Patent Application
20120286608
1. Field of the Invention
The present invention relates to a rolling bearing for inverter-driven motors, such as an air conditioner motor, which suppresses generation of electrolytic corrosion, and relates to an inverter-driven motor using the rolling bearing.
2. Description of Related Art
Recently, motors using pulse width modulation (hereinafter, referred to as a PWM), in which the motor is driven by an inverter, have increased. In such a PWM inverter driving method, since a neutral point potential of winding does not become zero, a potential difference (hereinafter, referred to as a shaft voltage) is often, generated between an outer ring and an inner ring of the rolling bearing which supports the shaft. This shaft voltage contains a high frequency component caused by switching, and when the shalt voltage reaches the dielectric breakdown voltage of the oil film in the bearing, small current flows at the inside of the bearing, and electric discharge is generated between the inner and outer rings and the rolling element of the bearing. As a result, local melting of material inside the bearing, so-called electrolytic corrosion, is generated. In the case in which this electrolytic corrosion progresses, a corrugation phenomenon occurs at the surface of the bearing inner ring, the bearing outer ring and the rolling element, so that poor lubrication or abnormal noises occur, and this is one of the primary factors of the problems in the motor.
As a method for suppressing the electrolytic corrosion in the rolling bearing, a technique in which withstand voltage is increased by strengthening insulation between the inner ring and the outer ring of the rolling bearing as much as possible, and a technique in which electric discharge is frequently repeated by making easier to flow electricity between the inner ring and the outer ring of the rolling bearing, so as to not accumulate electric charge between the inner ring and the outer ring of the rolling bearing, are known.
As a method for increasing the withstand voltage by strengthening the insulation, a technique in which the rolling elements retained between the inner ring and the outer ring are formed by press-sintering material having silicon nitride as a primary component, and the roughness of rolling surface thereof is set to be 0.2 Z or less, and therefore, discharge is not generated, even if relatively large voltage is applied between the inner ring and the outer ring, is disclosed in Japanese Unexamined Patent Application Publication No. H7-12129.
However, in this technique of increasing the withstand voltage by strengthening the insulation, although the electrolytic corrosion is avoided thanks to the perfect insulation obtained by the use of rolling elements made by silicon nitride, a bearing using the rolling elements made of silicon nitride becomes very expensive, and producing a motor with such bearing involves a problem of cost.
In addition, as a technique which does not accumulate the electric charge between the inner ring and the outer ring of the rolling bearing, a technique in which generation of the electrolytic corrosion is prevented by short-circuiting the inner ring and the outer ring using a discharge brush, whereby a discharge route excluding the rolling contact portion between the rolling element and the inner and outer rings is ensured, and a technique in which electric conduction frequency on the contact surfaces is increased and potential difference between the inner and outer rings is maintained to be low, and therefore, electrolytic corrosion damage is suppressed, by setting the center line average surface roughness of at least the contact surface of the rolling element to be 50 to 200 nm Ra, are disclosed respectively in Japanese Unexamined Patent Application Publications No. 2007-146966 and No. 2010-74873.
However, in the technique which provides the discharge brush, there is a problem in that, when conductivity of discharge brush is decreased by abrasion, electric resistance of the discharge brush increases becoming higher than that between the inner and outer rings and the rolling element, and electric conduction between the inner and outer rings is resumed. Another problem is that abrasion powder produced from the discharge brush may cause damage at inside of the bearing. In addition, in the technique in which electric discharge is made easier by roughening the contact surface of the inner and outer rings and the rolling element, small discharge is frequently repeated, so that large damage is not generated on the raceway surface. However, there is a problem in that although the discharges are small, roughness of the contact surface is increased and ultimately, service life of the bearing is shortened.
The present invention was completed by considering the above problems, and objects thereof are to provide a rolling bearing for an inverter-driven motor in which the oil film thickness in a steady operation condition is stably maintained in a specific range, the withstand voltage can be controlled, and whereby, the discharge due to the shaft voltage of the inverter-driven motor is prevented and electrolytic corrosion can be suppressed.
