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Thewattmeter is an instrument for measuring theelectric active power (or the average of the rate of flow ofelectrical energy) inwatts of any givencircuit. Electromagnetic wattmeters are used for measurement ofutility frequency and audio frequency power; other types are required for radio frequency measurements.
A wattmeter reads the average value of the productv(t)i(t) = p(t), wherev(t) is thevoltage with positive reference polarity at the ± terminal with respect to the other terminal of the potential coil, andi(t) is the current with reference direction flowing into the ± terminal of the current coil. The wattmeter readsP = (1/T) ∫0T v(t)i(t) dt, which in sinusoidal steady-state reduces toVrmsIrms cos(φ), whereT is the period ofp(t) and φ is the angle by which the current lags the voltage.[1]
The HungarianOttó Bláthy patented his AC wattmeter.[citation needed]
In 1974 Maghar S. Chana, Ramond L. Kraley, Eric A. Hauptmann Barry, and M. Pressman patented an early electronic wattmeter. This device is made up of power, current and voltage transformers, which measure the average power.[2]
The traditional analog wattmeter is anelectrodynamic instrument. The device consists of a pair of fixedcoils, known ascurrent coils, and a movable coil known as thepotential coil.
The current coils are connected inseries with the circuit, while the potential coil is connected inparallel. Also, onanalog wattmeters, the potential coil carries a needle that moves over a scale to indicate the measurement. A current flowing through the current coil generates anelectromagnetic field around the coil. The strength of this field is proportional to the line current and in phase with it. The potential coil has, as a general rule, a high-valueresistor connected in series with it to reduce the current that flows through it.
The result of this arrangement is that on adirect current (DC) circuit, the deflection of the needle is proportional toboth thecurrent (I)and the voltage (V), thus conforming to the equationP=VI.
ForAC power, current and voltage may not be in phase, owing to the delaying effects of circuitinductance orcapacitance. On anAC circuit the deflection is proportional to the average instantaneous product of voltage and current, thus measuringactive power,P=VI cosφ. Here, cosφ represents thepower factor which shows that the power transmitted may be less than the apparent power obtained by multiplying the readings of avoltmeter andammeter in the same circuit.
Electronic wattmeters are used for direct, small power measurements or for power measurements at frequencies beyond the range of electrodynamometer-type instruments.
A modern digital wattmeter samples the voltage and current thousands of times a second. For each sample, the voltage is multiplied by the current at the same instant; the average over at least one cycle is the real power. The real power divided by the apparentvolt-amperes (VA) is the power factor. A computer circuit uses the sampled values to calculate RMS voltage, RMS current, VA, power (watts), power factor, and kilowatt-hours. The readings may be displayed on the device, retained to provide a log and calculate averages, or transmitted to other equipment for further use. Wattmeters vary considerably in correctly calculating energy consumption, especially when real power is much lower than VA (highlyreactive loads, e.g.electric motors). Simple meters may be calibrated to meet specified accuracy only forsinusoidal waveforms. Waveforms forswitched-mode power supplies as used for muchelectronic equipment may be very far from sinusoidal, leading to unknown and possibly large errors at any power. This may not be specified in the meter's manual.
There are limitations to measuring power with inexpensive wattmeters, or indeed with any meters not designed for low-power measurements. This particularly affects low power (e.g. under 10 watts), as used in standby; readings may be so inaccurate as to be useless (although they do confirm that standby power is low, rather than high).[3] The difficulty is largely due to difficulty in accurate measurement of the alternating current, rather than voltage, and the relatively little need for low-power measurements. The specification for the meter should specify the reading error for different situations. For a typical plug-in meter the error in wattage is stated as ±5% of measured value ±10 W (e.g. a measured value of 100 W may be wrong by 5% of 100 W plus 10 W, i.e., ±15 W, or 85–115 W); and the error in kW·h is stated as ±5% of measured value ±0.1 kW·h.[4] If a laptop computer in sleep mode consumes 5 W, the meter may read anything from 0 to 15.25 W, without taking into account errors due to non-sinusoidal waveform. In practice accuracy can be improved by connecting a fixed load such as an incandescent light bulb, adding the device in standby, and using the difference in power consumption.[3] This moves the measurement out of the problematic low-power zone.
Instruments with moving coils can be calibrated for direct current or power frequency currents up to a few hundred hertz. Atradio frequencies (RF) a common method is arectifier circuit arranged to respond to current in atransmission line; the system is calibrated for the known circuit impedance.Diode detectors are either directly connected to the source, or used with a sampling system that diverts only a portion of the RF power through the detector.Thermistors andthermocouples are used to measure heat produced by RF power and can be calibrated either directly or by comparison with a known reference source of power.[5] Abolometer power sensor converts incident radio frequency power to heat. The sensor element is maintained at a constant temperature by a small direct current. The reduction in current required to maintain temperature is related to the incident RF power. Instruments of this type are used throughout the RF spectrum and can even measure visible light power. For high-power measurements, acalorimeter directly measures heat produced by RF power.[5]
An instrument which measures electrical energy inwatt hours is essentially a wattmeter which integrates the power over time (essentially multiplies the power by elapsed time). Digital electronic instruments measure many parameters and can be used where a wattmeter is needed:volts, current inamperes, apparent instantaneous power, actual power, power factor, energy in [k]W·h over a period of time, and cost of electricity consumed.
This article incorporates text from a publication now in thepublic domain: Chisholm, Hugh, ed. (1911). "Wattmeter".Encyclopædia Britannica. Vol. 28 (11th ed.). Cambridge University Press.
