Abstract:
A magnetic levitated pump that does not cause pulsation of a pumped liquid and can suppress the generation of particles, which are liable to be produced by contact of a sliding part, is disclosed. The magnetic levitated pump for magnetically levitating an impeller housed in a pump casing includes a motor configured to rotate the impeller, and an electromagnet configured to magnetically support the impeller. The motor and the electromagnet are arranged so as to face each other across the impeller, and the motor is arranged on the opposite side of a suction port of the pump casing.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This document claims priority to Japanese Patent Application Number 2014-226210 filed Nov. 6, 2014, the entire contents of which are hereby incorporated by reference. 
       BACKGROUND 
       [0002]    Conventionally, as a pump for transferring pure water or a chemical liquid, there has been commonly known a positive displacement pump that compresses a liquid to a predetermined pressure by using a reciprocating diaphragm or the like to deliver the liquid intermittently. It has also been practiced to transfer pure water or a chemical liquid by using a centrifugal pump having an impeller supported by a main shaft, which is rotatably supported by a bearing, in a pump casing. 
         [0003]    However, when the positive displacement pump is used, there arises a problem of generation of pulsation because the transfer of liquid does not become continuously smooth. On the other hand, when the centrifugal pump is used, the contact of a sliding part such as a shaft seal part or a bearing cannot be avoided, and thus particles are inevitably generated by this contact. Therefore, there is a problem of causing the particles to be mixed into the pumped liquid such as pure water or a chemical liquid and thus causing contamination of the pumped liquid. 
       SUMMARY OF THE INVENTION 
       [0004]    According to an embodiment, there is provided a magnetic levitated pump that does not cause pulsation of a pumped liquid and can suppress the generation of particles, which are liable to be produced by contact of a sliding part. 
         [0005]    Embodiments, which will be described below, relate to a magnetic levitated pump, and more particularly to a magnetic levitated pump having a structure which can suppress the generation of particles, which are liable to be produced by contact of a rotating portion, by rotating an impeller in a non-contact manner, and thus can prevent a pumped liquid such as pure water or a chemical liquid from being contaminated by the particles. 
         [0006]    In an embodiment, there is provided a magnetic levitated pump with an impeller housed in a pump casing and to be magnetically levitated, the magnetic levitated pump comprising: a motor configured to rotate the impeller; an electromagnet configured to magnetically support the impeller; wherein the motor and the electromagnet are arranged so as to face each other across the impeller; and the motor is arranged on the opposite side of a suction port of the pump casing. 
         [0007]    According to the embodiment, an axial thrust is applied by a pressure difference between a pressure in the pump casing and a pressure in the suction port during operation of the pump, and thus the impeller is pushed to the suction port side. However, the motor arranged on the opposite side of the suction port can apply an attractive force that pulls back the impeller to the opposite side of the suction port side, and thus the axial thrust generated by the differential pressure of the pump can be cancelled out. Therefore, control of the impeller in the thrust direction by the electromagnet during operation of the pump can be zero-power (no-electric power) control. 
         [0008]    In an embodiment, the motor is a permanent magnet motor having a permanent magnet on the impeller side. 
         [0009]    According to the embodiment, since the motor is a permanent magnet motor having a permanent magnet on the impeller side, an attractive force always acts on the impeller from the motor, so that the force that pulls back the impeller, which is pushed to the suction port side by the axial thrust, toward the opposite side can be exerted. 
         [0010]    In an embodiment, a ring-shaped permanent magnet is provided at an axial end portion of the impeller and a ring-shaped permanent magnet is provided at a position, of the pump casing, which radially faces the axial end portion of the impeller to allow the permanent magnet at the impeller side and the permanent magnet at the pump casing side to face each other in a radial direction, thereby constructing a permanent magnetic radial repulsive bearing. Here, the axial direction of the impeller refers to a direction of an axis of the rotating shaft of the impeller, i.e., a thrust direction. 
         [0011]    According to the embodiment, if radial rigidity obtained only by a passive stabilizing force is insufficient, the radial rigidity can be supplemented by the permanent magnetic radial repulsive bearing. Thus, the axial end portion of the impeller can be stably supported in a non-contact manner by the magnetic repulsive force. 
