Patent Publication Number: US-2015061425-A1

Title: Internally cooled servo motor with segmented stator

Description:
TECHNICAL FIELD 
     The present invention relates to active cooling of AC and DC electric motors, and more particularly, electric motors having a cooling tube formed in the back of the lamination segment and inside of the motor housing, which allows the use of water based or electrically conductive coolants to cool the stator coils. 
     BACKGROUND OF INVENTION 
     There are three main classes of prior art for cooling an electric motor. The first class of liquid cooling involves using a liquid tight housing that is installed over the stator housing. The second class of liquid cooling involves flooding the inside of the motor housing with oil, or a suitable dielectric cooling fluid. The third class of liquid cooling involves using a two-phase liquid/gas coolant as depicted in U.S. Pat. No. 5,952,748. 
     There are a variety of disadvantages associated with these classes. Such disadvantages are disclosed in U.S. application Ser. No. 13/164,128, which is owned by the assignee of the subject application. 
     SUMMARY 
     One aspect of the invention relates to a fluid cooled segmented servo motor lamination construction method that results in a very high power density. The high power density is achieved by incorporating a cooling tube into the back of the lamination segment and inside of the motor housing; a tapered pole body is used so that the magnetic flux density does not become elevated as it travels around the cooling tube; rectangular, square or ribbon wire may also be used to reduce thermal resistance and increase slot fill; round wire is also possible. 
     The output power of the servomotor can be increased by cooling the motor with a fluid. By placing the cooling tube in the back of the segmented lamination pole, the cooling path is shortened over external cooling. Also, the motor is smaller because an external fluid jacket is eliminated. By placing the cooling tube slot in the center of the lamination at the outer diameter and tapering the pole, the magnetic flux path can have minimum interruption, and the magnetic saturation is reduced. In one embodiment, the resistive losses in the motor are minimized by utilizing rectangular wire. By combining the cooling tube location, with the tapered pole, and the high slot fill, the power density of the motor is maximized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of this invention will now be described in further detail with reference to the accompanying drawings, in which: 
         FIG. 1  is a cross-section of an exemplary electric motor in accordance with aspects of the present invention. 
         FIGS. 2-4  are perspective views of an exemplary stator pole in accordance with aspects of the present invention. 
         FIGS. 5-6  are perspective views of an exemplary stator in accordance with aspects of the present invention. 
         FIG. 7  is an exploded view of a portion of the stator illustrated in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWING 
     An electric motor generates heat in the process of transforming electrical energy into mechanical energy. If this heat is not effectively dissipated to the surrounding environment the motor internal temperature will rise above the temperature rating of the individual components. Without an active cooling system such as a fan or liquid cooling system, the servo motor continuous output power can be extremely reduced from its full potential. 
     An exemplary motor control system  5  for a servo motor  10  is illustrated in  FIG. 1 . The servo motor  10  includes a stator  12  and a rotor  14 . The stator  12  and the rotor  14  are configured are coaxially aligned such that the rotor and the stator  12  is selectively actuable to induce rotation of the rotor  14 , as is conventional. A controller  16  may be used to control operation of the servo motor  10 . A cooling system  18  may also be coupled to the stator through a fluid tube, as discussed below. 
     Referring to  FIGS. 2-4 , the stator  12  may be formed from a plurality of segmented laminates, which is conventional. The stator  12  includes a plurality of poles  20 . Each of the poles  20  include a first surface  22  and a second surface  24 . The first surface  22  and the second surface  24  are spaced apart (e.g., a distance (d)). Each of the poles  20  also have a pair of slots  26 A,  26 B defined between the first surface and the second surface on respective sides of each pole. Each pair of slots is configured to receive at least one winding  28 ,  30 . 
     Each pole  20  further includes a cooling tube  32  coupled to the first surface  22 , wherein the cooling tube is at least partially encompassed within the first surface  22 . The first surface may include a recess  40 , which may also be in the form of through hole or the like for coupling the cooling tube  32  to the pole  20 . The cooling tube  32  is secured in a central portion of the first surface  22  along an axis of symmetry (A) of the pole. Preferably, the cooling tube  32  is embedded within the first surface or below the first surface  22  in such a manner that the cooling tube does not extend outward from the pole more than the first surface. In other embodiments, the cooling tube  32  may extend beyond surface  22 . In still other embodiments, it may be desirable for separate cooling tubes  32  for forming more than one cooling tube (e.g., separate cooling tubes having parallel flow paths). In still other embodiments, or cooling tube  32  may be formed from a single continuous tube or tubes. Likewise, the cooling tube or tubes may be formed from a plurality of cooling tubes that are coupled together in an appropriate manner. 
