Spacecraft flywheel systems are critical actuators for attitude control and
stability maintenance, and their operating reliability is strongly affected by
the health state of the supporting bearings. When local defects occur on the
bearing raceway or rolling elements, the rolling contact state changes abruptly,
which leads to time varying contact deformation, fluctuating support reaction
force, and additional rotational resistance. Unlike many ground based rotating
machines, spacecraft flywheel systems usually operate in compact and highly
integrated configurations, where direct vibration measurement is difficult in
practice. In contrast, motor current, voltage, and speed signals are easier to
acquire and are therefore more suitable for condition monitoring. To describe
how bearing defects propagate from the mechanical domain to electrical
responses, a simplified electromechanical coupling model is developed for a
flywheel rotor motor system with local bearing defects. First, a rolling element
contact model is established using the defect induced deformation relation and
Hertzian contact theory to derive the time varying bearing support force.
Second, the fluctuating support force is mapped to an equivalent time varying
load torque to represent the dominant pathway through which bearing defects
influence the motor side. Third, the derived load torque disturbance is coupled
with the transverse rotor dynamic equations, the rotational motion equation, and
the motor dq axis model to obtain a simplified electromechanical coupling
equation set. The proposed model preserves the main physical transmission chain
from bearing defect to electrical response. The model provides a clear
theoretical basis for analyzing fault propagation mechanisms and for developing
current based diagnostic methods for spacecraft flywheel systems.