Abstract:
A rotary electric machine such as a vehicle AC generator includes a multi-phase armature winding, a stator core, a rotor including a magnetic field coil and a plurality of poles, an unbalance detecting coil, and a comparator circuit for comparing an output signal of the unbalance detecting circuit with a threshold value. If the AC generator fails, a voltage induced in the unbalanced detecting coil becomes larger than the threshold value, and an alarm is be outputted.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is based on and claims priority from the following Japanese Patent Application: 2001-55352, filed Feb. 28, 2001, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a rotary electric machine, such as an AC generator, to be mounted in a passenger car or a truck or a generator-motor to be mounted in an electric vehicle or an industrial vehicle. 
     2. Description of the Related Art 
     As the rotary electric machine has become more powerful, compact and more accurate, it has unlimitedly come to use in industry and social life system. Accordingly, it has become very important for safety and life to detect a failure of the rotary electric machine so that a user can cope with the failure before an accident. For this purpose, it is necessary to provide an inexpensive, highly efficient and reliable failure detection device or system. In particular, the energy of the power system of the rotary electric machine (e.g. input side of a motor or output side of a generator) is so large that the failure may cause a terrible disaster, such as a fire. In order to prevent such a failure, the following arrangements of detecting the operation condition of the rotary electric machine have been proposed: a thermistor is inserted into a stator winding to detect abnormal temperature caused by a failure of the winding; and current transformers (CT) are respectively inserted into phase-windings of the three-phase stator winding to measure a current composite vector or difference vector so as to detect leakage of electricity or short circuit. 
     However, it is difficult to insert such a thermistor into the stator winding in an automatic winding process of the rotary electric machine. In addition, it is difficult to eliminate a big difference in reliability between the rotary electric machines. On the other hand, it becomes expensive to employ the current transformers in the rotary electric machine. 
     SUMMARY OF THE INVENTION 
     Therefore, a main object of the invention is to provide a reliable and inexpensive rotary electric machine whose failure can be surely detected. 
     According to a feature of the invention, a vehicle AC generator includes a multi-phase armature, a rotor, first means disposed in the armature for generating an output signal according to a balance condition of magnetic field of the armature winding and second means for judging whether the armature fails or not by comparing the output signal with a threshold value. Preferably, the first means includes an unbalance detection coil disposed in a portion of the armature where the unbalance detection coil is magnetically more influenced by a composite magnetic field caused by unbalance of current values of the multi-phase armature than a composite magnetic field of at least one of the rotor and the armature. The second means may be comprised of an alarm circuit for outputting alarm if the output signal of the first means becomes larger than the threshold value. 
     It is also preferable that the unbalance detection coil has a winding pitch that is equal to the quotient of the pole pitch divided by an odd number and is an even number of times as long as the winding pitch. The unbalance detection coil may be disposed in parallel with the stator core along coil ends of the armature winding. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and characteristics of the present invention as well as the functions of related parts of the present invention will become clear from a study of the following detailed description, the appended claims and the drawings. In the drawings: 
     FIG. 1 is a cross-sectional side view of a vehicle AC generator according to a first embodiment of the invention; 
     FIG. 2 is a circuit diagram of the vehicle AC generator shown in FIG. 1; 
     FIG. 3 is a fragmentary schematic view of an armature with an unbalance detection coil and a rotor; 
     FIG. 4 is a fragmentary schematic perspective view of the stator with the unbalance detection coil; 
     FIG. 5 is a graph comparing output voltage of the unbalance detection coil in normal operation of the vehicle AC generator with the output voltage of the unbalance detection coil in failure thereof; 
     FIG. 6 is a schematic fragmentary diagram of a vehicle AC generator according to a second embodiment of the invention; 
     FIG. 7 is a perspective view of a stator of the vehicle AC generator according to the second embodiment; and 
     FIG. 8 is a graph comparing output voltage of an unbalance detection coil in normal operation of the vehicle AC generator with the output voltage of the unbalance detection coil in failure operation thereof. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A vehicle AC generator  1  according to a first embodiment of the invention is described with reference to FIGS. 1-5. 
     As shown in FIGS. 1 and 2, the vehicle AC generator  1  includes a rotor  2 , an armature  3 , a frame  4 , a brush unit  5  and a rectifier unit  7 . 
     The rotor  2  is comprised of a cylindrically wound field coil  21 , a rotary shaft  22 , a pair of pole cores  23  and  24  and a pair of slip rings  25  and  26  fixed to a portion of the shaft away from a pulley. The field coil  21  is supplied with exciting current via the pair of slip rings  25  and  26 . The pair of pole cores  23  and  24  is fixed to the rotary shaft  22  and encloses the field coil  21  along the rotary shaft  22 . 
     The armature  3  is comprised of a stator core  31  having a plurality of slots and a three-phase armature winding  32  that is mounted in the plurality of slots. The armature winding  32  is comprised of X phase-winding, Y phase-winding and Z phase-winding, which have one ends being connected together to form a neutral point. 
