Motor

Abstract

A motor comprising a steel sheet used as a core material of the motor, wherein the steel sheet includes a composition including: by mass %, 0.010% or less of C; 2.0% to 7.0% of Si; 2.0% or less of Al; 0.05% to 1.0% of Mn; 0.005% or less of S; 0.005% or less of N; and balance Fe and inevitable impurities; the steel sheet includes a magnetic flux density changing area where a change ΔB in magnetic flux density to a change ΔH=50 A/m in a magnetic field, is equal to or higher than 0.50 T; a thickness of the steel sheet is 0.05 mm to 0.20 mm; and an eddy-current loss of the steel sheet, at 1000 Hz−1.0 T, is equal to or less than 0.55 of a total iron loss.

Claims

1. A motor comprising a steel sheet used as a core material of the motor, wherein the steel sheet includes a composition including: by mass %, 0.010% or less of C; 2.0% to 7.0% of Si; 2.0% or less of Al; 0.05% to 1.0% of Mn; 0.005% or less of S; 0.005% or less of N; and balance Fe and inevitable impurities; the steel sheet includes a magnetic flux density changing area where a change ΔB in magnetic flux density to a change ΔH=50 A/m in a magnetic field, is equal to or higher than 0.50 T; a thickness of the steel sheet is 0.05 mm to 0.20 mm; and an eddy-current loss of the steel sheet, at 1000 Hz−1.0 T, is equal to or less than 0.55 of a total iron loss, wherein a difference between a concentration of Si at a surface of the steel sheet and a concentration of Si at a center portion of the steel sheet is 0.5% to 4.0%, and a saturation magnetic flux density Bs of the steel sheet is equal to or greater than 2.0 T.

2. The motor according to claim 1, wherein the steel sheet further includes: by mass %, at least one element selected from 0.01% to 0.1% of P; 0.001% to 0.1% of Sn; 0.001% to 0.1% of Sb; and 0.001% to 0.01% of Mo.

3. The motor according to claim 2, wherein the magnetic flux density changing area is present in an area including a magnetic flux density equal to or higher than 1 T.

4. The motor according to claim 1, wherein the magnetic flux density changing area is present in an area including a magnetic flux density equal to or higher than 1 T.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an example graph of the magnetic flux density versus the external magnetic field of a steel sheet.

(2) FIG. 2 is a schematic drawing that illustrates an example configuration of a two-pole three-phase brushless DC motor.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(3) Requirements of a steel sheet, used as a material of the iron core of a motor according to aspects of the present invention, and reasons for the limitation will now be described. There is no limitation, other than being a magnetic motor, on the type of motor according to aspects of the present invention. In the following description, added components simply indicated in “percentage” are actually in “percentage by mass”.

(4) C: Equal to or smaller than 0.010%

(5) Since excessively adding carbon (C) to a steel sheet increases the hysteresis losses in the motor, the content of C should be equal to or less than 0.010%.

(6) Si: from 2.0% to 7.0%

(7) Silicon (Si) is an additive element effective for reducing the eddy-current losses in the motor by increasing the specific resistance of the steel sheet. Too much Si added to a steel sheet, however, makes the processing into a motor iron core more difficult and reduces the saturation magnetic flux density of the steel sheet. The content of Si should therefore be from 2.0% to 7.0%. Since adding Si equal to or higher than 4.0% impairs the rollability of the steel sheet, for example, Si may be added after cold rolling using the chemical vapor siliconizing method. More specifically, Si may be added such that the concentration difference of Si between the center layer and the surface layer of the steel sheet is in the range from 0.5% to 4.0% and that the saturation magnetic flux density Bs is equal to or higher than 2.0 T. This manner is advantageous in reducing the size of the motor while reducing the eddy-current losses.

(8) Al: equal to or less than 2.0%, Mn: from 0.05% to 1.0%

(9) Aluminum (Al) and manganese (Mn) are elements effective for reducing the eddy-current losses in a motor by increasing the specific resistance of the steel sheet. Excessive addition of Al and Mn, however, negatively affects the grain growth and increases the hysteresis losses in the motor. The content of Al should therefore be equal to or less than 2.0%, and the content of Mn should be in the range from 0.05% to 1.0%.

(10) S: equal to or less than 0.005%, N: equal to or less than 0.005%, the balance being Fe and unavoidable impurities

(11) Excessive addition of S and N causes generation of precipitates and inhibits the grain growth and increases the hysteresis losses in the motor. The content of each S and N is therefore equal to or less than 0.005%, and the balance is Fe and unavoidable impurities.