         [0012]    In an embodiment, the permanent magnet on the impeller side and the permanent magnet on the pump casing side are positionally shifted in the axial direction. 
         [0013]    According to the embodiment, because the permanent magnet on the impeller side and the permanent magnet on the pump casing side are positionally shifted in the axial direction, a force in a direction opposite to the attractive force which allows the motor to attract the impeller, i.e., a force for pushing the impeller to the suction port side, can be generated. Since the attractive force which allows the motor to attract the impeller can be reduced by the force for pushing the impeller to the suction port side, an electromagnetic force of the electromagnet can be reduced when performing the control of disengaging the impeller, which is attracted to the motor side at the time of pump startup, from the motor by the electromagnetic force of the electromagnet. Thus, the electric power of the electromagnet at the time of pump startup can be reduced. 
         [0014]    In an embodiment, a sliding bearing is provided between an axial end portion of the impeller and a portion, of the pump casing, which radially faces the axial end portion of the impeller. 
         [0015]    According to the embodiment, if the radial rigidity obtained only by the passive stabilizing force is insufficient, the radial rigidity can be supplemented by the sliding bearing. Thus, the axial end portion of the impeller can be supported in a stable manner. 
         [0016]    In an embodiment, the axial end portion of the impeller constitutes a suction port of the impeller or a portion projecting from a rear surface of the impeller. 
         [0017]    In an embodiment, the displacement of the impeller is detected based on impedance of the electromagnet. 
         [0018]    According to the embodiment, a sensor for detecting a position of the impeller as a rotor is not required, and thus the control of the electromagnet can be performed without a sensor. 
         [0019]    In an embodiment, a liquid contact portion that is brought into contact with a liquid to be pumped in the pump casing comprises a resin material. 
         [0020]    According to the embodiment, the liquid contact portion, such as an inner surface of the pump casing or the impeller, that is brought into contact with the liquid to be pumped is coated with the resin material such as PTFE or PFA, or all the constituent parts of the liquid contact portion are composed of the resin material. Therefore, metal ions are not generated from the liquid contact portion. 
         [0021]    The above-described embodiments offer the following advantages. 
         [0000]    1) The generation of particles which are liable to be produced by contact of a rotating portion or a sliding portion can be suppressed by rotating the impeller in a non-contact manner. Thus, a problem that particles are mixed into the pumped liquid such as pure water or a chemical liquid to contaminate the pumped liquid can be solved.
 
2) Since the magnetic levitated pump is constructed with a centrifugal pump, the liquid such as pure water or a chemical liquid can be transferred continuously and smoothly, and pulsation of the pumped liquid is not generated.
 
3) An axial thrust is applied by a pressure difference between a pressure in the pump casing and a pressure in the suction port during operation of the pump to push the impeller to the suction port side. However, the motor arranged on the opposite side of the suction port can apply an attractive force that pulls back the impeller to the opposite side of the suction port side, and thus the axial thrust generated by the differential pressure of the pump can be cancelled out. Therefore, control of the impeller in a thrust direction by the electromagnet during operation of the pump can be zero-power (no-electric power) control.
 
4) Since the liquid contact portion that is brought into contact with the liquid to be pumped in the pump casing is composed of the resin material such as PTFE or PFA, metal ions are not generated from the liquid contact portion.
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a vertical cross-sectional view showing a magnetic levitated centrifugal pump which is an embodiment of a magnetic levitated pump; 
           [0023]      FIG. 2  is a vertical cross-sectional view showing another embodiment of the magnetic levitated pump; 
           [0024]      FIG. 3  is a view showing an arrangement example of control magnetic poles (eight); 
           [0025]      FIG. 4  is a view showing an arrangement example of control magnetic poles (six); 
           [0026]      FIG. 5  is a view showing a first example of a permanent magnetic radial repulsive bearing; 
           [0027]      FIG. 6  is a view showing a second example of the permanent magnetic radial repulsive bearing; and 
           [0028]      FIGS. 7A and 7B  are views showing external appearance of the magnetic levitated centrifugal pump shown in  FIGS. 1 and 2 , and  FIG. 7A  is a front elevational view of the magnetic levitated centrifugal pump and  FIG. 7B  is a side view of the magnetic levitated centrifugal pump. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0029]    Embodiments of a magnetic levitated pump will be described below with reference to  FIGS. 1 through 7A, 7B . In  FIGS. 1 through 7A, 7B , identical or corresponding parts are denoted by identical or corresponding reference numerals throughout views, and will not be described in duplication. 
