Stepper motors can be divided mainly in two categories: those with permanent magnet and those with variable reluctance. As far as the former are concerned they consist of a main rotor, usually cylindrical, polarized and placed between stators on which some bobbins are wrapped. These are there to produce the magnetic fields which, acting upon the rotor's own polarization, (which tends to align in concordance with the generated field), make it move.

Variable reluctance motors on the other hand have a non-polarized star-shaped rotor, the star's points must be less than the number of stators around the rotor. The rotor's movement is given by the fact that, although it is not polarized, it tends to stabilize itself according to the magnetic field produced by the bobbins. Permanent magnet stepper motors divide into two subsystems: bipolar ad unipolar.

A bipolar motor is made up of a main rotor around which bobbins (generally two) are wrapped, the bobbins' terminals are connected to an external driver, in our case the i.c. 2918 Allegro, for example, which provides to feed them bi-directionally according to the working procedures and therefore of the desired magnetic field.

Unipolar motors differ from bipolar for the fact that they use double-wire bobbins (piloted one-way) to invert the flow of the field rather than single-thread bobbins piloted bi-directionally. The power supply of each single semi-bobbin and that is in one single direction and each is connected to a single driver output through one of its terminals (whereas the other is grounded). Operation modes, implemented by the external driver, are divided in one-phase-on, two-phase-on full step mode and half step mode.