Abstract:
A Variable Frequency Drive (VFD) supplies power to rotate a motor, controlling both the speed and direction. The method normally used for this power conversion by the VFD results in energy losses and line harmonics. When the motor is driven by the VFD to be rotating at the same speed and direction as if the motor was straight across the incoming AC power, a transfer in power supply to the motor can be made using contactors to bypass the VFD. When in this bypass mode, the energy losses of the VFD are greatly reduced, and the line harmonics due to the VFD are greatly reduced. When it is subsequently recognized that the speed reference has deviated from being the same speed and direction as if the motor was straight across the incoming AC power, a transfer in power supply can be made from using contactors to once again drive the motor from the VFD.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of provisional application No. 61/436,377 filed Jan. 26, 2011. 
     
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable. 
       MICROFICHE/COPYRIGHT REFERENCE 
       [0003]    Not Applicable. 
       FIELD OF THE INVENTION 
       [0004]    This invention relates to variable frequency drives and, more particularly, to the energy efficiency and harmonic reduction that can be achieved by automatically switching to a bypass mode when the motor speed as powered by the variable frequency drive closely matches the motor speed that would be achieved by connecting the motor directly to the incoming power. 
       BACKGROUND OF THE INVENTION 
       [0005]    One of the simplest and most common methods of driving an AC motor is applying input power directly to the motor through a switch (motor starter or contactor) in an across-the-line architecture, as shown in  FIG. 2 . The advantages of this architecture include simplicity, low cost, low energy losses while running, and minimal harmonics while running. Disadvantages include the motor can only run at one speed and in one direction, and switching on and off the load creates a large disturbance on the input power. The motor converts electrical energy to mechanical energy. The motor can drive a load, such as a fan or pump or some other form of machinery. In many applications it is desired to vary the output speed. Since the motor is across-the-line, it cannot be varied, so some other method must be employed. As an example, if the motor is turning a fan to create air-flow, one way to vary the air flow is to deflect the unwanted air away from the desired path. This results in wasted energy. 
         [0006]    An AC motor can also be driven by a Variable Frequency Drive (VFD) as shown in  FIG. 3 . Using a VFD to drive an AC motor overcomes the disadvantages of an across-the-line architecture. The motor speed and direction can be controlled by the VFD, and there is no large disturbance on the input power when starting the motor. The variable speed control of the VFD over the motor increases the efficiency of powering the motor, versus an across-the-line method, when it is desired to vary the output energy of the motor. However, when powering the motor at the same speed and direction as if the input power were connected across-the line, the efficiency is less when using a VFD. A VFD also adds undesirable harmonics to the input power, as will be explained below. 
         [0007]    A common VFD architecture changes AC power to DC power and then changes it back to AC power. There are variations of VFD technologies, but a common one will be described here. The incoming power is rectified to make DC power, called the DC bus, Transistors in the Inverter section are then turned on and off in a certain sequence (normally called Pulse Width Modulation, or PWM) by controlling logic which creates an AC signal on the output power leads. 
         [0008]    There are energy losses when the VFD converts power from AC to DC and back to AC. There are voltage drops across the diodes in the converter section, which cause energy loss. There is controlling circuitry that needs power, which causes energy loss. And when the transistors in the inverter section are turned on, there are energy losses across the transistors. 
         [0009]    Harmonics caused by a VFD are well documented. There are standards, such as IEEE 519, that have been created to limit harmonics because of the known detrimental effects. In general, a VFD, by its nature, is a non-linear load. The impedance of a non-linear load changes with the applied voltage. When a sinusoidal voltage is applied to a non-linear load, the load current will be non-sinusoidal. The non-sinusoidal currents contain harmonic currents, which then interact with the power system and can create voltage distortion. These harmonics can then affect the equipment in the power distribution and other loads. 
         [0010]    The present invention is directed to reducing the energy losses and reducing the VFD harmonics when the motor speed as powered by the variable frequency drive closely matches the motor speed that would be achieved by connecting the motor directly to the incoming power. 
         [0011]    It is known to use a bypass contactor to bypass the VFD and an output contactor to separate the output of the VFD from the motor. When the bypass contactor is closed and the VFD output contactor is open, this is known as bypass mode. In bypass mode, the motor is being powered through the bypass contactor in an across-the-line configuration. Bypass mode is shown in  FIG. 5 . When the bypass contactor is open and the VFD output contactor is closed, this is known as drive mode. In drive mode, the motor is being powered by the VFD and through the output contactor. Drive mode is shown in  FIG. 6 . 
         [0012]    The present invention is directed to improvements in switching between drive mode and bypass mode. 
       SUMMARY OF THE INVENTION 
