Sizing a 400 Hz Source for Inrush: kVA Is the Last Number to Pick

Aviation & 400 Hz · 5 min read · 33 cited facts

The first question asked about a 400 Hz source is how many kVA, and it is the wrong first question. Avionics loads are rectifier-fed: they draw their current in narrow peaks at the crest of the waveform, not smoothly across the cycle. A source sized by matching nameplate kVA to nameplate VA can hit current limit on the very first cycle — and a source in current limit is distorting the exact waveform your test depends on.

What the standard says the waveform must survive

Those three rows are voltage rows, but they size the current path. Holding distortion at ≤0.05 while a nonlinear load pulls its current in peaks means the source has to deliver that peak current without flattening the crest of the wave — the 1.31–1.51 crest factor window is gone the moment the output stage clips. Whether it clips is decided by the source's peak-current capability, not by its kVA rating.

The datasheet line that actually answers the question

For inrush and rectifier-fed loads, the figure to find is the peak-to-RMS output current ratio. The AFV-P bench series delivers a maximum of 4.5 peak/RMS — its inrush capability is 4.5 times the maximum RMS output current. Per model, on the low voltage range, that cashes out as:

• AFV-P-600 — 5 A RMS, 22.5 A peak
• AFV-P-1250 — 10 A RMS, 45 A peak
• AFV-P-2500 — 20 A RMS, 90 A peak
• AFV-P-5000 — 40 A RMS, 180 A peak

Read the list backwards from your load: the peak column is the number your DUT's front end actually meets on its first cycle, and it is what decides whether the start succeeds. A larger frame with a weaker peak ratio can fail an inrush event that a smaller frame with 4.5 peak/RMS clears. The same series holds slew under 300 µs from 0–90% of output voltage, which is what lets it run pre-compliance transient work to MIL-STD-704F and IEC 61000-4-11 rather than only steady-state soak.

Inrush is not the only overcurrent. Motor loads pull sustained overcurrent measured in seconds and minutes, and that is what an overload rating covers: the AMF series offers optional overload at 120% for 60 min, 150% for 60 s and 200% for 15 s. Overload ratings and peak ratios describe different phenomena on different lines of the datasheet — a per-cycle rectifier peak is untouched by the 60-minute rating, and a motor start cares more about seconds of sustained current than about a single-cycle peak.

When a bigger source is not the answer

• The failure is the transient envelope, not the load. Reproducing the 180 V for 10 ms surge and 80 V sag of Figure 3 takes voltage headroom and millisecond sequencing; extra kVA buys neither.
• The waveform is distorting under a nonlinear load. The fix is a source that holds the Table I distortion factor of ≤0.05 at your load's peak draw — a capability question that a larger frame with the same output stage does not resolve.
• The load returns energy. A motor-type EUT pushes energy back at the source; the AMF line is specified to handle reverse energy from motor-type EUTs and to operate into unbalanced loads, and a bigger source without that behaviour is a bigger problem, not a smaller one.
• The work is component-level. The AFV-P's 4.5 peak/RMS ratio across 600 VA – 5 kVA covers bench inrush work that would otherwise get argued into a cabinet-sized purchase it does not need.

When the answer genuinely is more power, the catalogue steps are wide. AFV-P bench units run 600 VA to 5 kVA single-phase. AMF models run from 500 VA single-phase to 100 kVA three-phase at 115/200 V, 400 Hz fixed or 350–450 Hz adjustable. AFV+ cabinets run 15 kVA to 2 MVA, with 400 Hz work sitting on the optional 300–840 Hz output band. But sizing lands on a specific model only after the peak current, the overload duration and the waveform-quality requirement are each known.