2) Actual rotating motor

Here, as a practical method of rotating electric machines, a method of producing a rotating magnetic field using three-phase alternating current and coils is introduced.
(Three-phase AC is an AC signal with a phase interval of 120°)

• The synthetic magnetic field in the above ① state corresponds to the following figure ①.
• The synthetic magnetic field in the state ② above corresponds to ② in the figure below.
• The synthetic magnetic field in the above state ③ corresponds to the following figure ③.

As described above, the coil wound around the core is divided into three phases, and the U-phase coil, V-phase coil, and W-phase coil are arranged at intervals of 120°. The coil with high voltage generates N pole, and the coil with low voltage generates S pole.
Since each phase changes as a sine wave, the polarity (N pole, S pole) generated by each coil and its magnetic field (magnetic force) change.
At this time, just look at the coil that produces the N pole, and change in sequence according to the U-phase coil→V-phase coil→W-phase coil→U-phase coil, thereby rotating.

Structure of a small motor

The figure below shows the general structure and comparison of the three motors: stepper motor, brushed direct current (DC) motor, and brushless direct current (DC) motor. The basic components of these motors are mainly 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 description of the structure associated with the example diagram. Since there may be other structures on a more granular basis, please understand that the structure described in this article is within a large framework.

Here, the coil of the stepper motor is fixed on the outside, and the magnet rotates on the inside.

Here, the magnets of the brushed DC motor are fixed on the outside, and the coils are rotated on the inside. The brushes and commutator are responsible for supplying power to the coil and changing the direction of the current.

Here, the coil of the brushless motor is fixed on the outside, and the magnet rotates on the inside.

Due to the different types of motors, even if the basic components are the same, the structure is different. The specifics will be explained in detail in each section.

brushed motor

Structure of brushed motor

Below is what a brushed DC motor often used in models looks like, as well as an exploded schematic of a common two-pole (2 magnets) three-slot (3 coils) type motor. Maybe many people have the experience of disassembling the motor and taking out the magnet.

It can be seen that the permanent magnets of the brushed DC motor are fixed, and the coils of the brushed DC motor can rotate around the inner center. The stationary side is called “stator” and the rotating side is called “rotor”.

The following is a schematic diagram of the structure representing the structure concept.

There are three commutators (bent metal sheets for current switching) on ​​the periphery of the rotating central axis. In order to avoid contact with each other, the commutators are arranged at an interval of 120° (360°÷3 pieces). The commutator rotates as the shaft rotates.

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

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

Rotation principle of brushed motor

① Rotate counterclockwise from the initial state

Coil A is on top, connect the power supply to the brush, let the left be (+) and the right be (-). A large current flows from the left brush to coil A through the commutator. This is the structure in which the upper part (outer side) of the coil A becomes the S pole.

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

The magnetic fields created in these coils and the repulsive and attractive effects of the magnets subject the coils to a counterclockwise rotating force.

② Further turn counterclockwise

Next, it is assumed that the right brush is in contact with the two commutators in a state where the coil A is rotated counterclockwise by 30°.

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

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

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

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

From ③ to ④, the upper coil continues to receive a force to the left, and the lower coil continues to receive a force to the right, and continues to rotate counterclockwise

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

In other words, the upper coil is repeatedly forced to the left, and the lower coil is repeatedly forced to the right (both in a counterclockwise direction). This keeps the rotor spinning counterclockwise all the time.

If you connect power to the opposite left (-) and right (+) brushes, opposite magnetic fields are created in the coils, so the force applied to the coils is also in the opposite direction, turning clockwise.

In addition, when the power is turned off, the rotor of the brushed motor stops rotating because there is no magnetic field to keep it spinning.

Three-phase full-wave brushless motor

Appearance and structure of three-phase full-wave brushless motor

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

On the left is an example of a spindle motor used to spin an optical disc in an optical disc playback device. A total of three-phase × 3 total of 9 coils. On the right is an example of a spindle motor for an FDD device, with a total of 12 coils (three-phase × 4). The coil is fixed on the circuit board and wound around the iron core.

