Abstract:
An excavator drive system wherein one of a pair of propulsion motors shares one of a pair of inverter power sources with another of a pair of motors dedicated to crowd and hoist motions. A pair of non-volatile bi-state switches, triggered under control of a controller, allow sharing of inverters between hoist and propel  1  motors. Another pair of switches allows inverter sharing between propel  2  and crowd motors. Each pair of switches enables change over and power transfer from one of the paired motors to the other motor. The bi-state switches enable quicker transfer of power between motors than transfer switches employing external motor-powered mechanical transfer linkages. Bi-state transfer switches also maintain transfer coupling status in the event of power failure to the switch actuators, allowing an excavator operator to continue the drive function in operation prior to the switch power failure.

Description:
BACKGROUND OF THE DISCLOSURE 
     1. Field of the Invention 
     The invention relates to excavator drive systems that control excavator propulsion and bucket motion, and in particular motor transfer switches in such drive systems that selectively couple and decouple drive system motors from shared power sources. 
     2. Description of the Prior Art 
     Excavator systems, such as electric shovels and drag lines, are critical high power-consuming equipment in the mining industry. As shown in  FIGS. 1 and 2 , a known shovel system  10  includes a chassis to which are mounted a pair of tracks  12 ,  14 , a boom  16 , a dipper arm  18  and a bucket  20 . The excavator  10  has a drive system  30  that enables the designated individual motions referred to as hoist, swing, crowd and propel. Those motions are typically powered by multi-phase AC asynchronous motors  40 ,  42 ,  44 ,  46  that are fed by active front end rectified inverters  32  including IGBT vector control. The inverters  32  reduce the harmonics associated with rectification and provide reactive power support at the shovel&#39;s point of common coupling (PCC). 
     The known excavator drive system  30 , shown in  FIG. 2 , has a propel motion system comprising two independently controlled motors  44 ,  46  powering respective tracks  12 ,  14 . For descriptive simplicity only a single phase of the multi-phase system is shown, it being readily apparent to those skilled in the art that the other phases have similar construction, function and operation. Transfer switches  34  allow sharing of power source inverters  32  between pairs of crowd and propel  1  motion motors  40 ,  44  and hoist and propel  2  motion motors  42 ,  46 . Each transfer switch  34  is controlled by a programmable logic controller (PLC)  38 ; they communicate with each other via communications bus  39  or other known communications pathway. 
     Typical known industrial transfer switches  34  have a motor module  35  that drives multiple switch modules  36  (often known multi-phase motor contactors) through an external mechanical linkage. The switch modules  36  typically are single pole double throw or double pole, double throw motor contactors having external mechanical “on”/“off” switches  37 . The motor module  35  mechanically interfaces with the switch module external mechanical switches  37  and functionally enables remote mechanical actuation under control of a programmable logic controller (PLC)  38 . As shown in  FIG. 2  a representative motor module  35  includes a reversable motor  35 A driving a mechanical linkage (here pinion  35 B engaging rack  35 C interface with on/off switches  37 ). 
     The known commercialized motor module  35 , being a mechanical device, typically needs greater than 2 seconds to perform a motor transfer, due to system response phase lag. Quicker transfer time is desired to increase excavator productivity. As an example, if in a typical operational hour there are 5 transfers between propel and digging modes and each transfer expends 3 seconds, that results in a loss of 30 seconds per hour. The expended time presents an opportunity to gain productivity of an additional digging cycle with an expensive piece of earthmoving equipment. 
     In the event of loss of power to the transfer switch  34 , some of the known switch module contactors  36  lose power and default to a driven motor “off” condition, in which case the excavator ceases motion. If prior to transfer switch  34  power loss the excavator  10  is performing a translation or digging motion the operation ceases, even though the excavator operator did not need to transfer motor power. If the prior state of motor connection was preserved after a transfer switch  34  power failure, as is possible with some known commercialized devices, the operator could continue excavator operation in that prior connection status mode. 
     Thus, a need exists in the art for an excavator, including an excavator drive system that is capable of transferring electric power from one drive motor to another drive motor without mechanical external transfer linkages and auxiliary motor drives that typically have long transfer time lags from initiation of a transfer command to completion of the power transfer. 
