Abstract:
Embodiments of the present invention provide novel techniques for using a switched converter to provide for three-phase alternating current (AC) rectification, regenerative braking, and direct current (DC) voltage boosting. In particular, one of the three legs of the switched converter is controlled with a set of pulse width modulation (PWM) control signals so that the input AC phase having the highest voltage is rectified and one of the switches in the two other legs is turned on to allow for added voltage. This switching activity allows for voltage from multiple AC line mains to be combined, resulting in an overall boost of the DC voltage of the rectifier. The DC voltage boost can then be applied to the common DC bus in order to ameliorate voltage sags, help with motor starts, and increase the ride-through capability of the motor.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to the field of electric motor drives such as those used to control electric motors and similar loads. More particularly, the present invention relates to systems and methods for using switched converters to rectify alternating current (AC) into direct current (DC), boost the overall voltage of a DC bus, and provide for regenerative braking capabilities while reducing the number of components in the system. 
         [0002]    Power rectifier systems are used in a wide range of applications. For example, power converters that perform rectification are used with centrifuges, magnetic clutches, pumps and more generally, in electric motor drive controllers to rectify and condition incoming AC voltage and to supply DC voltage to the motor. Many electric motor controllers also include some type of motor braking ability, in which energy from the motor is re-converted while slowing the driven load. The energy resulting from the braking operation can either be fed into a resistor, which will convert the energy into heat, or fed back into the supply network. Electric motor controllers with regenerative braking ability can feed the energy back into the supply network. This regenerative braking ability is very useful in reducing energy usage and in decreasing operational costs. Electric motor drives may also provide for the ability to boost DC voltage during certain low-voltage conditions such as sagging line voltage, motor starting, and heavy motor loading. The added boost in DC voltage during these periods allows for maintaining normal operating conditions and also increases the life of the motor. However, one drawback of electric motor drives that provide for motor drive, regenerative braking, and DC voltage boosting is that they require many extra components, lose energy due to constant switching activity, and are costly. 
       BRIEF DESCRIPTION 
       [0003]    Embodiments of the present invention provide novel techniques for using a fundamental front end (FFE) rectifier to provide for AC rectification, regenerative braking, and DC voltage boosting. The FFE rectifier is simple to operate, uses less expensive components, and is more energy efficient in its switching activity than other types of switched converters. In particular, the FFE rectifier can incorporate a low impedance reactor (typically 3% impedance) and exhibits less energy loss due to switching than comparable rectifiers such as active front end (AFE) rectifiers. Cost can be minimized by reducing the number of system components and by lowering the operational expenses where possible. 
         [0004]    In one embodiment, a method for controlling an electric motor via a controller and a rectifier is provided. The rectifier includes a positive solid state switch and a negative solid state switch for each of three phases of voltage. The rectifier may convert the three input phases of alternating current voltage to direct current voltage which may then be applied to a direct current bus. The method includes the detection of the voltage of the direct current bus and the voltage of each phase of input voltage, the identification of the phase of input voltage having the highest absolute voltage, the cycling of the positive and negative solid state switches of an identified phase, and the placing of a solid state switch for the two other phases in a conducting stated based upon which other phase exhibits the greater voltage difference from the identified phase. 
         [0005]    In a second embodiment, a system is provided which includes a controller controlling an electric motor and a rectifier. The rectifier includes a positive solid state switch and a negative solid state switch for each of three input phases of alternating current power. The rectifier is capable of converting three input phases of alternating current power to direct current power applied to a direct current bus. A detector is also included which is capable of detecting the voltage of the direct current bus and the voltage of each input phase. The controller can use the detector to detect the voltage of the direct current bus and the voltage of each phase of input power. The controller can then identify the input phase having the highest voltage and can cycle the positive and negative solid state switches of the rectifier at the identified phase. The controller can then place a solid state switch of the rectifier for the two other phases in a conducting state based upon which other phase exhibits the greater voltage difference from the identified phase. 
         [0006]    In a third embodiment, a method is provided for controlling an electric motor via a controller and a rectifier. The rectifier includes a positive and a negative solid state switch for each of three phases of power. The rectifier converts three input phases of alternating current power to direct current power applied to a direct current bus. The method detects the voltage of the direct current bus, and the voltage of each input phase of input power. The method identifies the phase of input power having the highest voltage, and cycles the positive and negative solid state switches of an identified phase. The method places a solid state switch for the two other phases in a conducting state based upon which other phase exhibits the greater voltage difference from the identified phase. The method also determines a duty cycle for cycling the positive and negative solid state switches of the identified phase based on electric power requirements, and wherein the positive and negative solid state switches of the identified phase are cycled at the determined duty cycle. 
