Abstract:
A permanent magnet canned motor pump features in its corrosion-protection housing which comprises a reinforced bracket, motor casing and a motor rear casing. The reinforced bracket is made of corrosion-resistance plastic and the motor casing and the motor rear casing are made of aluminum alloy. Accordingly, the corrosion-protection housing is capable of preventing the corrosion of the chemical liquid to the aluminum alloy components. Beside, the permanent magnet canned motor pump provides a heat dissipation mechanism while meets the structural need of the corrosion-protection housing, so that the motor can dissipate heat in a sufficient rate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No(s). 100140554 filed in Taiwan, R.O.C. on Nov. 7, 2011, the entire contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    One of the sealless pump is the permanent magnet canned motor pump, a device in which motor and pump are integrated as a complete unit, and an windings stator are isolated by corrosion-resistance containment shell, and an encapsulated inner rotor is directly in contact with the liquid to be transferred; another one is the magnetic pump driven by an induction motor, the sealless function is using a magnetic coupling to replace a mechanical seal; therefore, the sealless pump could meet the zero leakage requirement in industries, especially in high temperature, toxic, flammable and corrosive liquid transferred application. The invention focuses on a permanent magnet canned motor pump with a corrosion-protection housing, the corrosion-protection housing is comprised of an aluminum alloy motor casing, an aluminum alloy motor rear casing and a corrosion resistance plastic reinforced bracket, therefore, there will not damage the parts of the pump, even have some external chemical droplets dropping on, such as chemical liquid in the PCB manufacturing processes. The invention is also good for using in the filter tank system, the permanent magnet canned motor pump is installed under the filter tank, and is used to pressurize chemical liquid, so that while the filter is replaced, the problem of chemical liquid dropping on and corroding components of the motor of the pump is prevented. All the possible is through the plastic reinforced bracket fixed on the pump casing of the pump, and thus keeps zero leakage on the sealing surface. Owing to the plastic reinforced bracket, the heat dissipating capacity of the outer surface of the aluminum alloy motor casing is limited. Accordingly, another object of the invention is providing a new heat dissipation mechanism to dissipate the heat generated by the motor at a sufficient rate. 
         [0004]    2. Related Art 
         [0005]    The permanent magnet canned pump is a device that integrates a motor and a pump, in tradition the permanent magnet canned pump has an outer shell comprised of an aluminum alloy motor casing with windings stator and an aluminum alloy motor rear casing, thereafter, called motor casing and motor rear casing respectively. Those components of the outer shell have cooling fins for providing sufficient heat dissipation capability, and with corrosion resistance material coating, (e.g. fluor-resin) for the operation in the environment in which corrosive drops may drop on. However, the performance of this kind solution for long term operation does not satisfy in the corrosion-resistance ability. Specifically, in the filter tank system applications which is used to filter the chemical liquid, when the permanent magnet canned motor pump is installed under the tank used to pump chemical liquid, after a period of operation the cartridge filter inside the tank needed to take out and change the new, at this condition some chemical droplets may directly drop on the outer shell. And the capability of the outer shell for resisting the corrosive chemical liquid should be improved. One solution for improving the ability of the outer shell for resisting the corrosive liquid is that a motor guard made of corrosion-resistance plastic is placed on the permanent magnet canned motor pump. However, the motor guard is confined by the arrangement of the pipe lines, for example, the length of the motor guard is short and limited, and therefore, the metal parts exposed from the motor guard will suffer from the corrosion droplets of the chemical liquid. 
         [0006]    The invention is focused on a corrosion-protection housing of the permanent magnet canned pump, in which the corrosion resistance plastic reinforced bracket protects an aluminum alloy motor casing, an aluminum alloy motor rear casing. In addition, the invention also provides a new heat dissipation mechanism for heat dissipating that generated by motor guaranteeing at a sufficient rate. 
         [0007]    The followings are a conventional permanent magnet canned motor pump and a conventional magnetic drive pump, and none of them provide any effective solution to the chemical liquid droplet corrosion. 
