Patent Publication Number: US-2007113598-A1

Title: Induction motor for drum washing machine and drum washing machine using same

Description:
This application claims the benefit of Korean Patent Application No. 10-2005-0110837, filed on Nov. 18, 2005, which is hereby incorporated by reference for all purposes as if fully set forth herein.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a washing machine, more particularly, to an induction motor for a washing machine that can reduce vibration and noise as well as production cost.  
      2. Discussion of the Related Art  
      In general, drum washing machines may be classified as indirect drive drum washing machines and direct drive drum washing machines. The former indirectly transmits a driving force of a motor to a drum via a motor pulley and a drum pulley, and the latter directly connects a rotor of a motor to a drum to transmit the driving force of the motor to the drum.  
      The indirect drive drum washing machine has a problem with energy loss and noise during the transmission of the driving force. Thus, to solve this problem of the indirect drive drum washing machine, the direct drive drum washing machine recently has been more widely used. The direct drive drum washing machine has a motor mounted to a rear wall of a tub to directly transmit the driving force of the motor to the drum.  
      A motor in the conventional direct drive drum washing machine includes a rotor connected to a shaft of a drum, a permanent magnet on the rotor, and a stator wound with a coil. A conventional motor has a problem where it costs a lot to fabricate the motor and a washing machine, because the conventional motor needs an expensive permanent magnet.  
      The permanent magnet is installed around the circumference of the rotor with alternating magnetic poles along a circumferential direction of the rotor. When the rotor rotates, the permanent magnet interacts with space between teeth of the stator, that is, with a slot of the stator, such that cogging torque is generated. Moreover, a conventional motor with a permanent magnet has another problem where the efficiency may deteriorate due to mechanical vibration and torque ripple caused by frequency harmonics.  
      Meanwhile, a hall sensor should be provided in the conventional motor with a permanent magnet to detect the position of the rotor. Also, an algorithm that determines the position of the rotor is necessary when the motor is operated. Also, the position of the rotor should be controlled according to the phase of supplied voltage. Thus, this results in another problem where the cost of motor is high and the control of motor is complex.  
     SUMMARY OF THE INVENTION  
      Accordingly, the present invention is directed to an induction motor for a drum washing machine and a washing machine using the same that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.  
      An advantage of the present invention is to provide an induction motor for a drum washing machine that reduces the production cost.  
      Another advantage of the present invention is to provide an induction motor for a drum washing machine and a drum washing machine that reduces vibration and noise.  
      Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.  
      To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, an induction motor for a drum washing machine includes: a concentrated winding stator with a plurality of teeth that project from a circumference thereof and a coil wounded around each tooth; a rotor frame connected to a shaft directly fixed to a rotatable drum in the drum washing machine, wherein the concentrated winding type stator is located in the rotor frame; and a rotor conductor that generates induced currents in closed circuits along a circumferential direction of the rotor frame when electric currents are applied to the coils so that the rotor frame rotates, wherein the closed circuit is skewed.  
      In another aspect of the present invention, a drum washing machine includies: a tub that holds washing solution therein; a drum that rotates within the tub; a concentrated winding stator with a plurality of teeth that project from a circumference thereof and a coil wounded around each tooth, the concentrated winding stator fixed to a rear wall of the tub; a shaft directly connected to the drum, passing through the rear wall of the tub; a rotor frame directly connected to the shaft, wherein the concentrated winding type stator is located in the rotor frame; and a rotor conductor that generates induced currents in closed circuits along a circumferential direction of the rotor frame when electric currents are applied to the coils so that the rotor frame rotates, wherein the closed circuit is skewed  
      It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention.  
      In the drawings:  
       FIG. 1  is a diagram schematically illustrating a drum washing machine including an induction motor according to the present invention;  
       FIG. 2  is a plan view of the induction motor according to the present invention;  
       FIG. 3  is a sectional view along the line I-I of the motor shown in  FIG. 2 ;  
       FIG. 4  is a plan view partially illustrating a stator core of the induction motor shown in  FIG. 3 ;  
       FIG. 5  is a front view illustrating another embodiment of a rotor conductor of the induction motor according to the present invention shown in  FIG. 3 ;  
       FIG. 6  is a front view illustrating still another embodiment of the rotor conductor of the induction motor according to the present invention; and  
       FIG. 7  is a block diagram illustrating a parallel wiring of the induction motor according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED. EMBODIMENTS  
      Reference will now be made in detail to an embodiment of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.  
       FIG. 1  is a diagram schematically illustrating a drum washing machine including an induction motor according to the present invention. As shown in  FIG. 1 , the drum washing machine includes a tub  2  provided within a cabinet  1 , a drum  3  rotatably installed within the tub  2 , a shaft  4  fixed to a rear wall of the drum  3  passing through a rear wall of the tub  2 , and a motor  100  provided at the rear of the tub  2  to operate the drum  3 . The motor  100  is also connected to the shaft  4 . A door  5  is coupled to a front of the cabinet  1 , and a gasket  6  is provided between the door  5  and the tub  2 . Furthermore, the washing machine has a hanging spring  7  between an inner upper surface of the cabinet  1  and an upper surface of the tub  2  to suspend the tub  2  within the cabinet  1 . Also, a friction damper  8  is provided between a lower portion of the cabinet  1  and a lower outer surface of the tub  2  to attenuate vibration of the tub  2  generated by spinning the laundry.  
