As was mentioned in the previous sections 90 degrees even 45 degrees are very crude steps. How do actual motors achieve movements as low as 1.7 degrees. etc?
The actual stator (the stationary electromagnets) of a real motor has more segments on it than you see in our discussion and is arranged as shown Fig 28. The rotor is also a little bit different and is also shown in Fig 28.
If we have a situation such that the stator has eight individual sections on it there is 45 degrees between each section or pole piece. If the rotor has six sections on it as shown there is 60 degrees between each section. Refer fig 28.
Using the principle of a vernier mechanism the actual movement of the rotor for each step would be 60 - 45 or 15 degrees.
How is this so?
In our example pole pieces A, C, E, and G are wired together and so are pole pieces B, D, F, and H. To begin our example we assume that pole pieces A, C, D, and G are polarised such that
The rotor will be lined up accordingly
Lets say that we want the rotor to move 15 degrees clockwise
We would remove the current applied to the first set of pole pieces and reapply such that
If we wanted to rotate the rotor 15 degrees anti-clockwise pole pieces B, D, F, and H would have the opposite polarities on them.
Again practical motors would have many more sections than this but you should get some appreciation of just how small movements are achieved.
Other types of stepper motors do not have permanent magnets as their rotors but rather soft iron ones. But they work on a similar principle.
An excellent tutorial that will take you to take the next step in your understanding of stepper motor technology has been written by
Douglas Jones of the University of Iowa.
You may like to revisit the earlier section Stepper Motor Basic Theory (Part 2 (Half Step)