     Another need exists in the art for an excavator, including an excavator drive system, that does not cause disruption of power to drive system motors due to a transmit switch power failure, so that an operator can continue to use the excavator drive system in the manner preceding the power failure. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to create an excavator, including an excavator drive system that is capable of transferring electric power from one drive motor to another drive motor quicker than presently done using known transfer switches having mechanical external transfer linkages and auxiliary motor drives. 
     Another object of the present invention is to create an excavator, including an excavator drive system, that does not cause disruption of power to drive system motors due to a transmit switch power failure. 
     These and other objects are achieved in accordance with the present invention by excavator drive systems of the present invention, as well as excavators incorporating such drive systems. Excavator drive systems of the present invention have a first motor for powering an excavator bucket and a second motor for powering an excavator propulsion system and a power source for the motors. A first non-volatile bi-state switch, having a motorized electromagnetic actuator, selectively couples and decouples the first motor to the power source. Similarly, a second non-volatile bi-state switch, having a motorized electromagnetic actuator for selectively couples and decouples the second motor to the power source. At least one relay device controlling energization of the electromagnetic actuators is electrically coupled to at least one of the respective electromagnetic actuators. A controller, such as a programmable logic controller (PLC), is electrically coupled to and in communication with the at least one relay device. The PLC transmits control signals to the at least one relay device, for selective coupling of one of the motors to the power source while decoupling the other motor from the power source. 
     In practicing the present invention, the motors and power source often are multi-phase, and in which case the bi-state switches have separate poles for each phase. All the poles of each separate bi-state switch may be coupled to a common relay device controlling energization of electromagnetic actuators in each pole, so that all phases are switched simultaneously. The bi-state switches may incorporate a status indicator that communicates to the controller coupling or uncoupling state status. The controller may compare an intended bi-state switch coupling state with the state indicated by the status indicator and recognize a fault when the compared states differ. Upon recognition of a compared state difference fault the controller may re-synchronize the bi-state switches so that one couples its respective motor and power source while the second decouples its respective motor and power source. Desirably, the respective non-volatile bi-state switches are capable of holding a coupled or decoupled state until its electromagnetic actuator is energized again by its respective relay. 
     Another aspect of the present invention is directed to application in an excavator system, comprising a chassis having a drive system for moving a propulsion system including a pair of first and second tracks, a boom and an excavator bucket coupled to the boom. The excavator drive system includes a first motor for powering the excavator bucket hoist motion; a second motor for powering the propulsion system first track; a third motor for powering the excavator bucket hoist crowd motion; and a fourth motor for powering the propulsion system second track. The drive system has a first power source for the first and second motors and a second power source for the third and fourth motors. The drive system further has first non-volatile bi-state switch, having a motorized electromagnetic actuator for selectively coupling and decoupling the first motor to the first power source; and a second non-volatile bi-state switch, having a motorized electromagnetic actuator for selectively coupling and decoupling the second motor to the first power source. The drive system has a third non-volatile bi-state switch, having a motorized electromagnetic actuator for selectively coupling and decoupling the third motor to the second power source and a fourth non-volatile bi-state switch, having a motorized electromagnetic actuator for selectively coupling and decoupling the fourth motor to the second power source. At least one relay device controlling energization of electromagnetic actuators is electrically coupled to at least one of the respective electromagnetic actuators. A controller is electrically coupled to and in communication with the at least one relay device, transmitting control signals thereto, for selective coupling of one of the first or second motors to the first power source while decoupling the other motor from the first power source and for selective coupling of the third or fourth motors to the second power source while decoupling the other motor from the second power source. 
