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AC meters

Sangamo Weston meter

Sangamo Weston meter

The AC kilowatthour meter is mechanically a very simple device that works by electromagnetic induction. An aluminium disc rotates between the pole pieces of various magnetic circuits, driving a recording geartrain that turns the dial pointers. Alternatively, it can drive a Veeder mechanism with rotating numbered wheels that show through windows. The critical design task is getting the disc to rotate at a speed proportional to the instantaneous power consumption of the load circuit. Unlike DC measurement, with AC it is necessary to ensure that loads with power factors below unity are correctly measured, since only the real power that produces useful work is charged to ordinary customers. This is achieved by passing through the disc magnetic fields from voltage and current coils, that produce a torque on the disc in proportion to their instantaneous in-phase product, i.e. the real power. Permanent magnets also pass a field through the disc, that induces eddy currents resulting in a counter-torque proportional to the speed of rotation. Thus the speed is made proportional to the real power, integrating with respect to time by the geartrain to show energy used.

Three-phase meters

Meters for three-phase systems have multiple magnetic circuits working either on the same disc or separate discs mechanically interconnected. Many three-phase systems involve currents larger than can be easily accommodated by the current coils within the meter itself. These can be handled by external current transformers like those of panel ammeters, as can high voltages by means of potential transformers.

Ferranti 3-phase meter

Ferranti 3-phase meter

Layout of 3-phase meter

Layout of 3-phase meter

Inside 3-phase meter

Inside 3-phase meter


Telemetry outputs

Another feature often found on meters for large power applications is a telemetry output that allow the reading to be transmitted to a central location. On a typical induction meter, a contact set is arranged to impulse a low-voltage circuit after a fixed number of spindle rotations. The meter shown below is designed for substation work at 11kV, for demands up to 15MW. One impulse is sent for every 10kWh registered.

MWh meter dials

MWh meter dials

MWh meter impulsing contacts

MWh meter impulsing contacts

Ferranti megawatthour meter

Ferranti megawatthour meter

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