(12) At least one of elements selected from P: from 0.01% to 0.1%, Sn: from 0.001% to 0.1%, Sb: from 0.001% to 0.1%, and Mo: from 0.001% to 0.01%

(13) To the above composition, containing at least one of elements selected from P: from 0.01% to 0.1%, Sn: from 0.001% to 0.1%, Sb: from 0.001% to 0.1%, and Mo: from 0.001% to 0.01% can improve the magnetic flux density of the steel sheet. It is therefore preferable to add the selected element. However, excessive addition of whatever the element impairs productivity and magnetic characteristics of the steel sheet.

(14) Change ΔB in magnetic flux density to change ΔH=50 A/m in magnetic field being equal to or higher than 0.50 T

(15) A steel sheet subjected to a sharp change ΔB in the magnetic flux density equal to or higher than 0.50 T to a change ΔH=50 A/m in the magnetic field enables easy control of the magnetic flux density of the motor in the event of drive that is powered by a battery and has limited conditions on the power supply (the voltage, the current, or both of them).

(16) For example, FIG. 1 is an example graph of the magnetic flux density B versus the external magnetic field H of a steel sheet. In an example according to aspects of the invention (a curve L1), a change ΔB in the magnetic flux density B is equal to or higher than 0.50 T to a change AH =50 A/m in the external magnetic field H from a point A to a point B. In a comparative example (a curve L2), there is no areas that includes a change ΔB in the magnetic flux density B equal to or higher than 0.50 T to a change ΔH=50 A/m in the external magnetic field H. In the example according to aspects of the invention, for example, if the excitation magnetic flux density in use of a magnet is designed to a value around the point A, a magnetizing force of 50 A/m is necessary for a current control to weaken the magnetic flux density of the iron core to the level of the point B. In the comparative example, a magnetizing force of 140 A/m is necessary to weaken the magnetic flux density of the iron core from the level of the point A to the level of the point B. With power supply under limited conditions, however, such a strong magnetizing force is actually unavailable. It is therefore difficult to weaken the magnetic flux density, and the current control cannot effectively reduce the iron loss in the motor.

(17) The steel sheet used as a material of the iron core of a motor according to aspects of the present invention therefore has an area having a sharp change in the magnetic flux density where a change ΔB in magnetic flux density is equal to or higher than 0.50 T to a change ΔH=50 A/m in the magnetic field. Furthermore, if this area having a sharp change in the magnetic flux density is present in the area including the magnetic flux density equal to or higher than 1 T, the magnetic flux density of the iron core is maintained high with the above-described current control being effectively conducted. Downsizing of the motor is therefore achieved.

(18) Thickness of sheet: from 0.05 mm to 0.20 mm

(19) Although reducing the thickness of a steel sheet is effective in reducing the eddy-current losses in the motor, making the steel sheet thinner problematically increases the manufacturing cost and the cost for producing the motor iron core. The thickness of the steel sheet should therefore be in the range from 0.05 mm to 0.20 mm

(20) Eddy-current loss at 1000 Hz−1.0 T being equal to or less than 0.55 of total iron loss

(21) The excitation frequency of a small high-speed motor is usually several hundreds to 10 kHz. When the motor actually drives, the iron loss at high frequencies caused with excitation from a PWM inverter is a more important issue. Since the eddy-current loss is dominant at high frequencies, if the eddy-current loss at 1000 Hz−1.0 T is not smaller than the hysteresis loss, the loss generated in the iron core is large. The loss reduces the efficiency of the motor, and moreover, an increase in the size of the motor is inevitable to avoid heat generation. The eddy-current loss at 1000 Hz−1.0 T should therefore be equal to or less than 0.55 of the total iron loss. The eddy-current loss herein defined is calculated using, what is called, the dual-frequency method, on the magnetic characteristics measured using a method in compliance with JIS C 2550-1. If the motor iron core is magnetically closed, the magnetic characteristics may be measured as a ring iron core with primary and secondary coils wound. In this case, any of the magnetic characteristics may meet the above standards.

(22) A steel sheet used as a material of the iron core of the motor according to aspects of the present invention may be any desired steel sheet satisfying the above requirements. Such a steel sheet is preferably manufactured in the following conditions.

