Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Eddy current rotor
Reexamination Certificate
1999-03-01
2001-05-29
Brown, Glenn W. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Eddy current rotor
C264S255000, C264S328800, C264S108000, C073S862324, C073S770000, C029S598000
Reexamination Certificate
active
06239588
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to fabrication of electricity meters and, more particularly, to a method of fabricating a magnetic a plastic molded meter frame for use in meter components sensitive to external magnetic fields.
BACKGROUND OF THE INVENTION
Electricity meters are utilized to measure consumption of energy by a load and include components such as current and voltage coils which are sensitive to external magnetic fields. For example, in an electronic meter, a processor (e.g., an application specific integrated circuit) receives input current and voltage signals from current and voltage coils, and uses such signals to generate a measure of energy consumption. The current and voltage coils are coupled to the power lines that supply energy to the load. If an external magnetic field interacts with the coils, meter accurancy can be adversely affected.
In an electromechanical meter, an aluminum disk is supported on a shaft and is driven to rotate by magnetic fields at a speed proportional to electric power being consumed by the load. The shaft is supported by a bearing so that very little friction acts on the shaft. A voltage coil and a current coil, positioned on respective, opposite sides of the disk, are coupled to the power lines supplying energy to the load. A permanent magnet, sometimes referred to as a retarding magnet, is spaced from the voltage and current coils and is positioned so that its poles are on opposite sides of the disk. The energized voltage and current coils generate magnetic fields which impart a rotational torque to the disk while the field of the permanent magnet imparts a retarding torque to the disk. If an external magnetic field interacts with the coils, the bearing, or the retarding magnet, accuracy of the meter can be adversely affected.
Since the readings of an electricity meter form the basis for income to electric utilities, it is important that the meter accurately measure energy consumption and not be subject to inaccuracies that might be due to tampering. For example, accuracy of an electronic meter can be adversely affected by a strong permanent magnet positioned near enough to the current coils to affect signals generated by the coils. Similarly, accuracy of an electromechanical meter can be adversely affected by a strong permanent magnet positioned near the coils, the bearing, or the retarding magnet.
Until recently, compact permanent magnets with sufficient magnetic strength to adversely affect meter components had not been generally available. Since such magnets are now generally available, it would be desirable to design electricity meters in a way that protects meter components from tampering by use of such magnets.
Recently, both electronic and electromechanical meter frames have been molded from plastic. In an electronic meter, the meter frame is configured to support the voltage and current coils, and the circuit boards are utilized to process signals from the coils. In an electromechanical meter, the meter frame supports the voltage and current coils, as well as the bearing and the retarding magnets.
Plastic molded meter frames are lower in cost than previously used die cast meter frames, facilitating more cost efficient energy consumption metering. However, these lower cost plastic molded frames can be even more susceptible to tampering with a permanent magnet, as described above, than metallic frames. Therefore, it would be desirable to provide an effective way of substantially reducing the opportunity for tampering without significantly increasing the cost of the plastic molded frame, so that the cost advantages of the plastic molded frame are substantially preserved. In addition, the tamper resistant feature of such frame should preferably be relatively indiscernible so as to avoid drawing attention to the magnetic shielding structure of the frame.
SUMMARY OF THE INVENTION
Magnetic shields are provided for plastic molded meter frames in order to substantially prevent external magnetic fields from adversely affecting operation of the meter components. The shields can be utilized in connection with either electromechanical or electronic energy meters. For example, in an electromechanical watthour meter, the meter frame includes an integrally formed retarding magnet shield and an integrally formed bearing shield. The shields are coextensive with the frame, and extend from an inner surface of the frame toward the retarding magnet and the bearing, respectively.
To form the shields, and as a process step in the frame molding process, a magnetically permeable powder, such as a permeable ferrite powder, is mixed with the plastic material from which the meter frame is molded. Such mixed powder/plastic material is then supplied to jets which fill the portions of the mold for the shields while only the plastic material (i.e., not the mixed powder) is supplied by other jets to other portions of the meter frame mold. In this manner, magnetic and non-magnetic plastics are molded in different regions of the integral meter frame in a single operation.
In an alternative embodiment, a sheet metal stamping of magnetically permeable material, such as iron, is inserted into the mold prior to an injection molding step. The sheet metal stamping is located in the mold so that when the meter components are secured to the meter frame, the stamping at least partially blocks external magnetic fields from at least one component. With the sheet metal stamping located in the mold, the plastic is injected into the mold, including around the sheet metal stamping, thereby securing the sheet metal stamping in place by being integral with the mold.
The above described shields can effectively divert an externally applied field away from the retarding magnet and the bearing. In addition, the retarding magnet shield is sufficiently spaced from the retarding magnet so as not to divert its retarding magnetic field. In this manner the shields desirably prevent externally applied fields from adversely affecting the retarding magnet and the bearing without themselves adversely affecting the meter accuracy.
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patent: 59014699 (1982-01-01), None
patent: 57-109130 (1982-01-01), None
Brown Glenn W.
General Electric Company
Hamdan Wasseem H.
Snyder Marvin
Stoner Douglas E.
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