Abstract:
An electric motor having a snap-together construction without the use of separate fasterners. The construction of the motor removes additive tolerances for a more accurate assembly. The motor is capable of programming and testing after final assembly and can be non-destructively disassembled for repair or modification. The motor is constructed to inhibit the ready entry of water into the motor housing and to limit the effect of any water which manages to enter the housing.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to electric motors and more particularly to an electric motor having a simplified, easily assembled construction. 
     Assembly of electric motors requires that a rotor be mounted for rotation relative to a stator so that magnets on the rotor are generally aligned with one or more windings on the stator. Conventionally, this is done by mounting a shaft of the rotor on a frame which is attached to the stator. The shaft is received through the stator so that it rotates about the axis of the stator. The frame or a separate shell may be provided to enclose the stator and rotor. In addition to these basic motor components, control components are also assembled. An electrically commutated motor may have a printed circuit board mounting various components. Assembly of the motor requires electrical connection of the circuit board components to the winding and also providing for electrical connection to an exterior power source. The circuit board itself is secured in place, typically by an attachment to the stator with fasteners, or by welding, soldering or bonding. Many of these steps are carried out manually and have significant associated material labor costs. The fasteners, and any other materials used solely for connection, are all additional parts having their own associated costs and time needed for assembly. 
     Tolerances of the component parts of the electric motor must be controlled so that in all of the assembled motors, the rotor is free to rotate relative to the stator without contacting the stator. A small air gap between the stator and the magnets on the rotor is preferred for promoting the transfer of magnetic flux between the rotor and stator, while permitting the rotor to rotate. The tolerances in the dimensions of several components may have an effect on the size of the air gap. The tolerances of these components are additive so that the size of the air gap may have to be larger than desirable to assure that the rotor will remain free to rotate in all of the motors assembled. The number of components which affect the size of the air gap can vary, depending upon the configuration of the motor. 
     Motors are commonly programmed to operate in certain ways desired by the end user of the motor. For instance certain operational parameters may be programmed into the printed circuit board components, such as speed of the motor, delay prior to start of the motor, and other parameters. Mass produced motors are most commonly programmed in the same way prior to final assembly and are not capable of re-programming following assembly. However, the end users of the motor sometimes have different requirements for operation of the motor. In addition, the end user may change the desired operational parameters of the motor. For this reason, large inventories of motors, or at least programmable circuit boards, are kept to satisfy the myriad of applications. 
     Electric motors have myriad applications, including those which require the motor to work in the presence of water. Water is detrimental to the operation and life of the motor, and it is vital to keep the stator and control circuitry free of accumulations of water. It is well known to make the stator and other components water proof. However, for mass produced motors it is imperative that the cost of preventing water from entering and accumulating in the motor be kept to a minimum. An additional concern when the motor is used in the area of refrigeration is the formation of ice on the motor. Not uncommonly the motor will be disconnected from its power source, or damaged by the formation of ice on electrical connectors plugged into the circuit board. Ice which forms between the printed circuit board at the plug-in connector can push the connector away from the printed circuit board, causing disconnection, or breakage of the board or the connector. 
     SUMMARY OF THE INVENTION 
     Among the several objects and features of the present invention may be noted the provision of an electric motor which has few component parts; the provision of such a motor which does not have fasteners to secure its component parts; the provision of such a motor which can be accurately assembled in mass production; the provision of such a motor having components capable of taking up tolerances to minimize the effect of additive tolerances; the provision of such a motor which can be re-programmed following final assembly; the provision of such a motor which inhibits the intrusion of water into the motor; and the provision of such a motor which resists damage and malfunction in lower temperature operations. 
     Further among the several objects and features of the present invention may be noted the provision of a method of assembling an electric motor which requires few steps and minimal labor; the provision of such a method which minimizes the number of connections which must be made; the provision of such a method which minimizes the effect of additive tolerances; the provision of such a method which permits programming and testing following final assembly; and the provision of such a method which is easy to use. 
     In one form, the invention comprises an electric motor. A stator includes a stator core having a winding thereon. A rotor includes a shaft received in the stator core for rotation of the rotor relative to the stator about the longitudinal axis of the shaft. A housing connected together with the stator and rotor forms an assembled motor, the housing being adapted to support the stator and rotor. A printed circuit board having programmable components thereon controls operation of the motor, the printed circuit board having contacts mounted thereon for use in programming the programmable components, the printed circuit board being received in the housing. The housing has a port therein generally in registration with the contacts on the printed circuit board, the port being sized and shaped to receive a probe connected to a microprocessor into connection to the contacts inside the housing for programming the motor. 
     In another form, the invention comprises a method of assembling an electric motor comprising the steps of: 
     forming a stator including a stator core and a winding thereon; 
     forming a rotor including a rotor shaft; 
     forming a housing adapted to support and at least partially enclose the stator and rotor; 
     connecting a printed circuit board having a programmable component thereon to the winding; 
     assembling the stator, rotor and housing such that the printed circuit board is enclosed in the housing; 
     inserting a probe through a port in the housing into connection with contacts on the printed circuit board subsequent to said step of assembling the stator, rotor and housing; and 
     programming the programmable component through the probe connection to the printed circuit board. 
     Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded elevational view of an electric motor in the form of a fan; 
     FIG. 2 is an exploded perspective view of component parts of a stator of the motor; 
     FIG. 3 is a vertical cross sectional view of the assembled motor; 
     FIG. 4 is the stator and a printed circuit board exploded from its installed position on the stator; 
     FIG. 5 is an enlarged, fragmentary view of the shroud of FIG. 1 as seen from the right side; 
     FIG. 6 is a side elevational view of a central locator member and rotor shaft bearing; 
     FIG. 7 is a right end elevational view thereof; 
     FIG. 8 is a longitudinal section of the locator member and bearing; 
     FIG. 9 is an end view of a stator core of the stator with the central locator member and pole pieces positioned by the locator member shown in phantom; 
     FIG. 10 is an opposite end view of the stator core; 
     FIG. 11 is a section taken in the plane including line  11 — 11  of FIG. 10; 
     FIG. 12 is a greatly enlarged, fragmentary view of the motor at the junction of a rotor hub with the stator; 
     FIG. 13 is a section taken in the plane including line  13 — 13  of FIG. 5, showing the printed circuit board in phantom and illustrating connection of a probe to a printed circuit board in the shroud and a stop; 
     FIG. 14 is a section taken in the plane including line  14 — 14  of FIG. 5 showing the printed circuit board in phantom and illustrating a power connector plug exploded from a plug receptacle of the shroud; and 
     FIG. 15 is an enlarged, fragmentary view of the motor illustrating snap connection of the stator/rotor subassembly with the shroud. 
