From T6b Naval manual:

A dedicated permanent magnet alternator (PMA), mounted on the reduction gearbox, powers the PMU. The PMA supplies 32 VAC, which the PMU converts to DC. The PMU automatically switches to the 28 VDC battery bus when propeller RPM drops below 40-50% NP, or when the PMA fails.

  • What does the PMA really do? From looking online I only found that in other engines the PMA is good to convert mechanical energy into electrical not sure if this is the case for the PT6 engine.
  • I think it has to be with the startup process, not sure. During the startup the aircraft its connected to an Electrical Power source if battery is low but the engine does not require any gear accessory connection, only electrical power. I have seen how big jet engines need an accessory gear connection go turn the compressor, how does this PT6 engine actually starts? Im guessing electrical power somehow moves the compressor shaft?
  • The PT6-68 engine has an Starter/Generator that acts as an starter and then a generator, which is aft the accessory gear box. Is this related to the PMA? Does the starter somehow moves the compressor shaft?
  • I couldn’t find much in The P&W manual, its in the link below. Thank u in advance https://mikeklochcfi.files.wordpress.com/2018/08/training-pt6a-60-series.pdf

1 Answer 1


A permanent magnet alternator is just an alternator. It produces electricity and charges the battery while the engine is running. The type of alternator is a largely irrelevant detail here, but in case you're interested:

  • A permanent magnet alternator is maintenance-free, but output voltage is proportional to rpm, which is undesirable for most applications.

  • A conventional alternator has a spinning electromagnet, powered from the battery via brushes. The brushes wear out and must be replaced occasionally. This setup allows magnetic field strength to be adjusted by a regulator, so that the alternator produces a constant voltage regardless of rpm.

  • $\begingroup$ Thank you for the input. But based on the statement from the book, it looks like the PMU can get its power from the battery bus, so why putting an alternator at all, If you could power the PMU with the battery bus that is being powered by the Generator whenever the aircraft is working properly? $\endgroup$ Commented Oct 19, 2022 at 12:23
  • $\begingroup$ @YamchaAviator - Is there a separate generator/alternator somewhere else or is this the only one? If it is not the only one, then maybe for backup electricity when the battery bus fails while the engine is running? $\endgroup$
    – Rainer P.
    Commented Oct 19, 2022 at 12:37
  • $\begingroup$ @YamchaAviator - By the way, what is a PMU? You use that abbreviation a lot without ever explaining it. $\endgroup$
    – Rainer P.
    Commented Oct 19, 2022 at 12:46
  • $\begingroup$ @YamchaAviator Redundancy. You need to use ship's power/battery to power the engine electronics during start (and maintenance), when the PMA isn't spinning. Once the PMA reaches a minimum speed, the EEC/FADEC transfers over to it. If either the ship's power bus or PMA fails, engine operation continues with the operational source. I believe some engines also read PMA frequency as a tachometer, instead of/redundant with a separate sensor. $\endgroup$
    – user71659
    Commented Nov 19, 2022 at 0:49

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