Abstract:
A method for dynamically controlling regenerating energy flow and direct current bus voltage for an adjustable frequency drive system includes sensing a direct current voltage of a direct current bus; and sensing a plurality of alternating currents at outputs of an inverter. The sensed alternating currents are transformed to a stationary current vector. Voltage and frequency values are converted to a stationary voltage vector and an angle. The angle and the stationary current vector are transformed to a rotating current vector including torque and flux producing current components. An induction machine generating mode is determined when the torque producing current component reverses polarity. The voltage and frequency values limit the direct current voltage of the direct current bus at a predetermined threshold responsive to the generating mode. The stationary voltage vector and the angle are converted to pulse width modulated control inputs of the inverter.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to adjustable frequency drive (AFD) systems and, in particular, to such AFD systems, which control the speed, torque, horsepower and/or direction of an induction machine, such as an AC motor. 
     2. Background Information 
     An Adjustable Frequency Drive (AFD) system may be employed in a wide range of commercial applications, such as, for example, HVAC, fans, pumps, conveyors, material handling and processing equipment, and other general industries, such as, for example, forest products, mining, metals and printing. 
     If the stator terminals of an induction machine are connected to a three-phase AFD system, then the rotor will rotate in the direction of the stator rotating magnetic field. This is the induction machine motoring mode of operation. When load torque is applied to the motor shaft, the steady state speed is less than the synchronous speed. 
     When the induction machine speed is higher than the synchronous speed, and the induction machine rotates in the same direction as the stator rotating field, then the induction machine is in the generating mode of operation. A generating torque acting opposite to the stator rotating magnetic field is produced. For example, to stop an AFD system, the power supply frequency is gradually reduced. In the deceleration process, the instantaneous speed of the AFD system is higher than the instantaneous synchronous speed because of the system inertia. As a result, the generating action of the induction machine will cause the power flow to reverse and the kinetic energy of the AFD system is fed back to the power supply source. 
     For AFD systems, the braking or regenerative energy of the system flows, for example, from the motor, through diodes in the inverter section of the drive, and into DC bus capacitors. Typically, the input diodes of the upstream converter do not provide a path for this energy to be returned to the AC power line. Hence, the regenerative current flows into the DC bus capacitance and, thus, the DC bus voltage increases. 
     There are four common methods to deal with the high DC bus voltage due to the regenerating condition. The first method employs silicon controlled rectifiers (SCRs), insulated gate bipolar transistors (IGBTs) or gate controlled thyristors (GCTs or GTOs), as converters in order to both provide power to the DC bus when motoring and to regenerate from the DC bus back to the AC line when braking. This method has the disadvantage of relatively higher cost since the converter section is similar to or the same as the inverter section. 
     The second method simply initiates a drive fault and trip when the DC bus voltage becomes too high. The disadvantage of this solution is the disruption of the process because of a resulting shut down or nuisance trip. 
     The third method handles the regenerating energy without tripping by employing a braking resistor, which provides a path to dissipate the regenerative energy. A braking resistor control circuit senses the high voltage condition and, then, electrically connects the braking resistors across the DC bus. The braking resistors (e.g., bus clamps; snubbers; voltage limiters) dissipate the excess energy. For example, for 230 VAC drives, the DC bus is approximately 310 VDC, and for 460 VAC drives, the DC bus is approximately 620 VDC. The actual DC bus voltage is about 1.35 times the RMS AC line voltage. The current through the braking resistor is proportional to the DC bus voltage divided by its resistance. For example, a 20 Ω resistor module connected across a 460 VAC line dissipates about 10 kW, while the same resistor dissipates about 20 kW when electrically connected to a DC bus, which is produced by rectifying a 3-phase AC line for a 460 VAC drive. The costs of the braking resistor can be significant, while the physical size of the resulting drive assembly increases. Both of these are normally undesirable results. 
     The fourth method actively limits the DC bus voltage at a safe threshold by applying proper control algorithms in response to regenerating conditions. However, when the induction machine operates in its normal motoring mode, the high input (utility) AC line voltages can push the DC bus voltage to reach the regenerating voltage threshold. In order to prevent the control algorithms from limiting the DC bus voltage in this scenario, it is known to employ input AC potential transformers or other voltage amplifier apparatus to measure the AC line input voltages. This approach similarly has the disadvantages of relatively higher cost and relatively larger physical dimensions. 