     FIG. 16 is a block diagram of the microprocessor controlled single phase motor according to the invention. 
     FIG. 17 is a schematic diagram of the power supply of the motor of FIG. 16 according to the invention. Alternatively, the power supply circuit could be modified for a DC input or for a non-doubling AC input. 
     FIG. 18 is a schematic diagram of the low voltage reset for the microprocessor of the motor of FIG. 16 according to the invention. 
     FIG. 19 is a schematic diagram of the strobe for the Hall sensor of the motor of FIG. 16 according to the invention. 
     FIG. 20 is a schematic diagram of the microprocessor of the motor of FIG. 16 according to the invention. 
     FIG. 21 is a schematic diagram of the Hall sensor of the motor of FIG. 16 according to the invention. 
     FIG. 22 is a schematic diagram of the H-bridge array of witches for commutating the stator of the motor of FIG. 16 according to the invention. 
     FIG. 23 is a flow diagram illustrating the operation of the microprocessor of the motor of the invention in a mode in which the motor is commutated at a constant air flow rate at a speed and torque which are defined by tables which exclude resonant points. 
     FIG. 24 is a flow diagram illustrating operation of the microprocessor of the motor of the invention in a run mode (after start) in which the safe operating area of the motor is maintained without current sensing by having a minimum off time for each power switch, the minimum off time depending on the speed of the rotor. 
     FIG. 25 is a timing diagram illustrating the start up mode which provides a safe operating area (SOA) control based on speed. 
     FIG. 26 is a flow chart of one preferred embodiment of implementation of the timing diagram of FIG. 25 illustrating the start up mode which provides a safe operating area (SOA) control based on speed. 
     FIG. 27 is a timing diagram illustrating the run up mode which provides a safe operating area (SOA) control based on speed. 
     FIG  28  is a flow diagram illustrating the operation of the microprocessor of the motor of the invention in a run mode started after a preset number of commutations in the start up mode wherein in the run mode the microprocessor commutates the switches for N commutations at a constant commutation period and wherein the commutation period is adjusted every M commutations as a function of the speed, the torque or the constant air flow rate of the rotor. 
     Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, and in particular to FIGS. 1 and 3, an electric motor  20  constructed according to the principles of the present invention includes a stator  22 , a rotor  24  and a housing  26  (the reference numerals designating their subjects generally). In the illustrated embodiment, the motor  10  is of the type which the rotor magnet is on the outside of the stator, and is shown in the form of a fan. Accordingly, the rotor  24  includes a hub  28  having fan blades  30  formed integrally therewith and projecting radially from the hub. The hub  28  and fan blades  30  are formed as one piece of a polymeric material. The hub is open at one end and defines a cavity in which a rotor shaft  32  is mounted on the axis of the hub (FIG.  3 ). The shaft  32  is attached to the hub  28  by an insert  34  which is molded into the hub, along with the end of the shaft when the hub and fan blades  30  are formed. A rotor magnet  35  exploded from the rotor in FIG. 1 includes a magnetic material and iron backing. For simplicity, the rotor magnet  35  is shown as a unitary material in the drawings. The back iron is also molded into the hub cavity at the time the hub is formed. 
     The stator,  22  which will be described in further detail below, is substantially encapsulated in a thermoplastic material. The encapsulating material also forms legs  36  projecting axially of the stator  22 . The legs  36  each have a catch  38  formed at the distal end of the leg. A printed circuit board generally indicated at  40 , is received between the legs  36  in the assembled motor  10 , and includes components  42 , at least one of which is programmable, mounted on the board. A finger  44  projecting from the board  40  mounts a Hall device  46  which is received inside the encapsulation when the circuit board is disposed between the legs  36  of the stator  22 . In the assembled motor  10 , the Hall device  46  is in close proximity to the rotor magnet  35  for use in detecting rotor position to control the operation of the motor. The stator  22  also includes a central locator member generally indicated at  48 , and a bearing  50  around which the locator member is molded. The bearing  50  receives the rotor shaft  32  through the stator  22  for mounting the rotor  24  on the stator to form a subassembly. The rotor  24  is held on the stator  22  by an E clip  52  attached to the free end of the rotor after it is inserted through the stator. 
     The housing  26  includes a cup  54  joined by three spokes  56  to an annular rim  58 . The spokes  56  and annular rim  58  generally define a shroud around the fan blades  30  when the motor  10  is assembled. The cup  54 , spokes  56  and annular rim  58  are formed as one piece from a polymeric material in the illustrated embodiment. The cup  54  is substantially closed on the left end (as shown in FIGS.  1  and  3 ), but open on the right end so that the cup can receive a portion of the stator/rotor subassembly. The annular rim  58  has openings  60  for receiving fasteners through the rim to mount the motor in a desired location, such as in a refrigerated case (not shown). The interior of the cup  54  is formed with guide channels  62  (FIG. 5) which receive respective legs  36 . A shoulder  64  is formed in each guide channel  62  near the closed end of the cup  54  which engages the catch  38  on a leg to connect the leg to the cup (see FIGS.  3  and  16 ). The diameter of the cup  54  narrows from the open toward the closed end of the cup so that the legs  36  are resiliently deflected radially inwardly from their relaxed positions in the assembled motor  10  to hold the catches  38  on the shoulders  64 . Small openings  66  in the closed end of the cup  54  (FIG. 5) permit a tool (not shown) to be inserted into the cup to pry the legs  36  off of the shoulders  64  for releasing the connection of the stator/rotor subassembly from the cup. Thus, it is possible to nondestructively disassemble the motor  10  for repair or reconfiguration (e.g., such as by replacing the printed circuit board  40 ). The motor may be reassembled by simply reinserting the legs  36  into the cup  54  until they snap into connection. 
     One application for which the motor  10  of the illustrated in the particular embodiment is particularly adapted, is as an evaporator fan in a refrigerated case. In this environment, the motor will be exposed to water. For instance, the case may be cleaned out by spraying water into the case. Water tends to be sprayed onto the motor  10  from above and to the right of the motor in the orientation shown in FIG. 3, and potentially may enter the motor wherever there is an opening or joint in the construction of the motor. The encapsulation of the stator  22  provides protection, but it is desirable to limit the amount of water which enters the motor. One possible site for entry of what is at the junction of the hub  28  of the rotor and the stator  22 . An enlarged fragmentary view of this junction is shown in FIG.  12 . The thermoplastic material encapsulating the stator is formed at this junction to create a tortuous path  68 . Moreover, a skirt  70  is formed which extends radially outwardly from the stator. An outer edge  72  of the skirt  70  is beveled so that water directed from the right is deflected away from the junction. 