     
    
     
       DRAWINGS 
         [0007]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0008]      FIG. 1  is a schematic diagram of an exemplary embodiment of a rectifier being shown as a component in a larger circuit that is used to drive a three-phase motor. 
           [0009]      FIG. 2  is a three-phase voltage diagram alongside a switching diagram of a set of rectifier switches. 
           [0010]      FIG. 3  is an exemplary flow chart of the methodology used to select a set of rectifier switches to which control signals are applied. 
           [0011]      FIG. 4A  illustrates a timing diagram of a set of duty cycles that may be used for pulse width modulation (PWM) of rectifier switches in accordance with an embodiment of the present invention. 
           [0012]      FIG. 4B  illustrates a PWM control signal used to drive one of the rectifier switches. 
           [0013]      FIG. 4B  illustrates a PWM control signal used to drive a different rectifier switch. 
           [0014]      FIG. 5  is an exemplary flow chart that may be used to calculate the duty cycle d for pulse width modulation of switches of a rectifier. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  illustrates an embodiment of a three-phase motor controller  10 . The three-phase motor controller  10  may include an insulated gate bipolar transistor (IGBT) rectifier circuit  12  coupled to an inverter circuit  14 , which may be used to drive a three-phase motor  16 . In the illustrated embodiment, the rectifier circuit and the inverter circuit are controlled by a controller  18 . Three phases of AC voltage from the supply mains  20  are converted into DC voltage by the rectifier  12 . The DC voltage is then converted to controlled frequency AC voltage by the inverter circuit  14  to drive the motor  16 . In one embodiment of the invention the rectifier  12  may provide for AC rectification, braking regeneration, and DC voltage boosting. The rectifier circuit  12  comprises a set of solid state switches Sap  22 , San  24 , Sbp  26 , Sbn  28 , Scp  30 , and Scn  32 , each provided with a fly-back diode. In the present discussion, the subscripts “a”, “b” and “c” are used to designate each of three phases of voltage, while the subscripts “p” and “n” are used to designate “positive” and “negative” sides of the DC bus output of the converter circuitry  12  (although such designations are by convention only). During rectification, the switches of the circuit need not be controlled (i.e., switched). This allows for the diode components of the IGBT switches Sap  22 , San  24 , Sbp  26 , Sbn  28 , Scp  30 , and Scn  32  to act as a full wave rectifier that converts the incoming AC voltage  20  into DC voltage. The resulting DC voltage is transferred to a common DC bus and may then be converted into AC voltage by the inverter  14 . A DC bus capacitor Cdc  34  is connected between the two DC bus lines and is used to create a low impedance source which also helps filter DC ripples. Line reactor components comprised of inductors La  36 , Lb  38 , and Lc  40  are used as a filter to smooth converted power signals and to improve the harmonics of the circuit. 
         [0016]    In another phase of operation, the rectifier  12  allows for energy resulting from the braking of the three-phase motor  16  to be redirected back into the supply mains  20 . As will be appreciated by those skilled in the art, during regenerative braking, the motor  16  behaves as a three-phase generator. Consequently, the switches of the rectifier  12  are switched by controller  18  in such a way as to allow the alternating current flowing through the main bus to pass back into the supply network. Each one of the positive switches Sap  22 , Sbp  26 , and Scp  30  is turned on when its respective phase voltage is the most positive of the three (upper half of the respective wave). Similarly, each of the negative switches San  24 , Sbn  28 , and Scn  32  is turned on when its respective phase voltage is the most negative of the three (lower half of the respective wave). This switching activity is then able to recapture the energy resulting from the braking activity. 
