Inner stator¹⁾ or stationary rotor²⁾ - a part of an electric motor or generator, which looks similarly to an ordinary rotor, but is stationary and performs the function of the stator.

Inner stator (S) and outer rotor (R)^{3) USPTO, Public domain}

For instance, in an induction motor the stator envelopes the rotor which revolves inside. No electrical connections are needed to the rotor, but they can be made conveniently to the stator, which does not move.

However, in some applications it is more advantageous to have the outer part rotating, and the inner part stationary. Such construction is used for instance for direct drive of conveyor belts. The outer rotating part is designed to have the belt placed directly on it, whereas the inner part is stationary.

This concept is also used in some permanent magnet generators for wind turbines. The electrical windings and connections are made on the stationary inner part. The outer part is equipped with permanent magnets, whose rotation generates voltage in the stationary winding on the inner stator. The outer part also gives the possibility of mounting the turbine blades on it.

The difference between the two names is that inner stator refers to the actual mechanical function of the part, whereas stationary rotor to the appearance (the part is located inside, similarly to a rotor in an ordinary machine).

Inner stator

^{S. Zurek, E-Magnetica.pl, CC-BY-4.0}

Inner stator

^{S. Zurek, E-Magnetica.pl, CC-BY-4.0}

Inner stator, with wave winding visible

^{S. Zurek, E-Magnetica.pl, CC-BY-4.0}

Outer rotor

^{S. Zurek, E-Magnetica.pl, CC-BY-4.0}

• Outer rotor