A rolling bearing for an inverter-driven motor of the present invention includes an inner ring, an outer ring, rolling elements, and grease, in which a root mean square roughness on the raceway surface of at least one of the inner ring and the outer ring is in the range of 4 to 16 nm, and an oil film parameter Λ in a steady operation condition is at least 17.5. Another aspect of the rolling bearing for an inverter-driven motor of the present invention is that it includes an inner ring, an outer ring, rolling elements, and grease, in which a root mean square roughness on the raceway surface of at least one of the inner ring and the outer ring is in the range of 4 to 16 nm, and when the root mean square roughness of the raceway surface is set to be x in units of nm and kinematic viscosity at 40° C. of base oil of the grease is set to be y in units of mm2/s, the equation y≧(3x+12) is satisfied.
In addition, in the rolling bearing for the inverter-driven motor of the present invention, it is preferable that kinematic viscosity at 40° C. of the base oil of the grease be at least 24 mm2 is. Furthermore, in the rolling bearing for the inverter-driven motor of the present invention, it is preferable that withstand voltage at 1000 rpm be at least 3 V. Additionally, in the present invention, it is preferable that in the rolling bearing for the inverter-driven motor of the present invention, it is preferable that kinematic viscosity at 40° C. of base oil of the grease be at least 60 mm2/s.
According to the rolling bearing for the inverter-driven motor of the present invention, by setting the root mean square roughness of the raceway surface where the rolling elements roll to be in the range of 4 to 16 nm, and by setting the oil film parameter Λ in a steady operation condition to be at least 17.5, the formation condition of the oil film can be suitably controlled, and thereby discharge at a voltage lower than a specific voltage can be prevented and electrolytic corrosion can be prevented.
Quality of lubricated condition of rolling contact surfaces is evaluated by the oil film parameter Λ, which is the ratio of the thickness of an oil film formed between the contact surfaces and surface roughness of each contact surface. This oil film parameter Λ is expressed by the following equation.
Λ=hmin/σ (Equation 1)
In the above equation, hmin is EHL oil film thickness, σ is composite surface roughness √{square root over ( )}(σ12+σ22) (that is, the square root of (σ12+σ22)), and σ1 and σ2 are surface roughness (root mean square roughness) of the rolling element and the rolling groove which are in contact.
It should be noted that since the rolling bearing of the present invention uses grease lubrication, the oil film parameter is calculated using hmin measured from the grease by optical interferometry. In addition, as a conventional value of oil film parameter Λ, for example, a range of 0.8 to 3.0 is disclosed in paragraph [0006] of Japanese Unexamined Patent Application Publication No. 2000-179559 for a case of the rolling bearing under a usual bearing operational condition. This value is completely different from the numerical range of the oil film parameter Λ in the present invention.
Additionally, the inverter-driven motor of the present invention is characterized in that the motor shaft is supported by the above rolling bearing for the inverter-driven motor. According to the inverter-driven motor having such a construction, electrolytic corrosion can be suitably suppressed by applying the above rolling bearing for the inverter-driven motor to an inverter-driven motor which the shaft voltage is lower than the withstand voltage controlled in the above rolling bearing for the inverter-driven motor.
Furthermore, in the rolling bearing for the inverter-driven motor of the present invention, since the electrolytic corrosion is suppressed by controlling the range of base oil kinematic viscosity of grease, mechanical loss does not increase, and long service life is also achieved. Therefore, an inverter-driven motor in which a bearing can be smoothly and continuously rotated for long period can also be easily provided at low cost.
According to the rolling bearing for the inverter-driven motor of the present invention, the oil film thickness in a steady operation condition is stably maintained in a specific range, the withstand voltage can be controlled, and thereby, the discharge due to the shaft voltage of the inverter-driven motor is prevented and electrolytic corrosion can be suppressed.
Next, an embodiment of a rolling bearing for inverter-driven motor according to the present invention will be specifically explained.
In the metallic rolling bearing 1 used for the inverter-driven motor, since all of the inner ring 2, the outer ring 3, and the rolling elements 4 are made of metal, electric current flows between these components and damage due to electrolytic corrosion is generated. In order to solve this problem, in the present invention, the oil film thicknesses between the inner ring 2 and the rolling element 4, and between the outer ring 3 and the rolling element 4, when the bearing 1 steadily rotates, can be increased by controlling the root mean square roughness on the raceway surface of at least one of the inner ring 2 and the outer ring 3, and the oil film parameter Λ in a steady operation condition. As a result, the current becomes difficult to flow and electrolytic corrosion is suppressed.