       [0013]    In accordance with one aspect of the invention, an AC motor drive system comprises a variable frequency drive (VFD) for receiving AC power from a power source and developing variable frequency power at output terminals. A bypass contactor is operatively connected between the power source and an AC motor. An output contactor is operatively connected between the VFD output terminals and the AC motor. A bypass control is operatively associated with the VFD, the bypass contactor and the output contactor. The bypass control includes a drive mode in which the output contactor is controlled to connect the VFD to the motor and a bypass mode in which the bypass contactor is controlled to connect the power source to the motor. The bypass control monitors operation of the VFD to automatically switch to the bypass mode when motor speed as powered by the VFD approximates the motor speed that would be achieved by connecting the motor directly to the power source. 
         [0014]    It is a feature of the invention that the bypass control operates in a frequency mode wherein the automatic switching occurs based on the VFD controlling the motor at the input line frequency. 
         [0015]    It is another feature of the invention that the bypass control operates in a frequency and output current mode wherein the automatic switching occurs when both frequency and load current are within a preselect range to achieve a preselect energy savings. 
         [0016]    It is a further feature of the invention that the bypass control is operatively associated with the VFD to monitor a frequency reference, output current and output frequency. 
         [0017]    It is an additional feature of the invention that the bypass control develops a command to the VFD to increase a frequency reference prior to switching from the drive mode to the bypass mode. 
         [0018]    It is still another feature of the invention that the bypass control automatically switches to the bypass mode only after motor speed as powered by the VFD approximates the motor speed that would be achieved by connecting the motor directly to the power source for a preselect period of time. 
         [0019]    It is an additional feature of the invention that the bypass control develops a command to the VFD to stop developing output power in the bypass mode. 
         [0020]    It is yet a further feature of the invention that the bypass control controls the output contactor to open a preselect time period after determining motor speed as powered by the VFD approximates the motor speed that would be achieved by connecting the motor directly to the power source, and controls the bypass contactor to close a preselect time period after commanding the output contactor to open. 
         [0021]    It is yet another feature of the invention that the bypass mode monitors operation of the VFD to automatically switch from the bypass mode to the drive mode when motor speed as commanded by the VFD differs from the motor speed achieved by connecting the motor directly to the power source by a preselect amount. 
         [0022]    It is still a further feature of the invention that the VFD is operatively connected to the bypass control with a serial communication link. 
         [0023]    In accordance with another aspect of the invention, an AC motor drive system comprises a variable frequency drive (VFD) for receiving AC power from a power source and developing variable frequency power at output terminals. A bypass contactor is operatively connected between the power source and an AC motor. An output contactor is operatively connected between the VFD output terminals and the AC motor. A bypass control is operatively associated with the VFD, the bypass contactor and the output contactor. The bypass control includes a drive mode in which the output contactor is controlled to connect the VFD to the motor and an energy savings mode in which the bypass contactor is controlled to connect the power source to the motor. The bypass control monitors operation of the VFD to determine if operating in the energy savings mode provides a preselect energy savings and if so then automatically switching to the energy savings mode. 
         [0024]    More particularly, the bypass control recognizes that the VFD is running the motor in the same direction and at about the same speed as if it were running through the bypass contactor, the bypass control will automatically transfer control of the motor from drive mode to bypass mode. The bypass control can be logic contained within the VFD itself, or a separate device that communicates with the VFD. Running in bypass mode allows for energy savings and reduced harmonics. Furthermore, when the bypass control recognizes that the speed reference command changes so that it is no longer tolerable to operate the motor in bypass mode, the controller will transfer control of the motor from bypass mode back to drive mode. In addition, a visual indication will be made so the user knows they are in the energy savings mode, providing them feedback in case they would like to fine tune settings to achieve even greater efficiency. 
         [0025]    Other objects, features, and advantages of the invention will become apparent from a review of the entire specification, including the appended claims and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a general block diagram of a VFD in a bypass configuration including a control implementing the energy savings method in accordance with the invention; 
           [0027]      FIG. 2  is a block diagram showing how a motor is powered directly across the line; 
           [0028]      FIG. 3  is a block diagram showing how a motor is powered by a VFD; 
           [0029]      FIG. 4  is a generalized schematic of a VFD; 
           [0030]      FIG. 5  shows the bypass configuration when the motor is running from the bypass contactor; 
           [0031]      FIG. 6  shows the bypass configuration when the motor is running from the VFD; 
           [0032]      FIGS. 7A-D  comprise a flowchart diagram of the energy savings method implemented in the control of  FIG. 1 ; 
           [0033]      FIG. 8  is a diagram showing the sequence of events leading to the energy savings mode when only the VFD frequency reference and VFD output frequency are considered; 
           [0034]      FIG. 9  is a diagram showing the sequence of events leading to the energy savings mode when the VFD frequency reference, VFD output frequency, and VFD output current are considered; and 
