Abstract:
A protective device fitted to an electromagnetic coil to prevent damage to a thermal protective switch installed in the electromagnetic coil beneath the winding. In a preferred embodiment there is a subsequent encapsulation process following manufacture of the coil. Preferably, said device comprises a protective cap shaped to conform with the shape of the thermal switch and closely fitting the thermal switch and wiring connected thereto.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to the protection of a thermal switch device installed to protect the windings of an electromagnetic coil during the winding. Preferably, an electromagnetic coil embodiment of the present invention is subsequently encapsulated.  
         DESCRIPTION OF THE PRIOR ART  
         [0002]    Throughout this description and the claims which follow, less the context requires otherwise, the word “comprise′, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or step or group of integers or steps.  
           [0003]    The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that that prior art forms part of the common general knowledge in Australia.  
           [0004]    It is an essential requirement, mandated by electrical Standards that the electromagnetic coils used in electric motors and other electrical devices be protected against excessive temperature rises when operating normally, and when a malfunction has occurred to the electrical device or its load. Excessive temperatures in the coils can lead to damage to the coils or other parts, and can result in fire or the danger of electric shock. A common method of protecting the coils against such excessive temperatures is to install a miniature, enclosed, temperature sensitive, snap-action switch in close proximity to the windings. Such a switch is selected to open circuit at temperatures well below temperatures dangerous to the coil windings, and is electrically connected in series with the windings to ensure their electrical isolation in the event of the switch opening.  
           [0005]    One method of installing and connecting a thermal switch in an electromagnetic coil is described in U.S. Pat. No. 6,326,879 by Hangmann et al. In U.S. Pat. No. 6,326,879, the electromagnetic coils are wound onto a plastic winding former into which is inserted the iron laminations required to form the magnetic path of the coil. To protect the electromagnetic coil from excessive temperature, a thermal protection switch is placed on the outside of the coils and held in place for example, using an adhesive. The thermal protective switch is electrically connected in series with the coil windings such as to interrupt the flow of current in the event of the thermal switch operating.  
           [0006]    The arrangement of the thermal protection switch described by Hangmann is satisfactory for the general construction of these types of coils and motors, whereby the coil and lamination assembly is either operated in free air within a main enclosure, or is enclosed by a close fitting enclosure. However, certain applications require that the coils or motors so produced must operate in an adverse environment. In particular, in an environment whereby the coils and laminations are subjected to water droplets or spray such as within a humidifier or evaporative air cooler, it is particularly desirable to prevent any water from reaching the internal workings of the coil or motor. In such applications it has become normal practice to encapsulate the windings of the coil in a plastic material. One method of achieving such encapsulation is to place the wound coil on its former within the cavity of an injection moulding tool and fill the space between the coil and tool with thermoplastic material injected at high pressure. This method results in a sealed encapsulation of the electromagnetic coils, which are then unaffected by any moisture which may find its way into the motor or coil in normal service.  
           [0007]    The encapsulation process subjects the entire space within the injection moulding cavity to high hydrostatic pressures and temperatures. If the construction of the coil and thermal protective switch described in U.S. Pat. No. 6,326,879 were subjected to such a process, the thermal protective switch would undoubtedly be permanently damaged by the crushing hydrostatic pressure and direct contact with the molten thermoplastic during thermoplastic injection.  
           [0008]    These extremes of pressure and temperature can be reduced if the thermal protective switch is placed against the plastic former, or in a cavity space formed within the plastic former, prior to the winding of the coil onto the former. In this configuration, the thermal protective switch does not suffer the peak temperatures experienced by the conventional location at the outside of the coil due to the thermal resistance and thermal inertia of the coil now located between the molten thermoplastic and thermal switch. However, the thermal switch is still subjected to high hydrostatic pressures during thermoplastic injection, which have been found to result in permanent damage to a high proportion of switches in production trials. The thermal switch is also subjected to crushing forces from the coils wound over the top of any protrusion of the switch above the plastic former during the coil winding process. The internal construction of typical commercially available thermal switches relies on over-centre bimetallic springs attached to the casing of the switch. When subjected to high hydrostatic pressures, or crushing forces from the coil winding, the casing of the switch is subjected to distortion, which causes significant changes to the switching temperature and operation of the switch. This damage is generally permanent, resulting in scrapping of the manufactured electromagnetic coil. Moreover, since the thermal switch is a mandatory safety device to be incorporated in the coil winding, there must be no question as to its effective and reliable operation in the event of over-heating of the coil.  