In the rolling bearing for an inverter-driven motor of the present invention, it is necessary that the root mean square roughness on an raceway surface of at least one of an inner ring and an outer ring be in the range of 4 to 16 nm. The formation condition of the oil film can be controlled even when the root mean square roughness on the raceway surface of at least one of the inner ring 2 and the outer ring 3 is less than 4 nm, however, rather extreme accuracy is required in production, and the cost problem makes the mass production difficult. In contrast, when the root mean square roughness exceeds 16 nm, the kinematic viscosity of the base oil should be increased to keep the oil film parameter above a specific value and, depending on application, the required torque cannot be satisfied. Therefore, in the present invention, the root mean square roughness of the raceway surface of at least one of the inner ring 2 and the outer ring 3 is controlled to be in a range from 4 to 16 nm.
In addition, in the rolling bearing for the inverter-driven motor of the present invention, it is necessary that oil film parameter Λ in a steady operation condition be at least 17.5, and it is more preferable that it be at least 20. The greater this oil film parameter Λ, the greater is the suppression effect of electrolytic corrosion. However, it is not desirable that it be too large, since if bearing torque is too large, power consumption of the motor is increased.
Furthermore, grease 5 is supplied on contact surfaces between the inner ring 2 and the rolling element 4 and between the outer ring 3 and the rolling element 4, respectively. In the present invention, it is necessary that the kinematic viscosity at 40° C. of the base oil of the grease be at least 24 mm2/s. When the kinematic viscosity at 40° C. is less than 24 mm2/s, the service life of the bearing is shortened.
The inventors have conducted various research with respect to the oil film parameter Λ, the root mean square roughness of the raceway surface of the inner ring or the outer ring, and the withstand voltage, in the rolling bearing for inverter-driven motor of the present invention, and as a result, they have found each of the correlations shown in
The continuous line is an approximate curve based on measured values of withstand voltage at 1000 rpm measured while changing the oil film parameter Λ in the rolling bearing for an inverter-driven motor, according to the following method, and it shows the correlation between the oil film parameter Λ and the withstand voltage. The withstand voltage is measured by a measuring apparatus which is schematically shown in
In addition, the short-dashed line and the long-dashed line show the relationship between the root mean square roughness of the raceway surface of the inner ring or the outer ring and the oil film parameter Λ, with respect to the base oils in which kinematic viscosities at 40° C. are 24 mm2/s and 60 mm2/s, respectively. This relationship can be calculated from Equation 1. Here, hmin value is a value that measures grease at a rotational speed of 1000 rpm by optical interferometry.
In the graph of
In applications of inverter-driven motors for household electrical appliances (fan motors for air conditioners, washing machine motors, cleaner motors, etc.), fan motors for office equipment, etc., since potential difference between the rolling element and the shaft is less than 3 V, it is necessary to increase the withstand voltage of the rolling bearing above the shaft voltage in order to prevent the electrolytic corrosion in the inverter-driven motor, that is, it is necessary that the withstand voltage of the bearing be at least 3 V. In other words, it is necessary that the oil film parameter of the rolling bearing be at least 17.5 in normal operation condition. In addition, the kinematic viscosity of the base oil may exceed 60 mm2/s if the bearing is used in an application in which bearing torque is irrelevant. Therefore, the range defined by the present invention is shown by a halftone dot meshing region (gray area) of
In
y=3x+12 (Equation 2)
In the equation, x is root mean square roughness of the raceway surface (unit: nm), and y is base oil kinematic viscosity (unit: mm2/s). Therefore, with respect to the range defined by the present invention, when the root mean square roughness of the raceway surface x is 4 nm≦x≦16 nm, the base oil kinematic viscosity y (mm2/s) is at least (3x+12), i.e., y≧(3x+12).
As described above, according to the rolling bearing for the inverter-driven motor of the present invention, it is proven that the withstand voltage at 1000 rpm of at least 3 V is obtained by setting the root mean square roughness of at least one of the inner ring and the outer ring of the raceway surface to be 4 to 16 nm and by setting the oil film parameter Λ to be at least 17.5. Thus, in the rolling bearing for the inverter-driven motor of the present invention, since very high withstand voltage at 1000 rpm of at least 3 V can be obtained, electrolytic corrosion due to discharge can be efficiently suppressed.
