Abstract:
The present invention relates generally to a protection circuit for electrical apparatuses, and more particularly, to a method and apparatus for detecting when silicon controlled rectifier (SCR) or other control elements in an engine driven generator of a welding-type power source have failed.

Description:
BACKGROUND OF INVENTION  
         [0001]    The present invention relates generally to a protection circuit for electrical apparatuses, and more particularly, to a method and apparatus for detecting when silicon controlled rectifier (SCR) control elements or other rectifying control elements in an engine driven generator of a welding-type power source have failed.  
           [0002]    SCRs are known in the power electronics industry for their use as high power current switching devices. Such devices have found wide application in solid-state control systems where it is desired to control the voltage, current, and/or power supplied to a load. Generally, in single phase or poly-phase systems, one or more pairs of SCRs can be connected in a back-to-back relationship and in series with a load. The SCRs are further connected to an alternating current (AC) supply source, from which the current, voltage, or power desired to be controlled is provided. Typically, SCRs are triggered into conduction at an appropriate time in the positive and negative half cycles of the AC supply. The time between the zero crossing of the AC supply waveform and the moment at which the SCRs are driven into conduction is commonly referred to as the firing angle of the SCR. Adjusting the firing angle provides a means for controlling the voltage, current, and/or power delivered to the load.  
           [0003]    Power sources for welding-type systems commonly use an SCR bridge, transistors, or diodes to control the welding-type output current using conventional phase control techniques. For static welding-type machines such as those that are powered by utility line supplies, shorted SCR detection circuits monitor the input voltage to the SCR bridge. If a short is present in the system the input voltage to the SCR bridge will go to zero. Because the short is applied through a failed device within an output rectifier bridge, the short is applied for only one-half of an AC cycle. Moreover, since machines powered by a utility line source will always have an input voltage to the SCR bridge under no-fault conditions, detection of a zero voltage at the input of the SCR bridge is indicative of a fault present in the system.  
           [0004]    In contrast, engine driven generators will routinely have a zero voltage input to the SCR bridge during normal operation. For example, when the SCR bridge is turned on, the input voltage will be zero as a result of the high impedance of the generator. Further, the voltage may often collapse to zero when large currents are drawn from the generator. Because periods of zero voltages are not harmful and are expected in normal operation of an engine driven generator, detection circuits monitoring input voltage to the SCR bridge cannot discern between normal zero-going voltages or fault induced zero-going voltages. Therefore, electrical circuits that monitor input voltages are inapplicable with high impedance power sources such as engine driven generators.  
           [0005]    Because voltage monitoring is not preferred for engine driven power sources, a number of systems have been developed that focus on input current to the SCR bridge to detect and determine shorted or fault conditions in the SCR. For example, fuses have commonly been used in each of the three-phase AC lines to detect high current flow. Another approach uses current transformers placed in each AC line to detect high current. While both of these approaches will indicate a shorted SCR bridge of an engine driven power source, these approaches require expensive and bulky external hardware that significantly increase the size, weight, and cost of the power source.  
           [0006]    It would therefore be desirable to design an electrical circuit configured to detect shorted conditions of rectifying control elements in an engine driven generator of a welding-type power source.  
         BRIEF DESCRIPTION OF INVENTION  
         [0007]    The present invention is directed to an electrical circuit designed to detect shorted conditions in rectifying control elements of an engine driven generator of a welding-type power source that overcomes the aforementioned problems. The electrical circuit includes a bandpass filter to pass only those frequency components of a current signal indicative of current in the rotor of an engine driven generator that are indicative of shorted conditions. By monitoring the generator field current for specific frequency components a fault condition can be determined. The specific frequency components are induced by a DC component in the stator of generator which can only occur under a fault condition. The bandpass filter includes a high pass filter and a low pass filter that collectively allow frequency components ranging from idle to rated speed of the generator to be passed. By analyzing the frequency components that are passed by the bandpass filter, a fault condition can be determined. Moreover, if the fundamental frequency components exceed corresponding thresholds, the engine driven generator may be automatically disabled.  
