Stepper motors , also known as pulse motors, are actuators in digital control systems. Their function is to convert pulse electrical signals into corresponding angular or linear displacements. Reactive stepper motors are widely used in practice, and their structure and working principle are easy to understand. Their operating mode is different under different energizing conditions, and we can change their operating characteristics accordingly.
The rotor is constructed from magnetic materials such as silicon steel sheets or electrical pure iron rods. The outer surface of the rotor has a multi-tooth structure (the rotor's slots generate changes in magnetic reluctance during rotation, hence the name variable reluctance motor, abbreviated as VR, also known as a reactive stepper motor). When the stator coils are energized, the stator poles are magnetized, attracting the rotor teeth and generating torque, causing it to move one step. Compared to permanent magnet motors, which generate both magnetic attraction and repulsion torques, VR motors only generate attraction torque.
The structure and working principle of a VR-type variable reluctance stepper motor are shown in the figure below. In the upper part of the figure, 12 magnetic poles are evenly distributed on the stator, each 30° apart. Four coils, spaced 90° apart (three slots apart), form one phase winding. The rotor has 8 teeth. When one phase winding is energized, its stator poles attract the rotor teeth, minimizing the air gap reluctance and achieving a stationary position.
The working principle of the VR type variable reluctance stepper motor will be explained below, from step 1 to step 3.
Step 1: A simplified diagram of the first phase coil. The cross-section represents the first phase stator excitation. The rotor is attracted by the first phase stator magnetic pole, and the rotor teeth rotate below the stator magnetic pole.
Step 2: Turn off the current in the first phase winding, turn on the second phase winding, and rotate the rotor counterclockwise one step (15°) until it stops below the second phase stator magnetic pole.
The step angle of a VR variable reluctance stepper motor cannot be directly calculated using the formula θs=180°/PNr from the previous article. Instead, it is twice the value calculated using the formula θs=180°/PNr. That is, compared with the permanent magnet type, although the number of rotor teeth is the same, the VR type only has 1/2 the resolution.
Step 3: Similarly, energize the third phase winding, and the rotor will rotate 15° counterclockwise until it stops relative to the third phase stator pole. The next moment, energize the first phase winding, and the rotor will rotate 15° counterclockwise from its position in step 3 to below the first phase stator pole, returning to the state of step 1. This process of switching excitation phases—phase 1, phase 2, phase 3, phase 1…—is repeated, causing the rotor to rotate counterclockwise. This is the working principle of a VR stepper motor.
If rotating clockwise, the commutation sequence is phase 1, phase 3, phase 2. At this time, the step angle is 1/3 of the rotor tooth pitch, that is, the step angle is obtained by dividing the tooth pitch by the number of phases. The output torque is different from that of a permanent magnet motor, and it is proportional to the square of the excitation current.
VR-type variable reluctance stepper motors do not use permanent magnets. The magnetic field strength of their stator and rotor is proportional to the excitation current. To increase the magnetic field strength, a large excitation current is required, which usually results in a high temperature rise.
