Three equal impedances are first connected in delta across a three-phase balanced supply. If the same impedances are connected in star across the same supplya.phase current will be one-thirdb.line current will be one-thirdc.noned.power consumed will be one-third
Question
Three equal impedances are first connected in delta across a three-phase balanced supply. If the same impedances are connected in star across the same supplya.phase current will be one-thirdb.line current will be one-thirdc.noned.power consumed will be one-third
Solution
The answer to your question is:
a. False - The phase current will not be one-third. In a star connection, the phase current is equal to the line current. In a delta connection, the phase current is √3 times the line current. Therefore, if we switch from a delta to a star connection, the phase current will be divided by √3, not 3.
b. True - The line current will be one-third. In a delta connection, the line current is √3 times the phase current. In a star connection, the line current is equal to the phase current. Therefore, if we switch from a delta to a star connection, the line current will be divided by √3.
c. False - None is not a valid answer.
d. False - The power consumed will not be one-third. The total power consumed in a three-phase system is given by P = √3 * V_L * I_L * pf, where V_L is the line voltage, I_L is the line current, and pf is the power factor. Since the line voltage is the same in both delta and star connections, and the power factor is a property of the load and not the connection, the total power consumed will be the same in both cases.
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A balanced star connected load of (2+j3) Ω per phase and a balanced delta-connected load of(6+j6) Ω per phase are connected in parallel to a three phase, 400V, 50Hz supply. Calculate thephase current in each load, the total current in each supply line, the total power supplied and theoverall power factor.
a) To determine the magnitude and phase of each line current supplied by the source, we use Ohm's law (I = V/Z). Given that the phase voltage V = 240∠30° V and the impedance Z = 50∠25° Ω, we can calculate the current as follows: I = V/Z = 240∠30° / 50∠25° = 4.8∠5° A This is the phase current for phase A. Since it's a balanced star connection, the line current equals the phase current. Therefore, the line current for each phase is 4.8∠5° A. b) To determine the active power and the reactive power supplied by the source, we use the power formula P = √3 * VL * IL * cos(θ) for active power and Q = √3 * VL * IL * sin(θ) for reactive power. Given that the line voltage VL = √3 * phase voltage = √3 * 240 V = 415.69 V, the line current IL = 4.8 A, and the power factor cos(θ) = cos(25°) = 0.9063, we can calculate: P = √3 * 415.69 V * 4.8 A * 0.9063 = 2721.6 W And for reactive power, sin(θ) = sin(25°) = 0.4226, so: Q = √3 * 415.69 V * 4.8 A * 0.4226 = 1341.6 VAR c) To determine the reactive power requirement of each of the three capacitances to improve the power factor to 0.95 lag, we first need to calculate the new reactive power Q_new using the formula Q_new = P * tan(θ_new), where θ_new = cos^(-1)(0.95). Q_new = 2721.6 W * tan(cos^(-1)(0.95)) = 859.5 VAR The reactive power provided by the capacitors should be the difference between the original reactive power and the new reactive power, which is Q - Q_new = 1341.6 VAR - 859.5 VAR = 482.1 VAR. Since the capacitors are connected in star, the reactive power for each capacitor is 482.1 VAR / 3 = 160.7 VAR. ####
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