Siemens G120 Common DC Bus Sharing Regenerative Energy Between

Overview

Connecting the DC bus of two SINAMICS G120 drives allows a motoring drive to absorb regenerative energy produced by a braking drive, eliminating most of the heat normally dissipated in a brake resistor. In a vibratory conveyor with two mechanically coupled agitators running at slightly different frequencies (e.g. 49.5 Hz and 50.5 Hz), one agitator is always accelerating the load and the other is always retarding it. With a common DC bus, the retarding drive's inverter section returns energy to the bus, and the driving inverter pulls it back out — net braking energy is recycled instead of converted to heat in the external brake chopper resistor.

This reference covers the SINAMICS G120 modular drive (Control Unit + Power Module PM240/PM240-2/PM250) wired into a common DC bus, the protection topology required for safe paralleling, parameter adaptations on the Control Unit, and a commissioning and verification sequence suitable for industrial vibratory conveyor duty.

Application Context: Differential-Frequency Vibratory Conveyor

A linear or circular vibratory conveyor typically uses two out-of-phase eccentric weights (agitators) driven by induction motors. By phase-shifting the two motors electronically, the conveying direction and amplitude can be tuned without mechanical adjustment. A common technique is to set one drive to 49.5 Hz and the other to 50.5 Hz. Because the two motors are mechanically coupled through the conveyor trough, the actual mechanical speed locks to an average value, and a phase angle delta_phi develops between the two shafts.

When delta_phi is non-zero, one agitator's eccentric force has a component in the direction of motion and the other has a component opposing motion. The "opposing" agitator's motor therefore operates as an induction generator, returning power to its VFD. Over each electrical cycle the average power flow is:

• P_motor (driving side) = T_load * omega_m + losses
• P_gen (braking side) = -T_load * omega_m * sin(delta_phi_term)

where delta_phi_term is the torque-producing phase displacement between stator flux and rotor flux. The mismatch (driving losses + P_gen) is dumped into the brake resistor by the standard G120 chopper. In a small conveyor the continuous braking power may be only 50-300 W, but the resistor still has to be sized for continuous dissipation, not peak — which is why a standard 200 W / 100 % ED chopper resistor overheats.

Common DC Bus Architecture on SINAMICS G120

The PM240 and PM240-2 Power Modules expose DC link terminals DC+ (terminal 1 or DCP depending on frame size) and DC- (terminal 2 or DCN) directly on the input rectifier's output. The internal precharge resistors, DC bus capacitors, and brake chopper (if fitted) sit between these terminals. Connecting two PM240 modules in parallel creates a shared DC link, electrically equivalent to a single, larger DC bus feeding two inverter sections.

For a 400 V class system, the unloaded DC bus sits at approximately sqrt(2) * 400 V = 565 VDC. Under load the bus rises to ~750-800 VDC at full motoring, and the brake chopper activation threshold is ~770 VDC by default. Connecting two units in parallel means both inverters see the same bus voltage, so the chopper threshold applies to both.

Wiring Topology

The reference wiring below assumes two PM240-2 Power Modules (e.g. 6SL3210-1PE22-7AL0, 7.5 kW / 400 V) and the G120B or CU240E-2 Control Units. Topology is a "Y" configuration with each drive fed from its own supply and tied only at the DC link:

1. Each PM240-2 is fed from its own 3-phase 400 VAC supply through a dedicated circuit breaker (e.g. 3RV2011-1KA10 or similar, sized to drive FLA).
2. Each PM240-2 has its own line reactor (or the integrated one on Frame size FSA-FSC) to limit precharge inrush and DC link circulating currents.
3. DC+ of drive A is connected to DC+ of drive B through a DC-rated semiconductor fuse or a DC-link contactor/breaker arrangement.
4. DC- of drive A is connected to DC- of drive B through an identical fuse or breaker.
5. A single shared brake chopper / resistor is acceptable, or each drive retains its own internal chopper and external resistor for redundancy.
6. The PE/protective earth of each PM240-2 is bonded to the cabinet PE bus, but the DC link negative is not bonded to PE in this topology (it is a floating DC bus).

