[Arivaldo]

Originally Posted by Butcher

For what reason? They essentially spin a magnetic field which pulls the impellor round since it has attached magnets. That's not particularly crazy... It's not dissimilar to how a brushless fan works.

I'll try to explain (excuse for my bad english):

AC regular motors use iron build rotor (squirrel cage). They need any art to get direction. Usually shaded or delayed coils.

Originally Posted by wikipedia.org

A squirrel cage rotor is the rotating part commonly used in an AC induction motor. Internally it contains longitudinal conductive bars of aluminium or copper set into grooves and connected together at both ends by shorting rings forming a cage-like shape. The core of the rotor is built of a stack of iron laminations.
The field windings in the stator of an induction motor set up a rotating magnetic field around the rotor. The relative motion between this field and the rotation of the rotor induces electrical current flow in the conductive bars. In turn these currents flowing lengthwise in the conductors react with the magnetic field of the motor to produce force acting at a tangent to the rotor, resulting in torque to turn the shaft.
The iron core serves to carry the magnetic field across the motor. The thin laminations, separated by varnish insulation, reduce stray circulating currents that would result in eddy current loss. The material is a low carbon but high silicon iron with several times the resistance of pure iron, further reducing eddy-current loss. The low carbon content makes it a magnetically soft material with low hysteresis loss.

Originally Posted by wikipedia.org again

Small single-phase ac motors can also be designed with magnetized rotors. The rotors in these motors do not require any induced current so they do not slip backward against the mains frequency. Instead, they rotate synchronously with the mains frequency.
Because inertia makes it difficult to instantly accelerate the rotor from stopped to synchronous speed, these motors normally require some sort of special feature to get started. Various designs use a small induction motor (which may share the same field coils and rotor as the synchronous motor) or a very light rotor with a one-way mechanism (to ensure that the rotor starts in the "forward" direction).

Originally Posted by again

All single-phase motors require a means of producing a rotating magnetic field for starting. In the shaded-pole type, a part of the face of each field pole carries a copper ring called a shading-coil. Currents in this coil delay the phase of magnetic flux in that part of the pole enough to provide a rotating field. The effect produces only a low starting torque compared to other classes of single-phase motors.

These motors have only one winding, no capacitor nor starting switch, making them economical and reliable. Because their starting torque is low they are best suited to driving fans or other loads that are easily started.

The photo is of a common C-frame motor. With the shading coils positioned as shown this motor will start in a clockwise direction as viewed from the shaft end.

Butcher,
Our aquarium magnetic pumps (not iwaki) use ceramic/magnetic rotors and DO NOT use any artifice to get direction. Their impeller blades have to be "semi-crazy" (please, just remove a impeller and see) to get start without load. By absence of direction inductor their rotor can spin at any direction (if you rotate impeller with finger it will assume any direction that you want).

Dissimilarly DC brushless motors (computer fans) use 3 or 4 four coils, driven by eletronic circuit, to get rigth direction and variable speed control. By this way DC pumps (Laing, CSP, MCP) can use curved blade impeller and, better, curved blade at closed impeller.

Once more, excuse this "journal" and my english language.