Air-break magnetic blow-outs: For contactors and circuit breakers both A-C. and D-C.

Abstract

Magnetic blow-outs have been used in contactors, circuit breakers and controllers for many years for rupturing both a-c. and d-c. power circuits, but their commercial use, particularly on alternating current has been largely confined to relatively low voltages. Oil circuit breakers and switches have been generally used for rupturing high-voltage a-c. power circuits, and their development has reached a high state of perfection. The air break has the advantage of avoiding the possibilities which attend the use of any inflammable material — like oil, with its possible gasification and explosion on heavy short circuits. While there are many different types of magnetic blow-outs this paper deals largely with the “individual” type, in which a blow-out coil is connected in series with each pair of current-rupturing contacts, since it is with this type that most of the progress and studies have been made in recent years. Contactors and circuit breakers with the “individual” type of blow-out are now used almost exclusively in the main d-c. power circuits of (he 1500 and 3000-volt d-c. railway systems. Oil circuit breakers have been tried for this service, but they are rather unsatisfactory because there is no periodic zero point in the current wave at which the oil can form an insulating seal between contacts. The oil under d-c. arc conditions carbonizes rapidly and involves the possible danger from explosive gases. Recently the use of magnetic blow-out contactors on a-c. circuits has been extended to moderately high voltage and capacity. Short-circuit tests on a 6600-volt, 26,700-kv-a. alternator are described towards the end of the paper. During these three-phase tests the air-break magnetic blow-out contactors successfully ruptured 17,500 amperes, the full short-circuit current, at 5500 volts. This is 170,000 kv-a., three-phase. The maximum asymmetrical peak current through the contacts during this test was 67,500 amperes, but during a 2500-volt short-circuit test this peak current reached 80,000 amperes. Oscillograph records of the voltage and current in each phase are shown and also illustrations of the arcs. The contactors used were rated at 5000 volts, 3000 amperes, but they successfully ruptured a circuit of 9000 volts, 3500 amperes. The oscillographic records and illustrations of this test are shown in Figs. 25 and 26. Current-rupturing tests at 2300 amperes and 3500 amperes normal voltage are also shown for comparison in Figs. 22 to 24. In all of the tests the circuit was ruptured within the first half cycle after the tips started to part, indicating the effectiveness of this type of blow-out. The arrangement of the current-carrying and magnetic blow-out parts are shown in Fig. 15. The main current is carried through solid copper contacts mounted at the back. The auxiliary contacts in the arc chute and the blow-out coils carry current only during the time the circuit is being ruptured. These coils with their attending arcing horns are cut into the circuit in succession, so as to obtain the strongest possible final magnetic field without undue arcing at the contact tips and across the terminals of the coils when they are introduced into the circuit. Several arc suppressor plates are provided in each half of the arc chute which increases the cooling surface, and on heavy short circuits split the arc into a number of multiple paths. See Fig. 10. A brief description is given in the first part of the paper of a typical form of the “individual” type magnetic blow-out as used in contactors and circuit breakers, and photographs of a number of contactors for various a-c. and d-c. voltages are reproduced. Attention is directed to the tests with accompanying illustrations of successive positions of the arc in the chute taken with a high-speed camera, from which some interesting data were obtained on arc characteristics. The arc was photographed in its movement every three-thousands of a second. The time between pictures in the familiar motion picture camera is sixty-two-thousandths of a second. These tests bring out the effectiveness, in rupturing the circuit of the arc suppressor plates and narrow arc chutes.