Self-magnetizing motor and compressor having the same

A self magnetizing motor and a compressor having the same is configured to require a lower starting voltage and to deliver superior performance. In the self magnetizing motor, a magnetic material on an exterior of the rotor is magnetized when power is supplied to a magnetizing unit of the stator. A plurality of conductive bars are inserted on the outer circumference of the rotor core. A spacing distance between outer portions of the conductive bars and inner circumference of the magnetizable material is longer than an air gap between the stator and the rotor. As a result, a magnetic strength of the magnetic material may be increased.

The present application claims priority to Korean Patent Application No. 10-2007-0021664, filed on Mar. 5, 2007, which is herein expressly incorporated by reference in its entirety.

BACKGROUND

The present application discloses a self magnetizing motor and a compressor having the same.

In general, a motor is a device for converting electrical energy into kinetic energy. Motors may be divided in direct current (DC) motors and alternating current (AC) motors according to a power source to be used. AC motors include induction motors, synchronous motors and commutator motors. The induction motors may be classified into a single-phase induction motor and a three-phase induction motor.

The single-phase induction motor has a relatively simple structure. Also, it is relatively easy to obtain a single-phase power source for such a motor. As a result, single phase induction motors are widely used in all sorts of electrical devices and appliances for domestic, office, industry and architecture. The single-phase induction motor cannot start rotating using just its main coil. For this reason, in addition to the main coil, these motors include a sub coil which receives current having a phase which is approximately 90 degrees out of phase with the current supplied to the main coil. The current applied to the sub-coil generates a starting torque. The main and sub coils are wound on a stator using a predetermined winding method.

In the related art single-phase induction motor, when an AC power source is supplied to the main coil and the sub coil during the initial starting procedure, a rotating magnetic field is generated. Once the rotor begins to rotate, the current supplied to the sub coil is stopped by a current cut-off device. Current is then only supplied to the main coil. Also, as the rotor rotates, an induction current is generated in conductive bars of the rotor.

The AC current applied to the main coil generates a rotating magnetic field which rotates at a predetermined speed which depends on the physical configuration of the motor. However, in related art motors, the rotor will rotate at less than the speed of the rotating magnetic field. The difference between the rotational speed of the rotating magnetic field and the rotating speed of the rotor is typically called “slip.”

DETAILED DESCRIPTION

Reference will now be made in detail to preferred embodiments, examples of which are illustrated in the accompanying drawings.

As shown inFIG. 1, a compressor having a self-magnetizing motor may comprise a case11; a compression portion20disposed inside of the case to compress a refrigerant; and the self magnetizing motor100providing the compression portion20with a driving power. The case11may have a closed receiving space therein, and be provided with an intake pipe13and a discharge pipe15. An oil receiving portion17for receiving a lubricating oil may be formed at a lower portion of the inside of the case11.

The compression portion20for compressing the refrigerant may be disposed at an upper portion of the case11. The self magnetizing motor100may be installed below the compression portion20so as to provide the compression portion20with driving power. A plurality of support springs19for supporting the self magnetizing motor100may be provided at the lower side of the self magnetizing motor100.

The compression portion20may comprise a cylinder21forming a compressing space for the refrigerant therein and a piston27reciprocatingly installed inside the cylinder21to compress the refrigerant. An intake muffler23may be installed at one side of the cylinder21, which is to be connected to the intake pipe13for sucking the refrigerant. One end of a piston rod25may be connected to the piston27and the other end of the piston rod25may be connected to an eccentric portion formed on a rotation shaft152of the self magnetizing motor100. In other embodiments, different types of compression mechanisms may be used to compress the refrigerant, such as a rotating vane type compressor.

Meanwhile, as shown inFIGS. 1 through 3, the self magnetizing motor100may comprise a stator110with the main and sub-coils. A rotor140with a rotor core143is disposed inside of the stator110. A magnetic material160is disposed on an outer circumference145of the rotor core143. Conductive bars mounted in the rotor at locations spaced around the circumference145of the rotor core143. As shown inFIGS. 2 and 3, a spacing distance “t” between the outer edges of the conductive bars151and the inner edge of the magnetic material160is greater than a thickness “G” of an air gap between the magnetic material160and the stator110.

As shown inFIG. 2, a magnetizing unit135is used to magnetize the magnetic material160when power is supplied to the magnetizing unit135. The stator110may be provided with a stator core111and a stator coil121would on the stator core111. A cylinder receiving space may be formed inside of the stator core111so as to receive the rotor140. A plurality of teeth112and slots114are alternatingly formed in an inside diameter of the stator111. As shown inFIG. 2, a certain air gap (G) may be formed between the stator core111and the outer circumference of the rotor140.

The magnetizing unit135includes a magnetizing pole137protrudingly formed between the teeth112. A separate magnetizing coil139is wound on the magnetizing pole137. The magnetizing pole extends inwards towards the magnetic material160on the outer circumference of the rotor.

The rotor140may be provided with a rotor unit141having a rotation shaft142. The magnetic material160is formed on the exterior surface of the rotor unit141. An oil pumping unit149may be provided at the lower portion of the rotation shaft142so as to upwardly supply oil in the oil receiving portion17. An eccentric portion148for creating an eccentric movement may be formed at the upper end of the rotation shaft142.

The rotor unit141may be provided with a rotor core143having a shaft hole144at the middle portion thereof. The rotation shaft142is mounted within the shaft hole144. A plurality of conductive bars151are mounted in the rotor, and the bars are located along the outer circumference145of the rotor core143. The bars are oriented so that they extend radially towards the outer circumference. End rings153may be formed integrally with the conductive bars151, at both ends of the rotor core143, respectively.

