Short notes
Objective questions (Answers are given below)
- Which of the following connections is best suited
for 3-phase, 4-wire service ?
(a) Δ − Δ (b) Y − Y
(c) Δ − Y (d) Y − Δ - In a three-phase Y-Y transformer connection,
neutral is fundamental to the
(a) suppression of harmonics
(b) passage of unbalanced currents due to
unbalanced loads
(c) provision of dual electric service
(d) balancing of phase voltages with respect to
line voltages. - As compared to Δ − Δ bank, the capacity of the
V − V bank of transformers is ……… percent.
(a) 57.7 (b) 66.7
(c) 50 (d) 86.6 - If three transformers in a Δ − Δ are delivering
their rated load and one transformer is removed,
then overload on each of the remaining transformers
is ……… percent.
(a) 66.7 (b) 173.2
(c) 73.2 (d) 58 - When a V − V system is converted into a Δ − Δ
system, increase in capacity of the system is
……… percent.
(a) 86.6 (b) 66.7
(c) 73.2 (d) 50 - For supplying a balanced 3 − φ load of 40-kVA,
rating of each transformer in V − V bank should
be nearly ……… kVA.
(a) 20 (b) 23
(c) 34.6 (d) 25 - When a closed − Δ bank is converted into an
open − Δ bank, each of the two remaining transformers
supplies ……… percent of the original
load.
(a) 66.7 (b) 57.7
(c) 50 (d) 73.2 - If the load p.f. is 0.866, then the average p.f. of
the V – V bank is
(a) 0.886 (b) 0.75
(c) 0.51 (d) 0.65 - A T − T connection has higher ratio of utilization
that a V − V connection only when
(a) identical transformers are used
(b) load power factor is leading
(c) load power factor is unity
(d) non-identical transformers are used. - The biggest advantage of T – T connection over
the V – V connection for 3-phase power transformation
is that it provides
(a) a set of balanced voltages under load
(b) a true 3-phase, 4-wire system
(c) a higher ratio of utilization
(d) more voltages. - Of the following statements concerning parallel
operation of transformers, the one which is not
correct is
(a) transformers must have equal voltage
ratings
(b) transformers must have same ratio of transformation
(c) transformers must be operated at the same
frequency
(d) transformers must have equal kVA ratings
- Statement
An auto-transformer is more efficient in transferring
energy from primary to secondary
circuit.
Reason
Because it does so both inductively and
conductively.
Key
(a) statement is false, reason is correct and
relevant
(b) statement is correct, reason is correct but
irrelevant
(c) both statement and reason are correct and
are connected to each other as cause and
effect
(d) both statement and reason are false. - Out of the following given choices for poly phase
transformer connections which one will you
select for three-to-two phase conversion ?
(a) Scott (b) star/star
(c) double Scott (d) star/double-delta - A T − T transformer cannot be paralleled with
……… transformer.
(a) V − V (b) Y − Δ
(c) Y − Y (d) Δ − Δ - Instrument transformers are used on a.c. circuits
for extending the range of
(a) ammeters (b) voltmeters
(c) wattmeters (d) all of the above. - Before removing the ammeter from a current
transformer, its secondary must be shortcircuited
in order to avoid
(a) excessive heating of the core
(b) high secondary e.m.f.
(c) increase in iron losses
(d) all of the above.
1.c 2. a 3. a 4. c 5. c 6. b 7. b 8. b 9. d 10.b 11. d 12. c 13. a 14. b 15. d 16. d