A fan inside a drive is a paradox: it exists to remove heat, but it is also the most common failure point. In a distributed drive mounted directly on a conveyor motor — exposed to dust, fiber, and the occasional washdown — a fan ingests everything in the ambient air and eventually clogs, slows, or seizes. The SINAMICS G115D series offers both fanless and fan-cooled variants across the same power range, forcing a design decision: is natural convection enough for your application, or does the drive need forced air to survive?
The answer is not on the datasheet. It depends on your conveyor's duty cycle, ambient temperature, mounting orientation, and what happens to that drive at 2 a.m. when nobody is watching.
The Physics of Getting Heat Out of a Drive
A variable frequency drive dissipates roughly 3–5% of its output power as heat — primarily from IGBT switching losses and diode conduction losses in the rectifier. At 3 kW output, the drive generates approximately 90–150 W of waste heat. This heat must move from the semiconductor junction through a thermal interface material, into an aluminum heatsink, and finally into the surrounding air.
Natural convection relies on buoyancy: warm air rises across the heatsink fins, drawing cooler air in from below. The rate of heat transfer is proportional to the temperature difference between the heatsink and the ambient air, multiplied by the effective surface area. A fan-cooled drive adds forced convection — air moving at 2–4 m/s across the fins — which increases the heat transfer coefficient by a factor of 3 to 5. The same-sized heatsink can dissipate far more power, or a smaller heatsink can dissipate the same power, because the moving air strips heat from the fin surface far faster than buoyancy alone.
In the G115D, the fanless models use the entire cast-aluminum drive housing as a heatsink — the body itself is the thermal radiator. Fan-cooled models use a smaller internal heatsink with a dedicated axial fan pulling air through a ducted path, isolated from the electronics compartment to maintain the IP65 seal.
When Fanless Works — and When It Does Not
The 0.55 kW fanless G115D in FSA frame and the 3 kW fanless model in FSB can operate at full rated current in a 40°C ambient without derating — provided the drive is mounted with the heatsink fins oriented vertically and with at least 100 mm of clearance above and below for air circulation. Mount the drive horizontally with the fins facing down, and natural convection stalls — heat builds in the housing, and the drive derates or trips on overtemperature within minutes at full load.
The real advantage of fanless is not thermal performance — it is maintenance elimination. No fan means no bearing to wear out, no impeller to balance, no filter to clean, no dust ingress path through the fan opening. For a conveyor in a clean warehouse or a packaging line where scheduled maintenance is quarterly, fanless drives run for years without anyone opening the housing. The thermal trade-off is that the drive occupies a larger physical envelope for the same power rating — the FSB fanless 3 kW unit is 100 mm wide, using the full housing surface to shed heat.
Fanless fails when the ambient temperature climbs above 40°C, when the conveyor runs continuous duty at low speed (below 10 Hz, where the drive's output current concentrates in fewer switching cycles), or when the mounting location is enclosed — inside a machine frame with no vertical airflow path. A fanless drive in a sealed metal enclosure at 45°C ambient will trip on overtemperature within an hour of continuous operation, regardless of the nameplate rating.
When the Fan Earns Its Place
The fan-cooled G115D models — such as the 5.5 kW FSC PROFINET variant — use an internal fan that pulls ambient air across a dedicated heatsink and exhausts it without exposing the electronics. The fan enables the FSC frame to deliver 5.5–7.5 kW from a package that would need roughly twice the surface area to dissipate the same heat passively.
Fan-cooled is the right choice when any of these conditions apply: ambient temperature exceeds 40°C for more than 100 hours per year, the conveyor runs continuous duty at low speed, the drive is mounted in a confined space with restricted natural airflow, or the application cannot accommodate the larger physical envelope of a fanless drive at the same power rating.
The cost of the fan is a maintenance item. The G115D fan assembly is field-replaceable without opening the electronics compartment — a deliberate design choice that lets maintenance swap a noisy or failed fan in under 10 minutes. The expected service life is 30,000–40,000 hours at 40°C, which is 3.5–4.5 years of continuous three-shift operation. In a two-shift conveyor line, expect to replace the fan every 5–7 years. Factor that into the preventive maintenance schedule, and fan-cooled becomes an economic trade-off rather than a reliability risk.
Conveyor Duty Cycle: The Overlooked Variable
Most conveyor drives do not run at full load continuously. An accumulation zone starts and stops dozens of times per hour; a diverter actuates for two seconds every 15 seconds; a belt section runs at 80% speed for 80% of the shift and idles the rest. The drive's thermal time constant — roughly 10–20 minutes for the G115D aluminum housing — means that intermittent overloads are absorbed by the thermal mass of the housing without raising the steady-state junction temperature.
For an intermittent-duty conveyor, a fanless drive rated for 3 kW continuous at 40°C can handle 3.5 kW peaks for 30 seconds every 5 minutes without exceeding its junction temperature limit, because the housing absorbs the peak and dissipates it during the idle interval. A fan-cooled drive handles the same peaks with less temperature swing, but for intermittent duty, the fan adds no value — the natural thermal mass of the drive housing is sufficient.
The decision flips for continuous-duty conveyors: a pallet-handling line that runs at 90% load for 16 hours a day, six days a week. Here, the drive reaches thermal equilibrium at its continuous rating, and the margin between rated current and overtemperature trip depends entirely on ambient temperature and airflow. A fan-cooled drive at 5.5 kW continuous dissipates roughly 200 W of waste heat — natural convection alone cannot move that heat fast enough in a 40°C ambient without a heatsink roughly twice the size of the FSC housing.
How do I know if my drive is thermally derating?
The G115D reports its IGBT junction temperature and heatsink temperature over PROFINET or the USB service interface. When the heatsink temperature exceeds 85°C, the drive begins to reduce its switching frequency — you will hear the motor whine change pitch — and at 95°C it reduces output current. If you see switching frequency reduction in the diagnostic log, the drive is telling you it is running too hot. The fix is either more airflow, lower ambient temperature, or the next frame size up.
Can I retrofit a fan to a fanless G115D later?
No. The fanless and fan-cooled variants use different housing castings and internal thermal paths. A fanless drive relies on the entire housing surface as a heatsink; a fan-cooled drive routes heat to a smaller internal heatsink optimized for forced convection. The mounting points, connector positions, and internal airflow paths are different. If you discover that a fanless drive is overheating, the solution is either improve the mounting airflow or replace it with a fan-cooled model — not retrofit a fan.
A fanless drive is an investment in maintenance-free operation. A fan-cooled drive is an investment in power density. The conveyor duty cycle — not the nameplate power — determines which investment pays off. If the drive spends 60% of its life at idle, save the fan for an application that actually needs it.
