Fig. 5.62 shows the basic principle of how a rotor moves in steps in the case of a permanent magnet rotor. Consider a four stator poles and permanent magnet rotor, as shown in fig. 5.62(b). To start with in step 0, the rotor is in equilibrium since opposite poles are adjacent to each other and hence attracts each other. The rotor can remain in this position and can with stand the opposing torque called holding torque until the magnetization of the stator poles are changed. Once the magnetization of the stator poles are changed (step 0 to step 1) a troque is induced to the rotor causing it to move by 90° in the CW direction and the next equilibrium position is achieved as shown in step 1. When the magnetization is again changed (step 1 to step 2) again a torque is induced in the rotor and it rotates by another 90° and another equilibrium position is obtained as shown in step 2. Successive change of magnetization of stator poles thus rotate the poles in steps of 90°. The direction of rotation of the rotor depends on the direction of sequencing of magnetization of poles, i.e., counter clockwise sequence of magnetization of stator poles, rotates the rotor in CCW direction.
Variable reluctance stepper motor
The variable reluctance stepper motor has a ferro- magnetic rotor rather than a permanent magnet rotor fig. 5.63. Motion and holding are results of minimization of the magnetic reluctance between the stator and the rotor poles. The number of poles on the rotor will always be smaller in number than that on the stator. When current is passed through a pair of stator poles having maximum reluctance path, magnetic field is produced with lines offered trying to shorten themselves, rotates the rotor until the stator and rotor poles lines up which will be the shortest or minimum reluctance path. The steps of angles that are generally obtained with this type of motor is 7.5° or 15°. A variable reluctance motor has the advantage of a low rotor inertia and hence faster dynamic response.
Hybrid Stepper motor : This combines the feature of both permanent magnet and variable reluctance stepper motors. It consists of a permanent magnet mounted inside iron caps. These caps have teeth cut on it as shown in fig. 5.64. This unit forms the rotor of the hybrid stepper motor. The rotor unit itself has minimum reluctance position in response to a pair of energized stator coils. Such motors find its application extensively in high accuracy positioning (e.g., computer hard disc), with a typical step angles of 0.9° and 1.8°.