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A wattmeter is an instrument which gives a visual indication of the amount of electrical energy being supplied to a circuit. This indication is expressed in watts which is the standard unit of measure for electrical energy supply or consumption. There are two commonly used types of wattmeter: analog and digital. Analog meters indicate power supply via a needle and scale indicator while digital instruments display the power usage on a liquid crystal display (LCD). Wattmeters are typically rated for a set voltage range but may include features such as coil taps which allow for multiple voltages.
All electrical equipment consumes power subject to a set of known constants which include the rated voltage, the current usage expressed in amps, and the overall energy usage expressed in watts. Some types of electrical appliances or installations use far more energy than others of similar voltage ratings. A wattmeter allows for power usage to be monitored to establish whether circuits are operating correctly. This information is crucial in larger installations where large resistive loads are used. The wattmeters in such installations allow operators and technicians to keep track of individual circuit health and overall power supply balancing and consumption.
Wattmeters are generally presented in one of two basic formats. The first is the traditional analog wattmeter. These are electrodynamic instruments and consist of three internal coils — two static current coils and a movable potential coil which has the indicator needle attached to it. When an electrical current is passed through the two current coils, an electromagnetic field is generated. This field causes the potential coil to move and display a value on the scale behind the needle.
The second type of wattmeter is the electronic or digital meter. These devices differ from electrodynamic meters in the way they calculate power usage. Digital meters use a microprocessor to assess voltage and amperage values from the circuit at a frequency of several thousand samples per second. These samples are used to calculate an average power factor or power usage. The power factor reading is then displayed digitally on a LCD display.
The wattmeter is a fairly sensitive instrument and may be damaged by excessive current values. As with most electrical measurement instruments, wattmeters are usually rated for a specific voltage range. Many high end meters feature coil taps or series/parallel current coil switching which allows such devices to be used for multiple voltage ranges. All wattmeters are also rated for safe maximum current or amperage values; these ratings should be adhered to in order to avoid damage or destruction of the meter.