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While measuring power observed by the load using the arrangement shown in the figure, the obtained values voltmeter and ammeter readings are 150 V and 12 A respectively. If the load power factor is unity, the wattmeter reading will be- (the internal resistances of instruments are mentioned in the figure).

While measuring power observed by the load using the arrangement shown in the figure, the obtained values voltmeter and ammeter readings are 150 V and 12 A respectively. If the load power factor is unity, the wattmeter reading will be- (the internal resistances of instruments are mentioned in the figure). Correct Answer 1843.2 W

Wattmeter:

A wattmeter is used to measure the power in the circuits.

The wattmeter reading can be expressed as

W = V I cos ϕ Watts

V = RMS value of pressure coil voltage

I = RMS value of the current in the current coil 

cos ϕ = Power factor

Calculation:

Given that,

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Current through current coil (I) = ammeter reading = 12 A.

Power factor = cos ϕ = 1

Resistance of current coil = Rcc = 0.2 Ω

Ammeter resistance = RA = 0.1 Ω

The pressure coil voltage = The voltage drop across the current coil and ammeter + voltmeter reading.

V = I (Rcc + RA) + 150

V = 12 (0.2 + 0.1) + 150

∴ V= 153. 6 V

Watt meter reading is 

W = VI cos ϕ

= (153.6) (12) (1)

∴ W = 1843.2 W

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