The disk-shaped part to the right of the coil is the permanent magnet rotor. The periphery is a permanent magnet, the shaft of the rotor is inserted into the central part of the coil and covers the coil part, and the permanent magnet surrounds the periphery of the coil.

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

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

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

In this case, the coil is Y-connected, using a semiconductor element to supply the coil with current, and the inflow and outflow of current is controlled according to the position of the rotating magnet. In this example, a Hall element is used to detect the position of the magnet. The Hall element is arranged between the coils, and the generated voltage is detected based on the strength of the magnetic field and used as position information. In the image of the FDD spindle motor given earlier, it can also be seen that there is a Hall element (above the coil) for position detection between the coil and the coil.

Hall elements are well known magnetic sensors. The magnitude of the magnetic field can be converted into the magnitude of the voltage, and the direction of the magnetic field can be expressed as positive or negative. Below is a schematic diagram showing the Hall effect.

Hall elements take advantage of the phenomenon that “when a current I _{H flows through a semiconductor and a magnetic flux B passes at right angles to the current, a voltage V H} is generated in the direction perpendicular to the current and the magnetic field “, American physicist Edwin Herbert Hall (Edwin Herbert Hall) discovered this phenomenon and called it the “Hall effect”. The resulting voltage V _H is represented by the following formula.

V _H  = (K _H  / d)・I _H・B ※K _H : Hall coefficient, d: thickness of magnetic flux penetration surface

As the formula shows, the higher the current, the higher the voltage. This feature is often used to detect the position of the rotor (magnet).

Rotation principle of three-phase full-wave brushless motor

The rotation principle of the brushless motor will be explained in the following steps ① to ⑥. For easy understanding, the permanent magnets are simplified from circles to rectangles here.

①

Among the three-phase coils, it is assumed that coil 1 is fixed in the direction of 12 o’clock of the clock, coil 2 is fixed in the direction of 4 o’clock of the clock, and coil 3 is fixed in the direction of 8 o’clock of the clock. Let the N pole of the 2-pole permanent magnet be on the left and the S pole on the right, and it can be rotated.

A current Io is flowed into the coil 1 to generate an S-pole magnetic field outside the coil. Io/2 current is made to flow from Coil 2 and Coil 3 to generate an N-pole magnetic field outside the coil.

When the magnetic fields of coil 2 and coil 3 are vectorized, an N-pole magnetic field is generated downward, which is 0.5 times the size of the magnetic field generated when the current Io passes through one coil, and is 1.5 times larger when added to the magnetic field of coil 1. This creates a resultant magnetic field at a 90° angle to the permanent magnet, so maximum torque can be generated, the permanent magnet rotates clockwise.

When the current of coil 2 is decreased and the current of coil 3 is increased according to the rotational position, the resultant magnetic field also rotates clockwise and the permanent magnet also continues to rotate.

②

In the state rotated by 30°, the current Io flows into the coil 1 , the current in the coil 2 is made zero, and the current Io flows out of the coil 3 .

The outside of the coil 1 becomes the S pole, and the outside of the coil 3 becomes the N pole. When the vectors are combined, the resulting magnetic field is √3 (≈1.72) times the magnetic field produced when the current Io passes through a coil. This also produces a resultant magnetic field at a 90° angle to the permanent magnet’s magnetic field and rotates clockwise.

When the inflow current Io of the coil 1 is decreased according to the rotational position, the inflow current of the coil 2 is increased from zero, and the outflow current of the coil 3 is increased to Io, the resultant magnetic field also rotates clockwise, and the permanent magnet also continues to rotate.

※Assuming that each phase current is a sinusoidal waveform, the current value here is Io × sin(π⁄3)=Io × √3⁄2 Through the vector synthesis of the magnetic field, the total magnetic field size is obtained as ( √3⁄2) ² × 2=1.5 times. When each phase current is a sine wave, regardless of the position of the permanent magnet, the magnitude of the vector composite magnetic field is 1.5 times that of the magnetic field generated by a coil, and the magnetic field is at a 90° angle relative to the magnetic field of the permanent magnet.