     The drive system may have multi-phase motors, inverter power sources and bi-state switches having separate poles for each phase. All the poles of each separate bi-state switch may be coupled to a common relay device controlling energization of electromagnetic actuators in each pole, so that all phases are switched simultaneously. The bi-state switches may incorporate a status indicator that communicates to the controller coupling or uncoupling state status. The controller may compare an intended bi-state switch coupling state with the state indicated by the status indicator and recognize a fault when the compared states differ. Upon recognition of a “compared states differ” fault the controller may re-synchronize the bi-state switches so that one couples its respective motor and power source while the second decouples its respective motor and power source. Desirably, the respective non-volatile bi-state switches are capable of holding a coupled or decoupled state until its electromagnetic actuator is energized again by its respective relay. 
     The objects and features of the present invention may be applied jointly or severally in any combination or sub-combination by those skilled in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a schematic perspective view of an excavator and its ranges of motion; 
         FIG. 2  is a schematic of a known excavator drive system; 
         FIG. 3  is a partial schematic diagram of an excavator drive system of the present invention, simplified to show a single current phase for two motors and their shared power system; 
         FIG. 4  is a schematic block diagram of an excavator drive system the present invention; and 
         FIG. 5  is a schematic diagram of an exemplary excavator drive system of the present invention, showing four motors that provide the excavator ranges of motion shown in  FIG. 1  and their two shared power sources. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION 
     After considering the following description, those skilled in the art will clearly realize that the teachings of my invention can be readily utilized in excavator drive systems. The drive system of the present invention provides for quicker power transfer from one paired drive motor to the other motor, reducing transfer time from multiple seconds to a fraction of a second. The drive system of the present invention also preserves transfer status in the event of power failure to the non-volatile bi-state transfer switches electromagnetic actuators. 
     Drive System Architecture and Functional Overview 
       FIG. 3  shows a single phase (Φ N ) embodiment of the drive system  30  of the present invention. Alternating current power from inverter  32  is transferred between motor  42 , driving hoist motion H of the excavator  10  bucket  20  and motor  46 , driving propulsion motion P 2  of the excavator tread  14 , depending upon whether the excavator is being propelled between digging locations or is digging. 
     Power transfer between the motors  42 ,  46  is accomplished with a pair of respective first and second disconnect switches  50 ,  52 , designed to connect and isolate electrical circuits, rather than by known externally motorized transfer switches, the disadvantages of which were previously described. Each of the disconnect switches  50 ,  52  is a single pole single throw switch. In multiple phase applications a single pole switch can be dedicated to each phase, such as by ganging them together, or alternatively, a multi-phase single throw switch sharing a common housing can be constructed. 
     Single pole disconnector switches  50 ,  52  have a fixed contact  54  and a moving contact  56  for selectively coupling and decoupling the inverter power source  32  to the respective motors  42 ,  46 . The moving contact  56  is opened and closed with a motorized mechanism including an electromagnetic actuator, schematically shown as box and dashed line  58 . The disconnect switch  50 ,  52  operates by charging an electromagnetic coil in the actuator  58 , and is both non-volatile and bi-state. In other words, the switch can only be fully opened or fully closed state, and it “holds” state until caused to change state by the actuator  58 . The same actuator pulse is used to open and to close the contacts  54 ,  56  sequentially as needed. In the closed position/state the contacts  54 ,  56  are mechanically locked. In opened contact position/state a spring holds the status. 
     No externally applied electrical holding power is necessary to maintain switch  50 ,  52  state. In the event that actuator  58  energization power to either of the switches  50 ,  52  is disrupted, the affected switch will hold its pre-disruption state. Thus, unlike previously known transfer switches, the switches  50 ,  52  of the present invention do not decouple either of motors  42 ,  46  from the inverter power source  32  in the event of switch power disruption. An exemplary disconnect switch suitable for practicing the present invention is a model XMS disconnector switch sold by Secheron SA of Geneva, Switzerland. 
