Electricity: power supply or regulation systems – Including a transformer or an inductor – Current transformer
Patent
1991-11-06
1993-04-13
Stephan, Steven L.
Electricity: power supply or regulation systems
Including a transformer or an inductor
Current transformer
323361, 323910, 324107, 336 5, 336175, 363 87, H01F 4006
Patent
active
052026213
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The subject matter of the invention is a current transformer arrangement for three-wire three-phase systems, especially to detect the actual current value for controlled DC loads powered via rectifiers.
2. Description of the Related Art
The line-commutated converter with controllable semiconductors is an important actuator in drive control. When rectifiers are used for controlled DC drives, it is practically only a three-phase mains that is available. In this case, the rectifier fulfills two tasks, that is to say the conversion of three-phase current into direct current in the case of DC operation, or the conversion of direct current into three-phase current in the case of invertor operation, as well as the amplification of output of the controllers to that of the machine. The variable to be controlled is the direct current supplied by the rectifier to the machine.
In order to measure or detect this direct current, it is possible in the case of larger installations having mandatory electrical isolation to make use of shunts having potential isolation (shunt converters), magnetic amplifiers (Kramer converters), Hall probes with and without modulator amplifiers or search coils. Given power converter feed, however, measurement is generally undertaken via an equivalent three-phase current, to be precise by means of three-phase transformers. The three-phase transformers provide electrical isolation. There is a strictly proportional relationship between the three-phase current and the direct current in a rectifier circuit. In a measurement principle, generally known in the prior art, for a three-phase bridge circuit, the three-phase current is detected via three conventional transformers which are designed for 0.1 A, 1 A or 5 A secondary nominal current. The secondary current is rectified and conducted via a load resistor, at which a proportional DC voltage can be tapped. A disadvantage here is the large outlay for transformer iron and winding copper for the three rectifiers, which are therefore heavy, bulky and expensive. This outlay is also still large in the generally known two-current transformer V circuit. The latter additionally has the disadvantage that upon demagnetization the converters influence one another, since their demagnetization conditions fluctuate. This can lead to hunting in the control.
SUMMARY OF THE INVENTION
In an arrangement which does not have these disadvantages, a single current transformer is provided in the form of a bushing transformer, the latter is arranged on the three-phase side and only two of the three phase conductors are threaded or pushed through the current transformer, in order to avoid the occurrence of a zero resultant magnetic flux the two phase conductors are threaded or pushed through the current transformer with the same defined direction and a ratio of the number of turns per unit length of 1:2 or with the same defined direction and the same number of turns per unit length but with a current amount halved in a phase conductor by a shunt or with the same numbers of turns per unit length but in mutually opposite defined directions, the double measuring voltage occurring on the secondary side as a result of the resultant double magnetic flux occurring in this case is halved by a correcting circuit, and the commands for switching the correcting circuit in and out are derived from control pulses for the rectifiers of the power control.
The achievable advantage resides in the considerable reduction in the outlay for only one current transformer, it being possible for the required, less complicated correcting electronics to be supplied along with the power supply for the controllers.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in more detail with reference to exemplary embodiments by means of drawings, wherein:
FIG. 1 shows the block diagram of a conventional one-quadrant drive having a current control circuit and a speed control circuit,
FIGS. 2, 3 and 4 show arrangements similar in principle
REFERENCES:
patent: 3668513 (1972-06-01), Tsubouchi et al.
patent: 4096539 (1978-06-01), Scaturro
patent: 4683513 (1987-07-01), Miller
patent: 4725941 (1988-02-01), Watanabe
patent: 4787023 (1988-11-01), Thomas
Siemens Aktiengesellschaft Osterreich
Stephan Steven L.
Sterrett Jeffrey
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