Abstract:
A polyphase AC induction motor is connected to a power supply through a soft starter having three sets of inverse parallel connected silicon controlled rectifiers with each set corresponding to one particular phase. Low speed starting and operation of the motor can be accomplished through triggering circuits controlling the phases of the triggering pulses in relation to the phases of the supply. The low motor speeds are developed by a gating sequence that generates a low frequency waveform that is less than the main supply frequency to the motor. This low frequency waveform is current and voltage controlled by the gating sequence to permit the AC motor to smoothly operate at speeds less than 100% of rated while developing net positive torque at the low controlled operating frequency.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a controlled gating sequence utilized in conjunction with a polyphase silicon controlled rectifier-based solid-state starter to rotate an AC induction motor through control of current and frequency. Specifically, the controlled SCR gating sequence is used to rotate a standard three-phase AC induction motor at a speed corresponding to 1% to 44.4% of the rated motor speed. The controlled gating sequence enables low speed motor rotation, in addition to acceleration and deceleration of the motor, without additional hardware. The controlled gating sequence also provides a reduction of peak phase currents; minimization of motor heating; and generation of higher shaft torque than prior art.  
       BACKGROUND OF THE INVENTION  
       [0002]     An AC induction motor is the most common type of motor used in industrial control systems. The AC induction motor offers simple, rugged construction, easy maintenance, and cost-effective pricing. Three-phase AC induction motors are utilized in many industrial environments, including chemical plants, foundries, pulp and paper plants, waste management facilities and rock crushers.  
         [0003]     The basic components of an AC induction motor are the stator, rotor, and frame. The stator has coils of insulated wires, referred to as windings, which are directly connected to a power supply. The stator of a three-phase AC induction motor has three sets of windings. The stator windings are typically held stationary by the motor frame. Each stator winding is spaced at an equal distance from the other two windings and is connected to one of three lines of a three-phase power supply. The lines from the three-phase power supply provide current from each phase to the motor. As the windings magnetize in sequence, the phase currents also peak in sequence and create a rotating magnetic field within the stator. The rotating magnetic field produced by the stator windings produces a transformer-like effect, induces a current in the unpowered rotor windings, and causes the rotor to produce its own magnetic field. The magnetic interaction between the stator and the rotor magnetic fields is an attractive force resulting in rotor movement.  
         [0004]     Despite its usefulness and wide-ranging application, the AC induction motor has certain associated limitations. One limitation, in particular, is that it is inherently incapable of providing a wide range of variable speeds in operation, when connected to a typical utility power supply. However, the operation of an induction motor at less than its rated speed is a desirable and useful feature in industrial applications. For example, in the mining industry, a three-phase AC induction motor is commonly used on a conveyer belt. In this application, the user may desire to have slow speed control over the motor for the inspection and repair of the belt. To satisfy this function it is necessary to slowly turn the motor at an exact speed to properly position the belt.  
         [0005]     In order to operate AC induction motors at less than rated speed, various low speed techniques have been used in the past. In one method, the supply voltage to the motor is switched on for brief time periods to partially start the motor and then the motor is quickly disconnected from the source and the motor is allowed to coast. As a result, the motor slowly turns. Additional full voltage pulses at line input frequency are then applied to the motor windings intermittently to keep the motor turning. Power can be applied to the motor intermittently either through the use of an electromechanical contactor or through the use of a solid-state device such as a soft starter. The disadvantages of this method include high transient currents, high transient torques, and potential overheating of the motor. Furthermore, this method does not posses any inherent speed control because the voltage and frequency applied to the motor is not being altered other than being applied intermittently.  
         [0006]     Another method of controlling the speed of an AC induction motor is through the use of a variable frequency drive. A variable frequency drive converts the supply voltage and frequency to another voltage and frequency so the induction motor can operate at less than the rated speed. Pulse width modulation, or PWM, drives are the most common type of variable frequency drive. The PWM drive contains electronic circuitry to convert AC line power to DC power. The PWM drive then pulses the DC output voltage for varying lengths of time to mimic a voltage output at the frequency desired. More specifically, the PWM drive produces a voltage waveform which, when applied to the motor, results in a motor current waveform that is essentially sinusoidal and of the frequency corresponding to the desired fundamental output frequency. By varying output voltage and frequency, a variable frequency drive controls the torque, speed, and direction of an AC induction motor. However, the variable frequency drive tends to be more complex and expensive in comparison to other low speed methods, especially as motor horsepower and motor rated voltage increases.  
         [0007]     A solid-state reduced voltage starter, in addition to soft starting a motor, can be used to rotate an induction motor at less than rated speed. The solid-state starter is placed in series between the power supply and the motor and employs solid-state switches, such as Silicon Control Rectifiers or SCRs, to control the application of current flow and voltage to the motor. Each SCR in a soft-starter can be phase controlled or zero fired. Zero firing turns the SCR completely on so the voltage applied to the load is similar to that of an electromechanical switch or contactor.  
         [0008]     Phase-control firing requires manipulation of the SCR firing angle. The firing angle is defined as the number of degrees from the beginning of the associated half-cycle of the AC waveform to the angle at which the gate voltage is applied. By controlling the firing angle, the soft starter is able to control the output voltage by turning the appropriate SCR or other switching device on for a particular portion of each half-cycle. When the SCR is turned on, or gated, voltage is applied to the load. The magnitude of the voltage applied to the load depends on the timing of the input power supply and when the SCR is gated on. Phase-control provides infinitely variable adjustability voltage between zero and full input voltage to the load as timed gate pulses are fed to each SCR. For example, the earlier in the half-cycle the SCR is gated on, the greater voltage is applied to the load.  
         [0009]     Variables pertaining to the firing of the SCRs can be modified through the control electronics of the solid-state starter to increase or reduce the output voltage. The control electronics can be preprogrammed to provide a particular output voltage contour based on a timed sequence or the output voltage can be controlled based on measurements of current and/or motor speed.  
         [0010]     Controlling the speed of a motor through the use of a solid-state starter has a number of advantages. As stated, the output voltage can be easily altered to suit the required load conditions. Furthermore, the SCRs are solid-state devices. Therefore, SCRs have no moving parts and provide high reliability and low maintenance operation compared to electromechanical motor controllers. Finally, the mechanical stress and shock on the motor is greatly reduced due to the reduction of large torque transients as a result of phase control providing quieter motor operation, longer equipment life, less maintenance and increased uptime.  
         [0011]     Because of these advantages, the solid-state starter has found increasing utilization in connection with special SCR firing patterns to rotate a motor at slow speeds. One commonly used SCR firing pattern, known as a pulse skipping pattern, generates four pulses of current for each phase of each output cycle, two positive and two negative, to generate slow motor speed. When reduced speed is desired, the SCRs are controlled so that selected cycle portions from each phase of the power supply voltage are omitted from the voltage applied to the motor. Consequently, the fundamental frequency of the output voltage is a predetermined fraction of the fundamental frequency of the source voltage, and the running speed of the motor will be correspondingly reduced compared to full rated speed. Examples of solid-state starters utilizing a pulse skipping pattern include the Benshaw RediStart Microll product as well as soft starter products from Allen Bradley and SquareD.  
         [0012]     One disadvantage of the pulse skipping pattern is that in order to achieve a given average current in the motor to produce a required level of torque with only four pulses of current per phase per output cycle, results in the four pulses having very high peak currents. The high peak currents can tax the input supply system, causing various disturbances such as light flicker. These high peak currents further cause additional heating of the motor and source transformer due to the high levels of current and related harmonic heating losses. In addition, the limitation of four pulses of current for each cycle constrains the overall average motor current that can be achieved in the motor. This reduction in motor current reduces the maximum torque that can be generated by the motor during cycle skipping slow speed operation. Cycle skipping also creates a cogging motion, which further creates mechanical harmonics on the shaft of the motor.  
         [0013]     A particular example of a pulse skipping SCR firing pattern is disclosed in Rowan, et al., U.S. Pat. No. 4,996,470, issued for Feb. 26, 1991, for Electric Motor Speed Control Apparatus and Method, which discloses a device and method to reduce the speed of an induction motor. The speed of the motor is initially reduced through a combination of dynamic electrical braking and AC pulse skipping to a speed at which is it no longer synchronized to the AC input supply frequency. At such time, AC pulse skipping is employed to slow the AC motor down even further. The AC pulse skipping is continued for operation of the apparatus at a continuous speed until the point at which the user desires to reduce the motor speed to zero. Dynamic motor braking is again employed to break the motor out of synchronism with the pulse skipping frequency. The motor can then be slowed to a speed from which a very accurate final stoppage of the motor can occur to precisely position a work piece.  
         [0014]     The previously described disadvantages of pulse skipping apply to Rowan, et al. In addition, Rowan, et al., the slow motor speeds are limited to only a few defined speeds. Typically, pulse skipping methods are limited to two discrete forward speeds of 7% and 14% of rated speed and two discrete reverse speeds of 10% and 20% of rated speed because of the timing involved with the incoming power supply lines. The speeds utilized in Rowan, et al., correspond to an effective frequency that is a fundamental frequency component of the AC power supply input line frequency. This effective frequency is defined in Rowan, et al. by the known expression:  
         f   s       (       6   ⁢   n     +   1     )         
 
