Abstract:
A motor drive unit includes a controller, a voltage inverter, and a speed estimator. The controller is operable to generate at least one command signal for controlling a motor associated with the motor drive unit. The voltage inverter is operable to generate motor drive signals to be applied to the motor based on the command signal. The speed estimator is operable to estimate a speed of the motor when the voltage inverter is not providing the motor drive signals. The speed estimator is further operable to receive two-axis voltage measurements associated with the motor, determine flux angles for the motor based on the two-axis voltage measurements, and estimate the speed based on the determined flux angles.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable 
   BACKGROUND OF THE INVENTION 
   The present invention relates generally to AC induction motor drives and, more particularly, to a method and apparatus for monitoring motor status using induced motor voltage. 
   Induction motors have broad application in industry, particularly when large horsepower is needed. In a three-phase induction motor, three phase alternating voltages are impressed across three separate motor stator windings and cause three phase currents therein. Because of inductances, the three currents typically lag the voltages by some phase angle. The three currents produce a rotating magnetic stator field. A rotor contained within the stator field experiences an induced current (hence the term “induction”) that generates a rotor field. The rotor field typically lags the stator field by some phase angle. The rotor field is attracted to the rotating stator field and the interaction between the two fields causes the rotor to rotate. 
   A common rotor design includes a “squirrel cage winding” in which axial conductive bars are connected at either end by shorting rings to form a generally cylindrical structure. The flux of the stator field cutting across the conductive bars induces cyclic current flows through the bars and across the shorting rings. The cyclic current flows in turn produce the rotor field. The use of this induced current to generate the rotor field eliminates the need for slip rings or brushes to provide power to the rotor, making the design relatively maintenance free. 
   To a first approximation, the torque and speed of an induction motor may be controlled by changing the frequency of the driving voltage, and thus, the angular rate of the rotating stator field. Generally, for a given torque, increasing the stator field rate will increase the speed of the rotor, which follows the stator field. Alternatively, for a given rotor speed, increasing the frequency of the stator field will increase the torque by increasing the slip (i.e., the difference in speed between the rotor and the stator fields). An increase in slip increases the rate at which flux lines are cut by the rotor, increasing the rotor generated field and thus the force or torque between the rotor and stator fields. 
   Referring to  FIG. 1 , a rotating phasor  10  corresponding to a stator magneto motive force (mmf) generally has an angle, α, with respect to the phasor of rotor flux  12 . The torque generated by the motor is proportional to the magnitudes of these phasors  10 ,  12  but is also a function of the angle, α. Maximum torque is produced when the phasors  10 ,  12  are at right angles to each other, whereas zero torque is produced if the phasors  10 ,  12  are aligned. The stator mmf phasor  12  may therefore be usefully decomposed into a torque producing component  14  perpendicular to rotor flux phasor  12  and a flux component  16  parallel to rotor flux phasor  12 . 
   These two components  14 ,  16  of the stator mmf are proportional, respectively, to two stator current components: i q , a torque producing current, and i d , a flux producing current, which may be represented by quadrature or orthogonal vectors in a rotating or synchronous frame of reference (i.e., a reference frame that rotates along with the stator flux vector) and each vector i q  and i d  is characterized by slowly varying DC magnitude. 
   Accordingly, in controlling an induction motor, it is generally desired to control not only the frequency of the applied voltage, hence the speed of the rotation of the stator flux phasor  10 , but also the phase of the applied voltage relative to the current flow, hence the division of the currents through the stator windings into the i q  and i d  components. Control strategies that attempt to independently control current components i q  and i d  are generally referred to as field oriented control strategies (FOC). 
   There are many instances in which it is desirable to measure one or more parameters of motor operation. Typical parameters of interest include rotor speed, rotor direction, back EMF magnitude, and back EMF phase angle. During normal motor operation, adequate assumptions about these parameters can often be made based on the control that is implemented. For example, if a particular speed is commanded in an open loop control scheme, it is often adequate to assume that the control scheme is maintaining the actual motor speed at the commanded speed. In such cases the cost of the system may be reduced by eliminating the need for a rotor shaft speed sensor. The command frequency may be integrated to determine the flux angle, and the rotor speed may be determined by subtracting the slip frequency from the command frequency. 
