“The drive is rated for 7.5 HP” — but your pump draws 6.2 real kW. Now what?

You have a 1,800 rpm centrifugal pump that pulls 6.2 kW on the meter, nameplate says 7.5 HP (5.6 kW). The motor isn’t oversized; the pump curve puts you at 6.2 kW continuous. The VFD catalog says “7.5 HP / 5.5 kW — 120% overload for 60 s.” That’s the Delta MS300. A Danfoss VLT AutomationDrive FC 302 in the same frame size (say, 5.5 kW ND / 7.5 HP) is rated 110% overload for 60 s. Both datasheets say “7.5 HP.” But real watts don’t lie — the pump demands 6.2 kW, which is 113% of the 5.5 kW base rating. That one number, 113%, sits right where overload ratings diverge.

Let’s walk three cases that prove the point. No generic “efficiency is important” talk — only real watts, real overload, real relay.

Case A: Continuous load at 113% of base rating

The number: The Delta MS300 is dual-rated: Normal Duty (ND) 120% for 60 s, Heavy Duty (HD) 150% for 60 s. The Danfoss VLT AutomationDrive FC 302, per its datasheet, offers 110% overload for 60 s in its standard rating, with the ability to select a higher continuous current frame by going up one size — but doesn’t guarantee more than 110% in the same frame.

Mechanism: Overload rating is the time the drive can deliver >100% current before the IGBT junction exceeds the thermal limit. The MS300’s 120% ND means it can deliver 120% of its rated output current for 60 seconds out of a 5-minute window, then must return to ≤100%. For a 5.5 kW base, 120% = 6.6 kW (illustrative, assuming constant power factor). So 6.2 kW / 5.5 kW = 113% fits inside that 120% envelope. The Danfoss VFD at 110% = 6.05 kW (illustrative) — 6.2 kW exceeds that margin by about 2.5%. That 2.5% difference is the difference between a drive that thermally cycles into overload protection every few minutes and one that runs continuously within its overload budget.

Worked consequence: For a pumping application where 6.2 kW is the steady-state operating point (not a transient start or jam), the Delta MS300 can hold that load all day without tripping, as long as the overload duty cycle stays inside the 60 s/5 min window. The Danfoss in the same nominal frame (5.5 kW) would need to be up-sized to the next rating (e.g., 7.5 kW / 10 HP frame) to keep the load below 110% continuous. That upsize adds cost, panel space, and often a larger fan — real money.

Reversal: If the load is intermittent — say a conveyor that runs at 100% for 4 minutes, then drops to 30% for 3 minutes — the Danfoss’s 110% overload window can handle the transient better if the average load is lower, because its thermal model (VVC+ control, SIL2 STO) is designed for occasional peak torque events, not sustained overload. The Delta VFD’s 120% ND is more generous on sustained overload but its thermal capacity for rapid cycling (e.g., load/unload every 90 seconds) may be tighter because the 120% window resets on a 5-minute timer, not a true I²t thermal model. So a cycling load can favor the Danfoss despite the lower peak number.

Case B: Motor starting torque and locked-rotor watts

The number: The MS300 delivers 150% starting torque (HD rating) for 60 s. Danfoss VLT AutomationDrive with VVC+ control provides about 150% starting torque as well, but the torque rise is smoother at very low frequencies (0.5 Hz). Both claim similar peak torque, but the real difference is how that torque is delivered against actual motor losses.

Mechanism: A standard induction motor at locked rotor draws 6–8× FLA for a brief moment, then drops to ~1.5× FLA during acceleration. VFDs don’t deliver locked-rotor current — they ramp voltage/frequency, so the motor current stays near the overload limit. The critical number is the minimum continuous torque at low speed without overheating the motor. The Danfoss VVC+ algorithm maintains torque linearity down to 0.5 Hz, which is important for high-friction loads like screw compressors. The MS300 sensorless vector control also works well down to ~1 Hz, but the motor current waveform can contain more harmonics at very low speed, increasing motor copper losses (I²R) by an estimated 5–10% (illustrative).

Worked consequence: For a mixer that starts every 10 minutes with a high static friction load (say, 140% of rated torque), both drives can start it. But if the mixer runs at 10% speed for 30 seconds after start to clear product, the Danfoss’s smoother low-frequency control means the motor runs cooler (fewer harmonic losses) during that low-speed dwell. Over 8,000 cycles/year, that could shave ~20–30 kWh of motor heating — not a TCO killer, but it keeps the motor winding below Class F rise, which extends insulation life.

Reversal: If the load is a simple centrifugal pump that never needs less than 20% speed, the MS300’s V/f control (which runs at 1 Hz without torque compensation) is perfectly adequate. The extra control precision of VVC+ is wasted. Paying for Danfoss’s low-frequency control on a fan that only modulates between 30–100% speed is over-specification.

Case C: Efficiency at partial load — the real watts you don’t bill

The number: IEC 61800-9-2 defines IE2 efficiency classes for drives. The Danfoss VLT AutomationDrive is IE2-compliant with typical efficiency >97% at full load. The Delta MS300 also claims IE2 compliance, but its published efficiency data is less granular. At 25% load (a common pump duty point), a drive’s efficiency can drop to 93–95% due to fixed losses in the control board, fans, and switching losses that don’t scale with load.

Mechanism: Efficiency in a VFD is (output watts) / (input watts). At full load, switching and conduction losses are a small fraction. At 25% load, the fixed losses (control power, gate drive, fan) become a larger percentage. A drive with a larger fan (often in the Danfoss IP55 enclosure) consumes ~30–50 W continuously even at low load. The MS300 in its IP20 compact housing uses a smaller, temperature-controlled fan — it may consume ~15 W at low load. That 15–35 W difference at 25% load translates to about 0.5–1.0% efficiency difference in favor of the Delta at that operating point.

Worked consequence: For a pump that runs 8,000 hours/year at 25% average load (say, 1.5 kW output), the efficiency difference of 0.5% equals about 60 kWh/year. At $0.12/kWh, that’s a negligible $7.20/year. But if the site has 50 pumps, that’s $360/year. Meanwhile, the Danfoss’s higher full-load efficiency (~97.5% vs ~97.0%, illustrative) at 100% load saves about 0.5% × 5.5 kW × 2,000 hrs = 55 kWh/year per drive, or about $6.60. The numbers are close, but the Delta’s advantage at partial load is real for lightly-loaded continuous processes.

Reversal: In a variable-torque fan that operates mostly at 80–100% speed, the Danfoss’s slightly higher full-load efficiency wins. For a refrigeration compressor that runs 100% load for 12 hours then off for 12 hours, the efficiency difference is too small to matter — the overload case (Case A) dominates the sizing decision.

Rule-of-thumb: a threshold you can use tomorrow

If the steady-state motor current is ≤115% of the VFD’s base rating and the load is continuous (pump, fan, conveyor with >80% duty cycle), choose the Delta MS300 — it will fit without up-sizing. If the steady-state current is 105–110% of base rating but the load profile has sharp transients (crushers, mixers, punch presses), the Danfoss VLT AutomationDrive’s thermal model handles the spikes without nuisance tripping, even at a lower peak overload number. The tipping point is 110% continuous load with transient peaks >130%: Danfoss. Continuous load >110% with low transients: Delta.

That’s the real size rule — not “7.5 HP” nameplates, but real watts, real duty cycle, real overload budget. Spec the drive that stays inside its thermal envelope for your load, not for the motor nameplate.

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.