Abstract:
A method for detecting a fault in semiconductor components in electrical machines, e.g., three-phase generators, used in the automotive field is described. The electrical machine is equipped with multiple windings, each having a phase terminal U, V, W. The electrical machine is checked by determining the voltage in one of its phase terminals U, V, W both during operation and at rest. The phase terminals of phase terminals U, V, W of the electrical machine that are not taken into account are checked for voltage dips with respect to a potential or are checked for the output currents.

Description:
FIELD OF THE INVENTION  
       [0001]     Generators such as the three-phase generators used in motor vehicles are equipped with diodes for rectification of the output current.  
       BACKGROUND INFORMATION  
       [0002]     All three phase terminals U, V, and W are connected in the positive direction to the positive pole of the motor vehicle battery via the diodes and to ground with one diode in the reverse direction. Electronic regulators used to regulate the output voltage of three-phase generators require information about whether the generator is operating and, if so, at which rotational speed. This rotational speed information is obtained by observing one or more phase terminals of the generator.  
         [0003]     German Published Patent Application No. 43 08 769 relates to a method for monitoring and protecting high-current diodes. In the method known from German Published Patent Application No. 43 08 769, which is used in welding systems in particular, the high-current diodes are monitored by determining the temperature of the diodes by simulating the temperature of the diodes electrically by an analog method, preferably via an RC network. A comparison with a reference signal corresponding to a maximum allowed temperature of the high-current diodes is performed. First, the current flowing through the high-current diodes is detected, and then a current corresponding to the measured value is applied to a measuring line. A voltage is generated, allowing the thermal response of the high-current diode to be visualized by using an RC network. The simulated voltage is ascertained using a preselectable reference voltage; in the case when the simulated voltage exceeds the preselectable reference voltage, an error signal is generated. A system for implementing the method includes an ammeter that measures the current flowing across the high-current diode and maps a measuring signal. Furthermore, the arrangement includes a controlled current source which applies a current corresponding to the measuring signal delivered by the ammeter to a measuring line and an RC network block, which applies a voltage simulating the thermal response of the high-current diode to the measuring line. Finally, a monitoring circuit is provided and delivers a signal when the voltage applied to the measuring line exceeds a preselectable reference value.  
         [0004]     A rectifier is known from German Published Patent Application No. 100 05 183. This rectifier includes semiconductor rectifier elements for rectifying an alternating electric quantity applied to an input u, v, w of the rectifier and an output for picking up the rectified electrical quantity. A fusible cutout is connected between the rectifier elements and the output, the fusible cutout having a semiconductor chip made of the same semiconductor material as the rectifier elements. The semiconductor rectifier elements have rectifier semiconductor chips surrounded by a housing, the semiconductor chip of the fusible cutout being surrounded by a housing having essentially the same design as the housing of the semiconductor rectifier elements.  
         [0005]     If one of the diodes of a three-phase generator is defective, whether due to a short-circuit or an interruption, the generator is able to supply only a reduced output current, so an unacceptably high voltage dip may occur when a major electric load is connected to the vehicle electrical system. Electronic regulators are usually used to regulate the output voltage of a three-phase generator and require information about whether the generator is in operation and, if so, at what speed. This rotational speed information is made available by observing one or more of the phase terminals of the three-phase generator. The frequency of the alternating voltage is directly proportional to the rotational speed of the three-phase generator.  
         [0006]     All these methods and/or arrangements and devices usually have the disadvantage that the diode faults mentioned above, i.e., a short circuit and/or an interruption, is/are typically either not detected at all or are not detected adequately. Furthermore, according to the approaches outlined above, the design complexity for implementing this design in the regulator or for accommodating the sensors is usually substantial, thus greatly increasing the manufacturing cost.  
       SUMMARY OF THE INVENTION  
       [0007]     With the method proposed according to the present invention, existing diode faults may be detected by testing only one phase terminal, e.g., phase terminal V. These detected faults may either be displayed for the driver of the vehicle or stored in the fault memory of a control unit, from which they are read out via a diagnostic plug of the type used conventionally today. Via targeted shutdown of certain power consuming loads that are not absolutely necessary for basic operation of the vehicle, it is also possible to prevent the voltage from dropping below a critical level.  
         [0008]     With the method proposed according to the present invention, it is possible to detect a diode fault in advance before it results in restricted operation or charge balance problems or a loss of comfort with respect to the vehicle&#39;s electrical system due to a reduced maximum output power of the generator in the event of a diode fault.  
