Abstract:
A lockout circuit which limits the field voltage in an alternator while the vehicle starter is activated. The lockout circuit may be configured to limit the field voltage while the charging circuit voltage is below a threshold value. A timer circuit may advantageously be employed with the lockout circuit. A temperature compensating function may also be employed to change the threshold value in response to temperature changes. The disclosed circuit is particularly advantageous when employed in cold weather conditions. A method of starting the engine of a vehicle is also disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. 119(e) of U.S. provisional patent application Ser. No. 61/641,628 filed on May 2, 2012 entitled ALTERNATOR WITH LOCKOUT PHASE the disclosure of which is hereby incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to automotive alternators and similar electrical machines and their controls. 
         [0003]    Most modern automobiles include a charging system which include a battery, an alternator and a regulator. In such charging systems, the alternator is mechanically coupled to the engine whereby the engine will rotate the rotor of the alternator when the engine drive shaft is rotating. When the engine is operating, the alternator is used as a generator to recharge the battery and provide electrical power to various electrical loads, e.g., headlights, of the vehicle. Alternators are typically multiphase electrical machines, typically three-phase. The electrical power generated by the alternator is dependent upon several variables, two of the more significant variables are the engine speed, the rotational speed of the alternator rotor generally varies with the engine speed, and the voltage of the field coils on the rotor of the alternator. 
         [0004]    A regulator senses the voltage of the charging system and regulates the voltage of the field coils of the alternator rotor to maintain the voltage of the charging system at the desired level as the engine speed and electrical loads vary. The battery not only acts as an electrical power reservoir but also acts as a buffer dampening such variations. 
         [0005]    As a higher voltage is provided to the field coils, a greater torque will be required to rotate the alternator rotor at any one speed. In other words, when the voltage of the field coils is increased to increase the output of the alternator, the alternator will drain additional horsepower from the engine. 
         [0006]    When initially starting the engine of a typical automobile, the battery powers an electrical starter motor which turns a flywheel and thereby turns over the engine. The starter provides torque to the engine for a brief period of time until the engine starts to operate normally and no longer needs assistance. Under colder conditions, the time required to start the engine is lengthened and the starter may be required to provide torque for a longer period of time. 
         [0007]    In such cold start conditions when the starter is activated for a relatively long period of time, the voltage of the alternator field coils may be increased to normal operating levels while the starter is still activated. In such a situation, the alternator will be unnecessarily parasitic on the starter, draining mechanical energy from the engine as the starter is providing mechanical energy to the engine and thereby extending the crank time of the starter and lengthening the time required for the engine to reach a stable idle condition. Such a prolongation of the crank time and associated delay of stable idle condition is generally undesirable. 
       SUMMARY 
       [0008]    The present invention provides an alternator and regulator which minimizes the risk of prolonged crank times and delayed stable idle conditions due to the operation of the alternator. 
         [0009]    The invention comprises, in one form thereof, an alternator for a vehicle having an engine and a charging system with a battery. The alternator includes a stator having at least one stator winding and a field coil rotatable relative to the stator winding and which is adapted to be rotated by mechanical energy from the engine. A voltage regulator is configured to regulate an output voltage of the alternator by controlling a field current through the field coil. The regulator has a strobe mode and a normal duty mode wherein, in the strobe mode, the regulator introduces a pulsed current into the field coil and, in the normal duty mode, introduces an electrical current into the field coil at a controllably varied voltage to thereby control the output voltage of the alternator. During starting of the engine, the regulator is initially in the strobe mode and is released into the normal duty mode based upon properties of the electrical current generated in the stator winding. A lockout circuit is configured to maintain the field coil in the strobe mode until a voltage of the charging system exceeds a threshold value wherein the threshold value varies as a function of a temperature value. 
         [0010]    In some embodiments, the threshold value increases as the temperature value decreases. In still other embodiments, the temperature value is a function of the temperature of the lockout circuit. In yet other embodiments, the lockout circuit further includes a timing circuit configured to maintain the regulator in the strobe mode until the voltage of the charging system exceeds the threshold value for a predefined period of time. 