   However, situations exist in which such assumptions are not adequate. This is the case, for example, when a motor drive becomes disconnected from a motor (i.e., the power supply to the motor is interrupted, not necessarily the electrical connection between the motor drive and the motor) and open loop control is no longer present. In this case, with no control present, it is difficult to make any assumptions about the motor parameters. 
   There are a variety of reasons why a motor drive may become disconnected from a motor. For example, there may be a sudden temporary power loss at the power source that supplies power to the motor and motor drive. Alternatively, it may simply be the case that there are times when it is not necessary to operate the motor, and power is not supplied to the motor during these times. 
   The fact that the motor drive is disconnected from the motor does not prevent the motor from continuing to rotate. For example, if the motor is used in conjunction with a fan in an air conditioning system, a draft in the air conditioning system may drive the motor at an unknown speed and in an unknown direction. Similarly, if the motor is used in a conveyor system, the force of gravity acting on the motor by way of the conveyed articles and friction may drive the motor at an unknown speed and in an unknown direction. 
   When a motor drive becomes disconnected from a motor, it eventually becomes necessary to reconnect the motor drive to the motor. To perform the reconnection, it is desirable to determine the above-mentioned parameters, namely, rotor speed, rotor direction, back EMF magnitude and/or back EMF phase angle, before the motor drive is reconnected to the motor. Measuring these parameters is useful because it allows the motor drive to be synchronized to the motor, thereby reducing transients at the moment of reconnection. For example, if the speed of the motor is not determined before reconnection, then the motor drive must assume an initial speed of zero when reconnecting to the motor. This assumption may result in severe transients due to the difference between the frequency of the applied voltage and the frequency of the motor-induced back EMF. The transients are especially severe when the initial motor speed is high and when the motor is rotating in a reverse direction as compared to that commanded by the motor drive. If the current control circuitry or current limiting circuitry of the motor drive is not fast enough, the motor drive can fault due to an overcurrent condition. Additionally, when the motor operates as a generator (i.e., when the frequency of the voltage applied to the motor is less than the motor speed), the DC bus voltage may increase to unacceptable levels and cause damage to the power switches in the motor drive. 
   It is therefore desirable to determine motor parameters to allow the motor drive to be synchronized to the motor when the motor drive is reconnected and thereby to reduce transients upon reconnection. Additionally, when performing a reconnection, it is desirable to measure these parameters in as little time as possible so that operation may continue as smoothly as possible to make the temporary disconnection as imperceptible as possible. 
   This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. 
   BRIEF SUMMARY OF THE INVENTION 
   One aspect of the present invention is seen in a motor drive unit including a controller, a voltage inverter, and a speed estimator. The controller is operable to generate at least one command signal for controlling a motor associated with the motor drive unit. The voltage inverter is operable to generate motor drive signals to be applied to the motor based on the command signal. The speed estimator is operable to estimate a speed of the motor when the voltage inverter is not providing the motor drive signals. The speed estimator is further operable to receive two-axis voltage measurements associated with the motor, determine flux angles for the motor based on the two-axis voltage measurements, and estimate the speed based on the determined flux angles. 
   Another aspect of the present invention is seen in a method for monitoring a motor disconnected from a motor drive. The method includes receiving two-axis voltage measurements associated with the motor. Flux angles are determined for the motor based on the two-axis voltage measurements. A speed of the motor is estimated based on the determined flux angles. 
   These and other objects, advantages and aspects of the invention will become apparent from the following description. The particular objects and advantages described herein may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made, therefore, to the claims herein for interpreting the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
       FIG. 1  is a schematic view in cross section of an induction motor showing instantaneous locations of a rotor flux, a stator mmf, and the torque and flux components of the stator mmf; 
       FIG. 2  is a simplified block diagram of a motor drive unit capable of controlling a motor; 
       FIG. 3  is a simplified block diagram of a speed estimator in the drive unit of  FIG. 2 ; and 
       FIG. 4  is a diagram illustrating equations used by the speed estimator of  FIG. 3  for estimating a flux angle based on the values of induced voltages. 
   