         [0009]     The individual phase terminals of a three-phase generator are conductively interconnected so that they conduct at a low resistance via the generator windings. No voltage is induced in these windings when the three-phase generator is at rest. Since the semiconductor components that are used, namely diodes in the present case, usually have a high resistance in the reverse direction, this yields a potential at the phase terminals which depends on the residual currents of the diodes and the resistive load at the phase terminals of the three-phase generator. During operation, the positive diode becomes conductive in the positive half-wave of a phase voltage and the negative diode becomes conductive in the negative half-wave.  
         [0010]     In the event of a diode short-circuit fault, a diode connected to the positive pole of the motor vehicle battery is short-circuited or has a low resistance when the three-phase generator is at rest, so the potential of a phase terminal is at the battery voltage level at rest and cannot be lowered significantly by a defined low resistive load at one of the phase terminals of the three-phase generator. However, if a diode connected to ground is short circuited or has a low resistance, the potential is accordingly at ground and cannot be increased significantly by applying a low resistive load to the positive pole of the motor vehicle battery. In both cases, the fault is detectable by connecting a resistive load, for example, to the positive pole of the motor vehicle battery and to ground, and the resulting voltage potentials in each case may be analyzed.  
         [0011]     During operation of the three-phase generator, however, a short circuit of a positive diode to the positive pole of the motor vehicle battery at a phase terminal V causes the potential there to no longer be differentiated from the positive pole of the motor vehicle battery. A short circuit of the negative diode to ground, however, results in a continuously applied ground potential, so that rotational speed information cannot be obtained in either case.  
         [0012]     A short circuit of a positive diode to one of the other phase terminals, e.g., U or W, results in a voltage dip at the positive pole of the motor vehicle battery during the negative half-wave of the particular phase, this branch taking up current and thus reducing the output current. A short circuit at the negative diode of the three-phase generator at one of the other phase terminals (U or W) results in a voltage dip during the positive half-wave of the particular phase because no output current is flowing in this period of time.  
         [0013]     In the case of a diode interruption at a phase terminal V, the voltage at this terminal is no longer limited by the battery or the vehicle electrical system during the positive half-wave and therefore it may exceed the battery voltage by several volts. However, if the negative diode of phase terminal V of the three-phase generator is interrupted, the voltage in the negative half-wave is no longer limited by the ground potential and may thus fall several volts below the ground potential.  
         [0014]     With all the types of faults described above, it is apparent that the phase terminal affected by the fault is unable to deliver an output current to the electrical system of a vehicle, for example. Therefore, the output voltage of the three-phase generator will be lower during this phase than in the other phases. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  shows a basic diagram of a three-phase generator.  
         [0016]      FIG. 2  shows a basic curve of the phase voltages of an electrical machine such as a three-phase generator.  
         [0017]      FIG. 3  shows the effect of a short circuit at the positive diode of phase terminal W.  
         [0018]      FIG. 4  shows the effect of a short circuit at the negative diode of the phase terminal W.  
         [0019]      FIG. 5  shows the effect of an interruption in the positive diode at the phase terminal W of the three-phase generator.  
         [0020]      FIG. 6  shows the effect of an interruption at the negative diode in phase terminal W, and  
         [0021]      FIG. 7  shows the effect of a diode fault on the generator output voltage of a three-phase generator.  
     
    
     DETAILED DESCRIPTION  
       [0022]     The diagram in  FIG. 1  shows the basic design of a three-phase generator.  
         [0023]     A three-phase generator  1  whose U winding  8 , V winding  9 , and W winding  10  are connected to a generator star point  2  has phase terminals for phases U, V, and W. Each phase U, V, W of three-phase generator  1  is connected to a line that includes positive diodes  11 ,  13 , and  15 , acting in the forward direction, and blocking diodes  12 ,  14 , and  16 , each acting in the reverse direction. These lines are connected to a positive pole  4  of a battery (potential terminal) and are also connected to ground  3 .  
         [0024]     Individual phase terminals U, V, W are interconnected so they conduct with a low resistance via generator windings  8 ,  9 , and  10 . When three-phase generator  1  is stopped, no voltage is induced in these windings  8 ,  9 , and  10 . Semiconductor components  12 ,  14 ,  16 , preferably diodes, which act in the reverse direction, are usually designed for high resistance, so that a potential that depends on the residual currents of semiconductor components  11 ,  13 , and  15  and the resistive load on phase terminals U, V, W is yielded at phase terminals U, V, W. During operation of the three-phase generator, positive diodes  11 ,  13 , and  15 , respectively, become conductive in the positive half-wave of a phase voltage, while semiconductor components  12 ,  14 , and  16 , respectively, situated in the reverse direction become conductive in the negative half-wave of a phase voltage.  
