Abstract:
A motor controller for an electric motor having a stator and a rotor. The motor controller includes a power input for receiving AC power from a power source; a control input for receiving a control signal from a control; and circuitry for switching power from the power source to the electric motor in response to the control signal. The circuitry is operable to: apply a braking waveform to the stator while the rotor is rotating; monitor a reactive power of the stator; detect an increase in the reactive power of the stator to determine the rotor has substantially stopped rotating; and remove the braking waveform from the stator in response to detecting the increase in the reactive power.

Description:
RELATED APPLICATION 
     This is a continuation of U.S. application Ser. No. 13/657,335, filed Oct. 22, 2014, entitled SYSTEMS AND METHODS FOR BRAKING AN ELECTRIC MOTOR which is hereby incorporated by reference into the present application in its entirety. 
    
    
     FIELD 
     The present disclosure relates to systems and methods for braking electric motors. 
     BACKGROUND 
     Electric motors commonly include a stationary component called a stator and a rotating component called a rotor. The rotor rotates within (or around) the stator when the motor is energized with a driving waveform. When the driving waveform is removed from the motor, the rotor may coast to a standstill over time due to the inertia of the rotor and anything coupled to the rotor. 
     In many motor applications, it is desirable to stop rotation of the rotor as soon as the driving waveform is removed from the motor. For example, in washing machine applications, it is desirable to stop rotation of the washing machine motor after a high speed spin cycle so that the washing machine may be unloaded or switched to a slower speed wash or rinse cycle. 
     Accordingly, various techniques have been developed for braking electric motors. One such technique uses brake pads, pulleys, and/or other friction braking systems. Unfortunately, friction brakes add cost to a motor and are therefore not desirable for low cost applications such as washing machines. Friction brakes also eventually wear out with use and require repair or replacement. 
     Thus, many motor applications employ electric braking systems rather than friction brakes. One type of electric braking system employs regenerative braking technology. Although such technology is effective and energy efficient, it is far too complicated and expensive for lower cost applications such as washing machine motors. Another type of electric braking system is DC injection braking in which a direct current (DC) voltage is applied to a motor&#39;s stator windings to brake the rotor. The DC voltage creates a stationary magnetic field which applies a static torque to the rotor. This slows and eventually halts rotation of the rotor. As long as the DC voltage remains on the stator windings, the rotor is held in position and resists rotation. DC injection braking is relatively simple, cost-effective, and maintenance free and is therefore a popular choice of braking for many motor applications; however, it has not been used effectively in some applications as described below. 
     It is also often desirable to determine when a motor&#39;s rotor has stopped rotating so the rotor can be driven in the opposite direction, at a different speed, etc. This can be accomplished with a motor shaft sensor such as a Hall effect sensor, but such sensors increase the cost and complexity of motors and are therefore not desirable for many lower cost applications such as washing machine motors. 
     Thus, sensorless techniques for determining motor speed have been developed. One type of sensorless speed detection employs various algorithms for estimating when a rotor stops based on measured electrical parameters. However, the measured electrical parameters, and thus the results of the algorithms, are less accurate when the motor is being braked with the above-described DC injection braking techniques. Thus, DC injection braking techniques generally require a motor shaft sensor. 
     Another sensorless technique to ensure a motor has stopped rotating is to simply have a time delay that must lapse after power is removed from the motor. A similar technique uses DC injection braking to slow the motor as described above and continues the DC injection until a time delay has lapsed. The first of these methods unfortunately wastes time between motor cycles because the time delay must account for the maximum possible coast time of a motor, and the second of these methods wastes energy and time because the DC injection braking must be maintained longer to account for the maximum possible breaking time even though the motor may in fact stop sooner. 
     The above section provides background information related to the present disclosure which is not necessarily prior art. 
     SUMMARY 
     Embodiments of the present invention solve the above described problems by providing improved systems and methods for braking electric motors and for determining when the rotors of the motors have stopped rotating. For example, embodiments of the invention provide systems and methods for quickly and efficiently braking an electric motor with DC injection braking without the use of a motor speed sensor and without requiring time delays. 
