Patent Publication Number: US-2003234627-A1

Title: Motor with dynamic current draw

Description:
PRIORITY  
     [0001] This patent application claims priority from provisional U.S. patent application No. 60/390,261, filed Jun. 20, 2002, entitled, “MOTOR WITH DYNAMIC CURRENT DRAW,” and naming Mark Reinhold, Kenneth Hoffmann, Frank Cettina, and Steve Miller as inventors, the disclosure of which is incorporated herein, in its entirety, by reference. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] This invention relates generally to electric motors and, more particularly, this invention relates to devices and methods of controlling current draw in electric motors.  
       BACKGROUND OF THE INVENTION  
       [0003] In simplified terms, electric motors (e.g., DC electric motors) have a rotating portion (“rotor”) rotatably secured to a stationary portion (“stator”) that controls rotor rotation. More specifically, the rotor in various types of DC electric motors has a magnet that interacts with a fluctuating magnetic field produced by the stator. This interaction causes the rotor to rotate at a speed controlled by the stator. To produce the fluctuating magnetic field, the stator typically includes a metallic stator core, which is made up of a plurality of stacked metal laminations, a coil wrapped around the stator core, and circuitry for selectively energizing the coil. The circuitry detects the magnetic field produced by the magnet within the rotor and thus, selectively energizes the coil to provide the rotating energy.  
       [0004] Heat produced by motors operating at relatively high power levels undesirably can cause motor failure (e.g., the circuitry can overheat or the coils can fail). Consequently, electric motors typically are rated to operate at a specified power level (e.g., plus or minus ten percent). To ensure that they do not significantly exceed their rated power levels, motors typically have a current limiting circuit that permits no more than a pre-set current to be drawn. This current commonly is set to a value that, when drawn with expected input voltages, should not exceed the rated power level of the motor.  
       [0005] Motor speed and torque of a motor, however, both are a function of its current and thus, are limited by the pre-set current value. Undesirably, this pre-set current often is not high enough to permit the motor to operate at the rated power. Consequently, such a motor generally operates at a speed that is less than the speed it would operate if it received its rated power.  
       SUMMARY OF THE INVENTION  
       [0006] In accordance with one aspect of the invention, to improve performance, a motor varies its current draw based upon its input voltage. To that end, the motor has an input for receiving an input voltage, and a current controller operatively coupled with the input. The current controller is capable of detecting the current drawn by the motor. In addition, the current controller is capable of changing the current draw as a function of the input voltage.  
       [0007] The motor may have a rated power value, where the current controller is capable of controlling the current draw as a function of the rated power. In some embodiments, the current controller is capable of changing the current draw in an inversely proportional manner to the input voltage. Among other things, the current controller may include a pulse width modulator to control current draw.  
       [0008] In some embodiments, the current controller includes a programmable element capable of executing program code. The motor also may have a stator and a rotor that is rotatably attached to the stator. The rotor speed is controlled by the current controller. The motor also may have a coil. The current draw thus may be a function of at least one characteristic of the coil. The motor also may have voltage sensor that is 1) capable of measuring the input voltage and 2) in electrical communication with the current controller.  
       [0009] In accordance with another aspect of the invention, an apparatus and method for controlling motor speed receives an input voltage that powers the motor, and calculates the power produced by the input voltage. The power produced by the input voltage then is compared to a given power, and the current drawn by the motor is controlled as a function of this comparison. The power produced by the input voltage is a function of the current drawn by the motor.  
       [0010] The current drawn may be increased if the power produced is less than the given power. Conversely, the current drawn may be decreased if the power produced is greater than the given power. The current draw may be controlled by pulse width modulating the input voltage. In some embodiments, the given voltage is preprogrammed into a programmable element that controls the current drawn. The programmable element illustratively is capable of executing program code. In some embodiments, the value of the input voltage may be calculated.  