     Switching state of the non-volatile bi-state disconnect switches  50 ,  52  is changed by energizing the respective actuators  58  in each switch by means of relay devices  60 , each of which includes a fixed contact  64  and a moving contact  66 . Relay power supply  69  provides sufficient power to energize the bi-state switch  50 ,  52  actuators. Exemplary output specifications for power supply  69  are 5 kW with 125 V DC and (in worst case)  35  A to operate the motor transfer switches  50 ,  52  reliably in a temperature range of −40° C. to +50° C., with margin for a ±10% variation of the power supply output. The bi-state switches  50 ,  52  are capable of indicating their respective coupled status (i.e., opened or closed), such as through known “normally open” (N.O.) and “normally closed” (N.C.) auxiliary contacts status indicators that are in communication with the PLC  38  input ports via communications pathway  39 . The PLC  38  in turn causes the relay devices  60  to energize the actuators  58  in the respective bi-state switch pairs  50 ,  52 . 
     While the exemplary embodiments of the bi-state switches  50 ,  52  and relay devices  60  describe electromechanical components, one skilled in the art may substitute solid state switching components to perform either or both of their respective functions. 
       FIGS. 4 and 5  show application of the present invention to a three-phase excavator drive system  30 , having two separate pairs of drive motors  40 ,  44  for crowd and propulsion  1  drive and motors  42 ,  46  for hoist and propulsion  2  drive. As each bi-state disconnector switch  50 ,  52  is a single pole disconnector, 12 single switches will be required to achieve 12 phases (i.e., four motors each having three phases). Each of the motor pairs is selectively coupled to its corresponding inverter  32  power source through pairs of bi-state switches  50 ,  52 . Functionally in each phase, the individual single-pole single throw switches  50 ,  52  functions as a single pole double throw change-over switch that transfers power from one paired motor to the other. In operation, as shown in  FIG. 4 , six bi-state disconnector switches are in closed position (e.g., crowd/hoist functions) and six are in open position (e.g., their corresponding pair counterpart propulsion  1 /propulsion  2  functions). 
     As previously described, the bi-state switches  50 ,  52  are capable of indicating their respective coupled status (i.e., opened or closed) such as through known “normally open” (N.O.) and “normally closed” (N.C.) auxiliary contacts b 1 , b 2  that are in communication with the PLC  38  via communications pathway  39 . If each switch  50  or  52  in each of the three motor phases has its own dedicated relay device  60  and the PLC  38  is intended to monitor status of each individual phase, a total of 12 monitoring PLC inputs and 12 outputs (total 24) are required. While 24 separate monitoring inputs/outputs may be accommodated with an appropriately sized PLC input/output interface card, it is possible to reduce the switches  50 ,  52  control and monitoring interfaces down to 4 PLC control outputs and 2 inputs. 
     As shown in  FIG. 5 , the bi-state switches  50 ,  52  are grouped into four separate operational blocks corresponding to each of the drive motors  40 ,  42 ,  44 ,  46 . Each block of three mechanically-linked, single phase bi-state switches  50 ,  52  have the same “open” or “closed” status and shares one common relay device  60 . A single PLC  38  output causes the relay device  60  to energize all bi-state switch actuators  58  in its block. 
     In  FIG. 5 , the 2 PLC inputs B 1  and B 2  are used to monitor the status. While the blocks C (Crowd) and H (Hoist) are closed and P 1  (Propel  1 ) and P 2  (Propel  2 ) are opened B 1 =1 and B 2 =0. While the blocks P 1  and P 2  are closed and C and H are opened B 1 =0 and B 2 =1; B 1 =0 and B 2 =0 indicates a fault. It is not possible to have both B 1 =1 and B 2 =1. In other words, it is exactly known whether the machine is in Crowd/Hoist mode, in Propel mode or there is a fault situation. 
     The Logical Operation Sequence is presented below in Tables I and II. With the mechanical linking of each bi-state switch  50 ,  52  blocks there are  16  possible statuses (Case A-P) for the main contacts  54 ,  56  and the auxiliary contacts b 1 , b 2 . For each case, the status of the auxiliary contacts and what signal is given to the PLC (B 1 , B 2 ) is shown. Normal operation is change over. That means switchover between Case B and Case C. Otherwise there is some kind of fault. Not every fault is critical, e.g., at the beginning all switches  50 ,  52  are open so there is just a synchronization required. 
     For all fault cases the same special switching sequence is started to get the bi-state switches  50 ,  52  back to the cases B or C. This sequence makes it possible to resynchronize without knowing the exact status. Whether there is loss of synchronization or a power failure, the non-volatile bi-state switches  50 ,  52  will continue to maintain their existing status, so that excavator operation can continue until re-synchronization or power supply resumption. 
     The status of the main contacts is shown in Table I. There are  16  cases indicated as case A to case P. 
     
