Abstract:
A permanent split capacitor electric motor is constructed by using existing components of a known shaded pole motor design to reduce engineering, tooling, inventory and other manufacturing costs of the new motor and, potentially, the known design through economics of scale. The alterations to the known motor principally involve different winding circuits and the addition of a capacitor. The new motor can be reversed with a single switch circuit.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to improvements in fractional horsepower electric motor constructions. 
   PRIOR ART 
   Shaded pole electric motors are well known for their simplicity and low manufacturing costs. These attributes allow manufacturers to sell relatively high volumes of these types of motors. U.S. Pat. No. 3,158,769 discloses an efficient design for a shaded pole motor. 
   The cost of new tooling can make the difference in whether or not it is economically practical for a manufacturer to offer a new line of motors. Where initial tooling costs are minimized or largely avoided, a manufacturer can more readily introduce a new product line while avoiding significant financial risks, prolonged development time and consumption of other resources, including labor and allocations to inventory. 
   There exists a need for a reversible fractional horsepower motor of proven durability that can be used, for example, in the air circulation path of refrigeration units. 
   SUMMARY OF THE INVENTION 
   The invention involves a discovery that a class of shaded pole motors have most of their componentry usable, without modification, to construct a permanent split capacitor motor. The resulting permanent split capacitor motor, compared to an ordinary shaded pole motor can be more efficient and have a higher starting torque as well as being reversible. 
   More specifically, in accordance with the invention, a type of shaded pole motor having its poles slotted, apart from the shading coil slots, can be wound with separate field coils that correspond in function to the main and auxiliary windings of a conventional permanent split capacitor motor. The invention, thus, enables a manufacturer to produce a permanent split capacitor motor while avoiding major investments in new tooling and additional inventory of component parts dedicated solely to the production of such a motor. Additionally, a motor manufacturer can realize a savings in costs where an increased volume of component parts applicable to both the shaded pole and permanent split capacitor motor can reduce the individual costs of these components. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an axial view of an exemplary stator, formed of a stack of laminations, used in the present invention; 
       FIG. 2  is a view similar to  FIG. 1  of the stator wound with inner coils at its poles; 
       FIG. 3  is a view similar to  FIGS. 1 and 2  of the stator wound with inner and outer coils at its poles; 
       FIG. 4  is a schematic electrical diagram representing a circuit used with the motor of the invention; and 
       FIG. 5  is a longitudinal cross-sectional view of a motor constructed in accordance with the invention and using the wound stator of FIG.  3 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to the drawings,  FIG. 1  illustrates a stack of stator laminations  10  similar to that illustrated in aforementioned U.S. Pat. No. 3,158,769, the disclosure of which is incorporated herein by reference. The stack of stator laminations  10  form four salient poles  11 . Each of the poles  11  has a first or main portion  12  and a second or minor portion  13 . The pole portions  12 ,  13 , are separated by a large slotted opening  14  communicating with a face  16  of the pole  11 . Each first or main pole portion  12  has a slot  17  as is customarily found in shaded pole stators. Two diametrically opposed slots  17 , in the illustrated example, receive a self-shorted shading coil  18  in a known manner. 
   As taught in the aforementioned &#39;769 patent, at each pole motor the main pole portion  12  and then the main and minor pole portions  12 ,  13  together, are wound with coils in a series circuit to form the field coils of a shaded pole motor. 
   In contrast, in accordance with the present invention, at each pole  11 , the pole portions are wound with coils that are electrically independent of one another. More specifically, a first field coil or winding  21  is wound on a main portion  12 . As suggested in  FIG. 2 , adjacent sets of the pole portions  12  can be wound with a single continuous length of magnet wire. Leads from these coils are identified by the numerals  22 - 25 . 
   A second field coil or winding  26  is wound on both the major and minor pole portions  12 ,  13  at each pole  11  as shown in FIG.  3 . As with the coils  21 , adjacent sets of the coils  26  can be wound from a single continuous length of magnet wire. The leads from the four field coils  26  are identified by the numerals  27 - 30 . 