           [0035]      FIG. 10  is a hardware block diagram for a control board implementing functionality of the control of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0036]    Referring initially to  FIG. 1 , a VFD bypass system consisting of a variable frequency drive (VFD) in bypass configuration is illustrated. The VFD bypass system includes an AC source  1 , a VFD  2 , a bypass contactor  4 , a VFD output contactor  5 , a motor  3 , and a control  18  for an energy savings mode. The AC source  1  may comprise a drive or the like developing three-phase AC power on feeder conductors  6  labeled L 1 , L 2 , and L 3 . The AC source  1  is grounded. The L 1 , L 2 , and L 3  conductors are connected to both the bypass contactor  4  and input terminals of the VFD  2 . The VFD  2 , as described more particularly below, converts the AC power from the feeder conductors L 1 , L 2 , and L 3 , to DC power and converts it back to AC power at a select frequency which is then impressed across terminals  7  individually labeled U, V, and W. The terminals U, V, and W are connected to three (3) feeder conductors to the VFD output contactor  5 . Continuing past the output contactor, the three (3) feeder conductors  8  individually labeled T 1 , T 2 , and T 3  are connected to both the bypass contactor  4  opposite the L 1 , L 2 , and L 3  conductors, and to the three phase motor  3 . 
         [0037]    Referring to  FIG. 2 , the motor  3  is shown in an across-the-line configuration. In this configuration, the AC source  1 , as described above, develops three-phase power on feeder conductors labeled L 1 , L 2 , and L 3 . The AC source  1  is grounded. The L 1 , L 2 , and L 3  conductors are connected to the contactor  4 . The other side of the contactor  4  contacts are connected to conductors T 1 , T 2 , and T 3  and further connected to the motor  3 . To run the motor  3 , the contactor  4  is commanded to close and thus supply the input power  1  to the motor  3 . 
         [0038]    Referring to  FIG. 3 , the motor  3  is shown in a drive configuration. In this configuration, the AC source  1 , as described above, develops three-phase power on the feeder conductors  6  labeled L 1 , L 2 , and L 3 . The AC source  1  is grounded. The L 1 , L 2 , and L 3  conductors are connected to the variable frequency drive (VFD)  2 . The output terminals of the VFD  2  are connected to conductors T 1 , T 2 , and T 3  and further connected to the motor  3 . To run the motor  3 , the VFD  2  is commanded to run in a conventional manner. 
         [0039]    Referring to  FIG. 4 , a schematic diagram illustrates a typical circuit implementation for the VFD  2 . The VFD  2  includes an AC/DC converter  20  connected by a DC bus  21  to a DC/AC inverter  22 . Particularly, according to the illustrated embodiment of the invention, the AC/DC converter  20  comprises a full wave bridge rectifier circuit of conventional construction which is operable to convert three-phase AC power to DC power. The DC bus  21  includes a conventional filter  23 . The DC bus  21  has rails labeled “+” and “−”. The DC/AC inverter  22  comprises an inverter section. Particularly, the inverter section comprises a pulse width modulation (PWM) inverter, using insulated gate bipolar transistors (IGBTs)  24 . The six (6) IGBTs  24  are connected in a three-phase bridge configuration to the DC bus  21  to develop power at the output terminals  7  labeled U, V, and W. The IGBTs  24  are pulse width modulated by signals on lines from a VFD control, which may be the same control  18  as that which implements the invention, using a conventional control scheme. Particularly, the PWM inverter  22  is controlled to create a sinusoidal effect for the induction motor  3 . The pulse frequency used is fixed. The pulse width is varied to vary the sinusoidal frequency. The basic PWM control scheme may take any known form and does not itself comprise the invention. 
         [0040]    Referring to  FIG. 5 , the VFD bypass system consisting of a VFD in bypass configuration is again illustrated as in  FIG. 1 , without the control  18 , but this time showing the system in the bypass mode. In bypass mode, the output contactor  5  is commanded to open and the bypass contactor  4  is commanded to close by the control  18 . Power is then transferred from the input power source  1  through the conductors L 1 , L 2 , and L 3 , through the bypass contactor  4 , through the conductors T 1 , T 2 , and T 3 , and to the three-phase-motor  3 . The flow of this power is indicated by the arrows. This bypass mode is similar to the motor in an across-the-line configuration as shown in  FIG. 2 . To run the motor  3 , the control  18  commands the bypass contactor  4  to close and thus supply the input power  1  to the three-phase-motor  3 . 
         [0041]    In accordance with the invention, the bypass mode can be used to provide a redundant operation mode in the event of a VFD failure, as is known, and/or can be used to implement an energy savings function as described herein. 
         [0042]    Referring to  FIG. 6 , the VFD bypass system consisting of a VFD in bypass configuration is again illustrated as in  FIG. 1 , without the control  18 , but this time showing the system in the drive mode. In the drive mode, the bypass contactor  4  is commanded to open and the output contactor  5  is commanded to close by the control  18 . Power is then transferred from the input power source  1  through the conductors L 1 , L 2 , and L 3 , through the VFD  2 , through the conductors U, V, and W, through the output contactor  5 , through the conductors T 1 , T 2 , and T 3 , and to the three-phase-motor  3 . The flow of this power is indicated by the arrows. This drive mode is similar to the motor being powered by a VFD as shown in  FIG. 3 . To run the motor  3 , the control  18  commands the output contactor  5  to close and the VFD  2  to run, as described below. 
         [0043]    Referring to the Table below, parameters used to implement the bypass energy savings are listed. These parameters are used in the control  18  to determine when to enter and exit the energy savings mode, and how to transition from the drive mode to the energy savings mode and back. As is apparent, different range and default values could be used. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                 Parameter # 
                 Name 
                 Description 
                 Range 
                 Default 
               
