Abstract:
The invention concerns a method for detecting a failure in a three-phase alternator connected to a three-phase rectifier bridge. The method comprises the following steps — rectifying ( 70 ) the voltages generated by the three-phase alternator; — adding ( 76 ) the rectified voltages taken at the first and second phases in order to obtain a voltage sum; — subtracting ( 78 ), from said voltage sum, twice the rectified voltage taken at the third phase, in order to obtain a voltage difference; — comparing ( 80 ) the voltage difference to a high threshold and a low threshold; — detecting ( 82 ) a failure when the voltage difference is less than said low threshold or when the voltage difference is greater than said high threshold. The invention also concerns a device for detecting failures and an associated computer programme.

Description:
BACKGROUND AND BRIEF SUMMARY OF THE INVENTION 
     The invention relates to a method for detecting a failure in a three-phase alternator, and an associated failure detection device. 
     In particular, this detection method is adapted to detect an open circuit in a phase of the alternator and/or a short-circuit of a phase to ground. 
     This method may, for example, be used to detect failure of an alternator supplying a control unit of an engine, for example an engine of an aircraft. Such a control unit controls the operation of the engine according to the altitude, external pressure, external temperature, flow rate of the injected fuel, etc. In such an application, for passenger safety it is important to detect any failure of the alternator in order to quickly establish a backup power mode. The method and device for detecting failures may, however, be used in any other application where a three-phase alternator is used to supply power. 
     The failure detection is usually performed by measuring the phase shift between the three voltages delivered by the alternator. In the absence of failure, there is a 120° phase shift between each of these voltages. In the event of an open circuit or a short-circuit to ground, these phase relationships change. The phase shift is measured using voltages sampled at the diodes of the three-phase rectifier bridge, reshaped, and compared to reference levels. Logic signals representative of each phase are then analyzed by counting, directly by a programmable logic circuit or by an internal counter of the microprocessor. 
     However, the method of detecting a failure by measuring the phase shift does not detect all types of short-circuit to ground and all types of open circuit. Indeed, to protect electronic circuits from electromagnetic exposure (compliance with electromagnetic compatibility (EMC) standards—lightning), capacitors are mounted between each phase and the ground. If one phase is cut off, the corresponding input from the rectifier bridge still remains connected to the rest of the circuit via the resulting serial-parallel combinations of these capacitors. A residual voltage therefore appears at the corresponding input to the phase-shift measurement device. Under a high alternator load, this residual voltage may be higher than the normal voltage generated by the alternator when under a low load, and may be sufficient to activate the phase measurement circuit. Accordingly, this method of detecting a failure by measuring the phase shift does not allow differentiating a case of an open circuit under high load from a case with no failure when under a low load. 
     In addition, in case of a short-circuit to ground of one of the phases, common mode current flows in the current return wires and in the remaining phases, and causes a voltage drop proportional to the wire length. If the alternator and battery used in the alternator start-up stage are more than a few meters away from the load (for example, the engine control unit), the resulting voltage drop will disrupt the phase shift measurement. 
     Finally, the phase shift measurements are measurements made over time using a microprocessor or a programmable logic circuit. Such components are costly. In addition, the use of these components requires certifications which are also costly. 
     The object of the present invention is to provide a detection method and device for detecting an open circuit in one of the phases regardless of the presence of protective components which enclose the circuit and regardless of the operating conditions of the alternator. 
     Advantageously, the failure detection method and device of the invention are capable of detecting a short-circuit to ground, regardless of the length of the wiring. 
     Advantageously, the detection device can be implemented both with analog components or software components. 
     To this end, the invention relates to a method for detecting a failure of a three-phase alternator connected to a three-phase rectifier bridge; said method comprising the steps of:
         rectification of the voltages generated by the three-phase alternator in a first phase, a second phase, and a third phase,   addition of the rectified voltage sampled from the first phase, to the rectified voltage sampled from the second phase, in order to obtain a voltage sum;   subtraction, from said voltage sum, of twice the rectified voltage sampled from the third phase, in order to obtain a voltage difference;   comparison of the voltage difference to a high threshold and to a low threshold; and   detection of a failure when the voltage difference is less than said low threshold or when the voltage difference is greater than said high threshold.       