   While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
   DETAILED DESCRIPTION OF THE INVENTION 
   One or more specific embodiments of the present invention will be described below. It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. Nothing in this application is considered critical or essential to the present invention unless explicitly indicated as being “critical” or “essential.” 
   Referring now to the drawings wherein like reference numbers correspond to similar components throughout the several views and, specifically, referring to  FIG. 2 , the present invention shall be described in the context of a motor drive unit  100 . The motor drive unit  100  drives an electric motor  105  using an open loop control scheme. However, the application of the present invention is not limited to an open loop scheme, and may be used in a closed loop system with current feedback. The motor drive unit  100  includes a control system  110 , an AC to DC power converter  115 , and a PWM (Pulse Width Modulation) voltage inverter  120 . The control system  110 , the power converter  115  and the voltage inverter  120  cooperate to drive the motor  105  using power supplied by a three phase AC power source  125 . As will be described in greater detail below, the motor drive unit  100  monitors rotor speed indirectly (i.e., a rotor shaft sensor is not provided). The motor drive unit  100  uses command signals to determine rotor speed while it is driving the motor  105  and estimates rotor speed based on observations of induced voltages when not driving the motor  105 . 
   The control system  110  operates as a field oriented control strategy (FOC) system and regulates the current through the motor  105  by generating voltage commands Vq* and Vd*. (Herein, lower case letters a, b and c denote phase A, phase B, and phase C, respectively, lower case letters q and d denote D phase and Q phase, respectively, and an asterisk denotes a command signal.) Of course, since current and voltage are directly related, the fact that current is controlled means that voltage is simultaneously also controlled, and vice versa. 
   The control system  110  includes a 2-3 phase converter  130  for converting the d and q phase voltage commands Vq* and Vd* to three phase voltage commands Va*, Vb* and Vc* for use by the voltage inverter  120 . A speed monitor  140  is provided for determining the electrical flux angle, θ e , a necessary component of the transformation and for determining the rotor speed, ω r   * , while the motor drive unit  100  is driving the motor  105 . The speed monitor  140  subtracts the slip frequency, ω s   * , from the command frequency, (ω e   * , both of which are provided by the controller  145  to generate the rotor speed, ω r   * . The speed monitor  140  integrates the command frequency, ω e   * , to generate the electrical flux angle, θ e  These operations for determining rotor speed and flux angle based on the command and slip frequencies are well known to those of ordinary skill in the art. 
   The control system  110  is implemented in firmware executed by a microprocessor. The control system  110  includes a controller  145  which generates the command signals Vq* and Vd*. During normal connected operation, the controller  145  generates the commands based on a speed command received at a user input (not illustrated). Voltage feedback signals, V as , V bs , and V cs  are also provided by voltage sensors associated with the motor  105 . Although the controller  145  and other components are illustrated as separate entities, they may be integrated into a single application executed by a microprocessor. Separate units are illustrated to aid in the illustration of the present invention, not to require distinct hardware. 
   The outputs of the 2-3 phase converter  130  are the voltage commands Va*, Vb* and Vc*. The voltage commands Va*, Vb* and Vc* are received by the PWM voltage inverter  120 , which generates PWM control signals based on the voltage commands Va*, Vb* and Vc*. The PWM voltage inverter  120  includes a network of six solid state switches (not illustrated) which are switched on and off in accordance with the PWM control signals. The solid state switches convert the low power PWM control signals to high power current pulses to drive the motor  105  using power supplied by the three-phase AC source  125  via the AC-DC power converter  115 . The PWM voltage inverter  120  may also include a conventional PWM dead time compensation circuit (not illustrated) to compensate for the dead time necessary between PWM pulses to prevent short circuiting the power supply. 
   The control system  110  also includes a speed estimator  170  for estimating the speed of the rotor, ω r     —     est , while the motor drive unit  100  is disconnected from the motor  105 . The controller  145  selects controls a multiplexer  172  to select the output of the speed monitor  140 , ω r , when the PWM voltage inverter  120  is driving the motor  105 , and to select the output of the speed estimator  170 , ω r     —     est , when the PWM voltage inverter  120  is not driving the motor  105 . During a reconnection process, the controller  145  may use the monitored rotor speed, ω r     —     monitor , as selected by the multiplexer  172 , to reconnect the motor drive unit  110  to the motor  105 . 
   Turning now to  FIG. 3 , a simplified block diagram of the speed estimator  170  is provided. The speed estimator  170  includes a 3-2 phase coordinate transformer  200 , a flux angle unit  210 , and a rotor speed unit  220 . The 3-2 phase coordinate transformer  200  receives the 3 phase motor voltages, V as , V bs , and V cs , and transforms them using the following equations to generate corresponding d-q signals using the following equations: 
   
