10718-KB/14..'U6.95 -i 25 each of said first, second, third and fourth controlled rectifier elements, respectively; and current detecting means provided in association with said voltage limiting means, respectively, for detecting a current flowing through the associated voltage limiting means upon operation thereof. 3. A power control apparatus according to claim 2, wherein each of said voltage limiting means is constituted by an avalanche diode, and said current detecting means includes a current detector disposed in series to said avalanche diode for detecting an avalanche current thereof. C S4. A power control apparatus according to r. claim i, wherein said load is an AC motor. A power control apparatus according to claim i, Swherein said abnormal operation detecting means C C includes: voltage limiting means connected in antiparallel to each of said first, second, third and fourth controlled rectifier elements, respectively; and voltage detecting means provided in association with said voltage limiting means, respectively, for detecting a voltage appearing across the associated voltage limiting means upon operation thereof. 6. A power control apparatus according to claim wherein each of said voltage limiting means is constituted by an avalanche diode, and said voltage detecting means including a voltage detector connected in parallel with said avalanche diode for detecting an avalanche voltage appearing across said avalanche diode. 4d -1_11_1_- eect oo 4o o 0 4 9 4 44 4 4 4 4 C f 26 7. A power control apparatus according to claim 1, wherein each of said first, second, third and fourth controlled rectifier elements having a control terminal and an anode-cathode path extending from an anode to a cathode, the anode-cathode paths of said first to fourth controlled rectifier elements being connected in series between said DC power source and the ground, wherein upon application of a control signal from said control means to said control terminal of said controlled rectifier element, a current is caused to flow along said anode- cathode path in accordance with said control signal; and wherein each of said abnormal operation detecting means includes: a diode connected between the anode and the cathode 15 of each of said controlled rectifier elements in antiparallel therewith; a first diode connected between said control terminal and said anode of each of said controlled rectifier elements and activated to a conducting state in response to a predetermined voltage appearing between said control terminal and said anode; and a second shunt circuit connected between said control terminal and said cathode and activated to a conducting state in response to a predetermined voltage appearing between said control terminal and said cathode; and C C t C RA VC appear: ltage detecting means for detecting a voltage ing across said second shunt circuit. 8. A power control apparatus according to claim 7, said first shunt circuit including a series connection of a constant-voltage diode and a current limiting resistor; said second shunt circuit includes a second constant-voltage diode; and wherein said voltage detecting means is comprised of a voltage detector connected in parallel with said second constant voltage diode. I I A n O 27 9. A power control apparatus according to claim 1, each of said first, second, third and fourth controlled rectifier elements having a control terminal and an anode-cathode path extending from an anode to a cathode, the anode-cathode paths of said first to fourth controlled rectifier elements being connected in series between said DC power source and the ground, wherein upon application of a control signal from said control means to said control terminal of said controlled rectifier element, a current is caused to flow along said anode- cathode path in accordance with said control signal; and U wherein each of said abnormal operation detecting means includes: r a diode connected between the anode and the cathode s 15 of each of said controlled rectifier elements in t" t antiparallel therewith; t ta first shunt circuit connected between said control terminal and said anode of each of sad controlled rectifier elements and activated to a conducting state in response to a predetermined voltage appearing between said control terminal and said anode; and S. a second shunt circuit connected between said control terminal and said cathode and activated to a conducting S o' state in response to a predetermined voltage appearing between said control terminal and said cathode; and current detecting means for detecting a current flowing through said first shunt circuit upon operation thereof. A power control apparatus according to claim 9, said fi:rst shunt circuit including a series connection of a first constant-voltage diode and a current limiting resistor; said second shunt circuit includes a second constant-voltage diode; and wherein said abnormal operation detecting means is comprised of a current transducer disposed in series to Jsaid first constant- 1-ny t 28 voltage diode. C C r I 9, *l 0 91 *O V V I& 4 I 11. A power control apparatus according to claim i, each of said first, second, third and fourth controlled rectifier elements having a control terminal and an anode-cathode path extending from an anode to a cathode, the anode-cathode paths of said first to fourth controlled rectifier elements being connected in series between said DC power source and the ground, wherein upon application of a control signal to said control terminal of said controlled rectifier element, a current is caused to flow along said anode-cathode path in accordance with said control signal, wherein each of said abnormal operation detecting means includes: a diode connected between the anode and the cathode of each of said controlled rectifier elements in antiparallel therewith; a first shunt circuit connected between said control terminal and said anode of each of said controlled rectifier elements and activated to a conducting state in response to a predetermined voltage appearing between said control terminal and said anode; and a second shunt circuit connected between said control terminal and said cathode and activated to a conducting state in response to a predetermined voltage appearing between said control terminal and said cathode; and voltage detecting means for detecting a voltage ,appearing between said anode and said cathode. 12. A power control apparatus according to claim 11, said first shunt circuit including a series connection of a first constant-voltage diode and a current limiting resistor; said second shunt circuit includes a second constant-voltage diode; wherein said voltage detecting means is comprised of a voltage detector connected in parallel with each of said 44 *44 *i I I. I .O C I' II I 1s C Vt V C V C 29 controlled rectifier elements. 