               
                   
               
             
             
               
                 101 
                 Energy 
                 Energy Savings Mode Enable. 
                 0~2   
                 0 Disable 
               
               
                   
                 Saving En 
                 0: Disable 
               
               
                   
                   
                 1: Enable (Freq) 
               
               
                   
                   
                 2: Enable (Freq + Output Current) 
               
             
          
           
               
                 102 
                 Energy Freq 
                 Energy Savings Mode Frequency. 
                 0.0~120.0 
                 Hz 
                 60.0 
               
               
                   
                   
                 Sets the value of the VFD frequency 
               
               
                   
                   
                 reference for use in comparison to enter 
               
               
                   
                   
                 or exit Energy Savings mode. 
               
             
          
           
               
                 103 
                 Energy Iout 
                 Energy Savings Mode Output Current 
                  0.0~100.0% 
                 0.0% Disabled 
               
               
                   
                 Lvl 
                 Level 
               
               
                   
                   
                 Allows system to switch when the output 
               
               
                   
                   
                 current rises above this level of motor 
               
               
                   
                   
                 rated current for time specified in Energy 
               
               
                   
                   
                 Time. 
               
             
          
           
               
                 104 
                 Energy Fref 
                 Energy Savings Mode Frequency 
                 0.0~5.0 
                 Hz 
                 0.5 
               
               
                   
                 Dbnd 
                 Reference Deadband. 
               
               
                   
                   
                 Sets the tolerance around the VFD 
               
               
                   
                   
                 frequency reference value during 
               
               
                   
                   
                 comparisons to enter or exit Energy 
               
               
                   
                   
                 Savings mode. 
               
               
                 105 
                 Energy Freq 
                 Energy Savings Mode Output Frequency 
                 0.0~5.0 
                 Hz 
                 0.5 
               
               
                   
                 Dbnd 
                 Deadband. 
               
               
                   
                   
                 Sets the tolerance around the VFD 
               
               
                   
                   
                 output frequency value during 
               
               
                   
                   
                 comparisons to enter Energy Savings 
               
               
                   
                   
                 mode. 
               
             
          
           
               
                 106 
                 Energy Iout 
                 Energy Savings Mode Output Current 
                 0.0~30.0% 
                 15.0% 
               
               
                   
                 Dbnd 
                 Deadband. 
               
               
                   
                   
                 Sets the tolerance around the VFD 
               
               
                   
                   
                 output current value during comparisons 
               
               
                   
                   
                 to enter or exit Energy Savings mode, in 
               
               
                   
                   
                 percent of motor rated current. 
               
             
          
           
               
                 107 
                 Energy Time 
                 Energy Savings Mode Time. 
                 10~3600 
                 sec 
                 30 
               
               
                   
                   
                 Sets the time that the VFD frequency 
               
               
                   
                   
                 reference and VFD output frequency 
               
               
                   
                   
                 must be within the set limits before 
               
               
                   
                   
                 transferring to Energy Savings mode. 
               
               
                 108 
                 Energy Fref 
                 Energy Savings Mode Frequency 
                 0.0~10.0 
                 Hz 
                 6.0 
               
               
                   
                 Increase 
                 Reference Increase 
               
               
                   
                   
                 Sets the value to add to the VFD 
               
               
                   
                   
                 frequency reference prior to starting the 
               
               
                   
                   
                 transfer to bypass for Energy Savings 
               
               
                   
                   
                 Mode. 
               