       
         
               
             
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 STATUS OF MAIN CONTACTS 
               
             
          
           
               
                   
                   
                   
                 Propel 
                   
                 Propel 
               
               
                 Status 
                 Case 
                 Crowd 
                 1 
                 Hoist 
                 2 
               
               
                   
               
               
                 Start 
                 A 
                 0 
                 0 
                 0 
                 0 
               
               
                 C + H 
                 B 
                 1 
                 0 
                 1 
                 0 
               
               
                 working 
                   
                   
                   
                   
                   
               
               
                 P1 + P2 
                 C 
                 0 
                 1 
                 0 
                 1 
               
               
                 working 
                   
                   
                   
                   
                   
               
               
                 C + H 
                 D 
                 1 
                 1 
                 1 
                 0 
               
               
                 working, 
                   
                   
                   
                   
                   
               
               
                 P1 error 
                   
                   
                   
                   
                   
               
               
                 C + H 
                 E 
                 1 
                 0 
                 1 
                 1 
               
               
                 working, 
                   
                   
                   
                   
                   
               
               
                 P2 error 
                   
                   
                   
                   
                   
               
               
                 C + H 
                 F 
                 0 
                 0 
                 1 
                 0 
               
               
                 working, C 
                   
                   
                   
                   
                   
               
               
                 error 
                   
                   
                   
                   
                   
               
               
                 C + H 
                 G 
                 0 
                 1 
                 1 
                 0 
               
               
                 working, 
                   
                   
                   
                   
                   
               
               
                 C + P1error 
                   
                   
                   
                   
                   
               
               
                 C + H 
                 H 
                 0 
                 0 
                 1 
                 1 
               
               
                 working, 
                   
                   
                   
                   
                   
               
               
                 C + P2error 
                   
                   
                   
                   
                   
               
               
                 C + H 
                 I 
                 1 
                 0 
                 0 
                 0 
               
               
                 working, H 
                   
                   
                   
                   
                   
               
               
                 error 
                   
                   
                   
                   
                   
               
               
                 C + H 
                 J 
                 1 
                 1 
                 0 
                 0 
               
               
                 working, 
                   
                   
                   
                   
                   
               
               
                 H + P1 error 
                   
                   
                   
                   
                   
               
               
                 C + H 
                 K 
                 1 
                 0 
                 0 
                 1 
               
               
                 working, 
                   
                   
                   
                   
                   
               
               
                 H + P2 error 
                   
                   
                   
                   
                   
               
               
                 P1 + P2 
                 L 
                 0 
                 0 
                 0 
                 1 
               
               
                 working, 
                   
                   
                   
                   
                   
               
               
                 P1 error 
                   
                   
                   
                   
                   
               
               
                 P1 + P2 
                 M 
                 0 
                 1 
                 0 
                 0 
               
               
                 working, 
                   
                   
                   
                   
                   
               
               
                 P2 error 
                   
                   
                   
                   
                   
               
               
                 P1 + P2 
                 N 
                 1 
                 1 
                 0 
                 1 
               
               
                 working, C 
                   
                   
                   
                   
                   
               
               
                 error 
                   
                   
                   
                   
                   
               
               
                 P1 + P2 
                 O 
                 0 
                 1 
                 1 
                 1 
               
               
                 working, H 
                   
                   
                   
                   
                   
               
               
                 error 
                   
                   
                   
                   
                   
               
               
                 All ON 
                 P 
                 1 
                 1 
                 1 
                 1 
               
               
                   
               
             
          
         
       
     
     As has been described above, each block of 3 bi-state disconnect switches  50 ,  52  has two auxiliary contacts a 1  or b 1 ,a 2  or b 2 . As an example, the status of contacts for Crowd (a 1 ,b 2 ) and Propel  1  (a 2 , b 1 ) are shown in the Table II. Similar status would exist for Hoist and Propel  2  blocks. 
     