Furthermore, in the present invention, in the case in which the oil film parameter is fixed, the withstand voltage of the rolling bearing is improved by increasing the kinematic viscosity of grease base oil at 40° C., as shown in
The rolling bearing in the present invention may be a roller bearing, a needle bearing, or an angular type ball bearing, in addition to the deep groove ball bearing, and it may be optionally selected as to size, shape, quantity, material, etc., depending on usage conditions or usage purpose of the inverter-driven motor, or the like motor. In addition, the grease in the present invention may be grease using lithium soap or urea as the thickener.
Next, an inverter-driven motor of the present invention using the above rolling bearing for the inverter-driven motor will be explained with reference to drawings. In the inverter-driven motor of the present invention, the shaft voltage can be reduced to be low, specifically, to be 3 V or less as shown in a measured result of
In
A rotor 14 is inserted in the stator 10 through a gap. The rotor 14 comprises a rotor 30 in a disc shape including a rotor iron core 31 and a shaft 16 which fastens the rotor 30 while passing through the center of the rotor 30. The rotor 30 holds a ferrite resin magnet 32 as a permanent magnet, facing an inner circumference side of the stator 10 in a circumferential direction. In addition, although the details will be explained below, the rotor 30 has a structure in which an outer iron core 31a which constitutes an outer circumference portion of the rotor iron core 31, a dielectric layer 50, and an inner iron core 31b which constitutes an inner circumference portion of the rotor iron core 31 are arranged in this order from the outermost ferrite resin magnet 32 to the shaft 16 at an inner circumference side, as shown in
Two bearings 15 for supporting the shaft 16 are attached to the shaft 16 of the rotor 14. The bearings 15 are bearings in a cylindrical shape having a plurality of steel balls, and the inner ring sides of the bearings 15 are fixed to the shaft 16. In
Furthermore, in this brushless motor, a printed circuit board 18 which packages a driving circuit including a control circuit is contained. A brushless motor is formed by press-fitting the bracket 17 in the stator 10 after this printed circuit board 18 is built-in. In addition, in the printed circuit board 18, interconnect lines 20 such as a lead wire, a ground line of a control circuit, etc., which applies source voltage of winding Vdc, source voltage of control circuit Vcc, and control voltage for controlling rotational frequency Vsp, are connected.
It should be noted that a zero potential point portion on the printed circuit board 18 in which the driving circuit is packaged is isolated from the grounded earth and primary (power source) circuit, and potential of the grounded earth and the primary power source circuit means a floating condition. Here, the zero potential point portion means a wiring having 0 volt potential as a standard potential on the printed circuit board 18, and shows a ground wiring generally called a “ground”. The ground line which is included in the interconnect lines 20 is connected with this zero potential point portion, that is, the ground wiring. In addition, a power source circuit which supplies source voltage of winding connected on the printed circuit board 18 in which the driving circuit is packaged, a power source circuit which supplies source voltage of the control circuit, a lead wire which applies the control voltage, a ground line of the control circuit, etc., are electrically isolated from all of a primary (power source) circuit for a power source circuit which supplies power source voltage of the wiring, a primary (power source) circuit for a power source circuit which supplies power source voltage of the control circuit, grounded earth connected with these primary (power source) circuits, and grounded earth independently grounded. That is, since the driving circuit packaged on the printed circuit board 18 is electrically isolated from the primary (power source) circuit potential and potential of the grounded earth, the potential is in a floating condition. This is expressed as a potential floating condition, and this is well known. In addition, from such facts, constructions of a power source circuit which supplies power source voltage of the winding connected with the printed circuit board 18, and a power source circuit which supplies power source voltage of the control circuit, are expressed as a floating power source, and this is also well known.
With respect to the brushless motor constructed as described above, each power source voltage and control signal is supplied through the interconnect line 20, and a stator winding 12 is driven by the driving circuit on the printed circuit board 18. When the stator winding 12 is driven, driving current flows in the stator winding 12, and magnetic field is generated from the stator iron core 11. Then, by magnetic field from the stator iron core 11 and magnetic field from the ferrite resin magnet 32, attractive force and repulsive force are generated depending on polarity of these magnetic fields, and the rotor 14 rotates around the center of the shaft 16 by these forces.