         [0008]    Taiwan Patent No. TWM369391 (hereinafter referred to as &#39;391) which is issued in 2009 discloses an improved permanent magnet canned motor pump capable of operating at high temperature and resisting chemical corrosion. 
         [0009]    This solution is focused on the shaft stiffness improvement of the pump, one of the features is a high stiffness cantilever stationary shaft of the motor, and the motor with a radial magnetic gap, too. Owing to the application, the encapsulation thickness in the &#39;391 a 3 mm corrosion tolerance must be considered. That is the total width of the magnetic gap is at least 8 mm. The rigid composite stationary shaft is used to satisfy the working conditions are high temperature and high power requirements. The &#39;391 shows that the structure of the permanent magnet canned motor pump of &#39;391 is more concise than magnetic drive pump, since a magnetic coupler and an induction motor are replaced by a canned motor. As a result, the permanent magnet canned motor pump is more suitable for being installed in equipment which the size is limited. However, the &#39;391 does not provide any solution to the problem of the corrosion caused by the dropped corrosive chemical liquid. 
         [0010]    Another solution for the conventional filter tank system, please refer to  FIG. 1 , which is used for manufacturing printed circuit boards (PCB). The filter tank system  1  consists of a magnetic drive pump  12 , a main frame  114  and a filter chamber  113 . The magnetic drive pump  12  is assembled to the main frame  114 , an inlet pipe  121  is connected to a tank of a PCB manufacture device, wherein the tank is used to contain chemical liquid; an outlet pipe  122  is connected to an inlet of the filter chamber  113  for outputting the pressured chemical liquid to the filter chamber  113 . After passing through the filter, the chemical liquid flows back to the tank of the PCB manufacture device through the outlet  116 . However, after a period of time, the filters in the filter chamber  113  need to be replaced by opening a top cover  115  of the filter chamber  115 , and taking out the blockage filter, the chemical liquid droplets may drop out from the surface of the blockage filter. In order to prevent the chemical liquid from dropping onto the magnetic drive pump  12 , a motor guard  123  is used. In practice, the main frame  114  has a limit in height, and the height of the outlet  116  must be matched up to the height of an inlet pipe of the tank of the PCB manufacture device. As a result, the motor guard  123  can not completely cover all metal components of the magnetic drive pump  12 , for example, the bracket  124  in  FIG. 1  is made of cast iron, and the motor guard  123  interferes with the tank exit  116  in height. Therefore, chemical liquid may drop onto and then corrode the bracket. 
         [0011]    In view of the above, it is learned that the permanent magnet canned motor pump with the size of inlet and outlet of the pump meets a standard, it will have shorten the longitudinal length than the magnetic pump, and with this feature is easier installed inside the manufacture device. Besides, the area onto which chemical liquid may drop is reduced as well. However, demand for preventing the corrosion caused by the chemical liquid remains because the problem of the dropping chemical liquid can only be improved but can not be rooted out, furthermore, operator&#39;s oversight which causes the chemical liquid to drop out is unpredictable. Accordingly, the inventors recognize following problems needed to be overcome. 
         [0012]    Problem 1: the heat dissipation of the motor. 
         [0013]    Although the reinforced bracket made of corrosion resistance plastic can prevent the problem of the corrosion caused by the chemical liquid, the reinforced bracket also causes difficulties in arranging the cooling fins of the outer shell. Accordingly, a new heat dissipation mechanism for the motor is needed. The permanent magnet motor featuring in the high efficiency, which is higher than 1E3 efficiency class of IEC60034-30 standard, can bring about dramatic reduction in the loading of heat dissipation, but the problem still remains. 
         [0014]    Problem 2: the permeation of the chemical liquid. 
         [0015]    The aluminum alloy motor casing of the &#39;391 is connected to the pump casing and presses against the flange of the containment shell for preventing the leakage of the chemical liquid. However, the chemical liquid drops on the motor casing may permeate through the thread of the screw bolt into the tapped hole of the front flange of the motor casing. After that, the chemical liquid in the tapped hole may corrode and penetrate the aluminum alloy motor casing, and then the windings stator may be corroded. The improvement of the invention can prevent middle-size and small-size permanent magnet canned motor pumps from the problem of corrosion of the dropping chemical liquid with reasonable cost. And the problem of the heat dissipation is overcome as well due to the improvement. As a result, the permanent magnet canned motor pump of the invention is more suitable for being assembled in manufacture device with confined interior space. 