 where n is an integer and f is the frequency of the supply voltage. Rowan, et al., further teaches that only fundamental frequency components of 1/7 and 1/13 are preferred because lower frequencies tend to drive the motor at too slow of a speed for many applications. 
 
         [0015]     Another example of AC pulse skipping for speed control is represented by Asano, et al., U.S. Pat. No. 4,176,306, issued Nov. 27, 1979, for a Speed Control Apparatus. Asano, et al., discloses a speed control apparatus that includes a plurality of switches disposed between a three-phase AC power source and a motor in addition to a low speed control device for feeding power having a frequency lower than the frequency of the power source to the motor under the control of the switches.  
         [0016]     Similar to Rowan, et al., Asano, et al., is limited by the effective frequencies of defined by the above equation. By limiting the effective frequency of the method, Asano, et al., have determined specific set of operating speeds, as described above.  
         [0017]     What is lacking in the present art therefore, is a method of rotating a three-phase motor at speeds less than the rated synchronous speed while satisfying the following criteria: the speed of motor rotation should not be restricted to only a minimal set of speed selections; the available motor current and torque should be maximized the harmonic and flickering effects on the power supply should be minimized and motor heating should be minimized by utilizing the maximum number of current pulses per output cycle.  
       SUMMARY OF THE INVENTION  
       [0018]     A method rotating a polyphase motor at less than the rated speed using a controlled SCR gating sequence is disclosed that will enable low speed motor rotation without additional hardware above that contained in a solid state soft starter. The method is utilized in connection with a typical reduced voltage solid state starter connected to a polyphase power supply. The digital controller of the solid state starter is programmed to individually control the gating of each SCR. The solid state starter can therefore be programmed to apply a pulsed waveform to each of the phases of an AC induction motor. The motor preferably is a three phase motor, although six phase and other embodiments are contemplated. The output voltage of the solid state starter is a result of the overlap of the firing of certain SCRs, in accordance with a predetermined gating sequence and the voltage and phase relationship of the poly-phase power supply. This method of rotating a three-phase motor using the controlled SCR gating sequence generates higher torque than other prior art methods of slow speed induction motor operation. The method maintains the motor current at a predetermined level and can be applied to rotate the motor at user defined speeds from 1 to 44.4% of the rated motor speed. The speed selected by the user is correlated to a reference table that associates the number of input line cycles required to closely approximate the user-defined speed. The user-defined speed is further utilized to determine the total output period of the operation. The firing and conduction angle of each SCR is calculated based on the total output period and desired motor current.  
         [0019]     The output voltage per input line cycle is divided into distinct output states. The output states correlate to different SCR firing patterns and are used to produce the user-desired motor rotational speed. The selected SCR gating pattern is chosen based on the desired speed of the motor. If the desired speed in a three phase embodiment is less than or equal to 8⅓%, a 30° single or double conduction pattern is used. A 30° conduction pattern has twelve output states for which certain combinations of SCRs are gated. The combinations of SCRs are predetermined and contained in a lookup table within the operating system of the solid state starter. At speeds greater than 8⅓%, a 60° conduction pattern is used. A 60° conduction pattern has six output states for which certain other combinations of SCRs are gated. In a six phase embodiment, these patterns are 15° and 30°, respectively. It is considered well within the ambit of one of ordinary skill to establish appropriate patterns for other phase embodiments. These combinations of SCRs are also predetermined and contained in a lookup table within the operating system of the reduced voltage starter. The conduction patterns and output states are appropriate if the motor is rotated in a forward or reverse direction.  
         [0020]     Those skilled in the art will recognize that while specific implementations have been described herein, many others are possible in keeping with the ideas and approaches presented. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]      FIG. 1  is a diagrammatic view of a prior art six switch SCR-based solid state motor starter, as utilized in accordance with the present invention.  
         [0022]      FIG. 2  is a graphical illustration of a known waveform output of a three-phase power supply, in accordance with the present invention.  
         [0023]      FIG. 3  is a diagrammatic representation of the sequence of steps performed during the initialization of an SCR-based solid state starter, as utilized in accordance with the present invention.  
         [0024]      FIG. 4  is a diagrammatic representation of the sequence of steps performed in selecting and programming the proper output states and conditions for executing at line frequency of the solid state starter, in accordance with the present invention.  
         [0025]      FIG. 5  is a diagrammatic representation of the sequence of steps used to control the gating of the SCRs of the solid state starter.  
         [0026]      FIG. 6  is a graphical illustration of a series of SCR firing sequences for each phase of a three-phase power supply.  
         [0027]      FIG. 7  is a representative graphical illustration of a waveform output corresponding to phase A of one cycle of a SCR gating sequence. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     Referring to  FIG. 1 , a three-phase AC induction motor  10  is connected to a three-phase AC power supply  12  through a solid state starter  65 , represented diagrammatically. Power supply  12  is provided with three output phases comprising phase A, phase B and phase C corresponding to the phases shown in  FIG. 2 . However, it can be appreciated by one skilled in the art that power supply  12  can be any poly-phase power supply that supplies AC electrical power in overlapping phases. Referring to  FIGS. 1 and 2 , power supply  12  is connected to the stator (not shown) of the motor  10  through solid state starter  65  by three input lines that each correspond to a particular phase of power supply  12 . For example, phase A corresponds to phase A input line  14 ; phase B corresponds to phase B input line  16  and phase C corresponds to phase C input line  18 .  
         [0029]     An input cycle is defined as the time that it takes for one phase of a 23 Hz to 72 Hz power supply  12  to complete one 360° cycle. For example, at 60 Hz, each phase of the power supply requires 16.667 milliseconds to complete a 360° cycle. The output cycle, as further described herein, is the time that it takes the output waveform of the solid state starter to complete one 360° cycle. In the present invention, the output is always at a lower frequency than the input such that a full output cycle will always take longer than an input cycle.  
         [0030]     Each phase of power supply  12  carries power that is 120° or one-third of an input cycle offset in time from each of the other phases, as shown in  FIG. 2 . If a polyphase power supply other than three-phase power supply  12  is used in connection with the present invention, the input cycle offsets may vary from 120° but with a similar mathematical relationship.  
