Abstract:
A system and method for starting electric motors. A controller attempts to start a motor without applying a brake to the rotor. If the motor fails to start, the controller applies a strength of braking and then again attempts to start the motor. If the motor still fails to start, the controller iteratively increases the strength of braking and attempts to start the motor until a maximum strength of braking and/or a maximum number of attempts to start the motor is reached. Alternatively, a sensing system first determines whether the rotor is rotating. If the rotor is rotating, the sensing system determines the speed of rotation, the controller determines a strength of braking that will halt the rotation based on the speed of rotation, applies that strength of braking to halt the rotation of the rotor, and then attempts to start the motor.

Description:
FIELD 
       [0001]    The present invention relates to systems and methods for starting electric motors. 
       BACKGROUND 
       [0002]    Electric motors commonly include a stationary “stator” and a rotating “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 stop over time due to the inertia of the rotor and anything that may be coupled to it. 
         [0003]    The rotation may be stopped more quickly using a braking method. One braking method involves using brake pads, pulleys, or other such mechanisms to induce friction that reduces the rotor&#39;s rotational speed. Another braking method involves adjusting the frequency of the driving waveform to be less than the rotor frequency, such that the rotating magnetic field created by the stator induces rotational pressure on the rotor to reduce its rotational speed. Another braking method involves applying a direct current (DC) voltage to the stator windings which creates a stationary magnetic field that applies a static torque to the rotor to reduce its rotational speed. Furthermore, the existence of rotation can be determined using a sensing method. One sensing method involves coupling a sensor, such as a Hall effect sensor, to the motor&#39;s shaft to detect its rotation. Another sensing method uses various algorithms to estimate when the rotor stops rotating based on measured electrical parameters. 
         [0004]    The open-loop-controlled Volts per Hertz starting routine developed for electric motors used in, e.g., heating and air conditioning variable speed (HAC VS) applications, involves maintaining a particular ratio of the amplitude of the motor phase voltage (expressed in Volts) to the synchronous electrical frequency (expressed in Hertz) applied to a motor, in which the particular ratio is defined by the base point of the motor. The open-loop controller provides input based on the current state of the actual system and the expected state of a model system, rather than on feedback. This starting routine can be tuned to start when the motor is rotating before being energized, though it would then fail to start when the motor is not rotating before being energized. However, with some newer motor designs that have a higher winding resistance and high back-emf, the starting routine has limitations starting when the motor is rotating before being energized. The starting routine can be tuned to start motors based on their winding designs, but doing so requires two tuned sets: A first set for starting when the motor is not rotating and a second set for starting when the motor is rotating. 
         [0005]    This background discussion is intended to provide information related to the present invention which is not necessarily prior art. 
       SUMMARY 
       [0006]    Embodiments of the present invention solve the above-described and other problems and limitations by providing a system and method operable to reliably start electric motors without regard to their winding designs and without regard to whether their unenergized rotors are rotating or not. In particular, the present invention provides an improvement to the open loop volts per hertz original starting routine used in, e.g., HAC VS commercial motors. 
         [0007]    In a first implementation of a first embodiment, the system may broadly comprise a controller in communication with the electric motor and operable to control operation of the motor, and a braking system operable to reduce a rotation of the rotor, wherein the controller first attempts to start the electric motor without applying the braking system to the rotor. If the electric motor fails to start, the controller causes the braking system to apply an initial strength of braking, and then again attempts to start the electric motor. If the motor still fails to start, the controller iteratively causes the braking system to increase the strength of braking and attempts to start the electric motor until a predetermined maximum strength of braking is reached. 
         [0008]    In a second implementation of the first embodiment, the system may broadly comprise the controller and the braking system, wherein the controller first attempts to start the electric motor without applying the braking system to the rotor. If the electric motor fails to start, the controller causes the braking system to apply an initial strength of braking to the rotor, and then again attempts to start the electric motor. If the electric motor still fails to start, the controller iteratively causes the braking system to increase the strength of braking applied to the rotor and attempts to start the electric motor until a predetermined maximum number of attempts to start the electric motor is reached. 
         [0009]    In a third implementation of the first embodiment, the system may broadly comprise the controller and the braking system, wherein the controller first attempts to start the electric motor without applying the braking system to the rotor. If the electric motor fails to start, the controller causes the braking system to apply an initial strength of braking to the rotor, and then again attempts to start the electric motor. If the motor still fails to start, the controller iteratively causes the braking system to increase the strength of braking applied to the rotor and attempts to start the electric motor until a predetermined maximum strength of braking is reached. If the motor still fails to start, the controller iteratively causes the braking system to maintain the predetermined maximum strength of braking applied to the rotor and again attempts to start the electric motor until a predetermined maximum number of attempts to start the electric motor is reached. 