       SUMMARY OF THE INVENTION 
       [0016]    One of the objects of the invention is to prevent the corrosion from the chemical liquid to the components of a permanent magnet canned motor pump with a corrosion-protection housing, thereafter, called the canned pump, the corrosion-protection housing is comprised of an aluminum alloy motor casing, thereafter, called the motor casing, an aluminum alloy motor rear casing, thereafter, called the motor rear casing, and a reinforced bracket made of corrosion resistance plastic, and the permanent magnet motor of the canned pump, thereafter, called the canned motor; has the aluminum alloy motor casing and the aluminum alloy motor rear casing, thereafter, called the motor shell; and another object is to find a solution to the heat dissipation problem of the motor. 
         [0017]    First, the followings are the solution to the problem of corrosion of the chemical liquid to the components of the motor shell. 
         [0018]    The corrosion-protection housing protects the motor shell by taking advantage of the reinforced bracket. The shape of the reinforced bracket is a column with opening at both ends. The reinforced bracket is for preventing the corrosion of the dropping chemical drops onto the motor shell, although such material limits the heat dissipation ability of the motor shell. The bottom part of the reinforced bracket is a blanket foot plate which used to install the canned pump. A front flange of the corrosion-protection housing is formed by a bracket front flange of the reinforced bracket and a pump side flange of the motor casing, and tapped holes are formed on the front flange for tightly combining to the pump casing by bolt screws, and the front flange is pressed by the bolt screws to against the pump casing. And a shell flange of the containment shell for sealing and for preventing the leakage of corrosive liquid from the canned pump, is pressed by the front flange at back side, an o-ring is disposed on the front flange surface for protecting the windings stator in the motor casing. A sealing gasket may be located on a verge of the bracket front flange for preventing permeation of the dropping corrosive drops to the motor casing and the bolt screws, and therefore, preventing the permeation of the chemical liquid through the gaps between the bolt screws and tapped holes or through the thread of the bolt screws. 
         [0019]    The following is the process of assembling the canned motor. First, an inner space of the reinforced bracket is divided by a ring rib into a front inner space and a rear inner space, the windings stator of the canned motor is fixed in the motor casing, then the motor casing is put in the front inner space, and the motor rear casing is put in the rear inner space. The motor casing and the motor rear casing are tied together by bolt screws, and the ring rib in the reinforced bracket is between them fixed from two opposite sides, and positioning blocks on the ring rib are inserted in fixing slots of the motor casing, so that, the corrosion-protection housing are combined as a complete unit as the corrosion-protection housing. A power transmission line for the windings is electrically connected to terminals of a terminal box of the motor rear casing. After that, an impeller and an inner rotor are combined as a complete unit and are put in the inner space of the containment shell. Finally, the front flange is tightly combined to the pump case, and is pressed and seals against the shell flange of the containment shell. 
         [0020]    Second, the heat dissipation mechanism for the motor is described as following. 
         [0021]    The key factor of the heat dissipation mechanism for the motor is that heat generated by the windings stator is transferred to the cooling fins without reducing the heat transfer rate, and that the cooling fins have sufficient surface and space for heat dissipation. Some embodiments of the invention take advantage of the thermal conductivity of the aluminum alloy which is as high as four times the thermal conductivity of the magnetic steel sheet. When the heat generated by the windings stator is transferred outward in a radial direction through yokes of the stator, the thermal conductivity of the contact surface, between the stator and the motor casing, will reduce by the surface roughness and the insulating vanish. So, the radial cross section area of the motor casing is similar as one fifth of the outer surface area of the lamination of the magnetic steel sheets of the stator. In other words, the cross section of the motor casing is able to transfer heat in longitude direction from the silicon steel plate with lower thermal resistance. And the longitudinal length of the motor casing is short, so that has small differential temperature between the outer surface of the magnetic steel sheets lamination and the rear end of the motor casing, that is, the heat generated by the stator is able to be transferred to rear end of the motor casing smoothly. The cup shaped structure of the motor casing has a big contact surface adapted to transfer heat to the motor rear casing, and area of the contact surface is equal to or bigger than the radial sectional area of the motor casing; the shape of the motor rear casing is a circular disc, and the shaft rear metal support is at the center of the motor rear casing and protrudes inward. The vertical cooling fins and the terminal box of the motor rear casing, which have sufficient heat dissipation surfaces are located in the outer side for dissipating heat to the air by nature convection smoothly without accumulation of heat therein. 
