# Electrical Machines Test Questions Set - 11

1)   Use of an autotransformer is economical when its transformation ratio is

a. Near unity
b. Much greater than unity
c. Much lesser than unity
d. None of these
 Answer  Explanation  Related Ques ANSWER: Near unity Explanation: No explanation is available for this question!

2)   If an autotransformer having transformation ratio equal to 0.6 is supplying a load of 8kw then its power transferred from primary to secondary is given by

a. 3 kW
b. 3.2 kW
c. 3.4 kW
d. 3.5 kW
 Answer  Explanation  Related Ques ANSWER: 3.2 kW Explanation: No explanation is available for this question!

3)   In an autotransformer if the power transferred inductively is equal to the power conducted through, then transformation ratio is given by

a. 1
b. 0.5
c. 2
d. 0
 Answer  Explanation  Related Ques ANSWER: 0.5 Explanation: No explanation is available for this question!

4)   For a two winding transformer if VA rating is given by (VA)tw and K is the transformation ratio of an autotransformer then the VA rating of the autotransformer is given by

a. (K/(K-1)) * (VA)tw
b. (K/(1-K)) * (VA)tw
c. ((1-K)/K) * (VA)tw
d. ((K-1)/K) * (VA)tw
 Answer  Explanation  Related Ques ANSWER: (K/(K-1)) * (VA)tw Explanation: No explanation is available for this question!

5)   Whether the given autotransformer is step up or step down, its VA rating is always

a. Greater than the two winding transformer
b. Equal to the two winding transformer
c. lesser than the two winding transformer
d. cannot say
 Answer  Explanation  Related Ques ANSWER: Greater than the two winding transformer Explanation: No explanation is available for this question!

6)   Compared to the two winding transformer, in an autotransformer the leakage reactance and copper losses is

a. less, more
b. less, less
c. more, more
d. more, less
 Answer  Explanation  Related Ques ANSWER: less, less Explanation: No explanation is available for this question!

7)   If a two winding transformer is converted into an autotransformer  by applying additive polarity and subtractive polarity which results in the secondary voltages of 1840 and 1810 volts. Then the primary and secondary voltages of transformer are

a. 1800V, 50V
b. 1810V, 40V
c. 1820V, 30V
d. 1825V, 15V
 Answer  Explanation  Related Ques ANSWER: 1825V, 15V Explanation: No explanation is available for this question!

8)   A star connected three phase transformer is provided with tertiary delta connected winding which allows the flow of

a. Second harmonic of exciting current
b. Third harmonic of exciting current
c. Fifth harmonic of exciting current
d. Seventh harmonic of exciting current
 Answer  Explanation  Related Ques ANSWER: Third harmonic of exciting current Explanation: No explanation is available for this question!

9)   A ferro resonant transformer regulation is also known as

a. Constant current transformer
b. Constant voltage transformer
c. Variable current transformer
d. Variable voltage transformer
 Answer  Explanation  Related Ques ANSWER: Constant current transformer Explanation: No explanation is available for this question!

10)   In a constant voltage transformer, the primary winding and secondary winding is formed by the

a. capacitor, linear inductor
b. Non linear inductor, capacitor
c. Linear inductor and non linear inductor
d. Capacitor, non linear inductor
 Answer  Explanation  Related Ques ANSWER: Linear inductor and non linear inductor Explanation: No explanation is available for this question!

11)   A constant voltage transformer is fed with the sinusoidal input voltage. Its output is

a. Sinusoidal
b. Flat topped
c. Saw tooth
d. Zig-zag
 Answer  Explanation  Related Ques ANSWER: Flat topped Explanation: No explanation is available for this question!

12)   The output voltage of constant voltage transformer contains excessive harmonics which can be filtered out by using

a. RC filter
b. RL filter
c. LC filter
d. None of these
 Answer  Explanation  Related Ques ANSWER: LC filter Explanation: No explanation is available for this question!

13)   The constant voltage transformers are most commonly used in

a. Personal computers
b. Arc welding
c. Power distribution
d. Electric cars
 Answer  Explanation  Related Ques ANSWER: Personal computers Explanation: No explanation is available for this question!

14)   In the operation of constant current transformer, two magnetic fields are produced. The magnetic field produced in the secondary is

a. In the same direction of magnetic field produced in the primary
b. In the opposite direction of magnetic field produced in the primary
c. Perpendicular to the direction of magnetic field produced in the primary
d. Both (b) and (c)
 Answer  Explanation  Related Ques ANSWER: In the opposite direction of magnetic field produced in the primary Explanation: No explanation is available for this question!

