Objectives Or Mcqs On Transformer | All Student World

1) Open circuit in a transformer is prefered with

a)    Rated Transformer voltage

b)    Rated transformer current

c)    Direct Current

d)    High frequency supply

Answer: a) As the Name implies high voltage side is left open and the low voltage need to applied rated voltage to get the constant loss, Because constant loss depends on the supply voltage. Therefore rated voltage applied at the low voltage side for no load test.

2). Open circuit test on transformer gives

a)    eddy current loss

b)    hysteresis loss

c)    copper loss   

d)    both 1 and 2  

Answer: d) the no load current drown by the primary is around 2% to 6% of rated current.Therefore copper loss of primary is very low and can be neglected. As the copper loss is negligible, the watt-meter reading is considered as iron loss or constant loss i.e eddy current loss and hysteresis loss

3). In a single phase transformer the magnitude of leakage reactance is the twice the that of resistance of both primary and secondary. With Secondary short circuited, the input power factor is

a)    1/√2    

b)    1/√5

c)    2/√5

d)    1/3

Answer: b)

4). If P_i and P_c  represent core and full load copper loss respectively, the maximum KVA delivered to load corresponding to maximum efficiency is equal to rated KVA is

a)    p_i /Pc    

b)    (P_i /Pc )

c)    √(P_i /P_c )

d)    P_c /P_i 

Answer: c)

5). A Transformer has maximum efficiency at 3/4 load. The ratio of iron loss and full load copper loss is

a)    4/3  

b)    3/4

c)    9/16

d)    16/9

Answer: c)

6). When a short circuit test on a transformer is performed at 25 V, 50 HZ , the drown current is I₁. If the test is performed by 25 V and 25 Hz and power drown current is I₂ , then

a)    I₁ > I₂

b)    I₁ < I₂

c)    I₁ = I₂

d)    non of the above

Answer: b)

as frequency decreases, reactance will decrease and the current drown I₂ will be more then I₁

7). Which of the following statements is /are correct statement ?

a)    For ideal Transformer efficiency is 100%  

b)    For Ideal transformer voltage regulation is 100%

c)    both 1 and 2

d)    non of the above

Answer: a)

8). Scott connections are used for

a)    single phase to three phase transformer

b)    three phase to single phase transformer

c)    three phase to two transformer

d)    any of the above

Answer: c) Scott connections are used to convert three phase to two phase conversion, particularly to start two phase motors and two phase furnaces. it requires two single phase transformers with with adjustable tapping, one transformer is known as main transformer which is center tapped through teaser transformer primary with a suitable number of turns to get a balanced two phase supply

9). Two transformers operating in parallel will share the load depending on their

a)    Rating

b)    Leakage reactance

c)    Efficiency    

d)    Per unit impedance

Answer: d) In order to have perfect load sharing proportional to their KVA rating the leakage impedance should be inversely proportional to the kKVA ratings or per unit impedance of the transformer in parallel should be same. If the per unit impedance are not same then the transformer which have smaller pu will share greater amount of the load irrespective of their KVA rating

10). A 100/10 ,50 VA double winding transformer is connected to 100/110 V auto transformer. The rating of auto transformer is

a)    550 VA

b)    500 VA

c)    110 VA

d)    100 VA

Answer: d)