Electronic watthour meter

Abstract: An electric watthour meter comprises a magneto-electric conversion element (34) supplied with a current (i) proportional to a load voltage (V) and supplied with a magnetic field (B) proportional to a load current (I) for producing a voltage proportional to a load power. Further, there is provided a first switching means (32a) to periodically reverse in a predetermined cycle the direction of the current (i) proportional to the load voltage (V) that is supplied to the magneto-electric conversion element (34). An integrating means (37) to integrate the output voltage of the magneto-electric conversion element (34). A second switching means (32b) serves to periodically reverse the direction of the magnetic field (B) which is proportional to the load current (I) and which is supplied to the magneto-electric conversion element (34).

Description: This invention relates to an electronic watthour meter according to the preamble of claim 1, which is disclosed in, e.g., FR-A-2 594 230. A conventional electronic watthour meter is disclosed for example, in Japanese Patent Publications Nos. 53508/1984 being related to JP-A-63 115 157 and 54704 /1985 being related to JP-A-54 000 548. The conventional 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. Summary of the Invention 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. According to this invention, an electronic watthour meter as defined in claim 1 comprises: a magneto-electric conversion element supplied with a current proportional to a load voltage and applied with a magnetic field proportional to a load current for producing a voltage proportional to a load power given by the load voltage and the load current; a switching unit for periodically reversing in a predetermined cycle a direction of the current proportional to the load voltage and supplied to the magneto-electric conversion element and a direction of the magnetic field proportional to the load current and supplied to the magneto-electric conversion element; and an integrating unit for integrating the output voltage of the magneto-electric conversion element. In the above-mentioned electronic watthour meter according to this invention, the direction of the current proportional to the load voltage and supplied to the magneto-electric conversion element and the direction of the magnetic field proportional to the load current and applied to the magneto-electric conversion element are periodically reversed by the switching unit in a predetermined cycle. The output voltage of the magneto-electric conversion element is integrated by the integrating unit to remove an offset voltage component contained in the output voltage of 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 this invention; Fig. 3 shows a structure of a power-voltage conversion circuit for use in an electronic watthour meter according to a second embodiment of this invention; Fig. 4 is a view illustrating an arrangement of a core, a Hall element, and a primary conductor contained in the power-voltage conversion circuit illustrated in Fig. 4; and Fig. 5 is a view for describing an operation of the electronic watthour meter according to the second embodiment of this invention. Description of the Preferred Embodiment 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 aconsumed 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. Referring to Fig. 3, the auxiliary voltage transformer 11 produces an output signal proportional to a load voltage. The output signal is delivered through a switch 32a and a resistor 33 to be converted into a current which is supplied to a Hall element 34. On the other hand, a consumed current flowing through a load is supplied through a switch 32b cooperating with the switch 32a to a primary conductor 36 (shown in Fig. 4 in detail) wound around a core 35. The consumed current is converted by the core 35 into a magnetic field which is applied to the Hall element 34. A combination of the switches 32a and 32b serves as a switching unit. Fig. 4 shows an arrangement of the core 35, the Hall-effect element 34, and the primary conductor 36 in detail. As illustrated in the figure, the Hall element 34 is arranged within a gap defined by the core 35. The primary conductor 36 is wound around the core 35 to provide a single turn and a reverse single turn which are selectively activated by the switch 32b one at a time. In other words, the primary conductor 36 effectively passes through the core 35 only once. The switch 32b is for selectively switching a winding direction of the primary conductor 36 with respect to the core 35, namely, a direction of the current flowing through the primary conductor 36. Turning back to Fig. 3, the switch 32a is connected to an output terminal of the auxiliary voltage transformer 11 so as to reverse a direction of the current flowing from the auxiliary voltage transformer 11 to the Hall element 34. Accordingly, by on-off control of the switches 32a and 32b, it is readily possible to change the direction of the current i supplied from the auxiliary voltage transformer 11 through the switch 32a to the Hall element 34 and the direction of the magnetic field B applied from the primary conductor 36 through the switch 32b to the Hall element 34. As described above, the current i is supplied from the auxiliary voltage transformer 11 through the switch 32a to the Hall element 34 while the magnetic field B is supplied from the primary conductor 36 through the switch 32b to the Hall element 34. Consequently, the Hall element 34 produces an output voltage Vop represented by: Vop = kiB, where k represents a proportional constant. As described before, the Hall element 34 produces an offset voltage Vou even in absence of the magnetic field. This results from irregularity in shape of the Hall-effect element 34, thermal electromotive force of the Hall-effect element 34 itself, or the like. Accordingly, the Hall-effect element 34 produces a total output voltage Vout represented by: Vout = Vop + Vou. The total output voltage Vout is varied in dependence upon the directions of the current i and the magnetic field B as shown in Table 1. i B output positive positive Vop+Vou negative positive -Vop-Vou positive negative -Vop+Vou negative negative Vop-Vou As will be understood from Table 1, the offset voltage Vou is removed as follows. The switches 32a and 32b are controlled so that the directions of both the current i and the magnetic field B are positive during a particular cycle and negative during a next following cycle. The total output voltage produced by the Hall element 34 during these two cycles is integrated. Thus, the offset voltage Vou is removed as follows: Vout = (Vop+Vou) + (Vop-Vou) = 2Vop. Accordingly, when the switches 32a and 32b are controlled as described above and the output voltage of the Hall element 34 is integrated by an integrator circuit 37 as shown in Fig. 3, the offset voltage Vou is removed to improve the efficiency of the electronic watthour meter. In Fig. 3, a reference numeral 38 represents a control circuit for controlling the switches 32 and 32b. Referring to Fig. 5, an input signal is depicted in a first or an uppermost row. The input signal is representative of the current supplied through the auxiliary voltage transformer 11 and the magnetic field supplied through the primary conductor 36. The switches 32a and 32b are switched in every two cycles of the input signal as depicted in a second row in the figure. Every time when the switches 32a and 32b are switched, the difference 2Vou between +Vou and -Vou is produced in the output voltage of the Hall element 34, as shown in a third or a lowermost row in the figure. The offset voltage is compensated by integrating the output voltage in the successive two cycles before and after switching of the switches 32a, 32b. In the embodiment, description has been made about the case where the Hall element is used as the magneto-electric conversion element. However, a magnetic reluctance element or a superconductor conversion element may be used as the magneto-electric conversion element.

Claims: Electronic watthour meter comprising: a) a magneto-electric conversion element (34) that is supplied with a current proportional to a load voltage and to which a magnetic field proportional to a load current is applied for producing an output voltage proportional to a load power given by the load voltage and load current, b) first switching means (32a) to periodically reverse in a predetermined cycle the direction of the current proportional to the load voltage that is supplied to the magneto-electric conversion element (34), and c) integrating means (37) to integrate the output voltage of the magneto-electric conversion element (34), characterized by d) second switching means (32b) cooperating with the first switching means (32a) to periodically reverse the direction of the magnetic field which is proportional to the load current and which is applied to the magneto-electric conversion element. Electronic watthour meter according to claim 1, characterized in that it further comprises a) an auxiliary voltage transformer (11) feeding the magneto-electric conversion element (34) for producing a voltage output signal proportional to the load voltage of a distribution line, b) an auxiliary current transformer (12) feeding the magneto-electric conversion element (34) for producing a current output signal proportional to the consumed current, whereby the current output signal is converted into the magnetic field, and c) a voltage-frequency conversion circuit (14) supplied from the integrating means (37) with the output voltage proportional to the load power for integrating the output voltage 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 the magneto-electric conversion element (34) comprises a Hall element.

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