(23) There is no limitation on the conditions of hot rolling applied to steel slabs, and any desired known conditions may be used. It is preferable that the temperature to heat the slabs be under 1250° C. for energy efficiency and that the thickness of a finished hot-rolled steel sheet be equal to or smaller than 2.0 mm. If the sheet finally has a thickness of 0.05 mm to 0.20 mm after undergoing a high rate of reduction in cold rolling, crystal planes, which inhibit magnetization (111), are increased in the crystal texture after recrystallization. The thickness is, however, not necessarily in the range if the cold rolling is performed twice with intermediate annealing put therebetween. After hot rolling, and annealing treatment if necessary, the steel sheet undergoes cold rolling to have a thickness of 0.05 mm to 0.20 mm. The sheet then undergoes finishing annealing that heats and retains the sheet at temperatures in the range from 900° C. to 1250° C. in an oxidation atmosphere equal to or less than 0.010, given by P(H.sub.2O)/P(H.sub.2). The heating rate is set equal to or higher than 25° C./s in the temperature range from 600° C. to 900° C., which is beneficial in improving the crystal texture and obtaining excellent magnetic characteristics. The heating rate is preferably equal to or higher than 100° C./s, and is more preferably, equal to or higher than 200° C./s. Furthermore, in the finishing annealing process, the Si concentration distribution of the steel is controlled at temperatures higher than 1200° C. using the chemical vapor siliconizing method. With this control, more excellent magnetic characteristics are obtained. The steel sheet used for the iron core of the motor according to aspects of the present invention is manufactured in the above manufacturing conditions to be adjusted as appropriate. The motor iron core is made by punching, wire cutting, and other methods. The advantageous effects according to aspects of the present invention are exerted by any method satisfying the requirements. As is known, introducing strain into the core material by punching affects the magnetic characteristics of the core material. It is therefore preferable to perform stress relieving annealing if punching is employed.

EXAMPLE

(24) The steel slabs (steel marks A to F) containing the components of Table 1 were heated to 1200° C. and then formed into hot-rolled steel sheets having a thickness of 1.8 mm through the hot rolling process. The sheet underwent annealing treatment at 1000° C.×30 s and was finished as a sheet having a thickness of 0.05 mm to 0.20 mm through the cold-rolling process. Finishing annealing was performed under the conditions (test numbers 1 to 13) of Table 2. Magnetic characteristics (the maximum change ΔB(T) in the magnetic flux density to ΔH=50 A/m, and the rate of eddy-current loss at W.sub.10/1000) indicated in Table 2 were obtained.

(25) The magnetic characteristics were measured using a method in accordance with JIS C 2550-1. Some cold- rolled steel sheets having passed the cold-rolling process were subjected to siliconizing treatment in the finishing annealing process, at 1200° C. under the atmosphere of silicon tetrachloride using the chemical vapor siliconizing method. The time period of treatment and the magnetic characteristics are indicated in Table 3. With regards to the sheets subjected to the siliconizing treatment, since the siliconizing treatment changes the concentration of Si and C, values of these components after the treatment were added to the data. Components of the steel slab and components of the steel sheet used for the iron core were the same in other conditions.

(26) TABLE-US-00001 TABLE 1 Steel Components (mass %) Mark C Si Al Mn S N Others A 0.002 2.5 0.002 0.1 0.002 0.001 Sn: 0.04 B 0.003 3.6 0.1 0.6 0.003 0.002 — C 0.0015 3.1 1 0.2 0.001 0.001 P: 0.07 D 0.002 4 0.2 0.8 0.002 0.002 Mo: 0.05 E 0.01 3.3 0.2 0.1 0.001 0.002 Sb: 0.005 Sn: 0.007 F 0.0025 2.7 0.5 0.3 0.002 0.001 —

(27) TABLE-US-00002 TABLE 2 Maximum Retention Change ΔB(T) Rate of Temperature in Magnetic Eddy- (° C.) at Flux Density current Test Steel Thickness Finishing to ΔH = 50 Loss at No. Mark (mm) Annealing (A/m) W.sub.10/1000 Remarks 1 A 0.10 950 0.53 0.60 Comparative Example 2 A 0.05 1050 0.61 0.47 Example 3 B 0.10 1000 0.63 0.58 Comparative Example 4 B 0.07 1050 0.64 0.53 Example 5 C 0.20 950 0.65 0.79 Comparative Example 6 C 0.10 1000 0.66 0.60 Comparative Example 7 C 0.05 1100 0.69 0.50 Example 8 D 0.15 1000 0.56 0.62 Comparative Example 9 D 0.10 1050 0.59 0.55 Example 10 D 0.05 1100 0.60 0.43 Example 11 E 0.10 1050 0.49 0.63 Comparative Example 12 E 0.05 1000 0.51 0.52 Example 13 F 0.10 1100 0.65 0.59 Comparative Example