     The known solutions to handle AFD regenerating conditions have the disadvantages of shutting down industrial processes, relatively higher equipment costs and/or relatively larger physical sizes. 
     The generating mode of an AFD system causes the DC bus voltage to rise. The AC line input voltages to the system can be higher than the rated value in steady state. Also, momentary surges can occur in such voltages of any electrical distribution system, thereby causing a relatively higher DC bus voltage to be present in these cases. Therefore, it is difficult to determine which source is creating the energy that results in an over-voltage condition on the DC bus. 
     There is room for improvement in AFD systems and methods and apparatus for controlling the same. 
     SUMMARY OF THE INVENTION 
     These needs and others are met by the present invention, which identifies and controls regenerating energy flow for Adjustable Frequency Drives (AFDs). This regenerating energy flow is developed in both steady state and dynamic operating conditions of a three-phase induction machine, which is controlled by the AFD. The method and apparatus distinguish the energy source both in steady state and in dynamic transient conditions. 
     The three-phase currents of the AFD are converted into a stationary vector of current. The three-phase AC currents are measured by employing suitable current sensors (e.g., Hall effect) and are transformed into two-phase AC currents through the Clarke Transformation in the stationary reference frame. A space vector technique is employed to develop an angle. The Park Transformation employs the two-phase AC currents and the angle to produce two-phase DC current vectors in the rotating reference frame. These DC current vectors include the induction machine torque and flux producing components. 
     If the torque producing current vector reverses its polarity (e.g., direction; sign) when the commanded speed is less than the actual rotor speed, then the regenerating condition is confirmed. 
     If the regenerating condition is true, then a control algorithm turns on to limit the rising DC bus voltage at its predetermined threshold. When the induction machine is in the regenerating mode, the DC bus voltage is clamped at the predetermined threshold without tripping. The DC bus voltage is regulated dynamically by a compensation module, in order to stay at the predetermined threshold, thereby controlling the regenerating energy flow without necessarily tripping the AFD or adding costs and sizes to the overall electrical system. After the regenerating energy is dissipated and the induction machine is no longer in the generating mode, the DC bus voltage is automatically reset to the normal level in the motoring mode. 
     If the regenerating condition is not true, then the predetermined threshold adjusts itself based on the DC bus voltage level as determined by the AC line inputs. 
     The present invention provides a stable operating system and eliminates the disadvantages of the known prior art. In addition, the torque and flux producing current vectors in the rotating reference frame may be employed to conveniently calculate the induction machine power factor. 
     As one aspect of the invention, a method for dynamically controlling induction machine regenerating energy flow and direct current bus voltage for an adjustable frequency drive system comprises: sensing a direct current voltage of a direct current bus; sensing a plurality of alternating currents at alternating current outputs of an inverter; transforming the sensed alternating currents to a stationary current vector; converting a voltage value and a frequency value to a stationary voltage vector and an angle; transforming the angle and the stationary current vector to a rotating current vector including a torque producing current component and a flux producing current component; determining a generating mode of the induction machine when the torque producing current component reverses polarity; employing the voltage value and the frequency value to limit the direct current voltage of the direct current bus at a predetermined threshold responsive to the generating mode of the induction machine; and converting the stationary voltage vector and the angle to pulse width modulated control signals for the control inputs of the inverter. 
     As another aspect of the invention, an adjustable frequency drive for an induction machine comprises: a converter including a plurality of alternating current inputs and a direct current output having a first node and a second node; a capacitor electrically connected between the first and second nodes of the direct current output; an inverter including a direct current input, a plurality of switches, a plurality of control inputs, and a plurality of alternating current outputs, the direct current input electrically connected to the direct current output, the alternating current outputs adapted to electrically connect to alternating current inputs of the induction machine; each of the switches electrically connected between one of the first and second nodes and one of the alternating current outputs of the inverter, each of the control inputs controlling one of the switches; a voltage sensor sensing a direct current voltage at the direct current output of the converter and outputting a sensed voltage value; a plurality of current sensors sensing a plurality of alternating currents at the alternating current outputs of the inverter and outputting a plurality of sensed current values; and a processor comprising: a plurality of inputs for the sensed current values and the sensed voltage value, a plurality of outputs for the control inputs of the inverter, a Clarke Transform module transforming the sensed current values to a stationary current vector, a space vector module including a plurality of inputs and a plurality of outputs, the inputs of the space vector module comprising a set voltage value, a set frequency value, a change voltage value, and a change frequency value, the outputs of the space vector module comprising a stationary voltage vector and an angle, a pulse width modulation module including a plurality of inputs and a plurality of outputs, the inputs of the pulse width modulation module comprising the stationary voltage vector and the angle, the outputs of the pulse width modulation module providing the control inputs of the inverter, a Park Transform module transforming the angle and the stationary current vector to a rotating current vector, and a regeneration override module comprising a plurality of inputs and a plurality of outputs, the inputs of the regeneration override module comprising the sensed voltage value and the rotating current vector, the outputs of the regeneration override module comprising the change voltage value and the change frequency value, the regeneration override module dynamically controlling regenerating energy flow from the induction machine to the direct current output of the converter. 