         [0031]     The three-phase power supply  12  produces three separate waveforms, as shown in  FIG. 2 , with each waveform corresponding to a particular phase of power supply  12 . Assuming a three phase Wye power supply configuration,  FIG. 2  is a graphical representation of the variation of instantaneous line to neutral point voltage  70  with respect to time T for each of phase A, phase B and phase C.  FIG. 2  represents one cycle of a three-phase system as it moves through each electrical angle  76  of the cycle. The electrical angles  76  are appropriately labeled 0°, 90°, 180°, 270° and 360° that indicate the particular electrical angle  76  in degrees. When any one sine wave is at zero the other two may still be delivering power to the motor depending on the conduction state of the SCRs associated with those phases.  
         [0032]     Phase A is represented by sine wave  80 , phase B is represented by sine wave  83  and phase C is represented by sine wave  86 . Each phase has a positive half-cycle  84 ,  81  and  88 , which is the time in degrees that the instantaneous voltage is positive, and a negative half-cycle  82 ,  85  and  87 , during which time the instantaneous voltage is negative. At the end of each half-cycle, which lasts for 180°, each phase crosses the zero axis. For example, phase A crosses the zero axis at  90  and  90 ′ corresponding to 180° and 360°, respectively; phase B crosses the zero axis at  95  and  95 ′ and phase C crosses the zero axis at  100  and  100 ′.  
         [0033]     Referring again to  FIG. 1 , each of phase A, phase B and phase C of power supply  12  is provided with an inverse parallel pair of SCRs  20 ,  25 ,  30 ,  35 ,  40  and  45  that are used to pass or block current passing through for the representative phase. Alternatively, each SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  may be any type of current control device similar to an SCR. Although each phase of power supply  12  is associated with two SCRs  20 ,  25 ,  30 ,  35 ,  40  and  45 , it can be appreciated by one skilled in the art that, with regard to higher voltage or higher current starters, each phase may consist of more than one SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  in series or parallel. If more than one SCR is utilized in a switch, each SCR in that particular switch is gated on at the same time.  
         [0034]     Each SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  utilized in connection with the present invention is a conventional silicon controlled rectifier having a control means to regulate current and voltage flow to the motor. Each SCR consists of basic elements including a gate  50 , an anode  55  and a cathode  60 . SCRs  20 ,  30 , and  40  are in an inverse parallel relationship with another SCR  25 ,  35  and  45 . At least six SCRs are required in a three-phase solid state starter due to the inverse parallel relationship of the SCRs. Referring to  FIG. 1  and  2 , phase A has two associated SCRs. The first SCR  20  corresponds to the positive half-cycle  81  of sine wave  80  and a second SCR  25  that corresponds to the negative half-cycle  82 . Similarly, phase B has a first SCR  30  that corresponds to the positive half-cycle  84  of sine wave  83  and a second SCR  35  that corresponds to the negative half-cycle  85  of sine wave  83 . Finally, phase C has a first SCR  40  that corresponds to the positive half-cycle  88  of sine wave  86  and a second SCR  45  that corresponds to the negative half-cycle  87  of sine wave  86 .  
         [0035]     Without special gating control the solid state starter  65  would supply the input power supply frequency to motor  10 . To achieve the desired lower output frequency, each of SCRs  20 ,  25 ,  30 ,  35 ,  40  and  45  must be gated as further described with respect to  FIG. 7 , to provide only the appropriate polarity and width of the input phase waveforms  80 ,  83  and  86  illustrated in  FIG. 2 . SCRs are line-commutated devices, meaning that the conducting SCRs are turned off by reducing the current through the SCR to zero. The current is reduced to zero by the reversal of the voltage applied to the SCR and load by the power supply. This voltage is supplied by power supply  12 , as supplied through input lines  14 ,  16  and  18 . Each SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  is gated only once per input cycle. Therefore, only one pulse per SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  is produced for each input cycle. For example, when using a 60 Hz power supply, one input cycle is completed in 16.667 milliseconds as described above. This corresponds to one possible positive and one possible negative pulse per SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  every 16.667 milliseconds.  
         [0036]     Solid state starter  65  is provided with a digital controller having control electronics and power switching electronics, as would be well known to those skilled in the art. The control electronics provide the firing or gating impulses for each SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  as further described with respect to  FIG. 6 . The power switching electronics consist of the actual SCRs  20 ,  25 ,  30 ,  35 ,  40  and  45 . The digital controller is provided with a real time operating system to handle the tasks performed by solid state starter  65  with respect to the firing of the SCRs  20 ,  25 ,  30 ,  35 ,  40  and  45 . Examples of a real time operating systems can include Chimera, Lynx, MTOS, QNX, RTMX, RTX, uCOS-II, and VxWorks.  
         [0037]      FIG. 3  is a diagrammatic illustration of the basic operation of solid state starter  65  during the initialization of operations by the digital controller. A start command  100  is received by the digital controller. The start command  100  can result from pressing a key on a remote key pad, a voltage input, or over a communication network. Next, the user must indicate to the device through a signal input or a preset user parameter if the controlled gating sequence method  105  of the present invention should be enabled. If the controlled gating sequence method  105  has not been enabled, the device will perform a normal soft start  106  of the motor, as previously described with regard to the basic operation of a solid state starter. If the controlled gating sequence method  105  has been enabled, the controlled gating sequence method variables are initialized at step  110  based on the preset user inputs. Among these user inputs is the desired speed of the motor in a range of one to forty-four percent of rated input line frequency motor speed.  
         [0038]     The present invention can be used to output a speed between 0% and 44.4% of rated motor speed. However, certain selected speeds are more useful and favorable for use than others are. The motor speeds available for selection by the user to operate solid-state starter  65  in accordance with the method of the present invention are listed in Table 1. Speeds below 1% of rated are not typically necessary in most applications and therefore are not presently described.  
                                         TABLE 1                                   Number of input               line cycles per   Speed,           output cycle   % of Rated                                        100   1           68   1.470588235           60   1.666666667           58   1.724137931           54   1.851851852           50   2           40   2.5           38   2.631578947           36   2.777777778           34   2.941176471           32   3.125           30   3.333333333           28   3.571428571           26   3.846153846           24   4.166666667           22   4.545454545           20   5           18   5.555555556           16   6.25           14   7.142857143           12   8.333333333           11   9.090909091           10   10           9   11.11111111           8   12.5           7   14.28571429           6   16.66666667           5   20           4   25           3   33.33333333           2.861111   34.9514563           2.666667   37.5           2.5   40           2.25   44.44444444                      
 