         [0022]    A bracket rear end of the reinforced bracket has a circular opening, of which the upper portion, the upper cover plate, is longer than the lower portion, the edge of the cross section of the circular opening is an arc, the lower portion of the circular opening exposes the lower portions of the cooling fins, so that air with relative low temperature enters the narrow space between the cooling fins, the air absorbs heat from the surfaces and floats upward from the lower portion of the cooling fins to the upper portion of the cooling fins due to the nature convection, and then the hot air flows out from the top portion of the rear end of the reinforced bracket. The lower circular opening is adapted to electrically connect the terminal box to the power transmission line; the upper cover plate covers the rear portions of the terminal box and the cooling fins; and the height of the lower circular opening reaches the lower portion of the terminal box to protect the cooling fins and the terminal box. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The present disclosure will become more fully understood from the detailed description given herein below for illustration only, and thus are not limitative of the present disclosure, and wherein: 
           [0024]      FIG. 1  is a conventional filter system used for manufacturing PCBs; 
           [0025]      FIG. 2  is an embodiment of a permanent magnet canned motor pump with a corrosion-protection housing according to an embodiment of the invention; 
           [0026]      FIG. 3  is a schematic diagram of the reinforce bracket in the  FIG. 2 ; 
           [0027]      FIG. 4  is a schematic diagram of heat dissipation path of the canned motor; 
           [0028]      FIG. 5  is a schematic diagram of the cooling fins in  FIG. 2 ; and 
           [0029]      FIG. 6  is a schematic diagram f the motor casing in the  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    The detailed features and advantages of the disclosure are described below in great details through the following embodiments, the content of the detailed description is sufficient for those skilled in the art to understand the technical content of the present disclosure and to implement the disclosure hereof. Based upon the content of the specification, the claims, and the drawings, those skilled in the art can easily understand the relevant objectives and advantages of the disclosure. 
         [0031]    Refer to  FIG. 2 , which is an embodiment of a permanent magnet canned motor pump, thereafter, called the canned pump, with a corrosion-protection housing according to an embodiment of the invention. A reinforced bracket is adapted to protect a motor casing and a motor rear casing. A pump side flange of the motor casing and a bracket front flange of the reinforced bracket together form a front flange of the corrosion-protection housing. In addition, tapped holes are formed on the reinforced bracket, there are screw nuts inside, so that screws are put in through holes on the motor casing to tightly tie the reinforced bracket and seal the shell flange of the containment shell. As a result, the leakage of corrosive liquid from the permanent magnet canned motor pump is prevented. The canned pump comprises a pump casing  4 , a triangle front support  31 , an impeller  5 , a containment shell  41 , a stationary shaft  3  and a canned motor  8 . 
         [0032]    The pump casing  4  has a flow channel  47 , adapted for containing the impeller  5 , an inlet  44  and an outlet  45 . A front thrust ring  46  is located on an inner surface of the pump casing  4  and at a position near the inlet of the impeller  5 , so that the front thrust ring  46  and a thrust bearing  53 , which are near an inlet side of the impeller  5 , form an axial thrust bearing together. The pump casing  4  and a bracket front flange  911  of a reinforced bracket  9  are combined together and adapted to fix to and seal a shell flange  411  of the containment shell  41 . 
         [0033]    The triangle front support  31  is fixed near the inlet of the pump casing  4 , and axially passes through a hub aperture  54  of the impeller  5  for supporting an end of a stationary shaft  3 . 
