Configuring SINAMICS G120 Load Sharing for Coupled 22kW Motors

Configuring SINAMICS G120 Load Sharing for Coupled 22 kW Motors on Traversing Gantries

1. Application Overview

Traversing platforms and rail-mounted gantry cranes are commonly driven by two AC induction motors mounted on opposite sides of the moving structure. Each motor drives a pinion engaged with a rack or a wheel running on a rail. Because the rack or rail is rarely perfectly geometric, and because the two drive bases sit on independent structural members, the load on each motor is not naturally balanced. Mechanical skew, thermal expansion, and uneven wear mean that one motor will almost always see a slightly higher torque demand than the other.

For a 2 x 22 kW installation, the engineering question is therefore not whether one inverter can drive both motors electrically - it can - but how the drive system handles:

• Load sharing between mechanically coupled sides
• Position control (EPOS / SIMATIC S7-1200 technology object) on a multi-drive system
• Dynamic load disturbances without one motor stalling or losing synchronism
• Safety stop behavior (STO, SS1) across two independent drives

The reference application described here is a rail-mounted traversing platform / small railway carriage with two 22 kW squirrel-cage induction motors, rigidly coupled through their gearboxes, controlled by SINAMICS G: Load sharing for two coupled drives and an S7-1200 with EPOS.

2. Mechanical Prerequisites Before Any Drive Configuration

Load sharing on a gantry depends on the mechanical system more than on the inverter. Before configuring the drives, verify:

3. Drive Topology Options

3.1 Single Inverter, Two Motors (Parallel)

For very small motors (the original 1 kW case), a single SINAMICS G120 can run two motors in parallel because the relative load imbalance is small and the slip difference is absorbed by the rotor. For 22 kW class motors, this approach is generally not recommended:

• Both motors see identical frequency; load sharing is purely a function of slip and rotor resistance mismatch.
• Stall protection is shared - one motor can stall while the other runs normally.
• Thermal monitoring becomes ambiguous.
• No individual encoder feedback for position control on a single side.

Use single-inverter parallel operation only when:

1. Both motors are identical and cold.
2. The mechanical coupling between gearboxes is rigid.
3. Position accuracy is not required (open-loop V/Hz only).

3.2 Two Inverters, Rigid Mechanical Coupling

This is the recommended topology for the 2 x 22 kW traversing platform. Both SINAMICS G120 units operate in vector control (SLVC or VC with encoder) and exchange load information via PROFINET telegram 352.

3.3 Two Inverters, No Mechanical Coupling

Possible only if the two motors drive the platform through independent friction drives that can tolerate skew. Each inverter runs its own position loop and a supervisory controller arbitrates. This is a synchronism problem, not a load-sharing problem, and is outside the scope of this note.

4. SINAMICS G120 Load Sharing via Telegram 352

SINAMICS G drives support a native load-sharing protocol between two SINAMICS units that are mechanically coupled. The function is described in Siemens Application Note 109774232. The basic idea:

• One drive is configured as the master (position-controlled or speed-controlled leader).
• The second drive is the follower and receives the master's torque setpoint via PROFINET.
• The follower closes its own speed/torque loop on the shared mechanical axis.
• The two drives actively balance their torque contribution so neither motor is overloaded.

4.1 Telegram 352 PZD Layout

Telegram 352 is a Siemens-specific free telegram configured with SINAMICS Starter / Startdrive. A typical assignment:

4.2 Parameter Setup for Load Sharing

The following parameter set is a starting point; values must be tuned on site.

4.3 Wiring and Topology

Physical layer:

• SINAMICS G120 #1 (master) - PROFINET port X150 - PROFINET switch - S7-1200 CPU
• SINAMICS G120 #2 (follower) - PROFINET port X150 - PROFINET switch - S7-1200 CPU
• Direct PROFINET IRT between the two G120s is also supported, but for an S7-1200 controlled system, route the telegram through the CPU so that the load-share word is visible to the application.