 Based on the information provided by the user with respect to the speed input by the user at step  120 , the device calculated the output period at step  115  by the calculation:  
       Period   =       360   ⁢           ⁢   degrees   *   100       Speed   ⁢           ⁢   %   ⁢           ⁢   entered   ⁢           ⁢   by   ⁢           ⁢   user           
 
         [0039]     The output period calculation at step  115  corresponds to the output cycle&#39;s period with respect to the total input supply electrical angle in degrees during which solid state starter  65  employs the controlled gating sequence method to achieve the speed selected by the user at step  120 , as listed in Table 1. The output period calculated at step  115  is utilized in the current calculation loop and the SCR output state calculation of the Line Frequency Task that is further described herein with respect to  FIG. 4 . Solid-state starter  65  enters a standby state at step  130  until the Line Frequency task is triggered.  
         [0040]     The preferred solid-state starter  65  output speeds listed in Table 1 have been selected based on the criteria of reducing DC offsets that may be generated in the phase current. When the number of input line cycles per output cycle is either an odd or non-integer number, DC current may be generated that produce DC offsets in the output waveform of solid-state starter  65 .  
         [0041]     Typically, direct current does not exist simultaneously in the same system with alternating current. The resulting waveform with DC offsets results from the addition of DC voltage generated in the phase current to the AC voltage of power supply  12 . An offset can be produced which causes a vertical shift distortion in the output wave amplitude of solid-state starter  65 . This DC waveform distortion, if applied to the motor, can cause torque ripple, act as a braking influence due to the production of retarding torque and can result in excessive heating of the motor.  
         [0042]     With reference to Table 1, when the number of input line cycles per output cycle is an even number, an equal number of positive and negative current pulses are generated for each phase and thus prevents significant DC offset currents from occurring. At speeds that are available for selection as described in Table 1 equal to and slower than 8⅓%, the number of input line cycles per output cycle is always an even number. However, at speeds that are available for selection in accordance with Table 1 higher than 8⅓%, this is not always possible. For example, at a speed equal to 9.09% of the rated speed, the number of input line cycles per output cycle is eleven.  
         [0043]     Although it is not possible to constantly maintain an even number of input line cycles per output cycles, it is desirable to have an integer number of input line cycles per output cycle to ensure smooth operation of solid state starter  65 . When the number of input line cycles per output cycle is a non-integer, the number of associated current pulses is also a non-integer number, which can cause negative transient torques. For example, at speeds of 33⅓% and lower, each available speed corresponds to an integer number of input line cycles and, thus, an integer number of current pulses. However, at a higher speed, such as 34.95% of the rated motor speed, it is not possible to have an integer number of input cycles. At these speeds, direct current offsets and other similar effects can cause a distortion of the waveform shown in  FIG. 2 . Slight adjustments to the firing delay time of either the positive or negative SCR of each phase are necessary to prevent these DC offsets from occurring at these higher speeds.  
         [0044]      FIG. 4  is a diagrammatic flow chart of the Line Frequency Task with respect to solid-state starter  65 . The Line Frequency Task is a task or sequence of events during which the main calculation of each function of solid-state starter  65 , as applicable to the controlled gating sequence method is completed. During the Line Frequency Task, the individual control of the gating of each SCR, as previously described, is accomplished through the application of this method. Once solid-state starter  65  is initialized, each waveform corresponding to its associated phase of power supply  12  cycles through its input line cycle shown in  FIG. 2 . Upon the second zero voltage crossing  90 ′ of the phase A voltage at 180°, an interrupt is generated at step  135 . The interrupt generated at step  135  initiates the Line Frequency Task functions at step  140 .  
         [0045]     A synchronization verification step  145  of solid state starter  65  is performed by the digital controller with respect to the phase input cycles as delivered by the phase input lines  14 ,  16  and  18  of power supply  12  to soft starter  65 . During the synchronization verification step  145 , the digital controller determines if the input lines  14 ,  16  and  18  of the appropriate phases of power supply  12  are delivering power of the same frequency, voltage, and phase sequence as solid state starter  65 . The synchronization verification step  145  is necessary to ensure that each SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  is gated and fired at the proper time. If input supply synchronization did not occur, the gating of each SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  would be random and produce results that are contrary to the desired outcome of the controlled gating sequence method. The occurrence of a fault at step  346  causes a fault indication to be displayed or otherwise communicated with the user and operation is halted  
         [0046]     In order to initiate the rotation of an induction motor, the firing of each SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  is timed to ensure that the appropriate voltage is applied to the stator windings of the motor. In order to gate each SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  at the proper time, the current level flowing through each phase of the power supply  12  is measured and controlled. This measurement is performed by calculating the root mean square, or RMS, of the current level from the last input cycle at step  150 , which takes into account the current of each phase of power supply  12 . The RMS value of an AC waveform is the value of an alternating current that corresponds to the steady DC waveform that provides an equivalent power dissipation over a specific period of time. The RMS value of current is determined by the equation:  
       Irms   =         1   n     ⁢       ∑     i   =     1   ⁢           ⁢   …   ⁢           ⁢   n         ⁢       (     I   sampled     )     2               
 
 where I sampled  is the sampled current at point i. The calculation of the average three-phase RMS involves a calculation of the RMS value of the current for each of phase A, phase B and phase C with the three results being averaged together. The current measurement is performed through a current measuring device, such as a current transformer or Hall-effect based current sensor incorporated into solid state starter  65 . 
 