In some embodiments, insertion holes147may be formed in the rotor core143so that the conductive bars151can be inserted into the rotor core143. Because the bars are spaced back from the outer circumference of the rotor, a rib146having predetermined thickness “t” is formed between the outer edge of the insertion hole147and the outer circumference145of the rotor core143. Preferably, the thickness t of the rib146is longer than the air gap G. More preferably, the thickness t is approximately 2 to 5 times the air gap G.

In a preferred embodiment, the air gap G is 0.3 mm and the thickness t of the rib146is approximately 0.6 mm to 1.5 mm. The inventors have found that when the conductive bars are spaced back from the magnetic material160by the thickness t of the rib, an intensity of a magnetic force of the magnetic material160(after magnetizing of the magnetic material160) can be better maintained. In addition, the rib helps to enhance starting characteristics and performance of the motor.

Hereinafter, with reference toFIGS. 4 and 5, changes in the efficiency and starting voltage characteristics of the motor, and changes in EMF (electromotive force) of the stator coil121, which result from changes in the thickness t of the rib146will be described.

FIG. 4shows how changes in the thickness of the rib affect the efficiency and the starting voltage of the motor. The efficiency is plotted as line L1, and the starting voltage is shown as line L2. The results shown inFIG. 4are for a motor with an air gap G of 0.3 mm. When the thickness (t) of the rib146is in the range of 2 (0.6 mm) to 5 times (1.5 mm) the air gap, the starting voltage is decreased and the efficiency is increased. Particularly, in the range of three times (0.9 mm) to 4 times (1.2 mm) of the air gap (0.3 mm), the starting voltage is remarkably decreased, while, the efficiency is remarkably increased.

FIG. 5shows the changes in EMF of the stator coil121according to changes of the thickness of the rib146. The stator coil121is composed of a main coil and a sub coil, and the curved line L3indicates changes in the EMF of the main coil and the curved line L4indicates changes in the EMF of the sub coil. As shown, the EMF of the stator coil121gradually increases as the rib146gets thicker.

The conductive bars151may be formed to have an oval cross-sectional shape. The longer axis of the oval is oriented in the radial direction. This helps to offset deterioration of the starting characteristics, which may be caused by a leakage flux. When the diameter of the rotor core is 55-60 mm, one can fit 24-30 conductive bars onto the rotor. This would result in the bars each having a cross-sectional area of 20 mm2to 25 mm2, for ensuring adequate induction torque.

FIGS. 6 and 7illustrate how changes in the number of conductive bars affect the starting voltage and efficiency of the motor. As shown inFIG. 6, the efficiency L5of the motor is slightly reduced when larger numbers of conductive bars151are used. However, the decrease in efficiency is slight, representing only slightly more than one percent when the number of bars changes from 10 to 35.

However, the required starting voltage L6is remarkably decreased when additional bars are added. And the decrease in the required starting voltage is quite large compared to the reduction of the efficiency. Accordingly, it is possible to start the motor with a relatively low starting voltage when more conductive bars are used. The efficiency is only very slightly reduced while the starting voltage is greatly decreased when 24 to 30 conductive bars151are used in the rotor.

As shown inFIG. 7, if the cross-sectional area of the conductive bar151is increased, the efficiency L7of the motor is slightly reduced. However, here again, the starting voltage L8may be significantly reduced when the bars have a larger cross-sectional area. That is, when the cross-sectional area of each conductive bar151is between 20 mm2and 25 mm2, the efficiency may be slightly reduced but the starting voltage may be greatly reduced. The inventors have also found that when the cross-sectional area exceeds 25 mm2, the efficiency of the motor begins to deteriorate faster, and there is no corresponding benefit of a lower staring voltage. Thus, it appears that once the cross-sectional area of the bars reaches approximately 25 mm2, further increases in the cross-sectional area cause a large detrimental impact on efficiency.

As shown inFIG. 8, the magnetic material160may be formed of a plurality of segments162a˜162c. When the magnetic material is formed of a plurality of segments, it enhances residual magnetic flux density by increasing the density of the magnetic material160. In addition, it is easier to control a thickness of the magnetic material when it is formed of multiple small segments. Preferably, the magnetic material160may be formed of 3 to 5 segments, which enhances the density of the magnetic material more than 5.9 g/cm3. Further, preferably, each segment162a˜162cmay be formed by using a Nd bonded magnet. This helps to facilitate the magnetization/demagnetization of the magnetic material160.

When power is supplied to the stator coil121of a rotor as described above, a rotation magnetic field is formed by the stator coil121. An induction current is then generated in the conductive bars151by the rotation magnetic field. In addition, the magnetic material160may be initially magnetized by the rotation magnetic field with an intensity weaker than that of the magnetization caused by the magnetizing unit135. As a result of all these forces, the rotor may be rotated by an integrated torque which includes the induction torque generated by the induction current of the conductive bars151and a hysteresis torque generated by the magnetic material160.

Once the speed of the rotor140has increased to a predetermined speed, power is supplied to the magnetizing unit135for a very short period of time. As a result, the magnetic material160may be quickly magnetized. Typically, the current would only be supplied to the magnetizing unit for the time required for the rotor to make 1 to 5 revolutions. After the magnetization of the magnetic material160, the rotor140speed typically increases due to the interaction between a magnetic force of the magnetic material160and the rotation magnetic field of the stator coil121. When the rotor140rotates in the higher synchronous speed, the induction current may not slow in the conductive bar151, thereby the rotor140may operate as a synchronous motor rotating at the synchronous speed by the interaction between the magnetic material160and the stator110.

When a motor as described above is used in a compressor of a home appliance, it is capable of being easily started in a home having limited current capacity. The motor described above requires a significantly lower starting voltage than related art motors.