In a balanced three-phase delta-connected circuit, the phase voltage is 380 V and the phase current is 12 A - NSC Electrical Technology Power Systems - Question 2 - 2017 - Paper 1
Question 2
In a balanced three-phase delta-connected circuit, the phase voltage is 380 V and the phase current is 12 A. If the phase angle is 25°, calculate the following:
2.1... show full transcript
Worked Solution & Example Answer:In a balanced three-phase delta-connected circuit, the phase voltage is 380 V and the phase current is 12 A - NSC Electrical Technology Power Systems - Question 2 - 2017 - Paper 1
Step 1
2.1.1 The true power
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Answer
To calculate the true power (P) in a three-phase system, we can use the formula:
P = rac{3 imes V_{L} imes I_{ph} imes ext{cos} heta}{1}
Here, the line voltage (V_{L}) is 380 V, the phase current (I_{ph}) is 12 A, and the phase angle (θ) is 25°.
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Answer
The apparent power (S) can be calculated using:
S = rac{P}{ ext{cos} heta}
So first, we need to calculate the apparent power based on the line voltage:
S = rac{3 imes V_{L} imes I_{ph}}{1}
Using the known values:
S = rac{3 imes 380 imes 12}{1} = 13,680 ext{ VA or } 13.68 ext{ kVA}
Step 3
2.2 Calculate the line voltage
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To find the line voltage (V_{L}) when the alternator is connected in star:
= 220 imes ext{sqrt}(3) \
= 381.05 ext{ V} $$
Step 4
2.3 Give ONE reason why electricity is generated in three-phase and not in single phase.
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Electricity is generated in three-phase because it is more efficient for power generation. The three-phase system provides a constant power output, minimizing fluctuations, which enhances the performance of electrical machines.
Step 5
2.4 Name the type of power factor.
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A three-phase generator typically has a lagging power factor due to the inductive nature of the coils. This occurs because the current lags behind the voltage in inductive loads.
Step 6
2.5 State the function of a kilowatt-hour meter.
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The function of a kilowatt-hour (kWh) meter is to measure the amount of electrical energy consumed by a consumer over time, typically in kilowatt-hours.
Step 7
2.6 How many electrical degrees apart are the three coils placed in three-phase generation?
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The three coils in three-phase generation are placed 120 degrees apart from each other.
Step 8
2.7 Draw a voltage phasor diagram.
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The voltage phasor diagram for a three-phase system consists of three vectors (phasors) representing the voltages of each phase placed 120 degrees apart, showing their direction of rotation.
Step 9
2.8 Indicate whether the reading on a voltmeter is a maximum value, average value or effective/rms value.
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The reading on a voltmeter when measuring an AC voltage is typically an effective value or rms (root mean square) value.
