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| Solenoids in Electricity Meters |
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In the U.K., the usual method of billing for domestic electricity use is on a quarterly basis, based on the amount of units of power consumed and recorded on individual meters in each dwelling. Because of problems with nonpayment of the electricity bills, a prepayment meter was designed to be installed in those dwellings with a history of nonpayment. This meter incorporated a solenoid-operated switch. The switch was closed by the solenoid which in turn was energized when money was inserted into the meter. This then allowed an electricity supply into the dwelling. The major problem with this system was that the solenoid remained energized for the whole time that the meter was in credit whether power was being used or not; and the solenoid itself was consuming power that had to be paid for by the customer or the electric company.
In order to overcome the problem and to update the metering system, a redesign was done to introduce an electronic meter using a prepared card system and a magnetic latching solenoid developed by Ledex Products. The requirements were that on insertion of a prepaid credit card bought from the electric company, the meter would provide power to the dwelling in the value of that particular card. It was planned that various values of cards would be available for purchase. On insertion of the card and validation for credit, a pulse would be given to a magnetic latching solenoid which would operate the main power on switch. The solenoid would then remain stable in this position without energization for the whole time the meter was in credit. On completion of the use of the power available on that credit, a reverse pulse would be given to operate the solenoid and open the switch making more power unavailable until a new card was inserted.
The considerations for the solenoid had to include matching the hold-on force to the switch spring force required, manufacturing the push rod from a material capable of direct contact onto the switch blade at 240 volts 50 Hz and retaining the plunger in the de-energized position with sufficient force to prevent switch operation by misuse of the meter (bump) but such that it did not prevent correct pull-in when energized.
To match the force/stroke requirements of the switch to give correct contact resistance, an open frame solenoid was used.
In order to overcome the problem of the push rod operation on live contacts, a plastic cup was used on a standard stainless steel push rod, but gave problems with position tolerances. The design was changed to a molded push rod located in a shoulder recess in the plunger. This gave more consistent positioning and reduced costs.
To retain the plunger in the de-energized position against misuse or bumping of the meter in an attempt to operate the switch, a secondary magnet circuit was created. This uses a steel mounting bracket with a stroke limit feature operating on a small magnetic circuit area at the pull end of the plunger. The design of this area had to be such that the hold-on force in this position did not interfere with the pull-in performance requirements of the solenoid.
This system is being further developed to incorporate switching for off-peak power use by remote control. This will require two switches per meter and double solenoids.
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