               
                 109 
                 Energy 
                 Energy Savings Mode Contactor Open 
                 0.0~5.0 
                 s 
                 0.2 
               
               
                   
                 Delay Open 
                 Delay Time 
               
               
                   
                   
                 Sets the time delay after commanding 
               
               
                   
                   
                 the VFD output contactor or bypass 
               
               
                   
                   
                 contactor to open to ensure the contacts 
               
               
                   
                   
                 are actually open. 
               
               
                 110 
                 Energy 
                 Energy Savings Mode Contactor Close 
                 0.0~5.0 
                 s 
                 0.2 
               
               
                   
                 Delay Close 
                 Delay Time 
               
               
                   
                   
                 Sets the time delay after commanding 
               
               
                   
                   
                 the VFD output contactor or bypass 
               
               
                   
                   
                 contactor to close to ensure the contacts 
               
               
                   
                   
                 are actually closed. 
               
               
                 111 
                 Min 
                 Minimum Baseblock Time 
                 0.1~5.0 
                 s 
                 0.2 
               
               
                   
                 Baseblock 
                 Sets the minimum wait time for residual 
               
               
                   
                 Time 
                 motor voltage decay after power is 
               
               
                   
                   
                 removed from a running motor. This will 
               
               
                   
                   
                 occur when the VFD enters baseblock 
               
               
                   
                   
                 mode or after the bypass contactor is 
               
               
                   
                   
                 opened. 
               
               
                   
               
             
          
         
       
     
