Unit 3 - Transformers 1

UNIT 3 - TRANSFORMERS 1. What is meant by window space factor? The window space factor kw is defined as the ratio of copper area in window. kw= Ac /Aw= conductor area in window / total area of window 2. What are the advantages of having circular in a transformer? The excessive leakage fluxes produced during short circuit and over loads , develops severe mechanical stress in the coils. on circular coils these forces are radial and there is no tendency for the coil to change its shape but on the rectangular coils the forces are perpendicular to the conductors and tends to deform the coil in circular. 3. What are the different losses in a transformer? i) Core or Iron loss ii) copper loss 4. State the merits of three phase transformers over single phase transformer. i. A three phase transformer occupies less space for same rating, compared to a bank of three single phased transformers. ii. It weight less iii. It cost is less iv. The core will be of smaller size and the material required for the core is less. 5. Distinguish between shell type and core type transformer. Core type Shell type Simple in design Comparatively complex Reduction of leakage reactance is not easily Reduction of leakage reactance is highly possible possible Low mechanical strength High mechanical strength. 6. What are the salient features of a distribution transformer? i. The distribution transformers will have low iron loss and higher value of copper loss ii. The capacity of transformers will be upto 500KVA iii. The transformers will have plain walled tanks or provided with cooling tubes or radiators iv. The leakage reactance and regulation will be low 7. How the distribution transformer differs from that of a power transformer? i. The distribution transformers are designed to have low iron loss and higher copper loss, whereas in power transformers the copper loss will be lesser than iron loss ii. The distribution transformers are designed to have the maximum efficiency at a load much lesser than full load, whereas the power transformers are designed to have maximum efficiency at or near full load. iii. In distribution transformer the leakage reactance is kept low to have better regulation, whereas in power transformer the leakage reactance is kept high to limit the short circuit current. 8. In transformers, why low voltage winding is placed near the core. The winding and core made of metals and so insulation have to be placed in between them. The thickness of insulation depends on the voltage rating of the winding. In order to reduce the insulation requirement the low voltage winding is placed near the core. 9. The area of the yoke in a transformer is taken 15 to 20% larger than that of the core. Why? By keeping yoke area 15-20% higher the flux density in the yoke is reduced, resulting into reduction in iron losses for yoke. The reduced core area results working flux density and needed to increase the number of turns. 10. What you mean by stacking factor? What its usual value? In transformers, the core is made of laminations and the laminations are insulated from each other by a thin coating of varnish. Hence when the laminations are stacked to form the core, the actual iron area will be less than the core area. The ratio of iron area and total core area is called stacking factor. The usual value is 0.9 11. What is tertiary winding? Some three phase transformers may have a third winding called tertiary winding apart from primary and secondary. It is also called auxiliary winding or stabilizing winding. The tertiary winding is provided in a transformer for any one of the following reasons, i) to supply small additional load at a different voltage ii) to give supply to phase compensating devices such as capacitors which work at different voltages. iii) to limit short circuit current. iv) to indicate voltage in high voltage testing transformer. 12. The voltage per turn of a 500KVA, 11KV, ∆/Y three phase transformer is 8.7V calculate the number of turns per phase of LV and HV windings Sol: Phase voltage of LV winding = = 239.6V (star connected) Phase voltage of HV winding = 11,000V (Delta connected) Number of turns in LV winding = = 27.54=28turn Number of turns in HV winding= no of turns of LV winding x phase voltage ratio = 28 x =1285.5=1286turns 13. What are conservator and breather? A conservator is a small cylindrical drum fitted just above the transformer main tank. It is used to allow the expansion and contraction of oil without contact with surrounding atmosphere. When conservator is fitted in a transformer, the tank is fully filled with oil and the conservator is half filled with oil. The breather is a device fitted in transformer for breathing. In small oil cooled transformers some air gap is provided between the oil level and tank top surface. When the oil is heated, it expands and air is expelled out of transformer to the atmosphere through breather. When the oil is cooled, it shrinks and air is drawn from the atmosphere into the transformer through breather. This action of transformer is called breathing. 14. List out the advantages and disadvantages of stepped core transformer. Advantages: i) for same area of cross section the stepped cores will have lesser diameter of circumscribing circle than square cores. This results in reduction in length of mean turn of the winding with consequent reduction in both cost of copper and copper loss. Disadvantages: with large number of steps a large number of different sizes of laminations have to be used. This results in higher labor charges for shearing and assembling different types of laminations. 15. State the various methods of cooling of power transformers. i) oil natural-air forced ii) oil natural-water forced iii) oil forced-air natural viii) oil forced-air forced iv) oil forced-water forced 16. How the heat dissipation is improved by the provision of cooling tubes? The cooling tubes will improve the circulation of oil. The circulation of oil is due to more effective pressure heads produced by columns of oil in tubes. The improvement in cooling is accounted by taking the specific heat dissipation due to convection as 35% more than that without tubes. 17. Why is the core of a transformer laminated? The cores of a transformer laminated in order to reduce the eddy current losses. The eddy current losses are proportion to the square of thickness of laminations. This apparently implies that the thickness of laminations should be extremely small in order to reduce the eddy current losses to a minimum. 18. Write the relation between core area and weight of iron and copper for a single phase transformer. Ac = Tp δp + Ts δs 19. What is the cause of noise in transformer? The cause of noise in the transformer is mainly because of magnetostriction effect and also loosening of stampings and mechanical forces produced during working. 20. Comment on the hysteresis and eddy current loss when voltage and frequency of a transformer is doubled. 2 Hysteresis Loss = Kh f Bm Hysteresis loss is proportional to the supply frequency. 2 2 Eddy Current Loss = KCf Bm Eddy Current Loss is proportional to the square of the supply frequency. 21. Explain the significance of the ratio r=Φm/AT in the design of transformer. In transformers, the ratio of specific magnetic and electric loading is specified as r=Φm/AT; rather than actual value of specific loadings. 22. How the leakage reactance of the transformer is reduced? In transformers the leakage reactance is reduced by interleaving the high voltage and low voltage winding. ----- %%%% ------- .

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