A motor cooling apparatus and method to auto-circulate coolant to reduce
the weight and size of the cooling system. The cooling apparatus includes
a coolant path disposed at hot areas of the motor. A reservoir tank is
located above the coolant path and gravity feeds coolant to the coolant
path. Coolant in the vapor phase discharged from the coolant path is
liquefied by a condenser located in a coolant return path. Coolant in the
liquid phase is expelled to the reservoir tank under pressure of the
vapor-phase coolant in the coolant path to provide for the
auto-circulating coolant. Expansion pressure of the coolant supplied by
an motor inverter can further increase the pressure of the coolant in the
discharging coolant path to improve coolant circulation efficiency to
prevent dry out in the coolant path.