15)   The constant current transformer are mainly used for

a. Supplying series connected street light
b. Personal computers
c. Arc welding
d. Electric toys
 Answer  Explanation  Related Ques ANSWER: Supplying series connected street light Explanation: No explanation is available for this question!

16)   The high leakage impedance transformers are suitable for the applications of

a. Arc welding
b. Personal computers
c. Street lights
d. Electric lamp
 Answer  Explanation  Related Ques ANSWER: Arc welding Explanation: No explanation is available for this question!

17)   Small iron core transformers used in certain frequency range is also called audio frequency transformer. The audio frequency range is

a. 20 t0 2000 Hz
b. 20 to 20000 Hz
c. 2 to 2000 Hz
d. 200 to 20000 Hz
 Answer  Explanation  Related Ques ANSWER: 20 to 20000 Hz Explanation: No explanation is available for this question!

18)   Pulse transformers are small in size. The leakage inductance and permeability of alloy used is

a. Low, high
b. Low, low
c. High, low
d. High, high
 Answer  Explanation  Related Ques ANSWER: Low, high Explanation: No explanation is available for this question!

19)   In a three phase transformer, the angle between two consecutive cores is

a. 30 degree
b. 60 degree
c. 120 degree
d. 150 degree
 Answer  Explanation  Related Ques ANSWER: 120 degree Explanation: No explanation is available for this question!

20)   In a three phase transformer, the current flowing in three primaries produces three corresponding fluxes. The sum of these three fluxes at any instant is

a. Zero
b. Three times of any individual flux
c. One third of any individual flux
d. None of these
 Answer  Explanation  Related Ques ANSWER: Zero Explanation: No explanation is available for this question!

21)   A bank of three single phase transformer can be used for obtaining the three phase output. Three magnetic circuits produced in case of a bank of three single phase transformer and in case of single phase transformer are

a. Linked, independent
b. Independent, linked
c. Linked, linked
d. Both are independent
 Answer  Explanation  Related Ques ANSWER: Independent, linked Explanation: No explanation is available for this question!

22)   For the star - star connection of three phase transformer, the phase angle between the phase voltages and line voltages on both primary and secondary side is

a. 0 degree
b. 30 degree
c. 60 degree
d. 120 degree
 Answer  Explanation  Related Ques ANSWER: 30 degree Explanation: No explanation is available for this question!

23)   While connecting three phase transformer in star-star connection, the terminals of secondaries are wrongly joined. Then the mutual phase angle and magnitude of line to line voltages of secondaries will

a. Equal to 120 degree and equal as before
b. Not equal to 120 degree and different
c. Equal to 120 degree and different
d. None of these
 Answer  Explanation  Related Ques ANSWER: Not equal to 120 degree and different Explanation: No explanation is available for this question!

24)   For the parallel operation of three phase transformers, which among the following connection is not applicable?

a. Δ - Δ to Y – Y
b. Y - Δ to Δ - Y
c. Y - Y to Y – Y
d. None of these
 Answer  Explanation  Related Ques ANSWER: None of these Explanation: No explanation is available for this question!

25)   Due to presence of third harmonic component in the star-star connection of three phase transformer, the frequency of the circuit component becomes

a. Three times of the circuit frequency
b. One third of the circuit frequency
c. Remains same
d. None of these
 Answer  Explanation  Related Ques ANSWER: Three times of the circuit frequency Explanation: No explanation is available for this question!

26)   In star-star connection of three phase transformer, if VL is the line voltage and IL is the line current then phase voltage and phase current is given by

a. VL / √3, IL
b. VL , IL
c.3VL , IL / √3
d. VL, IL / √3
 Answer  Explanation  Related Ques ANSWER: VL / √3, IL Explanation: No explanation is available for this question!

27)   The phase shift in a star-star connected three phase transformer is

a. 0 degree
b. 30 degree
c. 60 degree
d. 120 degree
 Answer  Explanation  Related Ques ANSWER: 0 degree Explanation: No explanation is available for this question!

28)   If K is the transformation ratio, then the secondary phase voltage of delta-delta connected three phase transformer will be

a. 1 / K times of the primary phase voltage
b. Equal to the primary phase voltage
c. 1 / K ∧ 2 times of the primary phase voltage
d. K times the primary phase voltage
 Answer  Explanation  Related Ques ANSWER: K times the primary phase voltage Explanation: No explanation is available for this question!