         [0044]    In accordance with the invention, the VFD bypass system achieves energy efficiency and harmonic reduction by automatically switching to the bypass/energy savings mode when the motor speed as powered by the variable frequency drive closely matches the motor speed that would be achieved by connecting the motor directly to the incoming power. The VFD bypass system is configured to use one of two different energy savings modes. The first energy savings mode is a frequency mode, referred to in the Table as Freq. In the frequency mode the automatic switching occurs based on the VFD controlling the motor at the input line frequency. The second energy savings mode is a frequency and output current mode. In the frequency and output current mode the VFD bypass system will not switch to energy savings mode unless the switchover will achieve a preselect energy savings. This decision will be based on both frequency and load current, which is application dependent. For both modes, the switchover will not occur until the frequency or the frequency and output current are within a deadband amount for a user selectable period of time. These parameters are defined in the Table above. 
         [0045]    A hardware block diagram for a control board for the bypass control  18  is shown in  FIG. 10 . The bypass control comprises a conventional microprocessor based control including a CPU and associated memory. The associated memory stores software for implementing functionality of the energy savings features described herein. The software is described below relative to the flow diagrams of  FIGS. 7A-D . The control board may be mounted in a bypass enclosure along with the VFD  2  and other necessary hardware and will be used to control the bypass functions of the VFD bypass system including motor run and stop and energy savings functions. The control board will interface with a remote operator panel, not shown. This control board will also accept network communications via on-board Modbus protocol and also via standard communication cards. 
         [0046]    Referring back to  FIG. 1 , the control  18  may be implemented using a separate control board, such as shown in  FIG. 10 . If the control  18  is a separate control board, then the method used to communicate with the VFD  2  is typically serial, however other forms of communication may be used. Alternatively, the functionality of the control  18  can be logic contained within the VFD  2  itself. The present application describes the former embodiment, in which signals are transferred between the control  18  and the VFD  2 , it being understood that the features of the invention could also be implemented using the latter embodiment in which signals may be in the form of software data transferred within the VFD itself. As such, regardless of how the overall system is configured, the control  18  is operatively associated with the VFD  2  to achieve the energy savings functionality described herein. 
         [0047]    The control  18  receives various input signals and develops output commands for implementing energy savings. The control  18  develops a command via a signal on a line  10  to the bypass contactor  4  to open or close. The control  18  develops a command via a signal on a line  15  to the output contactor  5  to open or close. These two signals place the VFD bypass system into either the bypass mode as shown in  FIG. 5  or into the drive mode as shown in  FIG. 6 . The techniques used by the control  18  to command the bypass contactor  4  and the output contactor  5  are well known and thus will not be discussed herein. Current sensors of conventional design sense line current on each of the U, V, and W conductors and are likewise connected to the control  18  with signal lines  14 . This current sensing is already performed by the VFD  2  with well known methods and will not be discussed herein. The VFD  2  supplies the control  18  with inputs on a line  11  for VFD Frequency Reference, VFD Output Current, VFD Output Frequency, and VFD Acceleration Time. The control  18  supplies the VFD  2  with output signals for a Baseblock Command on a line  12  and VFD Frequency Reference Command Override on a line  13 . The control  18  also outputs an Energy Savings Indicator command on a line  17  to turn on or off an Energy Savings Indicator  16 . 
         [0048]    Referring to  FIGS. 7A ,  7 B,  7 C and  7 D, a flow diagram illustrates a program implemented in the control  18  of  FIG. 1 , and shown in detail in  FIG. 10 . The program begins at a startup node  200 . Various setup parameters are initialized in a block  201 . These parameters are shown in the above Table and are user selectable. Each parameter is identified with a parameter # in the Table. These parameters include the conditions to start energy savings. The conditions are defined in an Energy Savings Mode Enable parameter  101 . If the parameter  101  is set for disable, then the conditions to enter energy savings mode will never be met. If the parameter  101  is set to Enable (Freq), then the conditions to enter energy savings mode is when both the VFD Frequency Reference and the VFD Output Frequency received from the VFD  2  on the lines  11  are equal to the value set in the Energy Frequency parameter  102 , and are within the bounds of the Energy Frequency Reference Deadband parameter  104  and the Energy Output Frequency Deadband parameter  105 . If the parameter  101  is set to Enable (Freq+Output Current), then the conditions to enter energy savings mode are when the conditions of Enable (Freq) as just explained are met and when the VFD Output Current on the line  14  is equal to the value entered in the Energy Output Current Level parameter  103 , and is within the bounds of the Energy Output Current Deadband parameter  106 . 
         [0049]    A decision block  202  checks to see if the conditions to start the energy savings mode, as determined by Energy Savings Mode Enable parameter  101 , are true. If the conditions are true, then a timer is started at a block  203 . If the conditions are not true, then the program loops back around the decision block  202  and continues to wait for the conditions to be true. While timing from the block  203 , a decision block  204  continues to monitor the conditions to start the energy savings mode. If the conditions are no longer true, then the program exits back to the decision block  202  and waits for the conditions to be true again. If the conditions remain true at the decision block  204 , then a decision block  205  determines if the timer has exceeded the energy time of the parameter  107 . If not, then the program loops back to decision block  204  and continues to monitor the conditions to start the energy savings mode. If the conditions remain true for the time of the parameter  107 , then the program moves to an enter energy savings routine at a node  206 . 
