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an instrument for measuring the power of an electrical current in watts. The most common wattmeters are the dynamometer type, in which the mechanism consists of an immovable coil connected in series to a load (the current circuit) and then to a movable coil, which is connected through a large auxiliary resistor (the resistor circuit) parallel to the load.
The operation of the dynamometer wattmeter is based on the interaction of the magnetic fields of the movable and fixed coils as an electrical current passes through them. In the case of direct current the torque, which causes a deviation in the movable part of the device and in the needle (indicator) connected to the device, is proportional to the product of the current intensity and the voltage potential; in the case of alternating current, it is also proportional to the cosine of the angle of displacement of the phases between the current and the voltage potential. Ferrodynamic and, less frequently, induction, thermoelectrical, and electrostatic wattmeters are also used.
Soviet industry produces portable (laboratory) dynamometer wattmeters in efficiency classes 0.2 and 0.5; they are used for measuring direct and alternating currents (with a frequency of up to 5 kilohertz) in circuits. The power of an alternating current with a frequency greater than 5 kilohertz is measured by thermoelectric wattmeters. In power plants the power is measured by panel (static) wattmeters, which are usually ferrodynamic and, less frequently, of the induction type.
The power in three-phase circuits is measured by three-phase wattmeters, which are based on the structural joining of three (or two) single-phase wattmeters. The movable coils of three-phase wattmeters are mounted along a common axis, resulting in the summation of the torques created by them. Wattmeters are connected to high-tension circuits by current and voltage measuring transformers.
REFERENCEShkurin, G. P. Spravochnik po elektroizmeritel’ nym i radioizmeritel’nym priboram, 3rd ed., vol. 1. Moscow, 1960.
N. G. VOSTROKNUTOV
An instrument that measures electric power. See Electric power measurement
A variety of wattmeters are available to measure the power in ac circuits. They are generally classified by names descriptive of their operating principles. Determination of power in dc circuits is almost always done by separate measurements of voltage and current. However, some of the instruments described will also function in dc circuits, if desired.
Probably the most useful instrument in the measurement of ac power at commercial frequencies is the indicating (deflecting) electrodynamic wattmeter. It is similar in principle to the double-coil dc ammeter or voltmeter in that it depends on the interaction of the fields of two sets of coils, one fixed and the other movable. The moving coil is suspended, or pivoted, so that it is free to rotate through a limited angle about an axis perpendicular to that of the fixed coils. As a single-phase wattmeter, the moving (potential) coil, usually constructed of fine wire, carries a current proportional to the voltage applied to the measured circuit, and the fixed (current) coils carry the load current. This arrangement of coils is due to the practical necessity of designing current coils of relatively heavy conductors to carry large values of current. The potential coil can be lighter because the operating current is limited to low values. See Ammeter, Voltmeter
A thermal converter consists of a resistive heater in close thermal contact with one or more thermocouples. When current flows through the heater, the temperature rises. Thermocouples give an output voltage proportional to the temperature difference between their junctions, in this case proportional to the square of the current, and so make suitable transducers for the construction of thermal wattmeters. See Thermal converters, Thermocouple, Thermoelectricity
The electrostatic force between two conductors is proportional to the product of the square of the potential difference between them and the rate of change of capacitance with displacement. A differential electrostatic instrument may therefore be used to construct a quarter-squares wattmeter. In spite of the problems of matching the capacitance changes of the two elements and the small forces available, electrostatic wattmeters were used as standards for many years.
Digital wattmeters combine the advantages of electronic signal processing and a high-resolution, easily read display. Electrical readout of the measurement is also possible. A variety of electronic techniques for carrying out the necessary multiplication of the signals representing the current and voltage have been used. Usually the electronic multiplier is an analog system which gives as its output a voltage proportional to the power indication required. This voltage is then converted into digital form in one of the standard ways. Many of the multipliers were originally developed for use in analog computers. See Analog computer
The instruments described are designed for single-phase power measurement. In polyphase circuits, the total power is the algebraic sum of the power in each phase. This summation is assisted by simple modifications of single-phase instruments. See Alternating current