Abstract:
An early break bypass switch provides a modular system for bypassing an inverter when needed without allowing switching induced damage to the inverter. In an embodiment of the invention, a shaft-mounted switch provides rotatable cams and sliding followers for positive opening contacts while providing an auxiliary switch section for breaking and making contact to an inverter controller in a specific sequence relative to inverter power. In this way, the inverter is both isolated from the line power and also protected during switching from being damaged by switching-induced spikes and transient signals and surges.

Description:
TECHNICAL FIELD 
       [0001]    This patent disclosure relates generally to inverter bypass mechanisms and, more particularly to an inverter bypass safety switch for providing a plurality of power states while preventing damage to the inverter circuit during switching. 
       BACKGROUND 
       [0002]    An inverter drive, sometimes referred to simply as an inverter, is a solid state device used to control the speed of a three phase electric motor. As the name implies, an inverter drive operates by receiving AC electrical power, internally converting (or inverting) that power into DC electrical power, and then outputting the DC power in a variably pulsed form to control the speed of an associated three phase motor. Thus, for example, longer DC pulses would provide higher motor speeds, while shorter pulses would provide lower speeds. 
         [0003]    Soft starters and solid state starters operate in a manner similar to inverter drives except these types of starters do not incorporate a means for speed control. For the sake of simplicity we will describe and refer to inverter drives in the remaining part of this discussion, but by implication soft starters, solid state starters and other types of starters using solid state components are included and hereinafter are referred to as inverter drives. 
         [0004]    Such inverter drives are common in industry and are used in conveyors, fans, cooling towers, extruders, and many other applications. However, as solid state devices, inverter drives are vulnerable to damage from electrical disturbances such as lightning strikes, power surges, low voltages, and other disturbances in the electrical line. When a disturbance of sufficient magnitude occurs, the inverter drive may fail, thus preventing the motor from operating, and stopping the application for which the motor was being used. In addition to the damage or loss caused by the cessation of the application, the damage to the inverter itself may also be costly and/or time-consuming to remedy. 
         [0005]    Although certain methods have been attempted for the purpose of isolating inverters during electrical disturbances, these methods involve expensive and unreliable contactors, relays and timers for switching. Moreover, the switching used to isolate the inverter drive may itself cause the damage that was to be avoided. This is due to the possibility that sudden changes in the connectivity of the inverter can cause current surges and spikes within the device, damaging sensitive solid state circuitry. 
         [0006]    Various solutions have been developed for preventing stray/surge currents from harming the solid state components of inverter drives, including fuses and circuit breakers, voltage regulators and crowbar circuits. A potential disadvantage of fuses and circuit breakers is that they can take time to respond to circuit faults. The time required for the fuse or circuit breaker to blow out is sometimes long enough enough for inverter drive components to be damaged. It is also possible for a power surge to damage equipment without disturbing the fuse or circuit breaker under some conditions. 
         [0007]    Voltage regulators, i.e., circuits that regulate voltage output despite changes in input voltage or output load current demand, are widely used in power supplies for inverter drives, controllers and operating devices in equipment. However, as with fuses and circuit breakers, power source voltage regulators may not activate fast enough to protect inverter drives. While crowbar circuits that quickly form a deliberate short circuit across the power supply lines may eliminate some delay in fuse or breaker activation, these safeguards are also prone to failure in the event of very short, infrequent power line transients. 
         [0008]    It will be appreciated that this background description has been created by the inventors to aid the reader, and is not to be taken as a reference to prior art, nor as an indication that any of the indicated problems were themselves appreciated in the art. 
       SUMMARY 
       [0009]    It is an object of the invention to provide a simple and reliable means for isolating an inverter during electrical disturbances so that the motor can continue at full speed/full power operation until a safe stopping point is reached, while preventing damage to the inverter caused by switching. 
         [0010]    Another object of the present invention is to provide a mechanical means for isolating an inverter without damaging the inverter. The present invention does not require contactors, relays, solenoids, or coils that can consume power, stick, or burn out. 
