Abstract:
A regenerative variable frequency drive includes an active converter connected to an inverter. The converter has a filter capacitor, an inductor, two half bridges, bus bars that connect to the inverter and bus capacitors. The converter converts single phase AC power to DC power and DC power to single phase AC power, boosts the AC power, reduces input line harmonics, maintains input current in phase with utility voltage in order to achieve near unity power factor, and maintains constant DC voltage between the bus bars.

Description:
[0001]    This application claims the benefit under 35 U.S.C. §119(e) of the U.S. provisional patent application No. 61/561,313 filed Nov. 18, 2011. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to variable frequency drives and more particularly to a regenerative variable frequency drive with an active converter that converts single-phase AC input to three-phase variable frequency AC output. 
       BACKGROUND ART 
       [0003]    A variable frequency drive controls the speed and torque of an alternating current (AC) motor by varying the input frequency and voltage. Three-phase motors provide higher mechanical efficiency, higher power factor and less torque ripple than single-phase motors and are therefore a more desirable choice. Variable frequency drives in the past have generally included a diode rectifier, that converts AC power to direct current (DC) power, connected through a DC bus to an inverter that supplies three phase, variable frequency AC power to a three-phase motor. 
         [0004]    When a motor turns faster than the speed designated by the variable frequency drive, the motor acts as a generator, generating power that is returned to the DC bus. In a variable frequency drive with a diode rectifier, the rectification of the AC power to the DC bus is a one-way street and the generated power causes the voltage on the DC bus to rise. 
         [0005]    One known method of handling the generated power is to add a dynamic braking resistor to the variable frequency drive. When the voltage on the DC bus rises due to the generated power, the generated power is shunted to the dynamic braking resistor that converts the generated power to heat. Dynamic braking resistors add complexity and expense to a variable frequency drive installation. 
         [0006]    The generated power can alternatively be handled with a regenerative variable frequency drive that has an active converter instead of the one-way diode rectifier. An active converter allows power to flow from the AC source to the DC bus and from the DC bus back to the AC source. A regenerative variable frequency drive puts the generated power back onto the line, and thereby reduces the total power consumption of the load. 
         [0007]    Regenerative variable frequency drives with three-phase active converters are known. A conventional diode rectifier drive can convert AC power from a single-phase source to charge the DC bus. The known three-phase active converters cannot convert the power from a single-phase AC source to charge the DC bus. 
         [0008]    Three-phase AC power is generally supplied to industrial areas. However, only single phase AC power is available to most residential and rural areas. The single phase AC power available in most residential and rural areas is provided by a step down transformer connected to a high voltage line and, in the United States, is normally supplied as about 240 volts at 60 Hz between the first and second input lines. Many three-phase induction motors are operated at high voltage such as about 460 volts to reduce the current passing between the inverter of the variable frequency drive and the motor, and thereby reducing the required size of the connecting cables. Diode rectifier converters cannot directly boost the incoming 240 volts to 460 volts. 
         [0009]    Diode rectifiers distort the current drawn from the power grid. This distortion creates harmonic distortions that may affect other users on the grid. The distortion also reduces the power factor. A variable frequency drive with a diode rectifier therefore requires additional circuitry for power factor correction and harmonic filtering. 
       DISCLOSURE OF THE INVENTION 
       [0010]    A regenerative variable frequency drive for converting single phase AC power to variable frequency three phase AC power includes an active converter that converts single phase AC power to DC power and DC power to single phase AC power and an inverter that converts DC power to variable frequency, three phase AC power, and variable frequency, three phase AC power to DC power. The converter includes first and second input lines that connect to a single phase AC power source, first and second inductors, a filter capacitor, active half bridge first and second modules, a positive bus bar, a negative bus bar, first and second bus capacitors and a controller. The inductors each have two coils and are connected in series with the filter capacitor connecting between the coils, between the inductors. The input lines connect to the coils, with one coil connecting to the first module and the other coil connecting to the second module, opposite the terminals. The modules each have a pair of switches and a pair of diodes, and each connect to the positive and negative bus bars. The bus capacitors connect together in series and connect between the positive and negative bus bars. The bus bars connect to the inverter. The controller monitors voltages and input current, and drives the switches with a pulse width modulated signal having a modulation index. The controller adjusts the modulation index to maintain a selected voltage between the bus bars, to provide correctly phased sinusoidal current from and to the power grid and to boost the single phase AC input voltage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Details of this invention are described in connection with the accompanying drawings that bear similar reference numerals in which: 
           [0012]      FIG. 1  is a block diagram of a variable frequency drive embodying the features of the present invention. 
