Abstract:
A motor controller ( 4 ) and method for maximizing the energy savings in an AC induction motor ( 3 ) at every load wherein the motor is calibrated at two or more load points to establish a control line ( 6 ), which is then programmed into a non-volatile memory ( 30 ) of the motor controller. A DSP-based closed-loop motor controller observes the motor parameters of the motor such as firing angle/duty cycles ( 23 ), voltage ( 37 ), current ( 9 ) and phase angles to arrive at a minimum voltage necessary to operate the motor at any load along the control line. The motor controller performs closed-loop control to keep the motor running at a computed target control point, such that maximum energy savings are realized by reducing voltage through pulse width modulation.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application Nos. 60/933,706 filed Sep. 14, 2007; and 61/135,402 filed Jul. 21, 2008. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to a system and method for maximizing the energy savings in AC induction motors at every load, more particularly one that uses a digital signal processor that calibrates control lines to determine the most efficient operational characteristics of the motors. 
         [0003]    In prior systems and methods related to energy saving motor controllers using control lines of a motor, constant phase angle and/or constant power factor control were used to determine the control lines. This meant that the control lines were horizontal and the motor controllers were not able to control the motor to specific calibrated operating point at every load to maximize energy savings. 
         [0004]    Thus, a need exists for a method and system for AC induction motors which controls the motor to a specific calibrated operating point at every load. Operating points taken across all loads will define a control line or a control curve. Furthermore, a need exists for a method and system for AC induction motors which is capable of recognizing when a motor begins to slip and is about to stall and uses that information to determine calibrated control line so as to maximize energy savings at every load. 
         [0005]    The relevant patents of prior art includes the following references: 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Patent/Ser. No. 
                 Inventor 
                 Issue/Publication Date 
               
               
                   
                   
               
             
             
               
                   
                 2008/0100245 
                 Turner 
                 May 01, 2008 
               
               
                   
                 7,288,911 
                 MacKay 
                 Oct. 30, 2007 
               
               
                   
                 7,279,860 
                 MacKay 
                 Oct. 09, 2007 
               
               
                   
                 7,256,564 
                 MacKay 
                 Aug. 14, 2007 
               
               
                   
                 7,211,982 
                 Chang et al. 
                 May 01, 2007 
               
               
                   
                 7,081,729 
                 Chang et al. 
                 Jul. 25, 2006 
               
               
                   
                 6,643,149 
                 Arnet et al. 
                 Nov. 04, 2003 
               
               
                   
                 6,489,742 
                 Lumsden 
                 Dec. 03, 2002 
               
               
                   
                 5,506,484 
                 Munro et al. 
                 Apr. 09, 1996 
               
               
                   
                 5,350,988 
                 Le 
                 Sep. 27, 1994 
               
               
                   
                   
               
             
          
         
       
