Electronic watthour meter

Abstract: A first Hall element (18) is supplied with a current proportional to a load voltage and applied with a magnetic field proportional to a load current and produces a voltage proportional to a load power given by the load voltage and the load current. A second Hall element (21) has the same characteristic as that of the first Hall element and supplied with the current proportional to the load voltage and no magnetic field. The output voltages of the first and the second Hall elements are combined by combining means (23) to compensate and remove an offset voltage component contained in the output voltage of the first Hall element.

Description: The present invention relates to an electronic watthour meter comprising a first magneto-electric conversion element to which a current proportional to a load voltage and a magnetic field proportional to a load current are supplied for producing a voltage proportional to the load power given by the load voltage and the load current. Such a watthour meter is described in EP-A-0 322 802 and contains a multiplication-adding circuit with a current output, a current-frequency converter comprising a compensation circuit to compensate the ripple, offset and charge injections of electronic switches, and means that remove an offset voltage component contained in the output of the magneto-electric conversion element. Here, the directions of current supplied to the respective Hall-elements of the multiplication adding circuit are reversed and then the outputs of the respective Hall-elements corresponding to the directions of current are added without reversing the direction of magnetic field, so that the output of the multiplication adding circuit is fed to the current frequency converter. The outputs of the current frequency converter are fed back through the compensation circuit and used for the compensation of an offset component. A further conventional electronic watthour meter is disclosed, for example, in Japanese Patent Publications Nos. 59-53508/1984 corresponding to JP-A-53 115 157 and 60-54704/1985 corresponding to JP-A-54 000 548. This electronic watthour meter comprises an auxiliary voltage transformer for producing a voltage output signal proportional to a load voltage of a distribution line, an auxiliary current transformer for producing a current output signal proportional to a consumed current, a power-voltage conversion circuit for multiplying the voltage output signal and the current output signal from the auxiliary voltage transformer and the auxiliary current transformer to produce an output voltage proportional to a load power, a voltage-frequency conversion circuit supplied from the power-voltage conversion circuit with the output voltage proportional to the load power for integrating the output voltage to produce a pulse signal, a frequency dividing circuit for weighting the pulse signal from the voltage-frequency conversion circuit to produce a divided pulse signal proportional to electric power, and a display circuit responsive to the divided pulse signal from the frequency dividing circuit for displaying the amount of electric power consumption represented by the divided pulse signal. Fig. 1 shows a structure of the power-voltage conversion circuit including a Hall element (Hall-effect element) as a magneto-electric conversion element. Referring to Fig. 1, an auxiliary voltage transformer 1 produces a voltage output signal proportional to a load voltage. The voltage output signal from the auxiliary voltage transformer 1 is converted through a resistor 2 into a current i which is supplied to a Hall element 3. A consumed current supplied to an auxiliary current transformer flows through a primary conductor 5 wound around a core 4 to produce a magnetic field proportional to the consumed current. The magnetic field is applied to the Hall element 3. Supplied with the current i and the magnetic field having a field intensity B, the Hall element 3 produces, by a Hall effect, a voltage Vop proportional to electric power corresponding to a product of the current i and the field intensity B, as represented by the following formula: Vop = kiB, where k represents a proportional constant. The voltage Vop produced by the Hall element 3 has a low level. As shown in Fig. 1, the voltage Vop is amplified by an amplifier circuit 6 and integrated by an integrator circuit 7 of the voltage-frequency conversion circuit to be produced as a voltage signal having an appropriate level proportional to the electric power. Generally, the Hall element 3 produces an offset voltage even when no input is supplied, namely, when no magnetic field is applied. This results in deterioration of a linear characteristic of the Hall element 3. In particular, considerable deterioration is observed in case of a small load. In view of the above, it is an object of this invention to provide an electronic watthour meter which is capable of removing an offset voltage produced by a magneto-electric conversion element such as a Hall element and which can therefore improve a characteristic of the magneto-electric conversion element as well as a stability. In order to accomplish the above-mentioned object, an electronic watthour meter of the type outlined above comprises a second magneto-electric conversion element having the same characteristic as that of the first magneto-electric conversion element is supplied with the current proportional to the load voltage and is not supplied with the magnetic field proportional to the load current. In the electronic watthour meter according to the invention, the outputs of the first and the second magneto-electric conversion elements are combined in a combining means to thereby compensate the offset voltage component contained in the output of the first magneto-electric conversion element. In one embodiment, a Hall element is used as the magneto-electric conversion element. However, a magnetic reluctance element of a superconductor conversion element may be used as the magneto-electric conversion element. Brief Description of the Drawing Fig. 1 shows a structure of a power-voltage conversion circuit using a Hall element for use in a conventional electronic watthour meter; Fig. 2 is a block diagram of an electronic watthour meter according to a embodiment of this invention; Fig. 3 shows a structure of a power-voltage conversion circuit for use in the electronic watthour meter illustrated in Fig. 2; Referring to Fig. 2, an electronic watthour meter comprises an auxiliary voltage transformer 11 for producing a voltage output signal proportional to a load voltage of a distribution line, an auxiliary current transformer 12 for producing a current output signal proportional to a consumed current, a power-voltage conversion circuit 13 for multiplying the voltage output signal and the current output signal from the auxiliary voltage transformer 11 and the auxiliary current transformer 12 to produce a voltage proportional to a load power, a voltage-frequency conversion circuit 14 supplied from the power-voltage conversion circuit 13 with the output voltage proportional to the load power for integrating the output voltage to produce a pulse signal, a frequency dividing circuit 15 for weighting the pulse signal from the voltage-frequency conversion circuit 14 to produce a divided pulse signal proportional to electric power, and a display circuit 16 responsive to the divided pulse signal from the frequency dividing circuit 15 for displaying the amount of the electric power consumption represented by the divided pulse signal. As shown in Fig. 3, the power-voltage conversion circuit 13 comprises first and second power-voltage converting sections 13A and 13B arranged in parallel. The first power-voltage converting section 13A has a conversion element for conversion into electric energy by the use of a physical effect. In the embodiment, a first magneto-electric conversion element 18, such as a Hall element, is used as the conversion element. Specifically, the power-voltage converting section 13A comprises a voltage-current conversion circuit 17 for converting an output voltage of the auxiliary voltage transformer 11 into an output current, the first magneto-electric conversion element 18 supplied with the output current of the voltage-current conversion circuit 17 and applied with a magnetic field produced by the auxiliary current transformer 12 for producing a voltage output proportional to electric power equivalent to a product of the current and a magnetic field intensity, and an amplifier circuit 19 supplied with the voltage output from the first magneto-electric conversion element 18 for amplifying the voltage output to an appropriate level. The second power-voltage converting section 13B comprises a voltage-current conversion circuit 20 for converting the output voltage of the auxiliary voltage transformer 11 into an output current, a second magneto-electric conversion element 21 supplied with the output current of the voltage-current conversion circuit 20, and an amplifier circuit 22. The second magneto-electric conversion element 21 has the same characteristic as that of the first magneto-electric conversion element 18. The second magneto-electric conversion element 21 is supplied with no magnetic field, and the second magneto-electric conversion element 21 produces an offset voltage alone. The first and the second power-voltage converting sections 13A and 13B are connected to a combining circuit 23. Referring to Fig. 3, a circuit operation will be described. In Fig. 3, an input voltage is represented by V while a consumed current is represented by I. The auxiliary voltage transformer 11 produces an output voltage depicted by v. Each of the voltage-current conversion circuits 17 and 20 produces an output current i which is proportional to the output voltage v. The consumed current I is converted by a magnetic field detection core of the auxiliary current transformer 12 into a magnetic field intensity B. The first magneto-electric conversion element 18 such as a Hall element is responsive to the current i and the magnetic field B and produces, in a theoretical sense, a first output voltage V H1 calculated by the following formula: V H1 = kiB, where k represents a proportional constant. Practically, the first magneto-electric conversion element 18 produces a first offset voltage depicted by α. Accordingly, the first output voltage VH1 contains the first offset voltage as described in the preamble of the specification and therefore has a.c. and d.c. components. As a result, the first output voltage V H1 is represented by: V H1 = kiB + α. On the other hand, the second magneto-electric conversion element 21 of the second power-voltage converting section 13B produces a second offset voltage β alone because the magnetic field produced by the auxiliary current transformer 12 is not supplied. As a result, a second output voltage V H2 of the second magneto-electric conversion element 21 is represented by: V H2 = β. Summarizing, the first magneto-electric conversion element 18 produces the first output voltage represented by V H1 = kiB + α while the second magneto-electric conversion element 21 produces the second output voltage represented by V H2 = β. It is assumed here that the amplifier circuits 19 and 22 have gains G 1 and G 2 , respectively. The combining circuit 23 calculates a sum V H of the first and the second output voltages V H1 and V H2 as follows: V H = G 1 x V H1 + G 2 x V H2 = G 1 x kiB + G 1 x α + G 2 x β. The gain G 2 of the amplifier circuit 22 of the second power-voltage converting section 13B is selected to satisfy the following formula: G 2 = -G 1 x (α/β). In this event, the sum V H is given by: V H = G 1 x kiB. Thus, the first offset voltage α is compensated.

