Temperature Rise of Transformers/Motors

The less heat generated the more efficient the device would have to be given equal loads and/or equal work done...

You are right, low temperature rise will entail bigger equipment and thus higher cost.

That's why it is always appropriate to stick to International standards which ensure optimum design / cost as well as a life of over 25 years.

Of course, there has to be correction applied to the allowable temperature rise (per International standards) specification based on the local ambient temperature.

Your question is framed a bit oddly, and the fact that you posted this anonymously makes me think that this is a homework question, correct? If so, please use our responses as a start of your own searches for answers.

Insulation typically degrades faster with higher temperatures, but the equipment is designed to operate within certain parameters. Your deduction of "more cooling accessories, therefore more size and weight" is not true. Forced cooling (air & oil) of transformers allows for more 20%-30% more MW in a very similar package size, with lower weight - less oil is required for the same MW output. Look up the terms ONAN (oil natural, air natural, ONAF (oil natural air forced) & ODAF (oil driven air direct). There is some extra maintenance and failure modes, but there is a reason that it is in wide use.

"...makes me think that this is a homework question, correct?"

Sounds probable.

Of course higher temperature also means higher resistance of the conductors, which leads to more heat (Higher losses). If not controlled, this could become a destructive repeating loop until the magic smoke is released.

That's a worthy topic for an undergraduate research project.

Basically it is the temperature rise over the ambient taht matters .Various insultion levels have maximum temp + 10 Degrees allowance for hot spots.With Class F insulation mostly used in Motors normal practice is to consider ambient temp 40Deg C over which rise in case of class B insulation is limited to 80 Degrees as maximum temp allowed for cass B is 130 degrees and making an allowance for hot spots 10 dg is redused and hence total temp rise including ambient is limited to 120Degrees Same way for class F max is 145DegC and rise is lmited to 135 Deg.In certain countries higher ambient is specified like 50Degrees then rise automaticlly gets redused .Voltage variation i supply also cause over heating and all these to be factored while choosing a motor.Same principles apply to transformers .Appropriate trip devices are provided in higher capacity motors

Here is a graph showing the relationship between insulation life and temperature.

This would apply to transformers as well as motors.

Heat kills,and it is accumulative.

The heat damage will not recover when cooled.

A subsequent heat rise simply adds to the previous damage.