     
       SUMMARY OF THE INVENTION 
       [0006]    The primary object of the present invention is to provide a system and method of maximizing energy savings in AC induction motors at every load. 
         [0007]    Another object of the present invention is to provide a system and method which recognizes when a motor begins to slip and when the motor is about to stall. 
         [0008]    A further object of the present invention is to provide a system and method which controls the motor to a specific calibrated operating point at every load. 
         [0009]    Another object of the present invention is to provide a motor controller that is capable of observing the operational characteristics of AC induction motors. 
         [0010]    A further object of the present invention is to provide a motor controller capable of making corrections to the RMS motor voltage as an AC induction motor is running and under closed loop control. 
         [0011]    Another object of the present invention is to provide a motor controller capable of responding to changes in the load of an AC induction motor in real-time. 
         [0012]    The present invention fulfills the above and other objects by providing a motor controller system and method for maximizing the energy savings in the motor at every load wherein a motor is calibrated at one or more load points, establishing a control line or curve, which is then programmed into a non-volatile memory of the motor controller. A digital signal processor (DSP) a part of a closed loop architecture of the motor controller possesses the capability to observe the motor parameters such as current, phase angles and motor voltage. This DSP based motor controller is further capable of controlling the firing angle/duty cycle in open-loop mode as part of a semi-automatic calibration procedure. In normal operation, the DSP based motor controller performs closed-loop control to keep the motor running at a computed target control point, such that maximum energy savings are realized. The method described here works equally well for single phase and three phase motors. 
         [0013]    The preferred implementation of this method uses a DSP to sample the current and voltage in a motor at discrete times by utilizing analog to digital converters. From these signals, the DSP can compute key motor parameters, including RMS motor voltage, RMS current and phase angle. Furthermore, the DSP based motor controller can use timers and pulse width modulation (PWM) techniques to precisely control the RMS motor voltage. Typically the PWM is accomplished by using power control devices such as TRIACs, SCRs, IGBTs and MOSFETs. 
         [0014]    The above and other objects, features and advantages of the present invention should become even more readily apparent to those skilled in the art upon a reading of the following detailed description in conjunction with the drawings wherein there is shown and described illustrative embodiments of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    In the following detailed description, reference will be made to the attached drawings in which: 
           [0016]      FIG. 1  is a block diagram of a digital signal processor (DSP) with hardware inputs and outputs of the present invention showing hardware inputs and outputs; 
           [0017]      FIG. 2  is a block diagram of a DSP-based motor controller of the present invention; 
           [0018]      FIG. 3  is a diagram showing a phase rotation detection method of the present invention; 
           [0019]      FIG. 4  is a flow chart showing a phase rotation detection method of the present invention; 
           [0020]      FIG. 5  is a graph showing power control device outputs for positive phase rotation; 
           [0021]      FIG. 6  is a graph showing power control device outputs for negative phase rotation; 
           [0022]      FIG. 7  is a block diagram of a window comparator; 
           [0023]      FIG. 8  is a schematic of the window comparator; 
           [0024]      FIG. 9  is a graph of a current waveform and zero-cross signals; 
           [0025]      FIG. 10  is a schematic of a virtual neutral circuit; 
           [0026]      FIG. 11  is a graph showing power control device outputs for single phase applications; 
           [0027]      FIG. 12  is a three-dimensional graph showing a three-dimensional control line of the present invention; 
           [0028]      FIG. 13  is a three-dimensional graph showing a control line projected onto one plane; 
           [0029]      FIG. 14  is a graph showing a two-dimensional plotted control line; 
           [0030]      FIG. 15  is a graph showing a sweeping firing angle/duty cycle in a semi-automatic calibration; 