         [0034]    The impeller  5  is contained in the pump casing  4 . A hub plate  52  is connected to an axially extended part  76  of an inner rotor  7 , so that the impeller  5  and the inner rotor  7  are integrated as a one. 
         [0035]    The shape of the containment shell  41  is similar to a cup, of which the bottom has a blank rear shaft support  413 . In addition, the containment shell  41  has no through hole and therefore, leakage of the corrosive liquid from the containment shell  41  is prevented. The shell flange  411 , which is at a front end of the containment shell  41 , is fitted between the pump casing  4  and the pump side flange  811  of the motor casing  81  in order to prevent the leakage of the corrosive liquid from the canned pump. The blank rear shaft support  413  is located at the center of the bottom of the containment shell, and a rear thrust ring  414  installed on the verge which is mated with a ceramic bearing  79  (shown in  FIG. 4 ) of the inner rotor  7  to form an axial thrust bearing. The blank rear shaft support  413  is supported from the outer surface by a shaft rear metal support  824  (shown in  FIG. 4 ) of a motor rear casing  82  which is closely attached each other. 
         [0036]    The stationary shaft  3 , which is made of corrosion-resistance and abrasion resistance ceramic material, is supported at both opposite ends. Specifically, the front end of the stationary shaft  3  is supported by the triangle front support  31 ; the rear end of the stationary shaft  3  is supported and fixed by the blank rear shaft support  413  reaching out axially. The middle part between the front end and the rear end mated with the ceramic bearing  79  (shown in  FIG. 4 ) for supporting the inner rotor  7 , so that the inner rotor  7  is able to rotate along the stationary shaft  3 . 
         [0037]    The canned motor  8  of the canned pump with a corrosion-protection housing comprises a windings stator  83 , the inner rotor  7 , a motor casing  81 , the motor rear casing  82  and the reinforced bracket  9 . 
         [0038]    The windings stator  83  has windings wound around teeth (un-index in  FIG. 2 ) and fixed in the motor casing  81 . A PWM power source is connected to the windings for generating magnetic flux which interacts with the magnetic field of the inner rotor  7 , so that a torque is generated to drive the rotation of the inner rotor  7 , and the inner rotor  7  drives the impeller  5  to generate hydraulic power. The containment shell  41  protects the stator from corrosion caused by the corrosive liquid. 
         [0039]    The inner rotor  7  is a ring shape structure comprising a main magnet set, a main yoke and the axially extended part  76 . Besides, the inner rotor  7  is encapsulated by a corrosion resistance plastic and forms a sealed rotor resin enclosure  74  (shown in  FIG. 2  and  FIG. 4 ), which the shape is a ring and the ceramic bearing  79  (shown in  FIG. 4 ) is installed inside. The axially extended part  76  of the inner rotor  7  is connected to the hub plate  52 , so that inner rotor  7  and the impeller  5  are combined as a complete thing. 
         [0040]    The motor casing  81  is fixed to the reinforced bracket  9 , the pump side flange  811  pressed against the back surface of the shell flange  411  of the containment shell. The O-ring located at flange out diameter  811   a  (shown in  FIG. 6 ) of the pump side flange  811  is capable of preventing the leakage of the corrosive liquid. In the motor casing rear side  812  has a cup shaped structure with an aperture  812   a  (shown in  FIG. 6 ), so that the motor casing  81  has big area to transfer heat. The cup structure with the aperture  812   a  is used to fix the motor rear casing  82  by screws. The thickness of the motor casing is so designed that heat can be transferred to the cooling fin  821  and the terminal box  822  of the motor rear casing  82 . 
         [0041]    The motor rear casing  82  is fixed in the reinforced bracket  9  through the cup shaped structure of the motor casing  81 , and the cooling fins  821  and the terminal box  822  is capable of dissipating heat to the air through nature convection, and the shaft rear metal support  824  axially extended inward, offers strongly support to the stationary shaft  3 , and the windings stator  83  are electrically connected to terminals  825  of the terminal box  822  by wire passing through the aperture  812   a  of the motor casing  81 , the terminals  825  is connected to an electric source from the power line through a cable adaptor  823 . 