     A motor assembly constructed in accordance with an embodiment of the present invention may be used in a washing machine, HVAC system, pump system or any other application. The motor assembly broadly comprises an electric motor and a motor controller for powering and controlling the motor. The motor may be any type of motor and includes a stator and a rotor. In one embodiment, the motor may be a three phase AC induction motor. The motor controller powers and controls the motor and is programmed or otherwise configured to perform at least some of the methods described herein. In one embodiment, the motor controller is programmed or configured to apply a DC injection braking waveform to the stator of the motor while the rotor is rotating, monitor a reactive power of the stator, detect an increase in the reactive power of the stator to determine the rotor has substantially stopped rotating, and remove the applied braking waveform from the stator in response to detecting the increase in the reactive power. 
     In a related embodiment, the motor controller monitors both the reactive power and active power of the stator, determines a ratio between the reactive power and the active power to determine the power factor of the stator, and uses the power factor to determine when the rotor has substantially stopped rotating. In one embodiment, the motor controller determines the rotor has stopped rotating when the power factor is approximately  1 . Determining that the rotor has stopped by monitoring the power factor is advantageous because the power factor level that indicates the rotor has stopped is the same regardless of the magnitude of the DC injection braking waveform. 
     In another embodiment of the invention, the braking waveform is a controlled current waveform. The motor controller is programmed or configured in this embodiment to apply the controlled current braking waveform to the stator while the rotor is rotating, determine when the rotor has substantially stopped rotating, and remove the applied controlled current braking waveform from the stator in response to determining the rotor has substantially stopped rotating. 
     In yet another embodiment of the invention, the motor or the motor controller includes one or more current shunts. The motor controller is programmed or configured in this embodiment to apply a braking waveform to the stator while the stator is rotating, monitor a current through one of the current shunts while the braking waveform is applied to the stator, determine whether the rotor has substantially stopped rotating using the monitored current through the current shunt, and remove the applied braking waveform from the stator in response to determining the rotor has substantially stopped rotating. 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described in the detailed description below. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Other aspects and advantages of the present invention will be apparent from the following detailed description of the embodiments and the accompanying drawing figures. 
    
    
     
       DRAWINGS 
       Embodiments of the present invention are described in detail below with reference to the attached drawing figures, wherein: 
         FIG. 1  is a block diagram of a motor assembly constructed in accordance with an embodiment of the invention. 
         FIG. 2  is a schematic representation of the stator and rotor of the motor in the motor assembly of  FIG. 1 . 
         FIG. 3  is a graph of rotor speed over a time period according to one exemplary embodiment of the present disclosure. 
         FIG. 4  is a graph of stator reactive power over the same time period as  FIG. 3 . 
         FIG. 5  is a flow diagram depicting steps in a method of the invention and/or code segments of a computer program of the invention. 
         FIG. 6  is a flow diagram depicting steps in another method of the invention and/or code segments of another computer program of the invention. 
         FIG. 7  is a flow diagram depicting steps in yet another method of the invention and/or code segments of another computer program of the invention. 
     
    
    
     The drawing figures do not limit the present invention to the specific embodiments disclosed and described herein. The drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the invention. 
     DETAILED DESCRIPTION 
     The following detailed description of embodiments of the invention references the accompanying drawings. The embodiments are intended to describe aspects of the invention in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments can be utilized and changes can be made without departing from the scope of the claims. The following detailed description is, therefore, not to be taken in a limiting sense. The scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled. 
     In this description, references to “one embodiment”, “an embodiment”, or “embodiments” mean that the feature or features being referred to are included in at least one embodiment of the technology. Separate references to “one embodiment”, “an embodiment”, or “embodiments” in this description do not necessarily refer to the same embodiment and are also not mutually exclusive unless so stated and/or except as will be readily apparent to those skilled in the art from the description. For example, a feature, structure, act, etc. described in one embodiment may also be included in other embodiments, but is not necessarily included. Thus, the present technology can include a variety of combinations and/or integrations of the embodiments described herein. 
