Apparatus and process for deriving an AC voltage from a DC voltage including detecting direct current magnetic flux deflections of an electrical transformer

Abstract: An apparatus for detecting direct current magnetic flux deflection of an electrical transformer(10) comprising a magnetic detector (4) for generating an output voltage waveform, an integrater (12) connected to the magnetic detector (4) for integrating the output voltage waveform from the magnetic detector. Positive and negative peak value detectors (13a and 13b) are connected to the integrater for detecting positive and negative peak values of the output voltage waveform integrated by the integrater. A comparator (14) connected to the positive and the negative peak value detectors compares the positive and the negative peak values to provide a peak difference. A process for detecting the d.c. magnetic flux deflection is also disclosed.

Description: BACKGROUND OF THE INVENTION This invention relates to an apparatus and a process for deriving an AC voltage from a DC voltage including detecting a direct current magnetic flux deflection of an electrical transformer according to Claim 1 and Claim 4 respectively. The invention sets out to require minimum space and generate no loss or noise. Since the a.c. output voltage from an inverter is generally low, it is necessary to connect the output voltage to the primary coil of an electrical transformer to boost the a.c. voltage. However, with an inverter which utilizes transistors or the like, the a.c. output voltage may include a d.c. component due to the deviation of the voltage drop in the respective arms of the inverter circuit as well as the deviation of the conduction periods of the transistors in the inverter. When such the d.c. component is included in the a.c. output voltage from the inverter, an electrical transformer, which is connected as a load to the output terminal of the main circuit of the inverter, generates a d.c. magnetic flux deflection and the characteristics of the transformer is degraded because of the increase in iron loss, excitation current and noise. Therefore, the d.c. magnetic flux deflection in an electrical transformer must be detected and prevented. Japanese Patent Application No. 62-28353 discloses one example of a conventional apparatus and process for detecting d.c. magnetic flux deflection in an electrical transformer. According to this apparatus, electrical current detecting means in which a saturable reactor for detecting an electric current is connected in parallel to the main circuit of the inverter, and polarity determining means for determining by a photo-coupler the polarity of the current in the reactor detected by the current detecting means are used. However, with the conventional apparatus as above discussed, extra space for installing an additional bulky device, which is the saturable reactor, is necessary and the saturable reactor itself generates iron loss and noise. US 4 255 705 shows a watt hour meter which includes a compensation device which is capable of compensating for a DC component in a transformer core susceptible to DC magnetisation by a DC current superimposed on an alternating current being measured by use of the transformer. Unfortunately the compensation device is intrusive and must be connected into the circuitry of the meter in order to work, furthermore, the compensation device is susceptible to damage by excess voltage upon opening of the secondary side of the transformer. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an apparatus for deriving an AC voltage from a DC voltage including detecting direct current magnetic flux deflection of an electrical transformer which is free from the above-discussed problems. Another object of the present invention is to provide an apparatus for deriving an AC voltage from a DC voltage including detecting direct current magnetic flux deflection of an electrical transformer in which no extra space is necessary and no iron loss or noise are generated. A still another object of the present invention is to provide a process for deriving an AC voltage from a DC voltage including detecting direct current magnetic flux deflection of an electrical transformer which is free from the above-discussed problems. Another object of the present invention is to provide a process for deriving an AC voltage from a DC voltage including detecting direct current magnetic flux deflection of an electrical transformer in which no extra space is necessary and no iron loss cr noise are generated. With the above objects in view, the apparatus for deriving an AC voltage from a DC voltage including detecting direct current magnetic flux deflection of an electrical transformer according to the present invention comprises the features set out in accompanying Claim 1. A process for deriving an AC voltage from a DC voltage comprises the steps set out in accompanying Claim 4. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more readily apparent from the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic front view of a shell-type electrical transformer in which the apparatus for detecting d.c. magnetic flux deflection of the present invention is applied; Fig. 2 is a schematic side view of the transformer shown in Fig. 1; Fig. 3 is a horizontal sectional view taken along line III - III of Fig. 1; Fig. 4 is a block diagram of a direct current magnetic flux deflection detection circuit used in one embodiment of the present invention in which a search coil is used; Fig. 5 is a graph showing the waveforms of the magnetic field and the magnetic flux as plotted against time obtained from the direct current magnetic flux deflection detection circuit shown in Fig. 4 and where no direct current magnetic flux deflection is generated; Fig. 6 is a graph showing the waveform of the magnetic flux as plotted