         [0042]    The reinforced bracket  9  is a column with opening at both ends, made of corrosion-resistance plastic. Tapped holes (un-index in  FIG. 3 ) are formed on the reinforced bracket, there are screw nuts (un-shown in  FIG. 3 ) inside, so that screws (un-shown in  FIG. 2 ) is put in through holes (un-shown in  FIG. 2 ) on the motor casing  4  to tightly tie the reinforced bracket  9  and seal the shell flange  411  of the containment shell  41 . The bracket front flange  911  is fixed to the pump casing  4  by screws, to build a seal face between the bracket front flange  911  and the pump casing  4 , so that the chemical liquid is unable to permeate through the seal surface and enter the gap between the threads of the screws and the tapped hole of the motor casing  81 , which is made of aluminum ally. The sealing slot  911   a  (shown in  FIG. 3 ) of the bracket front flange  911  presses against the back side of the shell flange  411  of the containment shell  41 , as a result, the seal o-ring (un-index in  FIG. 2 ) between the shell flange  411  and the pump casing  4  is compressed for preventing leakage of the corrosive liquid. In some severe conditions, a sealing gasket (non-shown on  FIG. 2 ) may be installed on a verge of the bracket front flange  911  for preventing the corrosive drops permeate the seal faces between the bracket front flange  911  and the pump casing  4 , thus to protect the screws in the tapped holes free from the chemical liquid. In addition, the reinforced bracket  9  is long enough so that the rear end of the reinforced bracket  9  covers a terminal box  822  and cooling fins  821  of the motor rear casing  82 . 
         [0043]    A ring rib  916  with positioning blocks  917  (shown in  FIG. 3 ) is located in the middle part of the inner surface of the reinforced bracket  9 . The ring rib  916  divides the inner space of the reinforced bracket  9  into a front inner space  914  and a rear inner space  915  (shown in  FIG. 3 ). The motor casing  81  is fitted in the front inner space  914 , and the motor rear casing  82  is fitted in the rear inner space  915 , both are tightly assembled together by screws, and the positioning blocks  917  of the ring rib  916  is inserted in the fix slot  813  (shown in  FIG. 6 ) formed on the rear surface of the motor casing  81 , so that the motor casing  81 , the motor rear casing  82  and the reinforced bracket  9  are assembled as a complete unit, that is the corrosion-protection housing (shown in  FIG. 5 ), and the canned motor  8  is supported by the blanket foot plate  912  firmly. The corrosion-protection housing is firmly mounted by a blanket foot plate  912 . A front flange surface of the corrosion-protection housing is formed by a bracket front flange  911  of the reinforced bracket  9  and a pump side flange  811  of the motor casing  81 . An o-ring is located in a flange out diameter  811   a  (shown in  FIG. 2  and  FIG. 6 ) of the pump side flange  811  of the aluminum alloy motor casing  81 , is able to prevent eroding the pump side flange  811  and the windings of the stator  83 . The pump side flange  811  presses against the back side of the shell flange  411  of the containment shell  41 , and press an o-ring between the pump side flange  811  and the pump casing  4  for sealing and for preventing the leakage of corrosive liquid from the canned pump. 
         [0044]    When the pump is in operation, the flowing fluid along the flow stream line  6  enters the inlet of the canned pump, and the flowing fluid along the inlet stream line  61  through the impeller  5 , so that the flowing fluid out of the impeller  5  is pressurized, then, the pressurized fluid discharged through the outlet  45 . In addition, a small portion of the flowing fluid is along the flow stream line  62 , along the impeller exit &amp; turn back, then passes through the back of the impeller  5 , and then enters an inner space  415  of the containment shell  41 . After that, the flowing fluid in inner space  415  flows to the bottom of the containment shell  41  through the gap between the outer surface of the inner rotor  7  and the inner surface of the containment shell  41 , and then flows through the gap between the stationary shaft  3  and the ceramic bearing  79 , finally, the fluid flows out turn back to the inlet of the impeller  5  through the hub aperture  54 , shown as the stream line  65 . The fluid in such loop lubricates the ceramic bearing  79  and takes away the heat generated by the inner rotor  7 . 
