Abstract:
An insulation tester and oscillator circuit for use in same includes a transformer including a primary winding and at least one secondary winding to be coupled across an external load such as an electrode. First and second bipolar junction transistors are connected in a “push-pull” operating mode and coupled to the primary winding of the transformer for producing a high frequency voltage. The primary winding has two ends each respective one of which is coupled to a respective one of the collectors of the first and second bipolar junction transistors. First and second field effect transistors are respectively coupled in parallel with the first and second bipolar junction transistors. An actuator for activating the first and second field effect transistors to respectively conduct substantially synchronously with the first and second bipolar junction transistors shortly after start-up is provided, whereby the majority of current is shunted through the field effect transistors.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to an apparatus for testing insulated electrical conductors, and more particularly to a spark tester including a hybrid bipolar-FET power oscillator circuit for improved power efficiency.  
         BACKGROUND OF THE INVENTION  
         [0002]    In the continuous testing of the insulation of an insulated conductor, it is now common practice to employ a high voltage sine wave AC potential at a frequency between about 500 Hz to about 5000 Hz. One method of generating this test potential is by means of a self-excited oscillator operating at the anti-resonant frequency of the high voltage transformer and the capacitance of the product under test to the test electrode. A suitable circuit for this purpose is described in my U.S. Pat. No. 4,952,880, the disclosure of which is herein incorporated by reference.  
           [0003]    It is an object of the present invention to provide an improved oscillator circuit for use with an insulation testing apparatus that increases oscillator efficiency so as to increase power delivered to the load while eliminating the need for heat sinks or forced air cooling.  
           [0004]    It is a further object of the present invention to provide an insulation testing apparatus that permits the high voltage output to be short-circuited without damage to circuit components and to provide rapid recovery of the high voltage potential to its preset value upon removal of the short-circuit.  
         SUMMARY OF THE INVENTION  
         [0005]    In a first aspect of the present invention an oscillator circuit for use with an insulation testing apparatus includes a transformer having a primary winding and at least one secondary winding. The secondary winding is to be coupled across an external load. At least one bipolar junction transistor is employed in an oscillator loop and is coupled to the primary winding of the transformer for producing high frequency voltage. At least one field effect transistor is coupled in parallel with the bipolar junction transistor. Means for actuating the field effect transistor to conduct with the bipolar junction transistor upon excitation of the primary winding of the transformer is provided, whereby the current conducting through the primary winding is substantially shunted from the bipolar junction transistor to the field effect transistor to significantly reduce power loss otherwise occurring if the current were conducting through the bipolar junction transistor alone.  
           [0006]    In a second aspect of the present invention an oscillator circuit for use with an insulation testing apparatus includes a transformer having a primary winding and at least one secondary winding. The secondary winding is to be coupled across an external load. First and second bipolar junction transistors are connected in a “push-pull” operating mode and coupled to the primary winding of the transformer for producing a high frequency voltage. The primary winding has two ends each respective one of which is coupled to a respective one of the collectors of the first and second bipolar junction transistors. First and second field effect transistors are respectively coupled in parallel with the first and second bipolar junction transistors. Further provided is means for actuating the first and second field effect transistors to respectively conduct substantially synchronously with the first and second bipolar junction transistors upon excitation of the primary winding of the transformer, whereby the current conducting through the primary winding is substantially shunted from the bipolar junction transistors to the field effect transistors to significantly reduce power loss otherwise occurring if the current were conducting through the bipolar junction transistors alone.  
           [0007]    Preferably, where the oscillator is employed in a push-pull configuration, the transformer includes a second secondary winding, and the actuation means includes a low voltage switch coupled to the second secondary winding for being closed upon excitation of the additional secondary winding, and a driver circuit powered via the low voltage switch for triggering the first and second field effect transistors to respectively conduct substantially synchronously with the first and second bipolar junction transistors.  
           [0008]    The driver circuit preferably has first and second digital outputs respectively coupled to the gates of the first and second field effect transistors such that the first output of the driver circuit has an opposite digital state relative to the second output of the driver for actuating the first and second field effect transistors to respectively conduct substantially synchronously with the first and second bipolar junction transistors.  