     As another aspect of the invention, an adjustable frequency drive system comprises: a three-phase induction machine; and the adjustable frequency drive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
     FIG. 1 is a block diagram of an AFD system for an induction machine in accordance with the present invention. 
     FIG. 2 is a vector diagram defining a motoring mode reference frame. 
     FIG. 3 is a vector diagram defining a generating mode reference frame. 
     FIG. 4 is a vector diagram including a voltage space vector projection on rotating and stationary reference frames. 
     FIG. 5 is a vector diagram including a current space vector projection on rotating and stationary reference frames. 
     FIG. 6 is a vector diagram of the Clarke Transform. 
     FIG. 7 is a plot of the three Clarke Transform input current signals versus time. 
     FIG. 8 is a plot of the two Clarke Transform output current signals versus time. 
     FIG. 9 is a vector diagram of the Park Transform. 
     FIG. 10 is a plot of the two Park Transform input current signals and the Park Transform input angle versus time. 
     FIG. 11 is a plot of the two Park Transform output current signals versus time. 
     FIG. 12 is a vector diagram defining a space vector. 
     FIG. 13 is a diagram showing IGBT gating in six different active states or 60° sectors for the space vector PWM module of FIG.  1 . 
     FIG. 14 is a block diagram of the Regeneration Override Logic Module of FIG.  1 . 
     FIG. 15 is a sample plot of induction machine speed versus time for the AC motor of FIG.  1 . 
     FIG. 16 is a sample plot of induction machine torque versus time for the AC motor of FIG.  1 . 
     FIG. 17 is a sample plot of AFD DC bus voltage versus time for the AFD system of FIG.  1 . 
     FIG. 18 is a sample plot of torque producing current versus time for the AFD system of FIG.  1 . 
     FIG. 19 is a block diagram showing the application of the three outputs of the Regeneration Override Logic algorithms of FIG.  14 . 
     FIG. 20A is a plot of the amplitude of the output to input signal gain of the compensation modulator of FIG. 14 versus frequency. 
     FIG. 20B is a plot of the phase angle between the output and input signals of the compensation modulator of FIG. 14 versus frequency. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a three-phase Adjustable Frequency Drive (AFD) system  2  including an AFD  4  and an induction machine, such as AC motor (M)  6 . The AFD  4  includes a suitable converter, such as a full-wave rectifier bridge  8 , a DC bus capacitor bank  10 , an inverter  12 , three current sensors  14 , 16 , 18 , a DC bus voltage sensor  20 , and a control system  22  including a processor and a plurality of control algorithms. The rectifier bridge  8  converts three-phase AC voltages  24  (from AC phases A,B,C) to an intermediate constant DC bus voltage, V bus . Preferably, the rectifier bridge  8  effectively blocks most troublesome harmonics from reaching the AC power source (not shown). Since the rectifier bridge  8  employs diodes (not shown) and, thus, has no firing delays, the AFD system  2  may have a nearly constant power factor. 
     The DC bus capacitor bank  10  includes one or more energy storing capacitors (only one capacitor is shown), which help to keep the DC bus voltage, V bus , constant. Preferably, the DC bus capacitor bank  10  is able to ride through power system transients and is capable of keeping active the circuits of the control system  22 . 
     The inverter  12  inverts the DC bus voltage to an adjustable frequency, adjustable voltage AC output for the AC motor  6 . For example, during each cycle of the fundamental excitation frequency, the insulated gate bipolar transistors (IGBTs)  26 , 28 , 30 , 32 , 34 , 36  of the inverter  12  are switched on and off at up to about 20 kHz, in order to create a variable voltage output for the AC motor  6 . The benefits of relatively higher IGBT switching frequencies are lower motor noise, more sinusoidal current waveforms resulting in less motor harmonic heating, and higher control accuracy. The disadvantages of relatively higher IGBT switching frequencies include lower efficiencies and the possibility of peak voltage damage to the motor insulation. 
