## 29/03/2020

### Design of Transformer Interview Question Answer - 5

 61 Explain the term : Barriers Barriers It is partition of solid insulating materials inside oil ducts. 62 Why the cross over winding is divided into number of layers? How the coils are separated? Cross over winding The number of layers in the cross over winding is divided in order to reduce voltage between adjacent layers. The coils are axially separated by means of washers or axial ducts. 63 On which factors the numbers of coils in the cross over winding depends?                       OR How to decide number of coils in the cross over winding? The number of coils in the cross over winding is calculated on the basis that the maximum voltage between adjacent layers should not exceed 300 Volt. 64 What should be limit for maximum voltage between adjacent coils in the cross over winding? 800 – 1000 Volt 65 How many turn does the each coil of continuous disc type winding consists of? 8 ( Eight ) 66 How the leakage reactance is affected by height of window? Leakage reactance The leakage reactance of the winding depends upon distance between two limbs.  If the distance between two limbs is small, the width of the winding is limited and height is counter balanced it.   The winding is long and thin resulting low leakage reactance. Similarly if the distance between two limbs is large, the height of the winding is limited whereas the width increases.  The winding is short and thick resulting high leakage reactance. 67 Why the yoke area is kept larger than the core area in the transformer in hot rolled silicon steel? Yoke area The area of the yoke is kept 15 to 20% higher than the core area in the hot rolled silicon steel. This will reduce flux density in the core and therefore the iron losses and magnetizing current also reduces. 68 What is condition for maximum output considering width of window in the transformer? Maximum output The condition for maximum output in the transformer is             D = 1.7 d ( Where D is distance between centre of adjacent limbs and d is diameter of circumscribing circle ) 69 What should be thickness of paper is kept on the conductor? 0.5 mm 70 Which type of winding has higher value of current density used? HV or LV? The high voltage winding is used for higher value of current density because the high voltage winding has better cooling condition than the low voltage winding. 71 What should be limiting value of current density kept for HT side in the power transformer? The limiting value of current density for HT side in the power transformer should be in the range of 2.5 to 3.5 Amp / mm2. 72 Which type of winding is used for HT side of the power transformer? Cross over or cylindrical 73 Whether the reluctance of yoke affects the flux distribution in the transformer? Reluctance of yoke The reluctance of flux path through yoke is negligible therefore it does not affect the flux distribution in the transformer. 74 Why the HV winding is sandwich between two LV winding in the transformer? HV winding and LV winding If the HV winding is sandwich between two LV winding, the leakage reactance becomes small. This type of arrangement is done in the distribution transformer where close voltage regulation is essential. 75 Describe the effect of radial forces on the transformer winding. Radial forces The radial forces on the transformer winding produces tensile force on the outer winding whereas it produces compressive force on the inner winding.

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