Protection Requirements

Field experience documented in common-bus applications shows that improper protection is the dominant cause of failures. The minimum protection set is:

Semiconductor Fuses on the DC Link Ties

Standard AC line fuses are too slow. Use DC-rated semiconductor fuses (e.g. Ferraz Shawmut / Mersen A70QS or similar) with a melting I²t lower than the I²t of the smallest PM240-2 IGBT module. For 400 V class, 690 V DC rated fuses are required. Typical sizing is 1.5-2.0x the drive's rated DC bus current:

Anti-Circulating-Current Diodes

When two diode-rectifier drives are paralleled, slight differences in DC bus capacitor ESR and precharge state cause circulating currents that flow: rectifier A -> DC+ bus -> capacitor in B -> DC- bus -> capacitor in A -> DC+ bus. Over time this heats the bus capacitors and precharge resistors. The standard remedy is to insert a diode in the DC+ tie of each drive, oriented to conduct from that drive's rectifier to the shared bus, and to disable the second drive's rectifier by removing AC input (or use a DC-link-only "slave" drive fed exclusively from the bus). Reference: Yaskawa Common Bus and Line Regeneration white paper (WP.AFD.14).

Precharge Sequencing

Each PM240-2 contains precharge resistors (R_pre) and a precharge contactor/thyristor that closes after ~3 s of AC applied. If both drives are energized simultaneously, the second drive's precharge current is added to the first drive's bus, doubling inrush and possibly tripping the first drive's precharge fuse. Solutions:

• Sequence the AC feeds: close drive A first, wait 5 s (or until A is in RUN), then close drive B.
• Use a single shared 24 V control signal from a PLC to interlock the line contactors.
• Add an external precharge resistor in series with one drive's DC link tie, bypassed by a time-delay relay.

Parameter Configuration

On SINAMICS G120, the default commissioning assumes an isolated AC-fed drive. When two PM240-2 modules share a DC bus, the following parameters on each Control Unit (CU240E-2, CU250S-2, G120B) must be reviewed and adjusted via Siemens Industry Online Support (article ID 109751446 lists the G120 parameter list):

Use SINAMICS G120 List Manual (article ID 109751446) and Operating Instructions for SINAMICS G120 for the authoritative parameter list. Star commissioning (PC-based) with Startdrive V16 or higher; download parameter set to both Control Units; verify checksum p3981 matches.

Power Flow Calculation

Quantify the recovered energy to size wiring and verify thermal headroom. With the agitator pair locked mechanically at average speed omega_m and a torque-producing slip difference delta_omega:

P_gen (per drive, continuous) = T_brake * omega_m = T_load * (delta_omega / omega_rated) * omega_m

For a conveyor with motor rated at 1.5 kW / 50 Hz, T_rated = 1.5e3 / (2*pi*50/2) = 9.55 Nm. A 1 Hz slip delta (1/50 = 2 %) at T_load = 4 Nm:

P_gen = 4 Nm * (2 * pi * 49.75/2) = 4 * 156.2 = 625 W (continuous, per drive, while delta_f is held)

Conductor sizing on the DC link tie: I_DC = P_gen / V_DC = 625 / 750 = 0.83 A continuous. A 6 mm² / 55 A copper bus bar is grossly oversized for current but is standard for mechanical stiffness. Voltage drop is not a concern; I²R heating is the only loss to verify.

Compare to the equivalent brake resistor case: at 100 % ED the resistor must dissipate 625 W continuously, requiring at least a 1000 W resistor (typical 50 % derating for enclosed mounting). The common-bus topology removes this load entirely as long as the driving drive can absorb the regen current into its motoring load, which in this application it always can.

Commissioning Procedure

1. Verify the two PM240-2 Power Modules are the same frame size and firmware version (read r0018 in Startdrive). Mismatched firmware is a known cause of Vdc controller instability.
2. Confirm both Control Units are the same variant (e.g. both CU240E-2 PN, both with the same firmware per Siemens support).
3. Wire AC feeds and DC links per the topology above. Do not parallel DC- to PE.
4. Apply AC to drive A only. Use a multimeter to verify DC bus rises to ~565 V (unloaded). This confirms precharge and rectifier operation of A.
5. Apply AC to drive B with A still on. Verify DC bus voltage stays at ~565 V, and current draw on A's input does not spike above 2x the precharge inrush of B alone.
6. Set both drives' motor data (p0300-p0350) for their respective agitator motors. Run each drive independently with its default Vdc controller enabled; verify normal operation and that the brake chopper does not fire at 50 Hz no-load.
7. Apply the parameter table above. Set the slave (B) to disable Vdc controller (p1240 = 0) and disable chopper (remove chopper resistor from B if B is the slave, or set p1245 above the maximum possible bus).
8. Run both drives unloaded at 50 Hz. Use Startdrive trace to record r0026 (DC bus voltage) and r0080 (torque actual) for both drives simultaneously. Bus should be stable at 750-770 VDC with both running.
9. Introduce the differential frequency (49.5 / 50.5 Hz). Observe torque signs: drive A (49.5 Hz) should show positive torque (driving), drive B (50.5 Hz) should show negative torque (regenerating). DC bus voltage should not rise to the chopper threshold of 773 V.
10. Run for 30 minutes. Monitor both PM240-2 heatsink temperatures via r0037. Verify neither exceeds 80 °C. Check that the brake resistor remains cold.
11. Force an AC supply loss on drive A (the master). Verify drive B faults with F30002 (DC link undervoltage) and is taken to OFF3 — not destructive. Reset and re-energize in proper sequence.