     The openings  66  which permit the connection of the stator/rotor subassembly to be released are potentially susceptible to entry of water into the cup where it may interfere with the operation of the circuit board. The printed circuit board  40 , including the components  42 , is encapsulated to protect it from moisture. However, it is still undesirable for substantial water to enter the cup. Accordingly, the openings  66  are configured to inhibit entry of water. Referring now to FIG. 15, a greatly enlarged view of one of the openings  66  shows a radially outer edge  66 a and a radially inner edge  66   b . These edges lie in a plane P 1  which has an angle to a plane P 2  generally parallel to the longitudinal axis of the rotor shaft of at least about 45°. It is believed that water is sprayed onto the motor at an angle of no greater than 45°. Thus, it may be seen that the water has no direct path to enter the opening  66  when it travels in a path making an angle of 45° or less will either strike the side of the cup  54 , or pass over the opening, but will not enter the opening. 
     The cup  54  of the housing  26  is also constructed to inhibit motor failures which can be caused by the formation of ice within the cup when the motor  10  is used in a refrigerated environment. More particularly, the printed circuit board  40  has power contacts  74  mounted on and projecting outwardly from the circuit board (FIG.  4 ). These contacts are aligned with an inner end of a plug receptacle  76  which is formed in the cup  54 . Referring to FIG. 14, the receptacle  76  receives a plug  78  connected to an electrical power source remote from the motor. External controls (not shown) are also connected to the printed circuit board  40  through the plug  78 . The receptacle  76  and the plug  78  have corresponding, rectangular cross sections so that when the plug is inserted, it substantially closes the plug receptacle. 
     When the plug  78  is fully inserted into the plug receptacle  76 , the power contacts  74  on the printed circuit board  40  are received in the plug, but only partially. The plug receptacle  76  is formed with tabs  80  (near its inner end) which engage the plug  78  and limit the depth of insertion of the plug into the receptacle. As a result, the plug  78  is spaced from the printed circuit board  40  even when it is fully inserted in the plug receptacle  76 . In the preferred embodiment, the spacing is about 0.2 inches. However, it is believed that a spacing of about 0.05 inches would work satisfactorily. Notwithstanding the partial reception of the power contacts  74  in the plug  78 , electrical connection is made. The exposed portions of the power contacts  74 , which are made of metal, tend to be subject to the formation of ice when the motor  10  is used in certain refrigeration environments. However, because the plug  78  and circuit board  40  are spaced, the formation of ice does not build pressure between the plug and the circuit board which would push the plug further away from the circuit board, causing electrical disconnection. Ice may and will still form on the exposed power contacts  74 , but this will not cause disconnection, or damage to the printed circuit board  40  or the plug  78 . 
     As shown in FIG. 13, the printed circuit board  40  also has a separate set of contacts  82  used for programming the motor  10 . These contacts  82  are aligned with a tubular port  84  formed in the cup  54  which is normally closed by a stop  86  removably received in the port. When the stop  86  is removed the port can receive a probe  88  into connection with the contacts  82  on the circuit board  40 . The probe  88  is connected to a microprocessor or the like (not shown) for programming or, importantly, re-programming the operation of the motor after it is fully assembled. For instance, the speed of the motor can be changed, or the delay prior to starting can be changed. Another example in the context of refrigeration is that the motor can be re-programmed to operate on different input, such as when demand defrost is employed. The presence of the port  84  and removable stop  86  allow the motor to be re-programmed long after final assembly of the motor and installation of the motor in a given application. 
     The port  84  is keyed so that the probe can be inserted in only one way into the port. As shown in FIG. 5, the key is manifested as a trough  90  on one side of the port  84 . The probe has a corresponding ridge which is received in the trough when the probe is oriented in the proper way relative to the trough. In this way, it is not possible to incorrectly connect the probe  88  to the programming contacts. If the probe  88  is not properly oriented, it will not be received in the port  84 . 
     As shown in FIG. 2, the stator includes a stator core (or bobbin), generally indicated at  92 , made of a polymeric material and a winding  94  wound around the core. The winding leads are terminated at a terminal pocket  96  formed as one piece with the stator core  92  by terminal pins  98  received in the terminal pocket. The terminal pins  98  are attached in a suitable manner, such as by soldering to the printed circuit board  40 . However, it is to be understood that other ways of making the electrical connection can be used without departing from the scope of the present invention. It is envisioned that a plug-in type connection (not shown) could be used so that no soldering would be necessary. 
     The ferromagnetic material for conducting the magnetic flux in the stator  22  is provided by eight distinct pole pieces, generally indicated at  100 . Each pole piece has a generally U-shape and including a radially inner leg  100   a , a radially outer leg  100   b  and a connecting cross piece  100   c . The pole pieces  100  are each preferably formed by stamping relatively thin U-shaped laminations from a web of steel and stacking the laminations together to form the pole piece  100 . The laminations are secured together in a suitable manner, such as by welding or mechanical interlock. One form of lamination (having a long radially outer leg) forms the middle portion of the pole piece  100  and another form of lamination forms the side portions. It will be noted that one pole piece (designated  100 ′ in FIG. 2) does not have one side portion. This is done intentionally to leave a space for insertion of the Hall device  46 , as described hereinafter. The pole pieces  100  are mounted on respective ends of the stator core  22  so that the radially inner leg  100   a  of each pole piece is received in a central opening  102  of the stator core and the radially outer leg  100   b  extends axially along the outside of the stator core across a portion of the winding. The middle portion of the radially outwardly facing side of the radially outer leg  100   b , which is nearest to the rotor magnet  35  in the assembled motor, is formed with a notch  100   d . Magnetically, the notch  100   d  facilitates positive location of the rotor magnet  35  relative to the pole pieces  100  when the motor is stopped. The pole pieces could also be molded from magnetic material without departing from the scope of the present invention. In certain, low power applications, there could be a single pole piece stamped from metal (not shown), but having multiple (e.g., four) legs defining the pole piece bent down to extend axially across the winding. 
