the rotor thus has a predetermined axial position and runs just like a rotor which is mounted via an axial radial bearing if on the other hand the rotor 34 runs when the excitation coil 28 is acted upon by voltage in the wrong direction of rotation as is indicated by an arrow 64 in fig2 the propeller 54 sucks water in via the outlet star 44 and forces it back into the aquarium through the inlet screen 50 there is now obtained as a force of reaction with respect to the momentum imparted to the water an axial force on the unit formed by the rotor 34 and propeller 54 which force extends in the outward axial direction as a result of which the unit formed by the rotor 34 and the propeller 54 is moved towards the left in fig2 this leftward movement ends when the magnetic coupling between the stator 24 and rotor 34 has become so weak that it is no longer sufficient to turn the propeller 54 against the load constituted by the water under these conditions there is no longer any synchronisation angle wise between the rotor 34 and the alternating magnetic field however there remain axial components of the magnetic field which attempt to draw the rotor 34 back into the separating cup 32 in the axial direction in the course of this rearward movement the magnetic coupling between the stator 24 and rotor 34 is then.