           [0009]    It is an objective of the current invention to prevent damage to a thermal protective switch fitted to an electromagnetic coil during manufacture, where the switch is installed between a former and windings of the electromagnetic coil.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention provides, in one aspect, a protective device fitted to an electromagnetic coil to prevent damage to a thermal protective switch installed in the electromagnetic coil beneath the winding. In a preferred embodiment there is a subsequent encapsulation process following manufacture of the coil. Preferably, said device comprises a protective cap shaped to conform with the shape of the thermal switch and closely fitting the thermal switch and wiring connected thereto.  
           [0011]    In another aspect the present invention provides a method of making an electromagnetic coil having a thermal protective switch, said method comprising:  
           [0012]    positioning said switch on a winding former of said coil,  
           [0013]    covering said switch with a protective cap adapted to resist forces applied thereto by coil windings wound onto the former and forces and temperatures applied thereto by encapsulation of the coil after windings have been applied to the winding former, such that the thermal protective switch is protected from damage resulting from forces and temperatures applied during manufacture of the electromagnetic coil,  
           [0014]    applying coil windings to the winding former fitted with the switch and protective cap. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The present invention will now be described by way of example with reference to the accompanying drawings, in which:  
         [0016]    [0016]FIG. 1 is an exploded isometric view of a small synchronous permanent magnet motor and its electromagnetic coils where one coil is fitted with a thermal switch in accordance with an embodiment of the present invention;  
         [0017]    [0017]FIG. 1 a  is an isometric view showing the assembled motor of FIG. 1;  
         [0018]    [0018]FIG. 2 is an isometric view of the plastic former of FIG. 1, onto which coils of wire are wound, with the thermal switch in place in an indent formed in the former,  
         [0019]    [0019]FIG. 3 [shows] is an isometric view of the plastic former of FIG. 1 showing the protective cap in place over the thermal switch[,] and with the lamination stack inserted through the plastic former;  
         [0020]    [0020]FIG. 4 [shows] is an isometric view showing the coil winding in place on the former;  
         [0021]    [0021]FIG. 5 is a magnified view of FIG. 1 a;  and  
         [0022]    [0022]FIG. 6 is a transverse cross-sectional view through the coils and in situ thermal switch and protective cap of FIG. 5; and  
         [0023]    [0023]FIG. 6 a  is a magnification of circled region in FIG. 6. 
     
    
     DESCRIPTION OF EMBODIMENTS  
       [0024]    [0024]FIG. 1 illustrates an electromagnetic coil as used in the construction of a small alternating current synchronous electric motor  10 . While this application is typical of the type of electric motor for which an electromagnetic coil might be used, those skilled in the art will be aware that there are many applications of such coils. These applications include solenoid actuators, relay coils electromagnets etc, all of which require protection against over temperature in the event of a failure of the coil or its controller. The construction of the motor  10  illustrated starts with the former  24 , into which the thermal protective switch  14  is placed within a formed cavity  26  within the shape of former  24 . The protective cover  16 , subject of the current invention, is then placed in position covering the thermal switch  14 . The wire coils  12  are then wound onto the former. During the winding process, the thermal switch  14  can be subjected to considerable stress by crushing of the wires  12 , which are normally under some tension. The protective cap  16  provides protection against such stress by bearing the crushing load from the wires, and preventing that load from reaching the thermal switch  14 .  
