The reluctance of the magnetic circuit increases with an increase in the air gap length. To produce the same flux in the motor, the stator winding draws more current if the air gap length is increased. with an increase in air gap length, the fixed losses in the motor increase and the motor efficiency deteriorate.
Flux= MMF/Reluctance of the magnetic circuit
With an increase in air gap length the reluctance increase, and to maintain the same flux the motor draws more magnetizing current.