       [0011] Illustrative embodiments of the invention are implemented as a computer program product having a computer usable medium with computer readable program code thereon. The computer readable code may be read and utilized by a computer system in accordance with conventional processes.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0012] The foregoing and advantages of the invention will be appreciated more fully from the following further description thereof with reference to the accompanying drawings wherein:  
     [0013]FIG. 1 schematically shows an exemplary motor that may be implemented to incorporate illustrative embodiments of the invention.  
     [0014]FIG. 2 schematically shows a circuit diagram of a stator circuit that may be used in the motor shown in FIG. 1.  
     [0015]FIG. 3 shows a process of dynamically modifying current drawn by the coil shown in the circuit of FIG. 2 in accordance with illustrative embodiments of the invention. 
    
    
     DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS  
     [0016] In illustrative embodiments of the invention, an electric DC motor is configured to dynamically vary its current draw as a function of its input voltage and rated power. For example, if the input voltage increases, then embodiments of the invention decrease the current draw to maintain a substantially constant power. Conversely, if the input voltage decreases, then embodiments of the invention increase the current draw for the same purposes. The actual power of the motor thus should remain substantially at the rated power, consequently permitting maximum torque and speed. Details of illustrative embodiments are discussed below.  
     [0017]FIG. 1 schematically shows a motor that may implement illustrative embodiments of the invention. In particular, the motor  10  includes a propeller  12  and thus, is a part of a cooling fan. To that end, the motor  10  includes a housing  14  with venturi (not shown), a stator portion  18  secured to the housing  14 , and a rotor  20 , which includes the propeller  12 . It should be noted that although the motor  10  is implemented as a fan, illustrative embodiments apply to other motor applications. Accordingly, description of the motor  10  as a fan is by illustration only and not intended to limit various embodiments of the invention.  
     [0018] The stator  18  includes a stator core  22 , a molded insulation layer  24  on the stator core  22  (e.g., see co-pending U.S. patent application No. 10/078,648, the disclosure of which is incorporated herein by reference), a coil  26  wrapped about the stator core and the insulation layer  24 , and a circuit board  28  having electronics for controlling the energization of the coil  26  (discussed in greater detail below with reference to FIGS. 2 and 3). The insulation layer  24  includes an arbor  29  extending through a central tubular opening of the stator core  22 . Bearings  31  are secured within the arbor  29  for receiving a rotor shaft (discussed below).  
     [0019] The rotor  20  includes a steel cup (not shown) for supporting the propeller  12 , an annular permanent magnet circumscribing the interior of the steel cup, and a shaft  32  extending from the center of the steel cup. When assembled, the shaft  32  is received by the bearings  31  secured within the arbor  29 .  
     [0020]FIG. 2 schematically shows a stator commutation circuit (“stator circuit  34 ”) implementing illustrative embodiments of the invention. The stator circuit  34 , which is located on the circuit board  28 , includes a plurality of circuit elements that effectuate the underlying commutation function in the manner summarized above. Namely, the stator circuit  34  varies current draw to maintain motor power at a substantially constant level, consequently optimizing motor performance.  
     [0021] To these ends, the stator circuit  34  includes an input  36  for receiving an input voltage that energizes the entire circuit, and a Hall Effect sensor  38  to detect the magnetic field produced by the rotating rotor  20 . In addition, the stator circuit  34  also includes the above noted coil  26 , which is selectively energized by four switches. In illustrative embodiments, the switches are metal oxide semiconductor field effect transistors (MOSFETS) and identified in FIG. 2 as first switch Q 1 , second switch Q 2 , third switch Q 3 , and fourth switch Q 4 .  
     [0022] In accordance with illustrative embodiments, the stator circuit  34  also includes a microprocessor  40  for controlling the switches Q 1 -Q 4  in a manner that controls the current drawn by the coil  26 . The microprocessor  40  illustratively may be a model number PIC16C712 processor, distributed by Microchip Technology Inc. of Chandler, Ariz. In the embodiment shown in FIGS. 2 and 3, the microprocessor  40  sets an effective voltage across the coil  26  in accordance with conventional pulse width modulation techniques. In alternative embodiments, the effective voltage across the coil  26  can be set in other known manners. Moreover, as known by those skilled in the art, a set of software instructions control operation of the microprocessor  40  to implement illustrative embodiments of the invention. Details of executed microprocessor processes are discussed below with reference to FIG. 3.  
