Q.1 What is the power factor of a transformer? Ans. At no load, the power factor of a transformer is very low and lagging, whereas the power factor on load is nearly equal to the power factor of the load, which it is carrying. Q.2 What is the normal phase difference between the voltage and the no-load current in a transformer? Ans. The no-load current in a transformer normally lags behind the voltage by about 80° – 85°. Q.3 What are the essential parts of a transformer? Ans. The essential parts of a transformer are as follows: Magnetic circuit consisting of laminated iron core and clamping structure Primary winding Tank filled with insulating oil HT terminals with bushings LT terminals with bushings Conservator tank Breather Vent pipe Thermometer Q.4 What is the name of the winding to which supply is given? Ans. Primary winding. Q.5 What is the name of the winding from which the supply is taken for load connections? Ans. Secondary winding. Q.6 Which material is used for the core of a transformer and why? Ans. Laminations of specially alloyed silicon steel (silicon content 4–5 per cent) are used due to its high electrical resistance, high permeability, non-ageing characteristics and minimum iron loss. Q.7 What is the use of iron core in a transformer? Ans. The iron core is used in a transformer to provide continuous easy magnetic path of low reluctance. Q.8 How is magnetic leakage reduced? Ans. Magnetic leakage is reduced to a minimum by sectionalizing and interleaving the primary and secondary windings. Q.9 Why are iron cores in transformers made laminated? Ans. Iron cores are made laminated to reduce eddy current loss. Q.10 What determines the thickness of the lamination or stamping? Ans. Frequency determines the thickness of the lamination or stamping. Q.11 Why are the laminations insulated from each other? Ans. The laminations are insulated from each other by insulating varnish or thin paper to break the path of eddy currents and thus reduce eddy current loss. Q.12 What is stacking factor? What is its approximate value? Ans. Stacking factor is the ratio of iron content in the laminated varnished core by volume. Its value is about 90 per cent, that is, 10 per cent volume is occupied by varnish and air. Q.13 What is called grain-oriented laminations? Ans. Grain-oriented laminations are cold rolled laminations specially annealed to orient the iron crystals, that is, the grains in a uniform way in the direction of rolling to get very high permeability and low hysteresis loss. Q.14 What is the permissible maximum flux density in transformer core? Ans. 1.6–1.8 Wb/m2. Q.15 What is the phase relationship between the primary and secondary voltages of a transformer? Ans. The primary and secondary voltages of a transformer are 180° out of phase. Q.16 What is turn ratio of a transformer? Ans. The ratio of the number of turns in the primary to the number of turns in the secondary windings is called the turn ratio or the ratio of transformation of the transformer, which is indicated by a constant. images Q.17 What is voltage ratio of a transformer? Ans. Voltage ratio is the ratio of the voltage between the line terminals of one winding to that between the line terminals of another winding at no load. Q.18 What current flows in the transformer primary when its secondary is open? Ans. When the secondary is open, there is no current in the secondary of the transformer. The primary takes a small current I0) from the source called no-load current which has a magnetizing com-ponent (I0 sinθ0) producing the magnetic flux and a working component (I0 cosθ0) supplying real power for iron losses. Q.19 What is the formula for calculating no-load current? Ans. No-load current images Q.20 Why is the frequency not changed during transformation of electrical energy in a transformer? Ans. As the same flux having a definite frequency is responsible for the production of emf on both the primary and secondary windings, there is no question of change in the frequency. Q.21 What is the emf equation of a transformer? Ans. Erms = 4.44 Φm fN V where Φm = maximum value of the flux linkage with both the windings, f = frequency of the supply, N = number of turns. Q.22 What are the two basic types of transformers? Ans. The two basic types of transformers are: The isolation type in which the two windings are physically isolated and electrically insulated from each other. The autotransformer type in which one coil is used for both the windings. Q.23 What are the types of transformers according to the arrangement of iron cores? Ans. There are three types: (i) core type, (ii) shell type and (iii) distributed core or Berry type. Q.24 What magnetic circuit is formed in Berry-type constructions and why? Ans. Distributed magnetic circuits are formed in Berry-type construction because of its distributed cores. Q.25 What is called limb of a transformer? Ans. The vertical portion of the iron core where the windings are placed is called limb of a transformer. Q.26 Why are LT windings placed near the core? Ans. LT windings are placed near the core to reduce the total dielectric strength of the insulating mate-rials provided on the winding. Q.27 What are the types of windings according to the construction? Ans. Sandwich type and cylindrical type. Q.28 What is the difference between cylindrical-type and sandwich-type winding? Ans. In cylindrical-type winding, the length of the coils is equal to the length of the core limb. The pri-mary and secondary windings are placed one over the other, placing low-voltage winding nearer the core, whereas in sandwich-type winding HT and LT windings are placed lengthwise one above and other like a sandwich. Q.29 What are the types of transformers according to its location? Ans. (i) Indoor-type transformer (ii) Step-down transformer. Q.30 What do you mean by step-up and step-down transformers? Ans. When a transformer converts low voltage to high voltage it is called a step-up transformer, and when the transformer converts high voltage to low voltage it is called a step-down transformer. Q.31 What are the advantages of using a transformer in an AC supply? Ans. The transformer has the following advantages: Voltage can be transformed from low voltage to high voltage for transmission and high voltage to low voltage for distribution without changing the frequency. As the transformer has no rotating parts, there is no factional loss and wear and tear is minimum. A high voltage or an extra high voltage can be transformed easily by providing good insulation. Its maintenance cost is low. Practically it requires very little attention for its operation. Q.32 What is an ideal transformer? Ans. A transformer having an overall efficiency of 100 per cent is called an ideal transformer. Q.33 What do you mean by power transformer? Ans. Transformers that are used on transmission lines for the transmission and distribution of relatively large quantities of energy are called power transformers. Q.34 What do you mean by distribution transformers? Ans. When transformers are used for distributing the energy from transmission lines as well as net-works for local consumption and the secondaries are directly connected to the consumers load, they are called distribution transformers. Q.35 What do you mean by lighting transformer? Ans. A transformer used to supply a distribution circuit having no motors connected to it is called light-ing transformer. Q.36 How does a transformer contribute towards the widespread popularity of AC system over DC? Ans. High voltage of AC system can be obtained using a transformer for transmission of electrical power. Using a transformer, AC voltage can be increased or decreased without any power loss.
Posted on: Wed, 12 Nov 2014 18:31:08 +0000