Encoder wiring (both motors): HTL or TTL encoder to the G120 encoder interface (X521 on CU240E-2 PN, X521 on CU250S-2). Encoder ground must be tied to inverter PE at one point only.

5. Droop Function - Single-Fallback Option

If the application tolerates losing load sharing on a PROFINET failure, the droop function (parameter p1488 / p1489 in SINAMICS G120) can be used as a simpler alternative. Droop releases the follower speed setpoint proportionally to its torque output, allowing it to slip back slightly when loaded and recover when unloaded.

5.1 Droop Formula

The SINAMICS droop implementation uses a virtual speed offset:

n_offset = (M_actual / M_rated) * n_droop_max

where:
  n_offset      = virtual speed added to follower setpoint (rpm)
  M_actual      = current torque output (Nm)
  M_rated       = motor rated torque (Nm)
  n_droop_max   = p1489 scaling factor, default 0.5% of rated speed

Typical starting value for a 22 kW, 4-pole motor (M_rated ~ 140 Nm at 1470 rpm):

n_droop_max = 0.005 * 1470 = 7.35 rpm at full load

This 0.5% droop is enough to balance a 5-10% torque mismatch without measurable position lag.

6. Position Control with S7-1200 EPOS / Technology Object

The S7-1200 position controller is implemented as a technology object (TO) of type TO_PositioningAxis. The TO communicates with the SINAMICS G120 via PROFINET IRT using telegram 110 (SIEMENS servo telegram with position control word).

6.1 Telegram 110 PZD Layout

6.2 Interaction Between EPOS and Load-Sharing Telegram 352

The S7-1200 can simultaneously host both telegrams:

• Telegram 110 on the master drive, carrying the EPOS setpoints.
• Telegram 352 between master and follower drives, carrying the load-share torque setpoint.

The PLC only sees the master drive's actual position. The follower drive is position-transparent - its speed follows the master through the rigid mechanical coupling. This is the configuration that the source application note confirms works on S7-1200 + SINAMICS G.

6.3 TIA Portal Configuration Steps (Overview)

1. Add the SINAMICS G120 to the TIA Portal device configuration (GSD file from Siemens Support GSD downloads).
2. Configure telegram 110 as the IO data for the master drive.
3. Add the follower drive with telegram 352 only.
4. Add a TO_PositioningAxis referencing the master drive's telegram 110 actual position.
5. Wire the S7-1200 MC_Power, MC_MoveAbsolute, MC_MoveVelocity blocks to the TO.
6. Configure the load-share handshake in the PLC application: enable master follower only when MC_Power.Done = TRUE on both axes.

7. Alternative: Torque Coupling via Analog Signal

A second topology mentioned in field discussions is to operate the follower in torque mode and command its torque setpoint from an analog output of the master drive (SINAMICS G120 analog output 1 / 2, parameter p0771). This is technically possible but discouraged because:

• Analog signals (0-10 V or 4-20 mA) are vulnerable to noise, especially with PWM inverters on the same cable tray.
• Latency through the analog path (~5-10 ms plus any filter) reduces the bandwidth of the load-share loop.
• Calibration drift between analog output and analog input reduces long-term accuracy.

If analog torque coupling is unavoidable (legacy hardware, retrofit), use:

• 4-20 mA current loop instead of 0-10 V.
• Shielded twisted pair, grounded at one end.
• Set p0771 = r80 (torque actual value) on master; set p1502 = analog input 1 on follower.
• Bandwidth limit the follower torque controller to 30-50 Hz to filter analog noise.