         [0047]     The phase current can be measured on an individual input line cycle basis or on an output cycle basis. If the current is measured on an individual input line cycle basis, a full output cycle&#39;s current is not measured because the input cycle is always shorter than the output cycle. Measuring the phase current on an input line cycle basis does not account for the variation in the timing of the different output states and overlap of the firing of each SCR  20 ,  25 ,  30 ,  35 ,  40  and  45 . This may result in misinterpretation of the result as the current fluctuates due to the timing and overlap effects. However, if the calculated RMS current value at step  150  is based on an output cycle, each current measurement must be stored during multiple input line cycles to provide a value based on a full output cycle.  
         [0048]     The current control loop  160  provides closed loop control of the RMS current value. The user defines a reference current  155 , or set point, which is the current that should be applied to the motor. The current control loop  160  provides a continuous feedback loop to take any required corrective action whenever there is any deviation from the user-defined reference current  155 . Each time the input line cycle is repeated, the user-defined reference current  155  is compared to the calculated RMS current value  150 . Based on the deviation between the values, current control loop  160  adjusts the firing angle and hence the on-time of each SCR to increase or decrease the applied voltage to the motor. The adjustments made to the applied voltage through the firing of each SCR causes the motor current to closely approximate the user-defined current reference.  
         [0049]     It can be appreciated that calculating the RMS current value  150  on an output cycle basis requires a certain amount of time to implement. Only one RMS current value calculation  150  is performed for each output cycle, and, as described above, the output cycle is longer than the input line cycle. The current control loop  160  is provided with current feedback only once every output cycle. At very slow output speeds, such as 1%, this can be an unacceptably long time period and may be greater than 0.6 seconds.  
         [0050]     At the start of each subsequent input cycle, the current output angle is incremented by 360° at step  175  to account for the start of a new input cycle. Solid state starter  65  compares the current output angle, as incremented at step  175 , to the output period  115 . If the current angle is greater than the total period, the current angle is recalculated to a valid angle by removing one complete output cycle&#39;s total angle from the variable.  
         [0051]     Solid state starter  65  uses two similar firing patterns to produce the slow motor speeds. At speeds that are available for selection according to Table 1, which are less than or equal to 8⅓%, a 30° pattern is used for improved smoothness of rotation. The 30° pattern rotates the magnetic field of the motor in 30° increments and produces twelve possible output states, as further described with respect to Table 2. At speeds that are available for selection in accordance with Table 1 that are higher than 8⅓%, a 60° conduction pattern, as further described with respect to Table 3, is used to rotate the magnetic field of the motor in 60° increments and results in six possible output states.  
         [0052]     There is no upper or lower speed limit associated with either firing pattern. For example, the 30° conduction pattern can be used at higher speeds, however, a loss of motor torque occurs because of the reduced motor current due to only two SCRs conducting in six of the twelve given states. Conversely, the 60° double conduction pattern may also be used at lower speeds, but with an increase in cogging, which reduces the smoothness of the rotation of the motor. Cogging is a result of cycling the rotor through large discrete angles at a low speed. It should be noted that utilization of a 60° single conduction pattern can also be used, however, this pattern has been shown in simulation and laboratory testing to produce less torque than the 60° double conduction pattern due to reduced motor currents with no appreciable benefits.  
         [0053]     Referring again to  FIG. 4 , the firing pattern for each SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  is calculated on a per input line cycle basis. The firing pattern for each SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  is determined by the present output angle as calculated by the digital controller. At step  190 , the digital controller compares the desired motor speed to the low speed pattern limit, which is the maximum speed for each pattern. If the instantaneous speed of the motor is greater than the low speed pattern limit, the pattern is set to a 60° double conduction pattern at step  195 . If the current speed is less than the low speed pattern limit, the pattern is set to a 30° conduction pattern at step  200 . With respect to either pattern, the SCR gating sequence for each output state is predetermined and stored in a reference or look-up table in the digital controller. The SCR output state for the 30° conduction pattern as calculated at step  205  is equal to:  
         SCR   ⁢           ⁢   Output   ⁢           ⁢   State     =       INT   ⁢           ⁢     (     Current   ⁢           ⁢   Angle   *   12     )         Output   ⁢           ⁢   Period           
 
 Similarly, because the 60° double conduction pattern is rotated in 60° increments, there are six total output states. The SCR output state for the 60° double conduction pattern as calculated at step  210  is equal to:  
         SCR   ⁢           ⁢   Output   ⁢           ⁢   State     =       INT   ⁢           ⁢     (     Current   ⁢           ⁢   Angle   *   6     )         Output   ⁢           ⁢   Period           
 
         [0054]     Next, the digital controller of solid state starter  65  determines if the motor is operating in a forward or reverse direction at step  215 , with respect to the 30° conduction pattern, or at step  230 , with respect to the 60° double conduction pattern. If the motor is operating in a reverse direction, an additional calculation must be performed. With regard to the 30° conduction pattern, the final SCR output state at step  220  is equal to:
 
SCR Output State=11−SCR Output State
 
 Similarly, with respect to the 60° double conduction pattern, the SCR output state at step  235  is equal to:
 
SCR Output State=5−SCR Output State
 
 The digital controller of solid state starter  65  references the appropriate table to determine which of SCR  20 ,  25 ,  30 ,  35 ,  40  and  45 , or combination of SCRs, should be fired during the operation of the motor in the reverse direction. 
 
         [0055]     If the motor is being operated in a forward direction, the digital controller references the appropriate table to determine the SCR, or combination of SCRs, that should be gated for the particular SCR output state as determined at steps  205  and  210 .  
         [0056]     Table 2 is the reference table utilized with the 30° conduction pattern and correlates the present SCR output state, as determined from the above calculations at  205 ,  210 ,  220  and  235  with the conductive SCR  20 ,  25 ,  30 ,  35 ,  40  and  45 , or SCRs, for each state. For this conduction pattern, the table also references the appropriate magnetic field angle created when certain of SCRs  20 ,  25 ,  30 ,  35 ,  40  and  45  are gated on. In order to control current the SCRs are gated at a precise time relative to the line to neutral input power called the firing angle. The firing angle is the number of degrees from the beginning of a line to neutral input line cycle that the appropriate SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  is gated to an on status corresponding to electrical angle  76  of  FIG. 2 .  
         [0057]     For a particular output state, if an SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  is indicated as a gated SCR, the SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  is fired at the appropriate angle. If the SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  is not indicated as a gated SCR  20 ,  25 ,  30 ,  35 ,  40  and  45 , the current flowing through the SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  during that state is zero. In this case, the SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  acts as an open switch during the particular referenced angle of the associated output state.  
                                     TABLE 2                           Gated           Output State   SCRs   Firing angle                                0   A+, B−, C+   120       1   A+, B−   150       2   A+, B−, C−   180       3   A+, C−   210       4   A+, B+, C−   240       5   B+, C−   270       6   A−, B+, C−   300       7   A−, B+   330       8   A−, B+, C+   0       9   A−, C+   30       10   A−, B−, C+   60       11   B−, C+   90                  
 
