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2022.11.24 motor

Structure and Operating Principles of Hybrid Type Stepping Motors

Stepping Motors

Up to this point, we have explained the operation of PM type stepping motors as an example of stepping motor operation. In this article, the structure and operating principles of hybrid type stepping motors, which today are used in a broad range of applications, are explained.

Structure and Operating Principles of Hybrid Type Stepping Motors

A hybrid type stepping motor combines advantages of both VR type and PM type stepping motors. A VR (Variable Reluctance) type motor uses a gear-shape iron core as the rotor. This has the advantage of enabling smaller step angles through machining, but there are the drawbacks of a somewhat smaller torque, making it difficult to achieve both compact size and a large torque. A PM (Permanent Magnet) type motor can achieve large torques even when small in size through the use of a powerful permanent magnet, but there are limits to reduction of the step angle.

A hybrid type uses the VR type structure to realize finer step angles, and by combining this with a permanent magnet, large torques can be obtained as well, so that hybrid type motors are used in numerous applications.

The basic rotor structure is a cylindrical magnet, magnetized in the axial direction, enclosed between two iron rotors. Teeth are cut into the peripheries of the rotors. The two rotors are mounted such that, as seen from the axial direction, the teeth in the rotors are shifted by one-half of the pitch between the teeth. The stator has multiple poles with excitation coils, and each of the poles has teeth similar to those of the rotors.

In this diagram, there are four stator windings, connected to form sets of coils that face each other with the rotor in between. In the diagram, the upper and lower coils constitute the A phase, and the left and right coils are the B phase. The coils are connected such that a current causes two poles, N and S, to be opposed.

There are 15 teeth in each of the rotors in the diagram. If the white rotor is in front and is magnetized to be an N pole by the permanent magnet, then the blue rotor is positioned in the rear and is magnetized as a S pole.

Structure and Operating Principles of Hybrid Type Stepping Motors

The diagrams below are used to explain the operating principles of a hybrid type stepping motor. In the initial state (see the "Initial state" diagram above), current is flowing such that the upper pole of the A phase (upper/lower) is S and the lower pole is N. The white teeth are N, and so they attract the A phase S pole, while the blue teeth are S and so attract the A phase N pole. From this state, the coil energization state changes in the order ① to ⑤.

The operation is explained in the order ① to ⑤.

  • ①Current is passed such that, for the B phase (left-right), the right pole is S and the left pole is N. The white teeth in front (N) are attracted to the right pole (S), and the blue teeth in the rear (S) are attracted to the left pole (N).
  • ②Current is passed such that, for the A phase (upper-lower), the upper pole is N and the lower pole is S; the white teeth in front (N) are attracted to the lower pole (S) and the blue teeth in the rear (S) are attracted to the upper pole (N), and the rotor moves further in the counterclockwise direction.
  • ③Current is passed in the B phase (left-right) in the direction opposite to ①; the white teeth in front (N) are attracted by and move toward the left pole (S), and the blue teeth in the rear (S) likewise move toward the right pole (N).
  • ④In the A phase (upper-lower), current is passed in the direction opposite that in ②. The white teeth in front (N) are attracted by and move toward the upper pole (S), and the blue teeth in the rear (S) are attracted by and move toward the lower pole. The A phase is energized such that the upper pole is S and the lower pole is N; movement is as before, and the rotor has rotated by one tooth's worth relative to the initial state (see the previous "Initial state" diagram).
  • ⑤In the B phase (left-right), similarly to ① above, current is passed such that the right pole is S and the left pole is N; movement is as before, and the rotor has rotated by one tooth’s worth relative to the state ①.

Thus in the four steps through the states ① to ⑤, the rotor rotates s in the counterclockwise direction by one tooth pitch, and by repeating this process, the motor is made to rotate continuously. In order to rotate the motor in the opposite direction, clockwise, current is controlled in the order ⑤ to ①.

Image sources
Fig_1:Commercialization of a compact, high-resolution, high-rigidity hybrid type stepping motor (external dimensions 25x25 mm, step angle 1.8°)

Key Points:

・A hybrid type stepping motor combines advantages of both VR type and PM type stepping motors.

・The structure of a VR type motor is used to achieve finer step angles, and a permanent magnet is incorporated for greater torques.

・By switching the energization state of two coil pairs (four steps), the rotor is rotated by one tooth’s worth.

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