Design of Transformer Interview Question Answer - 7

91	Which type of winding is used for core type transformer?
	Winding in the Core type transformer Helical Double helical Multi layer helical Cross over Cylindrical Disc type
92	Why the vegetable oil is not used in the transformer tank?
	The vegetable oil is not used in the transformer tank because it form fatty acids and attack fibrous insulating materials.
93	Why aluminium is selected as transformer tank material?
	Aluminium as transformer tank material Low weight Less stray loss
94	Which type of cooling ducts is provided in the transformer core?
	Types of cooling ducts Horizontal Vertical
95	Which type of cooling duct is efficient for cooling of transformer core? Why?
	Horizontal cooling duct The horizontal type cooling duct is efficient for cooling of transformer core because the heat is conducted along the lamination. As the thermal resistivity of lamination is nearly 1/20 times that of core, the cooling is efficient due to small temperature gradient. Vertical cooling duct The vertical type cooling duct is very less efficient for cooling of transformer core because the heat is conducted across the lamination. As the thermal resistivity across the lamination is high, there is large temperature gradient between duct and hot spot of the core. This large temperature gradient does not improve the cooling condition effectively.
96	Which type of core construction is preferred in the large power transformer?
	Cruciform core
97	Why the core flux density in the distribution transformer is kept low?
	As the distribution transformer is designed for higher all day efficiency, the value of flux density is kept low in order to keep iron losses minimum in it.
98	Why the optimum value of flux density is used for transformer core?
	Flux density The core area of the transformer depends upon the flux density of the core. As the flux density increases, the core area decreases and therefore the weight of core material reduces. This will result in length of mean turns of copper winding and weight reduces. However higher value of flux density increases the core losses and also temperature rise. This will decrease the efficiency of the transformer.   Therefore the value of flux density is chosen by considering efficiency and weight of iron and winding material.
99	Why some margin is kept while selecting the flux density in the transformer core?
	Flux density The flux density is selected near knee point of the magnetizing current.  However some margin is kept for over fluxing, frequency and voltage variation.
100	Which are the factors to be considered while selecting current density for the transformer winding?
	Current density The copper losses and load at which maximum efficiency occurs depends upon current density of the winding. However the permissible value of current density depends upon heating condition in the transformer.
101	Whether the current density in the two winding of the transformer is kept equal?
	The current density in the two winding of the transformer should be kept equal in order to kept minimum copper losses.
102	Which type of winding is preferred for high voltage and low voltage winding?
	Low voltage winding Cylindrical winding Helical winding High voltage winding Continuous disc winding with rectangular conductors Cross over winding with circular conductors Cylindrical winding with circular conductors
103	Which type of winding is preferred for shell type transformer?
	Sandwich winding
104	Why the helical winding is not used for low voltage winding for low kVA transformer rating?
	Helical winding The helical winding is not used for low voltage, low kVA transformer because the number of winding turns at a given voltage increases as the kVA rating of transformer decreases. If there are large numbers of turns, the continuous disc type winding is used.
105	Write full form of s.r.b.p.
	s.r.b.p. – Synthetic resin bonded paper

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