Abstract:
A method of establishing motor speed control, the method comprising the acts of generating a desired speed command, indirectly measuring the slip of the motor, and adjusting the speed command in response to the slip to maintain a constant speed.

Description:
RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. §119 of U.S. Provisional Application No. 60/170,338, entitled METHOD AND APPARATUS OF ESTIMATING ROTOR SPEED, filed on Dec. 13, 1999. 
    
    
     BACKGROUND 
     This invention relates to induction motors. More particularly, the invention relates to a method and apparatus for indirectly measuring induction motor slip and using the measurement to establish speed control. 
     As an AC induction motor rotates, the magnetic fields of the rotor and the stator interact. The stator windings are typically connected to a supply in three-phase form or single phase form. By applying a voltage across the windings, a radial, rotating magnetic field is formed. The rotor has solid aluminum bars cast in a “squirrel-cage” configuration. The rotating magnetic fields produced by the stator produce a current in the aluminum bars of the rotor. This produces a magnetic field in the aluminum bars which interacts with the rotating magnetic field of the stator to generate torque on the rotor. The rotor reacts to the magnetic field, but does not travel at the same speed. The rotor actually lags behind the speed of the rotating magnetic field. This lag is called slip, and is essentially a comparison of the speed of the rotor and the speed of the magnetic field. The slip typically increases proportionately with increases in load. 
     In some applications where a variable speed drive (i.e., controller) is being used with an induction motor, it is desirable to establish a constant operating speed for the motor. However, the speed of the motor has a tendency to change depending upon the load applied to the motor. Therefore, to attain a constant speed with a changing load, the drive requires information about the actual rotor speed. Stated differently, it is necessary to provide electronic feedback of the rotor speed to the drive. In the prior art, this feedback to the controller is typically measured with a tachometer which can be both expensive and unreliable. Other methods utilize a model of the motor, in combination with phase voltage and phase current and complex mathematical algorithms to indirectly measure rotor speed. A much more complex and expensive circuit or microprocessor is required to perform these calculations. 
     SUMMARY OF THE INVENTION 
     Accordingly, the invention provides a method and apparatus for indirectly measuring induction motor slip and using the measurement to provide actual speed information to the drive to establish constant speed control of the induction motor. In the apparatus of the invention, measured analog DC bus voltage and analog DC bus current are input to a power factor circuit which uses the DC bus voltage and current along with AC motor voltage and current to calculate a power factor. The power factor circuit is connected to a slip calculator circuit which uses desired motor speed and power factor to calculate slip. That is, since the desired speed of the motor is known, the measured power factor and speed can be used to calculate the slip, thereby providing an indirect measurement of induction motor slip. The slip calculator circuit is connected to a speed conversion circuit which converts the indirect measurement of induction motor slip to an actual motor speed value. This value is “fed-back” to the regulator to provide feedback control to the drive which constantly updates the commanded drive speed until the commanded speed and the actual speed (also referred to as “estimated speed” or “measured speed”) are equal. 
     In the method of the invention, three phase AC power is supplied to energize the motor. A DC bus voltage and a DC bus current are measured. The measured DC bus voltage and current along with AC motor voltage and current are used to calculate the power factor for the commanded motor speed. The commanded motor speed and power factor are then used to calculate the motor slip at the given speed and power factor. Stated differently, the induction motor slip is indirectly measured based on the DC bus voltage and current. This technique is most effective for variable speed drives at higher speeds for two reasons. The first is that there are two slips for each power factor value, one at relatively low speeds and a second at relatively high speeds. The second is that, at different drive frequencies, the stator resistance becomes a larger percentage of the loss impedance of the motor thus changing the power factor to slip relationship. At higher speeds, the influence of the stator resistance on this relationship is nominal. 
     The slip is then used to calculate an actual motor speed value. This value is “fed-back” to the regulator to provide feedback control to the drive which constantly updates the commanded drive speed until the commanded speed and the actual speed (also referred to as “estimated speed” or “measured speed”) are equal. 
     The principal advantage of the invention is to provide constant speed control of an induction motor by indirectly measuring the motor slip. 
     Another advantage of the invention is the use of measured bus voltage and bus current in addition to phase voltage and phase current to simplify the rotor speed calculation. The simpler calculation allows the use of a simpler and less expensive microprocessor circuit. 
    