     The current sensors (e.g., Hall effect)  14 , 16 , 18  are placed on the AC outputs of the AFD  4  in order to measure the three-phase AC motor currents. Preferably, the DC bus voltage sensor  20  employs a suitably low cost circuit in order to produce a suitable analog, V actual , of the DC bus voltage, V bus . 
     The control system  22  preferably includes a suitable processor, such as digital signal processor (DSP)  38 , which implements the control algorithms for controlling the motoring or regenerating energy flow. The DSP  38  includes four analog inputs  38 A- 38 D corresponding to four DSP A/Ds (not shown) for the three current signals, I a ,I b ,I c , and the one voltage signal, V actual . The DSP  38  also includes a plurality of digital outputs  39  for the control inputs of the inverter  12 . A suitable interface (I/F)  40  buffers six gate control signals from the digital outputs  39  to the IGBTs of the inverter  12 . The control system  22  further includes a Clarke Transform module  42 , a Park Transform module  44 , a regeneration override logic module  46 , a space vector module  48 , and a space vector PWM module  50 , which are executed by the DSP  38 . The space vector PWM module  50  outputs (e.g., through the DSP outputs  39 ) the six gate control signals to the interface  40 . 
     As shown in FIGS. 2 and 3, the measured three-phase currents I a ,I b ,I c  from the sensors  14 , 16 , 18 , respectively, at the outputs of the inverter  12  of FIG. 1 are described in a stationary reference frame, wherein stationary real axes a,b,c are separated by 120 degrees. A stationary complex reference frame d,q includes a real axis d and an imaginary axis q. The complex real axis d is fixed to the stationary real axis a, which is the magnetic axis of the stator winding (not shown) of the AC motor  6 . Both the stator current, I, and the stator voltage, V, are complex space vectors, which rotate about these axes at the rate of the excitation angular velocity, ω, produced by the AFD  4 . 
     As shown in FIGS. 6-8, the three-phase currents I a ,I b ,I c  in the axes a,b,c, respectively, are transformed into two-phase quantities I d ,I q  in the axes d,q, respectively, using the Clarke Transform module  42  of FIG. 1, which employs the Clarke Transform Equation (Equation 1).              {               I   d     =     I   a                                I   q     =       (       2        I   b       +     I   a       )     /     3                       (     Eq   .              1     )                                
     The projected two-phase current vectors, I d ,I q , in the axes d,q, respectively, together with the angle θ as obtained from the space vector module  48  of FIG. 1 are fed into the Park Transform module  44  as shown in FIGS. 9-11. The Park Transform module  44  employs the Park Transform Equation (Equation 2).              {             I   D     =         I   d        cos                 θ     +       I   q        sin                 θ                     I   Q     =         I   d        sin                 θ     +       I   q        cos                 θ               }           (     Eq   .              2     )                                
     As a result, the stationary frame two-phase quantities I d  and I q  are transformed into rotating two-phase quantities I D  and I Q  on the rotating real axis D and the rotating imaginary axis Q, respectively. Thus, when viewed from the rotating reference frame D,Q, the current and voltage space vectors I,V become stationary. The real axis D of the rotating reference frame is located at the angle θ from the real axis d of the stationary reference frames. The angular velocity for θ is determined by the AFD commanded frequency, f, of FIG. 1 as input by the space vector module  48 . 
     As shown in FIGS. 4 and 5, φ 1  is the angle between the voltage space vector V and the D-axis, and φ 2  is the angle between the current space vector I (as shown in FIGS. 1-3 and  5 ) and the D-axis. The angle relationship is φ=φ 1 −φ 2 . In FIG. 2, the induction machine, such as the AC motor  6  of FIG. 1, is in the motoring mode when the voltage V leads the current I by φ. The load is inductive in this mode. In FIG. 3, the induction machine is in the generating mode with the voltage V lagging the current I by φ. The load behaves as a capacitive component. The angle φ may, thus, distinguish the induction machine motoring and generating modes of operation. 
