SINAMICS S120 PM340 Multimotor Application: CU310-2 Configuration

1. Multimotor Application Overview

A multimotor drive topology uses a single SINAMICS S120 inverter to power several induction motors in parallel, each protected by its own motor-protective circuit breaker (MPCB) or thermal overload relay and switched by an individual output contactor. The reference configuration in this guide is one PM340 7.5kW blocksize power module paired with a CU310-2 DP control unit driving ten 0.37kW induction motors, scaled to 17 such drives for a 163-motor installation.

This architecture reduces inverter cost, panel space, and cabling compared with a one-drive-per-motor layout, at the expense of dynamic switching flexibility and the need to observe strict rules on output-contactor switching. It is appropriate when motors are started infrequently, run continuously once enabled, and require only occasional individual isolation for maintenance. The architecture is not appropriate for high-cycle sequential switching, where individual drives per motor should be used instead.

Two product families are commonly considered for this duty: SINAMICS S120 (PM340 + CU310-2, the system in the source project) and SINAMICS G120 (CU250S-2 or similar). Both support multimotor operation; the S120 path gives finer control over Drive Data Sets, PROFIsafe, and a wider choice of pulse patterns. The configuration in this article is specific to the S120 + PM340 + CU310-2 DP combination.

2. Drive Sizing and Current Verification

Confirm that the inverter can supply the combined running and starting current of the active motor group. For ten 0.37kW motors at 400V 3-phase, nameplate data is typically 1.0A rated per motor; the combined full-load current is therefore approximately 10A plus service factor. The PM340 7.5kW (order number 6SL3210-1PE27-5AL0) is rated at 18A output and can deliver 150% of rated current for 60s, which comfortably covers the simultaneous inrush of several direct-on-line starts.

Use the following dimensional checks before commissioning:

• Total running current: I_total = N_active × I_motor = 10 × 1.0A ≈ 10A
• Per-drive apparent power: kVA_3φ = sqrt(3) × V_LL × I_line / 1000 = 1.732 × 400V × 18A / 1000 = 12.5 kVA
• Per-motor apparent power: kVA_1φ ≈ 0.4kW / (0.8 pf × 0.9 eff) = 0.56 kVA each, so 10 motors ≈ 5.6 kVA running
• Drive loading at 10 motors running: 5.6 / 12.5 = 45% — comfortable thermal margin
• Drive loading at DOL start of 5 motors (6× inrush each, 0.5s): peak I ≈ 5 × 6A = 30A — exceeds rated 18A but within short-time overload (27A for 60s typical)

3. IGBT Protection: Output Contactor Switching Rules

Switching an output contactor while the drive is pulsing its IGBT bridge is the single largest reliability risk in a multimotor installation. The PM340 IGBT modules are designed to drive a continuous motor load; opening or closing a contactor at the output terminals while the drive is enabled can produce destructive dV/dt, voltage reflection at the motor terminal, and high-frequency ringing that exceeds the IGBT reverse-bias safe operating area.

Two fault signatures are common when this rule is violated:

The mandatory rule is: de-energize the drive before opening or closing any output contactor. Implement this with the following sequence, controlled by the higher-level PLC:

1. Issue OFF1 (ramp down) or OFF2 (pulse inhibit) to the drive via PROFIBUS control word bit 1 or 2.
2. Wait for status word r0899.0 = 0 (pulse inhibit acknowledged, drive output stage disabled). Add a 50ms safety delay to allow the IGBT output to fully commutate.
3. Open or close the target contactor.
4. Issue ON to re-enable the drive. If the motors may still be coasting, configure flying restart (p1200 = 1 or 4) so the drive can pick up the rotating group without DC-link inrush.

4. Motor Identification for Multimotor Operation

Motor ID is a calibration procedure that measures stator resistance, leakage reactance, and rotor time constant so the closed-loop current regulator and (in vector mode) the flux model have an accurate motor equivalent circuit. With a single motor this is straightforward. With multimotor operation the situation is more complex.

Siemens guidance for multimotor drives is to use V/f open-loop control (p1300 = 0, 1, or 2) and to perform the motor ID with all motors connected to the drive. Vector control (p1300 = 20 or 22) is not recommended for multimotor applications because the model becomes invalid the moment any contactor opens. The relevant parameters are:

To perform the standstill motor ID on a multimotor installation:

1. Pre-position all 10 contactors to the closed state; all motors must be connected to the drive output when the ID runs.
2. Enter motor nameplate data for one motor into the active Motor Data Set (MDS).
3. Set p1900 = 2; acknowledge the prompt; issue ON.
4. The drive injects test pulses and identifies the equivalent-circuit parameters scaled for the total connected motor count.
5. The resulting parameter set is the parallel sum of all ten motors. This is exactly the value needed for normal operation with all motors running, but it is no longer correct if several motors are disconnected.

For installations where the number of active motors varies widely, do not scale the identified values by 1/N to represent one motor. Instead, leave the ID result as the parallel sum, and use Drive Data Sets (Section 5) to swap in pre-calculated equivalent circuits for the most common groupings. Identifying with the parallel sum works because the V/f controller does not need an exact motor model — it only needs rough leakage reactance for current-controller stability and the magnetizing current value for boost voltage calculation. A 10:1 variation in connected motor count is tolerated by V/f with no parameter change; it is not tolerated by sensorless vector.

