Magnetic Flux loss in PM Motors
There are two magnetic fields, a stator field and a rotor field. Torque is developed due to the attraction and repulsion forces between the two fields. The energy is supplied through electric current in the stator field. The stator is designed so that its field does not cause the rotor field to deteriorate. If that field is allowed to become too strong, the rotor can be partially demagnetized, but a proper controller design will prevent that from happening. Excess temperature is also a risk to the rotor magnets, so that also must be considered in the design. Be assured that a well designed motor and controller can operate for a very long time without significant deteriorating of the rotor magnets.

Understanding the Basic Physics

Remember that if you have a force without any motion, no work is being done, so no energy is transferred or dissipated. In an electric motor, there are two magnets with a force between them, but they don’t move with respect to each other. The distance between them may change a little, but one is pulling the other almost like scrap is lifted with a magnet in a scrap yard. The force moving the magnets does the work, not by the force holding then together.

In an AC motor, electric power makes the stator magnet rotate around the stator. The stator magnet exerts a force on the rotor magnet that holds it close. Holding the two together requires no energy. Only the electric current that is moving the stator magnet does work.

It may also be helpful to compare magnets to springs. When you compress a spring, work is done and energy is stored. When you release the spring it does work, but it doesn’t lose its strength to resist compression.