Siemens SINAMICS G120 vs G120C: Which Compact Drive

What separates the Siemens SINAMICS G120 from the G120C? The SINAMICS G120 is a modular general-purpose AC drive built from a separate Control Unit (CU) and Power Module (PM) rated to IEC 61800-2, spanning roughly 0.37 kW to 250 kW depending on the PM selected; the G120C is its single-housing derivative, control and power electronics fixed into one book-size frame from 0.55 kW to 132 kW. That construction split changes the whole ownership model: a G120 line lets you standardize on one Power Module and swap Control Units for encoder feedback, extra safety functions, or a different fieldbus, while a G120C ships as a closed unit you replace outright on failure or upgrade. This article compares frame construction, control methods, Safety Integrated and I/O options, fieldbus support, overload ratings, and the machine classes each drive actually fits.

Frame Construction: Modular CU/PM vs Book-Size Housing

The G120's Control Unit mounts onto a Power Module through a defined mechanical and electrical interface — CU230P-2 for positioning tasks, CU240E-2 for general I/O-heavy machines, CU250S-2 for closed-loop vector with Safety Integrated. Power Modules come in PM230 (IP55, motor-mounted), PM240 and PM240-2 (cabinet, with braking chopper options), and PM250 (regenerative). Because the CU and PM are separate field-replaceable units, a technician swaps only the failed half instead of the whole drive.

The G120C collapses that pair into one book-size housing, IP20, 0.55 to 132 kW, with the control type fixed at the factory. No CU-swap option exists. What we see in the field: panel builders standardize on a G120 PM240-2 across a machine line and only change the CU when a later project needs PROFINET or a safety card. A G120C doesn't allow that swap — a spec change after the BOM is frozen means ordering a different drive, not a different card.

Control Methods and Base Speed Performance

Both drives run Siemens' V/f and vector algorithms. Below base speed, the vector algorithm holds flux constant and regulates torque current directly, giving tighter speed and torque response than open-loop V/f — relevant for extruders, winders, and cranes that need holding torque near zero speed. Above base speed, voltage is limited to the DC bus and torque falls off in the field-weakening region.

The G120's CU250S-2 adds closed-loop vector with an encoder input, which corrects the small speed error that open-loop vector leaves under load. The G120C runs open-loop (sensorless) vector only — no encoder terminal exists on the standard unit. For a 4-pole motor at 50 Hz, both drives target the same 1500 rpm synchronous speed; the difference shows up in how tightly each holds that speed as load torque changes, not in the frequency math itself.

Safety Integrated and I/O Options

STO ships as standard on both drive lines. The gap opens above that: the G120 with a CU240E-2 or CU250S-2 adds Safe Stop 1 (SS1) and Safely Limited Speed (SLS), monitored through PROFIsafe over PROFINET or PROFIBUS. The G120C's Safety Integrated function set stops at STO — no SS1, no SLS, no PROFIsafe telegram.

I/O count follows the same logic. The G120's digital and analog I/O total depends on the CU ordered, so a machine builder can pick a CU240E-2 for I/O-heavy applications or a leaner CU for a simple pump. The G120C's I/O is fixed per frame size at the point of order.

Fieldbus and Communication

The G120 supports PROFINET, PROFIBUS DP, and — on the right CU — EtherNet/IP or Modbus RTU, selectable by which Control Unit goes on the Power Module. The G120C is ordered with PROFINET or PROFIBUS built in, plus Modbus RTU; once ordered, the protocol is fixed for the life of the unit. This matters on a retrofit: swapping a plant from PROFIBUS to PROFINET means a new CU on a G120, but a new drive on a G120C.

Overload Ratings and Motor Sizing

Both lines publish two overload classes per frame: a Low Overload rating for variable-torque loads like pumps and fans, and a High Overload rating for constant-torque loads like conveyors and mixers that need higher starting torque. Matching the duty class to the load — not reading the nameplate kW alone — decides whether a given frame handles a conveyor's breakaway torque or trips on the first start. This depends on frame size and voltage class as much as drive family, so a G120 PM240-2 and a G120C at the same nameplate kW do not automatically carry the same High Overload current.

G120 vs G120C at a Glance

Where Each Drive Fits

The G120 fits multi-variant machine lines: a builder selling the same conveyor platform into a plant needing PROFINET and another needing PROFIBUS uses one Power Module family and swaps the CU per order. It also fits retrofits where an encoder gets added later for tighter speed holding, and projects where a customer might ask for SLS during commissioning.

The G120C fits single-purpose OEM equipment — a fixed pump skid, a packaging conveyor sold as one SKU, a mixer that never needs an encoder. Lower parts count and a lower unit price offset the lack of field flexibility. This depends on lifecycle: a machine platform sold for five years across multiple markets justifies the G120's modularity; a machine built once and sold as a fixed configuration rarely needs it.

Neither drive uses ABB's Direct Torque Control; Siemens' vector method controls flux and torque current through a modulated PWM stage, a different structure from DTC's modulator-free control loop, covered in more depth in the VFD control methods comparison. For selecting between drive families in the first place, the VFD selection checklist and the broader ABB ACS580 vs Schneider ATV630 vs Siemens G120 comparison cover how the G120 stacks up against other brands' general-purpose lines. Both drives are built to the IEC and NEMA standards for VFDs that set their overload and EMC ratings. For the full modular architecture and sizing background, see the VFD engineering guide; both drive families are stocked as part of Stoklink's range of variable frequency drives.

Conclusion

The SINAMICS G120 and G120C run the same underlying V/f and vector control engine, so raw motor performance is not the deciding factor between them. The decision comes down to how much the machine platform needs to flex: a G120's modular CU/PM pairing supports encoder feedback, PROFIsafe safety functions, and a choice of fieldbus that can change per order, while a G120C trades that flexibility for a fixed, lower-cost housing suited to single-purpose OEM equipment. Match the drive to the platform's expected variant count over its service life, not just its nameplate kW.