         [0011]    Still another object of the present invention is to provide a means for isolating an inverter without incurring switching damage in a manner that uses positive break contacts. Should a contact “weld”, the switch handle cannot be turned. If the switch handle cannot be turned, the contacts cannot transfer. If the switch handle is able to be turned, the contacts transfer. This feature assures the operator that when the switch has been turned the contacts have been transferred. 
         [0012]    Yet another object of the present invention is to provide a switch having positive removal of the electrical power from the inverter drive while avoiding switching damage. In other words, the switch can be used to disconnect power from both the incoming lines into the inverter and outgoing lines from the inverter. 
         [0013]    A further object of the invention is to provide a manual or power operated disconnect switch having an early break auxiliary contact (or contacts) that opens before the standard power contacts so the inverter drive control circuit is interrupted before the standard power contacts of the disconnect switch open. 
         [0014]    Another object of the invention is to provide a disconnect switch with early break auxiliary contact modules that can be added or removed without using special tools. 
         [0015]    Still another object of the invention is to provide a disconnect switch having means for protecting an inverter drive from stray/surge currents that can be installed or removed without using special tools. 
         [0016]    Yet another object of the invention is to provide a disconnect switch having means for protecting an inverter drive from stray/surge currents that can be designed as a one, two, three, four or more power pole disconnect switch. 
         [0017]    Yet another object of the invention is to provide a disconnect switch having means for protecting an inverter drive from stray/surge currents that can be used when only a very small amount of current is available. 
         [0018]    Further and alternative aspects and features of the disclosed principles will be appreciated from the following detailed description and the accompanying drawings, of which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a system schematic including a bypass safety switch in accordance with an embodiment of the invention; 
           [0020]      FIG. 2  is a system schematic of an alternative system including a bypass safety switch in accordance with an embodiment of the invention; 
           [0021]      FIG. 3  is a timing plot showing main power and controller power switching sequence in accordance with an embodiment of the invention; 
           [0022]      FIG. 4  is a perspective view of a switch in accordance with an embodiment of the present invention; 
           [0023]      FIG. 5  is a side elevational view of a switch in accordance with an embodiment of the present invention, with the base shown in partial cutaway to show the auxiliary contacts; 
           [0024]      FIG. 6  is a cross sectional view of the inverter bypass safety switch, taken along line  3 - 3  of  FIG. 5 ; 
           [0025]      FIG. 7  is a cross sectional view of the inverter bypass safety switch, taken along line  3 - 3  of  FIG. 5 , the cam having been rotated approximately 60 degrees in a counterclockwise direction from the orientation shown in  FIG. 6 ; and 
           [0026]      FIG. 8  is a switch schematic showing activation ranges for a primary switching section and an auxiliary contact section in an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    This disclosure relates to inverter bypass mechanisms generally and specifically to an inverter bypass safety switch that provides a number of power states that may be switched into while preventing damage to the inverter circuit during switching. In this way, a user seeking to avoid inverter damage by switching the inverter out will not inadvertently cause the damage he sought to avoid via switching spikes and transients. 
         [0028]    An inverter bypass safety switch described herein allows for selectively routing electrical power either through or around an AC variable frequency inverter drive to allow a motor to continue operation during electrical disturbances. The switch works in a mechanical fashion, and comprises a contact block containing electrical contacts and cams operably connected to the contacts and mounted on a shaft that can be turned by an operator. The cams are shaped and mounted on the shaft in such a way to provide a desired switching pattern when the shaft is manually rotated. 
         [0029]    Although any switching pattern is possible, in an embodiment of the invention, stray/surge currents potentially caused during switching are avoided, thus preventing damage to the solid state components of the inverter drive, via an early break auxiliary contact(s). The early break contact opens prior to the opening of the standard power contacts when the switch is turned off by an operator and closes after the closing of the standard power contacts when the switch is turned on by an operator. 
