“But the motor nameplate says 30 A …” — Why Real Watts Trump FLA When Sizing an ABB vs Danfoss VFD

One question, three cases. You have a 7.5 kW fan motor that draws 28 A full load at 400 V — 19.4 kVA on paper. The VFD catalogue says “7.5 kW / 30 A.” Should you buy the next size up? The answer depends on whether you are sizing by real watts or by the nameplate FLA. Let’s walk through the three real-world cases that reveal the gap between datasheet and field.

Case 1: Constant-torque load — the base case that works

Take a 7.5 kW (10 hp) constant-torque conveyor motor, rated 28 A at 400 V. The Danfoss VLT AutomationDrive FC 302 is available in a 7.5 kW frame that delivers 30 A continuous at 400 V. The ABB ACS580 in the same power class delivers 30 A continuous at 110% overload for 1 minute every 5 minutes. Both drives are nominally sized.

But real watts? The motor’s shaft power at full load is 7.5 kW (mechanical). At a 90% motor efficiency, the electrical input is about 8.3 kW (illustrative). The drive’s output stage must deliver that 8.3 kW plus reactive current. At 400 V, 8.3 kW / (√3 × 0.75 PF) ≈ 16 A — well within 30 A. The 30 A rating is not the real power limit; it is the current limit, but the limiting quantity for thermal stress in a VFD is fundamental current × voltage × switching losses, which is closer to apparent power (kVA). For a constant-torque motor running at 50 Hz, the drive will never draw above 22 A in practice (illustrative). Both drives handle this with margin.

Worked consequence: If your load is constant-torque and the motor is not oversized by more than 10%, the nominal power rating (7.5 kW) is a safe sizing. No need to upsize.

When this flips: As soon as the load profile includes even modest overtorque transients (e.g., a crusher or a mixer), the drive’s overload capability becomes the real constraint — and that is not the same as real watts.

Case 2: Variable-torque (fan/pump) — the case where real watts are lower

Now the same 28 A motor drives a centrifugal fan. At 50 Hz the shaft power is 7.5 kW. But a fan follows the affinity laws: at 45 Hz, power drops to about (45/50)³ × 7.5 = 5.5 kW (illustrative). The drive never sees 7.5 kW for sustained periods. The Danfoss VLT HVAC Drive FC 102 is purpose-optimized for fan/pump loads and its overload rating is 110% for 60 s, vs the 150% for the AutomationDrive. That is fine — the real watts at design point are lower than the nameplate.

The ABB ACS580, being a general-purpose drive, also handles this easily. Its built-in features include a PID controller and a simplified assistant for fan/pump applications. The real watts here are below the nominal rating, so both drives are over-sized from a thermal perspective. The risk is not overheating, but the opposite: operating a VFD below 10% load for long periods can cause ripple current in the DC bus capacitors and shorten life — not a sizing problem by watts, but a real concern for reliability.

Worked consequence: For variable-torque loads, you can often size the VFD one frame down from the motor FLA — but only if the motor’s real watts at max flow are ≤ 80% of the drive’s rated power (illustrative threshold). The Danfoss FC 102 being a dedicated drive makes this transparent with software limits; the ABB ACS580 is equally capable but requires manual parameter setting.

When this flips: If the motor is running near its nameplate FLA continuously (e.g., a fan with a dirty filter causing constant maximum flow), the apparent power still matters — and the nameplate FLA becomes the governing number again.

Case 3: High starting torque / full torque at zero speed — the case that punishes the wrong size

This is the non-obvious insight. Consider a 7.5 kW extruder drive that requires 150% torque for 5 seconds during startup. The ABB ACS880 with Direct Torque Control (DTC) delivers up to 150% starting torque and full torque at zero speed without a separate encoder. The Danfoss VLT AutomationDrive FC 302 with VVC+ control delivers about 150% starting torque as well, but with slightly lower bandwidth during high-frequency starts.

Now the real watts: during that 5-second peak, the motor draws roughly 1.5× rated current → 42 A (illustrative). The drive’s overload rating: ABB ACS880 typically 150% for 60 s (Heavy Duty); Danfoss FC 302 offers 160% for 60 s in heavy-duty mode. Both can handle 42 A for 5 s — but the sizing trap is that if you had sized by the motor’s real continuous watts (7.5 kW) and used a 7.5 kW drive, you are at 100% of its continuous rating plus an overload cycle. That is acceptable per IEC 61800-5-1 thermal stress criteria. However, if you used the nameplate FLA (28 A) to select a 30 A-rated drive, you are fine. Where it fails: if the motor is oversized (e.g., a 7.5 kW motor driving a 5 kW load) and someone picks a 5.5 kW drive by the real watts, the overload capability of a smaller frame may be insufficient.

Worked consequence: Always size the VFD based on the motor’s full-load current, not the load’s absorbed power. The real watts of the load can mislead you into downsizing the drive, then failing on the first startup transient.

Failure mode — the case where this breaks: If the load is purely resistive (e.g., a brake resistor bank), the VFD’s current rating is the only relevant value — real watts equal apparent watts, and the drive must be sized by continuous current, not motor FLA. But that is a rare corner case.

Decision rule: when to size by real watts vs FLA

After these three cases, a usable threshold emerges:

• If the load is variable-torque (fan/pump) and the motor operates below 95% of its nameplate FLA continuously, you can size the VFD one frame down from the motor power — real watts are lower. Use the drive’s variable-torque rating (if available).
• If the load is constant-torque with no more than 10% short-term overload, size the VFD to the motor’s nameplate power — real watts and nameplate kW align.
• If the load requires high starting torque (≥150%) or runs at zero speed, size the VFD to the motor’s FLA plus 10% margin, regardless of real watts. The ABB ACS880 with DTC handles this with lower margin because it controls torque directly; the Danfoss VVC+ may need a slightly larger frame for the same margin.

Bottom-line, real-world advice: Stop looking at the motor nameplate kW as the “real watts” — it is a mechanical rating. The VFD sees electrical kVA. For the ABB VFD vs Danfoss VFD choice on typical industrial loads, both brands will serve you well if you size by FLA + overload. But if you try to size by absorbed power (real watts), the Danfoss VLT family’s application-specific variants (FC 102 for HVAC, FC 302 for industry) offer software guardrails that reduce the risk of under-sizing; the ABB ACS580/ACS880 require more upfront engineering but reward you with higher dynamic performance when the load is unpredictable.

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