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I know that DC power is essential for avionics and that in medium and big airplanes AC (28 V, 400 Hz) is used with DC (28 V).

Small airplanes have a starter-generator that provides DC, but do they have an inverter too, to provide AC? If yes, then why use AC, and in which systems? And when is AC 26 V used?

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    $\begingroup$ Related: Why do airplanes use AC electric power? $\endgroup$
    – Farhan
    Jun 24, 2015 at 14:10
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    $\begingroup$ The single-engine Cessna planes I've been checked out on (172P, 172RG) have no A/C electrical system. $\endgroup$ Jun 24, 2015 at 16:36
  • $\begingroup$ As far as I know, all electricity is AC at first. All generators and alternators initially generate AC, which can then be converted to DC. This is because of the rotating motion of the coils or magnets in the generator. $\endgroup$
    – Sami
    Jun 24, 2015 at 19:36
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    $\begingroup$ @Sami: Yes, but in DC motor and DC generator the conversion is done by commutator directly on the shaft, so there is no AC outside of the motor/generator. $\endgroup$
    – Jan Hudec
    Jun 24, 2015 at 21:39
  • $\begingroup$ All the light planes that I know of have electrical systems almost identical to automobiles. In fact, the alternator on my 60s-vintage Cherokee said "MoPar" on it. And what on Earth would you use A/C for, anyway? $\endgroup$
    – jamesqf
    Jul 1, 2015 at 18:55

2 Answers 2

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I have worked on large and small aircraft. You will find a 400hz power bus only on large aircraft. The reason is the greater quantity of poles in the generator and hence weight. Smaller aircraft use a 28v DC system for the avionics.

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  • $\begingroup$ What about AC 26 V? When is it used? $\endgroup$ Jul 1, 2015 at 8:35
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    $\begingroup$ 26VAC is used on some analog instruments. Especially ones that use air. I've seen 26VAC on instruments like gyros etc ... $\endgroup$
    – Mark
    Jul 1, 2015 at 14:30
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    $\begingroup$ And to answer a second part of your question the 26vac is provided by either a static or analog rotary inverter. They are just small boxes. $\endgroup$
    – Mark
    Jul 1, 2015 at 14:33
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For small airplanes as you said it is used DC generation mainly to feed the battery which provides energy for the starter generator. I do not know of such airplanes having DC generation and the inverter for AC but I know that the ART-42 uses double generation DC and AC as well.

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  • $\begingroup$ Sorry to disappoint you, but avionics (like all computers) use DC for sure. $\endgroup$ Jul 1, 2015 at 8:34
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    $\begingroup$ Honestly it is reasonable...otherwise in an emergency situation main systems could not be feed by the battery right ? $\endgroup$
    – Davide
    Jul 1, 2015 at 15:13

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