JPH06189516A - Brushless dc motor - Google Patents

Abstract

PURPOSE:To prevent a worker from getting burnt at low temperature when a motor is used in a hand grinder, and besides, sharply reduce its gyro effect. CONSTITUTION:This is a brushless DC motor which has a permanent magnet 4 fixed to the shaft 3 of a rotor part 1, a back yoke 5 fixed to the shaft 3 on the same core with this magnet 4, a motor case 7 at the stator part 2 accommodating the permanent magnet 4 and the back yoke 5, and a stator coil 8 fixed to the motor case 7, being arranged in the air gap G formed between the permanent magnet 4 and the back yoke 5. The back yoke 5 is molded thin in thickness suppressing the magnetic flux leakage low by using a magnetic material which has 20000Gauss or more saturation magnetic flux density, and besides the motor case 7 is molded with either one of ferrite stainless, mantensite stainless, low carbon steel, etc.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is mainly applied to 40,000 rpm.
The present invention relates to a brushless DC motor used in a hand grinder for dental or industrial use, which rotates at a high speed (revolutions per minute).

[0002]

2. Description of the Related Art Conventionally, as a brushless DC motor of this type, as shown in FIGS. 4 and 5, a stator core 21 made by stacking concentrically punched silicon steel sheets in a cylindrical shape is used. A stator portion 24 that is adhered to the inside of the case 22 and a stator coil 23 is attached to the inside of the stator core 21 with an adhesive, and a shaft 25
Rotor part 2 formed by fixing a permanent magnet 26 to the outside of the rotor
7 and a sensor permanent magnet 28 for detecting the position of the rotor portion 27 and a sensor Hall IC 29, and the motor is placed in an aluminum outer case 30 to form a blade. It was connected to the attachment 31 of the part to which it is attached.

[0003]

However, the conventional brushless DC motor as described above is put in the hand grinder H for dental or industrial use, and the rotor portion 27 is replaced by 40.
When rotated at a high speed of about 000 rpm, the permanent magnet 26
Since only the shaft 25 and the permanent magnets 26 constituting the magnetic circuit through which the magnetic flux generated by the motor rotates, iron loss is likely to occur in the stator core 21 of the stator portion 24, and the motor may generate heat. Therefore, when putting this motor in the hand grinder H and holding it by hand, the external case 3
If the temperature of the outer periphery of 0 is not kept below the body temperature, the worker will be burned at low temperature. Therefore, the cooling fan 32 must be attached to the inside of the tip of the hand grinder H for cooling, which is not sufficient.

Further, in the hand grinder H, it is important to keep the outer peripheral temperature of the outer case 30 below the body temperature because it is held by hand, but at the same time, the moment of inertia is reduced to maximize the gyro effect. Must be reduced. If the gyro effect is large, the movement of the hand is hindered by the gyro effect during fine work, so this type of brushless DC motor cannot be used for the hand grinder H.

Therefore, the present invention was created in order to solve the above-mentioned problems, and when it is put in a hand grinder and used, even if it is rotated at a high speed of about 40,000 rpm, an external case without a cooling fan is provided. A brushless DC motor that keeps the outer peripheral temperature of the body below the body temperature to prevent a worker from being burned at low temperature and also reduces the moment of inertia to remarkably reduce the gyro effect. Is.

[0006]

In order to achieve the above-mentioned object, the present invention has a permanent magnet fixed to a shaft of a rotor portion and a permanent magnet fixed to the shaft concentric with the permanent magnet. The back yoke, the motor case of the stator that houses the permanent magnet and the back yoke, and the cylindrical air gap formed between the permanent magnet and the back yoke are arranged and fixed to the motor case. In a brushless DC motor having a thin cylindrical stator coil that is
A magnetic material with a thickness of 0000 gauss or more is molded to a thin wall thickness that causes leakage of magnetic flux.

Further, the motor case is formed of either ferritic stainless steel, martensitic stainless steel, low carbon steel or the like.

[0008]

The shaft which constitutes the magnetic circuit through which the magnetic flux produced by the permanent magnet passes when the inertia moment is reduced by the above-mentioned technical means, and when it is put in the hand grinder for use. And the back yoke rotate at the same time, iron loss does not occur at all, 40000 r
Even at a high speed of about pm, the temperature rise of the motor case due to the leakage magnetic flux is prevented without the conventionally existing cooling fan.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

As shown in FIGS. 1 and 2, the brushless DC motor of the present invention comprises a rotor portion 1 which is a rotating portion and a stator portion 2 which is a fixed portion. The rotor portion 1 is formed of a ferromagnetic material. A permanent magnet 4, a back yoke 5 formed of a magnetic material having a high saturation magnetic flux density such as a permendur, and a sensor permanent magnet 6 are fixed to the shaft 3 with an adhesive or the like. In addition, the stator portion 2
Is the stator coil 8 in a state of being floated in the hollow in a motor case 7 formed of either ferritic stainless steel, martensitic stainless steel, or low carbon steel.
And the Hall IC for sensor 9 is fixed. Then, such a brushless DC motor is inserted and fixed in the outer case 10.

