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Subtituting an oversized motor is a very common mistake that will nearly always result problems.
There is a basic law in physics which states that energy can neither be created nor destroyed, we can only change its form.
By example a light bulb changes electrical energy to light energy.
In its simplest form we could then say an electric motor is a device designed to convert electrical energy into mechanical energy.
There is another law of physics which states that 746watts of electrical energy equals 1 Horsepower of mechanical energy thus if we require a 1/2HP motor we design a motor winding that will draw 746/2 = 373watts of electrical energy however that motor will continue to draw that amount of energy even if the attached load is much less.
When you installed a 1/2HP motor in place of the 1/4HP the motor was drawing its rated 373watts of electrical energy. 186.5watts of that energy was then being converted to mechanical energy to drive the fan but since energy cannot be created nor destroyed the remaining 186.5watts of surplus energy was then being converted to Heat Energy in the motor windings and the motor burned out prematurely.
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