    
     Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings. 
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an induction motor including an efficiency optimization circuit for controlling the induction motor. 
     FIG. 2 is a schematic view of another embodiment of the efficiency optimization circuit of the induction motor. 
     FIG. 3 is a schematic view of a constant speed drive for an induction motor embodying the invention. 
    
    
     Before one embodiment of the invention is explained in full detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including” and “comprising” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Shown in FIG. 1 of the drawings is a schematic view of an induction motor  10  embodying the invention. The details of the induction motor are commonly known in the art and form no part of the invention. Accordingly, the induction motor will not be described in detail. The motor  10  includes an efficiency optimization circuit  100  for controlling the excitation of the stator  38 . The efficiency optimization circuit  100  includes a measuring circuit  104  for measuring a DC bus voltage  108  and DC bus current  112 . The DC bus voltage and current are input to analog to digital (A/D) converter  122  which is electrically connected to a circuit  124 . The circuit  124  may be a microprocessor or may be comprised of discrete components. In the embodiment of the invention including a microprocessor, the method of the invention is preferably implemented using a computer software program or programs stored in the memory for the microprocessor. The circuit  124  includes a torque producing current circuit component  144 , a comparator circuit component  148 , a regulator circuit component  152 , and a feed back circuit component  156 . The circuit  100  also includes an inverter  132 , and an RMS conversion circuit  168  connected as shown. 
     In operation, the slip at which a motor operates with maximum efficiency (S max.eff .) is constant for a given induction motor. Thus, lowering the operating voltage V ø  until (S max.eff .) is attained is one way of operating the motor at maximum efficiency. When the motor is operating at any slip less than the rated slip, the efficiency optimization circuit  100  of the invention uses the relationship between the torque producing component I q  and flux producing component I d  of the current I ø  supplied to the motor. 
     The measuring circuit  104  measures a DC bus voltage  108  and a DC bus current  112 . These voltages are preferably measured via a voltage and current bus. The DC bus voltage  108  and the DC bus current  112  are analog signals that are converted by an analog-to-digital ((A/D) converter  122  to a digital voltage signal  116  (V dc ) and a digital current signal  120  (I dc ). 
     The digital voltage signal  116  and the digital current signal  120  are supplied to the torque producing current circuit  144  along with an initial condition value of the phase voltage  128  (V ø ). Assuming that the inverter  132  has no losses, the conservation of power equation in the inverter is: 
     
       
           P   in   =V   dc   ·I   dc   =|Vø|·|I ø|·cos(θ)= P   out ; 
       
     
     where θ is the angle between the phase voltage V ø  and the phase current I ø . Solving for the power factor yields: 
     
       
         
           
             
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     Assuming that θ and ø (the angle between the phase current and the torque producing current) are approximately equal, then the torque producing component of the current I q  is approximately defined by the following equation: 
     