Next, the structure of the brushless motor according to the present invention will be explained in detail.
First in the brushless motor of the present invention, a shaft 16 is supported by two bearings 15, as described above, and each of the bearings is also fixed and supported by brackets. Furthermore, in the present embodiment, in order to prevent the problem clue to creep as described above, each bearing 15 has a structure fixed by metallic brackets having conductivity. That is, in the present embodiment, conductive brackets having high dimensional accuracy, which are produced by previously processed steel plates, are adopted for fixing the bearings 15. In particular, in the case in which increasing output power of the motor is required, it is preferable that such a structure be used.
Specifically, a bearing 15b at a side opposite to output shaft side is fixed by the bracket 19 having an outer diameter approximately equal to that of the bearing 15b. In addition, this bracket 19 is molded with insulating resin 13 as one body. That is, as shown in
Next, a bearing 15a at an output shaft side is fixed by a bracket 17 having an outer diameter almost equal to an outer diameter of the stator 10. The bracket 17 has nearly a disk-like shape, and comprises a protruding portion having a diameter almost equal to an outer diameter of the bearing 15a at the central area of the disk, and the inside of this protruding portion is hollow. The present brushless motor is formed by press-fitting the inside of the protruding portion of such bracket 17 in the bearing 15a after a printed circuit board 18 is built-in, and by press-fitting the bracket 17 in the stator 10, so as to fit a connecting end provided on a circumference of the bracket 17 and a connecting end of the stator 10. By constructing as described above, facilitation of the assembly process can be attempted, and the problem due to creep can also be prevented, since the outer ring side of the bearing 15a is fixed to the metallic bracket 17.
In addition, a conductive pin 22 is previously electrically connecting in the bracket 19. That is, one tip portion 22a of the conductive pin 22 is connected with the collar portion 19b of the bracket 19, as shown in
Here, since an outer ring side of the bearing 15a is press-fitted in a protruded portion of the bracket 17, the outer ring of the bearing 15a and the bracket 17 are electrically connected, and in contrast, since an outer ring side of the bearing 15b is press-fitted in a cylindrical portion 19a of the bracket 19, the outer ring of the bearing 15b and the bracket 19 are electrically connected. Therefore, the outer ring of the bearing 15a and the outer ring of the bearing 15b are electrically connected by electrically connecting the bracket 17 and the bracket 19.
Then, in the present embodiment, in the rotor 30, a dielectric layer 50 is provided between the shaft 16 and the outer circumference of the rotor 30.
Here, in the case in which the bracket 17 is not connected with the bracket 19, impedance of the two brackets differs since the shape or configuration of the two brackets differs. Therefore, an imbalance is generated between potential induced in the bracket 17 and potential induced in the bracket 19. According to this imbalance of potential, there is a problem in that high-frequency current easily flows through the shaft 16 from the output shaft side to the side opposite to output shaft side or from the side opposite to output shaft side to the output shaft side.
In the present embodiment, by electrically connecting the bracket 17 and the bracket 19, the potentials of the two brackets are equalized and the imbalance of potentials are suppressed, and the high-frequency current hardly flows through the shaft 16.
In addition, in the case in which the conductive pin 22 for connecting the bracket 17 with the bracket 19 is also connected to the stator iron core 11, impedance at the stator side is decreased. When the impedance is reduced, the potential at the stator side, that is, at the outer ring side of the bearing, becomes in high condition, as described above. In contrast, in the present embodiment, by isolating the conductive pin 22 from the stator iron core 11, the reduction of the impedance is suppressed and the potential at the outer ring side of the bearing is held in low condition. In addition, impedances at a stator side and at a rotor side are easily balanced by the above effect, as explained below. Furthermore, in the present embodiment, the bracket 17 and the bracket 19 can be electrically connected while ensuring the isolation from the stator iron core 11, just by press-fitting the bracket 17 in the stator 10, as described above. Therefore, during a production process, potentials of both the brackets can easily be equalized while suppressing the reduction of the impedance at the stator side.