         [0010]    Any or all of these implementations may further include any one or more of the following additional features. The electric motor may be a variable speed electric induction or permanent magnet motor. The braking system may employ an opposing driving waveform to reduce the rotation of the rotor, or the braking system may employ an opposing magnetic field to reduce the rotation of the rotor. The initial strength of braking may be approximately between 1% and 3%, and the strength of braking may be increased by approximately between 1% and 3% for each iteration. The predetermined maximum strength of braking may be approximately between 6% and 10%. The predetermined maximum number of attempts to start the electric motor may be between 8 and 12. 
         [0011]    In an implementation of a second embodiment, the system may broadly comprise the controller, a sensing system operable to sense the rotation of the rotor, and the braking system, wherein the sensing system first determines whether the rotor is rotating. If the rotor is rotating, the sensing system determines the speed of rotation, the controller determines a strength of braking that will halt the rotation based on the speed of rotation, the controller causes the braking system to apply the strength of braking to halt the rotation of the rotor, and the controller attempts to start the electric motor. 
         [0012]    This implementation may further include any one or more of the following additional features. The electric motor may be a variable speed electric induction or permanent magnet motor. The sensing system may determine whether the rotor is rotating by sensing an electric current flowing through a power inverter coupled with the electric motor. 
         [0013]    Additionally, each of these systems may be alternatively characterized as methods based on their functionalities. 
         [0014]    This summary is not intended to identify essential features of the present invention, and is not intended to be used to limit the scope of the claims. These and other aspects of the present invention are described below in greater detail. 
     
    
     
       DRAWINGS 
         [0015]    Embodiments of the present invention are described in detail below with reference to the attached drawing figures, wherein: 
           [0016]      FIG. 1  is a block diagram of a motor system constructed in accordance with an embodiment of the invention; 
           [0017]      FIG. 2  is an exploded isometric view of a stator component and a rotor component of the motor system shown in  FIG. 1 ; 
           [0018]      FIG. 3  is a flow diagram of steps in a first implementation of a first embodiment of a method of the present invention; 
           [0019]      FIG. 4  is a flow diagram of steps in a second implementation of the first embodiment of the method of the present invention; 
           [0020]      FIG. 5  is a flow diagram of steps in a third implementation of the first embodiment of the method of the present invention; 
           [0021]      FIG. 6  is a schematic diagram of a power inverter component of the motor system of  FIG. 1 ; and 
           [0022]      FIG. 7  is a flow diagram of steps in an implementation of a second embodiment of the method of the present invention. 
       
    
    
       [0023]    The figures are not intended to limit the present invention to the specific embodiments they depict. The drawings are not necessarily to scale. 
       DETAILED DESCRIPTION 
       [0024]    The following detailed description of embodiments of the invention references the accompanying figures. The embodiments are intended to describe aspects of the invention in sufficient detail to enable those with ordinary skill in the art to practice the invention. Other embodiments may be utilized and changes may be made without departing from the scope of the claims. The following description is, therefore, not limiting. 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. 
         [0025]    In this description, references to “one embodiment”, “an embodiment”, or “embodiments” mean that the feature or features referred to are included in at least one embodiment of the invention. Separate references to “one embodiment”, “an embodiment”, or “embodiments” in this description do not necessarily refer to the same embodiment and are not mutually exclusive unless so stated. Specifically, a feature, structure, act, etc. described in one embodiment may also be included in other embodiments, but is not necessarily included. Thus, particular implementations of the present invention can include a variety of combinations and/or integrations of the embodiments described herein. 
         [0026]    Referring to  FIG. 1 , an electric motor system  10  constructed in accordance with a first embodiment of the present invention is shown. The motor system  10  may broadly include an electric motor  12 , a power source  14 , and a motor control system  16  operable to control operation of the motor  12 , with the motor control system  16  including a controller  18  and a braking system  20  operable to halt rotation of the motor  12 . The motor system  10  may drive a load. For example, the motor system  10  may be a fan or a pump which may be part of a heating and air-conditioning unit or an appliance, such as a washing machine or a clothes dryer, which may include additional electrical or mechanical components not described herein. 