         [0025]     The diagram in  FIG. 2  shows the basic curve of the phase voltages in a three-phase generator.  
         [0026]     The curves of individual phase voltages  20 ,  21 , and  22 , respectively, are plotted as a function of time. It follows from the diagram in  FIG. 2  that a vehicle electrical system voltage  23 , identified by reference numeral  23 , that is subject to fluctuations is maintained as a function of individual voltage maxima  24  of phases U, V, and W. The voltage curve of individual phases U, V, and W of three-phase generator  1  shown in  FIG. 2  is established according to the 120° offset of generator windings  8 ,  9 , and  10 . Reference numeral  20  indicates the curve of phase voltage U in winding  9  of three-phase generator  1 , while reference numeral  21  indicates the curve of phase voltage V. Reference numeral  22  denotes the curve of phase voltage W prevailing in winding  10  of three-phase generator  1 .  
         [0027]     Individual curves  20 ,  21 ,  22  of voltages U U , U V , and U W  fluctuate between above-mentioned voltage maximum  24  and prevailing voltage minimum  25  in particular phase U, V, or W. The diagram of voltage curves according to  FIG. 2  reflects the condition in which no faults, e.g., short circuits or interruptions, occur in the lines assigned to phase terminal U, V, W in the three-phase generator diagrammed schematically in  FIG. 1  together with semiconductor components  11 ,  13 , and  15  that act in the forward direction as well as semiconductor components  12 ,  14 , and  16  acting in the reverse direction.  
         [0028]     The diagram according to  FIGS. 3 and 4  shows the effects of short circuits in semiconductor components situated in the forward direction of the three-phase generator.  
         [0029]     If one of semiconductor components  11 ,  13 , or  15 , preferably diodes, connected to positive pole  4  (B+) is short-circuited or has a low resistance, the potential of three-phase generator  1  at rest is at the battery voltage level and may be lowered only to an insignificant extent by loading with a defined low resistive load at one of phase terminals U, V, W.  
         [0030]     However, if a semiconductor component  12 ,  14 , or  16  connected to ground  3  is short-circuited or has a low resistance, the potential corresponds to the potential of ground terminal  3  accordingly and cannot be increased significantly by applying a low resistive load to positive pole  4  of the battery (B+).  
         [0031]     In both cases outlined above, i.e., in the case of a semiconductor component  11 ,  13 , or  15  connected to positive pole  4  of an motor vehicle battery or in the case of a semiconductor component  12 ,  14 , or  16  connected in the reverse direction, a short circuit in the particular semiconductor component may be detected by connecting a phase terminal to a resistive load, e.g., to positive pole  4  of an motor vehicle battery, or connecting the terminal to ground  3  and subsequently analyzing the resulting voltage potentials.  
         [0032]     Although the detection described above is suitable for the case of a stationary three-phase generator  1 , one phase terminal U, V, W, e.g., phase terminal V, may be short-circuited during operation of the three-phase generator. A short-circuit in semiconductor component  13  (phase terminal V, where semiconductor component  13  is applied to positive pole  4  of the battery) during operation of the three-phase generator causes the potential to no longer differ from the potential applied to positive pole  4  of the motor vehicle battery (B+). The short circuit of semiconductor component  14 , which is in the reverse direction, at phase terminal V to ground potential  3  results in a continuously applied ground potential. As a result, no alternating voltage prevails when the ground potential is always applied and consequently there is no frequency indicating the rotational speed of three-phase generator  1  and therefore rotational speed information is not obtainable at phase terminal V.  
         [0033]     During operation of three-phase generator  1 , two remaining phase terminals U and W are tested for the occurrence of a short circuit on a semiconductor component  11  or  15  connected in the forward direction and tested during the negative half-wave of particular phase U or W for the occurrence of a voltage dip at positive pole  4  of the battery (B+). Particular phases U and W take up current in this case and therefore reduce the output current, as indicated by the diagram in  FIG. 3 . The diagram according to  FIG. 3  shows that in a short circuit of semiconductor component  15 , phase voltage W assumes a minimum during negative half-wave  33 ; in the diagram according to  FIG. 3  this is approximately  11  volts. The peak of voltages U U  and U V  in phases U and V is indicated by reference numeral  24  and occurs at approximately 13.5 volts, while the potential applied to positive pole  4  of the motor vehicle battery (B+), i.e., the battery voltage is approximately 13 volts. The relative differences in voltage values between the individual phase terminals are of interest.  
