Abstract:
A brushless DC motor operated by a microcontroller has a unique pole construction that enables it to reliably start and operate as a unipolar device so that a reduced number of electronic power switches can be used to reduce cost and complexity. The microcontroller calculated rotor position to eliminate the need for a separate sensor and thereby further reduce manufacturing cost.

Description:
The invention relates to fractional horsepower motors and, particularly, to the construction of a permanent magnet brushless DC motor. 
   PRIOR ART 
   Small electric motors such as shaded pole motors can be inexpensive to produce, but have relatively low efficiency. Many applications for such motors can be significantly benefitted from a motor with increased efficiency. An example of such an application is air circulation in a refrigeration system where inefficiency is compounded by the need to remove heat generated by the motor. U.S. Pat. Nos. 3,158,769, 3,959,678, 4,234,810 and 5,036,237 disclose examples of shaded pole motors useful in commercial refrigeration systems. Conventional brushless DC motors are known to achieve relatively high efficiency but involve increased componentry and manufacturing costs. 
   There continues to be a need for improving the efficiency and reducing the manufacturing costs of small electric motors. 
   SUMMARY OF THE INVENTION 
   The invention relates to small electric motors constructed with a unique brushless DC drive that is both relatively high in efficiency and relatively low in cost. The disclosed motor drive circuit utilizes a microcontroller to control the delivery of current to the field windings in response to voltage signals that are inherently produced in the windings. The disclosed controller arrangement and operating mode reduces the number of power switches from what has been customary and eliminates the need for a rotor position sensor to operate the motor. 
   A feature of the drive circuit is a unipolar field operation that reduces the number of required power switches from what is ordinarily required and, consequently, reduces the manufacturing cost of the motor. The unipolar operation is made possible by use of a pole shape that produces an air gap that varies across the face of the pole. This air gap variation assures that a start-up position can be obtained that is off a neutral position with reference to the pole axis of the coils that are energized for start-up. 
   An additional benefit of the control circuit is its ability to control speed. Still further, the stator laminations as well as the stator housing body of existing prior art shaded pole motors can be used to practice the invention. 
   More specifically, the motor drive circuit is arranged to enable the microcontroller to monitor the back EMF of the field coils. The microcontroller is programmed to calculate the angular position of the rotor during a quarter of each revolution and this calculation, in turn, is used to regulate the dwell or angular displacement of the rotor through which power is applied to the field coils. The maximum dwell, for each phase of the poles is less than 90° of shaft rotation. The actual dwell produced by the microcontroller can be adjusted up or down to maintain a desired speed for an imposed load. The position calculator feature obviates the need for a rotor position sensing device and its attendant cost. The DC power, derived from a full bridge rectifier is applied in a unipolar arrangement wherein the poles do not change in polarity. 
   In summary, the overall simplicity and reduced number of components used in the disclosed motor system result in potential savings in manufacturing costs and reliability of operation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view, taken in a plane on the rotor axis of a motor of the invention; 
       FIG. 2  is a diagram of the electrical control circuit that operates the motor; 
       FIG. 3  is a computer simulation of the magnetic field induced in the stator by current flowing to opposed poles of one phase of the stator coils and the permanent magnets of the rotor; 
       FIG. 4  is a computer simulation similar to  FIG. 3  of the magnetic field induced in the stator by the permanent magnets of the rotor without electrical energization of stator coils; 
       FIG. 5  is a graph schematically showing stator coil feedback and control signals existing in operation of the motor and control circuit; and 
       FIG. 6  is a graph showing the torque developed on the rotor when one phase of the field coils is energized and when it is not energized. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A system embodying the invention comprises an electrical motor  11  ( FIG. 1 ) operated by an electronic control circuit  12  ( FIG. 2 ). The illustrated motor  11  is a brushless permanent magnet type operating on direct current (DC). In the illustrated case, the motor  11  has a permanent magnet rotor  13  with four magnetic poles,  16 ,  17 , and a stator  18  with four field poles  21 ,  22 . The four rotor permanent magnets or poles  16 ,  17  are bonded to a ferromagnetic round tube  26  suitably supported for rotation about a central axis  27  by a bearing such as a unit bearing known in the art. The rotor magnets  16 ,  17  are oriented with their north and south poles alternating circumferentially about the axis  27 . 
   Diametrically opposite pairs of stator poles  21  or  22  are electrically wound and interconnected in a manner that when simultaneously energized with direct current, they produce magnetic fields oriented in the same direction. That is, at the inside diameter of the stator  18  when one pole  21  is North, the opposite pole  21  is also North. For purposes of explanation, one pair of opposed stator field coils are referred to as Phase  1  and the other pair are referred to as Phase  2 . Referring to  FIG. 2 , a connector  31  is provided to receive command signals associated with a machine or appliance on which the motor  11  is installed. For example, the motor  11  can be used to drive an air circulating fan in a commercial refrigeration cabinet and the command signals can be related to the desired time and/or speed at which the motor is operated. 