     The pole pieces  100  are held and positioned by the stator core  92  and a central locator member, generally indicated at  104 . The radially inner legs  100   a  of the pole pieces are positioned between the central locator member  104  and the inner diameter of the stator core  92  in the central opening  102  of the stator core. Middle portions of the inner legs  100   a  are formed from the same laminations which make up the middle portions of the outer legs  100   b , and are wider than the side portions of the inner legs. The radially inner edge of the middle portion of each pole piece inner leg  100   a  is received in a respective seat  104   a  formed in the locator member  104  to accept the middle portion of the pole piece. The seats  104   a  are arranged to position the pole pieces  100  asymmetrically about the locator member  104 . No plane passing through the longitudinal axis of the locator member  104  and intersecting the seat  104   a  perpendicularly bisects the seat, or the pole piece  100  located by the seat. As a result, the gap between the radially outer legs  100   b  and the permanent magnet  35  of the rotor  24  is asymmetric to facilitate starting the motor. 
     The radially outer edge of the inner leg  100   a  engages ribs  106  on the inner diameter of the stator core central opening  102 . The configuration of the ribs  106  is best seen in FIGS. 9-11. A pair of ribs ( 106   a ,  106   b , etc.) is provided for each pole piece  100 . The differing angulation of the ribs  106  apparent from FIGS. 9 and 10 reflects the angular offset of the pole pieces  100 . The pole pieces and central locator member  104  have been shown in phantom in FIG. 9 to illustrate how each pair is associated with a particular pole piece on one end of the stator core. One of the ribs  106   d ′ is particularly constructed for location of the unbalanced pole piece  100 ′, and is engageable with the side of the inner leg  100   a ′ rather than its radially outer edge. Another of the ribs  106   d  associated with the unbalanced pole piece has a lesser radial thickness because it engages the radially outer edge of the wider middle portion of the inner leg  100   a′.    
     The central locator member  104  establishes the radial position of each pole piece  100 . As discussed more fully below, some of the initial radial thickness of the ribs  106  may be sheared off by the inner leg  100   a  upon assembly to accommodate tolerances in the stator core  92 , pole piece  100  and central locator member  104 . The radially inner edge of each outer leg  100   b  is positioned in a notch  108  formed on the periphery of the stator core  92 . Referring now to FIGS. 6-8, the central locator member  104  has opposite end sections which have substantially the same shape, but are angularly offset by 45° about the longitudinal axis of the central locator member (see particularly FIG.  7 ). The offset provides the corresponding offset for each of the four pole pieces  100  on each end of the stator core  92  to fit onto the stator core without interfering with one of the pole pieces on the opposite end. It is apparent that the angular offset is determined by the number of pole pieces  100  (i.e., 360° divided by the number of pole pieces), and would be different if a different number of pole pieces were employed. The shape of the central locator member  104  would be corresponding changed to accommodate a different number of pole pieces  100 . As shown in FIG. 8, the central locator member  104  is molded around a metal rotor shaft bearing  110  which is self lubricating for the life of the motor  10 . The stator core  92 , winding  94 , pole pieces  100 , central locator member  104  and bearing  110  are all encapsulated in a thermoplastic material to form the stator  22 . The ends of the rotor shaft bearing  110  are not covered with the encapsulating material so that the rotor shaft  32  may be received through the bearing to mount the rotor  24  on the stator  22  (see FIG.  3 ). 
     Method of Assembly 
     Having described the construction of the electric motor  10 , a preferred method of assembly will now be described. Initially, the component parts of the motor will be made. The precise order of construction of these parts is not critical, and it will be understood that some or all of the parts may be made a remote location, and shipped to the final assembly site. The rotor  24  is formed by placing the magnet  35  and the rotor shaft  32 , having the insert  34  at one end, in a mold. The hub  28  and fan blades  30  are molded around the magnet  35  and rotor shaft  32  so that they are held securely on the hub. The housing  26  is also formed by molding the cup  54 , spokes  56  and annular rim  58  as one piece. The cup  54  is formed internally with ribs  112  (FIG. 5) which are used for securing the printed circuit board  40 , as will be described. The printed circuit board  40  is formed in a conventional manner by connection of the components  42  to the board. In the preferred embodiment, the programming contacts  82  and the power contacts  74  are shot into the circuit board  40 , rather than being mounted by soldering (FIG.  4 ). The Hall device  46  is mounted on the finger  44  extending from the board and electrically connected to components  42  on the board. 
     The stator  22  includes several component parts which are formed prior to a stator assembly. The central locator member  104  is formed by molding around the bearing  110 , which is made of bronze. The ends of the bearing  110  protrude from the locator member  104 . The bearing  110  is then impregnated with lubricant sufficient to last the lifetime of the motor  10 . The stator core  92  (or bobbin) is molded and wound with magnet wire and terminated to form the winding  94  on the stator core. The pole pieces  100  are formed by stamping multiple, thin, generally U-shaped laminations from a web of steel. The laminations are preferably made in two different forms, as described above. The laminations are stacked together and welded to form each U-shaped pole piece  100 , the laminations having the longer outer leg and wider inner leg forming middle portions of the pole pieces. However, one pole piece  100 ′ is formed without one side portion so that a space will be left for the Hall device  46 . 
     The component parts of the stator  22  are assembled in a press fixture (not shown). The four pole pieces  100  which will be mounted on one end of the stator core  92  are first placed in the fixture in positions set by the fixture which are 90° apart about what will become the axis of rotation of the rotor shaft  32 . The pole pieces  100  are positioned so that they open upwardly. The central locator member  104  and bearing  110  are placed in the fixture in a required orientation and extend through the central opening  102  of the stator core  92 . The radially inner edges of the middle portions of the inner legs  100   a  of the pole pieces are received in respective seats  104   a  formed on one end of the central locator member  104 . The wound stator core  92  is set into the fixture generally on top of the pole pieces previously placed in the fixture. The other four pole pieces  100  are placed in the fixture above the stator core  92 , but in the same angular position they will assume relative to the stator core when assembly is complete. The pole pieces  100  above the stator core  92  open downwardly and are positioned at locations which are 45° offset from the positions of the pole pieces at the bottom of the fixture. 