         [0011]    The invention comprises, in another form thereof, a charging system for a vehicle having an engine that includes a battery coupled with the charging system, a starter coupled with the charging system and adapted to be coupled with the engine and an alternator coupled with the charging system and adapted to be coupled with the engine. The alternator has at least one field coil rotatable with mechanical energy generated by the engine and at least one stator winding wherein rotation of the field coil when energized generates an electrical current in the stator winding. A voltage regulator is configured to regulate an output voltage of the alternator by controlling a field current through the field coil. The regulator has a strobe mode and a normal duty mode. In the strobe mode, the regulator introduces a pulsed current into the field coil and, in the normal duty mode, the regulator introduces an electrical current into the field coil at a controllably varied voltage to thereby control the output voltage of the alternator. During starting of the engine, the regulator is initially in the strobe mode and is released into the normal duty mode based upon properties of the electrical current generated in the stator winding. A lockout circuit is configured to block communication of electrical current from the stator winding to the regulator before the voltage of the charging system exceeds a threshold value and communicate electrical current from the stator winding to the regulator after the charging system exceeds the threshold value wherein the threshold value varies as a function of a temperature value and wherein the threshold value increases as the temperature value decreases. 
         [0012]    In some embodiments, the lockout circuit includes a MOSFET transistor configured to selectively block or permit communication of the electrical current generated in the stator winding to the regulator. The lockout circuit may further include a Zener diode and NPN transistor responsively coupled to the charging system and arranged to control operation of the MOSFET transistor. Advantageously, the temperature value is a function of the temperature of the Zener diode and the NPN transistor. 
         [0013]    The invention comprises, in yet another form thereof, a method of starting an engine of a vehicle having a charging system with a starter coupled with the engine, a battery, and an alternator. The alternator includes a stator with at least one stator winding, at least one field coil is rotatable relative to the stator winding, and a regulator configured to regulate an output voltage of the alternator by controlling a field current through the field coil. The method includes activating the starter, placing the field coil in a strobe mode by introducing a pulsed current into the field coil, and maintaining the field coil in the strobe mode until a voltage of the charging system exceeds a threshold value wherein the threshold value varies as a function of temperature. The method also includes monitoring the stator winding with the regulator after satisfying threshold value and entering a normal duty mode wherein the regulator introduces an electrical current into the field coil at a controllably varied voltage to thereby control the output voltage of the alternator when the regulator determines that the properties of the electrical current generated in the stator winding satisfy predetermined conditions. 
         [0014]    In some embodiments, the threshold value advantageously increases as the temperature value decreases. The method may also include the step of maintaining the field coils in the strobe mode until the voltage of the charging system exceeds the threshold value for a predefined period of time. 
         [0015]    In some embodiments, the step of maintaining the field coil in strobe mode includes blocking communication of electrical current from the stator winding to the regulator before the voltage of the charging system exceeds the threshold value and communicating electrical current from the stator winding to the regulator after the charging system exceeds the threshold value. The method may also include providing a MOSFET transistor configured to selectively block or permit communication of the electrical current generated in the stator winding to the regulator. In such an embodiment, the method may additionally include providing a Zener diode and NPN transistor arranged to control operation of the MOSFET transistor wherein the temperature value is a function of the temperature of the Zener diode and the NPN transistor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The above mentioned and other features of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: 
           [0017]      FIG. 1  is a schematic representation of a vehicle engine and charging system. 
           [0018]      FIG. 2  is a schematic drawing of a lockout circuit which can be used to impose a limitation on the voltage of the alternator field coils. 
           [0019]      FIG. 3  is a flow chart representing the use of the lockout circuit of  FIG. 2 . 
           [0020]      FIG. 4  is a chart illustrating measured parameters while starting a vehicle under cold conditions without the lockout circuit of  FIG. 2 . 
           [0021]      FIG. 5  is a chart illustrating measured parameters while starting a vehicle under cold conditions with the lockout circuit of  FIG. 2 . 
       
    
    
       [0022]    Corresponding reference characters indicate corresponding parts throughout the several views. Although the exemplification set out herein illustrates an embodiment of the invention, in one form, the embodiment disclosed below is not intended to be exhaustive or to be construed as limiting the scope of the invention to the precise form disclosed. 