In the brushless DC motor formed as described above, the permanent magnet 4 generates a magnetic flux that passes through the magnetic circuit formed by the shaft 3 and the back yoke 5, and the stator coil in which the current floating in the air flows Although electromagnetic force is generated by interlinking with 8, the force acts on the permanent magnet 4 and the torque is generated on the shaft 3 because the stator coil 8 is fixed. Further, the sensor permanent magnet 6 and the sensor Hall IC 9 constantly detect the position of the rotor portion 1, and the driver theoretically switches the current flowing through the stator coil 8 to constantly keep the constant direction. Torque can be obtained and the rotor unit 1 rotates.

One of the features of the present invention is that a specific material is used for the back yoke 5 and the motor case 7, which are the main parts.

That is, the reason for increasing the gyro effect is
Although the back yoke 5 exists, a material having a high saturation magnetic flux density is used here. Among metallic magnetic materials, for example, cobalt, iron, and alloys are materials with extremely high saturation magnetic flux densities, and those of the alloy of cobalt 50%, vanadium 2%, residual iron are 24000 gauss, or cobalt 27%, residual iron. Its alloy is 22000 Gauss.

Normally, as shown in FIG. 3, the saturation magnetic flux density of 0.8% low carbon steel A, 3% silicon steel B, PB permalloy C, electromagnetic pure iron class 0 D, etc. used as magnetic materials is 18,000 gauss or less. There is a magnetic flux density in the back yoke 5 which will be described later in comparison between the case where 18000 Gauss and the highest electromagnetic pure iron class 0 D are used and the case where 24000 Gauss cobalt-iron alloy E is used. Point 11 in Figure 2 where the maximum is
When the back yoke 5 is designed so that the magnetic flux densities at a and 11b become the saturation magnetic flux densities of the respective materials, the latter can significantly reduce the moment of inertia by 25%.

In the present invention, a metal magnetic material having a saturation magnetic flux density of 20000 gauss or more is used for the back yoke 5, and at the same time, as shown in FIG. 2, the magnetic flux generated by the permanent magnet 4 is focused in the back yoke 5. 2, the outer diameter of the back yoke 5 is made smaller than the theoretical dimension at which the magnetic flux density at points 11a and 11b in FIG. 2 at which the magnetic flux density becomes maximum becomes the saturation magnetic flux density of the material used, and at points 11a and 11b in FIG. Reduce the thickness of the back yoke 5 until the magnetic flux leaks,
Design to reduce the moment of inertia as much as possible. Usually, in the design of a magnetic circuit, the magnetic material used for the portion through which the magnetic flux passes is designed to have a magnetic flux density much smaller than its saturation magnetic flux density, and leakage flux is avoided as much as possible.

In the trial examples shown in Table 1, when pure electromagnetic iron having a saturation magnetic flux density of 18000 Gauss was used without magnetic saturation, and when cobalt and iron alloys having a saturation magnetic flux density of 24000 Gauss were partially magnetically saturated. comparing the moment of inertia when used, in the former design the J = 13.1 g -cm ² latter design was able to attempt a reduction of J = 6.5g-cm ^2, and the 50% moment of inertia. In this case, the gyro effect did not cause any problem in use, and the object could be achieved 100%. The presence / absence of the gyro effect can be determined when the operator holds the hand grinder H with his / her hand and works, but it is difficult to express it as a number, so it is determined by the value of the moment of inertia. Further, there was almost no decrease in torque due to leakage of magnetic flux in some parts, and there was almost no effect on other characteristics. This is because the magnetic flux leaked at points 11a and 11b in FIG. 2 is originally small.

[Table 1]

As described above, as a result, the magnetic flux density at the points 11a and 11b in FIG. 2 at which the magnetic flux density in the back yoke 5 is maximized becomes the saturated magnetic flux density of the material used. In the case of the design, the use of high saturation magnetic flux density material, cobalt, and iron alloy reduced the moment of inertia by 25% compared with the case of using pure electromagnetic iron. Also,
When the cobalt and iron alloys are used and magnetically saturated to partially leak the magnetic flux, when compared with the case where electromagnetic pure iron is used without magnetic saturation, the inertia moment of the former is that of the latter. It can be reduced to about 50% of the above, and practically it does not affect the characteristics. However, the following problems occurred and countermeasures for them were needed.

Another feature of the present invention is its countermeasure.

