ramp Tuning

For a centrifugal pump with no check-valve slam risk, the cleanest fix is to take the drive out of the energy-regeneration path entirely. The default OFF1 ramp-down tries to decelerate the motor at a fixed rate; if the load can keep the motor turning, the drive becomes a generator.

For many pump applications the most reliable answer is OFF2 coast-to-stop. Set the control word bit 1 source so that a stop command issues OFF2, the IGBTs are gated off, and the motor/impeller/water column decelerates naturally. F7404 cannot fire on a coast because no PWM edges are switching and no current is being drawn. Verify with the field wiring that the digital input assigned to OFF2 is fail-safe (use a normally-closed contact or wire-break detection).

Solution Path 2: Raise the Vdc-max Threshold Carefully

p1284 lets the integrator raise the trip ceiling, but it must stay below the PM240 hardware limit. For a 400 V class PM240 the DC bus capacitor and IGBT modules are rated to ~800 V continuous, with an absolute short-term peak of ~1000 V.

Solution Path 3: Reduce Switching Frequency

Carrier frequency is the dominant variable controlling the reactive current drawn by the cable capacitance. Halving the carrier roughly halves the cable charging current and the rate at which energy is dumped back into the bus on every edge.

For 2 kHz switching on a 45 kW PM240 the thermal derating is small. For 45 kW continuous the PM240-2 power module is rated to 4 kHz without de-rating; at 2 kHz the output current capability actually increases. The acoustic penalty is roughly 8 dB at 2 kHz versus 4 kHz, and that is exactly why the field fix is sometimes noisy.

Solution Path 4: Correct Filter Selection and Placement

The Schaffner FN 3410 / FN 3440 series output wave filters are sine filters designed to convert the IGBT PWM into a near-sinusoidal output. They are derated by cable length; fitting one and then running 500 m of cable is outside the design envelope of the smaller FN 3410 chassis.

1. Confirm the filter is the correct family for the cable. A true sine filter is sized for the load current and the carrier frequency, with a maximum motor-side cable length stamped on the nameplate. For 500 m, a dU/dt filter plus a separate reactor, or a Schaffner FN-series sine filter with a re-quoted cable length, must be specified.
2. Place the filter at the drive end, not at the motor end. The filter should sit as close as possible to the PM240 output terminals. Any cable between the drive and the filter is still stressed by the raw PWM waveform.
3. Use shielded, symmetric cable from the filter to the motor. A 3-core + symmetric earth (e.g., 2YSLCY-J) keeps the capacitive balance between phases and reduces the common-mode current that contributes to bus pumping.
4. Verify the filter does not introduce its own resonance with the cable. Sine filters are LC tanks. With a long cable the parasitic L and C of the cable shift the resonance. Lower p1800 in 0.5 kHz steps until the bus ripple measured in r0026 is below 4 % of nominal Vdc.

Solution Path 5: Add a Brake Chopper and Resistor

If the application cannot accept a coast stop and the bus keeps climbing, a brake chopper plus braking resistor is the only energy-dissipating solution. The PM240 45 kW frame accepts the integrated chopper that is fitted on PM240-2 FSA-G power modules. The optional braking resistor is sized to the load.

Approximate sizing for a pump ramp-down with a 30 s ramp from 50 Hz:

P_brake (W) = 0.5 × J_total × (ω_0² − ω_1²) / t_ramp
ω_0 = 2π × 50 = 314.16 rad/s
ω_1 = 2π × 5  = 31.42 rad/s
J_total = J_motor + J_pump + J_water (effective) ≈ 2 × J_motor for a centrifugal pump

For a typical 45 kW 4-pole motor (J ≈ 0.45 kg·m²) and a pump load, the total inertia is around 1.0 kg·m². The energy to be dissipated in a 30 s ramp is roughly:

E = 0.5 × 1.0 × (314.16² − 31.42²) ≈ 49 kJ
P_avg = 49,000 / 30 ≈ 1.65 kW continuous over the ramp

Add a 2× safety factor to cover pump reverse rotation and water-hammer events. A 3.3 kW continuous / 50 kJ pulse brake resistor from the Siemens 6SE70 / 6SL3200 family is a typical match. The chopper thresholds (p1236 / p1237 / p1238) are set to engage ~10 V above nominal Vdc and release ~5 V below.

Parameter Summary: Quick Reference Table

Commissioning and Verification Procedure

1. With the motor decoupled, run a no-load ramp from 0 to 50 Hz in 60 s and back. Capture r0026 (Vdc) at a 100 ms sample rate. Confirm Vdc never exceeds 0.95 × p1284.
2. Re-couple the pump. Issue a normal stop (OFF1) from 50 Hz and capture r0026 plus r0027 (output current). If Vdc stays below 0.95 × p1284, F7404 will not fire.
3. Force a worst-case: close a downstream valve so the pump runs against a closed head, then issue the stop. This is the realistic worst case for pump-back. Verify Vdc still inside the threshold.
4. If F7404 still fires, switch the stop command to OFF2 (coast) and re-test. For most pump applications OFF2 eliminates the trip entirely.
5. Record the r0947 fault code value at the moment of the trip (it carries the Vdc value at the trip point). Match this against p1284 to confirm the parameter is in fact the limiting one.
6. After the fix, leave a Starter / Startdrive trace running for 24 hours to catch intermittent trips. The pump may only pump-back hard during specific process events (valve closure, sensor flush, etc.).

Troubleshooting Matrix

Field-Proven Caveats

• 500 m is at or beyond the limit of any standard PM240 output filter. Field experience shows that reliable operation at 500 m usually requires moving the drive cabinet to within 50 m of the motor, or installing a dU/dt reactor plus a sine filter with a custom-engineered cable length. No parameter change alone will fix a 500 m unfiltered run.
• Schaffner FN 3410 / FN 3440 filters are silent at full load but add measurable loss (typically 2–4 % of drive rated kW) and they may derate the drive output current. Re-quote the drive current after the filter, not before.
• OFF2 coast is acceptable for centrifugal pumps but not for positive-displacement pumps because the moving fluid column can slam a check valve. For PD pumps, a brake chopper is mandatory.
• Motor insulation at 500 m is also stressed. A standard induction motor with 1000 V peak insulation rating is marginal with an unfiltered PM240; with a sine filter the peak is reduced to ~600 V and insulation life is preserved.
• Earth-leakage current rises with cable length and filter. A 300 mA RCD will nuisance-trip; a 500 mA or B-type RCD is required when the filter is in circuit.

Related Documentation

• SINAMICS G120 Operating Instructions (CU240E-2) – official parameter list including p0210, p1121, p1280, p1284, p1800.
• SINAMICS G120 List Manual – complete fault code reference for F07404 and related Vdc messages.
• SINAMICS G120 Function Manual – Vdc Control – background on the Vdc_max and Vdc_min controllers.
• SINAMICS G120 product page – configuration and selection tools including SIZER.