           [0031]      FIG. 16  is a graph showing a directed sweep of a firing angle/duty cycle; 
           [0032]      FIG. 17  is a graph showing plotted semi-automatic calibration data; 
           [0033]      FIG. 18  is a graph showing plotted semi-automatic calibration data; 
           [0034]      FIG. 19  is a graph showing plotted semi-automatic calibration data; 
           [0035]      FIG. 20  is a flow chart of a semi-automatic high level calibration; 
           [0036]      FIG. 21  is a flow chart of a semi-automatic high level calibration; 
           [0037]      FIG. 22  is a flow chart of a manual calibration; 
           [0038]      FIG. 23  is a flow chart of a fixed voltage clamp: 
           [0039]      FIG. 24  is a graph showing a RMS motor voltage clamp; 
           [0040]      FIG. 25  is a graph showing a RMS motor voltage clamp; 
           [0041]      FIG. 26  is a flow chart of a stall mitigation technique; and 
           [0042]      FIG. 27  is a graph showing the stall mitigation technique. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0043]    For purposes of describing the preferred embodiment, the terminology used in reference to the numbered components in the drawings is as follows:
     1 . digital system processor (DSP)     2 . hardware inputs     3 . motor     4 . motor controller     5 . observed phase angle     6 . control line     7 . observed calibration data curve from sweep of control space     8 . supply divider resistors     9 . current     10 . target phase angle     11 . phase error signal     12 . proportional integral derivative (PID) controller     13 . root square mean (RMS) motor voltage     14 . power control device outputs     15 . phase A line voltage zero crossing     16 . phase B line voltage zero crossing     17 . phase C line voltage zero crossing     18 . positive phase rotation     19 . negative phase rotation     20 . power-on-reset (POR)     21 . stall point     22 . a,c,b phase turn on times     23 . firing angle/duty cycle     24 . percent load     25 . parametrical control line     26 . operating point     27 . low output impedance amplifier     28 . phase error     29 . control voltage     30 . point b     31 . knee     32 . calibration button     33 . power control device     34 . point c     35 . voltage minimum (Vmin)     36 . phase zero crossing inputs     37 . phase line voltage     38 . phase motor voltage     39 . time is measured     40 . is time greater or less than 90°     41 . ABC rotation     42 . ACB rotation     43 . point d     44 . place in loaded configuration     45 . place in unloaded configuration     46 . run calibration     47 . control line ends calibrated     48 . calculate control line     49 . saves control line     50 . line voltages     51 . set firing angle/duty cycle to 90°     52 . measure motor parameters     53 . detects knee     54 . decrease firing angle/duty cycle by 2°     55 . save phase angle and motor voltage     56 . repeat four times     57 . compute average values     58 . firing angle/duty cycle is increased     59 . measure next step     60 . fixed voltage clamp     61 . synthesize control segment     62 . analog to digital converter     63 . phase computation     64 . phase error is computed     65 . voltage error is computed connection     66 . RMS motor voltage is compared to fixed voltage threshold     67 . is control target positive     68 . voltage loop is run     69 . control line loop is run     70 . motor placed on dynamometer     71 . motor is connected to computer     72 . firing angle/duty cycle is increased and voltage decreased     73 . record calibration point     74 . start motor     75 . firing angle/duty cycle is adjusted     76 . form control line     77 . differential-to-single-ended amplifiers     78 . input resistors     79 . attenuator     80 . feedback resistor     81 . ground reference resistor     82 . protection diodes     83 . summing amplifier     84 . DC blocking capacitors     85 . summing resistors     86 . neutral     87 . jumper block for alternate neutral     88 . window comparator     89 . motor current is provided     90 . positive voltage is provided     91 . negative voltage is provided     92 . voltage passes through two comparators     93 . voltage passes through operation (OR) gate     94 . zero-cross digital signal is created     95 . current waveform     96 . positive voltage half cycle     97 . negative voltage half cycle     98 . OR function     99 . DSP monitors for increase in current     100 . increase is observed     101 . motor voltage is turned to full on     102 . motor voltage is reduced to control line     103 . load on the motor     104 . power applied to motor     105 . point a     106 . count sweeps   
 