     The press fixture is closed and activated to push the pole pieces  100  onto the stator core  92 . The radially inner edges of the inner legs  100   a  of the pole pieces  100  engage their respective seats  104   a  of the central locator member. The seat  104   a  sets the radial position of the pole piece  100  it engages. The inner legs  100   a  of the pole pieces  100  enter the central opening  102  of the stator core  92  and engage the ribs  106  on the stator core projecting into the central opening. The variances in radial dimensions from design specifications in the central locator member  104 , pole pieces  100  and stator core  92  caused by manufacturing tolerances are accommodated by the inner legs  100   a  shearing off some of the material of the ribs  106  engaged by the pole piece. The shearing action occurs as the pole pieces  100  are being passed onto the stator core  92 . Thus, the tolerances of the stator core  92  are completely removed from the radial positioning of the pole pieces. The radial location of the pole pieces  100  must be closely controlled so as to keep the air gap between the pole pieces and the rotor magnet  35  as small as possible without mechanical interference of the stator  22  and rotor  24 . 
     The assembled stator core  92 , pole pieces  100 , central locator member  104  and bearing  110  are placed in a mold and substantially encapsulated in a suitable fire resistant thermoplastic. In some applications, the mold material may not have to be fire resistant. The ends of the bearing  110  are covered in the molding process and remain free of the encapsulating material. The terminal pins  98  for making electrical connection with the winding  94  are also not completely covered by the encapsulating material (see FIG.  4 ). The skirt  70  and legs  36  are formed out of the same material which encapsulates the remainder of the stator. The legs  36  are preferably relatively long, constituting approximately one third of the length of the finished, encapsulated stator. Their length permits the legs  36  to be made thicker for a more robust construction, while permitting the necessary resilient bending needed for snap connection to the housing  26 . In addition to the legs  36  and skirt  70 , two positioning tangs  114  are formed which project axially in the same direction as the legs and require the stator  22  to be in a particular angular orientation relative to the housing  26  when the connection is made. Still further, printed circuit board supports are formed. Two of these take the form of blocks  116 , from one of which project the terminal pins  98 , and two others are posts  118  (only one of which is shown). 
     The encapsulated stator  22  is then assembled with the rotor  24  to form the stator/rotor subassembly. A thrust washer  120  (FIG. 3) is put on the rotor shaft  32  and slid down to the fixed end of the rotor shaft in the hub  28 . The thrust washer  120  has a rubber-type material on one side capable of absorbing vibrations, and a low friction material on the other side to facilitate a sliding engagement with the stator  22 . The low friction material side of the washer  120  faces axially outwardly toward the open end of the hub  28 . The stator  22  is then dropped into the hub  28 , with the rotor shaft  32  being received through the bearing  110  at the center of the stator. One end of the bearing  110  engages the low friction side of the thrust washer  120  so that the hub  28  can rotate freely with respect to the bearing. Another thrust washer  122  is placed on the free end of the bearing  110  and the E clip  52  is shaped onto the end of the rotor shaft  32  so that the shaft cannot pass back through the bearing. Thus, the rotor  24  is securely mounted on the stator  22 . 
     The printed circuit board  40  is secured to the stator/rotor subassembly. The assembly of the printed circuit board  40  is illustrated in FIG. 4, except that the rotor  24  has been removed for clarity of illustration. The printed circuit board  40  is pushed between the three legs  36  of the stator  22 . The finger  44  of the circuit board  40  is received in an opening  124  formed in the encapsulation so that the Hall device  46  on the end of the finger is positioned within the encapsulation next to the unbalanced pole piece  100 ′, which was made without one side portion so that space would be provided for the Hall device. The side of the circuit board  40  nearest the stator  22  engages the blocks  116  and posts  118  which hold the circuit board at a predetermined spaced position from the stator. The terminal pins  98  projecting from the stator  22  are received through two openings  126  in the circuit board  40 . The terminal pins  98  are electrically connected to the components  42  circuit board in a suitable manner, such as by soldering. The connection of the terminal pins  98  to the board  40  is the only fixed connection of the printed circuit board to the stator  22 . 
     The stator/rotor subassembly and the printed circuit board  40  are then connected to the housing  26  to complete the assembly of the motor. The legs  36  are aligned with respective channels  62  in the cup  54  and the tangs  114  are aligned with recesses  128  formed in the cup (see FIGS.  5  and  14 ). The legs  36  will be received in the cup  54  in only one orientation because of the presence of the tangs  114 . The stator/rotor subassembly is pushed into the cup  54 . The free ends of the legs  36  are beveled on their outer ends to facilitate entry of the legs into the cup  54 . The cup tapers slightly toward its closed end and the legs  36  are deflected radially inwardly from their relaxed configurations when they enter the cup and as they are pushed further into it. When the catch  38  at the end of each leg clears the shoulder  64  at the inner end of the channel  62 , the leg  36  snaps radially outwardly so that the catch engages the shoulder. The leg  36  is still deflected from its relaxed position so that it is biased radially outwardly to hold the catch  38  on the shoulder  64 . The engagement of the catch  38  with the shoulder  64  prevents the stator/rotor subassembly, and printed circuit board  40  from being withdrawn from the cup  54 . The motor  10  is now fully assembled, without the use of any fasteners, by snap together construction. 
     The printed circuit board  40  is secured in place by an interference fit with the ribs  112  in the cup  54 . As the stator/rotor assembly advances into the cup  54 , peripheral edges of the circuit board  40  engage the ribs  112 . The ribs are harder than the printed circuit board material so that the printed circuit board is partially deformed by the ribs  112  to create the interference fit. In this way the printed circuit board  40  is secured in place without the use of any fasteners. The angular orientation of the printed circuit board  40  is set by its connection to the terminal pins  98  from the stator  22 . The programming contacts  82  are thus aligned with the port  84  and the power contacts  74  are aligned with the plug receptacle  76  in the cup  54 . It is also envisioned that the printed circuit board  40  may be secured to the stator  22  without any interference fit with the cup  54 . For instance, a post (not shown) formed on the stator  22  may extend through the circuit board and receive a push nut thereon against the circuit board to fix the circuit board on the stator. 
     In the preferred embodiment, the motor  10  has not been programmed or tested prior to the final assembly of the motor. Following assembly, a ganged connector (not shown, but essentially a probe  88  and a power plug  78 ) is connected to the printed circuit board  44  through the port and plug receptacle  76 . The motor is then programmed, such as by setting the speed and the start delay, and tested. If the circuit board  40  is found to be defective, it is possible to non-destructively disassemble the motor and replace the circuit board without discarding other parts of the motor. This can be done by inserting a tool (not shown) into the openings  66  in the closed end of the cup  54  and prying the catches  38  off the shoulders  64 . If the motor passes the quality assurance tests, the stop  86  is placed in the port  84  and the motor is prepared for shipping. 