         [0034]     The effects of a short circuit in semiconductor component  16 , which is situated in the reverse direction, are indicated by the diagram in  FIG. 4 . A short circuit in phases U and W in semiconductor component  14  or  16 , which is situated in the reverse direction, results in a voltage dip, labeled with reference numeral  42  in  FIG. 4 , during a positive half-wave  32  of particular phase U or W. An output current cannot be supplied because according to the diagram in  FIG. 4 , maximum phase voltage U, W (cf. reference numeral  40 ) is only a few volts, depending on the residual resistance of the short circuit, which is greatly below the voltage of approximately 12 or 24 volts prevailing in the electrical system of a vehicle in general.  
         [0035]     For the sake of thoroughness, it should be pointed out that the peaks of phase voltages U U  and U V  are identified by reference numeral  24  and the minima of phase voltages U U  and U V  are identified by reference numeral  25 .  
         [0036]     In the case of a fault in the form of an interruption in a semiconductor component  11 ,  12 ,  13 ,  14 ,  15 , or  16  in one of phase terminals U, V, W, reference is made to the diagrams according to  FIGS. 5 and 6 .  
         [0037]     If a semiconductor component  11 ,  13 ,  15  lying in the forward direction is interrupted, e.g., semiconductor component  15  at phase terminal V, the voltage at this terminal is no longer limited by the motor vehicle battery, i.e., the vehicle electrical system, during positive half-wave  32  and therefore the voltage applied to positive pole  4  of the motor vehicle battery may increase by several volts (as indicated by the curve of phase voltage U, W according to reference numeral  50 ). The resulting voltage overshoot is indicated by reference numeral  51 . The voltage overshoot is greatly above voltage maximum  24  of phase voltage U U  according to curve  20  of phase voltage U. However, if semiconductor component  14  of phase terminal V, which is situated in the reverse direction, is interrupted, the voltage during negative half-wave  33  is no longer limited by the potential of ground terminal  3 , so that for this case (according to voltage curve  52  shown in  FIG. 6  for phase voltage U, W) voltage U, W may fall below the ground potential by several volts, as indicated by voltage drop  53  according to the diagram in  FIG. 6 .  
         [0038]     If semiconductor components  11  and  15 , which are situated in the forward direction, as well as semiconductor components  12  and  16 , which are situated in the reverse direction, of phase terminals U and W are interrupted, phases U or W, each being subject to an interruption, also may not deliver any current to the electrical system of a vehicle.  
         [0039]     All the types of faults depicted here, whether short circuits occurring in the area of semiconductor components  11 ,  13 ,  15 , which are situated in the forward direction, or semiconductor components  12 ,  14 ,  16 , which are situated in the reverse direction, or interruptions in the semiconductor components, have in common the fact that phase terminal U or V or W affected by the particular fault is unable to deliver an output current to the vehicle electrical system. Because of this circumstance, the output voltage of the three-phase generator is lower during particular phase U, V, or W than in the other intact phases.  
         [0040]     Fault detection will now be described with reference to phase terminal V. If the directly observed phase, i.e., measured phase V, is affected, a simple analysis of the faulty behavior is possible whether three-phase generator  1  is at rest or in operation. When three-phase generator  1  is at rest, phase terminal V is loaded, e.g., by a well-defined load resistance to ground  3  or to positive pole  4  of the motor vehicle battery (B+). If there are potentials which are definitely different from the potential of ground terminal  3  or the potential of positive pole  4  of the motor vehicle battery (B+), a short circuit has presumably occurred in at least one semiconductor component  11 ,  13 ,  15  to positive pole  4  of the battery or a short circuit of a semiconductor component  12 ,  14 ,  16  to ground terminal  3 .  
         [0041]     During operation of the three-phase generator, phase voltage U V  in phase V is fixed in the case of a short circuit at the potential of positive pole  4  of the motor vehicle battery (B+) or at the potential of ground terminal  3  and thus does not allow a frequency analysis for lack of a frequency of an alternating voltage.  
         [0042]     An interruption in a semiconductor component—i.e., semiconductor component  13  or  14  with respect to phase V—results in easy-to-detect voltage curves, as indicated above, which either run clearly above the potential of positive pole  4  of an motor vehicle battery according to the diagrams in  FIGS. 5 and 6  or clearly below the potential of ground terminal  3  (cf. reference numeral  51  in  FIG. 5  and reference numeral  53  in  FIG. 6 ).  
         [0043]     When observing phase V, no output current is able to flow in remaining phases U or W during positive half-wave  32  of particular affected phase U, W, depending on the site of the fault, either in phase U or in phase W, resulting in reduced output voltages  58  and  59  as shown in  FIG. 7 .  
