Motor-driven roots pump with smooth activation in low-temperature environment

A motor-driven Roots pump, comprising:

• a housing;
• a drive shaft and a driven shaft that are rotationally supported by the housing, the drive shaft and the driven shaft having axial lines that are parallel with each other;
• a drive gear that is fixed to the drive shaft;
• a driven gear that is fixed to the driven shaft and meshes with the drive gear;
• a drive rotor that is provided on the drive shaft;
• a driven rotor that is provided on the driven shaft and meshes with the drive rotor;
• an electric motor that is configured to rotate the drive shaft;
• a motor chamber that is defined in the housing and accommodates the electric motor;
• a gear chamber that is defined in the housing and accommodates the drive gear and the driven gear, oil being sealed in the gear chamber;
• and
• a rotor chamber that is defined in the housing and accommodates the drive rotor and the driven rotor, wherein
• the motor chamber, the gear chamber, and the rotor chamber are arranged in order along the axial line,
• the housing includes
• a first partition that separates the gear chamber and the motor chamber from each other in an axial direction of the drive shaft and includes a first defining surface that defines the gear chamber,
• a second partition that separates the gear chamber and the rotor chamber from each other in the axial direction and includes a second defining surface that defines the gear chamber, and
• a relief recess that opens in at least one of the first defining surface and the second defining surface,
• when viewed in the axial direction, an addendum circle of the drive gear and an addendum circle of the driven gear intersect with each other at a first intersection point and a second intersection point,
• a plane that includes both of the axial line of the drive shaft and the axial line of the driven shaft is defined as an imaginary plane,
• the first intersection point is located on a side of the imaginary plane on which the drive gear and the driven gear start meshing with each other,
• the second intersection point is located on a side of the imaginary plane on which the drive gear and the driven gear finish meshing with each other, and
• an opening of the relief recess is arranged in a region on a side of the imaginary plane on which the first intersection point is located.