         [0030]    Turning to a more detailed discussion of an operating environment and exemplary principles of operation,  FIG. 1  is a system schematic showing a power generation and use architecture. In the illustrated embodiment of the invention, an electrical generator  1  provides electrical power to an electrical motor  2 , driving the motor  2 . The power produced by the generator  1  is conveyed to the motor  2  via a switch  3  in series with an inverter drive  4 . As discussed above, the inverter drive  4  is a solid state device for controllably varying the speed of the motor  2 . 
         [0031]    The inverter drive  4  includes a controller, not shown in this figure, for controlling the operation of the inverter drive  4 . The inverter drive controller is powered by a power source  5 , which may be a voltage-reduced version of the generator  1  output or otherwise, which is switched through the switch  3 . In this way, the switch  3  is operable to bypass the inverter drive  4  and also to interrupt power to the inverter drive controller. 
         [0032]    The schematic illustration of  FIG. 2  shows a system similar to that of  FIG. 1 , wherein the switch  3  has been relocated within the circuit. In particular, the illustrated system still includes the electrical generator  1  to provide electrical power to drive the electrical motor  2 . As in the system of  FIG. 1 , the inverter drive controller is powered by a power source  5  which is switched through the switch  3 . 
         [0033]    Although the switch  3  remains in series with the inverter drive  4 , it is located adjacent the motor  2  in the circuit of  FIG. 2 . In this way, the switch  3  does not control main power to the inverter drive  4 , but does control the conveyance of power from the inverter drive  4  to the motor  2 . As in the system of  FIG. 1 , the switch  3  also controls the connectivity of the inverter control power source to the inverter controller. 
         [0034]    In either case, the operation of the switch  3  to protect the inverter drive can be the same. In particular, as noted above, the switch  3  includes an early break element that disconnects power to the inverter drive controller prior to breaking the main connection (between the generator  1  and inverter drive  4  in the system of  FIG. 1  and between the inverter drive  4  and the motor  2  in the system of  FIG. 2 . 
         [0035]    The timing sequence of the main power and controller power circuit closure and opening are shown as a function of switch rotation in the plot  6  of  FIG. 3 . As can be seen, the early break contact removes inverter drive controller power  7  prior to removing main drive power  8  when the switch is turned towards the all-off state by an operator. Similarly, moving from right to left in the plot  6 , the early break contact closes after the closing of the main power contacts when the switch is turned from the all-off state toward the all-on state by the operator, such that main power  8  is restored before inverter drive controller power  7  is restored. 
         [0036]    There is shown in  FIGS. 4 to 7  an embodiment of an inverter bypass safety switch  10  according to aspects of the present invention. The switch  10  comprises a base  12 , a contact block  14  mounted on the base  12  and divided into sections  16 , electrical contacts  18  located within the sections  16 , cams  20  operatively connected to the electrical contacts  18 , and a shaft  22  connecting all of the cams  20  together. A handle assembly attached to the shaft  22  is manipulated by an operator to turn the shaft, thereby selecting a desired operating mode. An enclosure or housing may be used to enclose the device and allow for easy mounting of the switch  10  on a wall. 
         [0037]    The contact block  14  may include up to nine power contacts and two auxiliary contacts. The power contacts direct the incoming power either to and from the inverter drive, or directly to the motor, bypassing the inverter. The auxiliary contacts are used to control a drive control circuit in a particular manner as will be discussed later with reference to  FIG. 8 . 
         [0038]    In the illustrated embodiment, the five sections  16  above the base  12  contain the nine power contacts, and the section  16  below the base  12  contains the two auxiliary contacts. As shown in  FIGS. 6 and 7 , a cam  20  located within each section operates the electrical contacts  18 . 
         [0039]    In an embodiment of the invention, the contacts  18  are double make/break contacts. That is, each contact  18  has a moving contact  28  and a pair of stationary contacts  30 . In an optional embodiment, each moving contact  28  comprises a copper bar  32  with silver cadmium oxide contact points  34  on each end. The silver cadmium oxide contact points  34  carry the amp switching load. Each moving contact  28  is held in a follower  36  which is keyed into the housing  38  of the switch  10 . The follower  36  is acted upon by the shaft-mounted cam  20 . 