     [0023] The microprocessor  40  includes a current sense input  42  to monitor the current drawn by the coil  26 , and a voltage sense input  44  to monitor the input voltage. The voltage and current sensed at these inputs  42  and  44  are used to calculate the actual power being consumed by the motor (discussed below). The microprocessor  40  also includes a plurality of outputs to controllably deliver open and close signals to each of the switches Q 1 -Q 4 . In particular, the microprocessor  40  includes a first high side port  46  to control the first switch Q 1 , and a second high side port  48  to control the second switch Q 2 . The microprocessor  40  also includes first and second low side ports  49  and  50  to control the third switch Q 3 , and a third low side port  52  that, with the second low side port  50 , controls the fourth switch Q 4 . Details of the interaction of each of these circuit components (i.e., the microprocessor  40 , the four switches Q 1 -Q 4 , the coil  26 , the voltage sense input  44 , and the current sense input  42 ) are discussed below.  
     [0024] Specifically, the lower left corner of FIG. 2 shows a voltage input circuit  54  for producing the voltage that energizes the stator circuit  34 . To that end, the voltage input circuit  54  has that prior noted input  36  for receiving an incoming DC voltage from an external source (e.g., 48 volts DC), a first stage  58  to produce the DC voltage identified as +V (i.e., powering, among other things, the first through fourth switches and the coil  26 ), and a second stage  60  to generate 5.0 volts for energizing the microprocessor  40  and other portions of the stator circuit  34 .  
     [0025] The first stage  58  includes a fuse  62  to protect against current surges, a reverse polarity diode D 1 , and a capacitor C 1 . The voltage identified as +V is distributed about the stator circuit  34  by connecting to all locations in FIG. 2 having that same symbol. As discussed below, the voltage +V is the voltage used to determine motor power. Its value is a function of the prior noted incoming voltage, which can fluctuate.  
     [0026] The second stage  60  includes conventional regulation components, such as a coarse voltage regulation component (i.e., the transistor Q 5  and Zener diode D 2 ) and a regulator application specific integrated circuit  64 . In a manner similar to the +V voltage, the node having the +5 V symbol is connected to all locations in FIG. 2 having that +5 V symbol.  
     [0027] As noted above, the microprocessor  40  controls the first through fourth switches Q 1 -Q 4  to selectively energize the coil  26 . To these ends, the switches Q 1 -Q 4  are configured in an H-bridge configuration. The first and second switches Q 1  and Q 2  are considered to be “high side switches,” while the third and fourth switches Q 3  and Q 4  are considered to be “low side switches.” In a manner consistent with other H-bridge configurations, during each on/off cycle, one high side switch is on while one low side switch is on. The other two switches are off. To provide a current path and voltage, the two switches that are on are located diagonally across the coil  26 .  
     [0028] In illustrative embodiments, the first and second switches Q 1  and Q 2  are P-channel MOSFETS, while the third and fourth switches Q 3  and Q 4  are N-channel MOSFETS. Accordingly, the source nodes of the first and second switches Q 1  and Q 2  are connected to the +V terminal of the voltage input circuit  54 , while the source nodes of the third and fourth switches Q 3  and Q 4  are connected together. In addition, the drain nodes of the first and third switches Q 1  and Q 3  are connected, while the drain nodes of the second and fourth switches Q 2  and Q 4  are connected. The connected sources of the third and fourth switches Q 3  and Q 4  are coupled to ground via a current sense resistor R 1  (discussed below).  
