Frame size on a distributed drive is not just about power rating. It determines the physical envelope, the heat dissipation surface area, the mounting hole pattern, and — critically — whether the drive fits between two conveyor rollers or needs its own mounting bracket off to the side. The SINAMICS G115D comes in three frame sizes: FSA (small), FSB (medium), and FSC (large). Each covers a specific power band, but the real decision is about current delivery at low speed, ambient temperature headroom, and mechanical clearance.
This guide walks through what each frame size delivers — and where the limits bite — so you can size the drive to the conveyor zone, not the nameplate kilowatts alone.
FSA: The Compact Frame for Light Conveyor Duty
FSA covers 0.37 kW to 0.75 kW at 400 V three-phase input. At roughly 230 mm tall and 73 mm wide, it fits between the side frames of a standard roller conveyor — you can mount it directly to the motor terminal box or tuck it under the conveyor bed without encroaching on the product path. The 0.75 kW FSA PROFINET model and the 0.55 kW FSA fanless variant serve infeed rollers, small belt sections, and gravity-discharge accumulation zones where the driven load is a few dozen kilograms of conveyed product, not a pallet.
The FSA limitation is continuous current at low output frequencies. When a conveyor runs at 10 Hz for creep-speed positioning, the drive's output transistors spend more time conducting per cycle, and heat builds faster than at 50 Hz. The FSA frame's aluminum housing relies on natural convection or a small integrated fan, and the surface area is limited by the compact footprint. At 0.75 kW output, an FSA drive running at 5 Hz in a 40°C ambient can approach its thermal limit. If your zone runs extended low-speed duty — indexing conveyors, positioning tables — consider stepping up to FSB even if the nameplate power is within FSA range.
FSB: The Mid-Range Workhorse
FSB covers 1.1 kW to 3 kW. At approximately 260 mm tall and 100 mm wide, it adds surface area and internal volume for a larger heatsink. The 3 kW FSB fanless PROFINET model can dissipate 3 kW continuously at 40°C ambient without a fan — the aluminum body is large enough that natural convection handles the heat. The 1.5 kW FSB AS-Interface variant and the 1.1 kW and 3 kW wall-mounted models cover the middle of the G115D power range.
FSB is the sweet spot for belt conveyors in logistics and packaging. A 2.2 kW FSB drive runs a 1.5 m/s belt carrying 30 kg cartons indefinitely without derating. The extra current headroom compared to FSA means you can oversize the drive by one power step — run a 3 kW FSB at 2.2 kW load — and gain thermal margin for high-ambient or low-speed operation without jumping to the larger FSC frame.
The mounting footprint matters: FSB is 100 mm wide, which is wider than many conveyor side rails. Motor-mounted installation is possible on IEC frame sizes 80–132, but check the clearance between the drive body and the conveyor frame — on narrow conveyors, the drive may need a standoff bracket.
FSC: High Power for Heavy Conveyor Duty
FSC covers 4 kW to 7.5 kW. At roughly 340 mm tall and 150 mm wide, it is a substantial cast-aluminum housing with an integrated fan as standard. The 5.5 kW FSC PROFINET drive and the 7.5 kW FSC AS-Interface model serve pallet conveyors, heavy roller decks, chain conveyors, and incline sections where the driven load plus product weight demands high torque at low speed.
FSC at 7.5 kW delivers roughly 16 A continuous output current at 400 V. On a chain conveyor pulling loaded pallets at 0.5 m/s, this translates to approximately 5,000 N of tractive force — enough for a 30-meter accumulation section of 500 kg pallets. The integrated fan ensures the drive stays within thermal limits even at 50°C ambient, but the fan itself is a wear item with a typical service life of 30,000–40,000 hours. In a three-shift operation, that is 3–4 years before the fan needs replacement.
Physically, FSC is too large to motor-mount on most conveyor motors. Wall-mounting on a bracket adjacent to the motor is the standard installation method for this frame size. The 4 kW wall-mounted FSC variant and the 7.5 kW wall-mounted models are designed for this configuration — the mounting holes are pre-drilled for bracket attachment.
Frame Size Comparison
| Parameter | FSA | FSB | FSC |
|---|---|---|---|
| Power Range | 0.37–0.75 kW | 1.1–3 kW | 4–7.5 kW |
| Width | 73 mm | 100 mm | 150 mm |
| Typical Motor Mount | IEC 63–90 | IEC 80–132 | Wall-mount preferred |
| Cooling Option | Fanless or fan | Fanless or fan | Fan as standard |
| Best Conveyor Fit | Roller, small belt | Belt, accumulation | Pallet, chain, incline |
| Low-Speed Thermal Limit at 40°C | ~60% rated at 5 Hz | ~80% rated at 5 Hz | Full rated at 5 Hz |
When should I step up a frame size even if the power rating fits?
Three scenarios justify oversizing to the next frame: extended low-speed operation below 10 Hz (thermal stress), ambient temperature above 40°C (derating), or planned future line speed increases that require a larger motor. Stepping from FSA to FSB for a 0.75 kW zone adds roughly 30% to the drive cost but buys you thermal headroom and the option to upgrade the motor later without replacing the drive.
Can I mix frame sizes on the same conveyor line?
Yes — and nearly every real installation does. The infeed zone might use an FSA drive on a 0.55 kW roller motor, the main transport zones run FSB at 2.2 kW, and the incline section at the end gets an FSC at 5.5 kW. All three share the same G115D parameter set, the same communication interface, and the same spare parts pool. The frame size changes; the engineering does not.
Frame size selection is thermal engineering in a mechanical constraint. The kilowatt number on the nameplate gets you in the right ballpark. Whether the drive survives the first August heat wave depends on how much aluminum surface area you put between the IGBTs and the ambient air.