           [0013]      FIG. 2  is a schematic diagram of the active converter of  FIG. 1  with an LC filter. 
           [0014]      FIG. 3  is a schematic diagram of the active converter of  FIG. 1  with an LCL filter. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Referring to  FIG. 1 , a regenerative variable frequency drive  11 , embodying the features of the present invention, includes an active converter  14  connected to an inverter  15 . The inverter  15  connects to and drives a load  16 , such as a motor. A master controller  18  controls the drive  11 . The master controller connects to an input controller  19 , an output controller  20  and a human interface device  21 . 
         [0016]    The input controller  19  connects to and controls the converter  14 . The converter  14  connects to a single phase AC power source  23 . Generally, the source  23  will be a utility power grid. In the United States, the source  23  will typically provide power at 240V at 60 Hz. 
         [0017]    The converter  14  converts single phase AC power to DC power and DC power to single phase AC power. The converter  14  connects to and provides DC power to the inverter  15  through a positive bus bar  24  and a negative bus bar  25 . The drive  11  includes a positive terminal  27  that connects to the positive bus bar  24  and a negative terminal  28  that connects to the negative bus bar  25 , so that the drive  11  can provide DC power to a DC load. 
         [0018]    The output controller  20  connects to and controls the inverter  15 . The inverter  15  draws DC power from the positive and negative bus bars  24  and  25 , and provides variable frequency, three phase AC power to the load  16 . When the load  16  supplies power back to the inverter  15 , as with a motor overrunning, the inverter  15  converts the AC power generated by the load  16  to DC power, and supplies that DC power to the positive and negative bus bars  24  and  25 . The human interface device  21  allows a user to set the speed and direction of the load  16 . 
         [0019]    As shown in  FIGS. 2 and 3 , the converter  14  includes first and second input lines  31  and  32 , a precharging circuit  33 , a filter  35 , first and second modules  36  and  37 , first and second bus capacitors  39  and  40 , first and second resistors  42  and  43 , the positive and negative bus bars  24  and  25 , and the input controller  19 . The first and second input lines  31  and  32  connect to the source  23 . The precharging circuit  33  connects along the second input line  32  and includes a fuse  46 , two diodes  47  and a resistor  48  in connected in series circuit, and a switch  49  connected in parallel to the series circuit. 
         [0020]    The filter  35  in  FIG. 2  is an LC filter with a first inductor  51  and a filter capacitor  52 . The first inductor  51  has a first coil  53  that connects to the first input line  31  opposite the source  23  and a second coil  54  that connects to the precharging circuit  33  opposite the source  23 . The filter capacitor  52  connects from the first coil  53  to the second coil  54 , between the first inductor  51  and the source  23 . The filter  35  in  FIG. 3  is an LCL filter that additionally includes a second inductor  56  between the source  23  and the filter capacitor  52 . The second inductor  56  has a first coil  57  that connects to the first input line  31  at one end and to the first coil  53  of the first inductor  51  at the other end, and a second coil  58  that connects from the precharging circuit  33  to the second coil  54  of the first inductor  51 . 
         [0021]    The first and second modules  36  and  37  are each active half bridges. The first module  36  has an input  61 , a positive output  62 , a negative output  63 , first and second switches  65  and  66 , and first and second diodes  67  and  68 . The input  61  connects to the first coil  53  of the first inductor  51  opposite the source  23 . The first and second switches  65  and  66  are preferably solid state switches and more preferably Insulated Gate Bipolar Transistors (IGBT). Other switches such as bipolar junction transistors or devices developed in the future might also be used. 