         [0027]    The motor  12  may be an electric induction or permanent magnet motor. For example, the motor  12  may be a three-phase, four-pole alternating current (AC) induction or permanent magnet motor rated to operate at a maximum voltage of approximately between 190 Volts and 200 Volts and a maximum current of approximately between 4 Amps and 6 Amps. Referring also to  FIG. 2 , the motor may include a stationary stator  26 , a rotatable rotor  28 , and a shaft  30  which couples the rotor  28  with the load. The power source  14  may be a conventional AC power source, such as a standard 115 Volt or 230 Volt source available in residential and commercial buildings via standard electrical outlets. 
         [0028]    The motor control system  16  may be broadly operable to control operation of the motor  12 , including receiving power from the power source  14  and generating a driving waveform to power the motor  12 . To that end, the motor control system  16  may include a controller  18  operable to receive input power from the power source  14 , create the driving waveform, and communicate the driving waveform to the motor  12 . The controller  18  may include digital logic components, programmable logic devices, or general purpose computer processors such as microcontrollers or microprocessors. For example, the controller  18  may include a computer processor operable to execute a computer program to manage certain aspects of the operation of the motor  12 . The computer program may include a series of executable instructions for implementing logic functions in the controller  18 . The motor system  10  may further include a memory (not shown) that is accessible to the controller  18  and operable to store the computer program. The memory may be of any suitable type. 
         [0029]    Referring also to  FIG. 6 , the controller  18  may further include a DC-to-AC power inverter  34  operable to convert DC power to AC power at a required frequency and amplitude to power the motor  12 . The power inverter  34  may include three half-bridge rectifiers, with each rectifier including two transistors that are alternately turned on and off to produce three voltage signals, each 120 degrees apart in phase, to power the three-phase motor  12 . The braking system  20  may be of any type operable to reduce the rotational speed of the rotor  28 . For example, the braking system  20  may employ an opposing driving waveform or an opposing magnetic field in which the braking system  20  pulses voltage to the motor  12  by turning on and off the power inverter  34 , wherein the pulse time corresponds to the strength of braking at the motor  12 . 
         [0030]    In operation, the system  10  may function as follows. Referring to  FIG. 3 , in a first implementation of the first embodiment, the motor control system  16  attempts to start the motor by executing the starting procedure, as shown in step  100 . In this first attempt, no braking is applied to the motor  12 . Next, the motor control system  16  determines whether the motor  12  successfully started, as shown in step  102 . If the motor  12  successfully started, then the motor control system  16  proceeds with normal motor operation, as shown in step  104 . However, if the motor  12  did not successfully start, then the motor control system  16  applies an initial strength of braking, as shown in step  106 , and again attempts to start the motor  12 , as shown in step  100 . The motor control system  16  again determines whether the motor  12  successfully started, as shown in step  102 . If the motor  12  did not successfully start, then the motor control system  16  increases the strength of braking, as shown in step  106 , and again attempts to start the motor  12 , as shown in step  100 . This process is repeated until either the motor  12  successfully starts or a predetermined maximum strength of braking is reached. If the maximum strength of braking is reached, then the strength of braking may be returned to zero and the entire process may be repeated from the beginning 
         [0031]    The initial strength of braking may be approximately between 1% and 3%, or approximately 2%, and each subsequent increase in the strength of braking may be between 1% and 3%, or approximately 2%. The maximum strength of braking may be between 6% and 10%, or approximately 8%. The strength of braking may be controlled by the controller  18 , and the strength of braking values, including the maximum strength of braking, may be stored in the memory. 
         [0032]    Referring to  FIG. 4 , in a second implementation of the first embodiment, the motor control system  16  attempts to start the motor by executing the starting procedure, as shown in step  200 . In this first attempt, no braking is applied to the motor  12 . Next, the motor control system  16  determines whether the motor  12  successfully started, as shown in step  202 . If the motor  12  successfully started, then the motor control system  16  proceeds with normal motor operation, as shown in step  204 . However, if the motor  12  did not successfully start, then the motor control system  16  may increment a counter and apply an initial strength of braking, as shown in step  206 , and attempt again to start the motor  12 , as shown in step  200 . The motor control system  16  may again determine whether the motor  12  successfully started, as shown in step  202 . If the motor  12  did not successfully start, then the motor control system  16  may again increment the counter and increase the strength of braking, as shown in step  206 , and attempt again to start the motor  12 , as shown in step  200 . This process may be repeated until either the motor  12  successfully starts or a predetermined maximum number of attempts to start the motor is reached. If the maximum number of attempts is reached, then the counter may be reset to zero and the strength of braking may be returned to zero, and the entire process may be repeated from the beginning 
         [0033]    The maximum number of attempts to start the motor  12  may be approximately between 8 and 12, or approximately 10. The counter may be implemented on and strength of braking may be controlled by the controller  18 , and the amount(s) by which to increase the strength of braking and the predetermined maximum number of attempts may be stored in the memory. 