     It is possible with the motor of the present invention, to re-program the motor  10  after it has been shipped from the motor assembly site. The end user, such as a refrigerated case manufacturer, can remove the stop  86  from the port  84  and connect the probe  88  to the programming contacts  82  through the port. The motor can be re-programmed as needed to accommodate changes made by the end user in operating specifications for the motor. 
     The motor  10  can be installed, such as in a refrigerated case, by inserting fasteners (not shown) through the openings  60  in the annular rim  58  and into the case. Thus, the housing  26  is capable of supporting the entire motor through connection of the annular rim  58  to a support structure. The motor is connected to a power source by plugging the plug  78  into the plug receptacle  76  (FIG.  14 ). Detents  130  (only one is shown) on the sides of the plug  78  are received in slots on respective sides of a tongue  132  to lock the plug in the plug receptacle  76 . Prior to engaging the printed circuit board  40 , the plug  78  engages the locating tabs  80  in the plug receptacle  76  so that in its fully inserted position, the plug is spaced from the printed circuit board. As a result, the power contacts  74  are inserted far enough into the plug  78  to make electrical connection, but are not fully received in the plug. Therefore, although ice can form on the power contacts  74  in the refrigerated case environment, it will not build up between the plug  78  and the circuit board  40  causing disconnection and/or damage. 
     FIG. 16 is a block diagram of the microprocessor controlled single phase motor  500  according to the invention. The motor  500  is powered by an AC power source  501 . The motor  500  includes a stator  502  having a single phase winding. The direct current power from the source  501  is supplied to a power switching circuit via a power supply circuit  503 . The power switching circuit may be any circuit for commutating the stator  502  such as an H-bridge  504  having power switches for selectively connecting the dc power source  501  to the single phase winding of the stator  502 . A permanent magnet rotor  506  is in magnetic coupling relation to the stator and is rotated by the commutation of the winding and the magnetic field created thereby. Preferably, the motor is an inside-out motor in which the stator is interior to the rotor and the exterior rotor rotates about the interior stator. However, it is also contemplated that the rotor may be located within and internal to an external stator. 
     A position sensor such as a hall sensor  508  is positioned on the stator  502  for detecting the position of the rotor  506  relative to the winding and for providing a position signal via line  510  indicating the detected position of the rotor  506 . Reference character  512  generally refers to a control circuit including a microprocessor  514  responsive to and receiving the position signal via line  510 . The microprocessor  514  is connected to the H-bridge  504  for selectively commutating the power switches thereof to commutate the single phase winding of the stator  502  as a function of the position signal. 
     Voltage VDD to the microprocessor  514  is provided via line  516  from the power supply circuit  503 . A low voltage reset circuit  518  monitors the voltage VDD on line  516  and applied to the microprocessor  514 . The reset circuit  518  selectively resets the microprocessor  514  when the voltage VDD applied to the microprocessor via line  516  transitions from below a predetermined threshold to above the predetermined threshold. The threshold is generally the minimum voltage required by the microprocessor  514  to operate. Therefore, the purpose of the reset circuit  518  is to maintain operation and re-establish operation of the microprocessor in the event that the voltage VDD supplied via line  516  drops below the preset minimum required by the microprocessor  514  to operate. 
     Optionally, to save power, the hall sensor  508  may be intermittently powered by a hall strobe  520  controlled by the microprocessor  514  to pulse width modulate the power applied to the hall sensor. 
     The microprocessor  514  has a control input  522  for receiving a signal which affects the control of the motor  500 . For example, the signal may be a speed select signal in the event that the microprocessor is programmed to operate the rotor such that the stator is commutated at two or more discrete speeds. Alternatively, the motor may be controlled at continuously varying speeds or torques according to temperature. For example, in place of or in addition to the hall sensor  508 , an optional temperature sensor  524  may be provided to sense the temperature of the ambient air about the motor. This embodiment is particularly useful when the rotor  506  drives a fan which moves air through a condenser for removing condenser generated heat or which moves air through an evaporator for cooling, such as illustrated in FIGS. 1-15. 
     In one embodiment, the processor interval clock corresponds to a temperature of the air moving about the motor and for providing a temperature signal indicating the detected temperature. For condenser applications where the fan is blowing air into the condenser, the temperature represents the ambient temperature and the speed (air flow) is adjusted to provide the minimum needed air flow at the measured temperature to optimize the heat transfer process. When the fan is pulling air over the condenser, the temperature represents ambient temperature plus the change in temperature (Δt) added by the heat removed from the condenser by the air stream. In this case, the motor speed is increased in response to the higher combined temperature (speed is increased by increasing motor torque, i.e., reducing the power device off time PDOFFTIM; see FIG.  26 ). Additionally, the speed the motor could be set for different temperature bands to give different air flow which would be distinct constant air flows in a given fan static pressure condition. Likewise, in a condenser application, the torque required to run the motor at the desired speed represents the static load on the motor. The higher static loads can be caused by installation in a restricted environment, i.e., a refrigerator installed as a built-in, or because the condenser air flow becomes restricted due to dust build up or debris. Both of these conditions may warrant an increased air flow/speed. 
     Similarly, in evaporator applications, the increased static pressure could indicate evaporator icing or increased packing density for the items being cooled. 
     In one of the commercial refrigeration applications, the evaporator fan pulls the air from the air curtain and from the exit air cooling the food. This exhaust of the fan is blown through the evaporator. The inlet air temperature represents air curtains and food exit air temperature. The fan speed would be adjusted appropriately to maintain the desired temperature. 
     Alternatively, the microprocessor  514  may commutate the switches at a variable speed rate to maintain a substantially constant air flow rate of the air being moved by the fan connected to the rotor  506 . In this case, the microprocessor  514  provides an alarm signal by activating alarm  528  when the motor speed is greater than a desired speed corresponding to the constant air flow rate at which the motor is operating. As with the desired torque, the desired speed may be determined by the microprocessor as a function of an initial static load of the motor and changes in static load over time. 