     According to some particular embodiments, the detection method comprises one or more of the following features:
         it further comprises a step of filtering the rectified voltages sampled from the first, second, and third phases, said filtering step occurring prior to said addition and subtraction steps.   it further comprises a step of attenuating the rectified voltages sampled from the first, second, and third phases, said attenuation step occurring prior to said addition and subtraction steps.       

     The invention relates to a device for detecting a failure of a three-phase alternator, said detection device comprising:
         a three-phase rectifier bridge connected to a first phase, second phase, and third phase of said three-phase alternator;   a summation and subtraction unit adapted for adding a rectified voltage sampled from the first phase to a voltage sampled from the second phase in order to obtain a voltage sum; said summation and subtraction unit being adapted for subtracting, from said voltage sum, twice a voltage sampled from said third phase, in order to obtain a voltage difference;   a window comparator adapted for comparing said voltage difference to a high threshold and to a low threshold;   a monitoring unit adapted for transmitting a failure signal when said voltage difference is less than the low threshold or when said voltage difference is greater than the high threshold.       

     According to some particular embodiments, the detection device comprises one or more of the following features:
         said summation and subtraction unit comprises an amplifier having an inverting input and a non-inverting input, the inverting input being connected to the first phase via a first resistor and to the second phase via a second resistor, the non-inverting input being connected to the third phase via a third resistor; and the value of the first resistor is equal to the value of the second resistor, and the value of the third resistor is equal to half the value of the first resistor.   said window comparator is a window comparator with hysteresis.   said window comparator with hysteresis comprises a first comparator and a second comparator each having an output; a first isolating diode being connected to the output of the first comparator and a second isolating diode being connected to the output of the second comparator.   it comprises an earth ground, a first, a second, and a third filter circuits adapted for filtering the voltages generated by the three-phase alternator; the first, second, and third filter circuits being connected between said earth ground and the first, second, and third phases respectively.   it comprises an earth ground, and first, second, and third attenuation circuits adapted for attenuating the voltages generated by the three-phase alternator; the first, second, and third attenuation circuits being connected between said earth ground and the first, second, and third phases, respectively.       

     Finally, the invention relates to a computer program comprising instructions for implementing the method mentioned above, when executed by a processor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood by reading the following description, given by way of example only and with reference to the figures in which: 
         FIG. 1  is a general circuit diagram representing a circuit supplying a load and the filter and attenuation circuits of the failure detection device according to a first embodiment of the invention; 
         FIG. 2  is a diagram representing a portion of the failure detection device according to the first embodiment of the invention; 
         FIG. 3  is a diagram representing a second embodiment of the failure detection device according to the invention; and 
         FIG. 4  is a diagram representing the steps of the detection method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , the power supply circuit  2  comprises a three-phase alternator  4 , a non-controlled three-phase rectifier bridge  6  connected to the three-phase alternator  4 , and a load  8  electrically connected to the three-phase rectifier bridge  6 . 
     The alternator  4  comprises a rotor and a stator having a first phase  12 , a second phase  14 , and a third phase  16 . 
     The three-phase rectifier bridge  6  is implemented in a conventional manner using three pairs of diodes  10 . The pairs of diodes  10  are connected in parallel. The two diodes  10  of a same pair are serially connected in the same direction. The first  12 , second  14 , and third  16  phases of the alternator are each connected to the midpoint of each pair of diodes  10 . 
     The load  8  consists of any load that can be powered by a three-phase alternator  4 , for example, but without limitation, such as an engine control unit, for example mounted in an aircraft. The current return line  20  from the load  8  is connected to an earth ground  22 . 
     The power supply circuit  2  further comprises an auxiliary power source  18 , connected in parallel to the alternator  4 —rectifier bridge  6  assembly, through a diode  19 . This auxiliary power source  18  is used during the startup phase of the three-phase alternator  4 . It consists, for example, of a battery supplying a direct current of 28 volts. 
     The failure detection device  24  according to the invention is intended to be connected to the first phase  12 , second phase  14 , and third phase  16 , at the output of the three-phase rectifier bridge  6 , at the contact points respectively denoted A, B, and C. 
     The failure detection device  24  is adapted to detect a short-circuit of a phase to ground and an open circuit in a phase. It is based on the principle of equilibrium between phases of the alternator. In the absence of a failure, aside from the possible intrinsic imbalance due to manufacturing tolerances of the alternator and of the detection device components, the voltages supplied by the three phases  12 ,  14 ,  16  of the alternator are identical. The same is therefore true for the rectified voltages V A , V B , V C  sampled at contact points A, B and C. 
     At equilibrium, each phase contributes equally to the power supplied to the load  8 . One can therefore write:
 
