Answer / razaq ali

because electrical heater is constant load with constant
current.
overload relays are used when load current increases due
due to increase in load.

A 15 kw squirrel cage motor having a maximum efficiency of 90% on a continuous full load has temperature rise of 42 degrees celcius after 30 min and 52 degrees celcius after 1 hr under the above operation conditions. a) calculate its final steady state temperature rise on continuous load and the heating time constatnt. b) if the motor is operated on a short load for 9 min estimate its temperature rise. c) calculate the maximum overload that can be applied on a short, 9 min rating so that the temperature rise is the same as the final steady state temperature rise on continuous load.

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