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Measurement of Power (Wattmeter)

Power in DC circuit:P = VI (watt)

Ammeter and Voltmeter require Power for this operation Two connection can be used Figure & expressions

Power indicated by instrument = Power consumed in load + Power loss in instrument

For true value, a correction must be applied for power loss in instrument orma y, t e power oss n nstrument s very sma

However, when the output is very limited the Ammeter and Voltmeter may unnecessarily load the circuit

causing serious error

The wattmeter is used For ower measurement To gives direct indication of power

Accuracy is also increased

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Power in AC circuit: In case of AC, the instantaneous power varies continuously but we are interested in its average value over a cycle.

because the average power multiplied by time period gives the transfer of energyover a per o

Mathematical expression

Finally, we will get,P = (V mIm/2)Cos = VICos

V and I are rms values Cos is the power factor of the load

The factor Cos indicates toward the use of Wattmeter for measurement of power in AC circuit instead of Ammeter andVoltmeter

Since Ammeter and Voltmeter never involve power factor

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Electrodynamometer Wattmeter

Similar to the design and construction of Electrodynamometer (EDM) Ammeter and Voltmeter

The fixed coils are connected in series with the load and so carry thecurrent in the circuit The fixed coil form the Current Coil or sim l CC of wattmeter

The moving coil is connected across the voltage, therefore carries a

The moving coil is called the Voltage or Pressure Coil or PC of wattmeter

current to a small value

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Construction: The moving coil is mounted on a pivoted spindle

and is governed by the fixed coil

Spring control is used for the movement Both fixed and moving coils are air cored nce e curren o mov ng co s carr e y e ns rumen

spring It is limited to avoid the appreciable heating of spring Current is limited to 100A

The voltage rating is limited to 600V

,considerable heat is generated

For higher voltage, the PC circuit is designed for 110V and aP.T. is used to step-down the voltage

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The theory of EDM Wattmeter (mathematical expressionfor deflection) is explained on the board

Uniform shape of scale of EDM Wattmeter

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Errors due to PC Capacitance: In addition to the inductance y u - u

Effect is opposite to that produced by inductance Thus, Wattmeter reads high on lagging power factor of load

Quite low at power frequencies

But increases as the frequency increases

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There are two alternate methods of connecting a wattmeter

1. The PC is connected on the supply sideFigure

2. The CC is connected on the supply sideFigure

If I is small, the voltage drop in CC is small

So, connection 1 introduces very small error

If I is large, I p is very small and thus the power loss in PC is very less Therefore, connection 2 is preferable

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Edd Current Errors: Eddy currents are induced in the solid metal parts by alternating

magnetic field of CC

phase of CC field and causes error.

Stray Magnetic Field Error:

The EDM Wattmeter has a relatively weak operating field , Hence, these instruments should be shielded against this effect