       DETAILED DESCRIPTION 
       [0023]    Conventional regulators have different operating modes during which different levels of current are supplied to the field coils of the alternator. These different levels are often described as a percentage of the full duty cycle which corresponds to the maximum achievable current in the field coils. For example, many alternators have a nominal 2 ohm field (typically 1.8 ohms) and a maximum achievable current of about 6 to 7 amps. 
         [0024]    The different operating modes can be broadly categorized as either a strobe mode or a normal duty mode. In the strobe mode, the regulator introduces pulsed current into the field coil in a square wave pattern. In the normal duty mode, the regulator introduces an electrical current into the field coil at a controllably varied voltage to thereby control the output voltage of the alternator. 
         [0025]    During the initial start-up of the vehicle the regulator and field coil are in the strobe mode and the regulator applies a strobe voltage to the field at approximately 12% of the full duty cycle. The effective full duty cycle percentage of the strobe mode may differ for alternative embodiments and it is advantageously less than about 20% of the full duty cycle. This reduced voltage allows the regulator to sense the operating characteristics of the alternator but does not draw a significant torque from the engine. It is noted that this reduced level of the fully duty cycle is due to the strobed nature of the current supplied to the field coil. The individual peaks of the square wave pattern correspond more closely to the full duty cycle of the alternator. 
         [0026]    When the vehicle is first started, the voltage of the charging system is drawn down as the starter motor draws current from the battery and the voltage drops significantly, often to a level of about 5 volts in a common passenger car having a nominal 12 volt charging system. After the engine starts and the voltage of the charging system rises, the regulator will increase the field voltage of the alternator to a full field condition at about 98% of the full duty cycle. This full field condition percentage may vary slightly as the regulator adjusts the field voltage to account for changes in the engine speed and variations in the electrical loads. 
         [0027]    When the alternator is at full field condition, it is not uncommon in a conventional passenger car for the alternator to draw 3 to 4 horsepower from the engine. If the alternator reaches full field condition while the starter is still cranking the engine, this will likely prolong the cranking time and delay when the engine reaches a stable idle condition. This is most likely to occur under cold weather conditions. 
         [0028]    Under normal operating conditions, if the charging system reaches an upper level threshold, e.g., 14.5 volts, the regulator will lower the field voltage from the full field condition to a low duty condition which often corresponds to about 6 or 7% of the full duty cycle. Both the full field condition and the low duty condition are normal duty modes as that term is used herein. In such normal duty modes, the engine is intended to be operating stably and the regulator adjusts the voltage of the field coil to an appropriate percentage of the full duty cycle to control the output voltage of the alternator and thereby maintain a desired voltage in the charging system and battery coupled to the charging system. The use of a regulator to implement such a strobe mode and normal duty modes in a vehicle alternator is known and is well understood by a person having ordinary skill in the art. 
         [0029]      FIG. 1  schematically depicts a vehicle which includes the lockout function described herein. The vehicle, which may take the form of a passenger automobile, has an internal combustion engine  20  and a charging system  22 . Charging system  22  includes an alternator  24 , a battery  26 , and a starter motor  28  which creates a load on the charging system when activated to crank engine  20 . Also coupled with charging system  22  is an ignition switch  30  and an electronic control module (“ECM”)  32 . The operator of the vehicle cranks, i.e., activates, starter motor  28  by closing ignition switch  30  in a conventional manner. Closing of ignition switch  30  is communicated to regulator  46  through ECM  32  to terminal L of regulator  46  in the illustrated example. ECM  32  is a conventional ECM and controls the operation of the engine and many of the other vehicle systems. Line  33  provides electrical communication between regulator  46  and ECM  32  for the exchange of data and control signals. 
         [0030]    Alternator  24  includes a stator  34  having a plurality of stator windings  36 . In the illustrated example, alternator  24  is a three phase alternator and stator windings  36  are arranged in a delta configuration. Alternative embodiments, however, can also be used with the present invention. For example, a different number of phases or arranging the stator windings in a wye configuration could be used with alternative embodiments. Coupled with stator  34  and stator windings  26  is a rotor  38  with field coils  40 . Rotor  38  is mechanically coupled to engine  20  whereby operation of engine  20  imparts mechanical energy to rotor  38  and rotates field coils  40  relative to stator windings  36  in a conventional manner well-known to those having ordinary skill in the art. 