     FIG. 23 illustrates one preferred embodiment of the invention in which the microprocessor  514  is programmed according to the flow diagram therein. In particular, the flow diagram of FIG. 23 illustrates a mode in which the motor is commutated at a constant air flow rate corresponding to a speed and torque which are defined by tables which exclude resonant points. For example, when the rotor is driving a fan for moving air over a condenser, the motor will have certain speeds at which a resonance will occur causing increased vibration and/or increased audio noise. Speeds at which such vibration and/or noise occur are usually the same or similar and are predictable, particularly when the motor and its associated fan are manufactured to fairly close tolerances. Therefore, the vibration and noise can be minimized by programming the microprocessor to avoid operating at certain speeds or within certain ranges of speeds in which the vibration or noise occurs. As illustrated in FIG. 23, the microprocessor  514  would operate in the following manner. After starting, the microprocessor sets the target variable I to correspond to an initial starting speed pointer defining a constant air flow rate at step  550 . For example, I=0. Next, the microprocessor proceeds to step  552  and selects a speed set point (SSP) from a table which correlates each of the variable levels 0 to n to a corresponding speed set point (SSP), to a corresponding power device off time (PDOFFTIM=P min ) for minimum power and to a corresponding power device off time (PDOFFTIM=P max ) for maximum power. 
     It is noted that as the PDOFFTIM increases, the motor power decreases since the controlled power switches are off for longer periods during each commutation interval. Therefore, the flow chart of FIG. 23 is specific to this approach. Others skilled in the art will recognize other equivalent techniques for controlling motor power. 
     After a delay at step  554  to allow the motor to stabilize, the microprocessor  514  selects a PDOFFTIM for a minimum power level (P min ) from the table which provides current control by correlating a minimum power level to the selected level of variable I. At step  558  the microprocessor selects a PDOFFTIM for a maximum power level (P max ) from the table which provides current control by correlating a maximum power level to the selected variable level I. 
     At step  560 , the microprocessor compares the actual PDOFFTIM representing the actual power level to the minimum PDOFFTIM (P min ) for this I. If the actual PDOFFTIM is greater than the minimum PDOFFTIM (PDOFFTIM&gt;P min ), the microprocessor proceeds to step  562  and compares the variable level I to a maximum value n. If I is greater or equal to n, the microprocessor proceeds to step  564  to set I equal to n. Otherwise, I must be less than the maximum value for I so the microprocessor  514  proceeds to step  566  to increase I by one step. 
     If, at step  560 , the microprocessor  514  determines that the actual PDOFFTIM is less than or equal to the minimum PDOFFTIM (PDOFFTIM≦P min ), the microprocessor proceeds to step  568  and compares the actual PDOFFTIM representing the actual power level to the maximum PDOFFTIM (P max ) for this I. If the actual PDOFFTIM is less than the maximum PDOFFTIM (PDOFFTIM&lt;P max ), the microprocessor proceeds to step  570  and compares the variable level I to a minimum value 0. If I is less or equal to 0, the microprocessor proceeds to step  572  to set I equal to 0. Otherwise, I must be greater than the minimum value for I so the microprocessor  514  proceeds to step  574  to decrease I by one step. 
     If the actual PDOFFTIM is less than or equal to the minimum and is greater than or equal to the maximum so that the answer to both steps  560  and  568  is no, the motor is operating at the speed and power needed to provide the desired air flow so the microprocessor returns to step  552  to maintain its operation. 
     Alternatively, the microprocessor  514  may be programmed with an algorithm which defines the variable rate at which the switches are commutated. This variable rate may vary continuously between a preset range of at least a minimum speed S min  and not more than a maximum speed S max  except that a predefined range of speeds S 1 +/−S 2  is excluded from the preset range. As a result, for speeds between S 1 −S 2  and S 1 , the microprocessor operates the motor at S 1 −S 2  and for speeds between S 1  and S 1 +S 2 , the microprocessor operates the motor at speeds S 1 +S 2 . 
     FIG. 22 is a schematic diagram of the H-bridge  504  which constitutes the power switching circuit having power switches according to the invention, although other configurations may be used, such as two windings which are single ended or the H-bridge configuration of U.S. Pat. No. 5,859,519, incorporated by reference herein. The dc input voltage is provided via a rail  600  to input switches Q 1  and Q 2 . An output switch Q 3  completes one circuit by selectively connecting switch Q 2  and stator  502  to a ground rail  602 . An output switch Q 4  completes another circuit by selectively connecting switch Q 1  and stator  502  to the ground rail  602 . Output switch Q 3  is controlled by a switch Q 5  which receives a control signal via port BQ 5 . Output switch Q 4  is controlled by a switch Q 8  which receives a control signal via port BQ 8 . When switch Q 3  is closed, line  604  pulls the gate of Q 1  down to open switch Q 1  so that switch Q 1  is always open when switch Q 3  is closed. Similarly, line  606  insures that switch Q 2  is open when switch Q 4  is closed. 
     The single phase winding of the stator  502  has a first terminal F and a second terminal S. As a result, switch Q 1  constitutes a first input switch connected between terminal S and the power supply provided via rail  600 . Switch Q 3  constitutes a first output switch connected between terminal S and the ground rail  602 . Switch Q 2  constitutes a second input switch connected between the terminal F and the power supply provided via rail  600 . Switch Q 4  constitutes a second output switch connected between terminal F and ground rail  602 . As a result, the microprocessor controls the first input switch Q 1  and the second input switch Q 2  and the first output switch Q 3  and the second output switch Q 4  such that the current through the motion is provided during the first 90° of the commutation period illustrated in FIG.  27 . The first 90° is significant because of noise and efficiency reasons and applies to this power device topology (i.e., either Q 1  or Q 2  is always “on” when either Q 3  or Q 4  is off, respectively. PDOFFTIM is the term used in the software power control algorithms. When the first output switch Q 3  is open, the first input switch Q 1  is closed. Similarly, the second input switch Q 2  is connected to and responsive to the second output switch Q 4  so that when the second output switch Q 4  is closed, the second input switch Q 2  is open. Also, when the second output switch Q 4  is open, the second input switch Q 2  is closed. This is illustrated in FIG. 27 wherein it is shown that the status of Q 1  is opposite the status of Q 3  and the status of Q 2  is opposite the status of Q 4  at any instant in time. 
     FIG. 26 is a timing flow chart illustrating the start up mode with a current maximum determined by the setting of PDOFFTIM versus the motor speed. In this mode, the power devices are pulse width modulated by software in a continuous mode to get the motor started. The present start algorithm stays in the start mode eight commutations and then goes into the RUN mode. A similar algorithm could approximate constant acceleration by selecting the correct settings for PDOFFTIM versus speed. At step  650 , the value HALLIN is a constant defining the starting value of the Hall device reading. When the actual Hall device reading (HALLOLD) changes at step  652 , HALLIN is set to equal HALLOLD at step  654  and the PDOFFTIM is changed at step  656  depending on the RPMs. 
