AC MACHINES 2

AC MACHINES

1. Conservator ( for a transformer) is used

a) to take up the expansion of oil due to temperature rise.
b) to act as an oil storage
c) for better cooling of transformer
d) none of the above

ans. a

2. Sludge formation in transformer oil is due to which one of the following?

a) Ingress of dust particles and moisture in the oil.
b) Appearance of small fragments of paper, varnish, cotton, and other organic materials in the oil.
c) Chemical reaction of the transformer with oil insulating materials.
d) Oxidation of transformer oil.

ans. d

3. A Buchholz relay is used for

a) protection of transformer against all internal faults.
b) protection of transformer against external faults.
c) protection of transformer against both internal and external faults.
d) protection of induction motors.

ans. a

4. Which is the arrangement of windings in a core type single phase transformer?

a) Half lv inside and half hv outside on each core limb.
b) Lv on one core limb and hv on the other limb.
c) Sandwiched lv and hv discs on each core limb.
d) Half hv side and half hv outside on each core limb.

ans. a

5. Addition of tubes to the transformer tank improves heat dissipation capacity because of

a) additional cooling surface
b) additional dissipation by radiation only.
c) additional dissipation by convention only
d) additional dissipation by radiation and convention both.

ans. a

6. Transformer is rated in kVA instead of kW because

a) load pf is often not known.
b) kVA is fixed where kW depends on load pf.
c) total transformer loss depends on the volt-amperes.
d) it has become customary.

ans. c

7. Which three-phase connection can be used in a transformer to introduce a phase difference of 30 degrees between its output and corresponding input line voltages?

a) Star-Star
b) Star-Delta
c) Delta-Delta
d) Delta-ZigZag

ans. b

8. A 3-phase transformer has a rating of 20 MVA,220 kV(star)- 33kV(delta) with leakage reactance of 12%. The transformer reactance (in ohms) referred to each phase of lv delta connected side is

a) 23.5
b) 19.6
c) 18.5
d) 8.7

ans. b

9. The percentage resistance and percentage reactance of a 10kVA, 400V/200V,3-phase transformer are 2% and 10% respectively. If the constant losses in the machine are 1%., the maximum possible percentage efficiency of the transformer is

a) 98.32
b) 97.25
c) 96.85
d) 96.12

ans. b

10. A 3-phase,delta-star transformer is supplied at 6000 V on the delta connected side.The terminal voltage on the secondary side when supplying full-load at 0.8 lagging power factor at 415V. The equivalent resistance and reactance drops for the transformer are 1% and  5% respectively. The turns ratio of the transformer is

a) 14
b) 24
c) 42
d) 20

ans. b