     Turning now to the drawing figures, and initially  FIG. 1 , a motor assembly  10  constructed in accordance with embodiments of the invention is illustrated. The motor assembly  10  may be used in any application, such as in a washing machine, HVAC system, pump system, or appliance. In one particular embodiment, the motor assembly  10  is designed for use with a horizontal axis, front-loading washing machine, but the principles of the present invention are equally applicable to all uses of the motor assembly. 
     An embodiment of the motor assembly  10  broadly includes an electric motor  12  and a motor controller  14 . The motor assembly  10  may also include or be coupled with other components not relevant to the present invention. 
     As best illustrated in  FIG. 2 , the electric motor  12  includes a rotor  16  and a stator  18 . The motor  12  may operate on direct current (DC) or alternating current (AC), may be synchronous or asynchronous, and may be single phase or three phase. The motor  12  may be of any type, including but not limited to, a brushed DC motor, a switched reluctance motor, a coreless or ironless DC motor, a series wound universal motor, an induction motor, a torque motor, or a stepper motor. Moreover, the motor may be fixed speed, multi-speed, or variable speed and may have any horsepower (HP) rating. In one particular embodiment of the invention, the motor  12  is a ⅓-1 HP, three phase, reversible and variable speed switched reluctance type motor. Such a motor provides maximum drive performance at a competitive price for washing machine applications which require a wide range of operating speeds and a high start-up torque. However, the principles of the present invention are not limited to any particular motor type, technology, or size. 
     The motor controller  14  provides power to and controls operation of the electric motor  12  and is programmed or otherwise configured to perform one or more of the function or methods described below. The motor controller  14  may include any combination of circuitry, hardware, firmware, and/or software. In one particular embodiment, the motor controller  14  includes a custom application specific integrated circuit (ASIC) with a microprocessor that controls and drives a 3-phase inverter and various other electronic components. 
     As shown in  FIG. 1 , the motor controller  14  may receive power from a single phase AC supply voltage at 115 VAC supplied by connections L 1  and N, where L 1  represents the “hot” side of the AC supply and N represents neutral, which is typically at earth potential. The AC supply voltage may also be 230 VAC, in which case the neutral line would be replaced with another hot supply line. The AC supply voltage may also be three phase 480 VAC. 
     The motor controller  14  may receive commands or operating instructions from one or more controls  20  such as a keypad, switches, or buttons as are commonly found on washing machines and other appliances and devices. The controls may be one or more separate components or may be integrated in the motor controller  14 . 
     The motor controller  14  may also be coupled to a single current shunt  22  for determining a stator current as described below. This single current shunt  22  may be a discrete component coupled to the printed circuit board of the motor controller ASIC or may be incorporated in the stator of the motor or the inverter module of the motor controller. 
     Aspects of the invention may be implemented with one or more computer programs stored in or on computer-readable medium residing on or accessible by the microprocessor of the motor controller  14 . Each computer program preferably comprises an ordered listing of executable instructions for implementing logical functions in the motor controller  14 . Each computer program can be embodied in any non-transitory computer-readable medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device, and execute the instructions. In the context of this application, a “computer-readable medium” can be any non-transitory means that can store the program for use by or in connection with the instruction execution system, apparatus, or device. The computer-readable medium can be, for example, but not limited to, an electronic, magnetic, optical, electro-magnetic, infrared, or semi-conductor system, apparatus, or device. More specific, although not inclusive, examples of the computer-readable medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a random access memory (RAM), a read-only memory (ROM), an erasable, programmable, read-only memory (EPROM or Flash memory), an optical fiber, and a portable compact disk read-only memory (CDROM). 
     According to one aspect of this disclosure, the motor controller  14  is programmed or otherwise configured to implement a method  500  of braking the electric motor  12 . The method broadly includes the steps of energizing the stator  18  of the motor  12 , with a braking waveform determining a reactive power of the stator  18 , detecting an increase in the determined reactive power of the stator  18 , and removing the braking waveform from the stator  18  in response to detecting the increase in the determined reactive power of the stator. The increase in reactive power indicates that the rotor  16  has substantially stopped rotating. 
