Delta vs Danfoss VFD: when the load doubles, which one stays in control?

You picked a drive. Motor nameplate says 3.7 kW, load is a centrifugal pump. Then someone adds a second pump on the same shaft, or a winter viscosity spike doubles the torque demand. You need 150 % for a minute. Which drive actually delivers the torque without tripping? That's the failure-mode question. No one sizes a VFD for the happy path. They size it for the moment the load doubles.

Myth #1: "All drives give you 150 % overload — the rating is the rating"

Reality: The duration and thermal tolerance differ sharply. The Delta MS300 carries a dual rating: Normal Duty (ND) with 120 % overload for 60 s and Heavy Duty (HD) with 150 % overload for 60 s. The Danfoss VLT AutomationDrive FC 302 does not publish a blanket 150 % for 60 s in its standard spec—its overload capability is application-dependent; typical industrial setting offers 110 % overload for 60 s every 10 min, with high-overload versions reaching 160 % for 1 s (torque boost) only.

Mechanism: Overload capability is limited by the IGBT junction temperature rise. The Delta MS300 uses a conservative thermal design in the compact frame—it allows the 150 % for 60 s only in Heavy Duty mode, which derates the base current (e.g., a 5.5 kW HD rating means the drive is oversized in VA relative to motor). Danfoss VFD's VVC+ control can maintain torque at low speed, but maintaining 150 % torque for 60 s requires a larger frame size than the motor's base power.

Worked consequence: If you size a Delta MS300 at "HD" for a 5.5 kW motor, it can survive a 60‑second, 150 % load—that's 8.25 kW demand. The same motor with a Danfoss FC 302 sized at the standard 110 % overload would trip on a 60‑second, 150 % load unless you oversize the drive by one frame (e.g., choose a 7.5 kW drive). That oversizing costs ~30 % more in hardware.

When this myth flips: If your highest overload lasts 10 seconds (e.g., a breakaway conveyor), Danfoss's short‑term peak of 160 % for 1 s can handle the spike without frame oversizing. For long‑duration overload (>60 s), neither drive will hold 150 %—you must size for the continuous torque regardless of brand. The myth is only dangerous when someone assumes "150 % overload" means "any duration."

Myth #2: "Sensorless vector control is sensorless vector control — same torque precision"

Reality: Torque accuracy at low speed under doubled load diverges. The Delta MS300 offers sensorless vector control (SVC) plus V/f. The Danfoss VLT AutomationDrive uses VVC+ control, a proprietary voltage‑vector control that maintains full torque down to ~2 % of rated speed. Under a doubled load at low speed (e.g., 5 Hz extruder start), VVC+ keeps the flux angle correct within a few electrical degrees; the MS300's SVC may lose orientation, reducing torque to 80–90 % of demanded.

Mechanism: Sensorless vector control estimates rotor position via back‑EMF. At low speed (below ~5 Hz), back‑EMF is weak—noise from load current dominates. VVC+ uses a high‑bandwidth current regulator that compensates for stator resistance drop, whereas the MS300's simpler estimator relies on an open‑loop slip calculation. Doubling the load increases slip by proportion; if the slip estimator lags, the drive commands the wrong frequency and torque droops.

Worked consequence: On a 4‑pole, 5.5 kW motor (rated torque ~35 N·m), a doubled load demands ~70 N·m at 10 Hz. The Danfoss FC 302 can deliver that within ~5 % accuracy. The Delta MS300 under HD rating (150 % for 60 s) can also supply 70 N·m, but the torque accuracy may be ±15 % at 10 Hz, causing speed fluctuation of ±2–3 Hz—enough to trip a downstream pressure switch on a pump.

When this myth flips: If your load doubles at high speed (>20 Hz), back‑EMF is strong and both drives deliver nearly identical torque. SVC is fine for fans, conveyors, and mixers that never run below 15 Hz. The myth matters when someone picks a drive for a low‑speed, high‑torque application (extruder, centrifuge) and assumes "sensorless vector" means equal performance.

Myth #3: "Built‑in EMC filter means you don't need external hardware — same compliance"

Reality: The filter's operating current range and common‑mode attenuation differ. The Delta MS300 includes a built‑in C2/C3 EMC filter (optional with capacitive variants). The Danfoss FC 302 can be ordered with built‑in RFI filters, but the standard unit (IP20) often ships without a full Category C2 filter unless specified; the typical default meets Category C3 (industrial environment).

Mechanism: A C2 filter limits conducted emissions to 150 kHz–30 MHz for domestic/supply‑line environments, but it also imposes a leakage current to ground. When the load doubles, the DC‑bus current rises, and common‑mode voltage increases proportionally. The built‑in filter's choke may saturate at >1.5× rated current, reducing common‑mode attenuation by 10–15 dB.

Worked consequence: With a doubled load, the MS300's built‑in C2 filter may still meet C2 limits up to ~2× rated current (illustrative, based on choke design). A Danfoss FC 302 with a standard C3 filter (no C2 option step) would fail C2 limits at the same load, requiring an external C2 filter that adds ~€80–120. In practice, for a 5.5 kW drive running at 150 % load for 60 seconds, the MS300's built‑in filter maintains compliance; the Danfoss without a specified C2 filter would need an add‑on.

When this myth flips: If your installation is in an industrial environment (C3 only required), both drives meet the standard without extra cost. The myth is dangerous for commercial buildings (C2 required) where the Danfoss standard unit is C3—someone might assume "built‑in filter" equals "C2 compliance." The Delta VFD's standard C2 filter is an advantage only in that narrow case.