3_PHASE Solved Problems

8/3/2019 3_PHASE Solved Problems

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Solved Problems

Example 1. 3 identical coils, each of resistance 20 and inductance 0.5H areconnected in star to a 3phase 400V, 50Hz supply. Determine the line current and thepower consumed.Data given:

R=20, L=0.5H, VL=400V, f=50Hz.

Vph =3

VL = .V94.2303

400

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Example 2. Three impedances each of (15+j20) are connected in mesh across a 3-phase 400V A.C supply. Determine the phase current, line current, active power andreactive power drawn from the supply.

Data Given Zph=(15+j20), VL=400V=Vph (deltaconnection) and IL= 3 IPh

Iph = ph

ph

Z

V

0

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Example 3 A balanced 3phase star connected load of 150KW takes a leading currentof 100Amps with a line voltage of 1100V. Find the circuit constants of the load perphase.

Data given:P=150KW, IL=100A=IPh(star connection) VL=1100V, f=50Hz.

Given that current is leading, therefore circuit constants are resistance andcapacitance.

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Example 4.Two wattcmeters are connected to measure power input to a 3phasebalanced circuit indicate 8KW and 0.8KW; the later reading being obtained afterreversing the current coil connection. Find i) power factor of the load. ii) Active poweriii) reactive power iv) apparent power

Data given: W1=8KW, W2= -0.8KW (because the reading is obtained after reversing the

current coil connection)

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Example 5 Each of two wattmeters connected to measure the input to a 3phasesystem reads 10KW on a balanced load when the power factor is unity. What doeseach instrument reads when the power factor falls to i) 0.866 lagging ii) 0.5 leading thetotal 3phase power remaining unchanged.Data given: W1=W2=10KW, when pf is unity, i.e cos 0:1

Therefore total power =W1+W2=20KW (1)

i) When pf=0.866 lagging

ii) When pf=0.5 leadingKWandWKWWandSolving

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WWei

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3_PHASE Solved Problems