         [0031]    When an electrical current is introduced into field coils  40  and coils  40  are rotated relative to stator windings  36 , an electrical current is generated in stator windings  36 . A rectifier bridge  42  having a pair of diodes  44  for the stator windings  36  corresponding to each of the three different phases. In other words, rectifier bridge  42  has a pair of diodes  44  for each different phase coil  36 . Rectifier bridge  42  converts the alternating current generated by stator windings  36  into direct current which is output by alternator  24  into charging system  22  to recharge battery  26  and power loads on charging system  22 . 
         [0032]    Regulator  46  controls the electrical current introduced into field coils  40  to regulate the voltage of the electrical current output by alternator  24  into charging system  22 . Electrical communication between regulator  46  and field coils  40  is provided through terminal F+ on regulator  46 . Similarly, terminal B+ on regulator  46  provides a connection between regulator  46  and charging system  22  and, thus, couples regulator  46  with battery  26 . Terminal E on regulator  46  connects regulator  46  to ground which may be accomplished by a connection to the frame of the vehicle or other suitable means. Terminal P on regulator  46  provides electrical communication between regulator  46  and one of the stator windings  36  at location  62  on line  64  between one of the stator windings/phase coils  36  and rectifier bridge  42 . When electrical signals are communicated from location  62  to terminal P, regulator  46  can monitor the properties of the electrical current in the one stator winding  36 . A lockout circuit  50  is coupled between terminal P and location  62  and is used to control the communication of signals from location  62  to terminal P as discussed in greater detail below. 
         [0033]    In the embodiment depicted and described herein, regulator  46  includes a printed circuit board with an integrated circuit (“IC”) that controls the field voltage. Regulator  46  senses the voltage of the charging system (which includes the battery, the alternator and the various electrical loads, e.g., the starter motor). When starting the engine, regulator  46  enters the strobe mode and introduces a strobe voltage in the field coils of the alternator (e.g., 12% of the full duty cycle). The voltage of the charging system will initially be very low, e.g., 5 volts, as the starter motor draws current from the battery. Conventional regulators typically release the strobe mode and enter the normal duty mode and go to full field condition when the charging system voltage is at about 6 or 7 volts. The thresholds used to determine when this occurs may be based upon the frequency and voltage of the charging circuit and/or other parameters. For example, conventional regulators often monitor the frequency of the oscillations of the current in one stator winding/phase coil, which corresponds to the engine speed, and enter normal duty mode when the frequency reaches a threshold value. Under cold start conditions, this may result in the alternator going to a full field condition prior to the deactivation of the starter motor. 
         [0034]    The circuit illustrated in  FIG. 2  can be used with the regulator to reduce the risk that the alternator will go to a full field condition while the starter is still activated. In  FIG. 2 , Bplus represents a connection to the positive terminal of the battery. Phase_In represents a connection to one of the phase coils of the alternator stator. Phase_Out represents a connection to the IC of the regulator. During operation of a multiphase alternator, most commonly three phases, the charging system voltage (which is measurable at Bplus in  FIG. 2 ) will represent the combination of each phase after it has been converted to direct current. The lockout circuit depicted in  FIG. 2  may be located in only one of the phases with regulator  46  using the characteristics of one phase to control operation of the alternator. 
         [0035]    In the lockout circuit  50  depicted in  FIG. 2 , resistors R 1  and R 11 , capacitor C 4 , diode D 1  and capacitor C 1  act together as a DC blocking filter and act as a high low band pass filter separate from the lockout function. 
         [0036]    The remaining components of lockout circuit  50  function as a lockout wherein MOSFET X1 is used to shunt electrical current from location  63  to ground  48  or permit it to be communicated to Terminal P of regulator  46 . Lockout circuit  50  is advantageously configured to shunt the phase voltage to ground while the starter is still activated. By shunting the phase voltage to ground and blocking the communication of electrical current from location  63  to terminal P, regulator  46  will keep the field coils at the limited field strobe voltage, e.g., 12%. This is because, in the illustrated example, regulator  46  monitors the properties of the electrical current in one of the stator windings  36  and enters the normal duty mode based upon one or more of the properties exceeding a threshold value. By blocking the signal from location  63  to regulator  46 , the lockout circuit prevents regulator  46  acting on a change in the properties of the electrical current in stator windings  36  and thereby maintains the regulator  46  and field coils  40  in strobe mode. 