13. A power control apparatus for controlling an electric power supplied from a DC power source to a three-phase AC load comprising: a capacitive voltage divider circuit connected between said DC power source and the ground and having a center tap defining a neutral point; a first-phase rectifier circuitry including first, second, third and fourth controlled rectifier elements having respective anode-cathode paths connected in series between *e said DC power source and the ground in parallel with said capacitive voltage divider circuit; oa second-phase rectifier circuitry including fifth, sixth, seventh and eighth controlled rectifier elements having respective anode-cathode paths connected in series between said DC power source and the ground in parallel with said first-phase rectifier circuitry; a third-phase rectifier circuitry including ninth, C tenth, eleventh and twelfth controlled rectifier elements i Cc.C having respective anode-cathode paths connected in series between said DC power source and the ground in parallel with said second-phase rectifier circuitry; first to six clamp diodes connected between said neutral point on one hand and on the other hand a first junction between said first controlled rectifier element and said second controlled rectifier element, a second junction between said third controlled rectifier element and said fourth controlled rectifier element, a third junction between said fifth controlled rectifier element and said sixth controlled rectifier element, a fourth junction between said seventh controlled rectifier element and said eighth controlled rectifier element, a fifth junction between said ninth controlled rectifier element and said tenthcontrolled rectifier element, and a sixth junction between said eleventh controlled rectifier element and said twelfth controlled -rectifier element, respectively; 30 said three-phase AC load having first, second and third input terminals, said first terminal being connected to a junction between said second and third controlled rectifier elements, said second input terminal being connected to a junction between said sixth and seventh controlled rectifier elements, said third input terminal being connected to a junction between said tenth and eleventh controlled rectifier elements; control means for controlling conduction and non- conduction of said first to twelfth controlled rectifier elements in response to a control command signal supplied externally so that a three-phase electric power is supplied rioo o°s to said three-phase load in conformance with said control command signal; ~abnorma operation detecting means provided in association with said first to twelfth controlled rectifier elements, respectively, for detecting abnormal operation thereof to thereby generate an abnormal operation detection signal; and protecting means responsive to a detection signal .*outputted from said abnormal operation detecting means to thereby protect against breakdown the controlled rectifier element of which abnormal operation is detected. *0 14. A power control apparatus according to claim 13, wherein said abnormal operation detecting means includes: voltage limiting means connected in antiparallel to each of said first to twelfth controlled rectifier elements, respectively; and current detecting means provided in association with said voltage limiting means, respectively, for detecting a current flowing through the associated voltage limiting means upon operation thereof. A power control apparatus according to claim 14, wherein each of said voltage limiting means is 31 constituted by an avalanche diode, said current detecting means including a current detector disposed in series to said avalanche diode for detecting an avalanche current thereof. 16. A power control apparatus according to claim 13, wherein said load is an three-phase AC motor serving as a prime mover for an electric rolling stock, and said control command signal is issued from a controller of cc said electric rolling stock. t 17. A power control apparatus according to claim 13, i e wherein said abnormal operation detecting means r, includes: voltage limiting means connected in antiparallel to each of said first to twelfth controlled rectifier elements, respectively; and voltage detecting means provided in association CC with said voltage limiting means, respectively, for detecting a voltage appearing across the associated voltage limiting means upon operation thereof. 18. A power control apparatus according to claim 17, wherein each of said voltage limiting means is C constituted by an avalanche diode, said voltage detecting means including a voltage detector connected in parallel with said avalanche diode for detecting an avalanche voltage appearing across said avalanche diode. 19. A power control apparatus according to claim 13, each of said first to twelfth controlled rectifier elements having a control terminal, wherein upon application of a control signal from said first control means to said control terminals of said controlled rectifier elements, a current is caused to flow along said anode-cathode paths in 32- accordance with said control command signal, wherein each of said abnormal operation detecting means includes: a diode connected between the anode and the cathode of each of said controlled rectifier elements in antiparallel therewith; a first shunt circuit connected between said control terminal and said anode of each of said controlled rectifier elements and activated to a conducting state in response to a predetermined voltage appearing between said control terminal and said anode; and a second shunt circuit connected between said control terminal and said cathode and activated to a conducting state in response to a predetermined voltage appearing between said control terminal and said cathode; and voltage detecting means for detecting a voltage appearing across said second shunt circuit. A power control apparatus according to claim 19, said first shunt circuit including a series connection of a constant-voltage diode and a current :oi[ limiting resistor; said second shunt circuit includes a second o constant-voltage diode; wherein said abnormal operation detecting means is comprised of a voltage detector connected in parallel with said second constant-voltage diode. 