Electricity: electrical systems and devices – Control circuits for electromagnetic devices – Systems for magnetizing – demagnetizing – or controlling the...
Patent
2000-03-31
2000-12-12
Sherry, Michael J.
Electricity: electrical systems and devices
Control circuits for electromagnetic devices
Systems for magnetizing, demagnetizing, or controlling the...
361146, 361149, 361267, H01H 4700
Patent
active
061606979
ABSTRACT:
A method and apparatus for establishing and maintaining a preferred induction level in a magnetic body. A varying voltage is applied to a conductive winding (61) that magnetically interacts with a magnetic body (60). The voltage is controlled during a first phase in such a way that the magnetic induction level of the magnetic body is changed from an unknown induction level to a known induction level. During a second phase, the induced voltage across the winding is controlled so that the induction level is changed from the known induction level to the preferred induction level. An optional third phase may be utilized, during which the average value of induced voltage is kept near zero in order to keep the induction level near the preferred induction level. The invention is especially applicable to ordinary current transformers, which may be demagnetized automatically while remaining in service. Once demagnetized, ordinary current transformers are able to accurately sense non-symmetrical currents, including d-c currents and a-c currents that have d-c components. A demagnetizing mode, during which the current transformer is demagnetized, and a current-sensing mode, during which current transformer secondary current is sensed, are usually utilized sequentially. For a-c power system applications, a current transformer demagnetizing circuit may include an adjustable impedance with a suitable control circuit. A controllable electric energy source may also be used as part of a current transformer demagnetizing circuit, in which case an ordinary current transformer may used to sense d-c current as well as a-c current, and the preferred embodiment has the advantage of being able to reduce the burden of the entire secondary circuit to near zero ohms for increased current-sensing accuracy.
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