         [0017]    In yet another phase of operation, the rectifier boosts the DC voltage applied to the DC bus.  FIG. 2  is helpful in detailing how the rectifier is able to provide for a boost in DC voltage.  FIG. 2  is a phase diagram of the three input line phases Va  42 , Vb  44 , and Vc  46  showing the input phases going through a full phase cycle (i.e., 0°-360°). The input line voltages are plotted on the abscissa  60  against the elapsed time (msec) which is plotted on the ordinate  62 . In an exemplary embodiment of the invention, switching schemes as represented by diagrams  64 ,  66 , and  68  may be used to boost the available DC bus voltage produced by the rectifier  12 . In particular, switching diagram  64  is a timing diagram that shows when the switches Sbn  28 , Sap  22 , Scn  32 , Sbp  26 , San  24 , and Scp  30  should be controlled with a PWM switching pattern set at duty ratio d. Switching diagram  66  is a timing diagram that shows when the switches Sbp  26 , San  24 , Scp  30 , Sbn  28 , Sap  22 , and Scn  32  should be controlled with a PWM switching patter set at a duty ratio 1-d. The switches found on switching diagram  68  are not controlled with PWM switching, but are rather controlled as shown in  FIG. 2  (i.e., placed in a conducting or non-conducting state by application or removal of an appropriate gate drive signal from the controller). A method that results in the switch timing described in switching diagrams  64 ,  66  and  68  will result in both the rectification of AC voltage through all regions  1 - 6  ( 70 - 80 ) of the full phase cycle as well as the addition of voltage from two AC line mains which in turn will boost the DC voltage output of the rectifier  12 . An FFE rectifier implementing such a timing of switches requires that only one leg (i.e., two switches) of the three AC legs be controlled by PWM switching. This saves switching energy as compared to active front ends circuits, which would require constant switching of all three legs (i.e., six switches). It is to be noted that the values of line voltage (along the abscissa  60 ) and the times (along the ordinate axis  62 ) illustrated in the figure are but one possible embodiment of the invention. Other alternate voltages values and timeline time measurements may be used. 
         [0018]      FIG. 3  is a flow diagram of an exemplary flow diagram for a process that may be used by the controller  18  to implement the switch timing described in switching diagrams  64 ,  66 , and  68  of  FIG. 2 . The embodiment shown in  FIG. 3  may allow for the controller  18  to combine voltage from the AC line mains Va, Vb, and Vc during the occurrence of the highest line-to-line voltage Vab  82 , Vbc  84 , or Vca  86  in each region  1 - 6  of the full phase cycle of  FIG. 2 . The resulting voltage combination may then be used to boost DC voltage. The controller  18  first finds the absolute values of the three line-to-line voltages Vab  82 , Vbc  84 , and Vca  86 , as indicated by blocks  88 ,  90 , and  92 . The absolute values of the three line-to-line voltages are then compared at block  94 , and the largest of the three values is chosen. As a first example, if Vab  82  is the largest value then the controller  18  determines whether Vab  82  is greater than zero, as indicated at block  96 . If Vab  82  is a positive value, then the absolute values of line-to-line voltages Vbc  84  and Vca  86  are compared at block  102 . If |Vbc| is found to be greater than |Vca|, then switch Sbn  28  is selected to be controlled with a PWM duty ratio of d, as indicated at block  114 . The opposite leg switch Sbp  26  is also selected to be controlled with PWM duty ratio of 1-d in order to maintain full-wave DC rectification. Switch Sap  22  will also be turned on at the same block  114 . All other switches of rectifier  12 , San  24 , Scp  30 , and Scn  32  will be unswitched during this period of operation. This switching arrangement allows for the voltage combination of Va  42  with Vb  44  during the second half of region  1  of  FIG. 2 , which results in a boost in DC voltage over the standard operating DC voltage output of rectifier  12 . 
         [0019]    Returning to block  102 , if |Vbc| is not found to be greater than |Vca|, then switch Sap  22  is selected to be controlled with a PWM duty ratio of d at block  116 . The opposite leg switch San  24  is also selected to be controlled with PWM duty ratio of 1-d in order to maintain full-wave DC rectification. Switch Sbn  28  will also be turned on at the same block  116 . All other rectifier switches, Sbp  26 , Scp  30 , and Scn  32  will be unswitched. This switching arrangement allows for the voltage combination of Va  42  with Vb  44  during the first half of region  2  of  FIG. 2 , which results in a boost in DC voltage over the standard operating DC voltage output of rectifier  12 . 
