SINAMICS G150 vs S120: Master-Slave Drive Selection Guide

1. Application Context: DC-to-AC Retrofit with Master-Slave Coupling

Rotating heavy industrial machinery such as rotary cement kilns, rotary dryers, and large mills have historically been driven by DC motor pairs in a master-slave arrangement. A typical configuration is two 235 kW DC motors mechanically coupled to the same pinion or girth gear, where one motor (master) regulates speed and the other (slave) regulates torque, with a fast torque or speed equalising channel between the two armature converters. Replacing this drive train with asynchronous (induction) motors requires an equivalent electronic master-slave coupling, because the kiln's mechanical inertia and gear backlash make a single-motor arrangement impractical above roughly 250 kW.

Siemens offers two cabinet-format drive families suitable for this class of retrofit:

• SINAMICS G150 – a single-axis cabinet drive, one Control Unit (CU) per drive, one incoming line connection per drive, vector control of asynchronous machines only.
• SINAMICS S120 – a modular multi-axis drive system available in chassis and cabinet formats, with a single shared Control Unit, a common Line Module (infeed), and Drive-Cliq (DRIVE-CLiQ) interconnection between the Motor Modules.

The decision between G150 and S120 is governed by the type of master-slave coupling required (load sharing, speed synchronism, position synchronism), the available cubicle footprint, the line-side short-circuit power, the available DC bus structure, and the willingness to add PROFINET/Industrial Ethernet hardware (CBE20) for drive-to-drive communication. The remainder of this guide maps those constraints to a deterministic selection.

2. SINAMICS G150 Architecture and Capabilities

The SINAMICS G150 is a ready-to-connect cabinet drive for single-axis asynchronous motor applications. Each G150 cabinet contains a single power section rated for the motor's apparent power, an integrated line filter (where fitted), an optional braking unit, and a CU320-2 Control Unit. The cabinet has its own line-side incoming feeder, so two G150 cabinets in a master-slave configuration are essentially two independent drives with a peer-to-peer communication overlay.

Relevant characteristics for retrofit work:

• Vector control (closed-loop with encoder) and V/f (open-loop) modes are supported; sensorless vector control is also available for applications that do not require encoder feedback.
• Only asynchronous induction machines are supported; permanent-magnet and synchronous-reluctance machines are not licensed on G150.
• Drive-to-drive communication requires PROFINET or PROFIBUS hardware. The CBE20 Communication Board Ethernet is the standard option for SINAMICS Link peer-to-peer exchanges; OALINK (an Ethernet-based drive-to-drive protocol) is an alternative on CU320-2 firmware from V4.7 onwards.
• Master-slave on G150 is implemented at the application level, not in firmware: the integrator must wire the torque setpoint (or speed setpoint with droop) from the master to the slave and configure the DCC (Drive Control Chart) or free-function blocks accordingly.

Catalog and engineering references:

• SINAMICS G150 Catalog (Siemens Industry Mall)
• Application: Load sharing for two coupled drives – SINAMICS G (entry 109774232)
• SINAMICS Engineering Manual V6.6 (Feb 2020)

3. SINAMICS S120 Architecture and Capabilities

The SINAMICS S120 is a modular drive system. In its cabinet format (S120 Cabinet Modules) the system is built from a Line Module (Active Line Module, Smart Line Module, or Basic Line Module), one or more Motor Modules, optional Braking Modules, and a CU320-2 or CU320-3 Control Unit. All Motor Modules share a common DC link and are controlled by a single Control Unit, which means cross-coupling between axes is realised in firmware rather than over a communication bus.

Relevant characteristics for retrofit work:

• One Control Unit controls both Motor Modules. The cross-coupling block (torque share, speed synchronism) lives inside the same CPU, eliminating communication jitter between the two drives.
• Drive-Cliq (DRIVE-CLiQ) is the internal point-to-point fibre link between the CU, the Line Module, each Motor Module, and the encoder/Sensor Modules. This is not Ethernet; it is a Siemens proprietary ring with deterministic cycle times down to 125 µs at the current controller.
• Asynchronous, synchronous (including permanent-magnet), and reluctance machines are supported.
• Master-slave functions are available as firmware function blocks (DCC and DCB extension); no external communication is needed for tight coupling.

