To keep the thermal-cycle efficiency high throughout the APU power-delivery range, the UAAPU is operated at varying speeds.
The air bearings - more aptly named foil bearings - used in the UAAPU are self-acting hydrodynamic-fluid-film bearings.
High damping in the foil-bearing system permits stable rotor dynamic operation throughout the UAAPU operating range.
The PEU, which resides in the UAAPU between the vehicle bus and the turbogenerator, is required to drive the PM machine as a motor during start-up and to condition the electrical power out of the PM machine during its generating mode.
The modulation scheme, therefore, is used to regulate the UAAPU output voltage to the vehicle electrical bus voltage.