against the magnetic field obtained from the direct current magnetic flux deflection detection circuit shown in Fig. 4 and where no direct current magnetic flux deflection is generated; Fig. 7 is a graph similar to Fig. 5 but where a direct current magnetic flux deflection is generated; Fig. 8 is a graph similar to Fig. 6 but where a direct current magnetic flux deflection is generated; Fig. 9 is a sectional view of a core-type electrical transformer to which the present invention is applied; and Fig. 10 is a block diagram similar to Fig. 4 but illustrating another embodiment in which a Hall element is used as the magnetic detector. Throughout the drawings, the same reference numerals designate identical or corresponding components. DESCRIPTION OF THE PREFERRED EMBODIMENTS In Figs. 1 to 3, an electrical transformer 10 of the shell-type to which the apparatus for detecting magnetic flux deflection can be applied is illustrated. The transformer 10 comprises two substantially rectangular iron cores 1 supported by and contained within a tank 5. The iron cores 1 define magnetic circuits of the electrical transformer 10, and primary low voltage coils 3 and a secondary high voltage coil 2 are wound on the iron cores 1. The secondary coil 2 is connected to electrical bushings 6a and 6b of different size mounted on the tank 5 for external connection. It is seen that a magnetic detector 4 of the direct current magnetic flux deflection detecting apparatus of the present embodiment is disposed between two iron cores 1. The magnetic detector 4 is electrically connected by a lead 8 to output terminals 7 mounted on the tank 5 so that the output voltage from the magnetic detector 4 can be supplied to the external circuit which will be described later. As shown in Fig. 3, in an unloaded excitation condition in which no load is connected to the high voltage coil 2, when an excitation current i L flows through the respective low voltage coils 3, a magnetic flux  L is generated in the respective iron cores 1 by this excitation current i L and a magnetic field H L corresponding to the magnetization force of the iron cores 1 is generated. According to the present invention, a magnetic detector 4 is disposed in the vicinity of the iron cores 1. In the embodiment shown in Fig. 3, the magnetic detector 4 is a search coil. While the magnetic detector 4 shown in Figs. 1 to 3 is located substantially at the center of the iron cores 1, it may be positioned in any other position to the iron cores 1 so that the magnetic field H L can be detected by the magnetic detector 4. In Fig. 4 in which a block diagram of the apparatus for deriving an AC voltage from a DC voltage including d.c. magnetic flux deflection detection apparatus of the present invention is illustrated, it is seen that the apparatus comprises an electrical transformer 10 connected to an inverter 11. The inverter 11 is connected to the low-voltage primary coil 3 of the electrical transformer 10, and the high-voltage secondary coil 2 can be connected to a desired load (not shown) through the bushings 6a and 6b mounted on the tank 5. The magnetic detector 4 in the form of a search coil is disposed within the tank 5 and connected through the output terminal 7 to an integrator 12. The integrator 12 is connected at its output to a positive peak value detector 13a as well as to a negative peak value detector 13b. The positive and the negative peak value detectors 13a and 13b are in parallel to each other. The respective outputs of the positive and the negative peak value detectors 13a and 13b are connected to a comparator 14 which in turn is connected to an output terminal 15. When the transformer 10 is in operation, an output voltage E [V] is obtained at the output terminals 7 of the search coil 4. Since the output voltage E [V] from the search coil 4 represents a magnetic field H in the space, in which the search coil 4 is disposed, differentiated in terms of time, it can be expressed by: E = N · S · µdH/dt where, N: number of turns of the search coil S: cross-sectional area of the search coil [m 2 ] µ: permeability of the search coil core [H/m] H: magnetic field [A/m] This output voltage E is supplied to the integrator 12 where it is integrated to provide an integrated output voltage E H . Since this output voltage E H from the integrater 12 is an amount of the output voltage E from the search coil 4 integrated in terms of time, it can be expressed by: E H = A ∫ Edt Where, A: integration constant of the Integrater. By substituting equation (1): E H = (A/N · S · µ)H This output voltage E H thus integrated is supplied to the positive peak value detector 13a on one hand and to the negative peak value detector 13b on the other hand. In the positive peak value detector 13a, a positive peak value H +P of the integrated output voltage E H is detected, and a negative peak value E -P of the output voltage E H is detected by the negative peak value detector 13b. These detected positive and the negative peak values H +P and H -P are supplied to the comparator 14, where a peak differential ΔH which is expressed by: ΔH = H +P - H -P is generated and supplied to the output terminal 15. When there is no direct current magnetic flux deflection generated in the electrical transformer 10, since the magnetization force of the iron core 1 is symmetric in the positive and the negative directions, the positive peak value H +P and the negative peak value H -P of the magnetic field H are in symmetry as shown in Figs. 5 and 6. In the graph of Fig. 5, waveforms of the magnetic field and the magnetic flux as plotted against time obtained from the direct current magnetic flux deflection detection circuit shown in Fig. 4 and when no direct current magnetic flux deflection is generated in the transformer are shown. The graph of Fig. 6 shows waveform of the magnetic flux as plotted