Catalog and engineering references:

• SINAMICS S120 Catalog (Siemens Industry Mall)
• Application: SINAMICS S DCC load sharing (entry 38470057)
• Application: SINAMICS S120 synchronism with DCB extension (entry 109757232)

4. Side-by-Side Architecture Comparison

5. Master-Slave Operating Modes: Load Sharing vs Speed/Position Sync

Master-slave has three distinct meanings in drive engineering. Selecting the wrong one is the most common reason for poor load sharing or for instabilities (hunting, anti-phase oscillation) in a coupled drive pair.

For a rotary cement kiln the appropriate mode is torque coupling with the master in speed control and the slave in torque control. The kiln shell's mechanical time constant is in the order of seconds, so a 1–4 ms SINAMICS Link cycle is more than adequate. Position synchronism is not required; angular alignment of the pinion to the girth gear is a one-time mechanical alignment.

6. Communication Paths: CBE20 SINAMICS Link, OALINK, Drive-Cliq

6.1 CBE20 + SINAMICS Link

The CBE20 is a PROFINET-capable plug-in Communication Board for the CU320-2. SINAMICS Link is a peer-to-peer protocol layered on top of the CBE20 that allows up to 64 participants to broadcast and receive drive telegrams on a defined cycle (1 ms default, 0.5 ms and 0.25 ms on newer firmware). Up to 16 PZD words can be exchanged per participant per cycle. This is the standard recommendation for G150-to-G150 master-slave coupling.

Topology example (two G150 in torque coupling):

   PLC (optional)        CBE20 (master)        CBE20 (slave)
   ---|PROFINET|---      ---|SINAMICS|----     ---|SINAMICS|
       |                     |                      |
   G150 Master            CU320-2                CU320-2
   (speed ctrl)           PZD-OUT: M_torque      PZD-IN : M_torque_set
                          PZD-IN : n_set         PZD-OUT: n_actual

6.2 OALINK

OALINK is a real-time Ethernet protocol used in SIMOTION and SINAMICS applications. It requires a SIMOTION controller or an S120 with the appropriate firmware option; on a G150 it is generally not licensed for the same scope as SINAMICS Link. For a pure G150 pair, prefer SINAMICS Link.

6.3 Drive-Cliq (S120 internal)

Drive-Cliq is not a network – it is a ring of point-to-point fibre connections between the CU, the Line Module, each Motor Module, and the Sensor Modules. In an S120 multi-axis cabinet, the master and slave Motor Modules are linked to the same CU by Drive-Cliq, so the torque and speed setpoints are exchanged inside the CU firmware without any bus cycle. Latency is effectively the current-controller cycle (125 µs for chassis units).

7. Selecting the Right Topology for a Rotary Cement Kiln

The decision tree below is a deterministic recommendation; deviations must be justified on paper and signed off by the kiln mechanical designer.

Use G150 when:

• The two motors are electrically and mechanically independent at the cabinet level (each drive has its own line feeder, transformer, and protective earth).
• The control room PLC is already PROFINET-based and the existing SCADA/PLC has spare connection points for two CBE20s.
• The drive-to-drive communication requirement is load sharing with droop only; no fast torque cross-coupling is required.
• Future maintenance requires that one drive can be physically removed and the kiln run on a single motor for short periods.

Use S120 when:

• The two motors are powered from a shared DC link and the line infeed is sized once for the combined apparent power.
• Sub-millisecond torque cross-coupling is needed (typical for kilns above 600 kW per motor, where the mechanical time constant shrinks).
• Future expansion to three or more coupled motors is anticipated; S120 scales linearly with additional Motor Modules.
• Position/speed synchronism beyond a single ratio is required, e.g. if the kiln is part of a multi-station process with shared product angle.

8. Sizing the Replacement Asynchronous Motor

When replacing a 235 kW DC motor with an asynchronous motor of the same shaft power, the apparent power at the inverter output is roughly 1.05–1.15× the DC rating due to power factor (typically 0.85–0.92) and inverter harmonics. Use the formulas below to derive the drive frame size; the DC bus voltage, line voltage, and current type must be confirmed against the nameplate before ordering.