     FIG. 5  illustrates the stator laminations  10  and the field windings or coils  21 ,  26  in assembled relation with other parts of a permanent split capacitor motor  35 .  FIG. 4  is a schematic representation of the circuitry in which the stator field windings  21 ,  26  are arranged. The coils  21  on the main pole portions  12  are connected in series with each other and, similarly, the coils  26  wound on the first and second pole portions  12 ,  13  are connected in series with one another. Single phase power, e.g. 115 VAC power is supplied across lines  36 ,  37  to the motor  35 . The serially arranged windings  21 ,  26  are connected together at one side at leads  25  and  27  and through a thermal protector  38  to a common line  36 . A capacitor  39 , typically mounted on the exterior of the motor  35 , as is conventional with standard permanent split capacitor motors, is connected across the other leads coming from the windings  21 ,  26 , respectively. A reversing switch  41  in the form of a single pole double throw unit is connected to the leads  22  and  30  of the windings  21 ,  26 . The switch  41  is thereby enabled to selectively supply line power directly to the main winding lead  30  or the auxiliary winding lead  22  depending on its position. The winding  21  or  26  not directly connected to the switch  41  is energized through the capacitor  39 . 
   The function of the capacitor  39  is to cause the field winding  21  or  26  to which the pole of the switch  41  is not directly connected with the supply power at the line  37  to lead in phase the current and magnetic field development of the other winding coil. Under normal or forward operation of the motor  35  a squirrel cage rotor will turn counterclockwise in the view of FIG.  3 . This results from a shift in the magnetic field at each pole  11  from a point where it is centered approximately with the center of the coil  26  wound about both the first and second pole portions  12 ,  13  since in this normal or forward operation the capacitor  39  is connected to this winding. The coil  21  on the minor pole portion  14  subsequently reaches its maximum field strength (after the coil  26  reaches its maximum field strength). The squirrel cage rotor is caused to rotate to follow this shift in effective magnetic field position. Still further, the shaded pole portions associated with the shading coils  18  subsequently reach their maximum magnetic field strength thereby inducing further movement of the squirrel cage rotor in the counterclockwise direction, with reference to FIG.  3 . 
   As suggested above, the motor can be reversed by changing the position of the switch  41  so that it connects line power directly to the coils  21  and the other field coils  26  are connected through the capacitor  39 . A study of  FIGS. 2 and 3 , again, will reveal that in this case, the coils  21  first come to a maximum field strength and, thereafter, the coils  26  reach their maximum field strength. This action results in clockwise rotation of the rotor as it would be viewed in FIG.  3 . 
   It is well understood that a shaded pole motor is unidirectional and that the rotor will always turn from the part of a pole that is unshaded to the shaded part of a pole since the shaded pole port reaches its maximum field strength last and, therefore, the field turns from the unshaded portion to the shaded portion. 
     FIG. 5  illustrates an example of a motor  35  that benefits from the present invention. The motor  35  includes a housing  46  in which the stator  10  is received. A squirrel cage rotor assembly  47  is disposed in the bore of the stator  10  formed by the faces  15  of the poles  11 . The rotor assembly  47  is supported on a shaft  48  journalled in a bore  49  formed in the housing  46 . The rotor axis is indicated at  50 . A load on the motor  35  is represented by an axial fan diagramatically shown at  55  on the shaft  48 . The housing  46 , rotor assembly  47 , stator laminations  10  as well as the smaller components such as front and rear covers  51 ,  52 , felt oil reservoir  53  and various clips, washers and like accessories are typically used in a shaded pole motor such as disclosed in aforementioned U.S. Pat. No. 3,158,769. 
   As discussed, in accordance with the present invention, the same component parts used to construct a shaded pole motor can be used to produce a permanent split capacitor motor. For the most part, the only changes required to convert from the shaded pole motor to the permanent split capacitor motor are different windings on the stator laminations  10  and the addition of a capacitor  39  in a manner like that taught hereinabove. The rotor assembly can be modified to reduce the end ring cross-section to improve starting torque in the permanent split capacitor motor. It has been found that at least in one version of a motor constructed in accordance with the invention that the provision of two shaded pole portions provide a proper balance of efficiency in normal operation in one direction and adequate torque in reverse operation. Where reversibility is not needed, all of the poles of the motor can be provided with a shading coil and greater starting torque and efficiency can be expected. At least in certain instances, the provision of four shading coils makes reversible operation too inefficient to be practical. In some instances, the shading coils  18  can be omitted from all of the slots  17 . 
   The disclosed motor is useful in refrigeration cabinets for air circulation where it is desirable to reverse the air flow produced by the fan during a defrost cycle. 
   It will be understood that the principles of the invention are applicable to other styles of motors including two and six pole motors, for example. 
   It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.