Which of the following is a common cause of transformer overheating in operation?

• A. Increased cooling
• B. Low ambient temperature
• C. Excessive ventilation
• D. Overload

Answer: (D)

Overheating happens when the heat generated inside a transformer exceeds what its cooling system can remove. The main extra heat comes from copper losses in the windings, which rise with the current squared. When the transformer carries more current than it was designed to carry (an overload), those losses increase a lot, and the temperature climbs even if cooling is working as designed. Iron losses in the core stay more or less the same with load, so the big driver is the extra current causing more I^2R heating. If cooling is increased or ambient temperature is cooler, the transformer would stay cooler, not overheat. Excessive ventilation also helps remove heat. So the common cause of overheating during operation is overload, when the load exceeds the transformer’s rated capacity.