Wattmeter

The wattmeter is an instrument for measuring the electric power (or the supply rate of electrical energy) in watts of any given circuit.

Electrodynamic

The traditional analog wattmeter is an electrodynamic instrument. The device consists of a pair of fixed coils, known as "current coils", and a movable coil known as the "potential coil".

The current coils connected in series with the circuit, while the potential coil is connected in parallel. Also, on analog wattmeters, the potential coil carries a needle that moves over a scale to indicate the measurement. A current flowing through the current coil generates an electromagnetic 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-value resistor connected in series with it to reduce the current that flows through it.

The result of this arrangement is that on a dc circuit, the deflection of the needle is proportional to "both" the current "and" the voltage, thus conforming to the equation W=VA or "P"="VI". On an ac circuit the deflection is proportional to the average instantaneous product of voltage and current, thus measuring true power, and possibly (depending on load characteristics) showing a different reading to that obtained by simply multiplying the readings showing on a stand-alone voltmeter and a stand-alone ammeter in the same circuit.

The two circuits of a wattmeter can be damaged by excessive current. The ammeter and voltmeter are both vulnerable to overheating - in case of an overload, their pointers will be driven off scale - but in the wattmeter, either or even both the current and potential circuits can overheat "without" the pointer approaching the end of the scale! This is because the position of the pointer depends on the power factor, voltage and current. Thus, a circuit with a low power factor will give a low reading on the wattmeter, even when both of its circuits are loaded to the maximum safety limit. Therefore, a wattmeter is rated not only in watts, but also in volts and amperes.

Electronic wattmeter

Electronic wattmeters are used for direct, small power measurements or for power measurements at frequencies beyond the range of electrodynamometer-type instruments.

Vacuum tube

The matched triodes are operated in the nonlinear region of their characteristic grid-voltage, plate-current curves.

The symmetrical resistive T network between the generator and load will provide V₁ and V₂ voltages proportional to, and in phase with, the load current and voltage, respectively. A source of AC power is connected to the load through the series resistors R₁ and R₂. These two resistors are of equal value and are made small to prevent the voltage drop across them from reducing the load voltage appreciably. R₃ is made large enough to have negligible power consumption.

Therefore, the R₃ voltage is equal to the load voltage, and the voltage across either series resistor is proportional to the difference in the output currents of the tubes. The average value of the difference could be measured by a DC meter connected to read the voltage potential between the grids of V₁ and V₂. This method is adequate only at low frequencies. As the frequency increases, the stray capacitances and inductances also increase.

The frequency range of the electronic wattmeter can be extended up to 20 megahertz by using pentodes instead of triode tubes. The operating conditions in a pentode are adjusted so that plate current is proportional to the product of a linear function of plate voltage and an exponential function of grid voltage.

Digital

A modern digital electronic wattmeter/energy meter samples the voltage and current thousands of times a second. The average of the instantaneous voltage multiplied by the current is the true power. The true power divided by the apparent volt-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 simple models display that information on LCD. More sophisticated models retain the information over an extended period of time, and can transmit it to field equipment or a central location.

See also

* Ammeter
* Voltmeter
* Power factor
* Electricity meter
* Kill A Watt
* Cent-a-meter