Electrical Machines Multiple Choice Questions on “Three Phase Transformers”.
1. A V-V connected transformer can be connected in parallel to delta-delta connected transformer but not to ___________
A. delta-star
B. star-delta
C. star-V
D. all of the mentioned
Answer: A
Clarification: The V-V connected transformer and D-D connected transformers have same phase displacement, so they only can be connected in parallel to each other.
2. Three units of single phase transformers and one single three-phase transformer rating ___________
A. will be same for one rating
B. can never be made same
C. may be same
D. none of the mentioned
Answer: A
Clarification: Three single phase transformers and one single unit of three phase transformer will be same only. this is done to reduce the cost and spacing.
3. The transformer which is more feasible to use in the distribution ends should be ___________
A. star-delta
B. delta-star
C. scott
D. delta-delta
Answer: A
Clarification: Star-delta will have lower voltage at delta end.
4. For the given circuit below, the voltage across the terminal ‘a’ and ‘b’ is?
electrical-machines-questions-answers-three-phase-transformer-q4″>
A. 1.73*V
B. V/1.73
C. V
D. 3*V
Answer: A
Clarification: From delta to star, the voltage between the lines will increase by 1.73.
5. The voltage across the terminal ‘r’ and ‘y’ is?
electrical-machines-questions-answers-three-phase-transformer-q5″>
A. 1.73*V
B. V/1.73
C. V
D. 3*V
Answer: B
Clarification: From star to delta, the line voltage will reduce by 1.73.
6. The delta-delta connections are used in applications of ___________
A. large l.v. transformers
B. small h.v. transformers
C. large h.v. transformers
D. small l.v. transformers
Answer: A
Clarification: Delta connected winding handle line voltages so it needs more turns in the winding but thin wires.
7. Open delta transformers can be obtained from ___________
A. delta-delta
B. star-delta
C. delta-star
D. any of the mentioned
Answer: A
Clarification: If one of the transformers is removed from the bank of only delta-delta, then it behaves with 58% power delivery.
8. If one of the transformers is removed from the bank of only delta-delta, then it behaves with 58% power delivery.
A. True
B. False
Answer: A
Clarification: It is true as the circuit will still be closed and the transformer will operate will lesser operating point.
9. Below is the circuit diagram for delta-delta transformer have a transformation ratio ‘k’. The current flowing in the windings of phase r-y in the below diagrams is?
electrical-machines-questions-answers-three-phase-transformer-q9″>
A. k*I/1.73
B. I/k*1.73
C. 1.73*k*I
D. 3*k*I
Answer: A
Clarification: The phase current will be reduced by 1/1.73.
10. If para magnetic core is used in the place of the ferromagnetic core of the transformer, then magnetostriction will ___________
A. be vanished
B. reduce
C. increase
D. not be affected
Answer: A
Clarification: If the core is not ferromagnetic then the transformer will not operate at all so no humming sound.
11. When we magnetize the ferromagnetic core of the transformer, core length_______ and it ______ when demagnetized.
A. decreases, increases
B. decreases, decreases
C. increases,increases
D. none of the mentioned