Scan the SINAMICS G115D range and you will meet two cooling philosophies wearing the same family badge: sections that push air across their heatsinks with an integrated fan, and fanless sections that shed heat by natural convection through taller fins. The split follows physics and duty, and it matters for dust, acoustics and where you can legally hang the drive.
The power boundary
Natural convection loses to physics above a few kilowatts: heatsink area grows faster than the wall space available. The G115D's small sections — a 3 kW fanless FSB unit with PROFINET — run silent with no filter to clog, which is why they suit clean indoor corridors and low-duty conveyors. Push into 4 kW and up, and forced cooling takes over: the 4 kW fan-cooled FSB section keeps its compact footprint by moving air, and the 7.5 kW class rides on fans entirely.
What the fan costs you
A fan is a filter, a bearing and a noise source. In dusty plants the fan intake clogs and derates the drive silently; in quiet buildings the fan is the audible component. Fanless sections trade these for size and a power ceiling — the honest engineering trade is dust and dB against wall meters.
Duty decides more than rating
A 3 kW conveyor that runs 10% of the time may live happily on the fanless thermal design where a continuously rated 3 kW process fan would not. Overload reserve and convection cooling interact: check the duty cycle before assuming the fanless section "should be enough."
For utility rooms that mix small pumps and conveyor spurs, the distributed drive group and the wider drives and motors family let you standardize on one engineering workflow even when cooling differs per point.
Can a fanless section be upgraded to fan-cooled later?
No — cooling is frame hardware, not an accessory. If duty may grow, order the fan-cooled frame now; the parameter set migrates, the wall drilling does not.
Do fanless sections need derating in hot rooms?
They derate like any convection-cooled device, and their curves are honest in the datasheet. Above 40°C ambient, compare the derated current against the motor before trusting the nameplate.
