Electrical Machines Test Questions Set - 9

1)   An over excited synchronous motor operating on no load condition is called as

a. Synchronous capacitor
b. Synchronous condenser
c. Both (a) and (b)
d. None of these
Answer  Explanation  Related Ques

ANSWER: Both (a) and (b)

Explanation:
No explanation is available for this question!


2)   The synchronous motor as a synchronous condenser is used to

a. Improve power factor
b. Reduce hunting
c. Both (a) and (b)
d. None of these
Answer  Explanation  Related Ques

ANSWER: Improve power factor

Explanation:
No explanation is available for this question!


3)   In a step-up transformer when the voltage delivered is higher than that received, the current delivered is

a. Higher than that received
b. Lower than that received
c. Equal to that received
d. Equal to half of that received
Answer  Explanation  Related Ques

ANSWER: Lower than that received

Explanation:
No explanation is available for this question!


4)   What happens if dc supply is given to the transformer?

a. Transformer will work more efficiently
b. It will work less efficiently
c. It will work same as with ac supply
d. Not work at all
Answer  Explanation  Related Ques

ANSWER: Not work at all

Explanation:
No explanation is available for this question!


5)   For a transformer, permeability and hysteresis loss should be

a. Low, low
b. Low, high
c. High, low
d. High, high
Answer  Explanation  Related Ques

ANSWER: High, low

Explanation:
No explanation is available for this question!


6)   During starting, the magnetic field produced in a transformer is

a. 1.5 times the maximum
b. Equal to the maximum
c. 0.5 times of the maximum
d. Equal to zero
Answer  Explanation  Related Ques

ANSWER: Equal to zero

Explanation:
No explanation is available for this question!


7)   For transformers ratings are always given in kVA, not in kW because

a. It is conventional
b. Power factor of load is not known
c. For higher efficiency
d. To make calculation simple
Answer  Explanation  Related Ques

ANSWER: Power factor of load is not known

Explanation:
No explanation is available for this question!


8)   In a 5 kV / 400V, 75 kVA single phase transformer, the current flowing in the primary winding of transformer is 10A. what will be the current flowing in the secondary winding?

a. 100A
b. 120A
c. 125A
d. 130A
Answer  Explanation  Related Ques

ANSWER: 125A

Explanation:
No explanation is available for this question!


9)   The hysteresis loss in a transformer is minimized by using

a. Laminated sheets of steel
b. Special grade of heat treated grain oriented silicon steel
c. Bracing of transformer assemblies
d. Proper design of winding
Answer  Explanation  Related Ques

ANSWER: Special grade of heat treated grain oriented silicon steel

Explanation:
No explanation is available for this question!


10)   Laminations and varnish are used to reduce the eddy current losses. The laminations and varnish tends to present a

a. High resistance path to eddy current
b. Low resistance path to eddy current
c. Low reluctance magnetic path
d. High reluctance magnetic path
Answer  Explanation  Related Ques

ANSWER: High resistance path to eddy current

Explanation:
No explanation is available for this question!


11)   In a transformer sludging is principally due to

a. Moisture present in oil
b. Dust particles present inside the tank
c. Decomposition of oil
d. Vaporization of oil
Answer  Explanation  Related Ques

ANSWER: Decomposition of oil

Explanation:
No explanation is available for this question!


12)   Breather present in a transformer is used to

a. Absorb atmospheric moisture
b. Prevent insulation failure
c. Cooling
d. All of these
Answer  Explanation  Related Ques

ANSWER: Absorb atmospheric moisture

Explanation:
No explanation is available for this question!


13)   The globules of water present in oil may result directly in a breakdown. This can be prevented by

a. Proper insulation
b. Isolation of oil from air by using an inert gas
c. Heating the tank
d. None of these
Answer  Explanation  Related Ques

ANSWER: Isolation of oil from air by using an inert gas

Explanation:
No explanation is available for this question!


14)   In a transformer tank, grounding is given to protect it from lightening stoke. Grounding lug is usually installed near

a. Bottom of the tank
b. Top of the tank
c. In the middle of the tank
d. Anywhere on the tank
Answer  Explanation  Related Ques

ANSWER: Bottom of the tank

Explanation:
No explanation is available for this question!