     [0029] The microprocessor  40  controls the frequency and duty cycles of the four switches Q 1 -Q 4  by means of the above noted five high side and low side ports  46 ,  48 ,  49 ,  50 , and  52 . The first high side port  46  controls the first switch Q 1  by selectively providing a turn-on signal to a first level shift transistor Q 7 . Specifically, the emitter node of the first level shift transistor Q 7  is coupled to ground, while its base node is coupled to the first high side port  46 (via a resistor R 2 ). In addition, the collector node of the first level shift transistor Q 7  is coupled to the gate of the first switch Q 1  (also via a resistor R 3 ). Consequently, application of a turn on voltage to the resistor R 2  produces a current that turns on the first level shift transistor Q 7 . When on, the transistor Q 7  effectively connects the gate node of the first switch Q 1  to a voltage that is below that of the first switch source node. In other words, the gate node voltage of the first switch Q 1  is less than its source node voltage. Because it is a P-channel MOSFET, this voltage relationship causes the first switch Q 1  to turn on.  
     [0030] The second switch Q 2  operates in a similar manner to that discussed with regard to the first switch Q 1 . Accordingly, the second switch Q 2  also has an associated second level shift transistor Q 8  and is selectively energized by the second high side port  48  on the microprocessor  40 .  
     [0031] The first, second, and third low side ports  49 ,  50 , and  52  of the microprocessor  40  respectively control the third and fourth switches Q 3  and Q 4 . The first and third low side ports  49  and  52  control which one of the third or fourth switches Q 3  and Q 4  is to be on during a given time in the commutation cycle, while the second low side port  50  pulse width modulates the switch that is on (based upon the signals delivered by the first and third low side ports  49  and  52 ) in a manner that controls current draw as a function of power. Specifically, the gate node of the third switch Q 3  is coupled with a conventional AND gate  76  that receives its input voltages from the first low side port  49  and the second low side port  50 . In addition, as noted above, the source node of the third switch Q 3  (and fourth switch Q 4 ) is coupled to ground via the current sense resistor R 1 . Accordingly, because it is an N-channel MOSFET, application of a positive voltage to its gate (by its attached AND gates  76 ) causes the third switch Q 3  to begin conducting current (i.e., causing it to turn on).  
     [0032] When the third switch Q 3  is to be on, the AND gate  76  input from the first low side port  49  is high (i.e., logic level “one”) while the AND gate  76  input from the second low side port  50  is pulse width modulated to control current flow through the coil  26 . This causes the gate node of the third switch Q 3  to receive a pulse width modulated voltage. As a result, the third switch Q 3  turns on and off in a manner that is controlled by the frequency and duty cycle specified by the second low side port  50 . While the third switch Q 3  is being modulated, the fourth switch Q 4  is off.  
     [0033] The fourth switch Q 4  operates in a substantially identical manner to that of the third switch Q 3 . Accordingly, the fourth switch Q 4  also has a corresponding AND gate  76  that receives input from the second low side port  50  and the third low side port  52 .  
     [0034] When the third switch Q 3  is modulated to be on, current flows from the +V node of the regulator, through the second switch Q 2 , through the coil  26 , and then through the third switch Q 3 . The current continues to flow through the current sense resistor R 1  to ground. Conversely, when the third switch Q 3  is modulated off (i.e., by pulse width modulation signals from the second low side port  50 ), current flow through the coil  26  is interrupted. Accordingly, current flow generated by the second switch Q 2  is transmitted back to its source via a recirculating diode (not shown). Note that the fourth switch Q 4  operates in a corresponding manner in concert with the first switch Q 1 .  
     [0035] As noted above, the current flowing through either of the low side switches Q 3  or Q 4  necessarily flows through the current sense resistor R 1 . This current flow generates a relatively low voltage, which is amplified and then fed to the current sense input  42  of the microprocessor  40 . To that end, the stator circuit  34  has a current sense circuit  78  that includes both the noted current sense resistor R 1  and a smoothing circuit  80  (comprising a resistor R 4  and a capacitor C 2 ) for generating a filtered average voltage of the voltage through the current sense resistor R 1 . This filtered average voltage is forwarded to an operational amplifier  82 , which amplifies the filtered average voltage to produce an amplified voltage. The amplified voltage then is forwarded to the current sense input  42  of the microprocessor  40 . The microprocessor  40  includes an internal analog to digital converter that converts the amplified voltage to a digital value for processing as discussed herein. This digital value represents the current through the coil  26 .  
