How many volts does the typical airliner CSD (Constant Speed Drive) produce? I've read the CSD's output is typically 3-phase 115 VAC @ 400 Hz. Is that correct?

I've also read the typical airliner generator puts out "in-phase 28 VAC". Is that correct?

Does the CSD provide more voltage than the generator needs and the generator reduces the voltage via a transformer?

  • 3
    $\begingroup$ You keep referring to power but I think you mean voltage. A transformer changes voltage, and "115-VAC @ 400 Hz" gives no power indication. For electrical circuits power would be expressed in watts. $\endgroup$
    – TomMcW
    Aug 31, 2015 at 21:58
  • $\begingroup$ Sorry, yes, I mean voltage. I'll try to edit my question. $\endgroup$
    – Dan
    Aug 31, 2015 at 23:12
  • 1
    $\begingroup$ The CSD is not an electrical generator, its function is to rotate a shaft at a constant speed. The generator is then driven by this constant speed shaft. The CSD and the generator can be under the same casing, it's named an Integrated Drive Generator (IDG). $\endgroup$
    – mins
    Sep 1, 2015 at 9:36
  • $\begingroup$ Yes, I understand the CSD itself is not a generator. $\endgroup$
    – Dan
    Sep 1, 2015 at 15:43
  • $\begingroup$ anyone got a problem with 'typical airliner'? what happened to aviation SE? $\endgroup$ Nov 11, 2016 at 17:51

2 Answers 2


A constant speed drive (CSD) converts the rotation from the accessory gearbox to a predetermined constant rotation rate. The rate of the accessory gearbox is directly related to the high pressure spool (N2) rate and changes with power adjustments.

The CSD will ensure its rotation rate is constant. The Challenger 604 CSD rotates at 12,000 RPM and outputs 115 VAC @ 400 hertz. Its max output is something like 30 kVA or kilovolts-amps. Since we are talking AC power, it is not appropriate to call that watts. The AC power is converted to 28 VDC by the way of transformers called transformer rectifier units (TRU).

  • $\begingroup$ So in my comment above about CSDs producing "3-phase 115 VAC @ 400 Hz" and airline generators producing "in-phase 28 volts AC", is there a transformer in between the CSD and the generator??? $\endgroup$
    – Dan
    Sep 1, 2015 at 15:57
  • $\begingroup$ No. Most airlines have AC generators or a better term AC alternators. That power goes to power the AC buses and the TRUs. The power outut from the TRUs power the DC buses. The transformer is between the AC alternators and DC buses. There are no DC generators. At least on my airplane. $\endgroup$
    – wbeard52
    Sep 2, 2015 at 1:45

Smaller jets such as the Hondajet HA-420 and Embraer Phenom 100 & 300 use 28 VDC starter/generators (325 Amp to 400 Amp).

Larger jets may use a variety of 3 phase 115 VAC @ 400 Hz generators, 28 VDC generators, or in the case of the Boeing 787, a variable frequency starter generator -

787 Engine start system schematic—GEnx
The variable frequency starter generator is a six-pole machine within an aluminum housing driven directly from the main engine gearbox. The generator is a brushless, three-phase, alternating current, and variable frequency synchronous machine. It has a nominal rating of 235 volts alternating current (VAC), 250 kVA, three phases, and 360–800 Hz output.

787 Propulsion System


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