AC Circuits and Power Systems in Practice

10Energy Metering

Once practical uses for electricity were discovered, its value as a saleable item was immediately realised, and the need for accurate measurement of electrical energy arose. Early energy meters were induction disc instruments, and their design has changed little for over 100 years. These instruments are based on a small induction motor, driving a gear train which registers the energy used on a series of decadal dials, visible from the face of the meter. The speed of rotation of the disc and thus of the gear train, depends on the applied voltage and current, as well as the angle between them. Induction disc meters are designed to produce maximum torque at unity power factor where, for a given voltage and current, the power flow is greatest. The speed of the disc is then proportional to the load power factor, and thus to the power flowing to the customer. Temperature compensation is usually provided, as well as damping magnets for precise speed calibration. Depending on the class of the instrument, accuracies can lie in a range from 0.2% to 3%, though being a mechanical device, periodic calibration is necessary to ensure that this accuracy is maintained.

An example of an early single‐phase residential meter appears in Figures 10.1 and 10.2. This is a Shallenberger instrument made in the USA by the Westinghouse Electric Manufacturing Company, about 1900. It was owned by the Melbourne City Council (Australia), which built a power station in 1894 to provide street ...