       
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     The torque producing current circuit  144  calculates the approximation of the torque producing component of the current  158  (I q ) as I ø  cos θ, and inputs the current  158  to a comparator circuit  148 . The comparator circuit  148  compares the current  158  with a desired torque producing current value  160 . The desired torque producing current value (I ø  cos θ cmd )  160  is determined by the feedback circuit  156 . The phase current  136  (I ø ) is measured using a sensor (not shown) at the motor coils. The phase current  136  (I ø ) is input into the RMS circuit  168 , where the AC phase current  136  (I ø ) is converted to an RMS value  172  (I øRMS ) of phase current  136  (I ø ). The RMS value  172  is input into the feedback circuit  156 , along with the power factor for maximum efficiency cos θ cmd , which is a known constant for the motor. The feedback circuit  156  determines the appropriate angle, based on motor parameters, between the phase voltage V ø  and the phase current I ø  for maximum efficiency. More specifically, the feedback circuit  156  calculates the cosine of the angle, thus factoring the change of angle from I ø  to I q , resulting in the estimated torque producing current value I ø  cos θ cmd    160 . 
     Comparator circuit  148  compares the desired torque producing current value  160  (I ø  cos θ cmd ) and the current  158  to generate an error term  170  representing the difference between the calculated and desired torque producing current. The error term  170  is supplied to the regulator circuit  152 . If the calculated and desired torque producing current values are the same value, the error term  170  is zero. 
     The regulator  152  increments or decrements the commanded phase voltage  128  (V ø ) by the error term  170 , generating a new value for a commanded phase voltage value  128 . If the calculated and desired torque producing current values are the same, the error term  170  is equal to zero, and accordingly, the commanded phase voltage does not change. The new commanded phase voltage is then input into the inverter  132 , which converts the rectified DC power to three-phase AC power. The three-phase AC power is then fed to the terminals  38  of motor  10 . 
     The new commanded phase voltage  128  (V ø ) is also fed back from the regulator  152  and used as the next commanded phase voltage (V ø ) for determining the actual torque producing current, and the cycle repeats. By constantly updating the commanded phase voltage V ø , the motor  10  is able to continually operate at the slip, thereby increasing the efficiency of the motor. 
     FIG. 2 schematically illustrates another embodiment  200  of the efficiency optimization circuit of the induction motor. Like parts are identified using like reference numerals. As shown in FIG. 2, the input  160  to comparator circuit  148  is simply the desired power factor cos θ cmd , and the input  158  to comparator circuit  148  from the torque producing current circuit  144  is a calculated power factor (cos θ), which is calculated similar to I ø  cos θ, except that the additional division operation is performed using I ø . This is achieved by directly supplying the RMS phase current  172  (I øRMS ) to the torque producing current circuit  144  through A/D converter  122 , instead of through a feedback loop as is shown in FIG.  1 . The overall operation of the efficiency optimization circuit  200  is simpler than that of the efficiency optimization circuit  100 , and thus requires less time to update the commanded phase voltage (V ø ). 
     FIG. 3 schematically illustrates another embodiment of the motor  10  having a control circuit  300  for effecting constant speed control through an indirect measurement of slip. Like parts are identified using like reference numerals. The desired speed command, (W r     cmd   ) is input to a comparator circuit  148  which includes a feed back input for adjusting the desired speed command to achieve constant speed control of the induction motor. The output of the comparator circuit  148  is connected to a regulator  152  and from there to an inverter  132  to supply a three phase drive stimulus to the stator  38 . The estimated speed command (W e ) output from the regulator  152  is also connected to a frequency to voltage converter  302  which outputs a phase voltage to the inverter  132 . DC bus voltage and DC bus current as well as AC motor voltage and current are supplied to a power factor circuit  304  which calculates the power factor of the motor using the relationship V b I b /V 0 I 0 =Power Factor (PF). The output (PF) of the power factor circuit  304  is connected to a slip calculator circuit  308 . The slip calculator circuit  308  is connected to a power factor coefficient look up table  316 . Using the desired speed command (W r     cmd   ) as a reference, look up table  316  supplies to slip calculator circuit  308  a speed dependent power factor coefficient K n . Slip calculator circuit  308  calculates the actual slip of the motor at the commanded speed using the relationship:        S   =         ∑   m       n   =   o              K   n          PF   n                                
     The indirectly measured and calculated slip is input to a compensating circuit or speed conversion circuit  312  which, using the estimated speed command output from regulator  152 , calculates a compensation factor (W r ) which is then input to comparator  148  to adjust the commanded speed input to the regulator  152 . The compensation factor is calculated using the relationship: 
     
       
           W   r =(1 −S ) W   e   
       
     
     The control circuit  300  constantly updates the commanded drive speed until the commanded speed and the actual speed (also referred to as “estimated speed” or “measured speed”) are equal.