     [0036] As noted above and shown in FIG. 2, the microprocessor  40  includes a plurality of other ports. Among those is the above noted voltage sense input  44 , which detects the input voltage +V, a pair of Hall sense ports  84  to detect output from the Hall sensor  38 , an external PWM input  86  to permit external control of the motor  10  with some external PWM means, and an external clock port  88 . In addition, the microprocessor  40  also includes ports to receive 5.0 volts from the voltage input circuit  54 , and a ground port  90 .  
     [0037] As noted above, the microprocessor  40  modulates the two low side switches Q 3  and Q 4  to control the effective voltage across the coil  26 . In so doing, the microprocessor  40  effectively controls the current draw/flow through the coil  26 . Accordingly, consistent with goals of various embodiments, if the input voltage (e.g., +V, which is a function of the incoming voltage to the regulator) increases, then the microprocessor  40  provides modulation pulses to the relevant low side switch(es) Q 3  or Q 4  at appropriate times to decrease current flow through the coil  26 . In a corresponding manner, if the input voltage decreases, the microprocessor  40  provides the appropriate modulation pulses to the relevant low side switch(es) Q 3  or Q 4  to increase current flow through the coil  26 . These modulation pulses are selected with an appropriate frequency and duty cycle to maintain motor operation substantially at the rated operating power.  
     [0038]FIG. 3 shows an illustrative process (executed by the microprocessor  40 ) for controlling the current draw of the coil  26  based upon the rated power and thus, the input voltage. As noted above, in the embodiments shown in FIGS.  1 - 3 , the process is implemented as a series of computer instructions in any conventional programming language. Among other benefits, use of the microprocessor  40  and accompanying software code simplifies the design and modification of various system parameters (e.g., rated power of the motor  10 ).  
     [0039] The process begins at  300 , in which the microprocessor  40  measures the input voltage +V via its voltage sense input  44 . As noted above, the input voltage +V is a function of the incoming voltage. Accordingly, in the embodiment shown in FIG. 2, the input voltage +V effectively is the incoming voltage received at the voltage input circuit  54 . Its magnitude may be lower by about a diode voltage drop, but its other characteristics are substantially the same as the incoming voltage.  
     [0040] Either simultaneously, before, or after the input voltage is measured, the current draw is measured (step  302 ). Accordingly, the current through the current sense resistor R 1  is measured at the current sense input  42 . To that end, the microprocessor  40  may access a look up table to convert the voltage received by the current sense port to a current value. This current value is used in later steps of the process to calculate power. It should be noted that the order that these measurements are made should not have a significant effect on the overall results of the process.  
     [0041] The microprocessor  40  then calculates the power by multiplying the incoming voltage value with the current draw value (step  304 ). This calculated power, which represents the actual power consumed by the motor  10 , subsequently is compared against the rated power to determine if the current draw should be modified (step  306 ).  
     [0042] If the calculated power is equal to the rated power, then the current draw is not modified (step  308 ). In such case, the process returns to step  300 . If the calculated power is less than the rated power, however, then the process continues to step  310 , in which the microprocessor  40  causes the coil  26  to increase the current draw. To that end, the microprocessor  40  increases the duty cycle of the pulses modulated to the relevant low side switch Q 3  and/or Q 4 . In other words, the microprocessor  40  modulates the relevant low side switch Q 3  or Q 4  in a manner that increases current flow through the coil  26 . This increased current flow consequently increases motor torque and speed. The process then loops back to step  300 , thus repeating the process.  
     [0043] Conversely, at step  306 , if the calculated power is greater than the power limit, then the process continues to step  312 , in which microprocessor  40  causes the coil  26  to decrease the current draw. To that end, the microprocessor  40  decreases the duty cycle of its pulse width modulated pulses via low side port  50 . In other words, the microprocessor  40  modulates the relevant low side switch Q 3  or Q 4  in a manner that decreases current flow through the coil  26 .  