         [0050]    The enter energy savings routine begins at a node  207  of  FIG. 7B . This routine transitions the VFD bypass system from the drive mode to the bypass mode. There will be a brief period of time, on the order of ½ second, where no power will be supplied to the motor  3 . This routine allows the user to temporarily increase frequency so that when power to the motor  3  is removed the frequency will not drop to an undesirable level. 
         [0051]    The energy savings mode starts at a block  208  which sets the VFD Frequency Reference Override on the line  13  to the VFD  2  to be equal to the sum of the active VFD Frequency Reference from the VFD  2  on the line  11  and the Energy Frequency Reference Increase parameter  108 . The program also starts a timer. In a decision block  209 , the program determines if the VFD Output Frequency from the VFD  2  on the line  11  is greater than or equal to the just calculated VFD Frequency Reference Override on the line  13 . If so, then the program continues the sequence to enter the energy savings mode. If the VFD Output Frequency on the line  11  has not yet reached the calculated VFD Frequency Reference Override on the line  13 , then a decision block  210  checks to see if the timer has exceeded the time expected for the VFD Output Frequency on the line  11  to reach the value in VFD Frequency Reference Override on the line  13 . If the timer did not time out, then the program continues to loop back to the start of decision block  209 . If the timer does time out, then it may be that the VFD cannot achieve the VFD Frequency Reference Override on the line  13 , so the program continues the sequence to enter energy savings mode. At a block  211  the VFD  2  is given a Baseblock Command on the line  12 , the override of the VFD Frequency Reference ceases, and a Timer is started. The baseblock command sets the VFD  2  to a baseblock mode which shuts off the drive and stops firing the IGBTs  24 . The timer is used because the drive output shutoff will have a decay time. A decision block  212  continues to wait until the timer exceeds the value set in the minimum baseblock time parameter  111 . This provides a minimum time before restart to avoid residual motor feedback. When this minimum baseblock time is met, then at a block  213  the command on the line  15  to the VFD Output Contactor  5  is set to off and a timer starts. In a decision block  214 , the timer is continually monitored until it exceeds the value in the Energy Delay Open parameter  109 . This allows the contacts sufficient time to open. When the timer value is met, then at a block  215  the Bypass Contactor  4  is commanded by a signal on the line  10  to turn on. At this point the program moves to an energy savings routine at a node  216 . 
         [0052]    The energy savings routine begins at a node  217  on  FIG. 7C . In the energy savings routine, the Energy Savings Indicator  16  is turned on via the signal on the line  17  at a block  218 . The energy savings routine thereafter monitors the conditions necessary to stay in or exit from the energy savings mode. A decision block  219  monitors the VFD Frequency Reference on the line  11  from the VFD  2 . When the Frequency Reference is no longer equal to the value in the Energy Frequency parameter  102 , within the bounds of the Energy Frequency Reference Deadband parameter  104 , then the Energy Savings Indicator  16  is turned off at a block  220  and the program moves to an exit energy savings mode routine at a node  211 . 
         [0053]    The exit energy savings mode routine begins at a node  222  on  FIG. 7D . The command on the line  10  to the bypass contactor  4  is set to off at a block  223  and a timer is started. This timer allows the bypass contactor sufficient time to open. This deenergizes the motor  3 . At a decision block  224 , the timer is continually monitored until it exceeds the value in the energy delay open parameter  109 . At a block  225 , the timer is started again. This timer is used to allow the motor field to collapse. In a decision block  226 , the timer is continually monitored until it exceeds the value in the minimum baseblock time parameter  111 . When the timer exceeds this value, then at a block  227  a command on the line  15  to the VFD Output Contactor  5  is set to on and a timer is started to allow the contacts to close. A decision block  228  continually monitors the timer until it exceeds the value in energy delay close parameter  110 . When the timer exceeds this value, then at a block  229  the VFD Baseblock Command on the line  12  is set to off to return control to the VFD  2 . The VFD  2  starts firing the IGBTs  24  based on the VFD Frequency reference. The VFD  2  is otherwise operated to “catch” a spinning motor using its speed search function, which is a function already present in the VFD  2  and will not be discussed here. At this point the energy savings mode has ended at a node  230  and can then start again. 
         [0054]    Referring to  FIG. 8 , the normal sequence of events is shown when the Energy Savings Enable parameter  101  is set to Enable (Freq). At a time  51 , the control  18  places the system into Drive Mode by turning on the VFD Output Contactor  5  while maintaining the Bypass Contactor  4  off. At a time  52  the VFD  2  had been given a run command and a VFD Frequency Reference that the control  18  monitors. In this sequence diagram, the VFD Frequency Reference is shown to be equal to the Energy Frequency parameter  102  within the bounds of the Energy Frequency Reference Deadband parameter  105 . It can also be seen that the VFD Output Frequency starts to accelerate, which is also monitored by the control  18 . At a time  53  the VFD Output Frequency has reached the lower bounds, defined by the parameter  105 , of the targeted Energy Frequency parameter  105 . At this point the conditions to start energy savings mode are true, so a timer is started. At a time  54 , the VFD Output Frequency has reached its targeted frequency, which happens to be equal to the Energy Frequency parameter  102 , and has stopped accelerating. 
         [0055]    At a time  55 , the conditions to start energy savings mode have remained true since starting the timer at time  53 , and now the timer is equal to the value in Energy Savings Time parameter  107 , so it is time to transition into the energy savings mode. The control  18  calculates a temporary frequency reference by summing the existing VFD Frequency Reference with the value in Energy Frequency Reference Increase parameter  108 , then it commands the VFD  2  to use this value as its new frequency reference as can be seen in the step rise of the VFD Frequency Reference. The VFD  2  begins to accelerate to this new frequency reference value as shown by the VFD Output Frequency beginning to rise. At a time  56  the VFD Output Frequency has achieved the targeted VFD Frequency Reference, so the control  18  commands the VFD  2  to enter the baseblock mode, the override of the VFD Frequency Reference is terminated, and a timer is started. At a time  57 , the timer has achieved the value in the Minimum Baseblock Time parameter  111  so the VFD Output Contactor  5  is commanded off and a timer starts. At a time  58 , the timer has achieved the value in the Energy Delay Open parameter  109 , so the Bypass Contactor  4  is commanded on, placing the system in bypass mode and thus is now in energy savings mode. 
