Three Phase Systems Objective Questions and Answers -2
(A) balanced load
(B) unbalanced load
(C) balanced and unbalanced load
(D) balanced to external load
Ans- (C)
2. 3 phase The power factor of each phase in unbalanced load is ……….
(A) values that make the total value unit
(B) opposite sign
(C) various
(D) difficult to find
Ans- (C)
3. A single watt meter can be used to measure power in a 3 phase system only when the load is ……….
(A) balanced load
(B) unbalanced load
(C) balanced and unbalanced load
(D) Stable
Ans- (A)
4. The power factor of 3 phase loads is less than 0.5. When measuring power from a watt meter ………….
(A) Any one watt meter gives zero reading
(B) Any one watt meter gives a negative reading
(C) Both watt meters give positive readings.
(D) Any one watt meter gives back projection.
Ans- (B)
5. When measuring 3 phase power from a 2 watt meter, a measured one points in the opposite direction. Do the opposite to get the reading in the right direction.
(A) Connection of two lines from the supply
(B) Current coil of forward reading
(C) Pressure coil of the converted device
(D) Pressure coils connected to both scales
Ans- (C)
6. Which formula describes the phase angle in the two watt meter method?
(A) 𝜃 = 𝑡𝑎𝑛 − 1 (√3 (𝑊1 + 𝑊2) / (𝑊1 − 𝑊2))
(B) 𝜽 = 𝒕𝒂𝒏 − 𝟏 (√3 (𝑾𝟏 − 𝑾𝟐) / (𝑾𝟏 + 𝑾𝟐))
(C) 𝜃 = 𝑐𝑜𝑠 − 1 (√3 (𝑊1 + 𝑊2) / (𝑊1 − 𝑊2))
(D) 𝜃 = 𝑐𝑜𝑠 − 1 (√3 (𝑊1 − 𝑊2) / (𝑊1 − 𝑊2))
Ans- (B)
7. What is the relationship between phase voltage and line voltage in a system connected by a balanced three phase?
(A) 𝑉𝑝ℎ =√3 𝑉𝐿
(B) 𝑽𝑳 = 𝟑 𝑽𝒑𝒉
(C) 𝑉𝑝ℎ = 𝑉𝐿 /√2
(D) None of these
Ans- (B)
8. Voltage in three phase star combinations is as much as described below?
(A) phase voltage
(B) 1 / √3 volt
(C) √3 volt
(D) 3 volts
Ans- (C)
9. In a three phase system, the active power can be calculated in which of the following ways?
(A) P = VL IL cos𝜑
(B) P = √3 VL IL
(C) P = √3 VL IL cos𝝋
(D) P = √3 Vph Iph cos𝜑
Ans- (C)
10. A watt meter is 15A / 30A, 300V / 600V and its scale is up to 4500 watts. When the scale was combined to 30A, 600V, the indicator showed 2000 watts. Is the actual power in the circuit?
(A) 2000 watts
(B) 4000 watts
(C) 6000 watts
(D) 8000 watts
Ans- (D)
11. Voltage ampere determination in three phase method is expressed by
(A) 3 VL IL
(B) Vph Iph
(C) VL IL
(D) √3 VL IL
Ans- (D)
12. The phase order of 3 phase supply is generally?
(A) RYB
(B) RBY
(C) YRB
(D) BYR
Ans- (A)
13. In 3 phase balanced load, will there be current in the depressed wire?
(A) 2 Iph
(B) 3 IL
(C) zero
(D) IL
Ans- (C)
14. In a 3-phase system, every two phases have phase difference?
(A) 60 degree
(B) 30 degree
(C) 120 degree
(D) 360 degree
Ans- (C)
15. The total power measured by a single watt scale of 3 phase connected to a balanced load star combination will be equal to ……………….
(A) √2 W
(B) 0.707 W
(C) √3 W
(D) 3 W
Ans- (D)
16. What will be the output voltage in open delta, if the input voltage is V?
(A) 0.50V
(B) 0.57V
(C) 0.33V
(D) 0 V
Ans- (B)
17. What is the meaning of phase voltage?
(A) Voltage between R and Y, Y and B, B and R.
(B) Voltage between R, Y or B and neutrals
(C) Voltage which is always in phase with current.
(D) Voltage between the ground and any one line
Ans- (B)
18. What is the meaning of line voltage?
(A) Voltage between R and Y, Y and B, B and R.
(B) Voltage between R, Y or B and neutrals
(C) Voltage which is always in phase with current.
(D) Voltage between the ground and any one line
Ans- (A)
19. The power coefficient of the load when a watt meter shows a negative reading during the 3 phase power measurement using the two watt meter method.
(A) is above 0.5
(B) is below 0.5
(C) is a unit
(D) is above 0.707
Ans- (B)
20. What is the power used for balanced loads in star and delta connections?
(A) 3 VL IL cos𝜃
(B) √3 VL IL cos𝜽
(C) 3 VP IP cos𝜃
(D) √3 VL IL sin𝜃
Ans- (B)