5. Motor Data Sets (MDS) and Drive Data Sets (DDS)

SINAMICS S120 supports up to 32 Motor Data Sets and 32 Drive Data Sets per Control Unit. Each Drive Data Set references a Motor Data Set plus a complete setpoint-source, command-source, and ramp-function settings. The active DDS is selected at runtime via control word bits 15-16 in PROFIdrive profile, or via the CU310-2 onboard terminal DI0/DI1 if hardwired.

For a multimotor drive, configure one DDS per common motor-grouping pattern. Example for a 10-motor drive with three duty modes:

To calculate a reduced-bank equivalent manually, the rule of thumb is: scale the magnetizing current and the no-load loss components by N_active/N_total, and leave the stator resistance per phase unchanged. This is an approximation; for V/f control it is sufficient. For commissioning speed, the practical approach is to run the standstill motor ID (p1900 = 2) once for each common grouping, with exactly that subset of contactors closed. Capture the result into the appropriate MDS.

The SINAMICS S120 Function Manual section on Drive Data Sets and the List Manual parameter descriptions for p820 (number of DDS) and p1800 should be reviewed during this step.

6. Cable Length Limits and Output Filtering

The maximum permissible motor cable length is the single most commonly missed constraint in multimotor installations. The limit exists for three reasons: (a) capacitive charging current of the shielded cable loading the inverter output, (b) reflected-wave voltage doubling at the motor terminals on long cables, and (c) EMC compliance boundaries. The total cable length in a multimotor installation is the sum of all individual motor branch lengths from the drive output to each motor terminal, not the distance to the farthest motor.

For the 10-motor reference installation, if each motor branch averages 15m, the total is 150m — already over the 100m limit with a simple output reactor. Either reduce branch length, install a du/dt filter with VPL at the drive output, or use individual 100m-rated branches. The PM340 Power Module manual section on EMC-compliant installation is the canonical reference for cable lengths and shielding requirements.

7. Motor Failure Behavior and Drive Response

When a single motor in the bank fails, the drive sees a change in total load current that depends on the failure mode:

The drive monitors total output current. If the total exceeds the threshold set in p640 (current limit) or p2170 (warning threshold), the drive clamps or trips. Therefore individual motor protection is non-negotiable: every motor must have its own MPCB or thermal overload sized for that motor's nameplate current, set to the 0.37kW / 1.0A class. The drive's protection is a backstop, not a substitute.

8. Programming the CU310-2 DP for Multimotor

The CU310-2 DP control unit (6SL3040-1LA01-0AA0) handles PROFIBUS DP-V1 communication, setpoint processing, and the drive-side interface to the PM340. For the multimotor application, configure the following in the STARTER or Startdrive commissioning tool:

1. Set p2051 (PROFIdrive PZD select) to include the standard telegram 1 (control word 1 + speed setpoint) plus an additional 2-word extension for DDS selection bits 15-16.
2. Set p820 = 4 (or desired number of DDS). Each DDS has its own ramp-function generator, setpoint channel, and MDS reference.
3. Configure p010 (number of MDS) = 4; populate MDS0 (10-motor), MDS1 (5-motor), MDS2 (2-motor), MDS3 (9-motor).
4. Assign command and setpoint sources per DDS: in most installations DDS0 is controlled via PROFIBUS, DDS1-DDS3 can be hardwired via the CU310-2 onboard terminals or via the higher-level PLC.
5. Set p1200 = 1 (flying restart with search in both directions) so the drive can re-acquire a coasting motor bank after a contactor re-closure.
6. Enable pulse inhibit on PROFIsafe or hardwired STO if required; the CU310-2 supports STO via terminals EP and enable the safety functionality per application requirement.

The PROFIdrive control word (STW1) bit assignments used in this application:

9. Commissioning Procedure

1. Mechanical and electrical inspection: verify phase rotation at every motor, torque on terminal boxes, and bonding of cable shields at both ends.
2. Power up the CU310-2 DP without the PM340 main power (24V auxiliary only) and connect via PROFIBUS from the higher-level PLC or a PG/PC running STARTER / Startdrive.
3. Load the project offline; select the correct PM340 firmware version (typically V5.x for the latest S120 blocksize). Online firmware version must match the offline project.
4. Perform drive factory reset (p0970 = 1).
5. Enter motor nameplate data for one motor into the active MDS.
6. Close all 10 contactors; run p1900 = 2 (standstill motor ID) to identify the parallel-sum equivalent circuit. Verify p0350 (stator resistance), p0356 (leakage reactance), and p0360 (magnetizing current) update.
7. Set p1300 = 0 (V/f linear) for multimotor operation. If quadratic V/f is acceptable for the load (e.g., fans, pumps), p1300 = 2 saves energy.
8. Run the rotating measurement (p1900 = 3) only with a single motor connected, to verify the model, then revert to the parallel-sum MDS for production.
9. Configure the additional MDS entries for common reduced-bank cases; switch between DDS via the higher-level PLC to verify selection.
10. Set up the PLC interlocks: drive must be in OFF2 state (r0899.1 = 0 and r0046.0 = 0) before any contactor command is honored. Add a 100ms interlock delay.
11. Test fault responses: simulate MPCB trip on one motor, verify the drive logs the change, the motor is isolated, and the drive continues to supply the remaining healthy motors.

10. Verification Checklist

11. Frequently Asked Questions