         [0044]     Since leading edge  54  of phase voltage U V  of phase V and trailing edge  55  of phase voltage U V  in phase V coincide with positive half-wave  32  of phase voltage curves  20  and  22 , respectively, voltages U B+1  and U B+2  are detectable at positive pole  4  of the motor vehicle battery (B+) at respective defined instants t 1  and t 2  of leading edge  54  and trailing edge  55  of voltage U V  (see reference numeral  21  in  FIG. 7 ). The detected voltages may be compared. A fault in a semiconductor element is probable when a difference between voltages U B+1 , U B+2 , detected at detection intervals t 1  ( 56 ), t 2  ( 57 ) during a plurality of successive detection instants, exceeds a defined limiting value. Voltage curve U B+1  in phase W coincides with the battery voltage level for the case when no fault in the form of failure of the semiconductor component has occurred in phase W. Voltage curve U B+2  in phase W denotes the case in which a fault has occurred in phase W. Consequently, a lower output voltage is established in phase W in comparison with an undamaged phase W:ΔU=U B+1 −U B+2 (t 2 ). The measured phase is phase V.  
         [0045]     Similarly, at instant t 1  (reference numeral  56 ) a comparison of voltages in phase U may be performed in which—as described above—phase voltages in phase U may be compared, as a result of which a fault in phase U may be assumed in a similar manner. The voltage curve in phase W is indicated with a dotted line in the diagram according to  FIG. 7 , while the voltage curve in phase W occurring at U B+1  is indicated with a solid line.  
         [0046]     The method according to the present invention for detecting faults in semiconductor components  11 ,  12 ,  13 ,  14 ,  15 ,  16  on a three-phase generator at low rotational speeds and low generator load may be performed in an advantageous manner. Ripples in three-phase generator output voltages occurring at high rotational speeds and at high loads of three-phase generator  1  influence detection of a voltage difference due to the reverse recovery performance of semiconductor components  11 ,  12 ,  13 ,  14 ,  15 , and  16 . Furthermore, external connection of the vehicle electrical system for detection of a voltage difference ΔU must be taken into account in a case when a plurality of electric loads is connected.  
         [0047]     To transmit the detected fault, a fault display may be activated. Furthermore, there is the possibility of setting a fault flag which may be read out via a (data) interface by a control unit. On the other hand, it is also possible to output a fault actively via an interface for transmission of the fault. The method proposed according to the present invention and described above offers the opportunity of detecting a fault in a semiconductor component  11 ,  12 ,  13 ,  14 ,  15 ,  16  at an early point in time even before the occurrence of any restriction in operation of the vehicle electrical system, charge balance problems with respect to the motor vehicle battery or comfort restrictions due to the reduced maximum output current of the three-phase generator.  
       LIST OF REFERENCE NUMERALS  
       [0048]      1  Three-phase generator  
         [0049]      2  Generator star point  
         [0050]      3  Ground potential terminal  
         [0051]      4  Terminal of positive pole of battery (B+)  
         [0052]      5  U phase terminal  
         [0053]      6  V phase terminal  
         [0054]      7  W phase terminal  
         [0055]      8  U winding  
         [0056]      9  V winding  
         [0057]      10  W winding  
         [0058]      11  U positive diode  
         [0059]      12  U blocking diode  
         [0060]      13  V positive diode  
         [0061]      14  V blocking diode  
         [0062]      15  W positive diode  
         [0063]      16  W blocking diode  
         [0064]      20  Phase U voltage curve  
         [0065]      21  Phase V voltage curve  
         [0066]      22  Phase W voltage curve  
         [0067]      23  Vehicle electrical system voltage  
         [0068]      24  Voltage maxima per phase  
         [0069]      25  Voltage minima per phase  
         [0070]      30  Short-circuit voltage UW (positive diode)  
         [0071]      31  Battery voltage UB+ 
         [0072]      32  Positive half-wave  
         [0073]      33  Negative half-wave  
         [0074]      40  Short-circuit voltage UW (blocking diode)  
         [0075]      41  Ground potential  
         [0076]      42  Voltage dip  
         [0077]      50  Phase voltage UW during an interruption (positive diode)  
         [0078]      51  Voltage overshoot  
         [0079]      52  Phase voltage UW during an interruption (negative diode)  
         [0080]      53  Voltage drop  
         [0081]      54  Leading edge of phase voltage UV  
         [0082]      55  Trailing edge of phase voltage UV  
         [0083]      56  First point in time (t 1 )  
         [0084]      57  Second point in time (t 2 )  
         [0085]     UB+ 1  Output voltage without fault in phase W  
         [0086]     UB+ 2  Output voltage with fault in phase W