![]() On an AC circuit the deflection is proportional to the average instantaneous product of voltage and current, thus measuring active power, P= VI cos φ. The result of this arrangement is that on a direct current (DC) circuit, the deflection of the needle is proportional to both the current ( I) and the voltage ( V), thus conforming to the equation P= VI.įor AC power, current and voltage may not be in phase, owing to the delaying effects of circuit inductance or capacitance. The potential coil has, as a general rule, a high-value resistor connected in series with it to reduce the current that flows through it. The strength of this field is proportional to the line current and in phase with it. A current flowing through the current coil generates an electromagnetic field around the coil. Also, on analog wattmeters, the potential coil carries a needle that moves over a scale to indicate the measurement. The current coils are connected in series with the circuit, while the potential coil is connected in parallel. The device consists of a pair of fixed coils, known as current coils, and a movable coil known as the potential coil. The traditional analog wattmeter is an electrodynamic instrument. Electrodynamic Įarly wattmeter on display at the Historic Archive and Museum of Mining in Pachuca, Mexico ![]() This device is made up of power, current and voltage transformers, which measure the average power. ![]() ![]() Pressman patented an Electronic wattmeter. The Hungarian Ottó Bláthy patented his AC wattmeter. The wattmeter reads P = (1/T) ∫ 0 T v(t)i(t) dt, which in sinusoidal steady-state reduces to V rms I rms cos(φ), where T is the period of p(t) and φ is the angle by which the current lags the voltage. Electromagnetic wattmeters are used for measurement of utility frequency and audio frequency power other types are required for radio frequency measurements.Ī wattmeter reads the average value of the product v(t)i(t) = p(t), where v(t) is the voltage with positive reference polarity at the ± terminal with respect to the other terminal of the potential coil, and i(t) is the current with reference direction flowing into the ± terminal of the current coil. The wattmeter is an instrument for measuring the electric active power (or the average of the rate of flow of electrical energy) in watts of any given circuit. ![]()
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