Abstract:
A method of operating a direct current motor fan assembly is provided in which a motor controller operates to apply full power to kick-start a motor to overcome static forces. As soon as a sensor determines that the motor has begun to rotate, the motor controller changes the motor drive level from full power to a predetermined lower level to maintain a desired rotational speed.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention pertains to motor controllers for direct current motors, in general, and to a control arrangement for start-up of direct current motors, in particular. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is common to use Pulse Width Modulation (“PWM”) to control the speed of direct current, dc, motors that are utilized with fans in computer applications. Advantageously, PWM control provides for accurate motor speed control. 
         [0003]    To reduce fan generated noise and also to reduce power consumption, most modern computers utilize fan controllers to respond to temperatures within the computer and to vary the speed of a fan to provide enough cooling to maintain a desired temperature. 
         [0004]    Typically, the fan motor is not operated until such time as there is a need for cooling. It is desirable to be able to initially operate the fan at a low speed to provide cooling of slight temperature increases. However, the static friction of any motor is much higher than dynamic friction of the rotating motor. To assure quick starting of direct current fan motors, it is common to “kick-start” the motor by applying a dc current pulse of fixed time duration to get the rotor in motion and then switch to PWM signals that represent the desired motor rotational speed. The optimum period of the kick-start pulse is determined by the motor and the load inertia and is best determined experimentally as that which gives the most reliable starting performance. In many applications, the fan motor is run at full power for one second to assure that the static friction has been overcome and the motor is rotating. 
         [0005]    The drawback to the kick-start is that fan is initially run at full speed and then slows down to the needed speed. 
         [0006]    It is highly desirable to provide a motor controller arrangement that minimizes initial fan roar that occurs during timed kick-start operation. 
       SUMMARY OF THE INVENTION 
       [0007]    In accordance with the principles of the invention, a method of operating a direct current motor fan assembly is provided in which a motor controller operates to apply full power to kick-start a motor to overcome static forces. As soon as a sensor determines that the motor has begun to rotate, the motor controller changes the motor drive level from full power to a predetermined lower level to maintain a desired rotational speed. 
         [0008]    In the illustrative embodiment of the invention, the motor controller initially applies a 100% duty cycle PWM drive signal to the motor until the controller detects rotor movement by monitoring the output of a Hall effect sensor coupled to the motor and determining that rotor movement has occurred by a Hall sensor transition and once movement has been detected the motor controller changes the duty cycle to the a predetermined duty cycle. 
         [0009]    An embodiment in accordance with the principles of the invention, is a monolithic brushless DC motor controller that comprises all of the required functions for implementing fan speed control. The motor controller contains a pulse width modulator (PWM) consisting of a fixed frequency oscillator, comparator and a latch for speed control, commutation logic for proper drive sequencing, a kick-start circuit and a Hall sensor transition detector. 
         [0010]    In accordance with the principles of the invention the motor controller is formed on a single integrated circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0011]    The invention will be better understood from a reading of the following detailed description of the drawing in which like reference designators are used to identify like elements in the various drawing figures, and in which; 
           [0012]      FIG. 1  illustrates in graph form startup characteristics of a dc motor of a type to which the principles of the invention may be advantageously applied; 
           [0013]      FIG. 2  illustrates in graph form typical kick-start run characteristics of the motor of  FIG. 1 ; 
           [0014]      FIG. 3  is a graph illustrating operational ranges of a motor that has been started utilizing kick-start; 
           [0015]      FIG. 4  illustrates a kick-start operation in accordance with the principles of the invention; and 
           [0016]      FIG. 5  illustrates a motor controller and fan in accordance with the principles of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  illustrates in graphical form the start-up characteristics of a dc motor of the type to which the present invention may be advantageously applied. It is a characteristic of such a motor that starting the motor from standstill is a demanding and complex process. At the instant of switching all the energy necessary to magnetize the motor, to provide the acceleration force, and to supply the kinetic energy of the rotor and load, must be present together with the energy to overcome the mechanical and electrical losses. The net effect of all the forces that must be overcome is that the motor does not begin to rotate until a predetermined drive level is applied to the motor-in this instance the minimum drive level to start the motor is 35% of full drive. Coincident with that predetermined drive level is that the minimum starting motor speed is a predetermined speed that in the illustrative embodiment is 35% of full speed. The typical operational speed range for a fan motor is from the minimum start up speed to 100% speed. 
         [0018]    To assure that motor start-up is not delayed, it is common to utilize a kick-start” by applying full drive power to the motor for a predetermined fixed time period, for example, one second. As shown in  FIG. 2 , when full power is applied to the motor the, the motor speeds up to maximum speed. From the maximum speed, after the predetermined fixed time period, the motor speed is reduced to the desired motor speed. 