     Referring to FIGS. 3,  5  and  18 , the torque producing current component I Q  actually reverses its polarity (e.g., direction or sign) during the transition from the motoring mode to the generating mode of operation. This transition may be captured, for example, by the DSP  38  employing a suitably high sampling rate. This is the ultimate indication, under a relatively fast deceleration command, if the generating mode operation is in place. In the case of a relatively slower deceleration command, a combination of this method and the ability to detect the difference of the commanded speed and the actual induction machine speed may be employed. 
     When the induction machine is in its normal motoring mode operation, the regenerating DC bus voltage threshold, V threshold  of FIG. 14, moves up and down based on the three-phase AC voltages  24  of FIG. 1 from the AC power source (not shown). For example, the DC bus voltage, V bus , is approximately 1.35 times the AC line input voltage. For a 480 VAC input, the DC bus voltage is about 670 VDC. For this condition, the regenerating DC bus voltage threshold is preset to about 730 VDC (e.g., about 60 VDC above the actual DC bus voltage). For a 500 VAC input, the DC bus voltage is about 700 VDC. Here, the regenerating DC bus voltage threshold is preset to about 760 VDC (e.g., about 60 VDC above the actual DC bus voltage). Preferably, the DC bus voltage threshold is determined from an average of a suitable count of samples (e.g., without limitation, 8-10 samples) of the DC bus voltage. Accordingly, the DC bus voltage is adjusted dynamically solely based on the AC line input without actually utilizing any input AC line voltage measurement apparatus. Although an example offset of about 60 VDC between the actual DC bus voltage and the regenerating voltage threshold is disclosed, any suitable positive offset may be employed. 
     Once the regenerating mode is detected, the real time regeneration override logic module  46  of FIG. 1 initiates a series of algorithm commands (as discussed below in connection with FIG.  14 ), in order to provide regenerating energy flow control. The regeneration override logic module  46  allows the AFD  4  to accelerate and decelerate without a high DC bus voltage trip. For example, for a 4-pole, 3-phase, 460 VAC, 60 Hz induction machine, the performance may include: (1) acceleration/deceleration from/to 1800 rpm unloaded at a rate of 60 Hz/s (1 s time) at an input voltage of 480 VAC; (2) acceleration/deceleration from/to 1800 rpm unloaded at a rate of 120 Hz/s (0.5 s time) at an input voltage of 480 VAC; (3) acceleration/deceleration from/to 1800 rpm unloaded at a rate of 600 Hz/s (0.1 s time) at an input voltage of 480 VAC; (4) acceleration/deceleration from/to 1800 rpm unloaded at a rate of 60 Hz/s (1 s time) at an input voltage of 504 VAC; (5) acceleration/deceleration from/to 1800 rpm unloaded at a rate of 120 Hz/s (0.5 s time) at an input voltage of 504 VAC; and (6) acceleration/deceleration from/to 1800 rpm unloaded at a rate of 600 Hz/s (0.1 s time) at an input voltage of 504 VAC. 
     In the regeneration override logic module  46  of FIG. 14, the difference between the sampled DC bus voltage, V actual , and the predetermined voltage threshold, V threshold , is determined by a summing function (SUM)  52 , which inputs the negative predetermined voltage threshold. If the sampled DC bus voltage is below the predetermined voltage threshold, then the regeneration override logic is inactive. However, as soon as the threshold is reached, there are active algorithm paths through three algorithms  54 , 56 , 58  to control the regenerative energy flow. The first algorithm  54  applies a proportional regulator gain G 1  to the voltage difference  60 , which is output by the summing function  52 . As shown in FIG. 19, the resulting quantity P 1   62  is applied to acceleration/deceleration logic  64 . The motor set speed  66  is set by the frequency input f of FIG.  1 . The f ∘ requency input f goes through the acceleration/deceleration logic  64 , in order to ramp up or ramp down the motor  6  at a preset acceleration or deceleration rate. In the motoring mode, the quantity P 1   62  is zero. In the regenerating mode, the quantity P 2   68  represents the additional amount of frequency command to be added to the original set frequency f (which is determined externally to the DSP  38  by the user through a communication link (not shown) between the DSP  38  and another microcontroller (not shown)), in order to ramp up or ramp down the frequency at a predetermined rate. For example, the proportional gain G 1  may be based upon the six performance items of the preceding paragraph. In this example, if the proportional gain G 1  is suitably set together with gains G 2  and G 3 , then the DC bus voltage, V bus , is clamped as shown in FIG. 17 under the test conditions of the six performance items. Otherwise, if the gain G 1  is not suitably set, then the DC bus voltage, V bus , may lose control, thereby tripping the AFD  4 . 
