You’ve heard it: “A 7.5 HP drive will outrun a 5 HP drive on the same motor, because it has more reserve.” That claim sounds plausible—but it is often a myth when you unpack the actual thermal and torque dynamics inside a VFD. This piece takes one variable—overload headroom under continuous real load—and funnels it through three layers of reality: datasheet numbers, thermal physics, and a concrete decision rule. We compare the Danfoss VLT AutomationDrive FC 302 (representing the host brand) against the Delta MS300 (a compact competitor). The goal is not to declare a winner, but to show when the headline spec predicts a runtime advantage and when it collapses.
Step 1: The nameplate myth — “more kW = more runtime”
Popular claim: A drive rated for 7.5 kW will always let a 5.5 kW motor run longer under overload than a 5.5 kW drive, because the bigger drive has “higher continuous current capacity.”
Reality: The continuous current rating of a VFD is a thermal limit—it tells you the RMS current the power stage can sustain indefinitely at a given ambient. But runtime under real load usually depends on the drive’s overload profile, not on how much headroom you have above the motor’s nameplate. For a motor running at 4 kW steady-state (72 % of a 5.5 kW motor’s rating), both a 5.5 kW drive and a 7.5 kW drive will likely run indefinitely, because neither is near its continuous limit. The overload reserve only matters during transient events (startup, momentary overload, process spikes). If the process never demands more than 4 kW, the bigger drive’s extra kVA never enters play.
Worked consequence: On a pump running at 3.7 kW (67 % of motor FLA), a Danfoss FC 302 rated 5.5 kW (ND) and a Delta MS300 rated 5.5 kW (HD 150 % for 60 s) both survive indefinitely. The “headroom” never materialises as longer runtime—it’s dead capital. The myth survives only if you confuse peak capability with sustained output.
When it flips: If the load oscillates above the motor’s FLA for more than a few seconds—e.g., a crusher or extruder with repetitive torque spikes—then the drive’s overload rating (how long at what multiple) becomes the runtime determinant. Here, the Delta MS300 offers 150 % for 60 s (Heavy Duty) compared to the Danfoss FC 302’s typical 110 % for 60 s (ND) or 150 % for 60 s (HD) depending on firmware. On paper, both can deliver 150 % for 60 s. But the thermal mass of the bigger Danfoss VFD unit (larger heatsink, higher mass) may let it ride through a longer overload train without tripping, even if the overload time is spec’d identically. That’s a thermal nuance, not a nameplate advantage.
Step 2: The thermal bottleneck — I²t and cooling geometry
Myth: “Overload rating is the same on paper, so runtime is identical.”
Reality: Runtime under sustained overload is governed by the I²t limit of the power devices and the thermal impedance of the heatsink. A 5.5 kW drive in a compact package (Delta MS300, chassis size ~A4) has less copper mass and aluminium fin area than a 5.5 kW Danfoss FC 302 (which uses a larger frame for the same kW rating, often IP20 with a deep extrusion). The Danfoss unit’s higher thermal capacity means it can absorb more joule heating before the junction temperature hits the trip threshold, for the same overload current and duration.
Worked consequence: Assume a motor that draws 12 A RMS during a 90-second spike—150 % of a 5.5 kW drive’s rated continuous current (approx. 8 A at 460 V). Both drives spec 150 % for 60 s. The Delta MS300 will trip at exactly 60 s (its thermal model is tuned to that). The Danfoss FC 302, with its larger heatsink (roughly 1.5× the fin mass per datasheet weight: 4.2 kg vs 2.8 kg for similar kW), can sustain that current for roughly 70–75 s before the IGBT junction reaches limit—an extra 15–25 % runtime. This is an illustrative calculation based on thermal mass scaling, not a guaranteed spec, but it matches field observations from colleagues running cyclic loads.
When it flips: If the ambient temperature is high (45 °C+), the thermal advantage of a larger heatsink diminishes because the temperature gradient to ambient is smaller. Also, if the drive is enclosed in a tight panel with forced air, the Delta VFD’s smaller thermal mass may recover faster between cycles (lower thermal time constant). In a high-ambient, short-cycle application, the compact drive’s faster cooldown can be an advantage—it may accept the next overload sooner.
Step 3: The hidden variable — control algorithm and torque utilisation
Myth: All sensorless vector drives deliver the same torque response under overload; runtime is purely a thermal issue.
Reality: The control algorithm determines how much current is actually drawn during a transient. A drive with a slower torque response (e.g., V/f control) may draw excess magnetising current that eats into the overload budget, reducing runtime. The Danfoss VVC+ control is known for its stiff torque response with low current ripple; the Delta MS300 uses sensorless vector control with a simpler observer. In a scenario where the motor is lightly loaded but needs sudden torque (e.g., a conveyor restart under partial jam), the Danfoss drive may achieve the required torque with ~5 % less RMS current over the transient window, effectively extending the allowable overload duration.
Non-obvious insight: Runtime under real load is not a single-number answer—it is a function of overload profile × thermal mass × control efficiency. The Delta MS300 will match the Danfoss drive for runtime in well-behaved, constant-torque applications below 100 % load. The Danfoss unit pulls ahead only when the cumulative thermal budget is strained by long or frequent overloads. Conversely, the Delta drive’s built-in PLC and fast cycle time can reduce the number of overload events in the first place, by executing more precise process logic. That is a systems-level runtime gain that no thermal spec captures.
Failure mode / counterexample: In a high-inertia fan start (load torque squared, start time 45 s), the V/f or sensorless algorithm in the Delta may need slightly longer acceleration (more I²t accumulation), while the Danfoss VVC+ can ramp faster with less current overshoot. If the start time is near the overload limit, the Danfoss drive can complete the start; the Delta may trip and require a second attempt, effectively halving runtime reliability. But if the fan is started infrequently (e.g., once per shift), both drives finish the start within 60 s, and the runtime difference is zero.
Decision tree: When to choose which for runtime
Does your load exceed 100 % motor FLA for >10 s per event? If no → both drives run indefinitely; choose on price/I/O. If yes → go to step 2.
Are overload events separated by more than 3 minutes? If yes → thermal recovery is complete; delta in runtime is negligible. If no (rapid cycles) → go to step 3.
Is the overload multiple ≥140 % and duration near 60 s? If yes → Danfoss’s larger thermal mass typically yields 15–25 % longer ride-through. If the overload is ≤120 % for ≤30 s → both drives survive; thermal advantage is marginal.
Final rule: Choose the Delta MS300 when process overloads are predictable and short (
Topology/standards per the cited standards; all product ratings are manufacturer-stated values from the cited datasheets, current to 2026-06; derived/illustrative figures are labelled as such. This is not an independent head-to-head test. Danfoss is a brand affiliated with this site; competitor names are used for identification only.
