Basic working principle of stepper motor

Stepper motors come in two basic forms: variable reluctance and hybrid. The basic working principle of the stepper motor is described with reference to the structural diagram in Figure 1. 　　Figure 1 is a schematic structural diagram of a four-phase variable reluctance stepper motor. There are eight protruding teeth on the stator of this kind of motor, and there is a coil on each tooth. The connection method of the coil winding is that the two coils on the symmetrical teeth are connected in anti-phase, as shown in the figure. Eight teeth form four pairs, so it is called a four-phase stepper motor. Its working process is like this: when one phase winding is excited, the magnetic flux passes from the positive phase teeth, through the rotor with the soft iron core, and flows to the negative phase teeth in the shortest path, while the other six convex teeth have no magnetic flux. . In order to minimize the flux path, the rotor is forced to move under the action of the magnetic field force so that the nearest pair of teeth aligns with the excited pair. In Figure 1(a), the A phase is excited, and the tooth pointed by the big arrow on the rotor is aligned with the positive A tooth. From this position, the B phase is excited again, as shown in (b) in Figure 1, and the rotor rotates 15° counterclockwise. If the D phase is excited, as in (c) in Figure 1, the rotor rotates 15° clockwise. The next step is for phase C to be excited. Because phase C has two possibilities: A-B-C-D or A-D-C-B. One is for counterclockwise rotation; the other is for clockwise rotation. But each step causes the rotor to turn 15°. Motor step length (step angle) is one of the main performance indicators of stepper motors. Different applications have different requirements for step size. Changing the number of control windings (number of phases) or number of poles (number of rotor teeth) can change the step size. The mutual relationship between them can be calculated by the following formula: Lθ=360 P×N where: Lθ is the step size; P is the number of phases; N is the number of rotor teeth. In Figure 1, the step size is 15°, which means that it takes 24 steps for the motor to rotate once. Working Principle of Hybrid Stepper Motor In practical applications, the most popular type is the hybrid stepper motor. But the working principle is the same as the variable reluctance synchronous motor shown in Figure 1. But the structure is slightly different. For example, its rotor is embedded with permanent magnets. The excitation flux is parallel to the X-axis. Generally, this type of motor has four-phase windings with eight independent lead terminals, as shown in Figure 2a. Or connect it into two three-terminal forms, as shown in Figure 2b. Each phase is driven with a bipolar transistor and the connections must be connected with the correct polarity. 　　The circuit shown in Figure 3 is the basic method of a four-phase hybrid stepper motor transistor drive circuit. Its driving voltage is fixed. Table 1 lists the logic timing of all step switches. 　　It is worth noting that the motor steps are in the order of 1-2-3-4. At the same time, two phases are excited. But phase 1 and phase 2, phase 3 and phase 4 must not be activated at the same time. The four-phase hybrid stepper motor has a very useful feature. It can be driven in half-step mode. That is, at a certain time, the step angle is only half advanced. This can be achieved with a single hybrid or with a bidirectional switch. This logic sequence is listed in Table 2. Four-phase hybrid stepper motors can also operate at higher than rated voltages. This can be reduced with a series resistor. Because phase 1, phase 2, and phase 3 and phase 4 will not work at the same time, there is only one voltage-reducing resistor for each pair, connected in series between points X and Y in Figure 3. Therefore, a stepper motor with a rated voltage of 6V can operate on a 12V power supply. At this time, a 6W, 6Ω resistor needs to be connected in series.