New energy battery technology: How does the motor rotate and turn? The basic working principle of the stepper motor

Nearly half of the world's power consumption is consumed by motors, so improving the efficiency of motors is considered the most effective measure to solve the world's energy problems.

PART 01 Motor Type

Generally speaking, it refers to the conversion of the force generated by the flow of current in a magnetic field into rotational motion, but in a broad sense it also includes linear motion.

According to the type of power source used to drive the motor, it can be divided into DC motors and AC motors. And according to the principle of motor rotation, it can be roughly divided into the following types. (Except for special motors)

On Electric Currents, Magnetic Fields, and Forces

First, to facilitate the subsequent explanation of motor principles, let's review the basic laws/principles of current, magnetic field and force. Although it feels nostalgic, it is easy to forget this knowledge if you don't work with magnetic components often.

We use pictures and formulas to illustrate.

When the wire frame is rectangular, the forces acting on the current must be taken into account.

The force F acting on the sides a and c is

Generates a torque centered on the central axis.

For example, when considering the state where the rotation angle is only θ, the force acting at right angles to b and d is sinθ, so the torque Ta of the a part is expressed by the following formula:

Considering part c in the same way, the torque is doubled and produces the torque calculated by the following formula:

Since the area of ​​the rectangle is S = h・l, substituting it into the above formula gives the following result:

This formula applies not only to rectangles, but also to other common shapes such as circles. Electric motors use this principle.

PART 02 How does the motor rotate?

1) The motor rotates with the help of magnets and magnetic force

Around a permanent magnet with a rotating shaft, ① the magnet rotates (to generate a rotating magnetic field), ② according to the principle that N poles and S poles attract each other and like poles repel each other, ③ the magnet with the rotating shaft will rotate.

This is the basic principle of motor rotation.

When current flows through a wire, a rotating magnetic field (magnetic force) is generated around it, causing the magnet to rotate. This is actually the same operating state.

In addition, when the wire is wound into a coil, the magnetic force is synthesized to form a large magnetic field flux (magnetic flux), generating an N pole and an S pole.

Inserting an iron core into the coiled conductor makes it easier for magnetic lines of force to pass through, generating a stronger magnetic force.

2) Actual rotating motor

Here, as a practical method of rotating an electric machine, a method of generating a rotating magnetic field using three-phase AC and a coil is introduced.

(Three-phase AC is an AC signal with phases spaced 120° apart)

The synthetic magnetic field under the above state ① corresponds to the following figure ①.

The synthetic magnetic field under the above state ② corresponds to the following figure ②.

The synthetic magnetic field in the above state ③ corresponds to the following figure ③.

As described above, the coils wound around the core are divided into three phases, with the U-phase coil, V-phase coil, and W-phase coil arranged at intervals of 120°. The coil with a high voltage produces an N pole, and the coil with a low voltage produces an S pole.

Each phase changes in a sinusoidal manner, so the polarity (N pole, S pole) and magnetic field (magnetic force) generated by each coil will change.

At this time, if we only look at the coil that generates the N pole, the coil changes in the order of U-phase coil → V-phase coil → W-phase coil → U-phase coil, thereby causing rotation.

PART 03 Structure of small motor

The figure below shows the general structure and comparison of the three types of motors: stepper motors, brushed DC motors, and brushless DC motors. The basic components of these motors are coils, magnets, and rotors. In addition, due to different types, they are divided into coil-fixed type and magnet-fixed type.

The following is a structural description related to the example diagram. Since there may be other structures if divided more finely, please understand that this article introduces the structure under the general framework.

Here the stepper motor has its coil fixed on the outside and the magnet rotating on the inside.

The brushed DC motor here has magnets fixed on the outside and coils rotating on the inside. The brushes and commutator are responsible for supplying power to the coils and changing the direction of the current.

The brushless motor here has its coils fixed on the outside and the magnets rotating on the inside.

Due to the different types of motors, even if the basic components are the same, their structures are different. The details will be explained in detail in each section.

