As we all know, stepper motors are mainly classified according to the number of phases. Usually, there are four-phase, two-phase, three-phase and so on. This article mainly introduces the difference between a 2-phase stepper motor and a 3-phase stepper motor.

### 2 Phase Stepper Motor vs. 3 Phase Stepper Motor

#### Number of phases

It means that the number of coils inside the motor is different. The two-phase stepper motor is composed of two coils, while the three-phase stepping motor is composed of three coils.

#### Step angle

It refers to the angle of each step of the motor. Generally, the step angle of the two-phase motor on the market is 0.9°/1.8°, and the three-phase motor is 0.75°/1.5°.

#### Size

Three-phase motors are generally large motors, so the size is generally larger than that of two-phase motors, which also determines that three-phase stepping motors run more smoothly than two-phase motors.

#### Torque

The torque of the two-phase motor is slightly larger than the torque of the 3-phase motor of the same size.

#### Precision

The subdivision function of the 2-phase stepping motor driver is becoming more and more powerful, and the 2-phase can also achieve the accuracy that the 3-phase can achieve. The 3-phase stepping motor has good high-speed performance (harder characteristics), and has a smaller step angle and better accuracy than the 2-phase stepper motor. Since the torque decreases slowly with the increase of speed, it is usually used in occasions where high precision is required.

### 2 phase and 3 phase stepper motor step angle

#### Factors that determine the step angle

The higher the stepper motor resolution (number of steps per revolution, 360° divided by the step angle), the higher the position accuracy. In order to get high resolution, the number of poles should be designed. PM type rotor is that N and S poles are alternately placed on the outer surface of the rotor core at equal pitches. The number of rotor poles is the sum of the number of N poles and S poles. To simplify the explanation, it is assumed that the number of pole pairs is 1. Here, the step angle θ_{s} of the stepper motor with the rotor as a permanent magnet is determined by the following formula, where N_{r} is the number of pole pairs of the rotor, P is the number of stator phases, (the HB type stepper motor described later in this lesson N_{r} is the number of rotor teeth ):

**Θ _{s} = 180° / PN_{r}**

The physical meaning of the above formula is as follows:

The mechanical angle for one rotation of the rotor is 360. , if removed by the number of poles 2Nr, the mechanical angle occupied by one pole is 180°/N_{r}. That is to say, the step angle is obtained by dividing the mechanical angle of a pole by the number of stator phases. This concept is shown in the figure below.

It can be seen from the formula θ_{s} = 180° / PN_{r} that the smaller the step angle, the higher the resolution. Therefore, to improve the resolution of the stepper motor, it is necessary to increase the number of rotor pole pairs or use a multi-phase type with more stator phases P. The increase of N_{r} is limited by machining, so it is necessary to use both methods to manufacture a high-resolution stepper motor.

#### 2 phase stepper motor

The simplest configuration of a two-phase stepping motor is the case of N_{r} = 1, and the stepper motor structure is shown in the figure below. Generally, the number of stator poles of a two-phase motor is a multiple of 4, at least 4. The rotor is a two-pole rotor with one N pole and one S pole.

The stator is generally made of laminated silicon steel sheets. The number of magnetic poles of the stator is 4 poles, which is equivalent to two poles for one phase winding. The two poles of phase A are 180° apart in space, and the two poles of phase B are also 180° apart in space. The current flows positively and negatively in the one-phase winding (this driving method is called bipolar driving), the phase of the A-phase and B-phase current is 90° different, and the rectangular wave current flows alternately in the two-phase winding.

That is, the stator of a two-phase motor, when N_{r}=1, has a space difference of 90°, and a current phase difference of 90° in time. The current is similar to that of an ordinary synchronous motor. A rotating magnetic field is generated on the stator, and the rotor is attracted by the rotating magnetic field. The magnetic fields rotate synchronously.

The above figure shows the structure of the two-phase stepper motor (PM type) and stepper motor operation principle. It rotates 90° clockwise from Figure (a) to Figure (b), and then rotates 90° to Figure (c) and (d) in turn. Continuous operation becomes continuous rotation.

Take the above figure as an example, if the A phase has two coils, the unidirectional current flows through the two coils alternately, and the opposite magnetic flux directions can also be generated. This method is called a unipolar type coil.

As shown in the figure below, only unidirectional current flows through the coil, and this coil is called a unipolar coil; the other type, the coil that flows with positive and reverse currents in the coil is called a bipolar coil. The advantages and disadvantages of the two coils It will be covered in detail in later lessons. A monopolar coil can replace the bipolar coil shown above, operating with the same step angle.

The two-phase unipolar coil in the above figure is also called a four-phase stepper motor in some literatures. At this time, the number of rotor pole pairs, the number of teeth Nr, and the step angle θs are the same as those of the bipolar coil. The definition of the two-phase motor in this course conforms to the formula θ_{s} = 180° / PN_{r}, that is, the number of rotor teeth and the step angle θs are substituted into the formula θ_{s} = 180° / PN_{r}. If P=2, it is a two-phase motor. If N_{r} is the same, P = 4. If the step angle θ_{s} is only 1/2, the motor is a four-phase motor, so please pay special attention here.

Two-phase stepping motors are widely used now. The actual structure of the motor is more complicated than the picture (structure and operation principle of PM bipolar two-phase stepper motor). In addition to laminations, the stator also has a claw pole structure, but the basic principle can be referred to figure (structure and operation principle of PM bipolar two-phase stepper motor), the rotor shown in the figure is called PM type (permanent magnet or permanent magnet) rotor, and the outer surface of the magnetic cylinder forms the rotor magnetic pole.

#### 3 phase stepper motor

Stepper motors (VR type or reactive or variable reluctance type) whose rotors do not use permanent magnets have long been used in three-phase stepper motors. In 1986, a stepper motor was developed with permanent magnets on the rotor and teeth on the stator poles (the design principle of the magnetic pole teeth will be introduced in detail later). The combination of the stator and rotor tooth pitch can obtain higher angular resolution and torque. The number of main poles of the stator coil of the three-phase stepper motor is a multiple of three, so the number of main poles of the stator of the three-phase stepper motor is 3, 6, 9, 12 and so on.

The figure below is a comparison of the typical stator structure and drive circuit of stepper motors with different numbers of phases, ignoring the rotor structure diagram. It is assumed that the rotors are all PM type or HB type, and the corresponding rotors are equipped for two-phase, three-phase and five-phase according to the stator. The stator adopts the structure with the minimum number of main poles that does not generate unbalanced electromagnetic force (will be described in detail later, the sum of the rotor radial attractive forces cannot completely cancel each other, resulting in residual radial force), that is, the two phases are 4 main poles and three main poles. When the phase is 3 main poles and the fifth phase is 5 main poles, unbalanced electromagnetic force will be generated in the structure, and the above structure will not be used except for special purposes. In the figure, the structure of the stator is 8 main poles for two phases, 6 main poles for three phases, and 10 main poles for five phases, which is the simplest structure.

On the other hand, for the stepper motor drive circuit used by the bi-polar coil, the number of power tubes is 8 for two-phase, 10 for five-phase, and three-phase because the winding adopts Y or △ connection. According to the law, only 6 power tubes are enough for the drive of 3 outlet ports. Therefore, considering the integration of the motor and the driver, the structure of the three-phase stepping motor is the simplest, and the manufacturing cost of the two is the lowest.

Judging from the odd and even number of stator phases, the number of switching power tubes in the drive circuit is less in odd-numbered cases than in even-numbered cases. For example, a three-phase stepper motor has fewer power tubes than a two-phase stepper motor.