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Which of the following statements are true regarding parts of a transformer? (i) The thickness of laminations varies from 0.35 mm to 0.5 mm. (ii) The material used for breather is blue in colour when it is damp and whitish pink when dry. (iii) For a constant input voltage, the output voltage can be varied over a small range by providing few tapings.
(i) and (iii)
(i), (ii) and (iii)
(i) and (ii)
(ii) and (iii)
(i) and (iii)
Statement (i) is correct as transformer laminations are typically 0.35 mm to 0.5 mm thick to minimize eddy current losses. Statement (ii) is incorrect because silica gel in the breather is blue when dry (active) and turns pink or whitish when damp (saturated). Statement (iii) is correct because tap changers allow for small adjustments in output voltage by varying the number of turns in the winding.
IS 2026 (Part 1): 2011
Statement (i) is correct as transformer laminations are typically 0.35 mm to 0.5 mm thick to minimize eddy current losses. Statement (ii) is incorrect because silica gel in the breather is blue when dry (active) and turns pink or whitish when damp (saturated). Statement (iii) is correct because tap changers allow for small adjustments in output voltage by varying the number of turns in the winding.
PeтАЛ=KeтАЛтЛЕBmax2тАЛтЛЕf2тЛЕt2 тАФ Eddy current loss formula where t is lamination thickness
V2тАЛV1тАЛтАЛ=N2тАЛN1тАЛтАЛ тАФ Ideal transformer turns ratio relationship
Transformer laminations are kept thin to increase the electrical resistance path for eddy currents, thereby reducing eddy current losses (PeтАЛ=KeтАЛBmax2тАЛf2t2). The breather utilizes hygroscopic silica gel to remove moisture from the air entering the transformer during thermal contraction; the color change serves as a visual indicator of saturation. Tap changers operate on the principle that the turns ratio (a=N2тАЛN1тАЛтАЛ=V2тАЛV1тАЛтАЛ) dictates the secondary voltage; changing N2тАЛ effectively adjusts V2тАЛ.
Laminations are insulated with varnish to prevent electrical contact between sheets.
Silica gel turns from blue (dry) to pink/white (moist) due to cobalt chloride moisture absorption.
Off-circuit tap changers require the transformer to be de-energized before operation.
On-load tap changers (OLTC) allow voltage adjustment without interrupting the load.
Thin laminations minimize core heating.
Breathers prevent oil degradation and insulation failure due to moisture.
High moisture in silica gel can lead to dielectric breakdown of the transformer oil.
Too many tapings can increase winding complexity and leakage reactance.
Distribution transformers
Power transformers in transmission substations
Standard lamination thickness for power frequency (50Hz) is typically 0.35mm using cold-rolled grain-oriented (CRGO) steel.
Option B is wrong because statement (ii) reverses the color indication for moisture absorption.
A is correct тАФ (i) and (iii) are accurate technical statements regarding transformer construction and voltage regulation.
Always remember the mnemonic 'Blue-Dry' for silica gel; if the breather is pink, the transformer needs maintenance or the silica gel needs replacement.
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