8. Commissioning Procedure

1. Mechanical pre-check. Verify rack parallelism and gearbox backlash per Section 2.
2. Parameter reset. p0970 = 1 on both G120 drives to factory reset.
3. Motor commissioning. Run the motor data identification (p1900 = 2) on each drive separately, with the motors uncoupled from each other.
4. Encoder commissioning. p0400 = encoder type, then p1990 = encoder identification. Confirm direction with a slow manual jog.
5. Set telegram 352 on both drives (p0922 = 352). Verify PROFINET name and IP assignment.
6. Configure load-share parameters per Section 4.2.
7. Mechanical re-couple the gearboxes and the platform. Verify no binding.
8. Static load test. Run master in speed mode at 10% rated speed. Read r80 on both drives - they should differ by < 5% at constant load.
9. Position loop test. Switch master to EPOS mode via the S7-1200. Command a 1 m move at 0.1 m/s. Verify the platform tracks without oscillation.
10. Dynamic load test. Apply a step load (e.g., 50% rated torque) on one side using a calibrated brake. Verify the master compensates within 200 ms.
11. STO / SS1 test. Trigger Safe Torque Off from the F-CPU or hardwired circuit. Verify both drives shut down within the SIL 3 time budget.

9. Verification Matrix

10. Common Faults and Diagnostics

11. Motor Sizing Reference

For the 22 kW class used here, nameplate values typically used in sizing:

Motor: 3-phase squirrel-cage induction
Rated power P_r    = 22 kW
Rated voltage V_LL = 400 V (or 690 V for some installations)
Rated current I_L  = 41 A (at 400 V)
Rated speed n_r   = 1470 rpm (4-pole)
Rated torque M_r  = P_r * 60 / (2 * pi * n_r) = 22000 * 60 / (2 * pi * 1470) = 142.9 Nm
Power factor      = 0.86
Efficiency        = 0.93

Apparent power    = sqrt(3) * V_LL * I_L / 1000 = sqrt(3) * 400 * 41 / 1000 = 28.4 kVA

Continuous load per motor when sharing a 44 kW total load:

P_each = P_total / 2 = 22 kW
I_each = I_total / 2 (assuming symmetric torque sharing)

Worst-case imbalance: 70/30 split means one motor sees 30.8 kW continuously. Verify that this is within the motor's S1 rating with the actual ambient temperature and altitude derating; otherwise size up to 30 kW.

12. Safety Considerations

Two-drive load-sharing systems introduce safety considerations that a single drive does not:

• STO must be applied to both drives simultaneously. Use a fail-safe digital output of the S7-1200 F-CPU driving both STO inputs in parallel, or wire the STO loop in series.
• Encoder redundancy: If the application is SIL 2 or higher, both drives should have redundant encoders feeding the safety function.
• Mechanical braking: A spring-applied holding brake on each gearbox is recommended so that a single inverter losing power does not allow the platform to coast.
• Dynamic braking: Use a common braking resistor (if the DC links are paralleled) or two independent braking resistors with shared chopper threshold setting.

13. Field-Proven Recommendations

1. Use telegram 352, not analog coupling, for any new installation. The bandwidth and noise immunity are an order of magnitude better.
2. Configure droop only on the follower. Setting droop on both drives causes oscillation.
3. Set the PROFINET update time to 1 ms for load sharing. 4 ms is the maximum tolerable for a 22 kW drive at full load.
4. Always run motor data identification separately on each drive before coupling the mechanics.
5. Monitor r80 (torque actual) and r63 (speed actual) on both drives via the TIA HMI. Display the imbalance in real time.
6. Keep the EPOS position loop closed on the master drive. The follower's position loop must remain open during normal operation - it is a torque follower.
7. For absolute encoders (recommended for EPOS homing), use an SSI or EnDat encoder compatible with the SINAMICS G120 encoder interface.

14. Standards Reference

The following standards apply to this configuration and should be verified by the system integrator:

• IEC 61800-5-1 - Adjustable speed electrical power drive systems - Safety requirements
• IEC 61800-5-2 - Safety functions (STO, SS1, SS2, SOS)
• IEC 60204-1 - Safety of machinery - Electrical equipment of machines
• EN ISO 13849-1 - Safety-related parts of control systems (PL)
• IEC 61508 - Functional safety (SIL)

15. Frequently Asked Questions