 V   A   =V   B   =V   C  
 
     From this, we can deduce:
 
2× V   C −( V   A   +V   B )=0  (1)
 
     Conversely, any deviation from zero of this algebraic sum indicates the presence of a failure. 
     The failure detection method according to the invention is based on this principle. It is, for example, implemented by a failure detection device  24  illustrated in  FIGS. 1 and 2 . 
     Referring to  FIG. 2 , the failure detection device  24  according to the first embodiment of the invention comprises a summation and subtraction unit  38  adapted for calculating the algebraic sum (1), a window comparator  40  having two thresholds which is adapted for comparing the result of the algebraic sum to zero, and a monitoring unit  42  adapted for transmitting a failure signal if there is a difference exceeding a given threshold. 
     The summation and subtraction unit  38  comprises an operational amplifier  44  mounted in a summing and differential assembly. In particular, the inverting input − of the operational amplifier  44  is connected to the first phase  12  via a first resistor  46 , and to the second phase  14  via a second resistor  48 . The non-inverting input + is connected to the third phase  16  via a third resistor  50  and to the earth ground  22  via a fourth resistor  52 . The third resistor  50  and fourth resistor  52  are connected serially. Lastly, a fifth resistor  54  is assembled with negative feedback between the input − and the output of the operational amplifier  44 . 
     In order to double the voltage sampled from the third phase  16 , the value of the first resistor  46  is equal to the value of the second resistor  48  and the value of the third resistor  50  is equal to half the value of the first resistor  46 . 
     The voltage sampled Vana at the output D of the summation and subtraction unit  38  is representative of the equilibrium between phases of the alternator, in other words the difference in voltage 2×V C −(V A +V B ). This voltage Vana is compared to zero, using the window comparator  40 . 
     The window comparator  40  is connected to the output D of the summation and subtraction unit  38 . It comprises first  56  and second  58  open-collector comparators. The non-inverting input + of the first comparator  56  is connected to a first voltage source  60  adapted to deliver a high threshold voltage V H , called the high threshold V H . The inverting input − of the second comparator  58  is connected to a second voltage source  62  adapted to deliver a low threshold voltage V L , called the low threshold V L . The inverting input − of the first comparator  56  and the non-inverting input + of the second comparator  58  are connected to the output of the operational amplifier  44  of the summation and subtraction unit  38 . The output of first comparator  56  and the output of the second comparator  58  are electrically connected to a third voltage source  64  via a sixth resistor  66 , called the “pull-up” resistor. 
     The voltage Vlog measured at the output E of the window comparator  40  has two logic states: 
     Vlog is equal to 1 if V L &lt;Vana&lt;V H    
     Vlog is equal to 0 if Vana&lt;V L  or if Vana&gt;V H    
     The monitoring unit  42  is connected to the output E of the window comparator  40 . It is adapted to measure the voltage Vlog at this output E and to transmit a failure signal, for example to a control unit not shown, when the voltage Vlog is zero. 
     Referring to  FIG. 1 , in cases where the three-phase alternator  4  delivers high voltages, the failure detection device  24  further includes a first attenuation circuit  26  connected between the earth ground  22  and the first phase  12  in order to attenuate the voltage generated by the alternator. This attenuation circuit  26  consists, for example, of two serially connected resistors. In the same manner and for the same reasons, second  28  and third  30  attenuation circuits may be connected between the earth ground  22  and the second  14  and third  16  phases respectively. 
     As the voltages sampled at the output of the rectifier bridge are often noisy, a first filter circuit  32  is preferably connected between the earth ground  22  and the first phase  12  in order to filter the output voltages of the rectifier bridge  6 . This filter circuit  32  consists, for example, of a capacitor connected in parallel with the leg resistor of the attenuation circuit. Similarly, second  34  and third  36  filter circuits may also be connected between the earth ground  22  and the second  14  and third  16  phases respectively. 