(28) TABLE-US-00003 TABLE 3 Maximum Change ΔB(T) in Magnetic Rate of Time period Flux Eddy- Thick- of Components after Annealing (mass %) Density to current Test Steel ness Siliconizing Surface Average ΔH = 50 Loss at No. Mark (mm) (min) Si Si C (A/m) W.sub.10/1000 Remarks 14 A 0.20 16 6.5 5.2 0.002 0.31 0.47 Comparative Example 15 A 0.20 12 5.2 4.3 0.002 0.55 0.55 Example 16 A 0.10 19 6.5 5.2 0.001 0.30 0.41 Comparative Example 17 A 0.10 13 4.6 3.9 0.001 0.52 0.46 Example 18 A 0.10 11 4.1 3.6 0.001 0.59 0.52 Example 19 B 0.20 14 6.5 5.5 0.001 0.38 0.44 Comparative Example 20 B 0.20 11 4.5 4.2 0.002 0.54 0.52 Example 21 B 0.10 10 4.4 4.1 0.002 0.52 0.46 Example 22 C 0.10 15 5.5 4.7 0.001 0.45 0.45 Comparative Example 23 C 0.10 10 4.1 3.8 0.001 0.52 0.49 Example 24 D 0.10 10 5.2 4.8 0.001 0.47 0.43 Comparative Example 25 D 0.10 5 4.5 4.3 0.001 0.54 0.50 Example 26 E 0.15 12 5.2 4.6 0.003 0.48 0.48 Comparative Example 27 E 0.15 8 4.5 4.1 0.005 0.50 0.51 Example 28 F 0.10 8 4.1 3.6 0.001 0.51 0.48 Example 29 F 0.05 5 3.7 3.4 0.001 0.56 0.44 Example

(29) Iron cores were fabricated using the steel sheets indicated in Tables 2 and 3 to evaluate the efficiency of the motor. The evaluated motor is a two-pole 3-phase brushless DC motor (the drive voltage 25.2 V), the size of which is illustrated in FIG. 2. The iron core had a thickness of lamination of 15 mm, and the laminated steel sheets were bonded using the impregnation technique. The efficiency of the motor was evaluated based on driving conditions A (sine wave drive having current phase advance of 30 degrees at 50000 rpm−10 mNm), as conditions of drive at a rotational speed lower than the maximum rotational speed, and based on driving conditions B (sine wave drive having current phase advance of 0 degree at 85000 rpm−25 Nm), as conditions of drive at the maximum rotational speed. Table 4 demonstrates the results of evaluation. The example according to aspects of the present invention has an area having a sharp change in the magnetic flux density where a change ΔB in magnetic flux density is equal to or higher than 0.50 T to a change ΔH=50 A/m in the magnetic field and an eddy-current loss at 1000 Hz−1.0 T is equal to or less than 0.55 of the total iron loss. As demonstrated in Table 4, the example according to aspects of the present invention achieved high motor efficiency in both the driving conditions A and B with the average motor efficiency over 85%. The comparative example not satisfying the above requirements had low motor efficiency in comparison with the example according to aspects of the invention, in either or both of the driving conditions A or B with the average motor efficiency under 85%.

(30) TABLE-US-00004 TABLE 4 Driving Driving Conditions Conditions A B Motor Motor Motor Average Test Efficiency Efficiency Efficiency No. (%) (%) (%) Remarks 1 80.3 86.7 83.5 Comparative Example 2 86.1 89.0 87.6 Example 3 82.0 87.5 84.7 Comparative Example 4 85.7 88.3 87.0 Example 5 80.4 84.4 82.4 Comparative Example 6 82.3 87.3 84.8 Comparative Example 7 86.9 89.0 87.9 Example 8 80.6 86.5 83.5 Comparative Example 9 84.6 87.7 86.2 Example 10 86.6 89.6 88.1 Example 11 74.6 86.0 80.3 Comparative Example 12 83.9 87.8 85.8 Example 13 82.2 87.4 84.8 Comparative Example 14 77.8 87.5 82.6 Comparative Example 15 84.0 87.5 85.8 Example 16 78.6 88.4 83.5 Comparative Example 17 84.9 88.7 86.8 Example 18 85.1 88.2 86.6 Example 19 78.7 88.3 83.5 Comparative Example 20 84.3 87.9 86.1 Example 21 84.9 88.7 86.8 Example 22 79.3 88.5 83.9 Comparative Example 23 84.5 88.3 86.4 Example 24 79.8 88.9 84.4 Comparative Example 25 84.6 88.2 86.4 Example 26 79.2 88.2 83.7 Comparative Example 27 83.9 87.9 85.9 Example 28 84.5 88.4 86.4 Example 29 85.8 89.2 87.5 Example

INDUSTRIAL APPLICABILITY

(31) According to aspects of the present invention, it is possible to provide a motor capable of achieving higher efficiency, downsizing, and higher speed by reducing the iron loss during drive at a rotational speed lower than the maximum rotational speed.