## Analysis of working principle of motor

1.The basic working principle of stepper motor

Stepper motors come in two basic forms: variable reluctance and hybrid. The figure above is a schematic diagram of the structure of a four-phase variable reluctance type stepping motor. This motor has eight protruding teeth on the stator and one coil on each tooth. The coil windings are connected in such a way that the two coils on the symmetrical teeth are connected in reverse, as shown in the figure. The eight teeth form four pairs, so they are called four-phase stepper motors.

Its working process is as follows: when one phase winding is excited, the magnetic flux flows from the positive phase tooth through the soft iron core rotor and flows to the negative phase tooth 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 is aligned with the excited one. In Figure 1(a), phase A is energized and the tooth pointed by the large arrow on the rotor is aligned with the positive A tooth. From this position, the phase B is excited again, as in (b) of Fig. 1, the rotor is rotated by 15° counterclockwise. If phase D is energized, as in (c) of Figure 1, the rotor is rotated 15° clockwise. The next step is that phase C is motivated. Because the C phase has two possibilities: A-B-C-D or A-D-C-B. One is counterclockwise; the other is clockwise. But each step turns the rotor 15°. Motor step size (step angle) is one of the main performance indicators of stepper motors. Different applications require different step sizes. By changing the number of control windings (phase number) or the number of poles (number of rotor teeth), you can change the step size. The relationship between them can be calculated by:

Lθ=360 P×N

Where: Lθ is the step size; P is the number of phases; N is the number of teeth of the rotor. In Figure 1, the step size is 15°, which means that the motor needs 24 steps to make one revolution.

2. Working principle of mixed stepping motor

In practical applications, the most popular is the hybrid stepping motor. However, the working principle is the same as that of the variable reluctance synchronous motor shown in FIG. But the structure is slightly different. For example, its rotor is embedded with a permanent magnet. The excitation flux is parallel to the X axis. In general, this type of motor has a four-phase winding with eight independent lead terminals, as shown in Figure 2a. Or connect into two three-terminal form, as shown below. Each phase is driven with a bipolar transistor and the polarity of the connection is correct.  The circuit shown in the picture is the basic way of the four-phase hybrid stepping motor transistor drive circuit. Its drive voltage is fixed.
A four-phase hybrid stepping motor has a feature that is very useful. It can be driven in a half-step manner. That is to say, at a certain time, the step angle is only halfway forward. This can be done with a single mix or with a bidirectional switch.
The four-phase hybrid stepping motor can also work at higher than the rated voltage. This can be stepped down with a series resistor. Since the 1 phase and the 2 phase, the 3 phase and the 4 phase do not work at the same time, so only one step resistor per pair is connected in series between the X and Y points in FIG. Therefore, the stepping motor with a rated voltage of 6V can work under the power supply of 12V. At this time, it is necessary to string a 6W, 6Ω resistor.