“I sized the drive to the motor nameplate—so why did it trip on a real 5 kW load?”

You look at the motor plate: 5 kW, 480 V, 10.5 A. You pick a 5.5 kW drive—plenty of headroom, right? Then on a real pump curve with a slightly worn impeller, the drive goes into overcurrent at 4.8 kW shaft power. That mismatch is not a fluke. It is the gap between nameplate power and real watts, and it punishes drives that treat overload ratings as optional headroom instead of a design boundary.

Let’s walk through three cases. Each starts with a measurable number from the datasheets, traces the real chain of cause and effect, and ends with a decision you can bet the plant on. No “depends” — just thresholds.

Case 1: The 60-second overload that isn’t a safety factor

Delta MS300 standard duty (ND) is rated 120% overload for 60 seconds every 5 minutes; heavy duty (HD) is 150% for 60 seconds. Danfoss VLT AutomationDrive FC 302, in typical configuration, offers 110% for 60 seconds (standard) and up to 160% for 1 second with its VVC+ control. These numbers look similar until you apply them to real watts.

Suppose your motor is rated 5.5 kW (nameplate), but the actual process load — pump with reduced discharge head, conveyor with mechanical friction from packing — draws 5.2 kW steady-state. That’s 95% of the drive’s nominal rating. Now a momentary jam or a high-inertia start pushes the demand to 7.2 kW for 20 seconds. On the MS300 (HD rating: 150% = 8.25 kW for 60 s), 7.2 kW is 131% — still within HD limit. On the Danfoss VFD, 110% for 60 s = 6.05 kW peak; 7.2 kW is 131% of its nominal, which exceeds its standard overload by a wide margin. The Danfoss will trip in about 5–8 seconds unless you have configured its overload profile to “high overload” (typically 160% for 1 s, then rolls back to 110%).

The worked consequence: If your real load hovers at 90–95% of the drive’s nameplate rating, the Danfoss offers less usable headroom for short overloads. You must derate the drive by one frame size (e.g., pick a 7.5 kW unit for a 5.5 kW motor) to maintain the same 60-second margin. That adds cost and panel space. The MS300, with its 150% HD rating, can handle the same real-watt bulge without a frame upsize — as long as the overload is within 60 seconds and the cycle is respected (every 5 min).

When this flips: If your process has frequent, high-peak, short-duration overloads (e.g., crusher start at 180% torque for 200 ms), Danfoss’s 160% for 1 second can ride through that pulse better than the MS300’s 150% for 60 s. Here, MS300 would need an even larger frame to survive a 180% spike—Danfoss wins on surge-only profiles.

Case 2: The 75% load efficiency blind spot

Delta MS300 achieves >96% efficiency at full load (illustrative, about 96.5% at 5.5 kW ND). Danfoss VLT AutomationDrive FC 302 quotes typical efficiency >97% at full load for 5.5–11 kW units. At face value, Danfoss looks better. But the real-watt question: what efficiency at 40% load, where most variable-torque (fan/pump) drives live 80% of their life?

Measured motor losses are roughly fixed, but drive losses scale with current. At 40% load (2.2 kW on a 5.5 kW system), the MS300’s sensorless vector control maintains decent flux angle accuracy down to about 10:1 speed range, keeping efficiency above 94% (illustrative). The Danfoss with VVC+ control can also hold ~95% at 40% load. The difference, about 1 point, translates to roughly 22 W of losses at 2.2 kW output. Over 8,000 hours per year, that 22 W means 176 kWh — about $18–22 at typical industrial rates. Negligible for a single drive, but a 50-drive line: $900–1,100 per year.

Worked consequence: For a fan array with 40 VFDs averaging 40% load, the Danfoss saves about $800/year in electricity versus the MS300. The Danfoss unit costs perhaps $200–300 more per drive; payback in 10–15 years on electricity alone. Not compelling unless you also get soft-start savings, reduced wiring, or longer motor life.

When this flips: If your load is constant-torque (conveyor, extruder) running above 80% load, the full-load efficiency delta VFD vanishes — both are above 96%. The MS300’s lower upfront cost becomes the deciding factor, and the 1% efficiency gap shrinks to pocket change.

Case 3: The EMC filter that turns real watts into leakage

Delta MS300 offers built-in C2/C3 EMC filters as standard; C2 is stricter (internal, for first environment residential/commercial), C3 is industrial. Danfoss AutomationDrive ships with integrated C2/C3 filters on most frames, plus an option for C1 (very low emissions).

Here’s the less obvious link: The filter capacitors create leakage current to ground. At 480 V, a standard C2 filter on a 5.5 kW drive can leak 30–50 mA (illustrative). If you have ten drives on a single ground rod, total leakage can approach 0.5 A. That is real watts lost as heat in the ground path — not large, but enough to trip a 30 mA RCD unexpectedly. More relevant, leakage increases with cable length and switching frequency. The MS300’s standard filter provides C2 compliance, but if your real installation has long motor cables (over 50 m) and high switching frequency (8 kHz), leakage may force you to disable the filter or add a line reactor. Danfoss offers filter options that can be selectively removed via software (dU/dt filter).

Worked consequence: In a plant with 30 drives and motor cables averaging 75 m (common in conveyor lines), MS300’s fixed filter may cause nuisance ground-fault trips unless you switch to a lower switching frequency (4 kHz) or add external reactors — both add cost and reduce motor efficiency (more harmonic losses). Danfoss’s adaptive filter approach may let you keep 8 kHz with lower net leakage, saving $100–200 per drive in external filtering, but at a higher drive price.

When this flips: If your installation is short-cable (

A non-obvious insight: the “load factor” that the datasheet hides

The MS300’s overload rating (150% for 60 s) gives it a headroom advantage for real-watt bulges that last 10–60 seconds. The Danfoss’s 160% for 1 second gives it a different advantage: momentary surges. Most engineers look at “peak kW” without decomposing the time axis. The real decision: measure the duration of your peak. If it’s under 1 second, Danfoss wins; if it’s between 1 and 60 seconds, MS300 wins; if it’s longer, either drive requires a bigger frame.

Real-watts decision table (5.5 kW class, 480 V)

The one rule you can apply now

Go to your plant and measure the real-watt load profile on one representative drive for 30 minutes: plot the current every 5 seconds. Calculate the RMS current and the peak current duration. If the RMS is above 85% of the drive’s nominal current and the peak exceeds 125% for more than 5 seconds, you need a drive with a 150% overload rating (HD class). If the peak is brief (140%, a drive with a 160% surge rating is safer. That decision — not the motor nameplate — determines whether your VFD lives through the summer.

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. Delta is a brand affiliated with this site; competitor names are used for identification only.