8.1 Three-phase line current (RMS, sinusoidal equivalent)

kVA = sqrt(3) * V_LL * I_line / 1000
I_line = (P_mech * 1000) / (sqrt(3) * V_LL * eta_motor * pf)

For a 235 kW, 400 V three-phase motor with η ≈ 0.96 and pf ≈ 0.88:

kVA = 235 / (0.96 * 0.88) = 278 kVA
I_line = (235 * 1000) / (sqrt(3) * 400 * 0.96 * 0.88)
       = 235000 / 584.6
       = 402 A

This places the drive in the SINAMICS G150 / S120 400 V frame size of approximately 450 A (frame F-G on S120 chassis, 6SL3710-1GE41-0AA0 family). Verify against the published type list for the firmware version installed.

8.2 Single-phase current (if motor is single-phase – not typical for kilns)

kVA = V * I / 1000
I   = (P_mech * 1000) / (V * eta_motor * pf)

8.3 Derating for inverter output

SINAMICS drives are typically rated for 1.5 × rated current for 60 s and 1.36 × for 240 s (overload capability). Kiln starting torque is rarely above 1.2 × rated once the shell is rotating, so no additional derating is required, but the user-configurable pulse frequency (default 2 kHz on G150) must be checked against the motor insulation class.

9. Firmware, DCC, and DCB Extension Requirements

The DCC (Drive Control Chart) is a Siemens graphical function-block environment that runs inside the drive firmware. Typical DCC blocks for master-slave include:

Slave speed limiter:
  IN:  n_set_master (PZD-IN via SINAMICS Link)
  IN:  M_torque_master (PZD-IN)
  IN:  n_actual_slave
  OUT: n_set_slave (to speed controller)
  OUT: M_set_slave (to torque controller)

  Logic:
    n_set_slave = n_set_master - droop_function(M_torque_slave)
    if |M_torque_slave| > M_max_slave: clamp M_torque_slave to M_max

The DCB (Drive Control Block) extension for S120 is a higher-level library with ready-made synchronism blocks (electronic gear, electronic cam, load-share). It is the recommended path for kilns above 400 kW per motor.

10. Commissioning Procedure

1. Pre-conditions. Confirm motor and encoder nameplate data are entered into the drive parameters (p0300 series). Encoder wiring verified on the Sensor Module. DRIVE-CLiQ topology printed from the drive's online diagnostics.
2. Motor identification. Run the automatic motor identification routine (p1910) on each drive with the motor uncoupled mechanically from the kiln. This measures the equivalent circuit of the asynchronous motor and writes r0062, r0063 magnetising current values back to the drive.
3. Speed controller optimisation. Run the speed-controller auto-tuning (p1960) for each drive independently. Capture the proportional gain (p1460) and integral time (p1462) values; record for the load-share commissioning step.
4. Cable test. Verify the SINAMICS Link cables between the two CBE20s (or the Drive-Cliq fibres between Motor Modules) by sending a test telegram with parameter p2051[0] = 0 and observing receipt on the partner drive.
5. Master-slave activation (G150 path). Configure p2051 / p2061 on master to broadcast M_torque actual (r0031) on SINAMICS Link slot 0. Configure p2050 / p2060 on slave to receive this into the torque setpoint path. Set p1500 on slave to the received torque value with sign reversal (slave is mechanically on the same shaft, opposite rotation sense is impossible – the slave rotates in the same direction as the master).
6. Master-slave activation (S120 path). Instantiate the DCB load-sharing block. Link master torque actual to slave torque setpoint input. Enable the load-sharing block via p21000 = 1.
7. Droop and bias. Apply a small droop coefficient (typical 0.5–2.0 % at rated torque) on the master. This guarantees stable operation if the communication channel is briefly interrupted; the slave falls back to its own speed controller with a small steady-state speed error that the process can absorb.
8. No-load rotation test. Run both drives uncoupled to the kiln. Verify the slave tracks the master speed setpoint with < 2 rpm error at all speeds from 10 % to 100 %. Verify torque actual on both drives is < 5 % of rated at no load.
9. Loaded rotation test. Couple the drive train to the kiln. Ramp the master setpoint to 10 % rated speed. Check the master-slave torque split is within ±5 % of the commanded 50/50 share.
10. Communication-loss fallback. Disconnect the SINAMICS Link cable. Both drives should trip with F08501 (sign-of-life failure) and the kiln should stop in a controlled ramp-down. Reset and verify the system can be re-armed.

11. Verification and Acceptance Tests

12. Troubleshooting Matrix

13. Frequently Asked Questions