     In a related embodiment, the motor controller monitors both the reactive power and active power of the stator, determines a ratio between the reactive power and the active power to determine the power factor of the stator, and uses the power factor to determine when the rotor has substantially stopped rotating. In one embodiment, the motor controller determines the rotor has stopped rotating when the power factor is approximately 1. Determining that the rotor has stopped by monitoring the power factor is advantageous because the power factor level that indicates the rotor has stopped is the same regardless of the magnitude of the DC injection braking waveform. 
     The flow chart of  FIG. 5  shows the functionality and operation of a preferred implementation of the above described method  500  in more detail. In this regard, some of the blocks of the flow chart may represent the method  500  and/or a module segment or portion of code of the computer programs of the present invention. In some alternative implementations, the functions noted in the various blocks may occur out of the order depicted in  FIG. 5 . For example, two blocks shown in succession in  FIG. 5  may in fact be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order depending upon the functionality involved. 
     The method  500  begins while the rotor  16  of the motor  12  is moving as depicted in box  502 . For example, in the embodiment in which the motor assembly  10  is used in a washing machine, the motor  12  may be operating at a high RPM during a spin cycle. 
     While or shortly after the motor  12  is de-energized and begins to coast, the motor controller  14  may receive or internally process a brake signal as depicted in box  504 . For example, the motor controller  14  may receive a signal from an internal or external timing circuit that indicates a spin cycle or other motor cycle has ended and that the rotor should be slowed or stopped. 
     In response to the brake signal or other indication that the motor  12  has been de-energized, the motor controller  14  energizes the stator  18  to apply a braking force to the rotor in order to brake the rotor as depicted in box  506 . The stator  18  may be energized by driving the stator with a controlled current waveform as described in more detail below in connection with the method  600 . Alternatively, the stator  18  may be driven with a voltage controlled waveform. 
     The motor controller  14  then determines a reactive power of the stator  18  as depicted in box  508 . The reactive power may be determined in any conventional manner. For example, the reactive power may be determined based on detecting current through the current shunt  22  as described in more detail below in connection with the method  700 . The reactive power may also be determined through multiple current shunts, etc. 
     The motor controller  14  then detects an increase in the reactive power of the stator  18  as depicted in box  510 . The increase in the reactive power indicates the rotor  16  has substantially stopped rotating. As a part of this step, the motor controller may detect an increase in the magnitude of the stator reactive power. Alternatively, the motor controller may detect an increase in a ratio of the determined reactive power and a determined active power of the stator to determine the rotor has stopped rotating. 
       FIGS. 3 and 4  show graphs that illustrate a specific embodiment of the method  500 .  FIG. 3  shows a motor&#39;s rotor speed over a time period, and  FIG. 4  shows the stator&#39;s reactive power over the same time period. During a first time period  100 , the rotor  16  rotates and the stator  18  reactive power is zero. Braking begins at the beginning of a second time period  102  as the stator  18  is energized with a braking waveform, causing the rotor  16  to slow down. At the end of the second time period  102 , the rotor  16  approaches zero speed and the stator reactive power increases. The motor controller  14  detects the increase in stator reactive power, and at the end of a third period  104 , the motor controller  14  removes the braking waveform from the stator in response to the detected increase in stator reactive power. 
     The example shown in  FIGS. 3 and 4  may be altered without departing from the scope of the present invention. For example, the stator reactive power may be greater than zero during the first period  100 . Also, the stator reactive power may be zero during the second period  102 . The third period  104  may be very brief to minimize the power dissipated in the stator. 
     The above-described method  500  provides numerous advantages. For example, the method minimizes the time that the stator  18  is energized with the braking waveform in order to save energy and minimize power dissipation that may damage the stator. Specifically, stator power dissipation is minimized by de-energizing the stator  18  when the rotor  16  has substantially stopped rotating. Also, a shorter braking cycle time increases the throughput of devices driven by the electric motor  12 . For example, when the motor assembly  10  is used in a washing machine, the method  500  allows quick and efficient braking of the motor  12  after a high speed spin cycle so that the washing machine can be unloaded or operated in a different cycle. 