         [0020]    Continuing the example, if at block  96  the controller  18  determines that line-to-line voltage Vab  82  is not greater than zero, then the absolute values of line-to-line voltages Vbc  84  and Vca  86  are compared at block  104 . If |Vbc| is found to be greater than |Vca|, then switch Sbp  26  is selected to be controlled with a PWM duty ratio of d at block  118 . The opposite leg switch Sbn  28  is also selected to be controlled with PWM duty ratio of 1-d in order to maintain full-wave DC rectification. Switch San  24  will also be turned on at the same block  118 . All other rectifier switches, Sbp  26 , Scp  30 , and Scn  32  will be unswitched. This switching arrangement allows for the voltage combination of Va  42  with Vb  44  during the second half of region  4  of  FIG. 2 , which results in a boost in DC voltage over the standard operating DC voltage output of rectifier  12 . 
         [0021]    Continuing with block  104 , if |Vbc| is not found to be greater than |Vca|, then switch San  24  is selected to be controlled with a PWM duty ratio of d at block  120 . The opposite leg switch Sap  22  is also selected to be controlled with PWM duty ratio of 1-d in order to maintain full-wave DC rectification. Switch Sbp  26  will also be turned on at the same block  120 . All other rectifier switches, Sbn  28 , Scp  30 , and Scn  32  will be unswitched. This switching arrangement allows for the voltage combination of Va  42  with Vb  44  during the first half of region  5  of  FIG. 2 , which results in a boost in DC voltage over the standard operating DC voltage output of rectifier  12 . 
         [0022]    As a second example, if at block  94  the largest absolute value of the line-to-line voltages Vab  82 , Vbc  84 , and Vca  86  is determined to be Vbc  84 , then the controller  18  determines whether Vbc  84  is greater than zero at block  98 . If Vbc  84  is a positive value, then the absolute values of line-to-line voltages Vca  86  and Vab  82  are compared at block  106 . If |Vca| is found to be greater than |Vab| then switch Scn  32  is selected to be controlled with a PWM duty ratio of d at block  122 . The opposite leg switch Scp  30  is also selected to be controlled with PWM duty ratio of 1-d in order to maintain full-wave DC rectification. Switch Sbp  26  will also be turned on at the same block  122 . All other rectifier switches, Sbn  28 , Sap  22 , and San  24  will be unswitched. This switching arrangement allows for the voltage combination of Vb  44  with Vc  46  during the second half of region  3  of  FIG. 2 , which results in a boost in DC voltage over the standard operating DC voltage output of rectifier  12 . 
         [0023]    Continuing with block  106 , if |Vca| is not found to be greater than |Vab|, then switch Sbp  26  is selected to be controlled with a PWM duty ratio of d at block  124 . The opposite leg switch Sbn  28  is also selected to be controlled with PWM duty ratio of 1-d in order to maintain full-wave DC rectification. Switch Scn  32  will also be turned on at the same block  124 . All other rectifier switches Scp  30 , Sap  22 , and San  24  will be unswitched. This switching arrangement allows for the voltage combination of Vb  44  with Vc  46  during the first half of region  4  of  FIG. 2 , which results in a boost in DC voltage over the standard operating DC voltage output of rectifier  12 . 
         [0024]    If at block  98  the controller  18  determines that Vbc  84  is not greater than zero, then the absolute values of line-to-line voltages Vca  86  and Vab  82  are compared at block  108 . If |Vca| is found to be greater than |Vab| then switch Scp  30  is selected to be controlled with a PWM duty ratio of d at block  126 . The opposite leg switch Scn  32  is also selected to be controlled with PWM duty ratio of 1-d in order to maintain full-wave DC rectification. Switch Sbn  28  will also be turned on at the same block  126 . All other rectifier switches, Sbp  26 , Sap  22 , and San  24  will be unswitched. This switching arrangement allows for the voltage combination of Vb  44  with Vc  46  during the second half of region  6  of  FIG. 2 , which results in a boost in DC voltage over the standard operating DC voltage output of rectifier  12 . 
         [0025]    Continuing with block  108 , if |Vca| is not found to be greater than |Vab|, then switch Sbn  28  is selected to be controlled with a PWM duty ratio of d at block  128 . The opposite leg switch Sbp  26  is also selected to be controlled with PWM duty ratio of 1-d in order to maintain full-wave DC rectification. Switch Scp  30  will also be turned on at the same block  128 . All other rectifier switches, Scn  32 , Sap  22 , and San  24  will be unswitched. This switching arrangement allows for the voltage combination of Vb  44  with Vc  46  during the first half of region  1  of  FIG. 2 , which results in a boost in DC voltage over the standard operating DC voltage output of rectifier  12 . 