         [0040]    In a further optional embodiment of the invention, two stationary contacts  30  are located on either side of each moveable contact  28  and are made of the same materials as the moveable contacts  28 . The stationary contacts  30  are keyed into the housing or body  38  of the switch  10 , and are connected to electrical terminals  31  for fastening external body wires to the switch  10 . Some of the electrical terminals  31  are connected by electrically conductive bridges  33  to other terminals in the same section  16  of an adjacent section  16 . 
         [0041]    Each cam  20  has a profile cut into the cam  20  that determines the switching pattern. The profile includes one or more low sections  21  and one or more high sections  23 . Each follower  36  is connected to a spring  40  that biases the moveable and stationary contacts  28 ,  30  in a closed position when the cam  20  is rotated such that a low section  21  of the cam profile faces the follower  36 . The moveable and stationary contacts  28 ,  30  are mechanically opened when the cam  20  is rotated to a high section  23  of the cam profile. Notably, in this embodiment of the invention, opening the contacts  18  does not require use of the spring  40 , and therefore is not susceptible to failure of such a spring. In the illustrated embodiment of the invention, the shaft  22  connects all of the cams  20  together, and may itself be connected to an operating handle for ease of use. 
         [0042]    By varying the shape of the cams  20  and the orientation of the cams  20  on the shaft  22 , various switching patterns can be achieved. Four different switching patterns or positions shall now be described. In a DRIVE or normal position, electrical power is routed through the switch  10  to the inverter drive  11 , from the inverter drive  11  back to the switch  10 , then to the motor. In the DRIVE position, full functionality of the inverter drive is available to the motor. 
         [0043]    In a LINE or bypass position, electrical power is routed from the incoming power source through the inverter bypass safety switch  10  directly to the motor. Thus power is eliminated from both the input and output side of the inverter drive  11 . In the LINE mode, the inverter drive  11  can be physically removed from service while the motor is left operating at full speed/voltage. 
         [0044]    In an OFF position, electrical power is disconnected from both the inverter drive and the motor. In a TEST position, electrical power is routed from the incoming power source through the inverter bypass safety switch to the inverter. In this position, no power is sent to the application. 
         [0045]    In more general terms, the inverter bypass safety switch may be configured in either a two position, three position, four position unit, depending on the needs of the user. The two position unit may have, for example, a DRIVE and a LINE position. The three position unit may have, for example, DRIVE, LINE, and OFF positions. The four position unit may have, for example, DRIVE, LINE, OFF and TEST positions. As will be appreciated, other switching patterns can be achieved by changing the configuration of the cams. 
         [0046]    As noted above, the section  16  below the base  12  of the assembly  10  contains auxiliary contacts that are also operated via a cam linked to the shaft  22 . The orientation of the cam associated with the auxiliary contacts is fixed relative to the cams associated with the main power contacts. Moreover, the auxiliary contacts may be bridged (connected) to provide power to the inverter drive controller. 
         [0047]    In an embodiment of the invention, the auxiliary contacts are opened prior to the main power contacts when the switch is turned from the DRIVE state (inverter in use) to the LINE or bypass state (inverter not in use). In this way, the inverter drive control circuit is interrupted before the standard power contacts open. Likewise, when the operator turns the switch to the DRIVE position, the early break auxiliary contacts close after the standard power contacts so the inverter drive control circuit is powered up after the standard power contacts close. 
         [0048]    The operation of the auxiliary contacts (providing or discontinuing power to the inverter drive controller) in relation to the operation of the primary contacts (directing power through or around the inverter drive) is shown in the simplified schematic of  FIG. 8 . In particular,  FIG. 8  is a simplified switch drawing showing activation ranges for a primary switching section and an auxiliary contact section in an embodiment of the invention. As can be seen, the operational modes of the switch (OFF, DRIVE, LINE, TEST) are inscribed on the face  46  of the switch  45 . 