         [0058]     Table 3 shows the assigned output state number, the conductive SCRs and the firing angle for the 60° double conduction pattern.  
                                     TABLE 3                               Firing       Output State   Gated SCRs   angle                                0   A+, B−, C+   120       1   A+, B−, C−   180       2   A+, B+, C−   240       3   A−, B+, C−   300       4   A−, B+, C+   0       5   A−, B−, C+   60                  
 
         [0059]     While the SCRs may be fired in numerous combinations, only certain combinations have useful application, as will be further described with respect to  FIG. 6 . A useful SCR output state containing at least one positive and one negative SCR firing together such that current can flow through the motor. Combinations may include either one positive and two negative devices or two positive and one negative device to achieve such current flow.  
         [0060]     Once the Line Frequency Task functions of  FIG. 6  are complete, the SCR Firing Task is initialized at step  245 , as further described with respect to  FIG. 5 . The SCR Firing Task implements the information calculated in the Line Frequency Task functions including the sequence of firing and firing delay time of each SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  for the next input cycle. The information is implemented by an output comparative function contained within the digital controller of solid state starter  65 . For each phase, the digital controller gates the appropriate SCR  20 ,  25 ,  30 ,  35 ,  40  and  45 , or SCRs, at the predetermined time.  
         [0061]     As stated earlier, the firing pattern for the SCRs is calculated on a per input line cycle basis. This calculation is triggered by an input phase A voltage second zero crossing such as at point  90  or  FIG. 2 , causing an interrupt at step  135 . Ideally, whether an SCR is fired should be calculated individually immediately prior to the time that each SCR would fire. The once per input cycle calculation time basis is primarily used to reduce the resource utilization and computation time required of the digital controller.  
         [0062]     Referring to  FIG. 5 , at the end of each Line Frequency Task, as described in  FIG. 4 , the output state and the corresponding SCR  20 ,  25 ,  30 ,  35 ,  40  and  45 , or group SCRs, and the associated firing angles, are loaded into the control electronics of soft starter  65 . Although each SCR  20 ,  25 ,  30 ,  35 ,  40  and  45 , or group of SCRs, uses the same firing angle period during each input line cycle, it can be appreciated that the firing angle of each SCR  20 ,  25 ,  30 ,  35 ,  40  and  45 , or group of SCRs can be individually controlled. Thus, a certain SCR  20 , 25 ,  30 ,  35 ,  40  and  45 , or group of SCRs, can have different firing angle periods. Additionally, the SCR Firing Task as described below with respect to  FIG. 6  can be applicable if the motor is operating in a forward or reverse direction.  
         [0063]     The control electronics of the digital controller of soft starter  65  cycle through the control electronics of each SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  to load the appropriate firing angle of each SCR  20 ,  25 ,  30 ,  35 ,  40  and  45 . The digital controller associates each SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  to its position in the predetermined gating sequence through an output compare function and determines whether an SCR should be fired during the output cycle.  
         [0064]     In  FIG. 5 , the digital controller initializes the output compare function with respect to SCR  45  to determine if it should be fired during the output cycle, at step  250 . At step  255 , the digital controller evaluates the position of SCR  45  in each output state of the predetermined gating sequences, such as the gating sequences shown in Tables 2 and 3. If SCR  45  should be fired, the digital controller sends a signal to the control electronics for SCR  45  to set the appropriate firing angle at step  260 . If SCR  45  should not be fired during the output cycle, the digital controller initializes the output compare function with respect to SCR  30  at step  265  to determine if SCR  30  should be fired during the output cycle.  
         [0065]     At step  270 , the digital controller evaluates the position of SCR  30  in each output state of the predetermined gating sequences, such as the gating sequences shown in Tables 2 and 3. If SCR  30  should be fired, the digital controller sends a signal to the control electronics for SCR  30  to set the appropriate firing angle at step  275 . If SCR  30  should not be fired during the output cycle, the digital controller initializes the output compare function with respect to SCR  25  at step  280  to determine if SCR  25  should be fired during the output cycle.  
         [0066]     At step  285 , the digital controller evaluates the position of SCR  25  in each output state of the predetermined gating sequences, such as the gating sequences shown in Tables 2 and 3. If SCR  25  should be fired, the digital controller sends a signal to the control electronics for SCR  25  to set the appropriate firing angle at step  290 . If SCR  25  should not be fired during the output cycle, the digital controller initializes the output compare function with respect to SCR  40  at step  295  to determine if SCR  40  should be fired during the output cycle.  
         [0067]     At step  300 , the digital controller evaluates the position of SCR  40  in each output state of the predetermined gating sequences, such as the gating sequences shown in Tables 2 and 3. If SCR  40  should be fired, the digital controller sends a signal to the control electronics for SCR  40  to set the appropriate firing angle at step  305 . If SCR  40  should not be fired during the output cycle, the digital controller initializes the output compare function with respect to SCR  35  at step  310  to determine if SCR  35  should be fired during the output cycle.  
         [0068]     At step  315 , the digital controller evaluates the position of SCR  35  in each output state of the predetermined gating sequences, such as the gating sequences shown in Tables 2 and 3. If SCR  35  should be fired, the digital controller sends a signal to the control electronics for SCR  35  to set the appropriate firing angle at step  320 . If SCR  35  should not be fired during the output cycle, the digital controller initializes the output compare function with respect to SCR  20  at step  325  to determine if SCR  20  should be fired during the output cycle.  
         [0069]     At step  330 , the digital controller evaluates the position of SCR  20  in each output state of the predetermined gating sequences, such as the gating sequences shown in Tables 2 and 3. If SCR  20  should be fired, the digital controller sends a signal to the control electronics for SCR  20  to set the appropriate firing angle at step  335 . If SCR  20  should not be fired during the output cycle, the SCR Firing Task and is suspended at step  340 . Accordingly, the gating sequence and firing angle of each SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  during the most recent cycle is suspended at step  340  until the next interrupt occurs at step  135 . The occurrence of the next interrupt  135  signifies that a second zero voltage crossing has occurred, and the Line Frequency Task at step  140  is once again initiated.  
         [0070]     The operation of solid state starter  65 , as shown in  FIGS. 4, 5  and  6 , and the associated controlled gating sequence method of the present invention, repeat until the start command is removed at step  345 . Similar to the initiation of the operation of solid state starter  65 , the start command may be removed by the manipulation of a key on a user communications interface of the solid state starter  65 , a the change in other control inputs, or through other communication means. The occurrence of a fault at step  346 , as described above, can also cease the operation of solid state starter  65 .  
         [0071]      FIG. 6  illustrates a specific example of the 60° double induction SCR firing sequence in accordance with the present invention. In this particular example, an output speed of 16⅔% is selected as the desired output speed. Accordingly, the 60° double conduction pattern is used because the user-defined speed  120  is greater than 8⅓%. According to Table 1, solid state starter  65  has six input line cycles per output cycle at the desired speed of 16⅔%, thus solid state starter  65  requires 2160 degrees to complete one output cycle at 16⅔%.  
         [0072]     The SCR gating sequences rotate through the output states in the manner  0 ,  1 ,  2 ,  3 ,  4 ,  5 ,  0 ,  1 ,  2 ,  3 ,  4 ,  5  and so forth. In  FIG. 6 , the SCR output states and the corresponding electrical angles are applied as follows:  
                                                         TABLE 4                                   Reference   Output   Begins at   Ends at electrical           Numeral   State   electrical angle   angle                                        400   0   −300   60           405   1   60   420           410   2   420   780           415   3   780   1140           420   4   1140   1500           425   5   1500   1860                      
 