         [0150]    With reference to  FIG. 1 , a block diagram of a digital signal processor (DSP)  1  and hardware inputs and outputs of the present invention is shown. The DSP  1  can observe the operational characteristics of a motor and make corrections to root mean square (RMS) voltage for the motor that is running and under closed loop control. Hardware inputs  2  capture phase zero crossing inputs  36 , phase line voltage  37 , phase motor voltage  38  and current  9  and passed through the DSP  1  for processing and then onto power control devices through the power control device outputs  14 . 
         [0151]    Referring now to  FIG. 2 , a block diagram of a system and method of the DSP-based motor controller  4  of the present invention is shown. First, the motor controller  4  reads the voltages  37  of each phase A, B and C and current  9  to capture the zero-crossing inputs  36 . At this point voltage  13  and current  9  may be converted from analog to digital using converters  62 . Next, computations  63  of motor phase angle for each phase are calculated to yield an observed phase angle  5 . Next, a target phase angle  10  which has been derived from a preprogrammed control line  6  is compared to the observed phase angle  5 . The difference between the target phase angle  10  and observed phase angle  5  yields a resulting phase error signal  11  which is processed by a digital filter called a proportional integral derivative (PID) controller  12  which has proportional, integral and differential components. The output from the PID controller  12  is the new control voltage  13  to the motor  3 , which can be obtained through the use of power control devices  33 , such as TRIACs, SCRs, IGBTs or MOSFETS, to yield power control device outputs  14  of RMS motor voltage  13  supplied with line voltages  50  for each phase for maximum energy savings. 
         [0152]    In this closed loop system, the voltage  13  of each phase of the motor  3  and the current are continually monitored. The motor controller  4  will drive the observed phase angle  5  to the point on the calibrated control line  6  corresponding to the load that is on the motor. At this point, maximum energy savings will be realized because the control line  6  is based on known calibration data from the motor  3 . The motor controller  4  can control the motor  3  just as if a technician set the voltage  13  by hand. The difference is that the DSP  1  can dynamically respond to changes in the load in real-time and make these adjustments on a cycle by cycle basis. 
         [0153]    Referring now to  FIG. 3 , in a three-phase system, the motor controller  4  is used to automatically determine the phase rotation. Zero-crossing detectors on the line voltages provide an accurate measurement of the angle between the phase A line voltage zero crossings  15  and the phase B line voltage zero crossings  16 . For positive phase rotation  18 , the angle is nominally 120° and for negative phase rotation  19 , the angle is nominally 60°. 
         [0154]    Referring to  FIG. 4 , a flow chart for phase rotation detection is shown. After a power-on-reset (POR)  20 , it is easy for the motor controller  4  to determine positive phase rotation  18  and the negative phase rotation  19 . First, the time is measured from phase A line voltage zero crossings to phase B line voltage zero crossings  39 . Next it is determined if the time is greater than or less than 90 degrees  40 . If it greater than 90 degrees, than it is an ACB rotation  42 . If the time is less than 90 degrees, than it is an ABC rotation  41 . The motor controller  4  of the present invention can control three-phase or single-phase motors with the same basic software and hardware architecture. For the three-phase case, depending on the phase rotation, the motor controller  4  can drive power control device outputs  14 . 
         [0155]    Referring now to  FIG. 5  which shows power control device outputs for positive drive rotation, the motor controller drives phase A power control device outputs  14  and phase B power control device outputs  14  together during the phase A line voltage zero crossings  15  turn-on time as indicated by the oval  22   a.  Similarly, the motor controller drives power control devices which drive phase B  16  and phase C power control device outputs  14  together during the phase B turn-on time as indicated by the oval  22   b.  Finally, the motor controller  4  drives phase C  17  and phase A power control device outputs  14  together during the phase C power control device outputs  14  turn-on time as indicated by the oval  22   c.  Note that the example shown in  FIGS. 5 and 6  depicts a firing angle/duty cycle  23  of 90°. 
         [0156]    Referring now to  FIG. 6  which shows the TRIAC drive outputs for negative phase rotation, the motor controller  4  drives phase A power control device outputs  14  and phase C power control device outputs  14  together during the phase A line voltage zero crossings  15  turn-on time as indicated by the oval  22   c.  Similarly, the motor controller  4  drives phase B  16  and phase A power control device outputs  14  together during the phase B line voltage zero crossings  16  turn-on time, as indicated by oval  22   a.  Finally, the motor controller drives phase C power control device outputs  14  and phase B power control device outputs  14  together during the phase C line voltage zero crossings  17  turn-on time, as indicated by oval  22   b.    