   The circuit  12  includes a power supply generally bounded by the broken line  32 . Nominal 120 volt AC voltage is supplied to a connector  33 . A bridge rectifier  34  produces a nominal 160 volt supply on a positive line or “plus rail”  36 . Line  37  represents a “minus rail” or ground. Local or subcircuits  38 ,  39  produce control voltages for the circuit  12 . The power supply subcircuit  39  supplies current to a microcontroller  40  through a line  41 . Current is supplied from the power supply  32  to the motor stator windings, designated  42 ,  43  through a “high side” MOSFET power transistor or switch  46  and alternately through one of a pair of “low side” MOSFET switches  47 ,  48 . One of the alternate low side MOSFET transistors  47  controls current in one set of field coils  42  arbitrarily labeled Phase  1  and the other MOSFET switch  48  controls current through the other stator field coils or windings  43 , arbitrarily called Phase  2 . The field windings or coils  42  are connected across solder pads or terminals  51 ,  52  while, similarly, the other field windings or coils  43  of Phase  2  are connected across solder pads or terminals  53 ,  54 . A driver  56  interfaces, via line  61 , between the microcontroller  40  and high side MOSFET or switch  46  and separate operational amplifiers  57 ,  58  interface between the microcontroller  40  and an associated low side MOSFET power transistor or switch  47 ,  48  through the lines  62 ,  63 , respectively. 
   A study of the circuit  12  shows that the microcontroller or microprocessor  40  is arranged to selectively control current delivery to the stator field coils  42 ,  43 . Feedback lines  66 ,  67  allow the microcontroller  40  to monitor the back EMF produced in the respective stator coils  42 ,  43 . The microcontroller  40  is programmed with a routine for starting the motor  11  and then a routine for operating it at a desired speed. As mentioned, the term Phase  1  is associated with one set of opposed stator poles  21  and the term Phase  2  is associated with the other set of poles  22 . 
   The poles  21 ,  22  of each Phase  1  and  2  are symmetrical with one another and are such they are physically displaced from the poles of the other phase by 90°. The illustrated stator pole geometry is characterized by an air gap that varies circumferentially of the rotor, i.e. in an angular direction with reference to the axis  27  across the face of a pole  21 ,  22 . This geometry produces two stable rotor positions slightly but distinctly displaced from one another corresponding to whether or not a set of opposed poles of a phase is electrically energized. The microcontroller  40  uses this phenomena to reliably start the motor in a consistent direction. In a first step in the starting sequence, the microcontroller  40  energizes a pair of poles, say those of Phase  1 . Thereafter, the microcontroller  40  de-energizes this pair as well as the other pair of poles (Phase  2 ). As indicated in  FIG. 6 , the stator will tend to align with the energized phase poles where the torque is 0, i.e. −3° from a reference point where 0 is taken as the nominal geometric center of the opposed poles. The microcontroller  40  then re-energizes the pole coils (Phase  1 ) while Phase  2  remains de-energized. The rotor shifts from the energized Phase  1  angular rest position of 0 torque to a rest or stable position of 0 torque indicated at −5°. This position sets the stage for energization of the Phase  2  coils  43 . The microcontroller  40  then energizes the Phase  2  coils which rotate the rotor in a consistent known direction since the rotor  13  is off center of the Phase  1  coils consistently to the same direction at start-up as a result of the alignment step. Since the Phase  2  coils are displaced 90° from the Phase  1  coils, the rotor  13 , once it moves off of alignment with the Phase  1  coils, is out of a potential dead spot that exists when centered on the neutral or zero torque position of Phase  1  and, likewise, not being in this neutral position is not capable of rotation in an unwanted direction. The microcontroller  40  energizes the Phase  2  coils  43  to start rotation of the rotor  13 . Thereafter, the microcontroller  40  alternately energizes Phase  1  and Phase  2  coils to maintain rotation of the rotor. 
   Reference is made to  FIG. 5 . The microcontroller  40  operates with the following strategy. Coils of only one phase, Phase  1  or Phase  2 , are energized at one time. When the back EMF, as signaled to the microcontroller  40  through one of the lines  66 ,  67  of the coils  42 ,  43  of a phase not energized reaches 0 that phase is energized by the microcontroller through the line  63  or  62  activating the associated MOSFET transistor  48  or  47 . 
   The coils of an energized phase are de-energized by the microcontroller  40  before the rotor turns 90° from when it is energized. The position of the rotor  13  after a phase is energized is calculated by the microcontroller  40  by integrating the back EMF signal, which signal is proportional to rotor speed, of the non-energized phase. The microcontroller  40  determines how long an energized set of stator coils remains turned on as a portion of a one-quarter revolution of the rotor (e.g. represented as a set point limiting the integral of the back EMF so that power is always extinguished before full 90° of rotation) to apply enough average power over an extended time so that the motor will run at a desired speed. The microcontroller can measure speed, for example, by measuring the time between instants when the back EMF goes to 0 at the same or alternate phases. 
   The duration of the angle of rotation that current is applied to the individual stator coils by the microcontroller  40  can be increased to increase the average speed, or reduced to lower the average speed. The microcontroller is preferably programmed to limit the rate of change of the time power is applied to minimize over or under shoot. 
   Current to either phase is extinguished by the microcontroller  40  at the appropriate time, this being determined by calculating the angular position of the rotor, by shutting off the high side MOSFET drive transistor  46 . This allows the field energy to dissipate in the respective stator coil  42 ,  43  through a freewheeling diode  71 . 
   The microcontroller  40  can be programmed to detect locked rotor conditions and when such a condition exists the microcontroller places the motor  11  in a low power mode while periodically trying to start the motor. A thermistor  76 , appropriately positioned relative to the motor  11  can be provided to work with a subroutine in the microprocessor program to detect excessive temperature and place the motor in a low power mode where it will start and run periodically but will not continue to run unless the excessive load or abnormal condition is removed. 
   It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.