           [0008]    Therefore, in accordance with one aspect of the present invention, a shorted rectifying control element detection device includes an input stage connected to an engine driven generator to supply power for a welding-type process. The detection device includes a filtering stage to bandpass current signals acquired by the input stage as well as an output stage connected to the filtering stage and configured to at least indicate a fault condition if fundamental frequency components present in the current signals exceed a threshold.  
           [0009]    In accordance with another aspect of the present invention, an engine driven welding-type system includes a synchronous wound field generator having a rotor magnetically coupled to a stator as well as an SCR bridge connected to be supplied with power from the generator and continue to rectify the power into a form usable in a welding-type process. The welding-type system further includes a fault detection circuit designed to determine if stator induced fundamental frequency components are present in current flowing through the rotor as well as a fault indicator connected to the fault detection circuit and configured to output a fault present signal.  
           [0010]    According to another aspect of the present invention, a short circuit detection device for an engine driven generator includes means for detecting current in a rotor of a generator as well as means for isolating frequency components with current indicative of a stator induced DC component. The short circuit detection device further includes means for determining a short from the isolated frequency components.  
           [0011]    In accordance with another further aspect of the present invention, a method of detecting a shorted SCR of an engine driven welding-type power source includes the step of detecting current in a rotor of an engine driven generator as well as the step of isolating frequency components in the current indicative of a stator induced DC The method also includes the step of determining if the shorted SCR condition is present from the isolated frequency components.  
           [0012]    Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0013]    The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention.  
         [0014]    In the drawings:  
         [0015]    [0015]FIG. 1 is a perspective view of an engine driven power source connected to a wire feeder assembly of an MIG welding system.  
         [0016]    [0016]FIG. 2 is an exploded view of an engine driven generator for use with the power source illustrated in FIG. 1.  
         [0017]    [0017]FIG. 3 is a flow chart illustrating the steps of a control algorithm to regulate a power supply system in accordance with one embodiment of the present invention.  
         [0018]    [0018]FIG. 4 is a circuit diagram of a bandpass filter to detect a shorted SCR in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0019]    The present invention will be described with respect to an engine driven power source for a MIG or Gas Metal Arc Welding (GMAW) system.  
         [0020]    One skilled in the art will appreciate however, that the present invention is applicable with power sources for other types of welding systems such as stick welding and TIG welding systems as well as other high output welding-type systems such as induction heaters and plasma cutters.  
         [0021]    Referring now to FIG. 1, an engine driven power source  10  houses an in internal combustion engine (not shown) designed to supply raw power that when conditioned is usable for a welding-type process. The power source  10  also includes a base  12 , sidewalls  14 , and a top cover  16  that connects the base and sidewalls into a single integrated unit. End  18  has an end cover  20  containing louvers  22 . The louvers  22  allow airflow into the power source  10  to cool internal components during device operation. Control panel  24  includes controls  26  that can be used by an operator to select a desired welding current amperage value and other operational parameters for a welding-type process.  
         [0022]    For MIG welding applications, power source  10  is connected to a wire feeder  28  via control cable  30  and is connected to a work piece  32  via negative weld cable  34 . A clamp  36  is connected to an end of negative weld cable  34  and is designed to hold and present work piece  32  to a weld. Wire feeder  28  is designed to present a consumable wire electrode to the weld via a MIG gun or torch  38 . Gun  38  is connected to the wire feeder via a connecting plug  40 . In some applications, the wire feeder may be a volt sensed wire feeder and, as such, includes a voltage sensing clamp  42  that is secured to work piece  32  during welding. For MIG welding applications a gas cylinder  44  provides shielding gas to the wire feeder for use during the welding process through gas hose  46 .  