         [0030]      FIG. 1  is a vertical cross-sectional view showing a magnetic levitated centrifugal pump which is an embodiment of a magnetic levitated pump. As shown in  FIG. 1 , the magnetic levitated centrifugal pump  1  comprises a substantially cylindrical container-shaped casing  2  having a suction port  1   s  and a discharge port  1   d,  a casing cover  3  covering a front opening of the casing  2 , and an impeller  4  housed in a pump casing comprising the casing  2  and the casing cover  3 . A liquid contact portion, such as an inner surface of the pump casing comprising the casing  2  and the casing cover  3 , is formed in a resin canned structure made of PTFE, PFA, or the like. The inner surface of the pump casing comprises both flat end surfaces and a cylindrical inner circumferential surface, and the interior of the pump casing is designed not to have a recessed portion so that there is no air pocket. 
         [0031]    In the casing  2 , there is provided an electromagnet  6  for attracting a rotor magnetic pole  5  made of a magnetic material, such as a silicon steel sheet, embedded in a front surface of the impeller  4  to support the impeller  4  by magnetism. The electromagnet  6  has electromagnet cores  6   a  and coils  6   b.  In the casing cover  3 , there is provided a motor  9  for rotating the impeller  4  while attracting permanent magnets  8  embedded in a rear surface of the impeller  4 . The motor  9  has motor cores  9   a  and coils  9   b.  Because the electromagnet  6  and the motor  9  are configured to be sextupole type, respectively, the cores can be commonalized, thereby reducing the cost. 
         [0032]    The magnetic levitated centrifugal pump  1  shown in  FIG. 1  has a simple structure in which the electromagnet  6  and the motor  9  are arranged so as to face each other across the impeller  4 . An axial thrust is applied to the impeller  4  by a pressure difference between a pressure in the pump casing and a pressure in the suction port during operation of the pump, and thus the impeller  4  is pushed to the suction port side. However, since the motor  9  is a permanent magnet motor having the permanent magnets  8  on the impeller side, an attractive force always acts on the impeller  4 , so that the force that pulls back the impeller  4 , which is pushed to the suction port side by the axial thrust, toward the opposite side can be exerted. In other words, the motor  9  is arranged on the opposite side of the suction port  1   s  so that the attractive force by the permanent magnet motor and the axial thrust by the differential pressure of the pump can be balanced. 
         [0033]    On the other hand, the electromagnet  6  disposed on the front surface side of the impeller  4  is configured as a magnetic bearing that generates a Z-axis control force (control force in a thrust direction) which is balanced with the motor attractive force, and a control force for correcting the tilt of θx (about an X-axis) and θy (about a Y-axis) defined as the tilt (rotation) with respect to the X-axis and the Y-axis which are axes perpendicular to the Z-axis, so that the electromagnet  6  supports the impeller  4  in a non-contact manner in the pump casing. Further, the position of the impeller  4  can be detected by detecting the displacement of the impeller  4  as a rotor based on impedance of the electromagnet  6 , thus allowing a sensor-less structure which requires no position sensor. Since the position where the control force acts is detected, so-called collocation conditions are met, and thus a structure that allows the electromagnet  6  to be easily controlled can be employed. 
         [0034]    As shown in  FIG. 1 , the motor  9  and the electromagnet  6  are disposed so as to face the impeller  4  respectively, thus becoming a compact structure in a radial direction. In this manner, the axial-type motor is selected to make radial dimension of the pump compact, and the permanent-magnet type motor is selected to have an improved efficiency and to obtain a large torque. Thus, the impeller  4  as a rotor is reliably attracted to the motor side, and therefore the electromagnet is disposed on the opposite side to counteract such attractive force. With such arrangement, the structure that can control three degrees of freedom (Z, θx, θy) by the electromagnet disposed on one side can be realized. 
