In a balanced three-phase delta-connected circuit, the phase voltage is 380 V and the phase current is 12 A - NSC Electrical Technology Electronics - 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 Electronics - 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 balanced three-phase system, we use the formula:
P = rac{3 V_{Ph} I_{Ph} \cos heta}{\sqrt{3}}
Where:
VPh=380V (phase voltage)
IPh=12A (phase current)
heta=25° (phase angle)
Thus, the true power is:
P=3×380V×12A×cos(25°)=12,40kW
Step 2
2.1.2 The apparent power
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Answer
The apparent power (S) can be calculated using:
S=3VLIL
Where:
VL=380V (line voltage)
IL=12A (line current)
So, we find:
S=3×380V×12A=13,68kVA
Step 3
2.2 Calculate the line voltage in star connection
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Answer
In a star connection, the line voltage (V_L) can be calculated using:
VL=3VPh
Given that IPh=20A and the phase angle is 30°, we can compute:
VL=3×220V=381,05V
Step 4
2.3 Give ONE reason why electricity is generated in three-phase and not in single phase.
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For high power generation, the three-phase system is functional and efficient because the voltages between all phases (i.e. line voltages) are the same. This ensures greater stability and efficiency in power distribution.
Step 5
2.4 Name the type of power factor that a three-phase generator has, and explain why it has such a type of power factor.
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Answer
A generator has a lagging power factor due to its inductive nature. In an inductive load, the current lags behind the applied voltage, causing the phase difference.
Step 6
2.5 State the function of a kilowatt-hour meter.
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The function of a kWh meter is to measure the amount of power consumed by a consumer over a period of time, which is useful for billing purposes.
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.
Step 8
2.7 Draw a voltage phasor diagram that represents a three-phase supply.
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A voltage phasor diagram for a three-phase supply consists of three vectors representing phase voltages, each separated by 120 degrees. The resultant sum of these phase voltages gives a balanced three-phase system.
Step 9
2.8 Indicate whether the reading on a voltmeter when measuring an AC voltage 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 usually the effective value or rms value.