         [0037]    Zener diode D 7  in cooperation with NPN transistor Q 1  is used to open and close MOSFET X1 and thereby determine whether the phase voltage is shunted to ground  48  or communicated to regulator  46 . When the starter is cranked, the voltage of charging system  22  will collapse, e.g., to 5 volts in a 12 volt system. This low voltage will be communicated to lockout circuit  50  at terminal  56 , i.e., the B+ terminal. Initially, at low charging system voltages, Q 1  prevents the flow of current from B+ therethrough. As the voltage of the charging system rises, it eventually passes a threshold value, the breakdown voltage of Zener diode is exceeded providing a sufficient cut-in voltage to Q 1  and Q 1  permits the flow of electrical current from B+ therethrough. The flow of current through Q 1  turns MOSFET X1 off and allows electrical current from location  63  to be communicated to Terminal P. Zener diode D 6  functions as a surge protector while capacitor C 2  filters the signal from B+ and thereby acts as a low pass filter for Q 1 . 
         [0038]    When Q 1  blocks the passage of current therethrough, C 3  is charged and X1 is turned on shunting electrical current from the phase_in terminal  58  to ground  48 . When X1 turns off, X1 allows signals from stator winding  36  to be communicated from the phase_in terminal  58  to the phase_out terminal  60  and, from there, to terminal P of regulator  46 . 
         [0039]    Zener diode D 7 , NPN transistor Q 1  and resistors R 4  and R 5  form a temperature sensitive voltage threshold circuit  52  which performs a temperature compensating function such that the charging system voltage required to close Q 1  and thereby turn off X1 and allow regulator  46  to enter normal duty mode which will result in raise the voltage of the alternator field coils to full field condition is reduced as the temperature increases. In other words, the threshold value of the charging system voltage increases as a temperature value decreases. In the illustrated embodiment, the temperature value corresponds to the temperature of the lockout circuit and, even more specifically, is primarily, although not exclusively, dependent upon the temperature of Zener diode D 7  and transistor Q 1 . In other words, the temperature value is a function of the temperature of lockout circuit  50  and, more specifically, a function of Zener diode D 7  and NPN transistor Q 1 . 
         [0040]    In the illustrated embodiment, the voltage threshold drops by approximately  20  millivolts for each 1° C. increase in the temperature. Thus, in the illustrated embodiment, the threshold voltage will be approximately 9.2 volts when the temperature is 0° C. and will be approximately 7.2 volts when the temperature is 100° C. As mentioned above, this is the temperature of the lockout circuit, not the temperature of the surrounding environment. 
         [0041]    Although the lockout circuit will be at the temperature of the surrounding ambient environment when initially starting a vehicle in extremely cold conditions after the vehicle has been sitting for several hours without running, the temperature of the lockout circuit, which will be located within or proximate the alternator, will be significantly higher after the engine of the vehicle has reached normal operating temperatures. For example, it is not uncommon for alternators to be cooled with blown air which is at a temperature of 125° C. Thus, as a practical matter, once the engine has reached normal operating temperatures, the temperature compensating function of lockout circuit  50  will effectively prevent the lockout circuit from shunting the phase voltage to ground. As a result, lockout circuit  50  provides protection for extremely cold conditions and generally only provides lockout protection, i.e., shunting phase voltage to ground and maintaining field coils in a prolonged strobe mode, when low temperatures require it. 
         [0042]    Lockout circuit  50  also provides a time delay function with diode D 5 , capacitor C 3  and resistor R 3  forming a timing circuit  54  and cooperating to prevent transient spikes in the charging system voltage from repeatedly opening and closing MOSFET X1. In the illustrated embodiment, these components cooperate to provide an approximately 1 second delay. As a result, the charging system voltage must exceed the threshold value and turn on Q 1  for a predefined period of time of approximately one second before X1 will be turned off and circuit  50  will communicate electrical signals from stator winding  36  to regulator  46  instead of shunting them to ground  48 . In this regard, it is noted that the delay time period is predefined by the operating characteristics of circuit  50 , it does not require that each time period of delay be precisely equivalent. 