     Each of the pairs of slots  26 A,  26 B are tapered a prescribed amount along an axis of symmetry of associated with each pole. For example, the slots  26 A,  26 B may be tapered between 5 and 30 degrees with respect to the axis of symmetry (A), as illustrated in  FIG. 4 . Preferably, each pair of slots  26 A,  26 B is tapered the same prescribed amount (θ). In another embodiment, the taper angles may be different. 
     The windings  28  and  30  may be any type of winding. In one embodiment, each winding has a rectangular cross-section, in order to increase slot fill and provide for low resistive losses. A tapered insulator cap may be used to force the rectangular wire to sit flat against the pole, thus reducing thermal impedance. In another embodiment, the windings  28  and  30  may have a circular cross-section. Windings can be manufactured without welds, solder or brazing, which increases reliability. 
     As shown in  FIGS. 5-7 , the cooling tube  32  is continuous and is routed to each of the poles. The cooling tube  32  includes an input end and an output end that is coupled to a cooling system (not shown). The cooling tube  32  may be made from any desirable material. Such materials include, for example, a copper alloy, an aluminum alloy, a stainless steel alloy, a polymide, etc. Since the stator  12  is cooled internally, an external water jacket that is common in certain systems is eliminated. This results in reduced size, weight and cost of the motor  10 . A thermally conductive epoxy or paste can be applied between the cooling tube  32  and the pole  20  to further improve thermal transfer. 
     The cooling tube is suitable for transfer of fluid to provide cooling to the motor  10 . Such fluids may include, water, a mixture of water glycol, R134, oil, a two-phase liquid gas mixture. 
     A servo motor  10  in accordance to this invention can be constructed with any desired number of stator teeth and magnet segments on the rotor. That is, the claimed invention is not limited to a particular number of stator teeth, magnet segments, or a particular cooling tube travel path. The servo motor depicted in  FIGS. 1-7  is a permanent magnet synchronous servo motor. It is constructed with a rotor  14  that has permanent magnet segments attached circumferentially. The rotor  14  rotates on bearings, as is conventional. The stator  12  is constructed from electrical grade steel in the form of a stack of laminations in order to reduce eddy current and hysteresis losses. Coils of wire or windings  28 ,  30  are installed into the slots  26 A,  26 B between the laminations stacks (e.g., poles  20 ). A feedback device (not shown) is used to sense the rotor  14  position during motor operation. 
     During the operation of the servo motor, current is commanded through the motor windings  28 ,  30  that is a function of rotor position, and the commanded torque. Resistive losses in the motor windings  28 ,  30  and eddy currents and hysteresis losses in the lamination stack (e.g., poles  20 ) cause the motor to heat. The heat generated must be effectively removed from the motor or the motor will over heat. 
     The electric motor is equipped with a continuous cooling tube  32 , as set forth above. Due to the coupling of the cooling tube  32  to the pole, as described above, there is a shorter heat flow path from the heat source to the heat sink over conventional methods. There is also a low thermal resistance path from the heat source to the heat sink. 
     In order to reduce the complexity of the assembly it is preferred that the tube has a minimum number of interconnection within the motor body. Therefore, a single pass continuous tube is preferred. It is possible to assemble the motor with a single continuous tube if the motor stator is built in segments. Likewise, it is preferable to locate the fluid tube  32  along the axis of symmetry (A) for each pole. Such location provides for optimization of magnetic flux flow, minimized magnetic lamination saturation, and minimum motor size. For in-slot cooling, it is possible to maximize the thermal path from the winding to the cooling tube by maximizing the thermal contact between the cooling tube and the wires and then encapsulate the entire stator in a thermally conductive epoxy. The encapsulation process also protects the insulation from abrasion failures. 
     The internal cooling loop can be used along with external cooling method to make even further improvement to the servo motor performance. The internal cooling loop will remove the heat from the resistive losses while the external cooling on the housing can remove the eddy current and hysteresis losses in the electrical steel, for example. 
     This invention is not limited to permanent magnet synchronous servo motors. It can also work on induction motors, PM brushed motors, Universal motors, and variable reluctance motors. 
     Although the principles, embodiments and operation of the present invention have been described in detail herein, this is not to be construed as being limited to the particular illustrative forms disclosed. They will thus become apparent to those skilled in the art that various modifications of the embodiments herein can be made without departing from the spirit or scope of the invention.