           [0009]    In a third aspect of the present invention, an apparatus for high voltage testing of the insulation of electrical conductors includes an electrode through which an insulated electrical conductor passes. The apparatus further includes an oscillator circuit having a transformer including a primary winding and at least one secondary winding. The secondary winding is to be coupled across the electrode and ground. At least one bipolar junction transistor is employed in an oscillator loop and coupled to the primary winding of the transformer for producing high frequency voltage. At least one field effect transistor is coupled in parallel with the bipolar junction transistor. Further provided is means for actuating the field effect transistor to conduct with the bipolar junction transistor upon excitation of the transformer windings and upon an oscillator signal in the oscillator loop reaching a predetermined voltage, whereby the current conducting through the primary winding is substantially shunted from the bipolar junction transistor to the field effect transistor to significantly reduce power loss otherwise occurring if the current were conducting through the bipolar junction transistor alone.  
           [0010]    Preferably, the switching means includes a sample and hold circuit having a switch coupled to a charging capacitor for opening a voltage regulator loop and maintaining via the charged capacitor the supply voltage to the oscillator loop.  
           [0011]    An advantage of the present invention is that the resistance and corresponding voltage drop across the field effect transistors is extremely low compared with the voltage drop across the bipolar junction transistors. This results in the shunting of most of the current from the bipolar junction transistors to the field effect transistors such that power losses through the transistors is almost completely eliminated.  
           [0012]    Another advantage is that the apparatus embodying the present invention permits the high voltage output to be short-circuited without damage to circuit components and to provide rapid recovery of the high voltage potential to its preset value upon removal of the short-circuit.  
           [0013]    These and other advantages of the present invention will become more apparent in the light of the following detailed description and accompanying figures. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 schematically illustrates an insulation testing apparatus embodying the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]    Turning to FIG. 1, an insulation testing apparatus embodying the present invention is generally designated by the reference number  10 . The apparatus  10  employs high voltage to test the insulation of electrical conductors for faults such as bare wire or pin holes.  
         [0016]    The apparatus  10  includes an oscillator circuit having a high voltage transformer  12  with a primary winding  14  and first, second and third secondary windings  16 ,  18  and  20 , respectively. The first secondary winding  16  is a high voltage winding to be coupled to a load having a capacitance represented by  22  and a resistance represented by  24 , such that the winding  16  and the load capacitance  22  comprise an anti-resonant tuned circuit. The secondary winding  16  may, for example, be coupled to a high voltage test electrode  25  through which an insulated, grounded conductor moves. However, the secondary winding  16  may be coupled to other types of loads without departing from the scope of the present invention.  
         [0017]    The oscillator circuit further includes first and second bipolar junction transistors (BJTs)  26  and  28  arranged in a “push-pull” operating mode such that one transistor is in a fully conductive state while the other is in a cut-off state during one half of the oscillating cycle and with the transistors assuming opposite conductive states during the other half of the oscillating cycle. Although the oscillator circuit is shown in a push-pull configuration, it should be understood that the oscillator circuit may be implemented in single-ended or other types of configurations without departing from the scope of the present invention. The collectors of the transistors  26  and  28  are coupled to respective ends  29  and  31  of the primary winding  14  of the transformer  12 . The emitters of the transistors  26  and  28  are coupled to one another and to ground potential via a biasing resistor  33 . It will be recognized that a square wave is developed across the collector-to-emitter of each of the first and second transistors  26  and  28 . The imposition of a square wave on an antiresonant tuned circuit causes large heat losses in the transistors  26 ,  28  and in the transformer  12 . A sinusoidal waveform rather than a square wave is therefore desired to be impressed across the primary winding  14  of the transformer  12  and an inductor or choke  30  having a constant current characteristic is connected to a center tap  32  of the primary winding to absorb the difference in the two waveforms to produce the desired sinusoidal waveform. The other end of the choke  30  is connected to a DC potential which may be varied from, for example, about 0 volts to about +32 volts. Thus, the oscillator circuit has greatly improved efficiency achieved as a result of not impressing the square wave generated on the antiresonant tuned circuit. Accordingly, employment of the inductor  30  reduces the direct current necessary to supply a given power to the load resistance  24  connected across ends of the first secondary winding  16  of the transformer  12 . The second secondary winding  18  of the transformer  12  has ends  34 ,  36  coupled to the respective bases of the first and second transistors  26 ,  28  to provide positive feedback to produce oscillation.  
