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Three Phase (Poly Phase ) Circuits Practice Quiz Test Objective Questions
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Question 1 of 15
1. Question
Find V_{bn} of a balanced three phase system circuit where V_{an} = 120 <230^{0} and V_{cn} = 120 <20^{0} ?
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Answer : 120 <250^{0}
Explanation: Since V_{bn} leads V_{cn} by 120^{0}, This is an abc sequence
V_{an} = 120^{0} < 130 + 120
= 120^{0}<250 
Question 2 of 15
2. Question
In a balanced Three phase systems all the line voltages are equal to each other where all the line currents are different to each other.
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Question 3 of 15
3. Question
In a balanced Y connected three phase system, if the line voltage is 520 volts what is the phase voltage?
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Incorrect
Answer: 300 Volts
Explanation:
In Y connected three phase,
Phase Voltage = Line Voltage / √3
= 520 / √3
= 300 Volts 
Question 4 of 15
4. Question
In a three phase Delta connected system, If the phase voltage is 240 Volts. What is line voltage?
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Incorrect
Answer: 240 Volts
Explanation :
In Delta connected three phase system
Phase Voltage = Line Voltage
So
Line Voltage = 240 Volts 
Question 5 of 15
5. Question
In a wye (Y) connected three phase systems, if the line current is 68A, what is the phase current?
Correct
Incorrect
Answer : 68 A
Explanation :
IN Y connected
Phase current = Line Current
Phase Current = 68 A 
Question 6 of 15
6. Question
The phase current in a delta connected system is 72 A, What is the value of line current?
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Incorrect
Answer : 124 A
Explanation :
Line Current = √3 * Phase current
= 1.732 * 72
= 124 A 
Question 7 of 15
7. Question
The horse power rating of three phase motors is about ………………. greater than the single phase motors for the same frame size.
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Question 8 of 15
8. Question
In a three phase circuit the power delivered to the load is same at any instant. where as in single phase it falls to zero three times in each cycle.
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Question 9 of 15
9. Question
In a Delta connected three phase system the Line Voltage is higher than the phase voltage by a factor of square root of three (3).
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Question 10 of 15
10. Question
In a why (Y) connected three phase system the line current is equal to phase current.
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Question 11 of 15
11. Question
Find the power delivered by a three phase system to a resistive load, if the line voltage is 120 V and line current is 15 amps?
Correct
Incorrect
Answer: 3117.6 watts
Explanation :
power delivered = √3 * E_{L} * I_{L}
E_{L } = 120 V
I_{L } = 15 amps
Power = √3 * 120 * 15 = 3117.6 watts 
Question 12 of 15
12. Question
The voltages of three phase system are ……………….. out of phase with each other.
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Question 13 of 15
13. Question
A three phase resistive load has a phase voltage of 240 V and a phase current of 12 A. what is power of the load?
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Incorrect
Answer : 2.88 kW
Explanation :
Power = E_{ph} * I_{ph}
= 240 * 12 = 2880 Watts = 2.88 kW 
Question 14 of 15
14. Question
The size of the transmission conductors in a three phase system is ………….. compared to single phase system for the same kVA and same voltage.
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Question 15 of 15
15. Question
In a power system unbalance occurs due to ………………………..
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