That is, the motor case 7 is not a component of a magnetic circuit, and its material is not magnetically or electrically specified, but a brass alloy or an aluminum alloy is used because of its excellent machinability. When used, since these specific resistances are extremely low, overcurrent loss due to leakage magnetic flux leaking from the back yoke 5 occurred in the motor case 7, and the motor generated heat. Further, although the specific resistance is large, if the non-magnetic material of austenitic stainless steel is used, heat generation due to overcurrent loss in the motor case 7 is eliminated, but the external case 10 made of aluminum whose leakage magnetic flux has a very low specific resistance. Leaking up to
The outer case 10 generated heat due to overcurrent loss. The outer case 10 is usually made of an aluminum alloy because of its excellent machinability and weight reduction. Any of the overcurrent loss rotates at a high speed of about 40,000 rpm, and
It is fatal in the hand grinder H which has to keep the temperature of the outer case 10 below the body temperature.

As a countermeasure, it is necessary to select a material that does not leak magnetic flux to the outer case 10 and can suppress the occurrence of overcurrent loss even in the motor case 7 itself. It is required to be a body and have high specific resistance. Theoretically, silicon steel is superior,
Machinability is poor. Martensitic stainless steel, ferritic stainless steel and low carbon steel are ferromagnetic materials and have excellent machinability. The resistivity of low carbon steel is not so high, about 2.2 times that of brass, but there was no problem in practical use. Table 2 shows the characteristics of each material.

[Table 2]

[0021]

As described above, the brushless DC motor of the present invention has the permanent magnet 4 fixed to the shaft 3 of the rotor portion 1 and the back magnet fixed to the shaft 3 concentric with the permanent magnet 4. The yoke 5, the motor case 7 of the stator portion 2 that houses the permanent magnet 4 and the back yoke 5.
And a brushless DC motor having a thin cylindrical stator coil 8 fixed to the motor case 7 and arranged in a cylindrical air gap G formed between the permanent magnet 4 and the back yoke 5. In the above, since the back yoke 5 is formed of a magnetic material having a saturation magnetic flux density of 20000 gauss or more and a thin wall thickness that causes leakage of magnetic flux, the moment of inertia can be reduced, and the gyro effect is significantly reduced. be able to.

A permanent magnet 4 fixed to the shaft 3 of the rotor portion 1, a back yoke 5 fixed to the shaft 3 concentric with the permanent magnet 4, the permanent magnet 4 and the back yoke 5 are provided. A thin cylindrical shape fixed to the motor case 7 by being arranged in a cylindrical air gap G formed between the permanent magnet 4 and the back yoke 5 of the motor case 7 of the stator part 2 which houses In the brushless DC motor having the stator coil 8 and the motor case 7, the motor case 7 is formed of any one of ferritic stainless steel, martensitic stainless steel, low carbon steel, etc. In addition, since the shaft 3 and the back yoke 5, which form the magnetic circuit through which the magnetic flux generated by the permanent magnet 4 passes, rotate simultaneously, no iron loss occurs. Without the 40000 in high-speed rotation of about rpm, without cooling fan 32 it has been conventionally exist, preventing the temperature rise of the motor case 7 due to the leakage flux, the outer case 1
The outer peripheral temperature of 0 can be maintained below the body temperature, and the worker can be prevented from low-temperature burn.

[Brief description of drawings]

FIG. 1 is a partially cutaway front view showing an embodiment of a brushless DC motor of the present invention.

FIG. 2 is a sectional view taken along the line AA of FIG.

FIG. 3 is a diagram showing magnetization characteristics of various magnetic materials.

FIG. 4 is a partially cutaway front view showing a conventional brushless DC motor.

5 is a sectional view taken along the line BB of FIG.

[Explanation of symbols]

 1 rotor part 2 stator part 3 shaft 4 permanent magnet 5 back yoke 6 sensor permanent magnet 7 motor case 8 stator coil 9 sensor hall IC 10 outer case 11a point 11b point A 0.8% low carbon steel B 3% silicon steel C PB Permalloy D Electromagnetic Pure Iron Class 0 E Cobalt / Iron Alloy G Air Gap H Hand Grinder

Claims (2)

[Claims] 1. A permanent magnet fixed to a shaft of a rotor portion, a back yoke fixed to the shaft coaxial with the permanent magnet, and a motor case of a stator portion accommodating the permanent magnet and the back yoke. And a thin cylindrical stator coil arranged in a cylindrical air gap formed between the permanent magnet and the back yoke and fixed to the motor case. Is a brushless DC motor characterized by being molded with a magnetic material having a saturation magnetic flux density of 20000 Gauss or more to a thin wall thickness that causes leakage of magnetic flux. 2. The brushless DC motor according to claim 1, wherein the motor case is formed of ferritic stainless steel, martensitic stainless steel, low carbon steel or the like.