Abstract:
An apparatus, and a method of opening and closing electrical power feed lines using a hybrid contactor, which combines a traditional set of mechanical main contacts with a high voltage solid state switch. The solid state switch provides a parallel current path around the main contacts. When the main contacts are to be opened or closed, the solid state switch is first closed, diverting current away from the main contacts to prevent arc formation when the main contacts are being opened or closed. Once the main contacts are opened or closed, the solid state switch is opened, as the parallel current path is no longer needed. Optional auxiliary contacts are connected in series with the solid state switch to provide galvanic isolation between an input terminal and an output terminal.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to vehicle power systems, and more specifically, to direct current contactors. 
   Vehicles, such as aircraft, rely on contactors and relays for protection and control of opening and closing electrical power feed lines. A typical vehicle may contain a hundred or more contactors. In an alternating current voltage system, an electric current follows a waveform, typically a sine wave, and there exists a zero voltage cross over point on that waveform. If a contactor is opened at the cross over point, the arc problem described below that exists in direct current systems will not occur. 
   In a direct current voltage system, there is no zero voltage cross over point. If a set of DC contacts are opened, an electric arc will form in a gas-filled space between the contacts, and without intervention will continue until the space between the electrical contacts is too large to sustain the arc. An arc can produce a very high temperature and is undesirable in a vehicle power system, as it can damage a contactor and can decrease the life span of a contactor. 
   One solution to this problem is an arc chute. An arc chute is used to stretch an arc a sufficient distance so that the voltage cannot support the arc, and the arc will eventually break. However in a high voltage DC system, such a contactor becomes undesirably large due to the size required for the arc chute and the large spacing required between the contacts within the contactor. 
   Another solution to the DC arc problem is to create a hermetically sealed container to enclose the contacts. In this solution, the container is typically metal, and is typically soldered for an airtight seal. The container is then either hooked to a hard vacuum to remove air, or the container is filled with an inert gas. The absence of air decreases the distance that the arc can be maintained for the voltage in the atmosphere around the contacts. Side magnets are sometimes used in a hermetically sealed contactor to pull the arc and eventually break it. The hermetic cavity of the construction, however, makes the manufacture of the contactor difficult and costly. 
   There is a need for a low cost and/or non-hermetic contactor that can switch high voltage DC current with high reliability, preferably without the need for an arc chute. 
   SUMMARY OF THE INVENTION 
   The present invention addresses the problem of DC arc formation through the use of a hybrid contactor. The hybrid contactor combines a traditional set of mechanical main contacts with a high voltage solid state switch. The solid state switch provides a parallel current path to the main contacts. A set of secondary auxiliary contacts in series with the solid state switch may also be used. When the main contacts are to be opened or closed, the solid state switch is closed, diverting current away from the contacts so that no arc is formed when the main contacts are opened or closed. Once the main contacts are opened or closed, the solid state switch is then opened. Auxiliary contacts, if present, are closed prior to closing the solid state switch, and are opened prior to opening the solid state switch. 
   These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a contactor employing the present invention. 
       FIG. 2  illustrates a contactor employing the present invention, along with associated controller logic. 
       FIG. 3  illustrates a solid state switch for a unidirectional DC contactor. 
       FIG. 4  illustrates a solid state switch for a bidirectional DC contactor. 
       FIG. 5  illustrates the present invention in the example environment of an aircraft. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  illustrates a high-level representation of a contactor embodying the present invention. A contactor  10  combines a traditional set of mechanical main contacts  12  with a high voltage solid state switch  14 . The solid state switch  14  provides a parallel current path to the main contacts  12 . The main contacts  12  could comprise an incoming wire, an outgoing wire, and a moving part to connect them, or the main contacts  12  could comprise a plurality of incoming wires, a plurality of outgoing wires, and a moving part to connect them. A set of optional auxiliary contacts  20  is connected in series with the solid state switch  14 . A gate drive  16  operates to open and close the solid state switch  14 . When the gate drive  16  is turned on, the solid state switch  14  closes, and when the gate drive  16  is turned off, the solid state switch  14  opens. A contactor coil  18  is used to provide power for an actuator shaft  22 . The actuator shaft  22  mechanically opens and closes the main contacts  12  and the optional auxiliary contacts  20 . Line connections  24  and  26  connect the contactor  10  to external circuit components. Controller  28  controls gate drive  16  and contactor coil  18 . Power source  29  provides power to gate drive  16 . 
   When the controller  28  needs the contactor  10  to relay current, a command signal is given to close the contactor  10 , the auxiliary contacts  20  are closed, then the solid state switch  14  is closed, and then the main contacts  12  are closed. During the short period of time in which the main contacts  12  are closing, current flows through the solid state switch  14 . With this parallel path, the voltage across the main contacts  12  is close to zero when the contacts are closing. This prevents arcing when the main contacts  12  close, and also increases the life of the contacts. Once the main contacts  12  are closed, the solid state switch  14  is opened, and then the auxiliary contacts  20  are opened. The opening of the solid state switch  14  can be based on either timing or feedback. Despite the criteria used for the decision, the controller  28  would still make the decision about when to close the main contacts  12 . 