         [0049]    In the OFF mode, the controller is not powered, as is the case with the LINE mode. However, in the DRIVE and TEST modes, the controller is powered. Thus, the controller must be powered up when switching between the OFF mode and the DRIVE mode. In keeping with the principles described above however, the main power must be turned on before, and turned off after the controller power is turned off and on respectively. Thus, the cams of the primary and auxiliary sections are set up such that at a rotation angle A (from the OFF position), the primary power is applied to the inverter and at a greater angle B, the driver controller power is engaged. At rotation angles between A and B, the inverter power is on and the controller power is off. At angles greater than B, both are on, and at angles less than A, both are off. 
         [0050]    Inverter drive control circuits typically operate at very low power levels, which standard power contacts cannot reliably conduct or switch. Thus, the auxiliary contacts may be gold flash plated to allow trouble free operation of the contacts while using the low power available from the control circuit of the inverter drive. Gold plating would still allow the auxiliary contacts to be used in applications that use higher power control circuits. Such higher power control circuit applications may “burn off” the gold flash plating of the contacts, but the underlying conventional power contact material will enable the contacts to continue functioning. 
         [0051]    The auxiliary power contacts section of the switch may be manufactured as an integral part of the disconnect switch or may be manufactured as a module attachable to the disconnect switch. In this embodiment of the invention, the attaching and removing of modules may be executed at the factory or on site. In a further embodiment of the invention, no special tools are required to either add or remove the modules or to install or remove the disconnect switch from the power circuit. 
         [0052]    In operation, the desired switching pattern is achieved by turning the shaft, e.g., via an operating handle, to the desired setting. Turning the handle causes the cams  20  to rotate, which acts upon the moveable contacts  28 , either opening or closing the electrical connections between the moveable and stationary contacts. 
         [0053]    While not important to the present invention, it will be noted that the handle assembly, if used, may have any number of configurations. A “Selector Style” handle assembly may include a backplate, an operating handle mounted to the backplate, and a gasket mounted to the side of the backplate opposite the handle. A “Lockout Style” handle assembly may include all of the features of the Selector Style embodiment, and further include holes for mounting one or more padlocks to lock the handle in the LINE, OFF, DRIVE, or TEST positions. In this way, the switch  10  can be locked into a desired position. 
         [0054]    A “Panel Mount” style may include all of the features of the Lockout Style embodiment, and further include an extension shaft and an extension shaft coupling. The extension shaft and extension shaft coupling allow the switch  10  to be mounted on the rear panel of the enclosure while the handle assembly is mounted on the front of the enclosure  26 . 
         [0055]    It will be appreciated that the present invention provides positive removal of the electrical power from the inverter drive and features entirely mechanical operation. That is, the switch  10  does not depend on coils, relays, contactors, or other electromechanical devices to switch power. Moreover, the present invention features positive break contacts. This means that, should a contact  18  “weld”, the operating handle cannot be turned, thus assuring the operator that when the operating handle has been turned the contacts  18  have been transferred. If the operating handle is able to be turned, the contacts  18  transfer. 
         [0056]    It is anticipated that the switch  10  may be enclosed in a UL listed nonmetallic enclosure or other enclosure as desired, allowing for easy installation of the switch on a wall. It is also anticipated that the switch  10  may be connected to a fuseblock to protect the motor against short circuits. It is further anticipated that the switch  10  can be enclosed in a nonmetallic enclosure or other enclosure with a disconnect switch, thus combining the features of an inverter bypass switch with a disconnect switch. Finally, it is anticipated that the switch  10  can be used in conjunction with a manual motor starter. The manual motor starter would provide protection against excessive motor current and short circuits. The manual motor starter could be reset like a circuit breaker. 
         [0057]    It will be appreciated that the foregoing description provides useful examples of the disclosed system and device. In particular, there has been described a cam-operated disconnect switch that provides a simple and economical means for preventing the possibility of stray/surge current from damaging the solid state components of an inverter drive. The disconnect switch includes “early break” auxiliary contacts that open before the standard power contacts when the switch is deactivated (turned off) by an operator. Likewise, when an operator turns on the disconnect switch the early break auxiliary contacts close after the standard power contacts. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. 
         [0058]    All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for the features of interest, but not to exclude such from the scope of the disclosure entirely unless otherwise specifically indicated.