         [0035]      FIG. 2  is a vertical cross-sectional view showing another embodiment of the magnetic levitated pump. The magnetic levitated pump shown in  FIG. 2  is a magnetic levitated centrifugal pump as with  FIG. 1 . In the magnetic levitated centrifugal pump  1  shown in  FIG. 2 , a ring-shaped permanent magnet  10  is provided at an axial end portion  4   e  of the impeller  4  and a ring-shaped permanent magnet  11  is provided at a portion, of the casing cover  3 , which radially faces the axial end portion  4   e  of the impeller  4  to allow the permanent magnet  10  on the impeller side and the permanent magnet  11  on the casing cover side to face each other in a radial direction, thereby constructing a permanent magnetic radial repulsive bearing. 
         [0036]    Although radial rigidity is obtained by the passive stabilizing force generated by the attractive force of the electromagnet  6  and the motor  9  in the embodiment shown in  FIG. 1 , according to the embodiment shown in  FIG. 2 , if the radial rigidity obtained only by the passive stabilizing force is insufficient, the radial rigidity can be supplemented by the permanent magnetic radial repulsive bearing comprising the permanent magnet  10  on the impeller side and the permanent magnet  11  on the casing cover side. With this structure, the axial end portion of the impeller  4  can be stably supported in a non-contact manner by the magnetic repulsive force. 
         [0037]    The permanent magnet  10  on the impeller side and the permanent magnet  11  on the casing cover side are positionally shifted slightly in the axial direction. Because the permanent magnet  10  on the impeller side and the permanent magnet  11  on the casing cover side are positionally shifted slightly in the axial direction, a force in a direction opposite to the attractive force which allows the motor  9  to attract the impeller  4 , i.e., a force for pushing the impeller  4  to the suction port side, is generated. Since the attractive force which allows the motor  9  to attract the impeller  4  can be reduced by the force for pushing the impeller to the suction port side, an electromagnetic force of the electromagnet  6  can be reduced when performing the control of disengaging the impeller  4 , which is attracted to the motor side at the time of pump startup, from the motor  9  by the electromagnetic force of the electromagnet  6 . Thus, the electric power of the electromagnet  6  at the time of pump startup can be reduced. 
         [0038]    Further, as shown in  FIG. 2 , a sliding bearing  12  is provided between the outer circumferential surface of the suction port  4   s  of the impeller  4  and a portion, of the casing  2 , which radially faces the outer circumferential surface of the suction port  4   s  of the impeller  4 . The sliding bearing  12  may be composed of ring-shaped ceramics fitted on the inner circumferential surface of the casing  2 . The inner circumferential surface of the casing  2  may be composed of a resin material such as PTFE or PFA to thereby constitute the sliding bearing  12 . 
         [0039]    Although  FIG. 2  shows the example in which the permanent magnetic radial repulsive bearing and the sliding bearing are provided at both axial end portions of the impeller  4 , respectively, the permanent magnetic radial repulsive bearings may be provided at both the axial end portions of the impeller, respectively, or the sliding bearings may be provided at both the axial end portions of the impeller, respectively. Alternatively, the permanent magnet radial repulsive bearing or the sliding bearing may be provided at only one end portion, such as the suction port side, of the impeller. Other configurations of the magnetic levitated centrifugal pump  1  shown in  FIG. 2  are the same as those of the magnetic levitated centrifugal pump  1  shown in  FIG. 1 . 
         [0040]    Next, a control circuit of the magnetic levitated centrifugal pump  1  configured as shown in  FIGS. 1 and 2  will be described. 
         [0041]    As shown in  FIG. 3 , eight control magnetic poles are basically provided, and two adjacent poles are used as a pair. When all of (1), (2), (3) and (4) are energized, a control force in Z-direction is generated. When (1) and (2), and (3) and (4) are differentially energized, a control force for θy is generated. When (1) and (4), and (2) and (3) are differentially energized, a control force for θx is generated. 
         [0042]    As shown in  FIG. 4 , ideally, by providing six control magnetic poles, a more compact construction can be realized. Specifically, the six control magnetic poles have advantages to lessen the number of electromagnet coils and the number of current drivers. In this case, two adjacent poles are used as a pair as well. When all of (1), (2) and (3) are energized, a control force in Z-direction is generated. When (1), and (2) and (3) are differentially energized, a control force for θx is generated. When (2) and (3) are differentially energized, a control force for θy is generated. 