         [0022]    The first switch  65  has a collector  70 , a base  71  and an emitter  72 . The second switch  66  has a collector  74 , a base  75  and an emitter  76 . The first diode  67  has an anode  78  and a cathode  79 , and the second diode  68  has an anode  81  and a cathode  82 . The input  61  connects to the emitter  72  of the first switch  65 , the collector  74  of the second switch  66 , the anode  78  of the first diode  67  and the cathode  82  of the second diode  68 . The collector  70  of the first switch  65  and the cathode  79  of the first diode  67  connect to the positive output  62 . The emitter  76  of the second switch  66  and the anode  81  of the second diode  68  connect to the negative output  63 . The positive output  62  connects to the positive bus bar  24  and the negative output  63  connects to the negative bus bar  25 . 
         [0023]    The second module  37  has an input  85 , a positive output  86 , a negative output  87 , first and second switches  89  and  90 , and first and second diodes  91  and  92 . The input  85  connects to the second coil  54  of the first inductor  51  opposite the source  23 . The first and second switches  91  and  92  are preferably solid state switches and more preferably Insulated Gate Bipolar Transistors (IGBT). Other switches such as bipolar junction transistors or devices developed in the future might also be used. 
         [0024]    The first switch  89  has a collector  94 , a base  95  and an emitter  96 . The second switch  90  has a collector  98 , a base  99  and an emitter  100 . The first diode  91  has an anode  102  and a cathode  103 , and the second diode  92  has an anode  105  and a cathode  106 . The input  85  connects to the emitter  96  of the first switch  89 , the collector  98  of the second switch  90 , the anode  102  of the first diode  91  and the cathode  106  of the second diode  92 . The collector  94  of the first switch  89  and the cathode  103  of the first diode  91  connect to the positive output  86 . The emitter  100  of the second switch  90  and the anode  105  of the second diode  92  connect to the negative output  87 . The positive output  86  connects to the positive bus bar  24  and the negative output  87  connects to the negative bus bar  25 . 
         [0025]    The first and second bus capacitors  39  and  40  are connected together in series at connection node  108 . The first bus capacitor  39  connects to the positive bus bar  24  opposite connection node  108 , and the second bus capacitor  40  connects to the negative bus bar  25  opposite the connection node  108 . One end of first resistor  42  connects to the positive bus bar  24  and the other end of first resistor  42  connects to the connection node  108 . One end of second resistor  43  connects to the negative bus bar  25  and the other end of second resistor  43  connects to the connection node  108 . The first and second resistors  42  and  43  are balancing resistors that insure that the voltage between the positive bus bar  24  and the connection node  108  equals the voltage between the connection node  108  and the negative bus bar  25 . 
         [0026]    The input controller  19  connects to the bases  71 ,  75 ,  95  and  99  of the first switches  65  and  89  and the second switches  66  and  90  of the first and second modules  36  and  37 , and drives the first switches  65  and  89  and the second switches  66  and  90  of the first and second modules  36  and  37 . The input controller  19  connects to the first and second input lines  31  and  32 , to opposite ends of the filter capacitor  52 , and to the positive and negative bus bars  24  and  25  to monitor input current and voltage, voltage across the filter capacitor  52 , and the voltage between the positive and negative bus bars  24  and  25 . 
         [0027]    To avoid excessively high input current when power is applied to the converter  14 , switch  49  is initially open and pre-charging current is supplied through the diodes  47  and the current-limiting resistor  48  in the precharging circuit  33 . After the first and second bus capacitors  39  and  40  are charged, the switch  49  is closed to bypass resistor  48 . 
