An electromagnet can have the polarity of its poles changed instantaneously by reversing the flow of current through its windings See fig 12.

This will have the effect of reversing the North and South poles. As the bar magnet (now known as the rotor or armature) reaches the vertical position the current through the Stator is reversed, causing the magnetic poles to reverse. 

This effect causes the armature to be driven through 180^o until the armature north pole is in line with the stator south pole. When the rotor reaches the vertical position the stator field is reversed causing the rotor to turn through 180^o until it completes another half turn. The direction of movement in the rotor depends on several factors as shown in Fig 12.

By using the left hand rule the motion of a current carrying conductor in a magnetic field can be predicted
Hold out your left hand as shown in fig 13.

Fig 12

The index finger pointing along the direction of the magnetic force.

The middle finger pointing along the direction of current flow.

The thumb points in the direction of motion the conductor will take.

If rotor turns one half revolution for each half of the 50Hz cycle, it will turn 50 revolutions per second