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2. Wattmeter 2

3. Presented By UMAIR FAROOQ 2010-EE-10 3

4. 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. 4

5. Basic Types Watt meters are generally presented in two basic formats. Analogue Wattmeter Digital Wattmeter 5

6. 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. 6

7. 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. 7

8. Symbols of Wattmeter 8

9. 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 9

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14. 3 Wattmeter Method Three watt meter method has the advantage of measuring power in each phase, and phase to neutral voltages. In a three wire system with no neutral a false neutral can be created. Three watt meters are required for this method. 14 AdvantagesDisadvantages

15. 2 Wattmeter Method Advantages The method is applicable for balanced as well as unbalanced loads. Only two watt meters are sufficient to measure total 3 phase power. Disadvantages Disadvantage of this method are, Not applicable for three phase, 4 wire system The signs of W1 and W2 must be identified and noted down correctly otherwise it may lead to the wrong result. The two watt meter method only measures total power. 15 Advantages

16. Why wattmeter shows sometimes negative reading? Power = V*I*CosA Suppose, phase difference between V and I is "A"(angle). and wattmeter reads +ve. Now if phase difference became (A+180), wattmeter will read -ve of same magnitude. It simply means, direction of power flow gets changed just because of changing in direction of either current or voltage. 16

17. 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 cos75 0 =0.258 but in normal working we take the value of power factor is 0.8 17

18. 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” 18

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