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The Classification, Structure and Principle of the Stepper Motor
According to the principle of torque generation, stepper motors can be classified into exciting type, reactive type and hybrid stepping motor. According to the number of stator, stepper motor can be divided into single stator, double stator and multi stator stepper motor. The stepper motor can be divided into three phase, four phase, five phase and six phase stepper motor according to the number of excitation phases. The stepper motor can be divided into radial phase separation and shaft phase stepping motor according to the distribution rules of each phase winding.
3.1 The classification, structure and principle of the number of stator phases.
While stepper motor switching the magnetizing current of the stator winding for one time , the rotor will switch a fixed angle called step angle. The step angle is usually obtained by the rotational torque generated by the switching phase current, so every per phase pole number should be an even number. Stepper motors usually have more than two phases, of course, there are some special single-phase stepper motors with only one coil. Although it has just one phase, it is actually a magnetic flux direction reversal caused by a coil to drive the rotor to rotate. The phase number of practical stepper motor has one-phase, two-phase, three-phase, four-phase and five-phase.
Most of the stepper motors used nowadays use permanent magnet rotors. The reason for the widespread use of permanent magnets is efficiency and high resolution. Below we will introduce permanent magnet rotors.
1. The factors that decided the step angle
The higher resolution of the stepper totors, the higher the position precision. In order to acquire a high resolution , we should design more number of poles. The PM type rotor is placed between N and S poles on the outer surface of the core of the rotor. The number of the rotor poles is the sum of the N pole and the S pole, which is a simplified explanation, assuming that the logarithm of the pole is 1. The step angle of the stepper motor with rotor permanent magnet is θs, which is represented by formula (2.1), where Nr is the logarithm of rotor pole and P is the number of stator phases:Θs=180°/PNr (2.1） The physical meaning of formula（2.1）is as below:
Rotor rotates a circle, the mechanical angle is 360 degrees, if divides 2Nr, equivalent to a very mechanical angle of 180 degree /Nr. That is to say, a very mechanical angle can be obtained by dividing the stator phase with the step angle, which is shown in figure 2.1.
We can know from figure 2.1 that the smaller the step angle, the higher resolution it is, therefore, in order to improve the resolution of stepper motor, it is necessary to increase the logarithm Nr of the rotor or adopt the polynomial method with more P of the number of stator phases.However, the increase of Nr is limited by mechanical processing, so we need two ways to make the high resolution stepping motor.
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