         [0043]    In order to control the three degrees of freedom (Z, θx, θy), a plurality of displacement sensors are necessary. Basically, four displacement sensors are provided, and outputs from the respective sensors are computed by a computing unit into mode outputs. Specifically, the Z-direction displacement is calculated from the sum of (1), (2), (3) and (4), θy is calculated by an equation of ((1)+(2))−((3)+(4)), and θx is calculated by an equation of ((1)+(4))−((2)+(3)). 
         [0044]    Ideally, the number of sensors can be reduced to three, and Z, θx and θy can be determined by calculating respective outputs of the sensors. 
         [0045]    Control laws which are optimum from respective natural frequencies are applied to the three modes of Z, θx and θy, which have been determined in the above manner, thereby calculating control outputs of the respective modes. The calculated control outputs are computed by the computing unit to allocate respective electric currents to the three or four pairs of electromagnet coils. Therefore, the movements of Z, θx and θy of the impeller  4  as a rotor is controlled, and thus the impeller  4  can be rotated stably by the motor (θz). 
         [0046]    Further, since the differential pressure is generated during pump operation to generate a force for pushing the impeller  4  to the suction port side, if such force and the attractive force by the motor are controlled so as to be balanced, a control current can be reduced. 
         [0047]    Specifically, with respect to the Z-direction, basically, the system is configured to allow the motor attractive force to be equal to or greater than the pump differential pressure force, i.e., the motor attractive force≧the pump differential pressure force, and the force of the electromagnet is controlled to establish the following equation, i.e., the motor attractive force=the pump differential pressure force+the electromagnetic force. Ideally, the force of the electromagnet can be 0 (zero-power control). 
         [0048]    More ideally, if the technology of a sensor-less magnetic bearing (self-sensing magnetic bearing) for estimating a position of a gap based on impedance of the control coil is applied, the displacement sensors can be eliminated and the pump body can be further miniaturized and manufactured at a low cost. 
         [0049]    The remaining two degrees of freedom (X, Y) out of six degrees of freedom are passively stabilized by an attractive force acting between the permanent magnet and a stator yoke of the motor and by an attractive force acting between a stator yoke of the control electromagnet and the magnetic pole of the rotor. 
         [0050]    Since the passive stabilizing force lessens depending on the size or the gap of the motor, it is effective positively to add the radial repulsive bearing utilizing the repulsive force of the permanent magnets as described in  FIG. 2 . The radial repulsive bearing comprises a plurality of stacked ring-shaped permanent magnets and a plurality of permanent magnets arranged radially outwardly and having the same structure to generate a restoring force in a radial direction. 
         [0051]    Such bearing is constructed by stacking permanent magnets each of which is magnetized in the axial direction and has a magnetized direction opposite to the magnetized direction of the adjacent one as shown in  FIG. 5 . Ideally, as shown in  FIG. 6 , by combining permanent magnets which are magnetized in the axial direction and permanent magnets which are magnetized in the radial direction, greater radial rigidity can be obtained. 
         [0052]    This type of radial bearing has unstable rigidity in the axial direction, and thus the force acts to cause one side of the radial bearing to slip out in either of both directions. Thus, the permanent magnets on the stationary side and the permanent magnets on the rotor side are positionally shifted from each other so that the force acts on the rotor (impeller  4 ) toward the suction port side, whereby the attractive force caused by the permanent magnets of the motor can be reduced. 
         [0053]      FIGS. 7A and 7B  are views showing external appearance of the magnetic levitated centrifugal pump  1  shown in  FIGS. 1 and 2 .  FIG. 7A  is a front elevational view of the magnetic levitated centrifugal pump  1 , and  FIG. 7B  is a side view of the magnetic levitated centrifugal pump  1 . 
         [0054]    As shown in  FIGS. 7A and 7B , the magnetic levitated centrifugal pump  1  has a short circular cylindrical shape having both end surfaces and a circumferential surface, and has the suction port  1   s  formed on its one end surface and the discharge port  1   d  formed on its circumferential surface. As shown in  FIGS. 7A and 7B , the magnetic levitated centrifugal pump  1  has an extremely simple structure. 
         [0055]    Although the preferred embodiments of the present invention have been described above, it should be understood that the present invention is not limited to the above embodiments, but various changes and modifications may be made to the embodiments without departing from the scope of the appended claims.