         [0026]    If at block  94  the largest absolute value of the line-to-line voltages Vab  82 , Vbc  84 , and Vca  86  is determined to be Vca  86 , then the controller  18  determines whether Vca  86  is greater than zero at block  100 . If Vca  86  is a positive value, then the absolute values of line-to-line voltages Vab  82  and Vbc  84  are compared at block  110 . If |Vab| is found to be greater than |Vbc| then switch San  24  is selected to be controlled with a PWM duty ratio of d at block  130 . The opposite leg switch Sap  22  is also selected to be controlled with PWM duty ratio of 1-d in order to maintain full-wave DC rectification. Switch Scp  30  will also be turned on at the same block  130 . All other rectifier switches, Scn  32 , Sbp  26 , and Sbn  28  will be unswitched. This switching arrangement allows for the voltage combination of Va  42  with Vc  46  during the second half of region  5  of  FIG. 2 , which results in a boost in DC voltage over the standard operating DC voltage output of rectifier  12 . 
         [0027]    Continuing with block  110 , if |Vab| is not found to be greater than |Vbc|, then switch Scp  30  is selected to be controlled with a PWM duty ratio of d at block  132 . The opposite leg switch Scn  32  is also selected to be controlled with PWM duty ratio of 1-d in order to maintain full-wave DC rectification. Switch San  24  will also be turned on at the same block  132 . All other rectifier switches, Sap  22 , Sbp  26 , and Sbn  28  will be unswitched. This switching arrangement allows for the voltage combination of Va  42  with Vc  46  during the first half of region  6  of  FIG. 2 , which results in a boost in DC voltage over the standard operating DC voltage output of rectifier  12 . 
         [0028]    If at block  100  the controller  18  determines that Vca  86  is not greater than zero, then the absolute values of line-to-line voltages Vab  82  and Vbc  84  are compared at block  112 . If |Vab| is found to be greater than |Vbc|, then switch Sap  22  is selected to be controlled with a PWM duty ratio of d at block  134 . The opposite leg switch San  24  is also selected to be controlled with PWM duty ratio of 1-d in order to maintain full-wave DC rectification. Switch Scn  32  will also be turned on at the same block  134 . All other rectifier switches, Scp  30 , Sbp  26 , and Sbn  28  will be unswitched. This switching arrangement allows for the voltage combination of Va  42  with Vc  46  during the second half of region  2  of  FIG. 2 , which results in a boost in DC voltage over the standard operating DC voltage output of rectifier  12 . 
         [0029]    Continuing with block  112 , if |Vab| is not found to be greater than |Vbc|, then switch Scn  32  is selected to be controlled with a PWM duty ratio of d at block  136 . The opposite leg switch Scp  30  is also selected to be controlled with PWM duty ratio of 1-d in order to maintain full-wave DC rectification. Switch Sap  22  will also be turned on at the same block  136 . All other rectifier switches, San  24 , Sbp  26 , and Sbn  28  will be unswitched. This switching arrangement allows for the voltage combination of Va  42  with Vc  46  during the first half of region  3  of  FIG. 2 , which results in a boost in DC voltage over the standard operating DC voltage output of rectifier  12 . 
         [0030]    It is to be noted that the methodology in  FIG. 3  is but one possible embodiment of a methodology that may be used in the present invention to select appropriate switching of rectifier legs through all the regions of a full phase cycle. An alternate embodiment of a methodology could use the same switching diagrams  64 ,  66 , and  68  found in  FIG. 2  but would select when to PWM and when to turn on or off the switches based on the values of line-to-neutral voltages Va  42 , Vb  44 , and Vc  46 . Yet another embodiment of a methodology could select when to PWM and when to turn on or off the switches based on a set of software timing flags in combination with line-to-line voltages Vab  82 , Vbc  84 , and Vca  86  or in combination with line-to-neutral voltages Va  42 , Vb  44 , and Vc  46 . Any switching methodology that results in the switching activity depicted in switching diagrams  64 ,  66 , and  68  of  FIG. 2  may be used. 
         [0031]    The duty ratios d and 1-d mentioned in previous embodiments of the current invention is explained in more detail in  FIGS. 4A ,  4 B, and  4 C.  FIG. 4A  depicts a timeline of time-varying duty ratios. The duty ratio d  142  of a switch is defined as the pulse duration divided by the pulse period. The ratio of the pulse duration over the pulse period may then be expressed as the percentage of time that the switch is in active operation (i.e., switched to a conducting state). For example, a switch that turns on for 10 ms out of 100 ms would have a duty ratio d=0.10 (10%).  FIG. 4A  depicts duty ratios from 0 to 1.0 (100%). An embodiment of the current invention calculates the duty ratio d  142  that will be used as described in previous sections to PWM a switch in one of the six legs of the rectifier  12 . The switch found on the inverse leg of the switch being driven with duty ratio d  142  will be driven with duty ratio 1-d  152 . 