     
       
         
           
             
               
                 
                   V 
                   qs 
                 
                 = 
                 
                   V 
                   as 
                 
               
             
           
           
             
               
                 
                   V 
                   ds 
                 
                 = 
                 
                   
                     1 
                     
                       3 
                     
                   
                   ⁢ 
                   
                     ( 
                     
                       
                         V 
                         cs 
                       
                       - 
                       
                         V 
                         bs 
                       
                     
                     ) 
                   
                 
               
             
           
         
       
     
   
   The flux angle unit  210  determines the flux angle, θ e , based on the magnitudes of the d and q voltages generated by the 3-2 phase coordinate transformer  200 .  FIG. 4  illustrates the equations used to determine the flux angle based on the magnitudes and signs of the d and q voltages. For example, if V qs &lt;V ds , and both are positive, the equation from the lower portion of the 1 st  quadrant is used:
 
θ e =arctan(| Vds|/|Vqs |)
 
   If V qs &gt;V ds , and both are positive, the equation from the upper portion of the 1 st  quadrant is used:
 
θ e =90°−arctan(| Vds|/|Vqs |)
 
   When V qs =V ds , either equation may be used. 
   As the signs of V qs  and V ds  change, the equations provided in the other quadrants may by used. Table 1 below summarizes the equations presented in  FIG. 4 . 
   
     
       
             
           
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Flux Angle Determination 
             
           
        
         
             
               V qs  &gt; sign 
               V ds  sign 
               V qs  &gt; V ds   
               Flux Angle Equation 
             
             
                 
             
             
               + 
               + 
               Yes 
               θ e  = arctan(|V ds |/|V qs |) 
             
             
               + 
               + 
               No 
               θ e  = 90° − arctan(|V ds |/|V qs |) 
             
             
               − 
               + 
               Yes 
               θ e  = 180° − arctan(|V d s|/|V qs |) 
             
             
               − 
               + 
               No 
               θ e  = arctan(|V ds |/|V qs |) + 90° 
             
             
               − 
               − 
               Yes 
               θ e  = arctan(|V ds |/|V qs |) + 180° 
             
             
               − 
               − 
               No 
               θ e  = 270° − arctan(|V ds |/|V qs |) 
             
             
               + 
               − 
               Yes 
               θ e  = 360° − arctan(|V ds |/|V qs |) 
             
             
               + 
               − 
               No 
               θ e  = arctan(|V ds |/|V qs |) + 270° 
             
             
                 
             
           
        
       
     
   
   Returning to  FIG. 3 , the rotor speed unit  220  estimates rotor speed by averaging two subsequent values of θ e  over the sampling interval in accordance with the equation: 
   
     
       
         
           
             ω 
             r_est 
           
           = 
           
             
               
                 
                   θ 
                   e_n 
                 
                 - 
                 
                   θ 
                   
                     e_n 
                     - 
                     
                         
                     
                     ⁢ 
                     1 
                   
                 
               
               
                 
                   t 
                   n 
                 
                 - 
                 
                   t 
                   
                     n 
                     - 
                     1 
                   
                 
               
             
             = 
             
               
                 Δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   θ 
                   e 
                 
               
               
                 Δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 t 
               
             
           
         
       
     
   
   The motor drive unit  100  may use the estimated rotor speed information for various purposes including safety or status monitoring, or for reconnecting the motor  105 . Techniques for reconnecting the motor drive unit  100  to the motor  105  are well known in the art, once the rotor speed has been determined, and as such, they are not described in greater detail herein. By measuring rotor speed using induced voltage, the need for a rotor speed sensor is obviated, thereby simplifying and reducing the cost of the motor drive unit  100 . 
   The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.