       
         
               
             
               
               
               
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 STATUS OF AUXILIARY CONTACTS 
               
             
          
           
               
                   
                   
                 Crowd 
                 Crowd 
                 Propel 
                 Propel 
               
               
                 Status 
                 Case 
                 (a1) 
                 (b2) 
                 1 (a2) 
                 1 (b1) 
               
               
                   
               
               
                 Start 
                 A 
                 0 
                 1 
                 0 
                 1 
               
               
                 C + H 
                 B 
                 1 
                 0 
                 0 
                 1 
               
               
                 working 
                   
                   
                   
                   
                   
               
               
                 P1 + P2 
                 C 
                 0 
                 1 
                 1 
                 0 
               
               
                 working 
                   
                   
                   
                   
                   
               
               
                 C + H 
                 D 
                 1 
                 0 
                 1 
                 0 
               
               
                 working, 
                   
                   
                   
                   
                   
               
               
                 P1 error 
                   
                   
                   
                   
                   
               
               
                 C + H 
                 E 
                 1 
                 0 
                 0 
                 1 
               
               
                 working, 
                   
                   
                   
                   
                   
               
               
                 P2 error 
                   
                   
                   
                   
                   
               
               
                 C + H 
                 F 
                 0 
                 1 
                 0 
                 1 
               
               
                 working, C 
                   
                   
                   
                   
                   
               
               
                 error 
                   
                   
                   
                   
                   
               
               
                 C + H 
                 G 
                 0 
                 1 
                 1 
                 0 
               
               
                 working, 
                   
                   
                   
                   
                   
               
               
                 C + P1error 
                   
                   
                   
                   
                   
               
               
                 C + H 
                 H 
                 0 
                 1 
                 0 
                 1 
               
               
                 working, 
                   
                   
                   
                   
                   
               
               
                 C + P2 error 
                   
                   
                   
                   
                   
               
               
                 C + H 
                 I 
                 1 
                 0 
                 0 
                 1 
               
               
                 working, H 
                   
                   
                   
                   
                   
               
               
                 error 
                   
                   
                   
                   
                   
               
               
                 C + H 
                 J 
                 1 
                 0 
                 1 
                 0 
               
               
                 working, 
                   
                   
                   
                   
                   
               
               
                 H + P1 error 
                   
                   
                   
                   
                   
               
               
                 C + H 
                 K 
                 1 
                 0 
                 0 
                 1 
               
               
                 working, 
                   
                   
                   
                   
                   
               
               
                 H + P2 error 
                   
                   
                   
                   
                   
               
               
                 P1 + P2 
                 L 
                 0 
                 1 
                 0 
                 1 
               
               
                 working, 
                   
                   
                   
                   
                   
               
               
                 P1 error 
                   
                   
                   
                   
                   
               
               
                 P1 + P2 
                 M 
                 0 
                 1 
                 1 
                 0 
               
               
                 working, 
                   
                   
                   
                   
                   
               
               
                 P2 error 
                   
                   
                   
                   
                   
               
               
                 P1 + P2 
                 N 
                 1 
                 0 
                 1 
                 1 
               
               
                 working, C 
                   
                   
                   
                   
                   
               
               
                 error 
                   
                   
                   
                   
                   
               
               
                 P1 + P2 
                 O 
                 0 
                 1 
                 1 
                 0 
               
               
                 working, H 
                   
                   
                   
                   
                   
               
               
                 error 
                   
                   
                   
                   
                   
               
               
                 All ON 
                 P 
                 1 
                 0 
                 1 
                 0 
               
               
                   
               
             
          
         
       
     
     In the above tables, status 1 indicates a contact is closed and status 0 indicates a contact is open. To implement the excavator drive system  30  motor power transfer, the PLC  38  is programmed to enable the system to perform the changeover and the fault handling sequences. The PLC  38  has to hold the output signals for a defined pulse duration to assure completion of the bi-state switch  50 ,  52  state change. This pulse duration time defines the transfer time of the drive system  30 . 
     EXPERIMENTAL EXAMPLES 
     Two bi-state switches  50 ,  52  have been combined to create a single phase, single pole double throw switch, in order to confirm feasibility of the present invention for application in excavator drive systems. Six such units need to be combined to meet the requirements of an excavator drive system three-phase propel  1 -propel  2 /hoist-crowd transfer switch. The experimental testing determined transfer switch operational and performance parameters, and confirmed that the present invention is capable of performing transfer operations in fractions of a second rather than multiple seconds expended in known transfer switches. 
     A. Test Setup at Ambient Temperature:
     Two single pole non-volatile bi-state Model XMS disconnect switches (switch  50  switch  52 )   Power supply: 27.6 V/48.0 V   Logic circuit voltage: 27.6 V/27.6   Timer relays: CM1/UC24-60V   Power relays: Omron Model G9 EB-1-B and Siemens Model 3TH2022-0BB4   