         [0023]    Referring now to FIG. 2, an exploded view of a generator for use with the power source illustrated in FIG. 1 is shown. The generator  48  includes a stator  50  and rotor assembly  52 . The rotor assembly  52  includes a fan  54  and a ball bearing assembly  56 . Rotor assembly  52  is designed to be positioned within stator  50  and secured thereto by adapter plate  58 . Adapter plate  58  is designed to secure the generator  48  to an engine (not shown). Connected to an opposite end of stator  50  is an endbell  60  that includes an o-ring  62  and a pair of fasteners  64  for securing a brushholder assembly  66  thereto. Elongated stud  68  extends from the endbell  60  within stator  58  to adapter plate  58 . The elongated stud  68  is secured to endbell  60  via washer and nut assembly  70 .  
         [0024]    Generator  48  is used to supply power to and SCR bridge in the power source. Preferably, generator  48  is a synchronous wound field generator. In this regard, stator  50  includes stator windings that then supplied with current will induce currents in windings of the rotor assembly  52 . Typically, DC current is supplied to the rotor assembly to produce output power in the stator. Under normal three-phase loading of the stator, currents are not induced in the rotor assembly. However, a single-phase load supplied to the stator or windings will induce a 120 Hz current in the rotor assembly. In a shorted SCR condition, a half-wave short is applied to the generator due to the nature of the SCR bridge. Current flow for one-half of the AC cycle has a DC component that is half wave rectified. The DC component flowing in the stator will induce fundamental frequency currents in the rotor assembly. For a 60 Hz machine, the rotor currents will also be 60 Hz. When the machine is operating at other than rated speed such as at idle, the frequency is reduced in the same proportion as the engine speed. As will be described in greater detail below, these fundamental frequency components can only be caused by a DC component in the stator. Sustained D.C. components in the stator can only occur under fault conditions. Therefore, by monitoring the generator field current for specific frequency components a fault condition can be determined.  
         [0025]    [0025]FIG. 3 is a flow chart illustrating the steps of a control algorithm  72  to regulate a power supply system. An engine driven generator provides a three-phase power supply input  74  that is passed to a rectifying circuit  78  such as an SCR bridge. In one example, the SCR bridge  78  includes pairs of SCRs connected in a back-to-back relationship. The SCRs can comprise any silicon-controlled element capable of regulating electric current flow. A fault detector circuit monitors the components of the rectifying circuit to determine the onset of fault conditions, i.e. a shorted rectifying component, at step  80 . If the rectifying circuit components are operating properly and as such fault conditions are not present  80 ,  82 , the rectified input is passed to a filter circuit for filtering at  84 . The filter circuit is configured to filter any undesired frequency components from the electrical current, which is then made available as welder output  86  to perform welding operations at voltages and currents determined by operator settings of controls  26 .  
         [0026]    However, if a fault condition exists  80 ,  88 , for example, if one or more of the SCRs is shorted a timer is activated at  90 . Timer  86  counts up from a start value to an end value. The timer is activated so that a determination can be made as to whether a real fault condition resides in the system or if the short was caused by a transient or noise condition. Accordingly, a determination is made at step  92  as to whether the fault has cleared. If so  92 ,  94 , the detector circuit returns to step  80  for further monitoring of the components of the rectifying circuit components. If the fault condition persists and therefore has not cleared  92 ,  96 , the timer will reach the end value thereby signaling to a control unit that the gate drives to the rectifying components should be shut off at  98 . If the short persists after the gate drives have been shut off  98 ,  100 ,  102 , the generator is then caused to be shut down  104 . If shutting off of the gate drives to the rectifying components clears the faults, the generator is allowed to continue operating and producing auxiliary power.  
