What is a static frequency converter?
If you need 400 Hz from a 50 Hz supply, a transformer will not do it. Frequency is not something you can change with turns.
A static frequency converter takes the incoming mains at one frequency and synthesises an output at another. It does it electronically: rectify the input to DC, then invert that DC back to AC at whatever frequency, voltage and phase configuration the load requires.
Why 400 Hz exists at all
Aircraft. At higher frequency, transformers and motors deliver the same power with far less iron and copper, so the equipment is dramatically smaller and lighter — which matters more in an airframe than almost anything else. Aviation standardised on 400 Hz for that reason, and everything on the ground that has to supply or test an aircraft has to match it.
The same logic appears elsewhere: 60 Hz supplies for equipment built to American specification, and variable-frequency outputs for test benches where a motor or product has to be exercised across a range.
Static converter versus motor-generator set
Both convert frequency. Only one has moving parts, and that is the whole argument. A motor-generator does it mechanically — a motor on the incoming supply driving an alternator at the required speed. It works, it is well understood, and it brings bearings, noise, a foundation, alignment, and a maintenance schedule.
A static converter has none of those. It is quieter, more efficient, needs no rotating maintenance, starts instantly, and holds tighter output voltage and frequency regulation because the output is synthesised rather than dependent on shaft speed. It is why new ground installations are almost always static, and why MG sets that fail are usually replaced rather than rebuilt.
It is not a UPS, and not a stabilizer
Those three get confused in enquiries. A UPS provides continuity when the supply fails. A servo stabilizer corrects voltage magnitude at the incoming frequency. A frequency converter changes the frequency itself. If the requirement is "clean 400 Hz whenever the mains is present", the converter is the answer and adding backup is a separate decision.
What actually has to be specified
- Output frequency, and whether it is fixed or variable. A fixed 400 Hz unit and a variable-frequency test supply are different machines.
- Output voltage and phase configuration, including whether a neutral is required. 400 Hz aircraft supplies are commonly 200/115 V three-phase four-wire; test benches vary widely.
- Continuous rating in kVA and the load power factor.
- Load type. A resistive bench load, a motor with high starting current and an aircraft with switching converter loads present three quite different problems.
- Whether more than one frequency has to be served, and whether simultaneously — that is usually the difference between one machine and two.
- Cable run length from converter to the load, because at 400 Hz voltage drop and reactance matter far more than at 50 Hz.
The cable point is worth repeating
Reactance rises with frequency. A cable run that is unremarkable at 50 Hz can produce significant voltage drop at 400 Hz, so the distance between the converter and the aircraft or bench is part of the design rather than an installation detail. Give the intended run length at enquiry; changing it afterwards means changing the specification.
Documentation, on defence and aerospace work
Expect more than on commercial supply, and agree it before manufacture. Test certificates, defined inspection stages, traceability of major components and customer or third-party witness of testing are commonly specified, and each of them affects the build programme rather than just the paperwork at the end. Send the technical specification and the quality requirements together so the inspection plan is priced in.
Questions we get asked
Anything not covered here, call us — most of it is faster to answer on the phone.
01Why do aircraft use 400 Hz instead of 50 Hz?
Because transformers, motors and generators can be far smaller and lighter at higher frequency for the same power, and weight is the binding constraint in an airframe. Ground equipment supplying or testing aircraft has to match that, which is what creates the need for a converter.
02What is the difference between a static frequency converter and a motor-generator set?
A motor-generator converts frequency mechanically, with the bearings, noise, foundation, alignment and maintenance that implies. A static converter does it electronically — rectify to DC, invert at the target frequency — so it is quieter, more efficient, needs no rotating maintenance and holds tighter regulation. Most new installations are static for those reasons.
03Can one converter output both 400 Hz and 60 Hz?
Not usually from the same output at the same time, and it should be stated at enquiry rather than assumed. Output frequency, voltage and phase configuration are design decisions. Tell us every frequency and voltage the installation must serve and the answer may be one machine or two.
04Is a frequency converter the same as a UPS?
No. A UPS provides continuity when the supply fails; a frequency converter changes frequency while the supply is present. They are sometimes specified together, but neither performs the other’s function.
05Does a frequency converter also correct voltage?
It regulates its own output voltage, which is set independently of the input, so within its input tolerance it will hold the output steady. That is different from a servo stabilizer, which corrects magnitude at the same frequency and does not change frequency at all.
06Does cable length matter for a 400 Hz supply?
Considerably more than at 50 Hz, because reactance rises with frequency. A run that would be trivial on a mains circuit can cause meaningful voltage drop at 400 Hz, so the distance to the aircraft or bench belongs in the specification.
07What ratings are available?
Rating is driven by the load and the duty rather than by a catalogue, and units are built to the required output. Send the connected load, power factor, duty cycle and the output configuration and it can be sized against those.
08What documentation is provided for defence supply?
Test certificates, inspection stages, component traceability and witness testing where specified. Agree the quality requirements at enquiry rather than after manufacture, because they affect the build programme and not only the paperwork.