         [0025]    Following the winding process, the winding on former assembly is inserted into a thermoplastic injection moulding tool into which is injected molten thermoplastic material to fill the cavity space around the winding assembly. This results in the encapsulating material  18  completely sealing off the electrical winding and preventing casual water from reaching electrically live parts. During the thermoplastic injection process, the inside of the tool cavity is subjected to high pressures and temperatures, which would normally damage the thermal protector  14 . The inclusion of the protective cap  16 , which closely fits over the thermal switch  14  with a very small gap  20  between the protective cap  16  and thermal switch  14  (shown in FIG. 6), prevents any high pressure and high temperature thermoplastic from reaching the thermal switch  14 . The fitting of the encapsulated coil and former assembly to the motor laminations  22  completes assembly of the motor. In the pump application illustrated, the motor is then fitted to the plastic moulding  28 , which encloses the rotating components of the synchronous electric motor.  
         [0026]    [0026]FIG. 2 shows the former  24  with the protective thermal switch  14  in place. To assist with the fitting of the thermal switch, a depression  26  would normally be formed in the moulding of the former to ensure that the thermal switch is positioned as close as possible to the winding coils, but without significant protrusion which might subject the switch to excessive crushing forces during the winding of the coil. This positioning is required to ensure that the thermal switch responds as accurately as possible to the temperature of the winding wire. Ribs  25  formed as part of the moulding of former  24  guide the winding wires over the thermal switch  14  while still maintaining close proximity of the wires to switch  14 .  
         [0027]    In FIG. 3, the protective cap  16  is fitted in position over the thermal switch  14 . The shape of the protective cap  16  is designed to be close fitting to the shape of the thermal switch, while ensuring that the protective cap  16  does not actually touch the critical parts of the thermal switch  14  when in position. A small clearance is essential to ensure that forces from winding and thermoplastic injection are kept away from the thermal switch  14 . It is preferred that the surfaces of the protective cap  16  on the face adjacent to the winding wire be curved to provide increased resistance to winding forces and injection pressures. It is preferred that the material properties and thickness of sections of the protective cap  16  are such that the resultant deflection of the protective cap  16  during wire winding and thermoplastic injection does not result in contact between the protective cap  16  and the critical components of thermal switch  14  during manufacture. It is preferred that the protective cap  16  be manufactured from an electrically non-conductive material, and it is prefer-red that such electrically non-conductive material be an injection mouldable thermoplastic. It is preferred that the protective cap  16  material has sufficient mechanical strength at the injection moulding temperatures to which it is subjected during thermoplastic injection to withstand moulding pressures without allowing contact between the protective cap  16  and thermal switch  14 .  
         [0028]    [0028]FIGS. 4 and 5 illustrate the progressive manufacturing processes in making the coil. In FIG. 4, the coil  12  has been wound onto the former  24  and over the thermal switch  14  and its protective cap  16 , now not visible beneath coil  12 . The coil  12  completely covers the thermal switch  14  and its protective cap  16  ensuring that the thermal switch responds to the temperature of the coil with the least influence from other surroundings. In FIG. 5, the thermoplastic encapsulation  18  has been added by injection plastic moulding over the former and coil. The coil is now completely sealed against the external environment.  
         [0029]    [0029]FIG. 6 shows a cross-section through the coil winding  12  and thermal switch  14 . Details of the arrangement of the thermal switch  14  and protective cap  16  are magnified in the inset FIG. 6 a.  The protective cover  16  is shaped to provide a close fit to the thermal switch  14 , to keep the gap  20  as small as possible when the base of the protective cap  16  is in contact with the former  24 . The combination of ribs  25  moulded as part of the former  24  and protective cap  16  guide the wires of coil  12  over the top of protective cap  16  while maintaining physical contact between the wires and protective cap. The tension in the wires of coil  12  while being wound can result in a considerable force on the outer surface of protective cap  16 . During the course of injection moulding of the encapsulation  18 , all internal components are subjected to high hydrostatic temperatures and pressures. The protective cap  16  seals off the thermal switch  14  from the effect of these high pressures thereby protecting thermal switch  14  from possible crushing and mechanical damage. The small gap  20  between the protective cap  16  and thermal switch  14  allows the protective cap  16  to deflect under the imposed load of the winding of the coil and hydrostatic pressure without coming in contact with the thermal switch  14 . It will be appreciated that the curved shape of the protective cap  16  facing the coil  12  significantly improves the strength of the protective cap  16  when subjected to winding and thermoplastic injection forces and pressures.  
         [0030]    It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit and scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.