Electricity: power supply or regulation systems – Output level responsive – Phase controlled switching using electronic tube or a three...
Patent
1992-10-30
1995-12-26
Wong, Peter S.
Electricity: power supply or regulation systems
Output level responsive
Phase controlled switching using electronic tube or a three...
323908, G05F 1455
Patent
active
054790867
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method of reducing the inrush current when energizing an inductive load having a magnetizable core from an a.c. power grid with an at least occasional phase segment.
When energizing inductive loads, a magnetic remanence or remanent induction remains in the magnetizable core of such loads after being turned off. Depending upon the polarity of the a.c. voltage at the turning-off moment, the magnetic remanence or remanent induction can have different polarities and also different magnitudes.
When switching on inductive loads, the state of their remanence is generally unknown. A large inrush current arises especially, for example, in transformers with high induction and small air gaps, particularly also in toroidal transformers, upon coincidence of a switching-on point which, relative to the form of the a.c. voltage, is "unmatched" to the state of the remanence. Under certain circumstances, the inrush current can be up to fifty times the rated current and results in tripping of the protective elements. These large currents arise because the magnetization is here driven well into saturation.
On these grounds one is forced to reduce the induction to such an extent that the inrush current does not exceed a predetermined threshold value even in an unfavorable situation. However, for the same rated output, this requires substantially more iron for the core of, for instance, a transformer, and makes this correspondingly large and heavy. This is very disadvantageous, particularly for transformers of high output.
In the German patent 27 46 845 known from the prior art, the angle of current flow is gradually increased during a switching-on phase in order to effect a smooth start-up. A further problem solution resides in that a full through-connection of the a.c. switch is to be achieved after dimming from high.
It is disadvantageous here that, when the remanence at standstill and the first generated voltage signal have the same polarity, the induction of the transformer during dimming nevertheless increasingly shifts into saturation. Thus, a peak inrush current adds up over consecutive, segmented half-waves, particularly because of asymmetries in the segments of the positive and negative grid half-waves. The existing remanent polarity of, for example, a transformer, cannot be taken into account with this device so that peak inrush currents can occur here.
Peak inrush currents in an inductive load also cannot be prevented with the measures of the German patent 35 13 365 which is concerned with current limitation when switching on lamps. A similar circuit arrangement is known from the ELV Journal 45, pages 1 to 4. Here, a relatively small segment angle is initially present at the switching-on moment, i.e., a phase segment angle of 180 degrees and a correspondingly low start-up output, and this phase segment angle then shifts to 0 degrees, which represents the full output, in a typical time of 0.5 to 2 seconds after the zero intercept of each grid half-wave.
Moreover, it is known from the German Auslegeschrift 27 43 65 to switch on a transformer at a phase angle of 90 degrees or 270 degrees. However, the phase angle when switching off, and the polarity and magnitude of the respective remanence, are not considered.
The German Offenlegungsschrift 24 24 716 already shows a possibility for preventing or reducing the inrush current in transformers. To this end, the magnitude and phase of the actual remanence are measured and switching-on performed in the phase position with which the smallest inrush current is produced. To achieve this, measuring probes must be installed at one or more locations of the casing of the transformer core. However, this must already be taken into account during construction of the transformers so that this method cannot practically be used for existing, random transformers or the like.
The U.S. Pat. No. 3,925,688 and German Offenlegungsschrift 27 35 736 shows a.c. switching arrangements which cause switching-on at the zero intercept at the beginning of an a.c. voltage peri
REFERENCES:
patent: 3925688 (1975-12-01), Kalfus
patent: 4131927 (1978-12-01), Tsuchiya
patent: 4806838 (1989-02-01), Weber
Berhane Adolf
Fraunhofer -Gesellschaft Zur Forderung der Angewandten Forschung
Wong Peter S.
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