Which of the following Drying out method is note used in rotating machines?

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Examoogle · Accepted Answer

Circulating Oil Explanation: Drying out is a maintenance process to remove moisture from electrical machine windings after exposure to damp environments or prolonged storage. 'Circulating Oil' is a cooling and insulation medium used in power transformers, not a method for drying out the windings of rotating electrical machines like motors or generators. Working Principle: The principle involves raising the temperature of the insulation system above the ambient temperature to drive off moisture. Common methods include passing a low-voltage DC or AC current through the windings (Circulating current), placing the machine in a heating enclosure (Heating chamber), or applying external heat sources such as Incandescent lamps. Key Formulas: $P = I^2R$ — Power dissipation used in current-based drying; $Q = mc\Delta T$ — Heat energy required to increase temperature Key Points: Moisture in insulation reduces dielectric strength significantly.; Insulation resistance must be measured periodically during the drying process.; Temperature must be controlled to prevent damage to the varnish or insulation material.; Circulating oil is used for transformer cooling and insulation, not machine drying. Advantages: Prevents internal arcing and insulation breakdown; Restores original dielectric properties of the winding insulation Disadvantages: Time-consuming process requiring careful monitoring; Risk of insulation damage if temperature is not regulated Applications: Post-flood restoration of machines; Pre-commissioning of machines stored in high-humidity areas Additional Info: Standard drying temperature for Class B insulation is typically limited to 90-100°C.. Option D is incorrect because oil-filled cooling is a design feature of static equipment, not a drying technique for rotating machines. Answer: D is correct — Circulating Oil is a cooling medium for transformers and is not an accepted method for drying out rotating electrical machines. Exam Tip: Always distinguish between maintenance for static transformers (which use oil) and rotating electrical machines (which rely on air/hydrogen cooling and ohmic heating).

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