         [0018]    A low pass filter such as, for example, an LC filter including a current limiting resistor  42 , an inductor  43 , and a capacitor  44  which is connected between the bases of the transistors  26  and  28  respectively, is used to prevent radio frequency parasitic oscillations. Also connected between the bases are forward biasing resistors  45  and  47 . The parasitic oscillations may be caused by transformer leakage reactance or by mutual coupling between the various circuit components such as those that occur when a long connecting cable is used between the transistors  26  and  28  and the transformer  12 . Stability of the oscillator circuit may be maintained when distances up to several hundred feet are introduced between the high voltage transformer and the oscillator circuit components.  
         [0019]    When a DC voltage is applied to the inductor  30 , the transistors  26  and  28  are forward biased by a resistor  46  having one end coupled to a DC voltage of, for example, 32 volts, and another end coupled to the base of the transistor  28 . At the same time, a collector voltage is applied to the transistors  26  and  28  through the inductor  30  and the primary winding  14 . The second secondary winding  18  is connected to the bases of the transistors  26  and  28  so as to increase current flow in one of the transistors and decrease current flow in the other of the transistors as soon as current flow in the primary winding  14  induces voltage in the second secondary winding  18 .  
         [0020]    The first secondary winding  16  and the load capacitance  22  comprise a high Q anti-resonant circuit in the frequency range of, for example, about 2 to about 5 kHz, and any primary current change will cause an oscillatory voltage in all windings of the transformer  12  at some frequency in this range. The second secondary winding  18  applies additional base current to whichever of the transistors  26  and  28  is conducting most and cuts off the other transistor until the former transistor is in full conduction. The action of the anti-resonant transformer  12  then reverses the roles of the transistors  26  and  28 , and the transistors continue to alternate between cutoff and full conduction. The inductor  30  permits the anti-resonant transformer  12  to control the frequency and waveform of the resultant continuous oscillation. Capacitors  48  and  50  coupled between the base and the collector of the BJTs  26  and  28 , respectively, prevent spurious oscillation at high frequencies.  
         [0021]    Power field effect transistors (FETs), such as MOSFETs  52  and  54 , are coupled across the oscillator bipolar junction transistors  26  and  28 . As shown in FIG. 1, the sources of the first and second FETs  52  and  54  are coupled to each other and to each of the emitters of the first and second BJTs  26  and  28 . The drains of the first and second FETs  52  and  54  are respectively coupled to the collectors of the first and second BJTs  26  and  28 . The gates of the FETs  52  and  54  are coupled to ground potential via biasing resistors  55  and  57 , respectively. As the linear oscillator starts to oscillate the BJTs are in a conductive state and the FETs are in a non-conductive state. Further, a sine wave voltage is induced in the third secondary winding  20 , closing a low voltage switch  56  coupled across the winding  20  when an oscillation signal in the oscillator loop reaches a predetermined voltage so as to apply operating power to FET driver  58 . Simultaneously, the sine wave voltage is clipped by a resistor  60  and a crossed diodes  62 , each coupled to opposite ends of the winding  20 , and applied through a buffer  64  to the FET driver  58 , causing the gates of the first and second FET transistors  52  and  54  to be driven alternately positive and, in turn, causing the FETs  52  and  54  respectively to conduct in synchronism with that of the conduction of the first and second bipolar junction transistors  26  and  28 .  
         [0022]    The drain-to-source resistance and corresponding voltage drops of the first and second field effect transistors  52  and  54  are extremely low—even at high drain currents—compared to that of the collector-to-emitter resistance and corresponding voltage drop of the first and second bipolar junction transistors  26  and  28 . As a result, virtually all of the current to the primary winding  14  of the transformer  12  is carried by the first and second field effect transistors  52  and  54 , having been substantially shunted away from the first and second bipolar junction transistors  26  and  28 . Power losses in both the first and second field effect transistors  52  and  54  and the first and second bipolar junction transistors  26  and  28  are almost completely eliminated. Consequently, more power is delivered to the high voltage load, and overall efficiency is significantly increased as compared to conduction through the first and second bipolar junction transistors  26  and  28  alone.  