     According to another aspect of this disclosure, the motor controller  14  may be programmed or otherwise configured to implement another method  600  of braking the electric motor  12 . As with the method  500 , the method  600  broadly includes the steps of energizing the stator  18  of the motor  12  to brake the motor, determining a reactive power of the stator  18 , detecting an increase in the determined reactive power of the stator  18 , and removing the braking waveform from the stator in response to detecting the increase in the determined reactive power of the stator. As explained in more detail below, the method  600  energizes the stator of the motor with a controlled current braking waveform. 
     The flow chart of  FIG. 6  shows the functionality and operation of a preferred implementation of the above described method  600  in more detail. In this regard, some of the blocks of the flow chart may represent the method  600  and/or a module segment or portion of code of the computer programs of the present invention. In some alternative implementations, the functions noted in the various blocks may occur out of the order depicted in  FIG. 6 . For example, two blocks shown in succession in  FIG. 6  may in fact be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order depending upon the functionality involved. 
     Steps  602 ,  604 ,  608 ,  610 , and  612  of method  600  are substantially identical to steps  502 ,  504 ,  508 ,  510 , and  512 , respectively, of method  500  and will therefore not be described again. 
     In step  606  of the method  600 , the motor controller  14  energizes the stator  18  with a controlled current waveform to apply a braking force to the rotor  16  in order to brake the rotor. The motor controller  14  may also vary the amplitude of the controlled current waveform. In one embodiment, the amplitude of the controlled current waveform is varied at a fixed frequency. The fixed frequency is preferably between about 1 Hz and about 10 Hz. More preferably the fixed frequency is about 2 Hz. 
     Energizing the stator  18  with a controlled current waveform to brake the rotor  16  is superior to braking the motor with a controlled voltage. This is because the stator generates a magnetic field based on the stator current (not based on the stator voltage) and it is the magnetic field that interacts with the rotor to brake the rotor. A controlled stator voltage may generate variable stator current since the stator resistance changes as the stator heats. Thus, a controlled stator current will result in more consistent rotor braking than would a controlled stator voltage. 
     According to another aspect of this disclosure, the motor controller  14  is programmed or otherwise configured to implement another method  700  of braking the electric motor  12 . The method  700  broadly includes the steps of energizing the stator of the motor to brake the motor, determining when the rotor stops rotating, and de-energizing the stator  18  in response to determining that the rotor has stopped. As explained in more detail below, the method  700  determines the rotor  16  has stopped rotating by monitoring the current shunt  22 . 
     The flow chart of  FIG. 7  shows the functionality and operation of a preferred implementation of the above described method  700  in more detail. In this regard, some of the blocks of the flow chart may represent the method  700  and/or a module segment or portion of code of the computer programs of the present invention. In some alternative implementations, the functions noted in the various blocks may occur out of the order depicted in  FIG. 7 . For example, two blocks shown in succession in  FIG. 6  may in fact be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order depending upon the functionality involved. 
     Steps  702 ,  704 , and  706  of method  700  are substantially identical to steps  502 ,  504 , and  506 , respectively, of method  500  and will therefore not be described again. 
     In step  708  of the method  700 , the motor controller  14  determines a current through the single current shunt  22 . This step may include computing one or more motor phase currents as a function of the signal on the current shunt. The computed phase currents may provide the indication that the rotor has substantially stopped rotating. Further, the signal on the single current shunt may indicate an increase of a reactive power of the stator. Alternatively, the signal may indicate an increase of the motor power factor, etc. 
     A single current shunt is lower cost than multiple current shunts; accordingly, implementing this method may be lower cost than implementing a method that requires multiple current shunts. 
     In step  710  of the method  700 , the motor controller  14  de-energizes the stator  18  to remove the braking force on the rotor  16 . This may be done in response to a signal on the current shunt  22  indicating that the rotor  16  has stopped rotating. 
     Although the invention has been described with reference to the preferred embodiment illustrated in the attached drawing figures, it is noted that equivalents may be employed and substitutions made herein without departing from the scope of the invention as recited in the claims.