         [0034]    Referring to  FIG. 5 , in a third implementation of the first embodiment, which is a hybrid of the first and second implementations, the motor control system  16  attempts to start the motor by executing the starting procedure, as shown in step  300 . In this first attempt, no braking is applied to the motor  12 . Next, the motor control system  16  determines whether the motor  12  successfully started, as shown in step  302 . If the motor  12  successfully started, then the motor control system  16  proceeds with normal motor operation, as shown in step  304 . However, if the motor  12  did not successfully start, then the motor control system  16  may increment a counter and apply an initial strength of braking, as shown in step  306 , and attempt again to start the motor  12 , as shown in step  300 . The motor control system  16  may again determine whether the motor  12  successfully started, as shown in step  302 . If the motor  12  did not successfully start, then the motor control system  16  may again increment the counter and increase the strength of braking, as shown in step  306 , and again attempt to start the motor  12 , as shown in step  300 . This process is repeated until either the motor  12  successfully starts or a predetermined maximum strength of braking is reached. If the maximum strength of braking is reached, then the strength of braking is no longer be increased with each iteration but rather is held constant for the remaining iterations. Thus, once the maximum strength of braking is reached, this process is repeated with the same maximum strength of braking until either the motor  12  successfully starts or a predetermined maximum number of attempts to start the motor  12  is reached. If the maximum number of attempts is reached, then the counter may be reset to zero and the strength of braking may be returned to zero, and the entire process may be repeated from the beginning 
         [0035]    The initial strength of braking may be approximately between 1% and 3%, or approximately 2%, and each subsequent increase in the strength of braking may be between 1% and 3%, or approximately 2%. The maximum strength of braking may be between 6% and 10%, or approximately 8%. The maximum number of attempts to start the motor  12  may be approximately between 8 and 12, or approximately 10. For example, on the second attempt to start the motor  12  approximately 2% strength of braking may be applied to the motor  12 , on the third attempt to start the motor  12  approximately 4% strength of braking may be applied, on the fourth attempt to start motor  12  approximately 6% strength of braking may be applied, on the fifth attempt to start the motor the maximum approximately 8% strength of braking may be applied, and on the sixth through the maximum tenth attempts to start the motor  12  the maximum approximately 8% strength of braking may be applied each time, and thereafter the counter and the strength of braking may be reset to zero. The strength of braking may be controlled by the controller  18 , the counter may be implemented on the controller  18 , and the strength of braking values, including the predetermined maximum strength of braking, and the predetermined maximum number of attempts may be stored in the memory. 
         [0036]    In a second embodiment, the system  10  may further include a sensing system  22  operable to sense or otherwise determine whether the rotor  28  is rotating. For example, the sensing system  22  may employ a sensor, such as a Hall effect sensor, or may use an algorithm to determine whether the rotor  28  is rotating based on measured electrical parameters. Referring to  FIG. 6 , the sensing system  22  may determine whether the rotor  28  is rotating based on current flowing through the power inverter  34 . 
         [0037]    Referring to  FIG. 7 , in an implementation of the second embodiment, before attempting to start the motor  12 , the sensing system  22  determines whether the rotor  28  is rotating, as shown in step  400 . If the rotor  28  is not rotating, then the motor control system  16  attempts to start the motor by executing the starting procedure, as shown in step  402 , and after the motor starts, the motor control system  16  proceeds with normal motor operation, as shown in step  404 . However, if the rotor  28  is rotating, then the controller  18  may determine an appropriate strength of braking to stop the rotation, and apply this appropriate strength of braking via the braking system  20  to stop the rotation, as shown in step  406 , and then attempt to start the motor  12  by executing the starting procedure, as shown in step  402 . The determination of the appropriate strength of braking may be based on the magnitude of the sensed current, the speed of rotation, and/or the direction of rotation. 
         [0038]    The present invention provides advantages over the prior art, including that it can reliably start electric motors without regard to their winding designs and without regard to whether their unenergized rotors are rotating or not. In particular, the present invention provides an improvement to the open loop volts per hertz original starting routine used in, e.g., HAC VS commercial motors. 
         [0039]    Although the invention has been described with reference to the one or more embodiments illustrated in the figures, it is understood that equivalents may be employed and substitutions made herein without departing from the scope of the invention as recited in the claims.