   When the controller  28  needs the contactor  10  to stop relaying current, a command signal is given to open the contactor  10 , the auxiliary contacts  20  are closed, then the solid state switch  14  is closed, and then the main contacts  12  are opened. As in the case of the command to close the main contacts  12 , the parallel current path provided by the solid state switch  14  prevents the formation of a DC arc between the main contacts  12  by diverting the flow of current away from the main contacts  12 . Once the main contacts  12  are opened, the solid state switch  14  is opened, and then the auxiliary contacts  20  are opened. 
   A typical solid state switch  14  contains silicon, which heats up very quickly. The contactor  10  is designed so that the solid state switch  14  remains closed for an extremely short period of time. This prevents the solid state switch  14  from overheating, and this also prevents the need for a heat sink to cool the solid state switch  14 . 
   The auxiliary contacts  20  are optional, and provide additional safety, as they prevent the possibility of a high voltage existing at contactor output terminal line connections  24  and  26 . The solid state switch  14  is a transistor-based switch, and carries the risk that even if open, a partial flow of current can still cross the switch. The auxiliary contacts  20  prevent this problem by providing galvanic isolation on the output terminal line connections  24  and  26 . Thus, although auxiliary contacts  20  are optional, it is desirable to incorporate them into a contactor. 
     FIG. 2  illustrates a more detailed schematic diagram of a contactor  30  embodying the present invention and incorporating some features known in the art. An external controller unit  58  transmits commands to a controller  44  to either open or close the contactor  30 . A discrete output module  50  provides status information to a control connector  48 , which then transmits the status information to an external system controller  59 . A power supply  46  obtains power from an external power source  57  and provides power to a gate drive  36 , a controller  44 , and the control connector  48 . Contactor  30  further comprises main contacts  32 , a solid state switch  34 , a contactor coil  38 , a set of auxiliary contacts  40 , and an actuator shaft  42  that all operate as described above. The contactor  30  further comprises a current sensor  54  and a current sensor  56 . Current sensor  54  monitors current in the contactor coil  38 . Current sensor  56  is used to notify the controller  44  if a fault is detected. As in  FIG. 1 , the auxiliary contacts  40  are optional. 
   If controller  44  receives a message to close the contactor  30 , the controller  44  first checks to make sure that the main contacts  32  are actually opened. Controller  44  utilizes current sensor  54  to obtain confirmation from the contactor coil  38  that the main contacts  32  are actually open. If main contacts  32  already closed, then the command to close the main contacts  32  is cancelled. 
   If confirmation is received that the main contacts  32  are actually open, controller  44  utilizes pulse width modulation (PWM) driver  52  to activate the actuator shaft  42  to close the auxiliary contacts  40 . Controller  44  then closes the solid state switch  34 , and then closes the main contacts  32 . Once main contacts  32  are actually closed, the solid state switch  34  is opened, and the auxiliary contacts  40  are opened. As in  FIG. 1 , the solid state switch  34  is closed for only an extremely short period of time, and arc formation is prevented. 
   When controller  44  receives a command to open the main contacts  32 , it similarly confirms that the main contacts  32  are actually closed. If the main contacts  32  are already open, the command is cancelled. If the controller  44  receives confirmation from current sensor  54  that the main contacts  32  are actually closed, the controller  44  then utilizes PWM driver  52  to close the auxiliary contacts  40 . Controller  44  then closes solid state switch  34 , opens main contacts  32 , opens solid state switch  34 , and then opens auxiliary contacts  40 . 
     FIGS. 3 and 4  illustrate example solid state switches that can be interchangeably used in the contactors of  FIGS. 1 and 2 , depending on if a unidirectional or a bidirectional contactor is desired. A unidirectional contactor carries current in only one direction. An example unidirectional contactor could carry current from a vehicle power source to a load. A bidirectional contactor is able to carry current in either direction. Bidirectional contactors are, however, typically more expensive to produce. An example bidirectional contactor is a bow tie contactor. 
     FIG. 3  illustrates a solid state switch  60  for a unidirectional DC contactor. The solid state switch  60  comprises both a transistor  62  and a diode  64  connected in parallel. In one example the transistor  62  could be an IGBT or a high voltage MOSFET. The solid state switch  60  has three connections: a first line connection  66 , a second line connection  68 , and a gate drive connection  70 . In this example unidirectional DC contactor, current would flow in from line connection  66  and would flow out from line connection  68 . Gate drive connection  70  would be hooked up to an external gate drive which would be operable to turn the solid state switch  60  OFF or ON. 
     FIG. 4  illustrates a solid state switch  80  for a bidirectional DC contactor. The solid state switch  80  contains a first transistor  82  and diode  84  pair, and a second transistor  86  and diode  88  pair. Transistor  82  and diode  84  are in parallel to each other, and transistor  86  and diode  88  are in parallel to each other. The first transistor and diode pair is in series with the second transistor and diode pair. As in  FIG. 3 , in one example the transistors  82  and  86  could be IGBTs or high voltage MOSFETs. The solid state switch  80  has four external connections: a first line connection  90 , a second line connection  92 , and two gate drive connections  94  and  96 . Gate drive connections  94  and  96  would connect to a single gate drive, which would be operable to turn the solid state switch  80  OFF or ON. 
     FIG. 5  illustrates the present invention in the example environment of an aircraft. Contactor  30  is positioned between a power source  57  and a load  102 . A controller unit  58  provides commands to the contactor  30 , and a system controller  59  obtains data from the contactor  30 . 
   Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.