     [0044] In illustrative embodiments, the duty cycle is increased (step  310 ) or decreased (step  312 ) by a preselected amount that is preprogrammed into the microprocessor  40 . Accordingly, a change in the input voltage should produce a commensurate number of iterations of the process shown in FIG. 2 to cause the current draw to reach a desired value. As a result, the actual power of the motor  10  should be maintained at a substantially constant level (i.e., substantially at the rated power).  
     [0045] By way of example, if a given motor is rated to operate at 75 watts and the input voltage is 50 volts, then the microprocessor  40  dynamically selects the current draw to be 1.5 amperes. If the input voltage of such motor increases to 60 volts, then the microprocessor  40  lowers the current draw to 1.25 amperes, thus maintaining the rated 75 watt power. Maintaining the current in this manner maximizes torque and speed without exceeding the rated power.  
     [0046] In some embodiments, the rated power also is dynamically set at run time. For example, the microprocessor  40  may monitor the temperature of the coil  26 , and then calculate a rated power based on such measured temperature. The microprocessor  40  thus may use this dynamically calculated rated power to calculate the current draw.  
     [0047] Instead of being a single power value, the rated power being compared against in step  306  may be a range of values around a specified rated power. For example, the range may be within five percent of the rated power. Accordingly, if the calculated power is within the range, then the current draw is not changed.  
     [0048] In illustrative embodiments, the motor  10  is implemented as a cooling fan used to cool telecommunications equipment. Accordingly, the motor  10  is configured to nominally receive a 48 volt input voltage. Of course, illustrative embodiments can be used across a range of applications and technologies. Discussion of the motor  10  as a fan used in a telecommunications applications thus is exemplary and not intended to limit the scope of the invention.  
     [0049] It also should be noted that a motor with an H-bridge configuration was discussed for exemplary purposes only. Accordingly, other types of motors may be used with illustrative embodiment of the invention. For example, three phase and bifilar type motor arrangements also may be used. Discussion of an H-bridge type motor thus is not intended to limit the scope of all embodiments of the invention.  
     [0050] Various embodiments of the invention may be implemented at least in part in any conventional computer programming language. For example, some embodiments may be implemented in a procedural programming language (e.g., “C”), or in an object oriented programming language (e.g., “C++”). Other embodiments of the invention may be implemented as preprogrammed hardware elements (e.g., application specific integrated circuits, FPGAs, and digital signal processors), or other related components.  
     [0051] In an alternative embodiment, the disclosed apparatus and method may be implemented as a computer program product for use with a computer system. Such implementation may include a series of computer instructions fixed either on a tangible medium, such as a computer readable medium (e.g., a diskette, CD-ROM, ROM, or fixed disk) or transmittable to a computer system, via a modem or other interface device, such as a communications adapter connected to a network over a medium. The medium may be either a tangible medium (e.g., optical or analog communications lines) or a medium implemented with wireless techniques (e.g., microwave, infrared or other transmission techniques). The series of computer instructions can embody all or part of the functionality previously described herein with respect to the system.  
     [0052] Those skilled in the art should appreciate that such computer instructions can be written in a number of programming languages for use with many computer architectures or operating systems. Furthermore, such instructions may be stored in any memory device, such as semiconductor, magnetic, optical or other memory devices, and may be transmitted using any communications technology, such as optical, infrared, microwave, or other transmission technologies.  
     [0053] Such a computer program product may be distributed as a removable medium with accompanying printed or electronic documentation (e.g., shrink wrapped software), preloaded with a computer system (e.g., on system ROM or fixed disk), or distributed from a server or electronic bulletin board over the network (e.g., the Internet or World Wide Web). Of course, some embodiments of the invention may be implemented as a combination of both software (e.g., a computer program product) and hardware. Still other embodiments of the invention are implemented as entirely hardware, or entirely software (e.g., a computer program product).  
     [0054] Although various exemplary embodiments of the invention have been disclosed, it should be apparent to those skilled in the art that various changes and modifications can be made that will achieve some of the advantages of the invention without departing from the true scope of the invention.