         [0032]      FIG. 4B  shows typical control signal switching for a switch Sx  144  that is being controlled via PWM by controller  18  using a duty ratio of d  142 . Switch Sx  144  could be any one of the switches Sap  22 , San  24 , Sbp  26 , Sbn  28 , Scp  30 , Scn  32  that are found in the rectifier  12 . The duty ratio of d  142  can be transformed to an equivalent PWM control signal at a certain modulating frequency with the aid of the equation Frequency=1/Period. The exemplary frequency range for motor control of the current invention is between 2 and 4 kHz. The duty ratio d  142  is converted into a PWM control signal  148  which is used to drive switch Sx  144 . 
         [0033]      FIG. 4C  shows typical control signal switching for a switch Sy  150  that is being controlled via PWM by controller  18  using a duty ratio of 1-d  152 . Switching of switch Sy  150  is the inverse leg of that for switch Sx  144  shown in  FIG. 4B . For example, if switch Sx  144  corresponds to switch Sap  22 , then switch Sy  150  will correspond to switch San  24 , and vice versa. Switch Sy  150  could be any one of the switches Sap  22 , San  24 , Sbp  26 , Sbn  28 , Scp  30 , Scn  32  that are found in the rectifier  12  as long as it is an opposite of switch Sx  144 . Here again, the duty ratio of 1-d  152  can be transformed to an equivalent PWM control signal at a certain modulating frequency, by the equation Frequency=1/Period. The frequency used for duty ratio d  142  and duty ratio 1-d  152  is the same and is typically between 2 and 4 kHz. The duty ratio 1-d  152  is converted into a PWM control signal  154  which is used to drive switch Sy  150 . In one embodiment of the invention, the PWM control signals  148  of  FIG. 4B  are used to control all the switches in  FIG. 3  having a duty cycle of d. The complementary PWM control signals  154  of  FIG. 4C  are then used to control all the switches  FIG. 3  having a duty cycle of 1-d. 
         [0034]    The duty cycle d  142  determines the magnitude of the DC voltage boost that may be achieved.  FIG. 5  is a flow diagram in an exemplary embodiment of the invention which may be used by the controller  18  to calculate the duty cycle d  142  which then may be used to create the PWM control signals  148 . The controller  18  first finds the absolute values of the three line-to-line voltages Vab  82 , Vbc  84 , and Vca  86  at blocks  158 ,  160 , and  162 . The absolute values of the three line-to-line voltages are then compared at block  164  and the largest of the three values is assigned to the variable Vin  166 . The equation for d 
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         [0000]    is solved at block  168 . The variable V*dc is set to be nominally below the standard non-boosted DC bus voltage. Typically, when the input line-to-line voltage is 480 Vac, the V*dc voltage is set to 600 volts and may be increased to up to 629 volts for applications requiring an operational DC bus voltage of 630 volts. The constant k is in the range 0&lt;k&lt;1. The constant k may be set depending on the desired system stability under various conditions and is typically set to an exemplary value of 0.9. 
         [0035]    In another embodiment of the invention, controller  18  may automatically boost the DC voltage of the DC bus by employing embodiments of the current invention. Controller  18  can detect low voltage conditions such as when the DC Bus voltage falls below a percentage range, for example, below 5% of a rated or steady state voltage. Controller  18  may then boost the DC bus voltage to a desired level. Similarly, controller  18  may detect when one or more of the three phases of AC input voltage falls below a certain voltage, for example, during brownout conditions. Controller  18  may then boost the DC bus voltage thus allowing the electric motor to continue to operate normally. Controller  18  can also detect when the DC voltage and/or the AC input voltage has returned to a normal operating range, for example within 5% of operating voltage, and automatically turn off the boosting of DC voltage. In yet another embodiment of the invention, controller  18  may automatically solve for d  142 . The controller  18  may solve for d continuously during electric motor operation and then use d in conjunction with embodiments of the current invention to boost the DC bus voltage to a desired level. 
         [0036]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.