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The figure given below shows a three- phase delta connected load supplied from a 400V, 50Hz, 3-phase balanced source. The pressure coil (PC) and cur r ent coi l (CC) of a wat t met er ar e connected to the load as shown, with the coil polarities suitably selected to ensure a positive deflection.
The wattmeter reading will be
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- 0
- 1600 Watt
- 800 Watt
- 400 Watt
Correct Option: C
Assume Vab as reference phasor, we have
Vab = V ∠0°
Vbc = 400 ∠– 120°
Vca = 400 ∠– 240°
Current through current coil,
| Icc = | = | |||
| Z2 | 100 + 10 |
= 4 ∠– 240° A
and, voltage across pressure coil,
Vpc = Vbc = 400∠– 120°
Hence, wattmeter reading, P = Vpc Icc cos φ
φ is the angle between Vpc and Icc
= 400 × 4 × cos 120° = – 800 watts
