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Question

A 5 A, 110 V electrodynamic type wattmeter has a scale having 110 divisions. Its pressure coil is fed by a voltage of
[1102 cos (314t)+2sin (942t)V and its current coil carries a current of [52cos (314t+60)+22sin(628t+90)+2cos (642t+90)]A

The needle of the wattmeter will move to

A
110 divisions
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B
50 divisions
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C
54 divisions
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D
55 divisions
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Solution

The correct option is D 55 divisions
Pavg=VRMSIRMS cos ϕ

In VRMS and IRMS we consider frequency components which are present in both V and I.

Pavg=11022.522cos(60)

Pavg=110×5×12=275 W

Also at full scale, P = 110 × 5 = 550 W which require 110 divisions.

So, for 275 W, number of divisions needed

=275550×110=55

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