Suppression of Leakage Flux in Magnetic Air Circuit Breakers

The need for increased ratings of magnetic air breakers has required critical studies of and improvements in the various factors involved in the interrupting process in these devices. One important factor, the magnetic field, has been extensively explored by means of probe coils connected to integrating circuits in such a manner that instantaneous values of useful flux, leakage flux, and flux densities in the air gap could be directly recorded and observed on magnetic oscillograms or cathode-ray screens. By these means the effects on field magnitude and distribution in improved field structures may be readily determined. The large leakage, usually more than 50% of the total flux, indicated that a substantial improvement in the useful field strength could be effected by decreasing the leakage. It was found that most of the leakage could be suppressed by the addition of low-resistance conducting bands completely surrounding each of the two magnet poles. For a particular field structure, an increase of almost 75% in the useful field density resulted from the application of leakage suppressor bands. Together with the proper design of arc chutes that take advantage of this increase in magnetic field, the use of such suppressor bands can substantially increase magnetic air breaker ratings.