29-08-2013, 04:31 PM
Wattmeter
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Definition
Wattmeter is an instrument which is used to measure the power consumption of an Electric circuit or an appliance which is connected to the supply in terms of Watts.
Analogue Wattmeter
The first is the traditional analog wattmeter. These are electro dynamic 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.
Digital Wattmeter
The second type of wattmeter is the electronic or digital meter. These devices differ from electro dynamic 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.
Construction and Working
In our lab work we mostly used analogue wattmeter so we only discuss the working and construction of analogue wattmeter. Basic parts are:
Pointer
Movable Voltage Coil
Stationary Current Coil
Why is LPF wattmeter used for no load test?
In case of no load only magnetizing current is flowing which is somewhere around 75 degree out of phase and so the power factor is very low.
As cos750=0.258 but in normal working we take the value of power factor is 0.8
Precautions
The wattmeter is a fairly sensitive instrument and may be damaged by excessive current values. As with most electrical measurement instruments, watt meters 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 watt meters 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.
Range mostly “1mW to 10kW”