Abstract:
An electric motor includes a stator having a plurality of conductive windings radially spaced about a central axis. A rotor is located radially inward up the stator and is rotationally fixed with respect to a shaft along the central axis. The rotor and the stator are disposed in a frame. A heater is in operative communication with the windings. A thermostat is in operative communication with the heater and is configured to activate and deactivate the heater responsively to a motor temperature.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to the art of electric motors, which should be understood as described herein to include generators and other electromechanical machines that effect a conversion between mechanical and electrical power. More particularly, the invention relates to electric motors having protective heaters. 
     As should be understood by those skilled in this art, electric codes rate motors for use in various environments. For example, the most stringent criteria are reserved for “explosion-proof” motors. Below this level are motors rated for use in “division 2” areas. These motors need not meet the explosion-proof standards, and are therefore less expensive, but are restricted not to exceed a prescribed surface temperature. 
     Division 2 motors are sometimes exposed to air having a high relative humidity. To prevent condensation on the windings, space heaters have been applied on or within the motor frame, for example as a wrap on the windings or as a cartridge mounted in the frame. The heater may be controlled by the user or may be automatically activated upon deactivation of the motor. In either case, the heater is activated at a period when the motor is at or near its normal operating temperature. It may be possible, therefore, that the additional heat raises the motor&#39;s temperature above a maximum desired temperature, for example the temperature rated for a division 2 area. 
     SUMMARY OF THE INVENTION 
     The present invention recognizes and addresses disadvantages of prior art constructions and methods. 
     Accordingly, it is an object of the present invention to provide an improved electric motor. 
     This and other objects are achieved by an electric motor including a stator having a plurality of conductive windings radially spaced about a central axis. A rotor is located radially inward of the stator and is rotationally fixed with respect to a shaft along the central axis. The rotor and stator are disposed in a frame. A heater is in operative communication with the windings. A thermostat is in operative communication with the heater and is configured to activate and deactivate the heater responsively to a motor temperature. 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended drawings, in which: 
     FIG. 1 is a plan view, partly in section, of an electric motor constructed in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a partial perspective view of the motor in FIG. 1, particularly illustrating the stator core; 
     FIG. 3 is a schematic illustration of a space heater for use with an electric motor constructed in accordance with the present invention; and 
     FIG. 4 is an electrical diagrammatic illustration of an electric motor in accordance with a preferred embodiment of the present invention. 
    
    
     Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference will now be made in detail to presently preferred embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     FIGS. 1 and 2 illustrate a motor  10  constructed in accordance with one preferred embodiment of the present invention. While the term “motor” is used throughout the present specification, including the appended claims, it should be understood that this term includes any suitable electromechanical device effecting a conversion between mechanical and electrical power, for example including a generator. 
     Motor  10  has a rotatable shaft  12  extending along a central axis  13 . The internal components of motor  10  are enclosed by a housing that includes a main housing portion, or frame,  14 . One or more eyebolts  16  may be provided for lifting motor  10 . Main housing portion  14  defines an appropriate base  18  on which motor  10  rests during use. 
     The housing of motor  10  further includes end portions, such as end bell  20 , located at respective axial sides of main housing portion  14 . The end portions may be attached to the main housing portion by any appropriate means, such as by bolts. Typically, each end portion maintains a respective bearing assembly, such as bearing assembly  22 , to facilitate rotation of shaft  12 . 
     Shaft  12  continues through bearing assembly  22  and beyond end bell  20  for connection to other equipment. The opposite end of shaft  12  carries a fan  24  located within a shroud  26 . Due to the shroud&#39;s configuration, rotation of fan  24  causes cooling air to circulate around various cooling fins  28  defined on the exterior of main housing portion  14 . 
     Inside the housing, motor  10  includes a stator  30  that remains fixed during operation. Stator  30  includes a magnetically permeable core  32  preferably comprising a plurality of relatively thin laminations arranged in a stack. As indicated at  34 , longitudinal windings are located in parallel, axially-extending slots defined about the inside surface of core  32  to provide a flow path for flux-generating current. The windings turn at respective coil heads  36  and  38  to return along a parallel slot. 
