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CHAPTER 5 : POWER METERS
5.1 Power Meter Power meter is electric meter : and define as a meter for measuring the amount of electric power used in watt of any given circuit.
Figure 5.1 : Power Meter 5.2 Principles of Analogue Wattmeter W P
Figure 5.2 : Symbol of Power Meter
The traditional analog wattmeter is an electrodynamics 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. A typical wattmeter in educational labs has two voltage coils (pressure coils) and a current coil. We can connect the two pressure coils in series or parallel to each other to change the ranges of the wattmeter. Another feature is that the pressure coil can also be tapped to change the meter's range. If the pressure coil has range of 300 volts, the half of it can be used so that the range becomes 150 Volts.
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Figure 5.3 : Wattmeter connection for power measurement
KWH Meter An electric meter or energy meter is a device that measures the amount of electrical energy consumed by a residence, business, or an electrically powered device. The electrically-powered kilowatt hour is most commonly known as a billing unit for energy delivered to consumers by electric utilities. A unit of energy, especially electrical energy, equ to equal the work done by one watt acting for one hour and equivalent to 3,600 joules.
Figure 5.4 : KWH Meter
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Figure 5.5 : Connection KWH Meter for Bulding
Figure 5.6 : Clamp Meter In electrical and electronic engineering, a current clamp or current probe is an electrical device having two jaws which open to allow clamping around an electrical conductor. This allows properties of the electric current in the conductor to be measured, without having to make physical contact with it, or to disconnect it for insertion through the probe. Current clamps are usually used to read the magnitude of a sinusoidal current (as invariably used in alternating current (AC) power distribution systems), but in conjunction with more advanced instrumentation the phase and waveform are available. Very high alternating currents (1000A and more) are easily read with an appropriate meter; direct currents, and very low AC currents (milliamps) are more difficult to measure. In the same current flows down one conductor and up the other, with a net current of zero. Clamp meters are often sold with a device that is plugged in between the power outlet and the device to be tested. The device is essentially a short extension
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cord with the two conductors separated, so that the clamp can be placed around only one conductor. The reading produced by a conductor carrying a very low current can be increased by...