     The armature  3  includes a unbalance detection coil  33  besides the armature winding  32 . The unbalance detection coil  33  is a coil of several-turn (e.g.  10 ) that is inserted zigzag into every one of the slots of the stator core  31 , as shown in FIGS. 3 and 4. The armature winding  32  is comprises of a plurality of coils inserted zigzag into every third slot. That is, a pole pitch corresponds to three slot-pitches. 
     When the vehicle AC generator  1  operate normally, three-phase AC voltage having symmetric wave forms is generated. Accordingly, the composite vector of basic magnetic flux that interlinks the unbalance detection coil  33  is approximately zero. Therefore, no electromotive force appears at an output terminal  34  of the unbalance detection coil  33  except for voltage caused by the third high-frequency component having a half wave-length of 60° that corresponds to a third of the pole pitch. On the other hand, if a failure occurs, the composite vector of the basic magnetic flux interlinking the unbalance detection coil  33  is not balanced, and a significantly large electromotive force appears at the output terminal  34 . 
     In the meanwhile, the unbalance detection coil  33  has a winding pitch that is equal to the quotient of the pole pitch (in this case 3 slot-pitches) divided by an odd number (e.g. 3) and is an even number of times as long as the winding pitch. Thus, the composite vector becomes zero because of a completely symmetric magnetic flux wave at normal operation, while the composite vector appears if a failure occurs. 
     The frame  4  accommodates and supports the rotor  2  and the armature  3  so that the rotor  2  can rotate about the rotary shaft  21  within the armature  3 . The brush unit  5  includes a pair of brushes  51  and  52  in contact with the pair of slip rings  25  and  26  through which the exciting current is supplied to the field coil  21 . 
     The rectifier unit  6  is comprised of a three-phase full-wave rectifying circuit that converts AC power induced in the armature winding  32  into DC power. The rectifier unit  6  includes three negative-side diodes having anodes connected together and three positive-side diodes having cathodes connected together. The cathodes of the positive-side diodes are connected to output terminals  61 . 
     The voltage regulator  7  regulates the output voltage of the vehicle AC generator  1  by controlling the exciting current supplied to the field coil  21 . The voltage regulator  7  includes an alarm circuit  71 . The alarm circuit  71  includes a voltage level detection circuit  72  that compares a DC voltage level, which is a rectified voltage of the output terminal  34  of the unbalance detection coil  33 , with a threshold voltage. If the DC voltage level is higher than the threshold voltage, the alarm circuit  71  outputs an alarm signal. 
     As shown in FIG. 5, the unbalance detection coil  33  generates the output voltage of 7 volts at 6000 rpm when the vehicle AC generator  1  is in the normal condition, while it generates 10 volts if it is in failure. Thus, a failure of the vehicle AC generator  1  can be detected easily if the threshold level is set to a value between 7 and 10 (e.g. 8.5 volts). Because the rotation speed of the vehicle AC generator  1  varies widely as the rotation speed of the vehicle engine varies, it is preferable to provide a plurality of threshold values that correspond to a plurality of the rotation speeds of vehicle AC generator  1 . 
     A vehicle AC generator  1   a  according to a second embodiment of the invention is described with reference to FIGS. 6-8. In the meantime, the same reference numeral corresponds to the same or substantially the same part, portion or component as the vehicle AC generator  1  according to the first embodiment. 
     An unbalance detection coil  33   a  is disposed near the stator core  31  of an armature  3   a  and the rotor  2  and wound along the coil ends of the armature winding to be in parallel to the stator core  31  and perpendicular to the rotary shaft of the rotor  2 . Therefore, the unbalance detection coil  33   a  is disposed under a balanced rotating magnetic field when the vehicle AC generator  1   a  is in a normal operation condition. In this condition, no voltage due to high frequency components is induced in the unbalance detection coil  33   a . If a failure occurs in the vehicle AC generator  1   a , an unbalance armature reaction is caused by the armature  3   a . Accordingly, an unbalance condition is caused by a difference between the magnetic field of the rotor  2  and the armature reaction, and an output voltage due to the unbalance condition is induced in the unbalance detection coil  33   a , as shown in FIG.  8 . 
     As shown in FIG. 8, the output voltage of the unbalance detection coil  33   a  is 0.3 V whenever the vehicle AC generator  1   a  operates normally, while the output voltage increases as the rotation speed increases if it fails. For example, the output voltage is 3V at 6000 rpm when it operates in a failure condition. Therefore, the SN ratio of the unbalance detection coil  33   a  is 10 (3/0.3), so that the failure can be detected easily if the threshold level of the voltage level detection circuit  72  is set to a value between 0.3 v and 3 V (e.g. 1.5 V). 
     The unbalance detection coil  33  or  33   a  can be disposed at any portion of the vehicle AC generator  1  or  1   a  if it is magnetically more influenced by a composite magnetic field caused by unbalance of current values of multi-phase windings than a composite magnetic field of at least one of the rotor  2  and the armature  3 . For example, the unbalance detection coil  33   a  can be disposed along the opposite side of the armature winding shown in FIG.  6 . 
     In the foregoing description of the present invention, the invention has been disclosed with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made to the specific embodiments of the present invention without departing from the scope of the invention as set forth in the appended claims. Accordingly, the description of the present invention is to be regarded in an illustrative, rather than a restrictive, sense.