     Referring to  FIG. 3 , the failure detection method of the invention comprises a step  70  in which the voltages generated on each phase of the alternator  4  are rectified. 
     Then, during a step  72 , the rectified voltages of each phase are attenuated. During a step  74 , the rectified and attenuated voltages of each phase are filtered. 
     In a step  76 , the rectified voltage sampled from the first phase  12  is added to the rectified voltage sampled from the second phase  14 . Then, during a step  78 , twice the rectified voltage sampled from the third phase  16  is subtracted from the amount calculated in step  76 . 
     In the embodiment described above, these operations are carried out by the summation and subtraction unit  38 , which first adds and inverts the rectified voltage V A  of the first phase  12  sampled at contact point A to the rectified voltage V B  of the second phase  14  sampled at contact point B, to obtain an inverted voltage sum −(V A +V B ). 
     Then, the summation and subtraction unit  38  adds, to the inverted voltage sum −(V A +V B ), twice the rectified voltage V C  of the third phase sampled at contact point C in order to obtain a voltage difference 2 V C −(V A +V B ). 
     Then, during a step  80 , the window comparator  40  compares said voltage difference 2 V C −(V A +V B ) to the high threshold V H  and to the low threshold V L . 
     Finally, during a step  82 , the monitoring unit  42  measures the voltage Vlog at output E and detects a failure when said voltage difference is less than the low threshold V L  and when said voltage difference is greater than the high threshold V H . The monitoring unit  42  can then, for example, transmit a failure signal to a control device (not shown). 
     The detection device  84  according to a second embodiment of the invention is similar to the detection device  24  according to the first embodiment of the invention, except that the window comparator is replaced by a window comparator with hysteresis  90  and that two isolating diodes  86 ,  88  are added. 
     The components of the detection device according to the second embodiment which are identical or similar to the components of the detection device according to the first embodiment are identified by the same references and will not be described again. 
     Referring to  FIG. 4 , the hysteresis circuit of the window comparator includes: a seventh resistor  92  connected between the first voltage source  60  and the non-inverting input + of the first comparator  56 ; an eighth resistor  94  connected, with positive feedback, between the non-inverting input + and the output of the first comparator  56 ; and a first isolating diode  86  connected between the output of the first comparator  56  and the output E of the comparator window with hysteresis  90 . 
     In parallel, the hysteresis circuit comprises: a ninth resistor  96  connected between the output D of the summation and subtraction unit  38  and the non-inverting input + of the second comparator  58 ; a tenth resistor  98  connected, with positive feedback, between the non-inverting input + and the output of the second comparator  58 ; and a second isolating diode  88  connected between the output of the second comparator  58  and the output E of the window comparator with hysteresis  90 . The conduction direction of the first  86  and second  88  insulating diodes is from the output E to the respective output of the first  56  and second comparator  58 . Isolating diode  86  is desirable so that the switching thresholds of comparator  56  are not disturbed by state changes of comparator  58 . Similarly, isolating diode  88  is desirable so that the switching thresholds of comparator  58  are not disturbed by state changes of comparator  56 . 
     Advantageously, the detection device according to this second embodiment is more stable. 
     According to a less advantageous embodiment, the window comparator with or without hysteresis circuit is replaced with a digital device. In this case, the output voltage Vana is converted into a digital signal by an analog-to-digital converter, then compared digitally to digital thresholds. This is done either with software in a microprocessor, or with hardware in a dedicated logic circuit. 
     Alternatively, the steps of the detection method illustrated in  FIG. 3  are carried out by a computer program comprising instructions for implementing the method when executed by a processor of the programmable logic circuit or microprocessor type.