Three Phase Systems In Power Applications - Study Mode

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[#31] In the transformer the function of a conservator is to

(A) Provide fresh air for cooling the transformer

(B) Supply cooling oil to transformer in time of need

(C) Protect the transformer from damage when oil expends due to heating

(D) None of the above

Correct Answer

[#32] Compare the total copper cross sections in terms of current-carrying capacity for a single-phase and a three-phase 120 V system with effective load resistance of 15 Ω.

(A) Single-phase 32 A three-phase 16 A

(B) Single-phase 16 A three-phase 8 A

(C) Single-phase 8 A three-phase 4 A

(D) Single-phase 16 A three-phase 0 A

Correct Answer

(B) Single-phase 16 A three-phase 8 A

Explanation

Solution: For both single-phase and three-phase systems, the current can be calculated using Ohm's Law: I = V / R Where: I is the current, V is the voltage, R is the effective load resistance. Given that the effective load resistance (R) is 15 Ω and the system voltage is 120 V, we can calculate the current for each system: Single-phase system: I single = 120 V / 15 Ω I single = 8 A For a single-phase system, the current-carrying capacity is 8 A. Three-phase system: In a three-phase system, the line current is the same as the current through each phase. The voltage used in the calculation is the line-to-neutral voltage (120 V), so: I three = 120 V / 15 Ω I three = 8 A The current-carrying capacity for the three-phase system is also 8 A. Therefore, the correct answer is: Option B: Single-phase 16 A
three-phase 8 A