PART 05 Brushed Motor

The structure of a brushless motor

Below is the appearance of a brushed DC motor that is often used in models, as well as an exploded diagram of a common two-pole (2 magnets) three-slot (3 coils) type motor. Perhaps many people have the experience of disassembling a motor and taking out the magnets.

You can see that the permanent magnet of the brushed DC motor is fixed, and the coil of the brushed DC motor can rotate around the internal center. The fixed side is called the "stator" and the rotating side is called the "rotor".

Below is a simplified structural diagram representing the structural concept.

There are three commutators (bent metal pieces used to switch current) around the outer periphery of the rotating central shaft. To avoid contact with each other, the commutators are arranged 120° apart (360°÷3 pieces). The commutators rotate as the shaft rotates.

One commutator is connected to one coil end and the other coil end, and three commutators and three coils form a whole (ring) as a circuit network.

The two brushes are fixed at 0° and 180° to contact the commutator. An external DC power supply is connected to the brushes, and the current flows along the path of brush → commutator → coil → brush.

Rotation principle of brushless motor

① Rotate counterclockwise from the initial state

Coil A is at the top, and the power supply is connected to the brush, with the left side as (+) and the right side as (-). A large current flows from the left brush through the commutator to coil A. This is a structure in which the upper part (outside) of coil A becomes the S pole.

Since 1/2 of the current in coil A flows from the left brush to coils B and C in the opposite direction to coil A, the outer sides of coils B and C become weak N poles (indicated by slightly smaller letters in the figure).

The magnetic fields generated in these coils and the repulsive and attractive effects of the magnets cause the coils to be forced to rotate counterclockwise.

② Further counterclockwise rotation

Next, assume that the right brush is in contact with both commutators while the coil A is rotated 30° counterclockwise.

The current of coil A continues to flow from the left brush to the right brush, and the outside of the coil remains at the S pole.

The same current as that of coil A flows through coil B, and the outside of coil B becomes the stronger N pole.

Since both ends of coil C are short-circuited by the brushes, no current flows and no magnetic field is generated.

Even in this case, there is a counterclockwise rotation force.

From ③ to ④, the upper coil is continuously subjected to the force moving to the left, and the lower coil is continuously subjected to the force moving to the right, and continues to rotate counterclockwise.

When the coil rotates to states ③ and ④ every 30°, when the coil is located above the central horizontal axis, the outer side of the coil becomes the S pole; when the coil is located below, it becomes the N pole, and this movement is repeated.

In other words, the upper coil is repeatedly subjected to a force moving to the left, and the lower coil is repeatedly subjected to a force moving to the right (both in a counterclockwise direction). This causes the rotor to always rotate counterclockwise.

If power is connected to the opposing left (-) and right (+) brushes, magnetic fields in opposite directions are generated in the coil, so the force applied to the coil is also in the opposite direction, causing clockwise rotation.

Additionally, when the power is disconnected, the brushed motor's rotor stops spinning because there is no magnetic field to keep it spinning.

PART 06 Three-phase full-wave brushless motor

The appearance and structure of three-phase full-wave brushless motor

The figure below shows the appearance and structure example of a brushless motor.

On the left is an example of a spindle motor used to spin a disc in a disc player. There are 9 coils (3 phases x 3). On the right is an example of a spindle motor for an FDD device, with 12 coils (3 phases x 4). The coils are fixed to a circuit board and wound around an iron core.

The disc-shaped part on the right side of the coil is the permanent magnet rotor. The outer periphery is the permanent magnet, the rotor shaft is inserted into the center of the coil and covers the coil part, and the permanent magnet surrounds the outer periphery of the coil.

Internal structure diagram of three-phase full-wave brushless motor and coil connection equivalent circuit

Next is a simplified diagram of the internal structure and a schematic diagram of the coil connection equivalent circuit.

This schematic diagram of the internal structure is an example of a very simple 2 pole (2 magnets) 3 slot (3 coils) motor. It is similar to the brushed motor structure with the same number of poles and slots, but the coil side is fixed and the magnets can rotate. Of course, there are no brushes.