     FIG. 25 illustrates the microprocessor outputs (BQ 5  and BQ 8 ) that control the motor when the strobed hall effect output (HS 3 ) changes state. In this example, BQ 5  is being pulse width modulated while HS 3  is 0. When HS 3  (strobed) changes to a 1, there is a finite period of time (LATENCY) for the microprocessor to recognize the magnetic change after which BQ 5  is in the off state so that BQ 8  begins to pulse width modulate (during PWMTIM). 
     FIG. 24 illustrates another alternative aspect of the invention wherein the microprocessor operates within a run mode safe operating area without the need for current sensing. In particular, according to FIG. 24, microprocessor  514  controls the input switches Q 1 -Q 4  such that each input switch is open or off for a minimum period of time (PDOFFTIM) during each pulse width modulation period whereby over temperature protection is provided without current sensing. Specifically, the minimum period may be a function of the speed of the rotor whereby over temperature protection is provided-without current sensing by limiting the total current over time. As illustrated in FIG. 24, if the speed is greater than a minimum value (i.e., if A&lt;165), A is set to 165 and SOA limiting is bypassed and not required; if the speed is less than (or equal to) a minimum value (i.e., if A≧165), the routine of FIG. 24 ensures that the switches are off for a minimum period of time to limit current. “A” is a variable and is calculated by an equation that represents a PDOFFTIM minimum value at a given speed (speed is a constant multiplied by 1/TINPS, where TINPS is the motor period). Then, if PDOFFTIM is&lt;A, PDOFFTIM is set to A so that the motor current is kept to a maximum desired value at the speed the motor is running. 
     As illustrated in FIG. 18, the motor includes a reset circuit  512  for selectively resetting the microprocessor when a voltage of the power supply vdd transitions from below a predetermined threshold to above a predetermined threshold. In particular, switch Q 6  disables the microprocessor via port MCLR/VPP when the divided voltage between resistors R 16  and R 17  falls below a predetermined threshold. The microprocessor is reactivated and reset when the voltage returns to be above the predetermined threshold thereby causing switch Q 6  to close. 
     FIG. 19 illustrates one preferred embodiment of a strobe circuit  520  for the hall sensor  508 . The microprocessor generates a pulse width modulated signal GP 5  which intermittently powers the hall sensor  508  as shown in FIG. 21 by intermittently closing switch Q 7  and providing voltage VB 2  to the hall sensor  508  via line HS 1 . 
     FIG. 17 is a schematic diagram of the power supply circuit  503  which supplies the voltage V in  for energizing the stator single phase winding via the H-bridge  504  and which also supplies various other voltages for controlling the H-bridge  504  and for driving the microprocessor  514 . In particular, the lower driving voltages including VB 2  for providing control voltages to the switches Q 1 -Q 4 , VDD for driving the microprocessor, HS 2  for driving the hall sensor  508 , and VSS which is the control circuit reference ground not necessarily referenced to the input AC or DC voltage are supplied from the input voltage V in  via a lossless inline series capacitor C 1 . 
     FIG. 20 illustrates the inputs and outputs of microprocessor  514 . In particular, only a single input GP 4  from the position sensor is used to provide information which controls the status of control signal BQ 5  applied to switch Q 5  to control output switch Q 3  and input switch Q 1  and which controls the status of control signal EQS applied to switch Q 8  to control output switch Q 4  and input switch Q 2 . Input GP 2  is an optional input for selecting motor speed or other feature or may be connected for receiving a temperature input comparator output when used in combination with thermistor  524 . 
     FIG. 28 illustrates a flow chart of one preferred embodiment of a run mode in which the power devices are current controlled. In this mode, the following operating parameters apply: 
     MOTOR RUN POWER DEVICE (CURRENT) CONTROL 
     At the end of each commutation, the time power devices will be off the next time the commutation period is calculated. 
     OFFTIM=TINP/2. (The commutation period divided by 2=90°). While in the start routine, this is also calculated. 
     After eight commutations (1 motor revolution) and at the start routine exit, PWMTIM is calculated: 
     PWMTIM=OFFTIM/4 
     At the beginning of each commutation period, a counter (COUNT 8 ) is set to five to allow for four times the power devices will be turned on during this commutation: 
     PWMSUM=PWMTIM 
     PDOFFSUM=PWMTIM−PDOFFTIM 
     TIMER=0 
     (PDOFFTIM is used to control the amount of current in the motor and is adjusted in the control algorithm (SPEED, TORQUE, CFM, etc.). 
     Commutation time set to 0 at each strobed hall change, HALLOLD is the saved hall strobe value. 
     During motor run, the flow chart of FIG. 28 is executed during each commutation period. In particular at step  702 , the commutation time is first checked to see if the motor has been in this motor position for too long a period of time, in this case 32 mS. If it has, a locked rotor is indicated and the program goes to the locked rotor routine at step  704 . Otherwise, the program checks to see if the commutation time is greater then OFFTIM at step  706 ; if it is, the commutation period is greater than 90 electrical degrees and the program branches to step  708  which turns the lower power devices off and exits the routine at step  710 . Next, the commutation time is compared at step  712  to PWMSUM. If it is less than PWMSUM, the commutation time is checked at step  714  to see if it is less or equal to PDOFFSUM where if true, the routine is exited at step  716 ; otherwise the routine branches to step  708  (if step  714  is yes). 
     For the other case where the commutation time is greater or equal to PWMSUM, at step  718  PWMSUM and PDOFFSUM have PWMTIM added to them to prepare for the next pulse width modulation period and a variable A is set to COUNT  8 - 1 . 
     If A is equal to zero at step  720 , the pulse width modulations (4 pulses) for this commutation period are complete and the program branches to step  708  to turn the lower power devices off and exit this routine. If A is not equal to zero, COUNT 8  (which is a variable defining the number of PWMs per commutation) is set to A at step  722 ; the appropriate lower power device is turned on; and this routine is exited at step  716 . More PWM counts per commutation period can be implemented with a faster processor. Four (4) PWMs per commutation period are preferred for slower processors whereas eight (8) are preferred for faster processors. 
     The timing diagram for this is illustrated in FIG.  27 . In the locked rotor routine of step  704 , on entry, the lower power devices are turned off for 1.8 seconds after which a normal start attempt is tried. 
     In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. 
     As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.