         [0073]     The gating sequence, as shown in  FIG. 6 , is simplified for clarity and assumes that each SCR is gated at its maximum capability or on-time. When all six output states are applied for just one input cycle, each phase will apply three positive pulses using its positive SCR  20 ,  30  and  40  and three negative pulses using its negative SCR  25 ,  35  and  45  during each output cycle. For example, an arbitrary segment of a sequence is shown, illustrating the gating pattern of SCR  20 , which consists of three positive pulses  430 ,  435  and  440 , and the gating pattern of SCR  25 , which consists of three subsequent negative pulses  445 ,  450 , and  455 . Similarly, the gating pattern of SCR  35  consists of three negative pulses including  475 ,  480  and  485 , and the corresponding gating pattern of SCR  30  consists of three positive pulses  460 ,  465  and  470 . After the final positive pulse  470  of SCR  30 , SCR  35  repeats the gating pattern of three negative pulses with the first of such negative pulses represented at  490 . Finally, the gating pattern of SCR  40  consists of three positive pulses, including two initial pulses, which are not shown as preceding this arbitrary period and pulse  500 . The corresponding gating pattern of SCR  45  consists of three subsequent negative pulses  515 ,  520  and  525 . After the last negative pulse  525  of SCR  45 , the gating pattern of SCR  40  repeats the gating pattern of three positive pulses including  505 ,  510  and a third positive pulse that is also not shown.  
         [0074]     The duration of each positive and negative pulse is 180°. In actual operation, the firing angle of each SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  can be delayed depending on the command output from the current control loop  160 . In a low voltage system, each SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  can be gated longer for an additional 30° beyond what is represented in  FIG. 5 . Only the appropriate SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  is gated according to the output state. However, depending on the output state, the power factor of the motor, and the firing angle of the SCR  20 ,  25 ,  30 ,  35 ,  40  and  45 , a particular SCR  20 ,  25 ,  30 ,  35 ,  40  and  45  may still be conducting current well into the beginning of the next output state.  
         [0075]     As described earlier, a useful SCR output state is signified by at least one positive and one negative SCR firing together and is necessary for current to flow through the motor. Therefore, the resultant voltage applied to the motor is distributed over the pulses provided by the gating of each SCR  20 ,  25 ,  30 ,  35 , 40  and  45 . The applied voltage is not a result of blocking the conduction of the phases. Instead, all three phases of power supply  12  are conducting and the applied voltage to the motor results from the overlap of the firing and conduction of SCRs  20 ,  25 ,  30 ,  35 ,  40  and  45 .  
         [0076]     The method produces a rotating magnetic field in motor  10  that has a net positive average torque in the desired direction of rotation at the desired speed. To accelerate a motor to a desired speed, the motor torque must exceed the load torque at all times. If the torque delivered by the motor is less than the torque of the load at any speed during the start cycle, the motor will cease accelerating. By maintaining the applied voltage and frequency, sufficient torque can be generated to accelerate the motor.  
         [0077]     The voltage of phase A that is applied to the motor is represented by the overlap of phase A voltage in  FIG. 6 . The overlap of phase A voltage is either positive or negative of the zero voltage line  530  and is a combination of the firing of SCR  20  and SCR  25  concurrently with an oppositely poled SCR. With reference to Table 3, the applied voltage of phase A  535  in output state  0 , as represented at  400 , is a result of the overlap of the firing of the positive pulse  430  of SCR  20  and the negative pulse  480  of SCR  35  because both SCR  20  and SCR  35  are gated at the same time. A positive voltage of phase A is applied for the duration of the first positive pulse  430  which continues into output state  1 , represented at  405 .  
         [0078]     During output state  1 , represented at  405 , the applied voltage of phase A continues into output state  1  and corresponds to the positive pulse  480  of SCR  20  and negative pulse  480  of SCR  35 . However, as SCR  35  becomes nonconductive, SCR  45  becomes conductive with negative pulse  515 . The applied voltage of phase A remains positive until the cease of activity of SCR  20 . Immediately following the activity, phase A enters a nonconductive period  536 .  
         [0079]     As output state  1   405  ends, SCR  20  becomes conductive again with a second positive pulse  435 . The applied voltage of phase A at  540  at the end of output state  1  corresponds to the second positive pulse  435  of SCR  20  and the negative pulse  485  of SCR  35 . As output state  2   410  is entered, SCR  20  is still conductive as a result of the continued pulse of  435 , but SCR  35  ceases activity. At this time, the negatively poled SCR  45  has become conductive and fires negative pulse  520 . Thus, a positive phase A voltage is applied until the end of the second positive pulse  435  of SCR  20 . Immediately following this activity, phase A enters a nonconductive period  536 .  
         [0080]     At the end of output state  2   410 , SCR  20  becomes conductive again with positive pulse  440 . However, there is no negative pole SCR activity occurring at this time, thus, phase A voltage remains zero. At the beginning of output state  3   415 , SCR  20  is still firing positive pulse  440  as SCR  45  begins to fire negative pulse  520 . The positive applied voltage of phase A at  570  is maintained until SCR  20  ceases activity. Immediately subsequent to this cessation of activity of SCR  20 , SCR  25  fires negative pulse  445  causing the applied voltage of phase B to become negative at  575 . At this time during output state  3   415 , SCR  30  becomes conductive by firing positive pulse  465 . Phase A then enters another nonconductive period  536  until output state  4   420  is entered.  
         [0081]     In output state  4   420 , the negative applied voltage of phase A at  580  is a result of the firing of a negative pulse  450  of SCR  25 , a positive pulse  470  of SCR  30  and a positive pulse  505  of SCR  40 . Finally, in output state  5   425 , the negative applied voltage of phase A at  590  is a result of the firing of a negative pulse  455  of SCR  25  and a positive pulse  510  of SCR  40 .  
         [0082]      FIG. 6  further represents the applied voltage with respect to phase B and phase C. The overlap of each phase is determined by the conductive SCRs  20 ,  25 ,  30 ,  35 ,  40  and  45  of each output phase of the 60° double conduction pattern as represented by Table 3.  