     As shown in FIG. 14, a compensated voltage difference  70  to the second and third algorithms  56 , 58  is the positive voltage difference  60  as compensated by the compensation modulator  72 . The output  74  of the compensation modulator  72  feeds into the gains G 2  and G 3  of the respective algorithms  56  and  58 . The quantities P 2   68  and P 3   76  are both zero if the motoring mode is active. In the generating mode, the quantity P 2   68  is another additional amount of frequency, which summing function  77  adds to the frequency output  78  of the acceleration/deceleration logic  64 . The quantity P 3   76  represents the additional amount of voltage (dV) to be added to the motor  6 . At relatively very low frequencies, the amplitude of the compensation modulator output quantities P 2   68  and P 3   76 , with P 3  being about several times larger than P 2 , is nearly a constant value, with the voltage difference  60  at the compensation modulator input  80  being provided with a constant gain A  79  (as shown in FIG. 20A) by the compensation modulator  72 . The phase angle of the compensated voltage difference  70  of the compensation modulator output  74 , is nearly the same as the phase angle of the voltage difference  60  at the input  80  of the compensation modulator  72 . At intermediate frequencies, the amplitude of the compensated voltage difference  70  increases nonlinearly with frequency for a given input amplitude as shown in FIG.  20 A. As shown in FIG. 20B, the phase angle of the compensated voltage difference  7   {circle around (∘)}   0  has a bell shape and, as a result, the phase angle of the compensated voltage difference  70  leads the phase angle of the voltage difference  60  by more than about 40 degrees at the peak of the bell shaped curve. At relatively very high frequencies, the amplitude of the compensated voltage difference  70  is nearly a constant value, with the voltage difference  60  at the compensation modulator input  80  being provided with a new constant gain B  81 . The phase angle of the compensated voltage difference  70  is nearly the same as the phase angle of the voltage difference  60 . The transition from the low frequency control characteristics to the high frequency control characteristics is done automatically, as shown in FIGS. 20A and 20B, without using any switching apparatus. 
     As employed in FIGS. 14 and 20A, the signal at input  80  is denoted as V in (f), and the signal at output  74  is denoted as V out (f), with V in (f) and V out (f) changing with f (frequency). The value of the plot in FIG. 20A is 20 log (V out (f)/V m (f)). If V in (f) and the curve are known, then V out (f) can be determined by the previous expression. 
     The second algorithm  56  applies a proportional gain term G 2  to the compensated voltage difference  70 . As shown in FIG. 19, the quantity P 2   68  is summed with the frequency output  78  of the acceleration/deceleration logic  64 , while bypassing such logic  64  for faster system response. 
     The third algorithm  58  provides the quantity P 3   76  by applying a proportional gain term G 3  to the compensated voltage difference  70 . This third algorithm  58  increases the induction machine applied voltage during the regenerating mode in transient conditions. In the case of both low and high frequency controls (i.e., the compensation modulator behaviors, as discussed above, when the voltage difference  60  varies from relatively low, to intermediate, to relatively high frequencies) on the AFD system  2  of FIG. 1, if the voltage difference  60  is negative, then the induction machine  6  is in the motoring mode and all three algorithms  54 , 56 , 58  of the regeneration override logic module  46  of FIG. 1 are disabled. 
     A space vector generation function (not shown) of the space vector module  48  adjusts the voltage input V and the frequency input f of FIG. 1 by the quantity P 3   76  (dV) and the quantity  84  (df), respectively, in order to provide internal values ((V+dV)  86  and (f+df)  88 ) for the space vector module  48 . In turn, the space vector module  48  provides the outputs (V d , V q , θ). 
     Referring again to FIG. 1, the incremental voltage, dV, and the commanded frequency modification, df, are determined in the regeneration override logic module  46 . The incremental voltage, dV, is a voltage variation, which is added to the original induction machine commanded voltage, V. The commanded frequency modification, df, is a frequency variation, which is added to the original AFD commanded frequency, f. The resulting voltage space vector in FIG. 12 is V ref , which rotates counterclockwise at an angular velocity determined by the frequency f+df. This rotating voltage vector (i.e., as defined by V d , V q , θ) is fed into the space vector PWM module  50  of FIG. 1, which, in turn, outputs six (i.e., 3 gate signals and three complemented gate signals) gate signals for the interface  40  to the six IGBTs  26 , 28 , 30 , 32 , 34 , 36  of the inverter  12 . 