         [0019]    Advantageously, utilization of kick-start extends the useful low speed range from a typically minimum speed of 35% of maximum speed to a lower minimum speed after start up to, for example, 10% of maximum speed as shown in  FIG. 3 . Thus the use of kick-start allows fan system operation down to the minimum run speed thereby increasing the operating speed range. The lower speed operability allows for reduced acoustic noise. 
         [0020]    However, the existing kick-start approach results in the motor reaching maximum speed before the kick-start time is completed. The result is that excessive motor and fan noise occur during kick-start. In addition, the motor speed drastically overshoots the desired final speed. 
         [0021]    In accordance with the principles of the invention, when a motor is started, full power is applied to the motor as in the prior kick-start approach. However, rotor movement is monitored. When rotor movement is detected, the power is reduced immediately to the desired power level corresponding to the desired motor speed as shown in  FIG. 4 . 
         [0022]    Rotor movement is detected in the illustrative embodiment by a Hall effect sensor signal transition. Upon detection of rotor movement the power applied to the motor immediately is changed from full power to a desired lower level to maintain a desired speed. 
         [0023]    The motor reaches a desired speed before the kick-start fixed time period of the prior art arrangement is completed since the time is variable and dependent upon rotor movement. Advantageously, this eliminates excessive motor noise during kick-start and further, the motor speed most always does not overshoot the required final speed. 
         [0024]    The illustrative embodiment of the invention is a monolithic brushless DC motor controller  500  that provides functions for implementing fan speed control shown in  FIG. 5 . 
         [0025]    Controller  500  for speed control of motor  520  includes control and PWM logic  501 , commutation logic  503  for proper drive sequencing, motor drive  505 , current limiter  507 , and a programmable fault timer with time delayed restart and a power down low current mode block  509 . 
         [0026]    Controller  500 , fully integrated on a single chip  502  contains all required functions for implementing fan speed control. Control and PWM logic  501  comprises a fixed frequency oscillator, comparator, and a latch along with associated gates for motor speed control of motor  520 . Controller  500  also includes commutation logic  503  for proper drive sequencing, on-chip power MOSFETs for direct motor drive, cycle-by-cycle current limiting circuit, and circuit block  509  providing a programmable fault timer and a power down low current mode. 
         [0027]    Motor drive  505  may include on-chip power MOSFETs for direct motor drive. Alternatively, motor drive  505  and/or the power MOSFETs may reside off of chip  502  in another embodiment of the invention. 
         [0028]    Motor  520  includes a rotor and stator windings. A rotor sensor is provided with motor  520 . In a typical motor fan arrangement, a Hall effect device sensor  521  is utilized. Motor  520  includes connections Ø 1 , Ø 2 , a sensor output HALL, power and ground connections. 
         [0029]    Controller  500  utilizes pulse width modulation to provide an energy efficient means for controlling the motor speed of fan motor  520  by varying the average applied voltage to each stator winding during the commutation sequence. 
         [0030]    Control of motor  520  is accomplished by applying a control signal indicative of a desired motor speed at the control input  525 . 
         [0031]    Commutation logic  503  includes a rotor position decoder coupled to Hall sensor  521 . The rotor position decoder provides proper sequencing of the Phase 1, φ1, and Phase 2, φ2 drive outputs. Commutation logic  503  provides an output signal for monitoring the motor speed at output Tach. 
         [0032]    Motor start logic  509  includes a kick-start circuit  511  that provides for applying maximum power to motor  520  when the fan motor is to be started. Kick-start circuit  511  determines that motor  520  has been in an off state and that a control signal applied to the control input  525  of controller  500  has been received that is indicative of a non-zero motor speed. Kick-start circuit  511  applies maximum power to motor  520  and also enables start detect logic  513 . Start detect logic  513  monitors Hall effect sensor  521  and when it detects rotation, causes the kick-start circuit to be overridden and the control signal at the control input  525  that indicates the desired or indicated motor speed to control the fan motor. 
         [0033]    The invention has been described in conjunction with a specific illustrative embodiment. It will be understood by those skilled in the art that various changes, substitutions and modifications may be made without departing from the spirit or scope of the invention. It is intended that all such changes, substitutions and modifications be included in the scope of the invention. It is not intended that the invention be limited to the illustrative embodiment shown and described herein. It is intended that the invention be limited only by the claims appended hereto, giving the claims the broadest possible scope and coverage permitted under the law.