     A rotor  40  is secured to shaft  12  and rotates based on electromagnetic interaction between it and stator  30 . In the illustrated embodiment, motor  10  is an induction motor, wherein rotor  40  is constructed as a “squirrel cage” in a known matter. A plurality of radial vanes  42  may be provided at the periphery of the rotor ends to circulate cooling air inside the motor housing. 
     A nonconductive top coat  44 , such as a known protective paint, may be applied over coil heads  36  and  38 , as well as other exposed surfaces of stator  30 , to provide protection against corrosion. To reduce capacitive coupling between the stator and rotor, an electrostatic shield arrangement indicated generally at  46  may be disposed between rotor  40  and the conductive windings of stator  30 . Generally, shield arrangement  46  includes an insulative layer located on the inside surface of coil heads  36  and  38  and along the entire axial extent of each winding slot. Preferably, the conductive layer is in electrical communication with core  32 , such as by contact with the inside walls of the winding slots. The conductive layer may be a conductive paint applied by spraying or brushing. 
     A space heater  48  is wrapped around coil head  36 . Referring also to FIG. 3, the space heater includes a polymer sheath  50  enclosing a wire heating element  52  connected to leads  54  that are in turn connected to an AC power source  56 . Sheath  50  may be made from any suitable material, for example polyester. In one preferred embodiment, heating element  52  is made from fourteen gauge copper wire. A similar space heater may be wrapped about coil head  38  instead of or in addition to heater  48 . 
     A thermostat  58  within heating element  52  is connected in series between power source  56  and heater  48 . In a preferred embodiment, thermostat  58  is a bimetallic switch. When the switch is closed, electric current from power source  56  flows through heating element  52  to generate heat. Opening of the bimetallic switch electrically opens the heating element, thereby deactivating heater  48 . The construction and operation of bimetallic switches should be understood in this art and is therefore not described in detail herein. It should be understood, however, that the metallic connections within such switches open and close based on ambient temperature and that bimetallic switches are commercially available that open and close at various predetermined temperatures. 
     FIG. 4 provides an electrical schematic diagram of a motor  10  in which a space heater  48  (FIGS. 1 and 2) is wrapped around a coil head so that heating element  52  provides heat to the windings. The space heater is connected to power source  56  through an auxiliary pole  60  of a starter  62 . Starter  62  includes three primary poles that connect the stator windings to a three-phase power source  64 . The starter acts as a switch that controls the auxiliary pole responsively to the primary poles. Specifically, when the primary poles are closed (i.e. when motor  10  is activated), the auxiliary pole is open (i.e. the space heater is deactivated). Conversely, opening of the primary poles closes the auxiliary pole. 
     Accordingly, when the motor is activated, the space heater is deactivated, and motor-generated heat prevents condensation on the windings. Because the deactivated space heater provides no additional heat, the motor remains within its rated temperature limits as determined by its construction. Upon deactivation of the motor, however, auxiliary pole  60  is closed so that power source  56  is applied to the heater and the thermostat. Thermostat  58  is chosen, however, to open and close at a temperature within a range from a minimum temperature necessary to prevent condensation up to the motor&#39;s rated temperature. Preferably, the thermostat deactivates the heater at a temperature below the motor&#39;s highest rated temperature and activates the heater at a temperature above the minimum temperature necessary to avoid condensation. Depending on the thermostat&#39;s construction, these temperatures may be the same. Accordingly, upon deactivation of motor  10 , the thermostat does not activate the heater until the motor cools to a temperature such that application of the additional heat does not cause the motor to exceed its rated temperature. 
     While one or more preferred embodiments have been described above, it should be understood that any and all equivalent realizations of the present invention are included within the scope and spirit thereof. For example, the present invention may be used in conjunction with various industrial motors, including those for use in non-division 2 areas. Furthermore, various types of space heaters may be used, including known tube-like cartridge heaters that are mounted on and within the frame proximate to the windings. Thus, it should be understood by those of ordinary skill in this art that the present invention is not limited to these embodiments since modifications can be made. Therefore, it is contemplated that any and all such embodiments are included in the present invention as may fall within the literal or equivalent scope of the appended claims.