③

In the state of continuing to rotate by 30°, the current Io/2 flows into the coil 1 , the current Io/2 flows into the coil 2 , and the current Io flows out of the coil 3 .

The outside of the coil 1 becomes the S pole, the outside of the coil 2 also becomes the S pole, and the outside of the coil 3 becomes the N pole. When the vectors are combined, the resulting magnetic field is 1.5 times the magnetic field produced when a current Io flows through a coil (same as ①). Here, too, a resultant magnetic field is generated at an angle of 90° with respect to the magnetic field of the permanent magnet and rotates clockwise.

④～⑥

Rotate in the same way as ① to ③.

In this way, if the current flowing into the coil is continuously switched in sequence according to the position of the permanent magnet, the permanent magnet will rotate in a fixed direction. Likewise, if you reverse the current flow and reverse the resultant magnetic field, it will rotate counterclockwise.

The figure below continuously shows the current of each coil in each step ① to ⑥ above. Through the above introduction, it should be possible to understand the relationship between current change and rotation.

stepper motor

A stepper motor is a motor that can accurately control the rotation angle and speed in synchronization with a pulse signal. The stepper motor is also called a “pulse motor”. Because stepper motors can achieve accurate positioning only through open-loop control without the use of position sensors, they are widely used in equipment that requires positioning.

Structure of stepper motor (two-phase bipolar)

The following figures from left to right are an example of the appearance of the stepping motor, a schematic diagram of the internal structure, and a schematic diagram of the structure concept.

In the appearance example, the appearance of HB (Hybrid) type and PM (Permanent Magnet) type stepping motor is given. The structure diagram in the middle also shows the structure of HB type and PM type.

A stepping motor is a structure in which the coil is fixed and the permanent magnet rotates. The conceptual diagram of the internal structure of a stepper motor on the right is an example of a PM motor using two-phase (two sets) of coils. In the example of the basic structure of the stepping motor, the coils are arranged on the outside and the permanent magnets are arranged on the inside. In addition to two-phase coils, there are three-phase and five-phase types with more phases.

Some stepper motors have other different structures, but the basic structure of the stepper motor is given in this article to facilitate the introduction of its working principle. Through this article, I hope to understand that the stepping motor basically adopts the structure of fixed coil and rotating permanent magnet.

Basic working principle of stepper motor (single-phase excitation)

The following figure is used to introduce the basic working principle of a stepper motor. This is an example of excitation for each phase (set of coils) of the two-phase bipolar coil above. The premise of this diagram is that the state changes from ① to ④. The coil consists of Coil 1 and Coil 2, respectively. In addition, the current arrows indicate the current flow direction.

      ①

• The current flows in from the left side of the coil 1 and flows out from the right side of the coil 1 .
• Do not allow current to flow through coil 2.
• At this time, the inner side of the left coil 1 becomes N, and the inner side of the right coil 1 becomes S.
• Therefore, the permanent magnet in the middle is attracted by the magnetic field of the coil 1, becomes the state of the left S and the right N, and stops.

  ②

• The current of the coil 1 is stopped, and the current flows in from the upper side of the coil 2 and flows out from the lower side of the coil 2 .
• The inner side of the upper coil 2 becomes N, and the inner side of the lower coil 2 becomes S.
• The permanent magnet is attracted by its magnetic field and stops by rotating 90° clockwise.

  ③

• The current of coil 2 is stopped, and the current flows in from the right side of coil 1 and flows out from the left side of coil 1 .
• The inner side of the left coil 1 becomes S, and the inner side of the right coil 1 becomes N.
• The permanent magnet is attracted by its magnetic field and stops by turning clockwise another 90°.

  ④

• The current of the coil 1 is stopped, and the current flows in from the lower side of the coil 2 and flows out from the upper side of the coil 2 .
• The inner side of the upper coil 2 becomes S, and the inner side of the lower coil 2 becomes N.
• The permanent magnet is attracted by its magnetic field and stops by turning clockwise another 90°.