Claims: Electronic watthour meter comprising: a) a first magneto-electric conversion element (18) to which a current (i) proportional to a load voltage (V) is supplied and a magnetic field (B) proportional to a load current (I) is applied for producing a voltage proportional to the load power given by the load voltage (V) and the load current (I), characterized in that it further comprises b) a second magneto-electric conversion element (22) having the same characteristic as that of the first magneto-electric conversion element (18) which is supplied with the current (i) proportional to the load voltage (V) and to which the magnetic field (B) proportional to the load current (I) is not applied, and c) a combining means (23) that combines the outputs of the first and second magneto-electric conversion elements (18, 22) and thereby compensates an offset component contained in the output of the first magneto-electric conversion element (18). Electronic watthour meter according to claim 1, characterized in that it further comprises a) an auxiliary voltage transformer (11) for producing a voltage output signal (v) proportional to the load voltage (V) of a distribution line, b) an auxiliary current transformer (12) for producing a magnetic field (B) proportional to the load current (I), and c) a voltage-frequency conversion circuit (14) for integrating the output voltage proportional to the load power, which voltage has been supplied from the combining means (23), to produce a pulse signal. Electronic watthour meter according to claim 2, characterized in that it further comprises a) a frequency dividing circuit (15) for weighting the pulse signal from the voltage-frequency conversion circuit (14) to produce a divided pulse signal proportional to the load power, and b) a display circuit (16) responsive to the divided pulse signal from the frequency dividing circuit (15) for displaying the amount of electric power consumption. Electronic watthour meter according to one of the claims 1 to 3, characterized in that each of the magneto-electric conversion elements comprises a Hall element (18, 22).

Inventors: KOBAYASHI SHUNICHI, KOBAYASHI, SHUNICHI
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1992
Main classification: G01R21
IPC codes: G01R21