         [0083]     The voltage of phase B that is applied to the motor is represented by the overlap of phase B voltage in  FIG. 6 . The overlap of phase B voltage is either positive or negative of the zero voltage line  530  and is a combination of the firing of SCR  30  and SCR  35  concurrently with an oppositely poled SCR. With reference to Table 3, the overlap of phase B is in a nonconductive period  706  during a portion of output state  0   400 . Although a negative pulse  475  of SCR  35  is being fired, there are no oppositely poled SCRs being fired at this time. Thus the applied voltage during the initial portion of output state  0   400  is zero. During the remainder output state  0 , represented at  400 , a negative applied voltage of phase B  705  is a result of the overlap of the firing of a negative pulse  480  by SCR  35  and a positive pulse  500  of SCR  40 .  
         [0084]     During output state  1 , represented at  405 , the negative applied voltage  705  of phase B as negative pulse  480  continues into output state  1  corresponds to a positive pulse  430  of SCR  20  and negative pulse  480  of SCR  35 . Phase B enters then a nonconductive period  706  for the most of the duration of output state  1   405 .  
         [0085]     As output state  1   405  ends, SCR  35  becomes conductive again with a negative pulse  485 . The applied voltage of phase B at  710  at the end of output state  1  corresponds to negative pulse  485  of SCR  35  and a positive pulse  435  of SCR  20 . During output state  2   410 , SCR  20  and SCR  35  become nonconductive and SCR  30  and SCR  45  become conductive with positive pulse  460  and negative pulse  520 , respectively. This causes the applied voltage of phase B  715  to be positive. Immediately following this activity, phase B enters a period of inactivity  706 .  
         [0086]     At the beginning of output state  3   415 , SCR  30  becomes conductive with positive pulse  465 . During a portion of positive pulse  465 , SCR  45  is also firing a negative pulse  525 . The applied voltage of phase B  720  is thus positive. As SCR  45  becomes nonconductive, SCR  25  becomes conductive firing negative pulse  445 . The positive voltage of phase B  720  continues to be applied in output state  3   415  until the end of positive pulse  465 . At this time, phase B enters another nonconductive period  706 .  
         [0087]     In output state  4   420 , the positive applied voltage of phase B at  725  is a result of the firing of SCR  25 , which corresponds to negative pulse  450 , and SCR  30 , which corresponds to positive pulse  470 . A period of inactivity of applied phase B voltage  706  follows this activity. Finally, in output state  5   425 , the negative applied voltage of phase B  730  is a result of the firing of SCR  35 , corresponding to negative pulse  490 , and SCR  40 , which corresponds to positive pulse  510 .  
         [0088]     The voltage of phase C that is applied to the motor is represented by the overlap of phase C voltage in  FIG. 6 . The overlap of phase C voltage is either positive or negative of the zero voltage line  530  and is a combination of the firing of SCR  40  and SCR  45  concurrently with an oppositely poled SCR. With reference to Table 3, the positive applied voltage of phase C  735  in output state  0 , as represented at  400  is a result of the overlap of the firing of SCR  35 , corresponding to negative pulse  480 , and SCR  40 , which corresponds to positive pulse  500 , because both SCR  35  and SCR  40  are gating at the same time. The positive voltage of phase C  735  is applied for the duration of the positive pulse  500 . In output state  1   405 , SCR  40  becomes nonconductive and SCR  45  immediately fires a negative pulse  515  causing the applied voltage of phase C to become negative at  740 . As SCR  45  fires negative pulse  515 , SCR  20  is firing positive pulse  430 . Phase C applied voltage then enters a period of inactivity  736 .  
         [0089]     As output state  2   410  is entered, SCR  45  becomes conductive again with a negative pulse  520 . At this time, SCR  20  is also firing a positive pulse  435 , and SCR  30  is firing a positive pulse. Thus, the applied voltage  745  of phase C corresponds to SCR  45 ,  20  and  30 . After this period of activity, phase C enters a period of inactivity  736 .  
         [0090]     At the beginning of output state  3   415 , the negative applied voltage of phase C  750  is a result of the negative pulse  525  of SCR  45  and positive pulse  465  of SCR  30 . Again, phase C enters a period of inactivity  736 . In output state  4   420 , the positive applied voltage of phase C  755  corresponds to SCR  45 , which fires a positive pulse  505 , and SCR  25 , which fires a negative pulse  450 . Finally, in output state  5 , the applied voltage  760  is a result of SCR  40 , which corresponds to positive pulse  510 ; SCR  25 , which corresponds to negative pulse  455 ; and SCR  35 , which corresponds to negative pulse  490 .  
         [0091]      FIG. 7  represents the current waveform of phase A during the operation of the present invention at 16⅔%, as described with respect to the overlap of phase A of  FIG. 6 . The applied voltage at  535  of phase A is represented by the current waveform in  FIG. 7  at  600 . This represents the current flowing at the end of output state  0   400  and into output state  1   405 . The double peak of current is a result of the natural wave shift of the input waveform because of the three-phase power supply. Similarly, the current waveform at  605  of  FIG. 7  corresponds to the applied voltage at  540  in  FIG. 6  followed by the period of inactivity  536 . The current waveform at  610  and  615  of  FIG. 7  represent the positive to negative voltage shift as shown by the applied voltage  570  and  575  in  FIG. 6 . Again, a period of inactivity  536  follows the applied voltage. Finally, the current waveform at  620  in  FIG. 7  represents the applied voltage  580  of output state  4   420  in  FIG. 6 , and the current waveform at  625  in  FIG. 7  represents the output state  5   425  applied voltage  590  in  FIG. 6 . The combined total current waveform of phase A  700  approximates the continuous sinusoidal phase A waveform  80  of  FIG. 2 , but is a result of the overlap of pulses from each of phase A, phase B and phase C.  
         [0092]     It can be appreciated by one skilled in the art that a similar current waveform pattern is produced with respect to the overlap of phase B and the overlap of phase C. The sinusoidal waveform produced for each of phase A, phase B and phase C overlap have a 120° offset from each other. Further, during the period of inactivity of phase A, current is flowing in phase B and phase C due to this offset of the phases, as represented by the overlap of phase A, phase B and C.  
         [0093]     Although particular embodiments of the present invention have been illustrated in the accompanying drawings and described in the foregoing detailed description, it is to be further understood that the present invention is not to be limited to just the embodiments disclosed, but that they are capable of numerous rearrangements, modifications and substitutions, as identified in the following claims.