Hybrid liquid and air cooling of high-power permanent magnet machine rotor

An electric machine system, comprising:

• an auxiliary power unit;
• and
• an electric machine operably connected to the auxiliary power unit to drive the electric machine, the electric machine including:
• a housing:
• a stator disposed in the housing and including a stator winding;
• and
• a rotor disposed in the housing configured to rotate about a machine central axis and electromagnetically interact with the stator across an air gap between the rotor and the stator, the rotor including:
• a rotor core;
• a plurality of permanent magnets disposed at the rotor core;
• a rotor sleeve enclosing the rotor core and the plurality of permanent magnets;
• wherein the stator is configured to be cooled via a flow of cooling oil;
• and
• wherein the rotor is configured to be cooled via a flow of cooling air directed through the air gap between the rotor sleeve and the stator;
• further comprising a cooling airflow inlet at a first axial end of the housing and a cooling airflow outlet at a second axial end of the housing to direct the flow of cooling air through the air gap;
• wherein the flow of cooling air is ambient airflow directed into the cooling airflow inlet;
• and
• wherein the flow of cooling air is directed from the cooling airflow outlet to an air inlet of the auxiliary power unit.

A method of cooling an electric machine, comprising:

• directing a flow of cooling oil into a stator cavity defined between a stator sleeve and a housing of the electric machine and containing one or more stator windings;
• cooling the one or more stator windings via the flow of cooling oil;
• directing a flow of cooling air through an air gap defined between a rotor sleeve of a rotor of the electric machine and the stator sleeve;
• cooling one or more permanent magnets of the rotor via the flow of cooling air;
• flowing an ambient airflow into the air gap via a cooling airflow inlet as the flow of cooling air;
• flowing the flow of cooling air out of the electric machine via a cooling airflow outlet;
• and
• directing the flow of cooling air from the cooling airflow outlet into an air inlet of an auxiliary power unit operably connected to the electric machine.