         [0157]    Now referring to  FIG. 7 , a block diagram of a window comparator is shown. The DSP based motor controller of the present invention uses the window comparator  88  to detect zero-crossings of both positive and negative halves of a current wave form. When RMS motor voltage is reduced by the motor controller, it if difficult to detect zero crossings of current waveform because the current is zero for a significant portion of both half cycles. First, motor current is provided  89 , a positive voltage is provided  90  as a reference for a positive half cycle and a negative voltage is provided  91  as a reference. Next, the current, positive voltage and negative voltage are presented to two comparators  92  and are then passed through an operation (OR) gate  93  to create a composite zero-cross digital signal  94 . 
         [0158]    As further illustrated in  FIG. 8 , a schematic of the window comparator  88  is shown. The motor current is provided  89 , a positive voltage is provided  90  as a reference for a positive half cycle and a negative voltage is provided  91  as a reference. Next, the current, represented as a positive voltage and negative voltage, is processed by two comparators  92  and are then passed to an OR gate  93  to create a composite zero-cross digital signal  94 . 
         [0159]    Further,  FIG. 9  shows graphs of a current waveform  95 , a positive voltage half cycle  96 , a negative voltage half cycle  97  and an OR function  98 . 
         [0160]    Now referring to  FIG. 10 , a schematic of a virtual neutral circuit is shown. A virtual neutral circuit may be used as a reference in situations where three phase power is available only in delta mode and there is no neutral present for use as a reference. The virtual neutral circuit comprises three differential-to-single-ended amplifiers  77 . Because phase to phase voltages are high, input resistors  78  are used to form a suitable attenuator  79  together with feedback resistors  80  and ground reference resistors  81 . Because the danger exists of a loss of phase, protection diodes  82  are used to protect the differential-to-single-ended amplifiers  77 . The differential-to-single-ended amplifiers  77  are coupled to a summing amplifier  83  through DC blocking capacitors  84  and summing resistors  85  together with the feedback resistor  80 . The output of of the summing amplifier  83  is boosted by amplifier  27  thereby providing a low impedance output which is at neutral potential. Additional resistors divide a supply rail thereby allowing the summing amplifier  83  to handle alternating positive and negative signals. An alternate connection is available in the event that a neutral  86  is available along with a jumper block for alternate neutral connection  87 . 
         [0161]    Referring now to  FIG. 11  showing a power control device output  14  for a single-phase application, the output  14  for phase A is turned on each half-cycle based on a power control device output  14  derived from the voltage zero-crossing input  15 . The power control device output  14  for phase B line voltage zero crossings and phase C line voltage zero crossings are disabled in the DSP  1  and the hardware may not be present. The power control device outputs  14  are not paired as they were in the three-phase case. 
         [0162]    Referring now to  FIG. 12  which illustrates a three-dimensional control line for the motor operating space of a motor bounded by an observed phase angle  5  on the y-axis. A controlled firing angle/duty cycle  23  showing the decrease in voltage is shown on the x-axis and the percent load  24  on a motor is shown on the z-axis. 
         [0000]    Every motor operates along a parametrical control line  25  within its operating space. For example, when a given motor is 50% loaded and the firing angle/duty cycle  23  is set to 100°, a phase angle  5  of approximately 55° is observed. 
         [0163]    The parametrical control line  25  shown in  FIG. 12  is defined by five parametric operating points  26  ranging from a loaded case  44  in the upper left corner, to an unloaded case  45  in the lower right corner. Furthermore, the parametrical control line  25  has special meaning because it is the line where a motor is using the least energy possible. If the firing angle/duty cycle  23  is increased and the motor voltage  13  decreased then a motor would slow down and possibly stall. Similar results would be seen if the load on the motor  3  is increased. 
         [0164]    As illustrated in  FIG. 13 , the parametric control line  25  may be parameterized and projected onto one plane described by phase angle  5  in the vertical direction and the firing angle/duty cycle  23  in the horizontal direction. 
         [0165]    Further, as shown in  FIG. 14 , the parametrical control line  25  may be displayed on a two-dimensional graph. On the x-axis, increasing firing angle/duty cycle  23  may be equated with a decreasing motor voltage. This is because small firing angle/duty cycles result in high voltage and large firing angle/duty cycles result in low voltage. The motor controller will drive the observed phase angle  5  to the point on the control line  25  that corresponds to the load presently on a motor. To accomplish this, a DSP computes the phase angle  5  between the voltage and current. 
         [0166]    Referring back to the block diagram of  FIG. 2 , the DSP  1  then computes the next target phase angle  5  based on the present value of the RMS voltage  13 , or equivalently the present value of the firing angle/duty cycle. The difference between the observed phase angle and the target phase angle  10  results in a phase angle error, which is processed through a proportional-integral-differential (PID) controller  12  or similar device to generate a new control target. This control target changes the voltage in such a way as to minimize the phase angle error. The target phase angle  10  is dynamic and it changes as a function of the firing angle/duty cycle. 