29)   For large low voltage transformers, the most commonly used connections are

a. Star - star connection
b. Delta - delta connection
c. V connection
d. All of these
 Answer  Explanation  Related Ques ANSWER: Delta - delta connection Explanation: No explanation is available for this question!

30)   In case of delta-star connection of three phase transformer, secondary line voltage with respect of primary line voltage is at

a. 0 degree
b. 30 degree leading
c. 30 degree lagging
d. 60 degree lagging
 Answer  Explanation  Related Ques ANSWER: 30 degree leading Explanation: No explanation is available for this question!

31)   The most commonly used connections for power systems as a step - up and step - down transformers are

a. Star - delta, star - star
b. Delta - star, star - delta
c. Star - star, delta - delta
d. Star - delta, delta - star
 Answer  Explanation  Related Ques ANSWER: Star - delta, delta - star Explanation: No explanation is available for this question!

32)   The load carried by V - V connection is

a. 47.7% of the original load
b. 57.7% of the original load
c. 67.7% of the original load
d. 87.7% of the original load
 Answer  Explanation  Related Ques ANSWER: 57.7% of the original load Explanation: No explanation is available for this question!

33)   Consider three transformers connected in delta-delta fashion and supplying their rated load. Now if one of the transformer is removed then each of the remaining two transformer is overloaded. The overload on each transformer is given as

a. 1
b. 1.232
c. 1.732
d. 1.872
 Answer  Explanation  Related Ques ANSWER: 1.732 Explanation: No explanation is available for this question!

34)   The average power factor at which V-V bank is operating is less than that with the load. The power factor is

a. 57.7% of the balanced load power factor
b. 66.7% of the balanced load power factor
c. 86.6% of the balanced load power factor
d. None of theses
 Answer  Explanation  Related Ques ANSWER: 86.6% of the balanced load power factor Explanation: No explanation is available for this question!

35)   With a bank of two single phase transformers connected in V - V fashion supplying a balanced three phase load with Cosφ as power factor. The power factor of the two transformers is given by

a. Cosφ , Cosφ
b. Cosφ, Cos(30+φ)
c. Cos(30-φ), Cos(30+φ)
d. Cos(30-φ), Cosφ
 Answer  Explanation  Related Ques ANSWER: Cos(30-φ), Cos(30+φ) Explanation: No explanation is available for this question!

36)   In T-T connection, the percentage tapping of main transformer and teaser transformer are

a. 50%, 86.6%
b. 50%, 50%
c. 86.6%, 86.6%
d. 86.6%, 50%
 Answer  Explanation  Related Ques ANSWER: 50%, 86.6% Explanation: No explanation is available for this question!

37)   In T-T connection, the ratio of actual capacity to the available capacity is

a. 1
b. 0.928
c. 1.928
d. 0.5
 Answer  Explanation  Related Ques ANSWER: 0.928 Explanation: No explanation is available for this question!

38)   Scott connection is used for the conversion of

a. Single phase to three phase
b. Three phase to single phase
c. Single phase to two phase
d. All of these
 Answer  Explanation  Related Ques ANSWER: Single phase to two phase Explanation: No explanation is available for this question!

39)   In Scott connection, the voltage across the teaser leads the mains by

a. 30 degree
b. 60 degree
c. 90 degree
d. 120 degree
 Answer  Explanation  Related Ques ANSWER: 90 degree Explanation: No explanation is available for this question!

40)   In a single phase, full wave bridge circuit and in three phase, delta full wave bridge circuit, the ripple voltage frequency is always

a. Twice the line frequency, six times the line frequency
b. Both will be twice the line frequency
c. Both will be six times the lines frequency
d. None of these
 Answer  Explanation  Related Ques ANSWER: Twice the line frequency, six times the line frequency Explanation: No explanation is available for this question!

41)   Most familiar application of zig - zag transformer is as

a. Ground reference on an ungrounded system
b. Converting single phase to two phase
c. Reducing harmonics
d. All of these
 Answer  Explanation  Related Ques ANSWER: Ground reference on an ungrounded system Explanation: No explanation is available for this question!

42)   In double delta transformation, a double delta refers to the case where there are two delta transformations in

a. Parallel
b. Series
c. Both series and parallel
d. Neither series nor parallel
 Answer  Explanation  Related Ques ANSWER: Series Explanation: No explanation is available for this question!

43)   A differential relay comparator used for the protection of three phase transformers has

a. One comparator
b. Two comparator
c. Three comparator
d. Six comparator
 Answer  Explanation  Related Ques ANSWER: Three comparator Explanation: No explanation is available for this question!