A permanent-magnet alternator (PMA) in a jet engine is an alternator that converts mechanical energy into AC electricity using permanent magnets. Its rotor is mounted on the main alternator rotor shaft, and a stationary stator surrounds it. Because the magnetic field is fixed, the device is essentially maintenance-free.
Unlike a conventional alternator that relies on a spinning electromagnet, the PMA's output voltage is directly proportional to engine’s rpm. A dedicated unit mounted on the reduction gearbox supplies 32 VAC and powers the electronic engine control's individual channels, while another within the propeller shaft energises the PMU.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a permanent magnet alternator in an aircraft?
A permanent magnet alternator converts mechanical energy to AC electrical energy and supplies electrical power to the Electronic Engine Control/FADEC. It serves as a power source for each individual channel of the electronic engine control and charges the battery while the engine is running.
A permanent magnet alternator (PMA) in an aircraft is an electric generator that converts mechanical energy into AC electricity using permanent magnets embedded in steel rotors, creating a constant magnetic field without requiring a DC supply for an excitation circuit, slip rings, or contact brushes. Lightweight, maintenance-free, and less space-consuming, it is mounted on the aft side of the accessory gearbox at the 7 o'clock position or on the reduction gearbox, rotates about axis A within or adjacent to the propeller shaft, and delivers 32 VAC over a speed range exceeding 30,000 RPM and a power range from 50 W to 250 kW while operating at temperatures up to 250°C (482°F). Its main purpose is to supply electrical power directly to the Electronic Engine Control/FADEC and to the Power Management Unit (PMU), which converts the 32 VAC to DC and automatically switches to the 28 VDC battery bus when propeller RPM drops below 40-50% NP or when the PMA fails, assuring efficiency and reliability without needing an external power source. The PMA also provides electrical power directly to the rotating propeller hub and blades, creating continuous power irrespective of power demand with a high power-to-weight ratio and reliable in-service performance.
