400 Hz AC Source Buyer's Checklist: Sizing for Inrush, Crest Factor and Nonlinear Loads

Aviation & 400 Hz · 3 min read · 9 cited facts

The specification that gets a 400 Hz source bought is usually kVA. The specifications that determine whether it does the job are further down the datasheet.

Start with what you are actually powering

Supplying ground power at nominal and running qualification testing are different purchases. The first needs capacity and reliability. The second needs programmability, and a source that cannot sequence a transient envelope cannot run the clauses at all — no amount of headroom compensates.

Size on peak, not RMS

A source selected on RMS headroom alone will clip the surge. A clipped surge is not a harder test passed — it is a different, easier test that your device under test passes for the wrong reason, and it will not survive review.

Nonlinear loads eat headroom you did not budget

Avionics equipment is rarely a resistive load. Rectifier front ends draw current in narrow conduction windows, so the peak current is a multiple of the RMS figure. Sizing a source on RMS load current and discovering the crest factor at commissioning is a common and expensive sequence of events.

The capability checklist

• Peak voltage capability, not only RMS — the transient envelope decides this
• Frequency: fixed 400 Hz, or a programmable range if your platform is variable-frequency
• Programmable rate of change of frequency if you have to run variable-frequency clauses
• Three-phase with independent phase-angle control, since phase difference is specified
• Millisecond-resolution envelope sequencing rather than setpoint stepping
• Tolerance of unbalanced load conditions
• Reverse-energy handling for motor-type equipment under test
• Overload profile — how much, for how long, and how often
• Remote interface that your test executive already speaks

Chassis format is a real constraint

Bench units and cabinet systems are not interchangeable, and the crossover point arrives lower than people expect. Above a few kVA the decision stops being about the instrument and starts being about floor space, cable runs, service entrance and whether the unit needs to move between test cells. Trailer-mounted configurations exist for ground-power work, and that requirement should be raised at quote rather than discovered afterwards.