         [0027]    Referring now to FIG. 4, an example of operational circuitry to implement a shorted SCR detector circuit in accordance with the current invention is shown. One skilled in the art will fully recognize, however, that alternatives and variations to the circuitry specifically illustrated are possible, contemplated, and within the scope of the invention. Additionally, the illustrated circuitry is applicable with detecting shorts in other types of rectifying components such as transistors and diodes. Circuit  108  is designed to bandpass current signals provided by a Hall effect sensor and, based on an analysis of the current feedback, determined if a short or otherwise fault condition is present.  
         [0028]    Field current feedback from the rotor in the engine driven generator is provided by a Hall effect sensor  110 . The current signals are then bandpass filtered to pass only those frequency components that would occur under a fault condition. Specifically, a high pass filter, amplifier Al, cascading with a low pass filter amplifier, A 2 , bandpass the current feedback signal. The passed result includes frequency components in the range of 36 to 80 Hz being passed through. This particular range of 36 to 80 Hz illustrates one example of the fundamental frequency range of a generator at idle to a generator at its maximum rated speed. As such, the frequency range that is bandpassed will depend upon the frequency of the generator at idle as well as the frequency of the generator at its maximum rated speed. The high pass filter Al is designed to reject the normal operation DC component found in the current signal. The low pass filter A 2  is designed to reject the 120 Hz component that occurs under a normal single-phase load, i.e. auxiliary power. The bandpass signal is then rectified, by amplifier A 3  and two associated diodes D 1 , D 2 . The rectified signal is then passed to a buffer amplifier A 4 , which transmits a signal to comparator A 5  which detects excessive fundamental frequency components and transmits a signal to the microprocessor to shut off the gate drive to all SCRs. The circuit is latched by software. As such, the power source must be shut down to deactivate the latch.  
         [0029]    Circuit  108  is designed to compare isolated fundamental frequency components in the current signal to a threshold value to determine if the fundamental frequency components are at a level that warrants shutdown of the SCRs. Accordingly, circuit  108  may be constructed to provide various audio and visual warnings and/or indicators upon detection of fault conditions being present in the system, i.e., frequency components present.  
         [0030]    As one skilled in the art will fully appreciate, the heretofore description of welding devices not only includes welders, but also includes any system that requires high power outputs, such as heating and cutting systems . Therefore, the present invention is equivalently applicable with any device requiring high power output, including welders, plasma cutters, induction heaters, and the like. Reference to welding power, welding-type power, or welders generally, includes welding, cutting, or heating power. Description of a welding apparatus illustrates just one embodiment in which the present invention may be implemented. The present invention is equivalently applicable with many high power systems, such as cutting and induction heating systems, or any similar systems.  
         [0031]    Therefore, in accordance with one embodiment of the present invention, a shorted SCR detection device includes an input stage connected to an engine driven generator to supply power for a welding-type process. The detection device includes a filtering stage to bandpass current signals acquired by the input stage by as well as an output stage connected to the filtering stage and configured to at least indicate a fault condition if fundamental frequency components present in the current signals exceed a threshold.  
         [0032]    In accordance with another embodiment of the present invention, an engine driven welding-type system includes a synchronous wound field generator having a rotor magnetically coupled to a stator as well as an SCR bridge connected to be supplied with power from the generator and continue to rectify the power into a form usable in a welding-type process. The welding-type system further includes a fault detection circuit designed to determine if stator induced fundamental frequency components are present in current flowing through the rotor as well as a fault indicator connected to the fault detection circuit and configured to output a fault present signal.  
         [0033]    According to another embodiment of the present invention. A short circuit detection device for an engine driven generator includes means for detecting current in a rotor of a generator as well as means for isolating frequency components with current indicative of our stator induced DC component. The short circuit detection device further includes means for determining a short from the isolated frequency components.  
         [0034]    In accordance with another further embodiment of the present invention, a method of detecting a shorted SCR of an engine driven welding-type power source includes the step of detecting current in a rotor of an engine driven generator as well as the step of isolating frequency components in the current indicative of a stator induced DC component. The method also includes the step of determining if the shorted SCR condition is present from the isolated frequency components.  
         [0035]    The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.