     B. Measurements with 27.6 V, Switch  50  Opening and Switch  52  Closing 
     The measurements are for minimal pulse duration with 27.6 V control supply. During operation, both the bi-state switches exhibited similar behavior, i.e., switch  50  opening, switch  52  closing was similar to switch  50  closing, switch  52  opening.
     Pulse duration: 504 ms   Beginning of the pulse until closing of  50 : 184 ms   Beginning of the pulse until opening of  52 : 534 ms   Coil current maximum: 6.9 A   

     C. Measurements with 48 V, Switch  50  Opening and Switch  52  Closing 
     The measurements are for minimal pulse duration with 48 V control supply. This is the best possible switching time achieved.
     Pulse duration: 84.4 ms   Beginning of the pulse until opening of  50 : 116 ms   Coil current maximum: 10.6 A   

     One important finding was that operating at the higher end of the rated actuation coil voltage of actuator  58  resulted in shorter switching times. Table III and Table IV show the comparison of results for 27.6 V and 48 V. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE III 
               
             
             
               
                   
               
               
                 RESULTS FOR 27.6 V 
               
             
          
           
               
                   
                 Pulse 
                 t A   
                 t B   
                 I c   
               
               
                   
                 [ms] 
                 [ms] 
                 [ms] 
                 [A] 
               
               
                   
               
             
          
           
               
                   
                 I 
                 504 
                 184 
                 534 
                 6.9 
               
               
                   
                 II 
                 500 
                 532 
                 178 
                 6.7 
               
               
                   
                 III 
                 496 
                 146 
                 — 
                 6.7 
               
               
                   
                 IV 
                 496 
                 522 
                 — 
                 — 
               
               
                   
                 V 
                 223 
                 — 
                 250 
                 6.6 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE IV 
               
             
             
               
                   
               
               
                 RESULTS FOR 48 V 
               
             
          
           
               
                   
                 Pulse 
                 t A   
                 t B   
                 I c   
               
               
                   
                 [ms] 
                 [ms] 
                 [ms] 
                 [A] 
               
               
                   
               
             
          
           
               
                   
                 I 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                 II 
                 500 
                 528 
                 — 
                 12.8 
               
               
                   
                 III 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                 IV 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                 V 
                 84.4 
                 116 
                 — 
                 10.6 
               
               
                   
               
             
          
         
       
     
     In the tables:
     t A : time from beginning of the pulse till action of switch  50     t B : time from beginning of the pulse till action of switch  52     I c : coil current maximum   I:  50  closing,  52  opening   II:  50  opening,  52  closing   III: Synchronization  50  closing   IV: Synchronization  50  opening   V: Minimal pulse duration   

     V. CONCLUSIONS 
     One of the important findings from the test results was that opening a bi-state disconnect switch takes much longer than closing the same switch. Therefore the opening time is defined in the transfer switch time. The minimal transfer switch time which could be found on the actual bi-state switch at a supply of 48 V is 116 ms. Synchronization is necessary initially between the paired switches  50 ,  52  and after a fault. The internal logic connections of the auxiliary contacts b 1 , b 2  were proved and verified. 
     Advantages of the present invention over known external mechanical linkage transfer switches include fast transfer time of fractions of a second compared to many seconds, simple logic, lack of holding energy input to maintain non-volatile bi-state switch state, relatively simpler robust construction and very minimal maintenance. These application advantages of the present invention have to be weighed by those skilled in the art with potential increased switching noise, size, weight, higher actuator actuation coil current needs, and additional power relays to accomplish the switching status logic implementation described herein that minimizes PLC switch status inputs (or use a PLC I/O interface that accommodates more inputs). It is believed that in most excavator drive system motor transfer applications, those skilled in the art will appreciate that the advantages of the present invention are preferable to excavator drive systems using known transfer switches. 
     Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.