varying inductance of solenoid

[Antimatter90]

Can anyone explain the concept behind changing of inductance of a solenoid (when AC current is passing through solenoid) on moving a metal sleeve (hollow metallic pipe) over the solenoid enclosing the solenoid.

 

[jiripolivka]

An inductance of a coil or solenoid is usually calculated in free space.

Once the coil's alternating magnetic field is disturbed by a metallic (conductive) object like you wrote, a larger pipe, by induction, currents induced by the primary field are flowing in the pipe; the magnetic field generated by that pipe current affects the primary current in your coil.
The above is the principle of an AC transformer; the specialty is that your secondary "coil" is shorted ( a pipe). Similar structure is used in short-current AC motors.
For a detailed analysis, my advice is to study a book on electrical power engineering.

 

[karesz]

I think its more an situation with Eddy current & back inductions...
K.