Next, as shown in
In the rotor 30, the dielectric layer 50 is a layer formed by insulating resin as an insulator, and it isolates and separates the outer iron core 31a and the inner iron core 31b in series. At the same time, the dielectric layer 50 is formed by the insulating resin having a specific dielectric constant, and a high-frequency current can flow between the outer iron core 31a and the inner iron core 31b.
Additionally, in the case in which such a dielectric layer 50 is not provided, impedance between the brackets considering the stator iron core as reference is high, and in contrast, impedance between the shaft ends that electrically connect with the rotor is low, as described above. High-frequency current having pulse width modulation generated by the stator iron core, etc., flows in an equivalent circuit having such impedance components. Therefore, potential due to the high-frequency current is generated between the outer ring of the bearing electrically connected with the bracket and the shaft at an inner ring side of the bearing.
In the present embodiment, impedance of the rotor 14 is increased by providing a dielectric layer 50 as shown in
Furthermore, in the present embodiment, by electrically connecting between the bracket 17 and the bracket 19 through a conducting pin 22, potentials of both brackets are equalized, and flow of high-frequency current through the shaft can be suppressed. In addition, potential difference between the inner ring and the outer ring of the bearing 15a can be approximate or can be equalized to potential difference between the inner ring and the outer ring of the bearings 15b by equalizing the potentials of the two brackets. In such a structure, with respect to the bearing 15a and the bearing 15b, respectively, the potential difference between the inner ring and the outer ring of the bearings, that is, shaft voltage, can be lowered by appropriately adjusting the impedance at a rotor side using the dielectric layer 50. Therefore, a problem in which electrolytic corrosion can be suppressed in one bearing, but the electrolytic corrosion is generated in the other bearing, can be prevented. Thus, with respect to two bearings fixed by conductive brackets, respectively, the potential difference between the inner ring and the outer ring of the bearings can be maintained to be low, and therefore, the electrolytic corrosion of the bearing generated by high frequency caused by PWM or the like can be prevented, while fixing strength of the bearings is ensured.
In addition, since the static capacitance can be changed by changing width and material of the dielectric layer 50, impedance at the rotor 14 side can also be optimized. That is, the static capacitance generated by the dielectric layer 50 can be lowered by decreasing the dielectric constant of the insulating resin for forming the dielectric layer 50, by increasing thickness of the insulating resin (distance between electrodes), by decreasing surface area of electrodes, or the like. Thus, the impedance of the rotor 14 can be increased by decreasing the static capacitance generated by the dielectric layer 50.
Additionally, a low-dielectric constant can be attained by using syndiotactic polystyrene (hereinafter referred to as SPS) resin as an insulating resin for forming the dielectric layer 50, and impedance of the rotor 14 can be increased, even when the thickness of the insulating resin is small. That is, as resin usually used for the insulating resin of the motor, polybutylene terephthalate (hereinafter referred to as PBT) resin, polyethylene terephthalate (hereinafter referred to as PET) resin, etc., reinforced by an inorganic filler such as glass fiber, etc., can be mentioned, and the dielectric constants of these materials are about 3.5. In contrast, the dielectric constant is 2.6 for non-reinforced SPS resin and 2.8 for reinforced SPS resin, which means that the dielectric constants of the SPS are lower than those of usual resins. Therefore, when the upper limit of thickness of the insulating resin is structurally limited and the impedance of PBT resin or the like is low and insufficient, the static capacitance can be decreased by using the SPS resin.
Furthermore, the rotor 30 is constructed so that an outer iron core 31a and an inner iron core 31b are separated by the dielectric layer 50, as shown in
In the case in which the dielectric layer 50 is formed to be a perfect ring shape as shown in
Next, an outer rotor type motor in which a rotor is arranged on a circumference side of a stator will be explained.
1. Rolling Bearing for Inverter-Driven Motor
As shown in Table 1, grease having a kinematic viscosity at 40° C. of 55 nm2/s and comprising ester oil as a base oil and lithium soap as a thickener, was used. This grease was supplied at a required amount in a ball bearing (trade name: 608ZZ, produced by Minebea Co., Ltd., outer diameter: 22 mm, inner diameter: 8 mm, width: 7 mm) in which the root mean square roughness on the raceway surface of the inner ring was 8 nm. By this way, a rolling bearing for an inverter-driven motor of Example 1 according to the present invention having an oil film parameter Λ of 26 at 1000 rpm was produced.