      The motor  100  may be a Brushless DC (BLDC) motor. The motor  100  includes a stator  300  fixed to the rear wall  2 a of the tub  2  and a rotor  200  coupled to the shaft  4  to directly rotate the drum  3 . The motor  100  of the present invention directly transmits its driving force to the drum  3  via the shaft  4  directly connected to the drum  3  and the rotor  200  and not via an auxiliary pulley or belt in the prior art.  
      Referring to FIGS.  2  to  4 , the structure of the motor  100  will be described in detail.  FIG. 2  is a plan view of the induction motor according to the present invention,  FIG. 3  is a sectional view along the line I-I of the motor shown in  FIG. 2 , and  FIG. 4  is a plan view partially illustrating a stator core of the induction motor shown in  FIG. 3 .  
      As shown in  FIG. 3 , the stator  300  includes a stator core  310  and an insulator  320 . Thin conductor plates are stacked or a long and thin conductor band is wound in a spiral shape to form a multi-layered stator core  310 . The insulator  320  is attached to an upper surface and a lower surface of the stator core  310 . An opening  330  is formed in the center of the stator  300  to allow the shaft  4  to pass through the stator  300 , and a plurality of fastening holes  340  are formed along a circumference of the opening  330 . The stator  300  is secured to the rear wall  2 a of the tub  2  by a fastener such as a bolt and the fastening hole  340 .  
      As shown in  FIG. 4 , a plurality of teeth  301  project from the circumference of the stator  300  in a radial direction. The pitch of the teeth is the distance between the centers of adjacent teeth and is illustrated as “p” in  FIG. 4 . A pole shoe  302 , at the end of the tooth  301  has two opposite ends that project in a circumferential direction relative to the stator  300  to partially cover a slot  303  that is a space between the teeth  301 . Also, it is preferred that the middle portion of the pole shoe  302  is thicker than the two opposite ends of the pole shoe  302 . When the pole shoe  302  has this shape, the air gap between the inner surface of the rotor  200  and the pole shoe  302  of the stator  300  may maintain a substantially constant value. Thus, when the rotor  200  rotates, the interaction between the rotor  200  and the stator  300  may vary little to prevent the air gap from abruptly changing. As a result, vibration and noise may be reduced.  
      A coil  350  is wound around each tooth  301 , and it is preferred that the coil  350  is a concentrated winding. That is, the coil  350  is wound around each tooth  301  of the stator  300  so that the number of slots corresponding to each pole, formed when electrical power is applied to the coil  350  wound around the each tooth, is 1 or less than 1. Hence, although the stator core  310  having a small number of the teeth  301  is used, a lot of magnetic poles may be formed. When a concentrated winding is employed to the motor  100 , the stator  300  may be small in size, compared to a dispersed winding. Also, it is easier to control the motor using an inverter due to the increased number of the magnetic poles.  
      Meanwhile, as shown in FIG,  1 , the rotor  200  surrounds the stator  300 . The rotor  200  includes a rotor frame  210  that has a cup shape. Also, as shown in  FIG. 1 , the shaft  4  is fixed to a rear wall of the rotor frame  210 , and a side wall of the rotor frame  210  encompasses the stator  300 .  
      The motor  100  for a drum washing machine of the present invention is an induction motor, and accordingly, the rotor  200  includes a rotor conductor  220  to generate induced currents when electric currents are applied to the coil  350  instead of a permanent magnet. Here, the rotor conductor  220  is attached to an inner surface of the side wall of the rotor frame  210  that faces the teeth  301  of the stator  300  wound by the coil  301 . Moreover, the side wall of the rotor  200  includes a rotor core  230  to form a magnetic flux path.  FIG. 3  illustrates that the rotor conductor  220  is arranged along the inner surface of the side wall of the rotor frame  210  to have an “I” shaped cross-section and that the rotor cores  230  are arranged on opposite sides of the rotor conductor  220 .  
      For example, the rotor conductor  220  may be made of aluminum or copper and formed substantially as one body with the rotor core  230  by die-casting. Because aluminum or copper has little electric resistance and relatively less strength than a conventional material, such as steel, used to make the rotor core  230 , the rotor conductor  220  may be easily attached to the rotor core  230  by process such as die-casting.  
      Alternatively, the rotor conductor  220  may be formed in another embodiment, unlike the one shown in  FIG. 3 .  FIGS. 5 and 6  illustrate other embodiments of the rotor conductor  220  that will be described below.  
      A rotor conductor  220  as shown in  FIGS. 5 and 6  is formed of a thin plate along an inner circumferential surface of a rotor core  230  attached to an inner surface of a side wall of the rotor frame  210  to face the stator  300 . Because the coil  350  of the induction motor  100  according to the present invention is a concentrated winding, multiple-magnetic poles are alternatively formed along the circumferential direction of the stator  300 .  