     The space vector PWM module  50  employs digital computations for AFD applications and calculates the IGBT switching times of the inverter  12 . This provides a 15% increase in DC bus voltage utilization compared with the conventional Sine-Triangle technique and reduces harmonic contents at relatively high modulation indices compared with such Sine-Triangle technique. 
     As shown in FIG. 13, there are eight inverter states including six active states: S 1 -S 6 , and two zero states: S 0  and S 7  (not shown). The six active states S 1 -S 6  occur when either: (a) one upper (e.g., one of IGBTs  26 , 30 , 34 ) and two lower (e.g., two of IGBTs  28 , 32 , 36 ) inverter IGBTs; or (b) two upper and one lower inverter IGBTs conduct simultaneously. The two zero states occur when either: (a) the three upper inverter IGBTs; or (b) the three lower inverter IGBTs are turned on. These two zero states are often referred to as freewheeling states, since all motor currents are freewheeling during operation in these configurations. There are six 60° sectors (as shown by the states S 1 -S 6  of FIG. 13) and, in each sector, a particular switching pattern is provided (where “0” corresponds to IGBT  26 , “1” corresponds to IGBT  30 , and “2” corresponds to IGBT  34  as shown in FIG.  13 ). The switching times are calculated at up to about a 20 kHz rate and the PWM voltages are applied to the induction machine  6 . In order to close the loop, the resulting output currents (e.g., I a , I b , I c  of FIG. 1) are sensed by the current sensors  14 , 16 , 18  and fed into the Clarke Transform module  42  of FIG. 1 for processing. 
     FIGS. 15-18 show various sample AFD system performances in accordance with the present invention. For example, the AFD system  2  may be operated on a 3-phase, 15 HP, 460V, 60 Hz, 4-pole induction machine, such as AC motor  6 . In FIG. 15, the motor  6  initially operates at about 377 rad/s (i.e., 60 Hz excitation frequency). After about 0.35 s, a speed command, f, of 40 Hz is issued and the motor speed is set to decelerate at a rate of −120 Hz/s. In turn, the regenerating mode takes place (i.e., between about 0.35 s and about 0.65 s) as best shown in FIG.  17 . 
     FIG. 16 shows the induction machine torque response. Because of the presence of negative slip, the torque changes abruptly (at ΔT) from 10 Nm to nearly −20 Nm. Also, at this point in FIG. 17, the DC bus voltage, V bus , transitions (at ΔV) from the nominal 680 V to a level of 740 V. 740V is the predetermined voltage threshold, V threshold , for the regeneration override logic module  46 . The DC bus voltage, V bus , is clamped at the 740 V level for as long as the regenerating condition exists. 
     FIG. 18 shows an important aspect of the invention for detecting the regenerating mode of operation. In this example, the torque producing current component, I Q , switches from about +4 A to about −7 A when the regenerating mode of the induction machine  6  initiates. If the regenerating condition is not true, then the predetermined threshold, V threshold , adjusts itself (e.g., about 60 VDC above the actual DC bus voltage) based upon the level of the DC bus voltage, V bus , as determined by the three-phase AC voltages  24 . In other words, the 740 V voltage threshold changes as the three-phase AC voltages vary. 
     In addition, the amplitude of the current I  94  of FIG. 1 is shown in Equation 3, which reflects the peak amplitude of the measured three-phase currents (e.g., I a , I b , I c  of FIG.  1 ). 
     
       
           I={square root over (I D   2   +I   Q   2 )}   (Eq. 3)  
       
     
     The load power factor, cos φ, may readily be calculated from Equation 4, wherein φ is the power factor angle. 
     
       
         cos φ= I   Q   I   (Eq. 4)  
       
     
     It will be appreciated that while reference has been made to the exemplary DSP  38 , a wide range of other suitable processors such as, for example, mainframe computers, mini-computers, workstations, personal computers (PCs), microprocessors, microcomputers, and other microprocessor-based computers may be employed using internal and/or external A/D converters. 
     It will be appreciated that while reference has been made to the exemplary Space Vector PWM module  50 , other suitable PWMs, such as, for example, a Sine-Triangle PWM, may be employed. 
     While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.