         [0167]    As stated above, the motor controller  4  will drive the observed phase angle  5  to the point on the control line  25  that corresponds to the load presently on the motor  3 . This operating point  26  provides the maximum energy savings possible because the control line  25  is calibrated directly from the motor  3  that is being controlled. 
         [0168]    This preferred method for calibration is called semi-automatic calibration. The semi-automatic calibration is based on the DSP  1  sweeping the control space of the motor. As shown in  FIG. 15 , sweeping the control space means that the DSP increases the firing angle/duty cycle  23  and records the current  9  and firing angle/duty cycle  23  of each phase at discrete points along the way. Thus, in this manner it is possible to see the beginning of the stall point  21  of the motor. A well-defined linear portion of observed calibration data curve obtained from sweeping the control space  7 , which is used to determine points on the control line  6 , has a constant negative slope at lower firing angle/duty cycles  23 . Then, as the firing angle/duty cycle  23  continues to increase, the current  9  begins to flatten out and actually begins to increase as the motor  3  begins to slip and starts to stall, called the “knee”  31 . 
         [0169]    As shown in  FIG. 16 , subsequent sweeps can be directed at smaller ranges of motor voltages to “zoom in” on the knee. The motor controller  4  requires multiple sweeps in order to get data that is statistically accurate. There is a tradeoff between the number of sweeps and the time required to calibrate the control line  25 . A measure of the quality of the calibration can be maintained by the DSP  1  using well known statistical processes and additional sweeps can be made if necessary. This is true because the DSP  1  has learned the approximate location of knee  31  from the first sweep. 
         [0170]    There is little danger of stalling during the semi-automatic sweep because of the controlled environment of the setup. A technician or operator helps to insure that no sudden loads are applied to the motor  3  under test while a semi-automatic calibration is in progress. 
         [0171]    The process of sweeping the control space can be performed at any fixed load. For example, it can be performed once with the motor  3  fully loaded and once with the motor  3  unloaded. These two points become the two points that define the control line  25 . It is not necessary to perform the calibration at exactly these two points. The DSP  1  will extend the control line  25  beyond both these two points if required. 
         [0172]    There are many numerical methods that can be applied to find the stall point  21  in the plot of the current motor voltage  23 . As shown in  FIG. 17 , the preferred method is to use the “least squares” method to calculate a straight line that best fits the accumulated data tabulated from the first five motor voltages  23 . 
         [0173]    The continuation of this method is shown in  FIG. 18 . Using the previous data points the value of the current  9  may be predicted. Graphically, the DSP  1  is checking for one or more points that deviate in the positive direction from the predicted straight line. 
         [0174]    As shown in  FIG. 19 , the DSP  1  is looking for the beginning of the knee in the curve. The first point that deviates from the predicted control line may or may not be the beginning of the knee  31 . The first point with a positive error may simply be a noisy data point. The only way to verify that the observed calibration data curve obtained from sweeping the control space  7  is turning is to observe data obtained from additional sweeps. 
         [0175]    Semi-automatic calibration may be performed in the field. Referring now to  FIG. 20 , a flow chart showing how semi-automatic calibration is performed is shown. First the motor  3  is placed in a heavily loaded configuration  44 . Ideally this configuration is greater than 50% of the fully rated load. Next a calibration button  32  on the motor controller  4  is pressed to tell the DSP  1  to perform a fully-loaded measurement. The DSP  1  runs a calibration  46  which requires several seconds to explore the operating space of the motor  3  to determine the fully-loaded point. The motor controller  4  indicates that it has finished this step by turning on an LED. 
         [0176]    Next the motor  3  is placed in an unloaded configuration  45 . Ideally this configuration is less than 25% of the rated load. Then a calibration button  32  on the motor controller  4  is pressed  47  to tell the DSP  1  to perform an unloaded measurement. The DSP  1  runs the calibration  46  to determine the unloaded point. The motor controller  4  indicates that it has finished calibrating both ends  47  of the control line  25  by turning on a light emitting diode (LED). The DSP  1  then determines the control line  48  using the two measurements and applies this control line when it is managing the motor  3 . The values of the control line  25  are stored in non-volatile memory  49 . 
         [0177]      FIG. 21  shows a more detailed flow chart of the semi-automatic calibration. First a first calibration sweep is run  46  with the motor voltage set at a certain degree  51 , depending on if it is a first sweep or previous sweeps have been run  106 , in which the motor controller measures the motor  52  until the motor controller detects a knee  53 . If a knee  53  is detected the firing angle/duty cycle is decreased by two degrees  54  and the phase angle and the motor voltage are recorded to the memory  55 . This process is repeated to obtain at least four sweeps  56  to get a computed average value  57  of the phase angle and the firing angle/duty cycle. If during any step along the calibration sweep, the knee is not detected, then the firing angle/duty cycle is increased by at least one degree  58  and the nest step is measured  59 . 