21. A power control apparatus according to claim 13, C I "each of said first to twelfth controlled rectifier elements having a control terminal, wherein upon application of a control signal from said control means to said control terminal of said controlled rectifier cc C element, a current is caused to flow along said anode- cathode path in accordance with said control command signal, wherein each of said abnormal operation detecting means includes: .09.95 33 a diode connected between the anode and the cathode of each of said controlled rectifier elements in antiparallel therewith; a first shunt circuit connected between said control terminal and said anode of each of said controlled rectifier elements and activated to a conducting state in response to a predetermined voltage appearing between said control terminal and said anode; and a second shunt circuit connected between said control terminal and said cathode and activated to a conducting state in response to a predetermined vctage appearing between said control terminal and said cathode; and t 'c t current detecting means for detecting a current flowing through said first shunt circuit upon operation thereof. 22. A power control apparatus according to claim 21, said first shunt circuit including a series connection of a first constant-voltage diode and a current o limiting resistor; a second:^ x, C said second shunt circuit includes a second constant-voltage diode; wherein said abnormal operation detecting means is comprised of a current transducer disposed in series to said first constant-voltage diode. 23. A power control apparatus according to claim 13, each of said first to twelfth controlled rectifier elements having a control terminal, wherein upon application of a control signal to said control terminal of said controlled rectifier element, a current is caused to flow along said anode-cathode path in accordance with said control command signal, wherein each of said abnormal operation detecting means includes: a diode connected between the anode and the cathode I Rof each of said controlled rectifier elements in antiparallel S34 therewith; a first shunt circuit connected between said control terminal and said anode of each of said controlled rectifier elements and activated to a conducting state in response to a predetermined voltage appearing between said control terminal and said anode; and a second shunt circuit connected between said control terminal and said cathode and activated to a conducting state in response to a predetermined voltage appearing between said control terminal and said cathode; and voltage detecting means for detecting a voltage appearing between said anode and said cathode. 24. A power control apparatus according to claim 23, said first shunt circuit including a series connection of a first constant-voltage diode and a current limiting resistor; said second shunt circuit includes a second constant-voltage diode; wherein said voltage detecting means is comprised of a voltage detector connected in parallel with each of said controlled rectifier elements. An electric power control apparatus for controlling an electric power supplied to a DC load from an AC power source, comprising: a capacitive voltage divider circuit connected in parallel with said DC load and having a centre tap defining a neutral point; a series connection of first, second, third and fourth controlled rectifier elements said series connection being connected in parallel with said capacitive divider circuit; a first clamp diode connected between said neutral point and a first junction between said first controlled rectifier element and said second controlled rectifier element; a second clamp diode connected between said neutral Rpoint and a junction between said third controlled rectifier 35 element and said fourth controlled rectifier element; said AC power source being connected between said tap and a junction between said third and fourth controlled rectifier elements; control means for controlling conduction and non- conduction of said first to fourth controlled rectifier elements in response to a control command signal to thereby control electric power supplied to said DC load; voltage limiting means connected in antiparallel to each of said controlled rectifier elements; abnormal operation detecting means provided in association with said first to fourth controlled rectifier 'l elements, respectively, for detecting abnormal operation S thereof to thereby generate an abnormal operation detection 1 signal; and Sprotecting means responsive to a detection signal outputted from said abnormal operation detecting means to thereby protect against breakdown the controlled rectifier element of which abnormal operation is detected. rtj 26. An electric power control apparatus according to J claim wherein said DC load comprises an inverter to which an AC motor is connected. t 27. An electric power conversion apparatus c, substantially as herein described with reference to any one embodiment as shown in the accompanying drawings. 28. A three-phase inverter circuit substantially as herein described with reference to any one embodiment as shown in the accompanying drawings. Dated this 3rd day of August 1994 MITSUBISHI DENKI KABUSHIKI KAISHA By their Patent Attorney GRIFFITH HACK CO. ABSTRACT OF THE DISCLOSURE A three-phase inverter includes a capacitive voltage divider circuit connected between a DC power source and the ground and having a tap defining a neutral point, a series connection of first, second, third and fourth controlled rectifier elements for each phase, the series connection being connected between the DC power source and the ground in parallel with the capacitive voltage divider circuit, a first clamp diode connected between the neutral point and a first junction between the first and second controlled rectifier elements, a second clamp diode connected between the neutral point and a junction between the third and fourth controlled rectifier elements, a control circuit for controlling conduction and non-conduction of the first to fourth controlled rectifier elements in response to a control command signal to thereby control electric power supplied to the load, an abnormal operation detecting circuit provided in association with the first to fourth controlled rectifier elements, respectively, for detecting abnormal operation thereof to thereby generate an abnormal operation detection signal, and a protecting circuit which responds to a detection signal outputted from the abnormal operation detecting circuit to thereby protect the controlled relevant rectifier element against breakdown. The abnormal operation detecting circuit is constituted by a voltage detector or a current detector connected in parallel or in series to each of the controlled rectifier elements. C t S *rhr I'