         [0043]    In the illustrated embodiment, lockout circuit  50  results in regulator  46  entering normal duty mode and initiating full field condition (e.g., 98% full duty cycle) when the voltage of the charging system  22  is approximately 10 volts and the lockout circuit is at −40° C. The illustrated lockout circuit, however, only imposes such an elevated the threshold voltage when the vehicle is subjected to cold temperatures which are likely to make starting engine  20  difficult. It is also noted that it is possible for the regulator  46  to jump immediately to a low duty condition instead of a full field condition when it enters normal duty mode, however, the vast majority of starting conditions will result in the immediate entry into a full field condition when regulator  46  enters normal duty mode after the start of engine  20 . 
         [0044]    The flow chart set forth in  FIG. 3  represents the starting of the vehicle schematically depicted in  FIG. 1 . Box  80  represents when the driver of the vehicle closes ignition switch  30  and cranks, i.e., activates, starter  28  which then draws current from battery  26  to impart mechanical energy to engine  20 . As starter  28  is cranked, regulator  46  enters strobe mode and places field coils  40  into strobe mode by introducing pulsed current into coils  40 . As represented by box  82 , the field coils are maintained in the strobe mode by the operation of circuit  50  until the voltage of charging system  22  exceeds a threshold value. As discussed above, the threshold value of the charging system voltage varies as a function of a temperature value. Once the threshold value of the charging system voltage is exceeded, at least one of the stator windings/phase coils  36  is monitored by regulator  46  as represented by box  84 . In the illustrated embodiment, lockout circuit  50  communicates electrical signals from location  62  to terminal P to allow for such monitoring. 
         [0045]    As depicted by box  86 , regulator  46  enters normal duty mode when regulator  46  determines that the properties of the electrical current generated in the stator winding satisfy one or more predetermined conditions. For example, lockout circuit  50  is well suited for use with a conventional regulator  46  that monitors the frequency of the oscillations of the voltage of the electrical current in one of the stator windings  36  and enters normal duty mode when the frequency of the oscillations exceeds a predetermined threshold. These oscillations correspond to the compression cycle of the engine and, thus, the engine speed. In the embodiment depicted in  FIG. 1 , regulator  46  sets a threshold value for such oscillations and enters the normal duty mode upon exceeding a predetermined frequency. Thus, if circuit  50  were removed from the vehicle depicted in  FIG. 1 , the vehicle would have a conventional structure and operate in a conventional manner. While setting a threshold for entering normal duty mode based on engine speed will generally prevent entry into the normal duty mode while the starter is still cranking, under cold weather conditions, this may not be the case. The use of lockout circuit  50 , however, greatly reduces this unwanted possibility. 
         [0046]      FIGS. 4 and 5  are testing data charts that illustrate the impact of lockout circuit  50 .  FIG. 4  illustrates testing data wherein a vehicle without lockout circuit  50  was started under cold conditions.  FIG. 5  illustrates a similar vehicle started under similar cold conditions but which included a lockout circuit  50 . 
         [0047]    It is noted that  FIG. 1  schematically depicts the vehicle with the operating characteristics of  FIG. 5 . The vehicle with the operating characteristics of  FIG. 4 , is the same as that depicted in  FIG. 1  except that the vehicle with the operating characteristics of  FIG. 4  does not include lockout circuit  50 . In this regard, it is noted that lockout circuit  50  can be added to some existing vehicles without any other modification to the vehicle to enhance the cold-weather starting of the vehicle. 
         [0048]    The charts of  FIGS. 4 and 5  show that the time required for the engine to go from 600 to 800 rpm was reduced from 2.5 second to 0.5 seconds by the use of lockout circuit  50 . The time required to go from 600 to 800 rpm is not identical to the starting or cranking time of a vehicle but it is representative of the required time and a vehicle that requires a shorter time period to go from 600 rpm to 800 rpm will generally take a shorter time to start. 
         [0049]    Turning first to the chart of  FIG. 4 , line  70  represents the starter motor current with the oscillations corresponding to the compression cycle of the engine. Line  72  represents the engine speed and line  76  represents the charging system voltage. Line  74  represents the phase voltage, in other words, the output of the alternator. The chart in  FIG. 5  uses similar lines but the phase voltage is not shown for purposes of graphical clarity. 
         [0050]    While this invention has been described as having an exemplary design, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles.