  Harmonic stator-rotor coupling offers a promising approach for the
interconnection of rotating subsystems in the simulation of electric machines.
This paper studies the stability of discretization schemes based on harmonic
coupling in the framework of mortar methods for Poisson-like problems. A
general criterion is derived that allows to ensure the relevant inf-sup
stability condition for a variety of specific discretization approaches,
including finite-element methods and isogeometric analysis with harmonic mortar
coupling. The validity and sharpness of the theoretical results is demonstrated
by numerical tests.
