The Structure and Working Principle of Three-phase Reactive Stepper Motor

Based on the principle of torque generation, the stepper motor can be divided into reactive, permanent magnet, and hybrid stepper motor. I will take the reactive stepper motor as an example to illustrate the structure and working principle.

1, The Basic Structure of Stepper Motor

The basic structure of the three-phase reactive stepper motor shows in picture 1. The center of the stepper motor is the rotor, which has four teeth. Around the rotor, there is a stator, which has evenly distributed six magnetic poles. There are excitation windings on the magnetic pole, and two relative windings consist of a phase. And its geometric axis stagger to the rotor teeth axis, namely A opposite tooth 1, B and tooth 2 stagger to right by 1/3て. C and tooth 3 stagger to the right by 2/3.（The distance between two adjacent rotor tooth axis is tooth pitch, which is expressed byて)

2, The Working Principle of Stepper Motor

The working principle diagram of stepper motor shows in picture 2. when stepper motor works, due to the function of the magnetic field, the rotor always rotates minimum reluctance position. Therefore, when A phase is energized, B and C is not energized, the rotor must rotate the position as shown in picture 2(a). The tooth 1, 3 of rotor align with A, A’. At this time, the rotor is not subjected to any force. According to the same principle, when B phase winding is energized, A, and C phase are not energized, the tooth 2, 4 should align with B, B’. So, the rotor rotates to right by 1/3て, at this time, the offset of tooth 1, 3, and c phase winding are 1/3て, the offset of the tooth 1, 4, and B phase winding are 2/3て. Likewise, when the C phase is energized, A and B phase is not energized, the tooth 1, 3 should align with C, C’. So, the rotor rotates to the right by 1/3て.

     At this time, the offset of tooth 1, 4, and A phase winding are 1/3て. If A phase is energized, B, and C phase are not energized, the tooth 1, 4 will align with A, A’. the rotor rotates to the right again by 1/3て. The power is sequentially supplied through A→B→C→A, the rotor of motor rotates to right by 1/3て per step. If the power is supplied through A→C→ B→A, the motor will rotate reversely. From the above procedure, we can see that the position and speed of stepper motor have a one-to-one correspondence with the number of electrify (the number of pulses) and frequency, its rotation direction is determined by power-on sequence. In addition, considering the torque, noise, the power-on state A→AB→B→BC→C→CA→A will be selected. In this way, the 1/3 て of rotation will be changed into 1/6て per step. So, if the stator of stepper motor has m phases excitation windings, its axis offset from the teeth axis of the rotor by 1/mて、2/mて……(m-1)/mて. From the theory, we can manufacture more precision stepper motor.

3, The Main Characteristic of Stepper Motor

Step angle and step error

When the stepper motor rotates a step, there exists a step error between actual angular displacement ant the designed step angle. When rotating many steps, the error will be accumulated. Because the rotor rotates a circle, it will return to the stable position of the previous revolution. So, this error will not be accumulated for a long time. The cumulative error of step refers to the maximum value of the cumulative error of step within one rotation of the stepper motor. These errors are usually expressed in degree, a minute, or percentage of step angle. The main factor that affecting the step error and cumulative error are indexing accuracy and assembly accuracy of teeth and magnetic pole.

Static moment angle characteristic and maximum static torque characteristic

The stepper motor doesn’t change the power-on state, and the rotor is stationary, this working state is called static. When there is no load, a phase of the stepper motor is connected with DC current, the corresponding stator, rotor teeth are aligned, at this time, the rotor doesn’t have torque output. If a load torque in the clockwise direction is added on the motor shaft, the rotor of the stepper motor in the clockwise direction rotates a small degree θ, which is called an offset angle. At this time, the rotor electromagnetic torque T is equal to load torque. The torque angle characteristic is a curve, which describes the relationship between electromagnetic torque and the offset angle θ.

The Torque Frequency Characteristic

The torque frequency characteristic is a curve that describes the relationship between output torque and continuous working torque during continuous stable operation. The torque corresponding to each frequency on the curve is called dynamic torque. Except for the structure and material, the dynamic torque is also connected to the winding connection, drive circuit, and drive voltage.

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