         [0028]    The input voltage is V 12 =V 1 −V 2 , the voltage across the filter capacitor  52  is v ab =V a −V b , the voltage at the connection node  108  is V z , and the voltage between the positive and negative bus bars  24  and  25  is 2V dc =(V POS −V z )+(V z −V NEG ). The first module  36  is driven to produce a pulse width modulated signal at input  61  that has an average value given by: 
         [0000]        v   az   =V   dc   M  cos(ω o   t ),
 
         [0029]    The second module  37  is driven to produce a pulse width modulated signal at input  85  that has an average value given by: 
         [0000]        v   bz   =−V   dc   M  cos(ω o   t ).
 
         [0030]    where M is the modulation index (0&lt;=M&lt;=1), ω o  is the frequency of the input voltage, and cos(ω o t) is the cosine of the input voltage frequency (1=&gt;cos(ω o t)=&gt;−1). The difference in voltage between the input  61  of the first module  36  and the input  85  of the second module  37  is given by: 
         [0000]        v   ab   =v   az   −v   bz =2 V   dc   M  cos(ω o   t ).
 
         [0031]    As long as the value 2V dc  is greater than the peak value of the input voltage V 12 , at any instant in time the value of M can be adjusted to make v ab  at that instant either less than, equal to, or greater than the input voltage V 12 . The input voltage V 12  is separated from the voltage v ab  by the first inductor  51  in  FIG. 2 , and the first and second inductors  51  and  56  in  FIG. 3 , so that: (1) if V 12  and v ab  are equal there will be no change in the current through the inductors, (2) if V 12  is greater than v ab  the current through the inductors will increase, or (3) if V 12  is less than v ab  the current through the inductors will decrease. The input controller  19  can make instantaneous adjustments to the value of M to induce any desired value of input current. The input current can be controlled so that the average value of the voltage 2V dc  remains constant even though electrical charge is being removed from the first and second bus capacitors  39  and  40  by the inverter  15 . The value of M can also be adjusted so that the input current is sinusoidal. If the converter  14  is delivering power to the inverter  15  and subsequently to the load  16 , then the input current will need to be in phase with the input voltage. If the load  16  is delivering power to the converter then the input current will need to be 180 degrees out of phase with the input voltage. 
         [0032]    The converter  14  can also boost the voltage of the incoming power. As an example, and not as a limitation, the converter can boost single phase 240 volt AC power to 460 volts. When V 12 &gt;0, the second switch  66  of the first module  36  is turned on, and current flows from the source  23  through the first coil  53  of the first inductor  51 , through the input  61  and second switch  66  of the first module  36 , through the negative bus bar  25 , through the second diode  92  and input  85  of the second module  37 , through the second coil  54  of the first inductor  51  and back to the source  23 . The current will be a steadily increasing ramp which will stop increasing only when the second switch  66  of the first module  36  is turned off. Thus the maximum current is determined by the width of the controller pulse to the second switch  66  of the first module  36 . The ramp rate is determined by the inductance values, the value of V 12 , and the equation V 12 =L dI/dt. 
         [0033]    When the second switch  66  of the first module  36  turns off, the first inductor  51  will develop a voltage which keeps the current constant during the transition. Current now flows from the source  23  through the first coil  53  of the first inductor  51 , through the input  61  and first diode  67  of the first module  36 , through the positive bus bar  24 , through the first and second bus capacitors  39  and  40 , through the second diode  92  and input  85  of the second module  37 , through the second coil  54  of the first inductor  51  and back to the source  23 . This current charges the first and second bus capacitors  39  and  40 . During the charging cycle the current decays at a rate determined by the inductance value of the first inductor  51 , the voltage (V 12 −V POS +V NEG ) and the equation (V 12 −V POS +V NEG )=LdI/dt. The amount of charging current can have any desired value, determined only by the width of the pulses, and the first and second bus capacitors  39  and  40  can be charged to any desired value. The charging sequence could also have been implemented by switching the first switch  89  of the second module  37 . When the voltage V 12 &lt;0, charging can be implemented by switching either the first switch  65  of the first module  36  or the second switch  90  of the second module  37 . 
         [0034]    Although the present invention has been described with a certain degree of particularity, it is understood that the present disclosure has been made by way of example and that changes in details of structure may be made without departing from the spirit thereof.