against the magnetic field obtained from the direct current magnetic flux deflection detection circuit shown in Fig. 4 and when no direct current magnetic flux deflection is generated. Therefore, when no direct current magnetic flux deflection is generated, the peak difference ΔH obtained from the output terminal 15 is zero. When a direct current magnetic flux deflection is generated in the electrical transformer 10, since the magnetization force of the iron cores 1 becomes asymmetrical in the positive and the negative directions, the positive and negative peak values H +P and H -P of the magnetic field H become asymmetrical as shown in Figs. 7 and 8, resulting in, for example, a smaller negative peak value H -P . Therefore, the generation of the direct current magentic flux deflection by the peak difference ΔH supplied from the comparator 14 can be detected. The electrical transformer to which the present invention is applied to the shell type transformer 10 as shown in Figs. 1 to 3, or of the core type transformer 10A as shown in Fig. 9, providing similar advantageous results. While the search coil 4 is used as the magnetic detector in the above-described embodiments, a Hall element shown in Fig. 10 may equally be used. In this case, while an additional electric power source is necessary for driving the Hall element, a direct current magnetic flux deflection detection circuit similar to that shown in Fig. 4 may be utilized. As has been described, in an apparatus and a process for deriving an AC voltage from a DC voltage according to the present invention, a direct current magnetic flux deflection is detected by utilizing the fact that the magnetization force of the iron core becomes asymmetric when a direct current magnetic flux deflection is generated by providing a magnetic detector in the vicinity of an iron core of the electrical transformer, integrating an output voltage from the magnetic detector, and by comparing a positive peak value and a negative peak value of a waveform of the integrated output voltage, providing a process for detecting a d.c. magnetic flux deflection of an electric transformer in which no extra space is necessary and no iron loss or noise are generated.

Claims: An apparatus for deriving an AC voltage from a DC voltage, the apparatus comprising: an inverter (11) for converting the DC voltage to an AC voltage; an electrical transformer (10) comprising a primary winding (3), a secondary winding (2) and a core (1) around which the primary and secondary winding are wound for providing a magnetic circuit path; means for connecting the AC voltage from the inverter to the primary winding (3) of the transformer; and means for detecting direct current magnetic flux deflection of the transformer (10), the means for detecting direct current magnetic flux deflection comprising: a magnetic detector (4) separate from the magnetic circuit path for detecting the magnetic flux deflection of the transformer and for generating an output voltage waveform in response to the magnetic flux deflection; an integrator (12) connected to said magnetic detector (4) for integrating said output voltage waveform from said magnetic field detector; a positive peak value detector (13a) connected to said integrator (12) for detecting positive peak values of the output voltage waveform integrated by said integrator; a negative peak value detector (13b) connected in series to said integrator (12) and in parallel to said positive peak value detector (13a) for detecting negative peak values of the output voltage waveform integrated by said integrator; and a comparator (14) connected to said positive and said negative peak value detectors (13a, 13b) for comparing the positive and negative peak values from said peak value detectors (13a, 13b) to provide peak difference values indicative of the direct current magnetic flux deflection of said transformer. An apparatus (10) for deriving an AC voltage from a DC voltage including detecting direct current magnetic flux deflection of an electrical transformer as claimed in Claim 1, wherein said magnetic detector comprises a search coil (4). An apparatus (10) for deriving an AC voltage from a DC voltage including detecting direct current magnetic flux deflection of an electrical transformer as claimed in Claim 1, wherein said magnetic detector comprises a Hall element (16). A process for deriving an AC voltage from a DC voltage, the process comprising: inverting the DC voltage to an AC voltage; applying the inverted AC voltage to a primary winding (3) of a transformer (10) which transformer further comprises a secondary winding (2) and a core (1) around which the primary winding and secondary winding are wound for providing a magnetic circuit path; and detecting direct current magnetic flux deflection of the transformer, the process for detecting direct current magnetic flux deflection comprising: detecting a magnetic flux deflection of the transformer and generating an output voltage waveform in response to the magnetic flux deflection using a magnetic field detector which magnetic field detector is separate from the magnetic circuit path; integrating said output voltage waveform from said magnetic field detector (4); detecting positive peak values of the output voltage waveform integrated by said integrator; detecting negative peak values of the output voltage waveform integrated by said integrator, comparing the positive and negative peak values from said peak detecting steps to provide peak difference values indicative of the direct current magnetic flux deflection of said transformer.

Inventors: KOUAN TOYOKAZU, SHIONO KATSUMI, KOUAN, TOYOKAZU, SHIONO, KATSUMI, KOUAN, TOYOKAZU C/O AKO SEISAKUSHO, SHIONO, KATSUMI C/O AKO SEISAKUSHO
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1988
Main classification: G01R15
IPC codes: H02H7, G01R15, G01R33