15)   For the calculation of induced emf in a transformer the flux used in the equation is in its

a. RMS value
b. Average value
c. Maximum value
d. Double of its maximum value
Answer  Explanation  Related Ques

ANSWER: Maximum value

Explanation:
No explanation is available for this question!


16)   If the primary winding is excited by a purely sinusoidal alternating voltage, than the nature of the flux produced is

a. Square wave
b. Rectangular wave
c. Saw tooth wave
d. Sinusoidal wave
Answer  Explanation  Related Ques

ANSWER: Sinusoidal wave

Explanation:
No explanation is available for this question!


17)   The average emf induced per turn in a transformer is given by

a. 1.11f Φm
b. 0.5f Φm
c. 4f Φm
d. 8f Φm
Answer  Explanation  Related Ques

ANSWER: 4f Φm

Explanation:
No explanation is available for this question!


18)   A transformer is said to be isolation transformer if its primary to secondary ratio is

a. Equal to one
b. More than one
c. Less than one
d. Equal to 0.5
Answer  Explanation  Related Ques

ANSWER: Equal to one

Explanation:
No explanation is available for this question!


19)   A 5kV / 250V, 50Hz single phase core type transformer with average flux in the core is 0.05wb. The number of turns in the primary winding is

a. 273
b. 278
c. 280
d. 287
Answer  Explanation  Related Ques

ANSWER: 287

Explanation:
No explanation is available for this question!


20)   If V1 is the primary applied voltage and E1 is the primary induced emf. For an ideal transformer

a. V1 > E1
b. V1 = E1
c. V1 < E1
d. V1 = E1 / 2
Answer  Explanation  Related Ques

ANSWER: V1 = E1

Explanation:
No explanation is available for this question!


21)   For an ideal transformer winding resistances and leakage flux should be

a. Least, maximum
b. Both should be maximum
c. Both should be zero
d. Maximum, least
Answer  Explanation  Related Ques

ANSWER: Both should be zero

Explanation:
No explanation is available for this question!


22)   When a resistance is to be transferred from primary to secondary side of transformer with the turns ratio as k, it must be

a. Multiplied by k ∧ 2
b. Multiplied by k
c. Divided by k
d. Divided by k ∧ 2
Answer  Explanation  Related Ques

ANSWER: Multiplied by k ∧ 2

Explanation:
No explanation is available for this question!


23)   If the power factor of load is near unity and leading, the full load voltage regulation of a power transformer will be

a. Zero
b. One
c. 0.5
d. 0.707
Answer  Explanation  Related Ques

ANSWER: Zero

Explanation:
No explanation is available for this question!


24)   If the applied voltage of a certain transformer is increased by 50% keeping the frequency constant, the maximum flux density will become

a. Remains same
b. 1.5 times the original value
c. Double the original value
d. Half of the original value
Answer  Explanation  Related Ques

ANSWER: 1.5 times the original value

Explanation:
No explanation is available for this question!


25)   A 10 kVA, 2000 / 100v transformer has R1=1.5 ohm, R2=0.005 ohm, X1=2.5 ohm and X2=0.08 ohm. The equivalent resistance referred to primary is

a. 3 ohm
b. 3.25 ohm
c. 3.5 ohm
d. 3.8 ohm
Answer  Explanation  Related Ques

ANSWER: 3.5 ohm

Explanation:
No explanation is available for this question!


26)   A transformer when connected to a 230V, 50Hz supply, under no load draws a current of 4A at a power factor of 0.2 lagging. The magnetizing current (Im) and core loss (Pc) is equal to

a. 3.919A, 184W
b. 1.84A, 391.9W
c. 39.19A, 184W
d. 3A, 180W
Answer  Explanation  Related Ques

ANSWER: 3.919A, 184W

Explanation:
No explanation is available for this question!


27)   If the supply voltage is kept at constant value and the supply frequency is increased. The magnetizing and core loss component

a. Both will decrease
b. Magnetizing component will increase and core loss will decrease
c. Magnetizing component will decrease and core loss will increase
d. Both will increase
Answer  Explanation  Related Ques

ANSWER: Both will decrease

Explanation:
No explanation is available for this question!


