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Transformer interview questions paper-6

91.What material is used to fill in the breather?

Ans: Moisture absorbing silicon gel or CaCl2 is used to fill the breather which turns pink when it absorbs moisture . When fried again it gets its original form by removing its moisture contents.

92. What is emergency release ?

Ans : It is a large diameter explosion vent pipe fitted on the top of the tank of transformer projecting upwards and incorporating diaphragms on the mouth of it.

93. What is the purpose of explosion vent pipe on the transformer ?

Ans : If there is a large volume of gas formed within the tank due to short circuit fault in the winding the pressure is released from the transformer through the explosion vent pipe by rupturing the diaphragms mounted on the mouth of it.

94.How can the temperature of the hot oil be determined in a transformer ?

Ans : The temperature of the hot oil be determined with the help of a thermometer by putting its bulb inside a low pocket meant for thermometer situated near the top of the transformer tank where the oil is hottest.

95. What do you know about the temperature indicator of the transformer winding ?

Ans : The temperature indicator is nothing but a dial type thermometer fitted in such a way that its bulb is immersed in the region where the transformer oil is hottest and the bulb is associated with a small encircled heating coil connected to the is so designed that the bulb temperature is increased by an amount equal to the greatest winding to oil temperature gradient and an appropriate value of temperature is indicated directly on the dial.

96. What are the causes of temperature rise in a transformer ?

Ans: Bad cooling and overloading are the causes of temperature rise in a transformer .

97. What are the losses in a transformer ?

Ans : The losses in a transformer are – (1) Core losses or iron or constant losses consisting of hysteresis and eddy current losses in the core and (2)Copper losses in the primary and secondary winding .

98. Is hysteresis objectionable ?

Ans: Since hysteresis is a kind of loss and affect the efficiency of the transformer, so it is always objectionable.

99.What are the factors on which the hysteresis loss depends?

Ans: The hysteresis loss depends on (1)the quality and constituents of the iron core, (2) the frequency and (3) the flux density.

100. Which iron loss is more in a transformer?

Ans: Hysteresis loss is more than eddy current loss in a transformer .

101. What is the main purpose of using silicon steel in the laminations of transformer core ?

Ans : To reduce the hysteresis loss.

102. Is there any variation of core flux between no load of full load ?

Ans: No, the core flux is always constant on every loads.

103. What are the functions of no load current in a transformer ?

Ans : No load current produces core flux and supplies iron losses and copper loss on no load.

104. What do you mean by the load current of a transformer ?

Ans : When a load is connected to the secondary side of a transformer the transformer is said to be on load and the current circulating through the secondary winding vai load is called the load current.

105. What do you mean by the primary current on load ?

Ans : When the load current (I2) flows through the secondary winding a counterbalancing current (I1,) is induced on the primary winding varying inversely to their respective turns . The total primary current on load (I1) is equal to the vector sum of the primary counterbalancing current (I1) and the no load current (I0) which will be approximately equal to (I1,) as I0 is practically very small.

106. What will be the effect on primary said if the load current increases ?

Ans : Primary current increases with the increase in load current.

107. What is called the equivalent circuit of the transformer ?

Ans : The equivalent circuit of the transformer means the simplified corresponding transformer circuit modeling the behavior of a real transformer which represents the primary circuit as well as the secondary circuit in one circuit with suitable parameters for easy calculation with the addition of a shunt circuit to account for no load current.

108. What is called the approximate equivalent circuit of the transformer ?

Ans : The approximate equivalent circuit of the transformer means a slight modification of the equivalent circuit by transferring the shunt circuit to the primary terminal with close approximation affecting very little change in the result.

109. How do you calculate the equivalent resistance and reactance of the transformer referred to the primary and secondary ?

Ans: Equivalent resistance referred to primary r1, =r1+ (N1/N2)2 r2
Equivalent reactance referred to primary x1, =X1 + (N1/N2 )2 x2
Equivalent resistance referred to secondary r2, = r2 + (N2/N1 )2 r1
Equivalent reactance referred to secondary X2, = x2+(N2/N1 )2 x1
Where, r1 and r2 are the primary and secondary resistances respectively. And x1 and x2 are the primary and secondary reactances respectively. N1 and N2 are the primary and secondary turns respectively.

110. A 50 KVA transformer has a turn ratio on (N1/N2 ) = 10. The impedance of the primary winding is 4+j7 ohms while that of the secondary winding is o.6 + j 1.1 ohms. What will be the impedance of the transformer when referred to the primary?

Ans : 64 + j 117 ohms.

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