Verification Checks

Troubleshooting Matrix

Design Considerations and Caveats

Servicing and isolation: A common DC bus cannot be serviced on a single drive without first isolating the DC link tie. Add a DC-rated disconnect switch or DC contactor (e.g. Eaton DILM series with DC-3 utilization category) on each tie. Reference the Rockwell Automation Drives in Common Bus Configurations (drives-at002_-en-p) for isolation best practices that translate directly to G120.

24 V control power: An undersized shared 24 V supply is one of the most common root causes of cascaded faults. If the supply sags during heavy switching, all drives on the bus may simultaneously register control undervoltage and attempt to discharge into the same DC bus. Use independent supplies sized to 2x the total Control Unit consumption.

Fault energy dissipation: A fault on a single inverter (e.g. IGBT short) dumps bus capacitor energy back into the fault. The fuse must clear before the second drive's capacitors back-feed through the tie. The fuse I²t must be lower than the parallel capacitor's stored energy divided by the minimum fault impedance. For a 750 V / 1000 µF capacitor bank, E = 0.5 * C * V² = 0.5 * 1e-3 * 750² = 281 J. The fuse must clear before 281 J is dissipated in the fault path — fast-acting semiconductor fuses at 690 VDC are required.

AC supply loss handling: If the master drive's AC supply is lost, the DC bus collapses and the slave drive trips on undervoltage. Coordinated shutdown is required: route a "line contactor closed" aux contact from each drive to the other's enable input, and configure p2100 to issue OFF2 on the loss of its own supply (using the internal monitoring of the rectifier via r0026 < 350 V).

Motor brake energy vs. bus capacitance: The bus capacitor of a PM240-2 stores 0.5 * C_dc * V_dc². For 7.5 kW frame, C_dc ≈ 330 µF total, E ≈ 0.5 * 330e-6 * 770² = 98 J. A single rapid stop from 50 Hz can return more than this from a high-inertia load; the chopper remains essential for emergency stops. Disable the chopper on the slave (where applicable) but keep it enabled on the master for transient absorption.

Alternate Architectures

Active Line Module (ALM) / Regenerative front-end: If the application has heavy continuous regeneration that exceeds the motoring load, install an SINAMICS G120 PM250 with regenerative line-side IGBTs, or a separate Active Line Module (6SL3130-7TE25-5AA3 family) sized to dump the excess back to the AC mains. This eliminates the brake resistor entirely but adds line harmonics filtering and cost.

Line regen unit (Bonitron-style):** A standalone line regen module tied to the DC bus returns energy to AC at the line voltage. This is appropriate when multiple drives share a DC bus and the total regen exceeds total motoring (e.g. a section of the machine where most axes decelerate simultaneously).

Hybrid chopper + bus sharing:** Leave both drives' choppers enabled but use a smaller resistor (e.g. 200 W peak) on each. The shared bus handles the continuous low-level regen from the differential-frequency conveyor; the choppers handle transients. This is the most robust topology and is recommended for field retrofits where the existing brake resistor must remain functional.

Verification Sign-Off Checklist

• All parameter changes documented in Startdrive project and uploaded to both Control Units.
• Firmware versions of both Power Modules and Control Units recorded (r0018, r1920).
• DC link fuse ratings and part numbers recorded in the maintenance log.
• 24 V control supply sized and verified: measured voltage under worst-case load > 22.5 V.
• Thermal run completed: 30 min at full differential frequency, both heatsinks < 80 °C, no chopper firing.
• Fault simulation: AC loss on master -> slave faults F30002 -> both drives safe torque off within 100 ms.
• Maintenance procedure: isolation of one drive from DC bus documented in EPLAN / circuit diagram.