         [0056]    At a time  59 , the control observes the VFD Frequency Reference starting to decrease, but still within the bounds of the Energy Frequency parameter  102 . At a time  60  the VFD Frequency Reference has decreased to a point outside the bounds of the Energy Frequency parameter  102 , so it is time to exit energy savings mode. The Bypass Contactor  4  is commanded off and a timer is started. At a time  61  the VFD Frequency Reference has stopped decreasing, but it is still out of the bounds of Energy Frequency parameter  102 . At a time  62 , the timer has exceeded the Energy Delay Open parameter  109  and the timer starts again. At a time  63 , the timer has exceeded the Minimum Baseblock Time parameter  111 , so the control  18  turns on the VFD Output Contactor  5  placing the system back to drive mode and starts a timer. At a time  64  the timer has reached the value in Energy Delay Close parameter  110  and thus the control  18  commands the VFD to leave the baseblock mode. The VFD  2  then begins its speed search mode to start controlling an already spinning motor. At a time  65  the VFD has completed its speed search mode and the VFD Output Frequency  11  is equal to the VFD Frequency Reference. 
         [0057]    Referring to  FIG. 9 , the normal sequence of events is shown when the Energy Savings Enable parameter  101  is set to Enable (Freq+Output Current). At a time  71 , the control  18  places the system into Drive Mode by turning on the VFD Output Contactor  5  while keeping off the Bypass Contactor  4 . At a time  72  the VFD  2  had been given a run command and a VFD Frequency Reference that the control  18  monitors. In this sequence diagram, the VFD Frequency Reference is shown to be equal to the Energy Frequency parameter  102  within the bounds of the Energy Frequency Reference Deadband parameter  105 . It can also be seen that the VFD Output Frequency starts to accelerate and the VFD Output Current starts to increase, both of which are monitored by the control  18 . At a time  73  the VFD Output Frequency has reached the lower bounds, defined by the parameter  105 , of the targeted Energy Frequency parameter  105 . The VFD Output Current is also within the bounds of the Output Current Level parameter  103 . At this point the conditions to start energy savings mode are true, so a timer is started. At a time  74 , the VFD Output Frequency has reached its targeted frequency, which happens to be equal to the Energy Frequency parameter  102 , and has stopped accelerating. 
         [0058]    At a time  75 , the conditions to start energy savings mode have remained true since starting the timer at the time  73 , and now the timer is equal to the value in Energy Savings Time parameter  107 . It is time to transition into the energy savings mode. The control  18  calculates a temporary frequency reference by summing the existing VFD Frequency Reference with the value in Energy Frequency Reference Increase parameter  108 , then it commands the VFD  2  to use this value as its new frequency reference as can be seen in the step rise of the VFD Frequency Reference. The VFD begins to accelerate to this new frequency reference value as shown by the VFD Output Frequency beginning to rise. At a time  76  the VFD Output Frequency has achieved the targeted VFD Frequency Reference, so the control  18  commands the VFD  2  to enter the baseblock mode, the override of the VFD Frequency Reference is terminated, and a timer is started. At a time  77 , the timer has achieved the value in the Minimum Baseblock Time parameter  111  so the VFD Output Contactor  5  is commanded off and a timer starts. At a time  78 , the timer has achieved the value in the Energy Delay Open parameter  109 , so the Bypass Contactor  4  is commanded on, placing the system in bypass mode and thus is now in energy savings mode. 
         [0059]    At a time  79 , the control observes the VFD Frequency Reference starting to decrease, but still within the bounds of Energy Frequency parameter  102 . At a time  80  the VFD Frequency Reference has decreased to the point outside the bounds of Energy Frequency parameter  102 , so it is time to exit energy savings mode. The Bypass Contactor  4  is commanded off and a timer is started. At a time  81  the VFD Frequency Reference has stopped decreasing, but it is still out of the bounds of Energy Frequency parameter  102 . At a time  82 , the timer has exceeded the longer of either the Energy Delay Open parameter  109  or the Minimum Baseblock Time parameter  111 , so the control  18  turns on the VFD Output Contactor  5  placing the system back to drive mode and starts a timer. At a time  83  the timer has reached the value in the Energy Delay Close parameter  110  and thus the control  18  commands the VFD  2  to leave the baseblock mode. The VFD  2  then begins its speed search mode to start controlling an already spinning motor. At a time  84  the VFD  2  has completed its speed search mode and the VFD Output Frequency is equal to the VFD Frequency Reference. 
         [0060]    The present invention has been described with respect to flowcharts and block diagrams. It will be understood that each block of the flowchart and block diagrams can be implemented by computer program instructions. These program instructions may be provided to a processor to produce a machine, such that the instructions which execute on the processor create means for implementing the functions specified in the blocks. The computer program instructions may be executed by a processor to cause a series of operational steps to be performed by the processor to produce a computer implemented process such that the instructions which execute on the processor provide steps for implementing the functions specified in the blocks. Accordingly, the illustrations support combinations of means for performing a specified function and combinations of steps for performing the specified functions. It will also be understood that each block and combination of blocks can be implemented by special purpose hardware-based systems which perform the specified functions or steps, or combinations of special purpose hardware and computer instructions. 
         [0061]    Thus in accordance with the invention a variable frequency drive includes a bypass configuration. Energy efficiency and harmonic reduction are achieved by automatically switching to bypass mode when the motor speed as powered by the variable frequency drive closely matches the motor speed that would be achieved by connecting the motor directly to the incoming power.