         [0178]    An alternative method for calibration is called manual calibration.  FIG. 22  shows a flow chart of manual calibration. First a motor is placed on a dynamometer  70 . Next the motor is connected to a computer for manual control  71  which allows the motor to be run in a open-loop mode and the firing angle/duty cycle of the AC induction motor to be manually set to any operating point. Then the motor is placed in a fully unloaded configuration  45 . Next the firing angle/duty cycle is increased and the RMS motor voltage is reduced  72  until the motor is just about to stall. The firing angle/duty cycle and phase angle are recorded and this becomes a calibrated point which is recorded  73 . Then the motor is started with drive elements fully on  74 . Then the motor is placed in a fully loaded configuration  44 . Next the firing angle/duty cycle is increased or decreased until the RMS motor voltage is chopped by the motor controller  75  until the motor is just about to stall. The firing angle/duty cycle are recorded and this becomes another calibrated point which is recorded  73 . Finally the two calibrated points are used to form a control line  76 . 
         [0179]    When the RMS line voltage is greater than a programmed fixed-voltage, the DSP controller clamps the RMS motor voltage at that fixed voltage so energy savings are possible even at full load. For example, if the mains voltage is above the motor nameplate voltage of 115V in the case of a single phase motor then the motor voltage is clamped at 115V. This operation of clamping the motor voltage, allows the motor controller to save energy even when the motor is fully loaded in single-phase or three-phase applications. 
         [0180]      FIG. 23  shows a flow chart of the fixed voltage clamp. First a phase error is computed  64 . Next a voltage error is computed  65 . Then the RMS motor voltage of the AC induction motor is determined and compared to a fixed voltage threshold  66 . If the RMS motor voltage is greater than the fixed voltage threshold then it is determined whether or not control target is positive  67 . If the control target is positive then a voltage control loop is run  68 . If the RMS motor voltage of the AC induction motor is less than a fixed-voltage threshold, then the a control line closed loop is run  69  and the entire process is repeated. If the control target is determined not to be positive then a control line loop is run  69  and the entire process is repeated again. 
         [0181]    In some cases, it may not be possible to fully load the motor  3  during the calibration process. Perhaps 50% is the greatest load that can be achieved while the motor is installed in the field. Conversely, it may not be possible to fully unload the motor; it may be that only 40% is the lightest load that can be achieved.  FIG. 24  shows an example of both load points being near the middle of the operating range. On the unloaded end  45  at the right of the control line  25 , the DSP  1  will set the fixed voltage clamp  60  of the voltage at minimum voltage  35 . When the load on the motor increases, the DSP  1  will follow the control line moving to the left and up the control segment  61 . This implementation is a conservative approach and protects the motor  3  from running in un-calibrated space. 
         [0182]    As further shown in  FIG. 25 , on the fully loaded end  44  at the left, the DSP  1  will synthesize a control segment  61  with a large negative slope. This implementation is a conservative approach and drives the voltage to full-on. 
         [0183]    Referring now to  FIG. 26 , the DSP-based motor controller uses a special technique to protect a motor from stalling. First, the DSP actively monitors for a significant increase in current  99  which indicates that load on the motor has increased. Next, if a significant increase is observed  100  then the DSP turns motor voltage to full on  101 . Next, the DSP will attempt to reduce motor voltage to return to the control  102  and the DSP returns to actively monitoring for a significant increase in current  99 . This technique is a conservative and safe alternative to the DSP attempting to track power requirements that are unknown at that time. 
         [0184]    As further shown in  FIG. 27 , a graph of the stall mitigation technique, the load on the motor is represented on an x-axis and time is represented on a y-axis. The bottom line represents the load on the motor  103  and the top line represents the power applied to the motor by the DSP  104 . Prior to point a  105 , the DSP is dynamically controlling the motor at a fixed load. In between point a  105  and point b  30 , the load on the motor is suddenly increased and the DSP turns the motor voltage to full on. At point c  34 , the DSP reduces the motor voltage to point d  43 . 
         [0185]    Although a preferred embodiment of a motor controller method and system for maximizing energy savings has been disclosed, it should be understood, it is not to be limited to the specific form or arrangement of parts herein described and shown. It will be apparent to those skilled in the art that various changes may be made without departing from the scope of the invention and the invention is not be considered limited to what is shown and described in the specification and drawings.