         [0023]    It is theoretically possible to employ field effect transistors alone in a linear mode to start oscillation. From a practical point of view, however, it is very difficult to bias the transistors in the linear mode and still use it as a low-loss high current switch. The center of the normal operating current range for a typical small power field effect transistor could be  15  amperes or more with an operating gate bias of +6 volts. A change in bias of 0.4 volts would change the collector current by 5 amperes. This is suitable for the described purpose, but linear operation could not result without introducing large resistance values in the emitter-drain circuit or using elaborate biasing circuits.  
         [0024]    The bipolar junction transistors are easily biased in their linear regions to start oscillation efficiently. This allows the power field effect transistors to take over the load as soon as oscillations reach a predetermined oscillator output voltage.  
         [0025]    Continuing with reference to FIG. 1, the apparatus  10  further includes a precision rectifier  100  using a voltage from the secondary winding  20  of the transformer  12  to generate a 0 to +10 VDC which is directly proportional to the 0 to 15 kV RMS high voltage at the test electrode  25 . This DC is connected to one input of a voltage comparator  102 . The other input of the comparator  102  connects through a sample and hold circuit, comprising switch  103  and capacitor  107 , to a voltage regulator  106  which supplies a 0 to +32 VDC to the oscillator inverter circuit, thereby controlling the amplitude of the high voltage.  
         [0026]    The voltage regulator  106  ensures that the output of the precision rectifier  100  is close to the control voltage so as to cause the electrode AC test potential to vary in direct proportion to the control voltage. The sample and hold circuit, and switch  104  serving as a comparator voltage stabilizer, function only when a pinhole or bare wire interval passes through the test electrode  25 . During the time period required for the fault to complete its passage through the electrode  25 , the sample and hold circuit  103 ,  107  opens the regulator loop, but maintains the DC regulator output voltage at its pre-fault level by means employing the charged capacitor  107  to maintain a supply voltage to the voltage regulator. At the conclusion of the event, the sample and hold circuit  103 ,  107  and the voltage regulator  106  revert to non-fault operation without causing the high voltage either to rise too slowly or to overshoot its proper value. This characteristic is important to ensure detection of closely spaced faults and to avoid the application of excessive test potentials.  
         [0027]    A conventional bare wire detector  108  responds to DC current flow from the conductor under test to the electrode  25  by superimposing a low voltage DC on the high voltage, and then detecting any direct current flow to ground. This produces a bare wire indication for any ohmic contact between the electrode and product conductor, and is independent of high voltage test conditions.  
         [0028]    The apparatus  10  may include a fault indicating circuit  110 , including a timer circuit  112 , counter  116 , and relay  118 . The DC current drawn by the oscillator circuit varies in accordance with the power loss in the load presented to the high voltage electrode  25  by the product under test. The current is measured by a DC current limiter  120  and, should the current exceed a preset level, the limiter output acts on the voltage regulator  106  to reduce the DC voltage supplied to the oscillator circuit, thus maintaining current at the preset level. The fault indicating circuit  110  and the bare wire detector  108  also operate a current range switch  114  so as to reduce the preset current level of current limiter  120  in the event any product fault passes through the electrode  25 .  
         [0029]    The preset level of current is automatically reduced, for example, by a factor of eleven when a bare or pinhole fault event occurs. This is accomplished by switching the range of the current range switch  114  coupled to the current limiter  120 . This reduces the current in an arc occurring within the high voltage electrode  25 , and also reduces the shock hazard for operating personnel. Whenever the low voltage detector operates, drive to the field effect transistors  52  and  54  is removed. Oscillation is then maintained by the bipolar junction transistors  26  and  28 , which also restart oscillation immediately after a bare wire event.  
         [0030]    The abrupt transient changes in the current of the high voltage transformer secondary  16  that occurs when a pinhole fault occurs are used to trigger the timer  112  which generates a series of pulses as a pinhole fault passes through the electrode  25 .  
         [0031]    Although the invention has been shown and described in a preferred embodiment, it should be understood that numerous modifications can be made without departing from the spirit and scope of the present invention. Accordingly, the present invention has been shown and described by way of illustration rather than limitation.