28)   For a transformer, no load primary current ( I0 ) has two components, magnetizing component and active components. The magnetizing component is given by

a. I0 cos Φ0
b. I0 sin Φ0
c. I0 cot Φ0
d. I0 tan Φ0
Answer  Explanation  Related Ques

ANSWER: I0 sin Φ0

Explanation:
No explanation is available for this question!


29)   For a certain transformer if E1 is the emf across primary winding, E2 is the emf across secondary winding and K is the ratio of secondary to primary turns. The value of secondary emf referred to the primary side is

a. E2 / K
b. E2 / K ∧ 2
c. K * E2
d. K ∧ 2 * E2
Answer  Explanation  Related Ques

ANSWER: E2 / K

Explanation:
No explanation is available for this question!


30)   In transformers for maximum voltage regulation to occur, the power factor and magnitude is

a. Lagging, Zpu
b. Leading, Zpu
c. Unity, Zpu
d. None of these
Answer  Explanation  Related Ques

ANSWER: Lagging, Zpu

Explanation:
No explanation is available for this question!


31)   If the voltage supply of a single phase transformer is doubled and the supply frequency is halved, then the eddy current loss will become

a. remains same
b. double of the original
c. four times of original
d. half of the original
Answer  Explanation  Related Ques

ANSWER: four times of original

Explanation:
No explanation is available for this question!


32)   For a single phase no load transformer, which among the following losses will be minimum?

a. hysteresis losses
b. eddy current losses
c. copper losses
d. mechanical losses
Answer  Explanation  Related Ques

ANSWER: copper losses

Explanation:
No explanation is available for this question!


33)   In the design of transformer, higher flux density is used. Due to which

a. weight per kVA increases
b. weight per kW increases
c. weight per kW decreases
d. weight per kVA decreases
Answer  Explanation  Related Ques

ANSWER: weight per kVA decreases

Explanation:
No explanation is available for this question!


34)   If the applied voltage is increased by 10% and the applied frequency is increased by 20% of a single phase transformer, the percentage change in eddy current losses will be

a. 19%
b. 20%
c. 21%
d. 40%
Answer  Explanation  Related Ques

ANSWER: 21%

Explanation:
No explanation is available for this question!


35)   A 600kVA transformer has core losses of 1w at 25 Hz and 2.5w at 50 Hz (neglecting resistances and assuming flux density to be constant). The power required at 60 Hz is

a. 3 watt
b. 3.5 watt
c. 3.24 watt
d. 3.54 watt
Answer  Explanation  Related Ques

ANSWER: 3.24 watt

Explanation:
No explanation is available for this question!


36)   What happens to the hysteresis losses and eddy current losses, if the frequency of a transformer is increased keeping the supply voltage constant?

a. Both will decrease
b. Hysteresis losses will increase and eddy current losses will decrease
c. Hysteresis losses will remain same and eddy current losses will decrease
d. Hysteresis losses will decrease and eddy current losses will remain same
Answer  Explanation  Related Ques

ANSWER: Hysteresis losses will decrease and eddy current losses will remain same

Explanation:
No explanation is available for this question!


37)   For maximum efficiency in a transformer

a. Core losses = 2 * copper losses
b. Core losses = copper losses / 2
c. Core losses = copper losses
d. Core losses = copper losses ∧ 2
Answer  Explanation  Related Ques

ANSWER: Core losses = copper losses

Explanation:
No explanation is available for this question!


38)   If the power factor of a transformer increases, then its efficiency will

a. Increases
b. Decreases
c. Remains constant
d. Not related to each other
Answer  Explanation  Related Ques

ANSWER: Increases

Explanation:
No explanation is available for this question!


39)   When a transformer is supplied by full load at 0.8 p.f. lagging its copper losses is given by Pcu. if it is supplied by half load then copper losses will be

a. Pcu
b. 2Pcu
c. Pcu / 2
d. Pcu / 4
Answer  Explanation  Related Ques

ANSWER: Pcu / 4

Explanation:
No explanation is available for this question!


40)   A 600 kVA transformer has iron losses of 400 kW and copper losses of 500 kW. Its kVA rating for maximum efficiency is given by

a. 537 kVA
b. 548 kVA
c. 555 kVA
d. 585 kVA
Answer  Explanation  Related Ques

ANSWER: 537 kVA

Explanation:
No explanation is available for this question!