As shown in Table 1, grease having a kinematic viscosity at 40° C. of 55 mm2/s and comprising ester oil as a base oil and lithium soap as a thickener, was used. This grease was supplied at a required amount in a ball bearing (trade name: 608ZZ, produced by Minebea Co., Ltd., outer diameter: 22 mm, inner diameter: mm, width: 7 mm) in which the root mean square roughness on the raceway surface of the inner ring was 14 nm. By this way, a rolling bearing for an inverter-driven motor of Comparative Example 1 according to the present invention having an oil film parameter Λ of 16 at 1000 rpm was produced.
As shown in Table 1, grease having a kinematic viscosity at 40° C. of 26 mm2/s and comprising ester oil as a base oil and lithium soap as a thickener, was used. This grease was supplied at a require amount in a ball bearing (trade name: 608ZZ, produced by Minebea Co., Ltd., outer diameter: 22 mm, inner diameter: 8 mm, width: 7 mm) in which the root mean square roughness on the raceway surface of the inner ring was 8 nm. By this way, a rolling bearing for an inverter-driven motor of Comparative Example 2 according to the present invention having an oil film parameter Λ of 14 at 1000 rpm was produced.
With respect to the rolling bearings for inverter-driven motor of Example 1 and Comparative Examples 1 and 2 produced as described above, measurement of withstand voltage and electrolytic corrosion reproduction tests were carried out by the following method.
The withstand voltage was measured by a measuring device in which each rolling bearing for inverter-driven motor of Example 1 and Comparative Examples 1 and 2 was fixed to a metallic shaft, an electric circuit as schematically shown in
Considering these graphs in detail, since an insulation condition is maintained by an oil film, the current did not flow at low voltage and the current value was zero. However, when the voltage was gradually increased, it suddenly dropped at certain time t1. At this time t1, thickness of the oil film was insufficient for maintaining the insulation condition at the increased voltage, and the current began to flow, and therefore, the current value rose. When the voltage was maintained in this condition, the current value was also maintained at almost a constant value, and thus, the graph became substantially horizontal after t1, as shown in
As is apparent from Table 1, it was shown that the withstand voltage of 5.3 V could be obtained in the rolling bearing for the inverter-driven motor of Example 1 in which the root mean square roughness on the raceway surface of the inner ring and the oil film parameter Λ were in the range defined by the present invention. In contrast, in the rolling bearings for the inverter-driven motor of Comparative Examples 1 and 2 in which the oil film parameter Λ was less than 17.5, it was shown that the withstand voltages were 2.3 and 1.9 V, respectively, and were less than the withstand voltage of 3 V which is required for inverter-driven motors in household electrical appliance application.
Next, the electrolytic corrosion reproduction test was carried out by continuous operation for a total of 504 hours at a rotational speed of 1000 rpm under acceleration test conditions in which high frequency rectangle pulse voltage having a maximum voltage of 3 V and frequency of 1.2 MHz was applied between the shaft and the outer ring, using the above measuring device in which the rolling bearings for the inverter-driven motor of Example 1 and Comparative Examples 1 and 2 were fixed to the shaft. For each of the Example and the Comparative Examples, four ball bearings were tested. The Anderon Medium band (M band) value was measured at the start of test and at every 168 hours (24 hours times 7 days is 168 hours). The average values of these measured values are shown in the graph of
As is apparent from
2. Inverter-Driven Motor
A brushless motor having a structure shown in
With respect to this brushless motor, the shaft voltage was measured using a direct current stabilized power supply, under a specific operating condition in which power supply voltage of winding Vdc was 391 V, power supply voltage of control circuit Vcc was 15 V, and rotational speed was 1000 rpm. It should be noted that the rotational speed was adjusted by control voltage Vsp and the brushless motor in the operation was arranged to have the shaft in a horizontal position.
The shaft voltage was measured by the following method: voltage waveform was observed using a digital oscilloscope (DPO7104 model, produced by Tektronix, Inc.) and a high voltage differential probe (P5205 model, produced by Tektronix, Inc.) to check the deformation of the waveform, and peak-to-peak voltage was measured as shaft voltage. This measured result of the shaft voltage is shown in
As shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-105203 | May 2011 | JP | national |