         [0045]    Refer to  FIG. 3  which is a  3 D diagram of the reinforced bracket  9  in the  FIG. 2 . The reinforced bracket  9  is a column with opening at both ends, which is made of corrosion-resistance plastic, the bracket front flange  911  with the sealing slot  911   a  is at one end, the blanket foot plate  912  which is a flat plate at the bottom part, and the ring rib  916  with the positioning blocks  917  is located in the middle of the inner surface, also the ring rib  916  divides the inner space of the reinforced bracket  9  into the front inner space  914  and the rear inner space  915 , and the motor casing  81  (shown in  FIG. 2 ) is fitted in the front inner space  914 , the motor rear casing  82  (shown in  FIG. 2 ) is fitted in the rear inner space  915 . The reinforced bracket  9  includes a bracket rear end  913  with a lower circular open  913   b  (shown in  FIG. 5 ), and the lower portion of the cooling fins  821  (shown in  FIG. 5 ) of the motor rear casing  82  (shown in  FIG. 5 ) is exposed. The bracket rear end  913  further comprises an upper cover plate  913   a  cover the rear ends of both the terminal box  822  (shown in  FIG. 5 ) and the cooling fins  821 . Besides, the height of the lower circular open  913   b  (shown in  FIG. 5 ) reaches the lower portion of the terminal box  822 , so that the cooling fins  821  (shown in  FIG. 5 ) and the terminal box  822  (shown in  FIG. 5 ) are protected. 
         [0046]    Refer to  FIGS. 4 and 5 , which are schematic diagrams of heat dissipation path and the cooling fins of the canned motor. Specifically, the  FIG. 4  is a sectional view of the canned motor  8  for illustrating the heat conduction mechanism and the function of the cooling fins  821 . While the heat generated by the windings stator  83  is transferred outward in a radical direction from the tooth and through the yoke of the windings stator  83 , the heat is first transferred to the motor casing  81  shown as the heat transfer path  66  from the stator. The thermal conductivity of the contact surface between the stator  83  and the motor casing  81  will little reduce by the surface roughness and the insulating vanishes. The radial cross section area of the motor casing  81  is no less than one fifth of the outer surface area of the lamination of the magnetic steel sheets of the stator  83 . In other words, the radial cross section area of the motor casing  81  is able to transfer heat in longitude direction from the lamination of the magnetic steel sheets with lower thermal resistance. The longitudinal length of the motor casing  81  is short, so that has small differential temperature between the outer surface of the magnetic steel sheets lamination and the rear end of the motor casing  81 , shown as the heat transfer path  67  from the motor casing. In addition, the cup shaped structure of the motor casing  81  in the motor casing rear side  812  has a big contact surface which is equal to or bigger than 1.5 times the radial sectional area of the motor casing  81 , so that heat is easily transferred to the motor rear casing  82 , shown as the heat transfer path  68  to the cooling fines. The cooling fins  821  and the terminal box  822  of the motor rear casing  82  have sufficient heat dissipation surfaces and protrude outward. The sum of the areas of the heat dissipation surfaces of the cooling fins  821  and the terminal box  822  is above four times the area of the outer surface of the stator  83 , so that heat can be dissipated to the air by nature convection smoothly without accumulation of heat therein, shown as nature convection flow streamline  69 . 
         [0047]    In addition to the sufficient heat dissipation surfaces, the nature convection velocity is another important factor as well. The lower circular open  913   b  of the rear end of the reinforced bracket  9  exposes the lower portion of the cooling fins  821  of the motor rear casing  82 , so that it is easy for the air with relative low temperature floating over the surface of the cooling fins  821  to absorb heat due to the nature convection. Then, the hot air flows out from the top portion of the rear end of the reinforced bracket  9 , shown as the nature convection flow streamlines  69   a,    69   b  and  69   c.