How do you calculate derating of squirrel cage induction motor when operating at higher ambient temperature than mentioned on name plate?

There is no "slam dunk" answer for calculating the derating of a motor operated beyond its manufacturer's rated temperature. There is a very good reason for this. The "curve" of a motor's performance with regard to temperature isn't linear.In fact, it's far from it.

And it will vary a great deal depending on a number of characteristics associated with a given machine. These will include its rating at "ambient" temperature, its size, and number of other things about its shape and mechanical construction. If an individual needs to operate a motor outside the manufacturer's ratings, it's a no-brainer that the manufacturer should be contacted to get information.

The hook is economy A lot of motors can be "pushed" a bit over their operating temperatures for a short time, but extended operation in the "red zone" will bring early failure. "Over-rating" a motor isn't a good idea if the objective is to save a few bucks.It will cost more in the long run to "cook" the unit and have to replace it than to get one that is rated for the operating conditions or to run cooling to the motor. This is really a question of engineering, and the smart engineer will cool a given motor or will get a different motor that is rated for the operating conditions.

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