Electrical Machines - II Mid - I, February - 2012

1.In two winding T/F the emf per turn in secondary winding is always the induced emf per turn in primary
  • egual to K times
  • equal to 1/K times
  • equal
  • none
Answer: C
2.The equivalent resistance of a primary of a T/F having k=5 and R1=0.1 ohm when referred to secondary becomes
  • 0.5
  • 0.02
  • 04
  • 2.5
Answer: D
3.During S.C test the iron losses of a T/F is negligible because
  • the i/p is sufficient to meet copper loss
  • flux produced is a small fraction of normal flux
  • iron core is saturated
  • supply frequency is constant
Answer: B
4.Transformer are rated in KVA instead of KW because
  • load power factor is often not known
  • KVA is fixed where as KW depends on load
  • total T/F loss depends on volt ampere
  • it has become customary
Answer: C
5.Two T/F A and B having equal o/p and voltage ratios but unequal percentage impedances of 4 & 2 are operating in parallel .T/F A will be running overload by[ ]%
  • 50
  • 66
  • 33
  • 25
Answer: C
6.The ordinary efficiency of a given T/F is maximum when
  • it runs at half full load
  • it runs at full load
  • its copper loss equal to iron loss
  • it runs slightly over load
Answer: C
7.When a 400 hz T/F is applied at 50 hz its KVA rating is
  • reduced to 1/8
  • increased 8 times
  • unaffected
  • increased 64 times
Answer: A
8.Which of the following connection is best suitable for three phase 4 wire system
  • delta-delta
  • Y-Y
  • delta-Y
  • Y-delta
Answer: C
9.The voltage applied on H.V side of a T/F during short circuit test is 25of its rated voltage. The core
loss will be [ ] % of rated core loss
  • 4
  • 0.4
  • 0.25
  • 0.04
Answer: D
10.When a v-v system is converted into a delta –delta system increases in capacity of the system is
  • 86.6
  • 66.7
  • 73.2
  • 50
Answer: C
11.Any two losses on which hysteres is losses depends are ____________ and ____________
Answer: flux density or frequency or quality and amount of iron in core
12.The main purpose of using core in T/F is to ----------reluctance of the common magnetic circuit.
Answer: decreases
13.In a ideal T/F the permeability of the core is----------
Answer: infinite
14.The iron losses of a T/F at 400hz is 10w assuming that eddy current and hysteres is losses vary square
of flux density the iron losses of T/F at rated voltage but at 50hz would be-------watts.
Answer: 640
15.As the regulation of the T/F decreases, its performance----------------
Answer: increases
16.If the two winding T/F is converted to a auto T/F using additive polarity then the common current
flows-----------the common terminal.
Answer: towards
17.The phase shift between the primary and secondary line voltages of a 3 phase T/F operated in Y/delta connection is----------.
Answer: 30 deg
18.The connection of a 3 phase T/F suitable at the substation end of the transmission line where
the voltage is to be stepped down is --------connection.
Answer: Y/delta
19.Copper losses varies------with the power factor
Answer: inversely
20.The main purpose of performing open circuit test on a T/F is to measure its ---------- losses.
Answer: core losses