Rotating machines, like transformers, are also subject to failure due to overfluxing. This
can be caused by overvoltage or underfrequency, exactly as in transformers. Synchronous
machines are subject to overvoltage due to self excitation, which can be imposed on the
machine due to purely external loading conditions.
Rotating machines are also complicated because of the fact that important parts of the
machine are in mechanical motion. This gives rise to problems related to vibration, bearing
failure, mechanical resonance, and other purely mechanical failure modes that can, nonetheless,
cause the machine to be removed from service. Moreover, mechanical parts are subject to aging
and wear that require periodic monitoring in order to preserve the expected life of the machine
and maintain it in good working condition.