      Thus, the induction motor of the present invention might generate torque ripple or mechanical vibration due to cogging torque, overlapped phase, or frequency harmonics that may be generated by interaction between the slot  303  of the stator  300  and the rotor  200 . However, this problem arises much less than in conventional motors having permanent magnets. In spite of the reduction of torque ripple, it is desirable to reduce even minute torque ripple.  
      As shown in  FIG. 5 , the rotor conductor  220  has a vertical member  221  may be skewed with respect to a horizontal member  223 . The rotor conductor  220  has two horizontal members  223  in parallel along a circumferential direction and a plurality of vertical members  221  are spaced apart by a predetermined distance and extend between the horizontal members  223  to connect the horizontal members  223  to each other.  
      Two adjacent vertical members  221  and the two horizontal members  223 , form one closed circuit. Each closed circuit is skewed, and the skew distance “d 1 ” indicates how much each closed circuit is skewed. The skew distance“d 1 ” corresponds to one pitch of the teeth  301 . Because, the vertical member  221  of the rotor conductor  220  is skewed at a predetermined angle(θ sk ), there is a skew distance “d 1 ” between an upper end and a lower end of the vertical member  221  as shown in  FIG. 5 . The skew distance “d 1 ” is equal to about one pitch “p” of the teeth  301 .  
      As shown in  FIG. 6  illustrating the other embodiment of the rotor conductor  220 , each closed circuit is symmetrically skewed about the center of the circuit, and the skew distance “d 2 ” indicates how much each closed circuit is skewed. The skew distance “d 2 ” falls within about one to two of the pitch of the teeth  301 , i.e, p≦d 2 ≦2xp. Because, a middle portion of the vertical member  221  is projected and each closed circuit is as skewed symmetrically in a vertical direction, the skew distance “d 2 ”, is the distance between the middle portion and an upper or lower end as shown in  FIG. 6 .  
      When the rotor conductor  220  is skewed as shown in  FIGS. 5 and 6 , the interaction between the rotor  200  and the slot  303  of the stator  300  may be prevented from changing abruptly. Thereby, torque ripple, cogging torque and mechanical vibration may be reduced.  
      The rotor frame  210  may also include a permanent magnet  150 . The permanent magnet  150  is for detecting a position of the rotor  200  and the rotor speed. A hall sensor (not shown) may be provided on a circumference of the stator  300  to sense a magnetic field of the permanent magnet  150 .  
      The permanent magnet  150 , as shown in  FIG. 1 , may be also placed on the shaft  4 , different from the above. In this case, the hall sensor for detecting the magnetic field of the permanent magnet  150  might be at an inner circumference of the stator  300 . This has the advantage of resulting in a smaller motor  100  because the space for fixing the hall sensor to an outer portion of the stator  300  is not needed.  
      Next, the operation of the induction motor according to the present invention will be described.  
      When three phase voltage is applied to the coil  350  of the stator  300  via an inverter, a rotational magnetic field is generated in the stator  300 . Hence, the rotor conductor  220  generates an induced voltage that produce induced currents and the rotor  200  starts to rotate due to the interaction between the rotational magnetic field and the induced currents.  
      When the rotor  200  rotates, the hall sensor (not shown) senses the, magnetic field of the permanent magnet  150  and a controller (not shown) finds out the position and rotation speed of the rotor  200  by using the information sensed by the hall sensor. The controller (not shown) controls the inverter based on the position and rotation speed of the rotor  200  to control the motor  100  more precisely.  
      Meanwhile, as shown in  FIG. 7 , a connection structure of the coil provided on the stator, which forms each phase separately, may be changed into a parallel connection structure. That is, high torque should be generated by high electric currents to operate the motor at a high speed, and the size of the electric current is limited in a serial connection structure due to high resistance. As a result, it is difficult to produce high torque as well as operate the motor at a high speed.  
      Accordingly, to solve the above problem, as shown in  FIG. 7 , a parallel connection structure is used for the coil provided on the stator  300 . In the parallel connection structure, the resistance value of the parallel connection is smaller than a resistance value of a serial connection so that high electric currents may be applied to the coils.  
      An embodiment of the rotor  200  has been described where the rotor  200  has a cup shape to cover the stator  300 . However, the rotor  200  may have a cage shape, like in a conventional induction motor.  
      As mentioned before, the present invention has the following advantages.  
      First, because the induction motor for a direct driven drum washing machine is provided and not a permanent magnet motor which costs much more, production cost for a washing machine may be reduced.  
      Second, because cogging torque and high frequency harmonics generated by a permanent magnet is reduced, vibration and noise caused by torque ripple may be reduced.  
      Third, because the coil wound around the stator is a concentrated winding with a high number of magnetic poles, it is easier to control the motor.  
      Finally, because an auxiliary algorithm for determining the position of the rotor is not needed, the control algorithm for the motor may be simpler.  
      It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.